Reference block for nondestructive testing of resin-based composite material

Abstract

The invention relates to the technical field of nondestructive testing of composite materials, in particular to a reference block for nondestructive testing of a resin-based composite material. The reference block comprises a resin-based composite material matrix and a plurality of hollow particles dispersed in the matrix, wherein the hollow particles are graphitized hollow carbon spheres modified by a silane coupling agent; the graphitized hollow carbon spheres have a particle size of 5-100 [mu] m and a bulk density of 0.1-0.5 g / cm < 3 >, and are randomly dispersed in the composite material matrix. The graphitized hollow carbon spheres modified by the silane coupling agent are adopted to construct a hollow inclusion structure with a closed cavity, a stable interface and clear acoustic impedance difference, so that the pore-like defect in the composite material is stably reserved in the forming process, and the morphology, the scale and the acoustic response of the composite material have controllability and repeatability.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of composite material nondestructive testing, and particularly relates to a contrast test block for nondestructive testing of resin-based composite materials. BACKGROUND

[0002] In modern high-end manufacturing, composite materials have become the core structural materials in the fields of aerospace, wind power blades, and automotive lightweighting due to their excellent properties such as high specific strength, high specific modulus, and strong designability. For example, in the aerospace field, the composite material usage of new-generation passenger aircrafts such as Boeing 787 and Airbus A350 exceeds 50%, significantly improving fuel efficiency and flight performance; in the wind power industry, a hundred-meter carbon fiber reinforced composite blade is a key component for achieving high-power generation. However, during the manufacturing process of composite materials, various defects are prone to occur due to factors such as material properties and molding processes.

[0003] From the perspective of material molding processes, problems such as uneven resin flow, insufficient fiber wetting, and unreasonable curing system are common in common processes such as autoclave molding and vacuum infusion molding. For example, during the autoclave molding process, if the temperature rising rate is not properly controlled, the resin gel time will be advanced, which will lead to incomplete fiber impregnation and form resin-poor areas; in the vacuum infusion process, insufficient vacuum extraction or abnormal resin viscosity can easily produce defects such as air holes and delamination. The existence of these defects can significantly reduce the mechanical properties of composite materials, such as a 40% - 60% reduction in interlaminar shear strength caused by delamination defects, which seriously threatens the quality and safety of structural components.

[0004] Nondestructive testing technology is a key means to ensure the quality of composite materials. Ultrasonic testing achieves the positioning and quantification of defects by analyzing the reflection and transmission characteristics of ultrasonic waves in the material; X-ray testing visually presents the internal structure of the material by utilizing the differences in X-ray absorption of different substances; and industrial CT can obtain three-dimensional tomographic images of the material to achieve accurate identification of defects. However, the accuracy of these detection technologies depends on the standard reference provided by the contrast test block.

[0005] At present, there are still many technical bottlenecks in the preparation of composite material comparison test blocks at home and abroad. The traditional air hole defect simulation mainly adopts the methods of reserving holes or filling foamed particles. The former is difficult to control the shape and size precision of the holes, and the latter is prone to produce false detection signals due to the weak interface combination between the foamed particles and the resin. In the simulation of inclusion defects, when using polytetrafluoroethylene film, the thickness of the material itself is too large, which exceeds 0.1 mm, while the defects in the composite material are usually micron level, so the material cannot simulate the actual defect state. When using traditional filling materials such as metal particles, there are problems such as too large difference in density with the matrix and poor dispersibility, which leads to excessive enhancement of the detection signal or the occurrence of agglomeration interference. In addition, the existing comparison test blocks generally have the problems of single defect characteristics, inability to simulate complex defect morphology and complicated preparation process. For example, the composite material used in aerospace often has layered and inclusion coexisting composite defects, but the existing test blocks are difficult to effectively simulate such conditions. Moreover, the preparation of some test blocks needs to use precision molds or special equipment, which is high in cost and limits its popularization and application.

