Method for characterizing crack propagation
Patent Information
- Application Number
- PCT/EP2026/058233
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2025-03-26
- Filing Date
- 2026-03-24
- Publication Date
- 2026-10-01
Smart Images
Figure EP2026058233_01102026_PF_FP_ABST
Abstract
Description
[0001] METHOD FOR CHARACTERIZING CRACK PROPAGATION
[0002] Technical Field
[0003] The present invention relates to methods for characterizing crack propagation and / or cracking failure in samples. It further pertains to quality control and selection methods based on the outcome obtained from the methods for characterizing crack propagation and / or cracking failure.
[0004] Background Art
[0005] In the pharmaceutical and medical devices industries, many products, like for example needle hubs of syringes’ needles, undergo quality control tests to verify that they meet required quality standards related to resistance to cracking failure and crack propagation.
[0006] Needle hubs are often produced by injection moulding, and they exhibit susceptibility to crack propagation and cracking failure when attached to syringes. Particularly, it is often observed that a cracking failure is visible in correspondence to an injection gate of the needle hub which acts as starting point of the crack propagation.
[0007] A common method of assessing susceptibility of the needle hubs to cracking failure when being attached to a syringe involves attaching them to syringes and applying a predefined load, a force or a torque depending on the design of the needle hub. A needle hub passes this test if it can withstand the applied load without breaking, indicating that it meets a basic mechanical strength criterion. To verify that a production batch of needle hubs meets this mechanical strength criterion, a statistical approach is typically employed. This involves testing a representative sample of multiple needle hubs from the batch, allowing for an inference about the overall quality of the entire production batch.
[0008] Typically, these quality control tests are attributive in nature, providing limited information about the properties of the needle hubs. In general, attributive tests involve a binary evaluation, focusing solely on whether a specific attribute, such as the ability to endure a predefined load, is present (pass) or absent (fail), without providing further quantitative details about the attribute's magnitude or variation.Moreover, attributive tests require a comparatively large sample size to achieve statistically significant results. Consequently, they demand considerable resources, making the process costly and inefficient.
[0009] Additional methods, such as Izod and Charpy tests, are employed in the testing industry for cracking failure characterization. However, these methods offer limited insights into specimen properties and behaviour. Furthermore, these tests necessitate specialized equipment that is not commonly available in laboratories. Moreover, adapting these setups for testing needle hubs for syringes is particularly challenging, as these are usually designed for material specimens with predetermined shapes. Specifically, these methods are not suitable for characterizing propagation and failure of longitudinal cracks along specimens, like those that for example can propagate on needle hubs.
[0010] Therefore, there is a need for a resource-efficient method for cracking failure and crack propagation characterization, in particular for axisymmetric hollow specimens, such as needle hubs. This method should provide continuous quantitative data and allow for a comparatively comprehensive characterization of cracking failure and crack propagation. Further, there is a need for resourceefficient quality control and selection methods, in particular suitable for axisymmetric specimens such as needle hubs, based on their capacity to withstand crack propagation and cracking failure.
[0011] More in particular, there is a need for a method to analyse longitudinal cracking failure and / or propagation in axisymmetric hollow specimens, such as needle hubs.
[0012] Disclosure of the Invention
[0013] The following definitions of the general terms, expressions and concepts used in the present description apply irrespectively of whether the terms in question appear alone or in combination with other embodiments.
[0014] The term “characterize” and its derivatives, such as “characterization”, as used herein, refer to the process of analyzing and describing features, mechanisms, and stages of a physical phenomenon. In particular, wherein the physical phenomenon is crack propagation or cracking failure.A "sample," as intended herein, can consist of either a single specimen, multiple specimens from a statistical population or an entire statistical population. Accordingly, the methods, features and considerations referring to a sample can refer to a single specimen, multiple specimens, a statistical population The characteristics and behaviours of a statistical population can indeed be represented by a sample drawn from it. The conclusions, characterizations, and outcomes derived from a sample can be applied to the sample or inferred to represent the statistical population from which the sample is drawn. In particular, in the context of the selection method according to the present invention, samples can differ from each other for example by material, manufacturing process, design, production batch, sterilizing process or the like. In particular, samples can represent statistical populations or types of specimens differing from each other in the listed or other features. In case the specimens composing the sample are needle hubs, the samples can differ from each other for example by shape of the hub, dimensions, location of an injection gate, dimensions of a specific structural feature or the like.
[0015] The expression "characterizing model," as used herein, refers to a construct that represents characteristics related to crack propagation in a sample and / or of its cracking failure behaviour. The characterizing model can take various forms, including equations, graphical plots, qualitative observations or categorizations, as well as specific or aggregate values such as the recorded maximum compressive force, displacement at failure, or statistical descriptors like the mean of a distribution. The characterizing model can be derived from empirical data. Further examples and clarifications are provided throughout this disclosure, particularly in the section titled "Characterizing model".
[0016] The expression “qualitative observation”, as used herein, refers to an attributive assessment of a characteristic related to crack propagation in a specimen or to cracking failure of a specimen that do not report numerical outcomes, but can be based on numerical outcomes. For example, it can relate to the presence, location or extent of a crack. Examples of qualitative observations can be “crack is not present”, “no cracking failure under 50N compressive force”, “crack on the tested location”.The term “suitability”, as used herein in the context of the quality control and the selection methods, refers to the degree to which a product is "suitable," meaning it is appropriate and well-fitted, for meeting the specific requirements and conditions of an intended use. A suitable product is fit for purpose and capable of operating effectively within defined parameters and constraints.
[0017] The term “hoop stress”, as used herein, refers to circumferential stress experienced by a material of an axisymmetric structure, generally as a result of internal or external pressure acting perpendicular to the axisymmetry axis. It is a tensile stress that occurs in the direction tangential to the cross-section of the structure perpendicular to the axisymmetry axis.
[0018] The expression "weak point," as used herein, refers to a specific location / feature within the specimen that is considered more likely to fail under applied load conditions than other locations / features. The term "weakest point" refers to the "weak point" where the failure actually occurs.
[0019] The expression “tested location,” as used herein, refers to a specific area along the substantially axisymmetric surface that is of particular interest for crack propagation and / or cracking failure characterization. This location can include features such as notches or manufacturing marks, which are anticipated to act as stress concentrators and potential initiation sites for crack propagation.
[0020] The concept “substantially axisymmetric surface”, as used herein, relates to a surface that is largely axisymmetric, accounting for both the technical tolerances and defects inherent in manufacturing processes and intentional design features that introduce asymmetries or non-uniform ities. These can include surfaces that are not perfectly axisymmetric due to planned discontinuities or variations, as well as sections of an axisymmetric surface. In particular, substantially axisymmetric surfaces can be substantially conical or cylindrical surfaces. Moreover, the term “surface” refers to a three-dimensional boundary, incorporating a thickness. Examples of specimens with hollow parts delimited by substantially axisymmetric surfaces, can be syringe needles, needle hubs, syringe barrels and the like. These are often designed with specific features that deviate from perfect symmetry to meet functional or manufacturing requirements, such as injection gates or reinforced sections and can also include defects like material non-uniform ities. The cross-sections perpendicular to the axisymmetry axis of a substantially axisymmetric surface delimiting a hollow part of a specimen will be substantially annuluses. Hoop stress tangential and parallel to such cross-sections can arise under specific loading conditions.
[0021] The term “axisymmetry” and its derivatives, like “axisymmetric”, as used herein, refer to a type of symmetry where an object or surface is invariant under rotations around a specific axis. This means that if the object or surface is rotated about this axis, its appearance does not change. Axisymmetric objects and surfaces are characterized by having a consistent shape or profile around their axisymmetry axis. In particular, axisymmetry is characteristic of objects and surfaces that can be generated by revolving a shape or profile around an axisymmetry axis.
[0022] The term “discontinuity”, as used herein, relates to any non-uniform ity of the substantially axisymmetric surface. Examples of non-uniform ity include geometrical discontinuities such as notches, moulding lines, depressions, bulges, thinner surface, thicker surface; material discontinuities such as different crystallized states or different materials and design or structural features like injection gates or reinforced sections.
[0023] The term “injection gate area”, as used herein, refers to a part of an injection gate or the whole injection gate of a part manufactured by injection moulding.
[0024] The expression “crack propagation”, as used herein, refers to the process by which a crack in a specimen evolves and extends under the influence of stress, encompassing both the initial development of a crack and its subsequent growth. This process can involve the initiation, extension, and / or possible branching of cracks. In particular, it can involve the initiation of a crack where no crack was previously present.
[0025] When a compressive force or compressive displacement is applied to a specimen, it results in a corresponding compressive displacement or compressive force, respectively. The terms "compressive displacement" and "compressive force", as used herein, can refer to either the applied or resulting conditions, unless otherwise specified. Values for both the applied and / or the resulting compressive force and / orcompressive displacement can be recorded when performing the method according to the first aspect of present invention.
[0026] The expression "and / or" is used herein when it is desired to highlight that the elements listed before and after the term can be considered individually or in combination. For instance, "mathematical model obtained from at least one value of the applied and / or resulting compressive force and / or compressive displacement" include the options of a mathematical model obtained from only values of the resulting compressive force, only values of the resulting compressive displacement, only values of the applied compressive force, only values of the applied compressive displacement, or any combination of these such as values of the applied compressive force and values of the resulting compressive displacement, values of the applied compressive displacement and values of the resulting compressive displacement. The same relationship exists among all elements of a list separated by commas and "and / or." For example, "a, b and / or c" includes the options: only element a, only element b, only element c, a combination of a and b, a combination of a and c, a combination of b and c, or a combination of a, b, and c.
[0027] The concept of “calibrating” a characterizing model or its parameters involves adjusting the parameters and structure of the characterizing model to reflect the observed data. This process aims to minimize the discrepancies between the model's outputs and real-world observations. Calibration often involves using statistical methods and optimization techniques to find the best-fit parameters that align the model with empirical data. This is a well known process in the field of mechanical characterization of samples.
[0028] The expression “longitudinal crack”, as used herein, refers to a crack that develops relatively parallel to an axis of axisymmetry of a substantially axisymmetric surface of a specimen. These cracks can occur due to stresses acting perpendicular to this axis. In particular, in the context of the present invention, they can occur due to hoop stress.
[0029] The expression “upper compressive force limit” as used herein, relates to a value of compressive force that is determined to be the maximum allowable force to be applied onto the specimen while performing the method according to the first aspectof present invention. The term “displacement limit” as used herein, relates to a value of compressive displacement that is determined to be the maximum allowable displacement to be applied onto the specimen while performing the method according to the first aspect of present invention.
[0030] The term “measurement noise”, as used herein, relates to a noise in a recorded value of the compressive force or compressive displacement. For example, when using a uniaxial tension machine to perform the method according to the first aspect of present invention, before both compression plates are in contact with the specimen, artificial compressive force noise can be recorded. Such artificial compressive force noise can invalidate part of recorded compressive force values.
[0031] The term "compressive force at failure", as used herein, refers to the compressive force at which the specimen is deemed to undergo cracking failure. This force can be identified in various ways.
[0032] The term “reference value”, as used herein, relates to a value which represents or quantifies a characteristic or an aspect of a sample related to crack propagation in the sample and / or to its cracking failure behaviour. The term “selection reference value”, as used herein, refers to a reference value used in the context of the selection method according to the present invention.
[0033] The term “outcome”, as used herein, can refer to:
[0034] • the values or qualitative observations recorded when performing any embodiment according to the method according to the first aspect of the present invention;
[0035] • a characterizing model.
[0036] Furthermore, in the claims the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single unit or step can fulfil the functions of several features recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. The terms “essentially”, “about”, “approximately”, “substantially” and the like in connection with an attribute or a value particularly also define exactly theattribute or exactly the value, respectively. The term “about” in the context of a given numerate value or range refers to a value or range that is, e.g., within 20%, within 10%, within 5%, or within 1% of the given value or range unless differently specified. Any reference signs in the claims should not be construed as limiting the scope.
[0037] There is a need for a more resource-efficient method for cracking failure and / or propagation characterization. This method should provide continuous quantitative data and allow for a comprehensive characterization of these phenomena. Further, there is a need for more resource-efficient quality control and selection methods for needle hubs based on their capacity to withstand crack propagation and / or cracking failure.
[0038] In particular, there is a need for a method to analyse longitudinal cracking failure and / or propagation in substantially axisymmetric specimens, like needle hubs.
[0039] According to the invention these needs are fulfilled by a method as it is defined by the features of independent claim 1 , by a quality control method as it is defined by the features of independent claim 11 and by a selection method as it is defined by the features of independent claim 13. Further embodiments are subject of the dependent claims.
[0040] In a first aspect, the present invention relates to a method for characterizing crack propagation and / or cracking failure in a sample comprising a specimen comprising a hollow part delimited by a substantially axisymmetric surface, wherein the method comprises the steps of:
[0041] a) orienting the specimen so that an axisymmetry axis of the substantially axisymmetric surface is substantially perpendicular to a compression direction;
[0042] b) applying a compressive force or a compressive displacement onto the substantially axisymmetric surface of the specimen substantially along a compression direction;
[0043] c) recording at least one value of the applied and / or a resulting compressive force and / or compressive displacement or at least onequalitative observation related to crack propagation and / or cracking failure.
[0044] In an embodiment of the method according to the first aspect of present invention, the method is used for characterizing cracking failure due to longitudinal cracks and / or crack propagation of longitudinal cracks.
[0045] In a third embodiment of the method according to the first aspect of present invention, the substantially axisymmetric surface includes a tested location, in particular wherein the tested location comprises a discontinuity of the substantially axisymmetric surface; and the method further comprises a step, preceding steps b and c, of: orienting the specimen so that a tangent to the substantially axisymmetric surface of the specimen passing through a centre of the tested location is substantially parallel to the compression direction;
[0046] In a further embodiment of the method according to the first aspect of present invention, the applied compressive force or applied compressive displacement in step b is progressively increased.