[0006] In addition, in the new nondestructive testing technology, some researches have proposed to introduce glass beads between the layers of composite material prepreg to construct a comparison test block for nondestructive testing of composite materials. For example, CN104407060B discloses a manufacturing method of a composite material porosity comparison test block, which uniformly spreads glass beads with a particle size of tens of microns between the layers of prepreg, and after curing and forming, the air gap formed between the glass beads is used to simulate the pore type defects to establish the correspondence between the porosity and the ultrasonic attenuation.

[0007] Although this method can be used for the preparation of comparison test blocks to some extent, it still has obvious deficiencies: in the process of hot pressing and curing, the resin viscosity stage drops suddenly, the flowability is too large, the air gap is backfilled with resin and the air is squeezed out, which leads to the instability of the "air hole" and the signal distortion and the increase of the missing detection rate; at the same time, the defect morphology is mainly constrained by the "particle gap" geometry, it is difficult to finely control the size and distribution of the micron level real defects, and it is more inclined to the single ultrasonic attenuation calibration scene. SUMMARY

[0008] The present application covers the following technical solutions:

[0009] According to one aspect of the present application, a comparison test block is provided, which comprises a resin-based composite material matrix and a plurality of hollow particles dispersed in the matrix, the hollow particles being graphitized hollow carbon spheres modified by a silane coupling agent; the graphitized hollow carbon spheres have a particle size of 5-100 μm and a bulk density of 0.1-0.5 g / cm³, and are randomly dispersed in the composite material matrix.

[0010] According to another aspect of the present application, a preparation method of the comparison test block is provided, which comprises the following steps:

[0011] 1) providing the graphitized hollow carbon spheres modified by silane coupling agent, mixing them with the matrix resin of the composite material to obtain a resin mixture of the pore inclusion simulation layer;

[0012] 2) coating or laying the resin mixture between the layers of the composite material prepreg and laminating;

[0013] 3) curing the laminated structure to obtain the comparative test block containing the pore inclusion simulation layer.

[0014] According to still another aspect of the present application, a non-destructive testing method of resin-based composite material is provided, which comprises the following steps:

[0015] Under the same non-destructive testing conditions, the comparative test block and the resin-based composite material to be tested are respectively tested to obtain defect echo signals and target echo signals;

[0016] The two echo signals are compared to determine whether the resin-based composite material to be tested contains defects corresponding to the corresponding defect simulation units in the comparative test block.

[0017] The present application uses graphitized hollow carbon spheres modified by silane coupling agent to construct a hollow inclusion structure with a closed cavity, stable interface and clear acoustic impedance difference, so that the internal pore-like defects of the composite material are stably retained during the molding process, and the morphology, size and acoustic response are controllable and repeatable. Compared with the existing scheme of simulating pores by relying on particle accumulation voids, the present application has significant advantages in defect simulation accuracy, molding consistency, detectable signal distinguishability and adaptability to different material systems. BRIEF DESCRIPTION OF DRAWINGS

[0018] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the drawings needed in the specific embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are some embodiments of the present application, and those skilled in the art can obtain other drawings according to these drawings without creative labor.

[0019] Figure 1 It is a physical diagram and a schematic diagram of the defect test block.

[0020] Figure 2 It is the non-destructive testing data of the defect-free area, and the first wave and the bottom wave are clearly visible, and there is no defect wave in the middle.

[0021] Figure 3The nondestructive testing data of the pre-embedded defect near the surface is shown in Fig. 1. The defect wave is obvious, the defect depth is 0.4 mm, and the initial wave and the defect wave are close. The bottom wave is weak because too much ultrasonic wave is consumed near the surface.

[0022] Figure 4 The nondestructive testing data of the pre-embedded defect in the middle layer of the test block is shown in Fig. 2. The defect wave in the middle is obvious, and the defect depth is 1 mm.