[0047] In a fifth embodiment of the method according to the first aspect of present invention, step b is stopped after a drop in the compressive force occurs and / or when an upper compressive force limit and / or a compressive displacement limit is reached.
[0048] In a further embodiment of the method according to the first aspect of present invention, the hollow part is made by injection moulding.
[0049] In a further embodiment of the method according to the first aspect of the present invention, the specimen comprises a needle hub.
[0050] In a further embodiment of the method according to the first aspect of present invention, the hollow part is the needle hub.
[0051] In a tenth embodiment of the method according to the first aspect of the present invention, step b is characterized in that the applied compressive force or compressive displacement is applied with a uniaxial tension machine.In an eleventh embodiment of the method according to the first aspect of the present invention, the method further comprises a step of obtaining a characterizing model of the crack propagation and / or cracking failure for the sample, wherein the characterizing model comprises at least one of:
[0052] • a mathematical model obtained from at least one value of the applied and / or resulting compressive force and / or compressive displacement recorded at step c;
[0053] • a representative value calculated from at least one value of the applied and / or resulting compressive force and / or compressive displacement recorded at step c;
[0054] • a graph constructed from at least one value of the applied and / or the resulting compressive force and / or compressive displacement recorded at step c;
[0055] • a qualitative categorization based on at least one qualitative observation recorded at step c.
[0056] In a further embodiment of the eleventh embodiment of the method according to the first aspect of present invention, the characterizing model is related to a compressive force at failure and / or a compressive displacement reached at the compressive force at failure.
[0057] In a further embodiment of the eleventh embodiment of the method according to the first aspect of present invention, the characterizing model comprises a representative value and the representative value represents a compressive force at failure and / or a compressive displacement reached at the compressive force at failure.
[0058] In a further embodiment of the method according to the first aspect of the present invention, a progressively increased compressive displacement is applied and the progressively increased displacement is increased at a substantially constant displacement velocity.In a further embodiment of the method according to the first aspect of the present invention, the application of the compressive force or of the compressive displacement is stopped when a compressive displacement limit is reached and, in particular the compressive displacement limit is equal to around two times a thickness of the specimen along the substantially axisymmetric surface.
[0059] In a further embodiment of the method according to the fifth embodiment of the first aspect of the present invention, the upper compressive force limit and / or the compressive displacement limit is selected to prevent damage of an equipment used for performing the method.
[0060] In a further embodiment of the method according to the first aspect of the present invention, the method further comprises a step of applying a static compressive pre-load onto the specimen before performing the step of applying the compressive force or the compressive displacement.
[0061] In a further embodiment of the method according to the first aspect of the present invention, the method further comprises a step of applying a static compressive pre-load onto the specimen before performing the step of applying the compressive force or the compressive displacement and the static compressive pre-load is set to exceed a measurement noise.
[0062] In a further embodiment of the method according to the first aspect of the present invention, the hollow part is made at least partially of a material prone to crack propagation and / or cracking failure under tensile stress.
[0063] In a further embodiment of the method according to the eleventh embodiment of the first aspect of the present invention, the characterizing model comprises a representative value and the representative value represents the compressive force at failure and / or the compressive displacement reached at the compressive force at failure.
[0064] In a further embodiment of the method according to the eleventh embodiment of the first aspect of the present invention, the sample comprises multiple specimens and the characterizing model is obtained from at least one recording of step c for at least two of the specimens comprising the sample.In a second aspect, the present invention relates to a quality control method for a sample comprising a specimen comprising a hollow part delimited by a substantially axisymmetric surface, wherein the substantially axisymmetric surface undergoes hoop stress in an intended use of the sample; the quality control method comprising the steps of:
[0065] a) performing the method according to anyone of the preceding claims on the sample;
[0066] b) obtaining a reference value from an outcome of step a;
[0067] c) comparing the reference value with a defined criterion;
[0068] d) assessing suitability of the sample for an intended use on the basis of step c.
[0069] In an embodiment of the quality control method according to the second aspect of the present invention, the reference value is the compressive force at failure or the compressive displacement reached at the compressive force at failure or a value derived from at least one of these; the criterion consists in a defined threshold value and; step d comprises: considering the sample suitable for the intended use when the reference value is higher than the criterion or considering the sample suitable for the intended use when the reference value is lower than the criterion.
[0070] In a third aspect, the present invention relates to a selection method for samples comprising a specimen comprising a hollow part delimited by a substantially axisymmetric surface, wherein the substantially axisymmetric surface undergoes hoop stress in an intended use of the specimen; and wherein the selection method comprises the steps of:
[0071] a) characterizing multiple samples according to a method according to an embodiment of the first aspect of the present invention;
[0072] b) selecting, on the basis of an outcome of step a, the sample which showed the best performance for the intended use.
[0073] In a second embodiment of the third aspect of the present invention, step b comprises obtaining for each sample a selection reference value from the outcomeof step a and selecting the sample with the best selection reference value, in particular wherein the selection reference value is a numeric value and wherein the best selection reference value is the highest or the lowest.
[0074] In a further embodiment of the second embodiment of the selection method according to the third aspect of the present invention, the selection reference value is, for each sample, the compressive force at failure or the compressive displacement reached at the compressive force at failure or a value derived from at least one of these.
[0075] In a further embodiment of the third aspect of the present invention, the samples differ from each other in material, manufacturing process, sterilization process and / or design.
[0076] In a fourth aspect, the present invention relates to a testing system configured to perform the methods according to the present invention, wherein the testing system comprises a uniaxial tension machine and a specimen holder, in particular wherein the specimen holder comprises a vise-like structure configured to orient the position of the specimen.
[0077] The method according to the first aspect of present invention advantageously offers an efficient framework for characterizing crack propagation and / or cracking failure. By requiring a comparatively small number of straightforward inputs, namely a compressive force or a compressive displacement, the method minimizes the introduction of external noise, thereby enhancing the precision, repeatability and reliability of the results. This reduction in experimental variability ensures that the characterization more accurately represents the intrinsic behavior of the specimen under examination. Additionally, the method is compatible with standard laboratory equipment, facilitating its incorporation into existing testing workflows and further contributing to its efficiency.
[0078] As cracking failure can be obtained for each specimen of each sample, and so the test conditions are comparatively stringent, it is possible to identify and quantify differences in performance that are not visible using other existing methods. For example, the methods described herein, which are based on variable quantitative data, can identify the different probability of failure between different types ofspecimens. This is particularly beneficial in the context of the selection process, as identifying differences in performance among various types of specimen facilitates the identification and selection of the optimal specimen type.
[0079] In particular, the method for characterizing crack propagation and / or cracking failure herein described can be applied to characterize cracking failure due to longitudinal cracks and / or crack propagation of longitudinal cracks, specifically in specimens comprising axisymmetric hollow parts, more specifically in needle hubs.
[0080] Utilizing a mathematical model, a representative value, or a graph derived from multiple recorded values of the applied and / or resulting compressive force and / or compressive displacement can be particularly advantageous. This approach captures the variable outcomes of the method, providing a detailed and comprehensive characterization of crack propagation within the specimen and its overall cracking failure behavior. The continuous data generated enables a nuanced understanding of the specimen's response to hoop stress and a comprehensive characterization of crack propagation and / or cracking failure, thereby facilitating more informed assessments of the mechanical behavior of the specimen and allowing to identify potential weak points. This is particularly advantageous when using the method for characterizing crack propagation and / or cracking failure for longitudinal cracks in a specimen, particularly for needle hubs, because it provides a continuous characterization of such phenomena.
[0081] Specifically, the method according to the first aspect of present invention, providing continuous numerical data along a broad range of compressive force and compressive displacement obtained before, during and after the crack propagation and cracking failure, allows to characterize different stages of these physical phenomena and so to provide a comprehensive characterization.
[0082] The method according to the first aspect of present invention can be particularly advantageous to identify the weakest point of the specimen and to characterize its cracking failure. More specifically, when the substantially axisymmetric surface is subjected to a compressive force, it experiences hoop stress (a tensile stress oriented tangentially to an axisymmetric surface and perpendicular to the relative axisymmetry axis), which promotes crack propagation from existing weak points. Therefore, it will promote the cracking failure of the weakest point.The step of orienting the specimen such that a tangent to the substantially axisymmetric surface at the tested location is substantially parallel to the compression direction is particularly effective for inducing and maximizing hoop stress at the tested location. This approach ensures that optimal conditions for analyzing crack initiation and propagation at the tested location are achieved, enhancing the precision, reliability, and repeatability of the method. Therefore, such approach advantageously allows to particularly characterize the mechanical behavior the tested location, and of any discontinuity or feature it comprises. However, it is important to note that failure will (most likely) occur at the specimen's actual weakest point, which not necessary coincides with the tested location. While the method focuses stresses on the tested location, it effectively assesses the mechanical behaviour of the entire specimen by inducing stress conditions that promote cracking failure at weak points. Therefore, this method not only allows for a quantitative and continuous analysis of cracking failure behaviour but also enables to determine whether the tested location corresponds to the weakest point of the specimen. This dual capability enhances the method's utility in quality control, specimen type selection and mechanical characterization. Further, this approach can be particularly advantageous when used in the context of the selection method according to the third aspect of present invention if the tested location is a feature that differentiate the multiple samples compared. In fact, this approach allows to differentiate the samples focusing on the resistance of the tested location to crack failure and / or crack propagation.
[0083] Progressively increasing the compressive force or compressive displacement is particularly advantageous as it facilitates the recording of multiple values of the applied compressive force or compressive displacement and / or the resulting compressive displacement and / or compressive force. Therefore, it facilitates the continuous, precise and accurate characterization of the specimen along a range of compressive force and / or compressive displacement. A continuous characterization of the specimen cracking failure and crack propagation is particularly advantageous as it provides a comprehensive characterization of a specimen behaviour.Further, the method according to the first aspect of the present invention, being continuous and quantitative, enables reliable statistical inference while requiring fewer specimens compared to attributive methods.
[0084] The displacement velocity can be chosen to be high enough to generate a cracking failure in the specimen and a consequent drop in the compressive force. Advantageously, the displacement velocity can also be chosen to not be so high as to induce noise in the data recording. For example, a particularly advantageous displacement velocity can be of 50 millimetres / minute.
[0085] In particular, the displacement velocity can be selected to be low enough to avoid measurement noise due to the inertia of the equipment used for applying the compressive force or compressive displacement. Further, the displacement velocity can be selected to be high enough to deliver energy at a sufficient rate to initiate crack propagation and / or cause cracking failure. The displacement velocity necessary to achieve this effect will be influenced by the material's fracture toughness and Young's modulus. The minimum displacement velocity to initiate crack propagation and / or cause cracking failure for a type of specimen can be identified experimentally.
[0086] Advantageously, when performing the method according to the first aspect of present invention, it can be chosen if to stop the application of the compressive force or compressive displacement when cracking failure occurs, when crack propagation is observed, when an upper compressive force limit is reached and / or when a compressive displacement limit is reached. Consequently, while performing the method it can be chosen if to proceed until failure of the specimen or not.
[0087] The upper compressive force limit and the displacement limit can be advantageously chosen to prevent a damage to a used equipment while performing the method according to the first aspect of present invention. The upper compressive force limit can correspond to a load cell safety limit. For example, the upper compressive force limit can be of about 350 Newtons. The displacement limit can be of about twice a thickness of the substantially axisymmetric surface of the specimen. For example, the displacement limit can be of about 5 millimetres.Further, the method according to the first aspect of present invention can be used also as an attributive method recording a qualitative observation and, potentially, using a qualitative characterization as characterizing model. For example, the application of the compressive force or compressive displacement can be stopped when an upper compressive force limit and / or a compressive displacement limit is reached. The qualitative observation can consist in verifying if there was cracking failure and the specimen can be categorized as “passed” or “failed”: if the specimen fails before reaching the upper compressive force limit and / or the compressive displacement limit it is considered “failed”, otherwise it is considered “passed”. This approach can be advantageous in the context of the quality control method. For instance, the representative value can be the percentage of “passed” specimens, with the criterion being that a minimum percentage of specimens of the sample is categorized as "passed." If this criterion is met, the sample or the represented statistical population is considered suitable for the intended use.
[0088] An intended use in the context of the present invention can be an intended use in which the substantially axisymmetric surface is subject to hoop stress, in particular, a use in the medical industry. For example, the specimen could be a needle hub and the use could be its coupling to and use with a syringe.
[0089] It can be particularly advantageous to select the upper compressive force limit and / or the displacement limit such to be sure that the cracking failure of the specimen is observed. For example, when characterizing the cracking failure for a specific type of specimen, it can be advantageous in a first phase to perform the method according to the first aspect of present invention setting the upper compressive force limit and / or the displacement limit such to prevent damage of the used equipment. Subsequently, the upper compressive force limit and / or the displacement limit can be set, on the basis of the data obtained in the first phase, such that the cracking failure will be always observed for the type of specimen that is being characterized. This can be done as in the first phase it can be possible to sufficiently characterize the type of specimen. This approach can be particularly advantageous in the context of the quality control according to the second aspect of the present invention. Specifically, the parameters determined through this approach can be employed to consistently apply the quality control method tosamples of the same type of specimen on different occasions, thereby increasing efficiency in performing the quality control method.
[0090] Stopping the application of the compressive force or compressive displacement after a drop in the compressive force can be particularly advantageous as such drop can correspond to the failure of the specimen. Therefore, it can correspond to the end of the cracking failure of the specimen that is being characterized and thereby it can be assured to comprehensively describe the crack propagation and the cracking failure. In particular, it can be advantageous to stop the application of the compressive force or compressive displacement after a drop in the compressive force for example of at least 1 %, in particular of at least 5%, more in particular of at least 10%. While performing the method according to the first aspect of present invention, it can be advantageous to choose as stopping point for the application of the compressive force or of the compressive displacement the event that happens before between a drop in the compressive force, the upper compressive force limit or the displacement limit. This approach can allow to characterize in a continuous and complete manner the crack propagation and / or cracking failure of the specimen without risking damaging the used equipment.
[0091] It can be particularly efficient to cease the application of compressive force or displacement immediately after sufficient values have been recorded to characterize the drop in compressive force.
[0092] The compressive force at failure can be efficiently and effectively identified with a peak value recorded in the compressive force followed by a drop in compressive force, for example of at least 1%, in particular of at least 5%, more in particular of at least 10%. In this case, the compressive displacement reached at the compressive force at failure corresponds to the compressive displacement reached at the peak value recorded for the compressive force.
[0093] For example, the compressive force at failure can be determined by applying the tangent intersection method on a compressive force versus compressive displacement curve.
[0094] The application of the static compressive pre-load can improve the repeatability and reliability of the method according to the first aspect of present invention as itensures that the starting point of the method is the same for different specimens. For example, when a uniaxial tension machine comprising two compression plates is used to perform the method according to the first aspect of present invention, applying the pre-load ensures that the two compression plates are in contact with the specimen before starting the step of applying the compressive force or the compressive displacement on the specimen. The static compressive pre-load can consist in a defined compressive displacement or, preferably, a defined compressive force.
[0095] The static compressive pre-load can be a force of about 0.5 Newtons.
[0096] In the method according to the first aspect of present invention the specimen can be made at least partially of a material prone to crack propagation under tensile stress. For example, the specimen can be an injection needle comprising a hub made of a material prone to crack propagation and a needle made of a metallic material not prone to crack propagation.
[0097] Examples of materials prone to crack propagation under tensile stress can be Polymethyl Methacrylate (PMMA), polycarbonate (PC), Acrylonitrile Butadiene Styrene (ABS), Polyethylene Terephthalate (PETG) glass, ceramic materials, alumina, zirconia, cast iron.
[0098] The method according to the first aspect of the present invention can be used for specimens where the thickness of the substantially axisymmetric surface is significantly smaller than the outer diameter of the corresponding cross-section perpendicular to the axis of symmetry. For instance, the thickness-to-outer diameter ratio may be less than 1 / 3, about 1 / 3, less than 1 / 5, about 1 / 5, less than 1 / 10, about 1 / 10, and similar ratios. The hollow part can possess a longitudinal length that is significantly greater than the average outer diameter of the crosssections perpendicular to axisymmetry axis. For example, the outer diameter-to-longitudinal-length ratio can be less than 1 / 3, about 1 / 3, less than 1 / 5, about 1 / 5, less than 1 / 10, about 1 / 10, and similar ratios. The hollow part can also feature at least one open end. For example, the hollow part can be a needle hub with an average thickness of about 0.5 mm and an average outer diameter of about 5 mm. Utilizing the method of the first aspect of the present invention for specimens withthese characteristics can be advantageous, as such specimens are more prone to experiencing hoop stress, crack propagation and / or cracking failure.
[0099] The method according to the first aspect of present invention can be particularly advantageous to characterize crack propagation and / or cracking failure in a hub of an injection needle, particularly when the hub of the injection needle is made by injection moulding and presents an injection gate. This method indeed allows to characterize crack propagation and / or cracking failure in the hub in a continuous manner, providing the characterization along a broad range of compressive force and / or compressive displacement providing comprehensive data. Moreover, the injection gate constitutes a discontinuity of the substantially axisymmetric surface of the needle hub and it can constitute a weak point of the hub. The method according to the first aspect of present invention allows to apply a tensile stress tangentially to the substantially axisymmetric surface of the hub. Therefore, any crack will propagate starting from the weakest point of the substantially axisymmetric surface. In case the crack would generate and propagate starting from the injection gate this can be identified as the weakest point of the hub and the crack propagation and / or crack failure at this position can be characterized.
[0100] The quality control method is particularly advantageous because it is based on the method according to the first aspect of present invention which provides continuous data. Therefore, it is based on a comprehensive characterization of the sample which allows for smaller sample populations to represent an entire statistical population. Therefore, this quality control method is more efficient and effective than attributive methods used for quality control.
[0101] The selection method is particularly advantageous as it can clearly distinguish the differences in behaviour between different samples. In particular, it can exacerbate the difference in cracking failure probability or differences related to crack propagation in different samples. This is possible because, thanks to the used equipment and method, the cracking failure of the specimen can always be reached and so the sample can be characterized comprehensively in its behaviour.
[0102] The reference value and the selection reference value can be obtained from a recording of a value of the applied and / or a resulting compressive force and / or compressive displacement or a qualitative observation related to crack propagationand / or cracking failure. In particular, wherein the sample is composed of multiple specimens, the reference value and the selection reference value can be an aggregated value derived from the values or qualitative observations recorded for the multiple specimens. This aggregated value can involve central tendencies, such as the mean, median, or mode, or other statistical metrics, such as variance or standard deviation. For example, the reference value for a sample can be the average compressive force at failure or the average compressive displacement reached at the compressive force at failure measured across all the specimens composing the sample.
[0103] In particular, the reference value and the selection reference value can be obtained from or can be a value included in a characterizing model.
[0104] Further, multiple reference values can be considered simultaneously to assess quality of a sample or to define a best performing sample.
[0105] The holder can advantageously comprise a vise-like structure configured to fix the specimen. This configuration allows to effectively and easily tune and fix the position and orientation of the specimen. In the methods according to the present invention, it can be advantageous to fix the specimen such that it keeps the desired orientation throughout the whole performance of the method.
[0106] The present invention covers further embodiments with any combination of features from different embodiments described above and below.
[0107] The advantages and considerations illustrated in regards to the method according to the first aspect of the present invention are applicable also to the quality control and the selection methods.
[0108] Characterizing model
[0109] The term "characterizing model," as used herein, refers to a construct that represents characteristics related to crack propagation in a sample and / or of its cracking failure behaviour.Characterizing models can, for example, describe the phenomenology of the observations, also relating it to the specimen’s attributes, design characteristics, processing parameters or the like.
[0110] The characterizing model can take various forms, comprising:
[0111] mathematical model: this involves equations or formulas that describe the relationship between applied and resulting compressive force and compressive displacement. For example, a mathematical model might express the compressive force as a function of the compressive displacement, such as y = kx, where y is a dependent variable representing the compressive force in Newtons, k is a constant, and x is an independent variable representing the compressive displacement. A mathematical model can also be used to represent the dependency of the compressive displacement from the compressive force or of the hoop stress from the strain generated in a specimen.
[0112] When a sample is composed of multiple specimens, it can be advantageous to choose a mathematical model as characterizing model and to fit it to one or more values of the compressive force and / or of the compressive displacement recorded for at least two of the multiple specimens.
[0113] For example, when characterizing a statistical population of needle hubs, a sample of multiple needle hubs representing the population can be defined. A characterizing model in the form y’=k’x’ can be chosen wherein y’ represents the compressive force, x’ represents the applied compressive displacement and k’ is a scaling factor. The scaling factor k’ can be calibrated by fitting the characterizing model to the recorded values of the resulting compressive force and of the applied compressive displacement for the needle injection hubs composing the sample.
[0114] In a further example, when the method according to the first aspect is applied to multiple specimens of a sample, the characterizing model can be in the form of a Gaussian distribution:
[0115]
[0116] wherein:
[0117] p is the mean of the distribution;
[0118] $ is the standard deviation;
[0119] e is the base of the natural logarithm;
[0120] x’” is the independent variable and represents the compressive displacement reached at the compressive force at failure;
[0121] and the characterizing model can be calibrated based on the data recorded for a sample representing a statistical population of specimens. A value for p can be chosen as criterion for the quality control method, and the statistical population can be considered suitable for the intended use if the value of p calculated through the data obtained by the data recorded for the sample is lower than the chosen criterion.
[0122] In general, it can be advantageous not to consider the values recorded for some of the specimens to calibrate or calculate the characterizing model. For example, values recorded for specimens which did not show a cracking failure can be discarded to be more conservative. The same approach can be advantageous to be more conservative when obtaining the (selection) reference value in the context of the quality control method or of the selection method according to the present invention. These considerations can be applied to all the forms of characterizing model.
[0123] representative value: this is a specific or aggregate numerical value that summarizes a key characteristic of the sample's behaviour, such as the maximum recorded compressive force or the displacement at failure. For instance, the median, mean or mode compressive force at failure determined for a sample of multiple specimens can serve as a representative value.
[0124] Using a representative value as characterizing model can be particularly advantageous when performing the quality control method or the selection method according to the invention. Particularly, this form of characterizing model can ease the choice of the reference value or of the selection reference value. Specifically,the representative value can be used as reference value or selection reference value.
[0125] A particularly advantageous characterizing model can be in the form of “compressive force at failure (Newtons)” = 8; wherein 8 represents the numerical value of the compressive force at failure. This characterizing model can effectively and efficiently represent the cracking failure of the specimen. Alternatively, 8 can represent the compressive displacement reached at the compressive force at failure and similar advantages in efficiency and effectiveness can be achieved in characterizing the cracking failure.
[0126] In the context of the quality control method, the characterizing model can be for example in the form “compressive force at failure (Newtons)” = 8; wherein 8 represents the numerical value of the compressive force at failure. A minimum acceptable value of compressive force at failure can be chosen as criterion. A specimen having a value of 8, used as reference value, higher than the chosen criterion can be considered suitable for the intended use.
[0127] When a sample is composed of multiple specimens, it can be advantageous to choose a representative value as characterizing model and to calculate it based on one or more values of the compressive force and / or of the compressive displacement recorded for at least two of the multiple specimens.
[0128] For example, an advantageous characterization model for a sample composed of multiple specimens can be in the form of y”=8”, wherein y” represents an aggregate quantity measured for the sample and 8” is the numerical value of y”. For example, y” can represent the mean, median or mode compressive force at failure in Newtons of the sample.
[0129] graph: This form includes graphical representations, such as plots or charts, that illustrate the relationship between variables like the compressive force and the compressive displacement. For example, a compressive force - compressive displacement curve showing a specimen’s response under compressive load conditions can be used to represent the behavior of the sample when subject to crack propagation. This form of characterizing model can be particularly advantageous to comprehensively represent the behavior of a sample.qualitative categorization: this involves categorizing the sample's behaviour based on observed characteristics, such as the location and pattern of crack propagation. Examples might include classifications like "crack at tested location”, “crack at unexpected weak point” or "no crack", advantageously providing qualitative insights into where and how cracking failure occurs.
[0130] It can be advantageous, in the context of the method according to the first aspect of the present invention, to use characterizing models typically employed to represent the data of compressive displacement and compressive force in a mechanical compression test. These same models can be used to represent the data of compressive force and compressive displacement obtained from the method according to the first aspect of the present invention. Within the context of the present invention, these characterizing models can be utilized to describe the characteristics of crack propagation and / or cracking failure. Such methods have the advantage to be familiar to the skilled person and so to ease the interpretation and assessment of the outcome of the methods according to the present invention.
[0131] Brief Description of the Drawings
[0132] The method for characterizing crack propagation and / or cracking failure, the quality control method and the selection method according to the invention are described in more detail hereinbelow by way of exemplary embodiments and with reference to the attached drawings, in which:
[0133] Fig. 1 shows a representation of a needle hub after having been characterized according to an embodiment of a method for characterizing crack propagation and / or cracking failure according to the invention;
[0134] Fig. 2 shows a schematic view of a cross-section of the needle hub of Fig. 1 being compressed by a uniaxial tension machine while being characterized;
[0135] Fig. 3 shows a graph representing the relationship between the resulting compressive force in Newtons (Kraft in N) and the applied compressive displacement in millimeters (Standardweg in mm) for the needle hub shown in Fig.
[0136] 1 and for other needle hubs of the same sample;
[0137] Fig. 4 shows a scatterplot representing the relationship between the compressive force at failure in Newtons (F Max [N]) and the compressive displacement reached at the compressive force at failure in millimeters (Displacement at Fmax [mm]) formultiple needle hubs representing different batches analyzed and compared according to an embodiment of the selection method according to the invention; Fig. 5 shows a boxplot representing the distribution and the mean values of the compressive force at failure in Newtons (F Max [N]) for the batches of needle hubs introduced in Fig. 4; and
[0138] Fig. 6 shows a boxplot representing the distribution and the mean values of the compressive displacement reached at the compressive force at failure in Newtons (F Max [N]) for the batches of needle hubs introduced in Fig. 4.Examples
[0139] Hereinafter, the present invention will be more specifically described based on examples, but the present invention is not limited to the following examples.
[0140] References are made to the Drawings described in chapter “Brief Description of the Drawings”.
[0141] NEEDLE HUBS - characterization and quality control
[0142] The cracking failure and crack propagation of a sample of multiple needle hubs (10), as example of specimens, were characterized according to a method according to the first aspect of present invention.
[0143] Setup
[0144] A uniaxial tensile machine (20), schematically shown in Fig. 2, was employed to perform the method according to the first aspect of present invention. The method was performed on a batch of 50 needle hubs (10), example of sample of specimens.
[0145] The parameters used for the uniaxial tensile machine were:
[0146] Pre-load: 0.5 Newtons
[0147] Displacement velocity: 50 mm / min
[0148] Compressive displacement limit: 5 mm
[0149] Upper compressive force limit: 550 Newtons
[0150] Methods
[0151] The following procedure was conducted for each needle hub (10):
[0152] The needle hub (10), example of specimen with a hollow part delimited by a substantially axisymmetric surface, was positioned with no gaps on the lower plate (22) of the uniaxial tensile machine (20) ensuring it could be compressed by the upper plate (23). The needle hub (10) was inserted into a vise-like structure. The needle hub (10) was oriented such that the tangent to its external surface (16) -example of a substantially axisymmetric surface - passing through the centre of the injection gate (11), example of tested location, was substantially parallel to the compression direction (31). The needle hub (10), was further oriented such that its axisymmetry axis was perpendicular to the compression direction. The position of the needle hub (10) was fixed using the vise-like structure.
[0153] The upper plate (23) was advanced towards the needle hub (10) at a speed of 5 mm / min until a static compressive pre-load of 0.5 Newtons was applied onto the needle hub (10), at which point the upper plate (23) was halted.
[0154] A compressive displacement was applied to the needle hub (10) by lowering the upper plate (23) at a constant displacement velocity of 50 mm / min.
[0155] During the lowering of the upper plate (23), multiple data points comprising relative values of the applied compressive displacement and the resulting compressive force were recorded.
[0156] The upper plate (23) was stopped upon meeting one of the following conditions:
[0157] A peak value followed by a drop of at least 5% was recorded for the resulting compressive force (the peak value was identified as the compressive force at failure).
[0158] Achievement of the upper compressive force limit of 350 Newtons.
[0159] Achievement of the compressive displacement limit of 5 mm.
[0160] The upper plate (23) was subsequently raised.
[0161] The needle hub (10) was removed from the vise-like structure.
[0162] The needle hub (10) was examined and classified, on the basis of qualitative observations, into one of the following categories, example of qualitative categorization:
[0163] “gate” - wherein the crack (12) propagated through the injection gate (11);• “opposite” - wherein the crack (12) propagated on the opposite side of the needle hub (10) relative to the injection gate (11 );
[0164] • “both sides” - wherein the crack (12) propagated through both the injection gate (11 ) and the opposite side;
[0165] • “just whitening” - wherein no crack propagation was observed.
[0166] Fig. 2 illustrates a schematic cross-sectional view of a needle hub (10) undergoing compression. The image highlights the concentration of hoop stress (30a and 30b) within the cross section at the tested location (15), distributed along the thickness (14) of the needle hub (10).
[0167] Results
[0168] Upon completion of the steps for all needle hubs (10), a graph, example of characterizing model, was generated, as depicted in Fig. 3, illustrating the relationship between the resulting compressive force in Newtons (Kraft in N) and the applied compressive displacement in millimetres (Standardweg in mm) for each characterized needle hub (10).
[0169] The censored data (40) indicated by the first rectangle (41) were excluded from subsequent analysis as they related to needle hubs (10) which did not exhibit cracking failure before reaching the predetermined upper compressive force limit of 350 Newtons.
[0170] Conversely, the valid data (42) highlighted by the second rectangle (43) in Fig. 3 and pertaining to needle hubs (10) which exhibited crack propagation, as suggested by the drops in compressive force (44), were utilized for further analysis.
[0171] In Fig. 1 , a representation of needle hub (10) can be seen after having undergone the characterization. It presents a crack (12) propagated through the injection gate (11), example of tested location and discontinuity.
[0172] Quality control
[0173] A mean peak value of 160 N was calculated for the compressive force at failure of the samples of the batch. This value was used as reference value. The chosencriterion was a mean value for the compressive force at failure of 200 N. Therefore, the characterized sample and the type of needle hubs represented by it were considered unsuitable for being used with a predefined syringe, example of intended use. Further needle hub types were therefore evaluated.
[0174] NEEDLE HUBS - Selection method
[0175] Multiple batches of different types of needle hubs, example of samples, were compared and one type of needle hubs, example of statistic population, was selected as preferred according to the selection method according to the invention.
[0176] Setup
[0177] A uniaxial tensile machine (20) was employed to perform the method according to the first aspect of present invention for the different needle hub types.
[0178] The parameters used for the uniaxial tensile machine were:
[0179] • Pre-load: 0.5 Newtons
[0180] • Displacement velocity: 50 mm / min
[0181] • Compressive displacement limit: 5 mm
[0182] • Upper compressive force limit: 550 Newtons
[0183] The selection method was performed on each specimen of a batch of multiple specimens, for each type of needle hub. The types of needle hubs differed from each other in their material.
[0184] The three needle hub types and the corresponding batches are referred to as:
[0185] • Reference
[0186] • Optimized#!
[0187] Optimized#2The batches were composed of, respectively: 30, 90 and 30 specimens.
[0188] Methods
[0189] To characterize the needle hubs (10) and the types of needle hubs the following procedure was conducted on each specimen of each batch:
[0190] The needle hub (10), example of specimen with a hollow part delimited by a substantially axisymmetric surface, was positioned with no gaps on the lower plate (22) of the uniaxial tensile machine (20) ensuring it could be compressed by the upper plate (23). The needle hub (10) was inserted into a vise-like structure. The needle hub (10) was oriented such that the tangent to its external surface (16) -example of a substantially axisymmetric surface - passing through the centre of the injection gate (11), example of tested location, was substantially parallel to the compression direction (31). The needle hub (10) was further oriented such that its axisymmetry axis was perpendicular to the compression direction. The position of the needle hub (10) was fixed using the vise-like structure.
[0191] The upper plate (23) was advanced towards the needle hub (10) at a speed of 5 mm / min until a static compressive pre-load of 0.5 Newtons was applied onto the needle hub (10), at which point the upper plate (23) was halted.
[0192] A compressive displacement was applied to the needle hub (10) by lowering the upper plate (23) at a constant displacement velocity of 50 mm / min.
[0193] During the lowering of the upper plate (23), multiple data points comprising relative values of the applied compressive displacement and the resulting compressive force were recorded.
[0194] The upper plate (23) was stopped upon meeting one of the following conditions:
[0195] • A peak value followed by a drop of at least 5% was recorded for the resulting compressive force (the peak value was identified as the compressive force at failure).
[0196] • Achievement of the upper compressive force limit of 350 Newtons.
[0197] • Achievement of the compressive displacement limit of 5 mm.The upper plate (23) was subsequently raised.
[0198] The needle hub (10) was removed from the vise-like structure.
[0199] The needle hub (10) was examined and classified, on the basis of qualitative observations, into one of the following categories, example of qualitative categorization:
[0200] • “gate” - wherein the crack (12) propagated through the injection gate (11);
[0201] • “opposite” - wherein the crack (12) propagated on the opposite side of the needle hub (10) relative to the injection gate (11 );
[0202] • “both sides” - wherein the crack (12) propagated through both the injection gate (11 ) and the opposite side;
[0203] • “just whitening” - wherein no crack propagation was observed. The whitening was associated to plastic deformation.
[0204] A characterizing model was generated including Table 2 and the graphs shown in Fig. 4, Fig. 5, and Fig. 6.
[0205] Results
[0206] The data indicated by a first ellipsis (50) in Fig. 4 highlight the behaviour of the needle hubs (10) of batch Optimized#2 which did not exhibit cracking failure before reaching the upper compressive force limit of 350N.
[0207] Fig. 5 shows a boxplot representing the distribution and the mean values of the compressive force at failure in Newtons (F Max [N]) for the analyzed batches. A second ellipsis (60) encloses a data point associated with a needle hub (10) of batch Optimized#1 that did not exhibit cracking failure. This data point can be discarded to be more conservative in the characterization of batch Optimized#1. Fig. 6 shows a boxplot representing the distribution and the mean values of the compressive displacement reached at the compressive force at failure in mm (Displacement at Fmax [mm]) for the analyzed batches.The batch 0ptimized#2 demonstrated better performance compared to the Reference and Optimized#! batches. Specifically, none of the needle hubs (!0) from batch Optimized#2 showed cracking failure before reaching the upper compressive force limit of 350 Newtons, indicating the best performance.
[0208] In this evaluation, the compressive force at failure was used as the selection reference value, with a higher value indicating better performance and so being a better selection reference value. A second selection reference value was the presence or absence of cracking failure and not presenting cracking failure was considered to denote a better performance. Consequently, the performance ranking from best to worst is as follows: Optimized#2, Optimized#! , and Reference. Therefore, the needle hub type from Optimized#2 is considered the best for use with a syringe, the intended use. The batches utilized in the selection method were regarded as samples representative of the statistical populations corresponding to the types of needle hubs.
[0209] Table !, example of characterizing model, presents a summary of various aggregate values, serving as examples of representative values, for each type of needle hub tested. To evaluate performance and select the best type of needle hub, a combination of these aggregate values or criteria derived from them may be considered. For instance, the best needle hub type might be identified as the one with the highest ratio of "mean compressive force at failure to mean displacement at compressive force at failure".
[0210] Table 1 - aggregate values for needle hubs types: “Reference”, “Optimized#!” and “Optimized#2”.
[0211] Batch Percent Percent Percent Percent Compres Displace age of age of age of age of sive ment at “gate” “oppos “both “just force at compres failures ite” sides” whiteni failure in sive (%) failures failures ng” Newtons force at (%) (%) failures (Mean - failure in (%) Standard millimete deviation rs - Min - (Mean - Max) Standard deviation - Min - Max)
[0212]
[0213] Referen 75 15 10 0 131.659 - 1.1595 - ce 15.5635 - 0.349673
[0214] 100.916 - 160.283 0.61
[0215] 1.71 Optimiz 0 100 0 0 158.884 - 2.02542 - ed#1 9.78957 - 0.201009
[0216] 149.134 - - 1.775 - 226.848 3.058 Optimiz 0 0 0 100 n / a - n / a n / a - n / a - ed#2 - n / a - n / a - n / a n / a
[0217]
[0218] REFERENCE NUMBERS LIST 10 needle hub;
[0219] 11 injection gate;
[0220] 12 crack;
[0221] 14 thickness of the needle hub (10); 15 tested location ;
[0222] 16 external surface;
[0223] 20 uniaxial tension machine;
[0224] 22 lower plate;
[0225] 23 upper plate;
[0226] 30a, 30b hoop stress;
[0227] 31 compression direction;
[0228] 40 censored data;
[0229] 41 first rectangle;
[0230] 42 valid data;
[0231] 43 second rectangle;
[0232] 44 drops in compressive force;
[0233] 50 first ellipsis;
[0234] 60 second ellipsis.
Claims
CLAIMS1. A method for characterizing crack propagation and / or cracking failure in a sample comprising a specimen (10) comprising a hollow part delimited by a substantially axisymmetric surface (16), wherein the method comprises the steps of:a. orienting the specimen (10) so that an axisymmetry axis of the substantially axisymmetric surface (16) is substantially perpendicular to a compression direction (31);b. applying a compressive force or a compressive displacement onto the substantially axisymmetric surface (16) of the specimen (10) substantially along a compression direction (31);c. recording at least one value of the applied and / or a resulting compressive force and / or compressive displacement and / or at least one qualitative observation related to crack propagation and / or cracking failure.
2. The method of claim 1 , wherein the method is used for characterizing cracking failure due to longitudinal cracks (12) and / or crack propagation of longitudinal cracks (12).
3. The method according to anyone of the preceding claims, wherein the substantially axisymmetric surface (16) includes a tested location (15), in particular wherein the tested location (15) comprises a discontinuity (11 ) of the substantially axisymmetric surface (16); and wherein the method further comprises a step, preceding steps b and c, of:a1. orienting the specimen (10) so that a tangent to the substantially axisymmetric surface (16) of the specimen (10) passing through a centre of the tested location (15) is substantially parallel to the compression direction (31);4. The method according to claim 3, wherein the hollow part is made by injection molding and the tested location (15) comprises an injection gate area (11).
5. The method according to anyone of the preceding claims, wherein the applied compressive force or applied compressive displacement in step b is progressively increased.
6. The method according to anyone of the preceding claims, wherein step b of claim 1 is stopped after a drop in the compressive force occurs and / or when an36upper compressive force limit and / or a compressive displacement limit is reached.
7. The method according to anyone of the preceding claims, wherein the specimen (10) comprises a needle hub (10), in particular wherein the hollow part is the needle hub (10).
8. The method according to anyone of the preceding claims, wherein step b of claim 1 is characterized in that the applied compressive force or compressive displacement is applied with a uniaxial tension machine (20).
9. The method according to anyone of the preceding claims, wherein the method further comprises a step of obtaining a characterizing model of the crack propagation and / or cracking failure for the sample, wherein the characterizing model comprises at least one of:i. a mathematical model obtained from at least one value of the applied and / or resulting compressive force and / or compressive displacement recorded at step c of claim 1 ;ii. a representative value calculated from at least one value of the applied and / or resulting compressive force and / or compressive displacement recorded at step c of claim 1 ;iii. a graph constructed from at least one value of the applied and / or the resulting compressive force and / or compressive displacement recorded at step c of claim 1 ;iv. a qualitative categorization based on at least one qualitative observation recorded at step c of claim 1.
10. The method according to claim 9, wherein the characterizing model is related to a compressive force at failure and / or a compressive displacement reached at the compressive force at failure.
11. A quality control method for a sample comprising a specimen (10) comprising a hollow part delimited by a substantially axisymmetric surface (16), wherein the substantially axisymmetric surface (16) undergoes hoop stress (30a, 30b) in an intended use of the sample; the quality control method comprising the steps of:a. performing the method according to anyone of the preceding claims on the sample;b. obtaining a reference value from an outcome of step a;c. comparing the reference value with a defined criterion;37d. assessing suitability of the sample for an intended use on the basis of step c.
12. The quality control method of claim 10, wherein the reference value is the compressive force at failure or the compressive displacement reached at the compressive force at failure or a value derived from at least one of these; wherein the criterion consists in a defined threshold value and; wherein step d comprises: considering the sample suitable for the intended use when the reference value is higher than the criterion or considering the sample suitable for the intended use when the reference value is lower than the criterion.
13. A selection method for samples comprising a specimen (10) comprising a hollow part delimited by a substantially axisymmetric surface (16), wherein the substantially axisymmetric surface (16) undergoes hoop stress (30a, 30b) in an intended use of the sample; and wherein the selection method comprises the steps of:a. characterizing multiple samples according to a method according to anyone of claims 1 -10;b. selecting, on the basis of an outcome of step a, the sample which showed the best performance for the intended use.
14. The selection method of claim 13, wherein step b comprises obtaining for each sample a selection reference value from the outcome of step a and selecting the sample with the best selection reference value, in particular wherein the selection reference value is a numeric value and wherein the best selection reference value is the highest or the lowest.
15. The method according to claim 14, wherein the selection reference value is, for each sample, the compressive force at failure or the compressive displacement reached at the compressive force at failure or a value derived from at least one of these.
16. The selection method according to anyone of claims 13-15, wherein the samples differ from each other in material, manufacturing process, sterilization process and / or design.