[0023] Figure 5 The nondestructive testing data of the pre-embedded defect near the bottom surface is shown in Fig. 3. The defect wave is obvious, the defect depth is 1.6 mm, and the bottom wave and the defect wave are close, resulting in signal superposition, so that no bottom wave is generated. DETAILED DESCRIPTION

[0024] Reference will now be made in detail to the embodiments of the present application, one or more examples of which are set forth below. Each example is provided by way of explanation of the present application, not limitation of the present application. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present application without departing from the scope or spirit of the present application. For example, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment.

[0025] Unless otherwise defined, all terms (including technical and scientific terms) used in disclosing the present application have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. By further guidance, the following definitions are set forth to better define the present teachings. The terminology used in the description of the present application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application.

[0026] In the present application, unless otherwise defined, the scientific and technical terms used herein have the meanings commonly understood by a person of ordinary skill in the art. Also, the composite material science, ultrasonic nondestructive testing related terms and laboratory operation steps used herein are the terms and conventional steps widely used in the corresponding fields. At the same time, in order to better understand the present application, the definitions and explanations of related terms are provided as follows.

[0027] The terms "containing", "including" and "comprising" used in the present application are synonymous and are inclusive or open-ended, and do not exclude additional, unrecited members, elements or method steps.

[0028] The numerical ranges used in the present application expressed in endpoints include all numbers and fractions subsumed within the range, as well as the recited endpoints.

[0029] The term "about" or "approximately" used in the present application means within 20%, preferably within 10%, and more preferably within 5% of a given value or range. It also includes specific numbers, for example, about 20 includes 20.

[0030] Furthermore, in describing representative embodiments of the present application, the specification can have presented the method and / or process of the present application as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the

[0031] As used in the present application, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0032] In the present application, the description of "a plurality of," "a plurality of kinds," and the like, if not particularly limited, means greater than or equal to 2 in number.

[0033] In the present application, the technical features described in an open-ended manner include both a closed technical solution consisting of the listed features and an open technical solution including the listed features.

[0034] In the present application, "preferably," "more preferably," "even more preferably," "suitably," and the like, merely mean that the described implementation or embodiment is better, and should be understood as not constituting a limitation on the scope of protection of the present application. In the present application, "optionally," "optional," and "may" mean that either of the two parallel schemes "yes" or "no" is acceptable, i.e., selected from the two parallel schemes. If multiple "optionally" appear in a technical solution, and there is no special description, and there is no contradictory relationship or mutual restriction, each "optionally" is independent.

[0035] In the present application, "a contrast test block" refers to a resin-based composite test sample used for calibrating the detection sensitivity and comparing and analyzing the defect features in the non-destructive testing of composite materials. The contrast test block has a defect simulation layer inside for simulating the potential defect morphology in the composite material, so as to establish the correspondence between the defect type, size, and distribution and the detection echo signal. In use, the contrast test block and the resin-based composite material to be detected do not require complete consistency in resin formulation and reinforcing fiber type, but should be the same or similar in acoustic impedance, layer structure, and thickness, so as to ensure the comparability of the propagation, reflection, and scattering behavior of acoustic waves in both, so that the echo signal corresponding to the defect simulation layer in the contrast test block can be used to identify the corresponding micro-scale defects in the material to be detected. The application basis of the contrast test block described in the present application is the acoustic detection condition mapping property, rather than the complete consistency of the material system.

[0036] In the present application, "non-destructive testing" refers to a testing method for identifying the type, location, size and distribution of internal defects of a material by the propagation, reflection, scattering and energy attenuation characteristics of acoustic waves, electromagnetic waves, rays, thermal fields or other detection signals without destroying the overall structure and performance of the tested composite material. The core purpose of non-destructive testing in the present application is to obtain the echo or response signal of the internal defects of the material and perform corresponding analysis with the reference signal of the comparative test block, thereby realizing the judgment and grading evaluation of the internal micro-scale defects of the composite material.

[0037] In the present application, "graphitized hollow carbon sphere" refers to a spherical or near-spherical particle with a hollow core-shell structure, taking carbon material as the main body, and the shell layer is composed of carbon layers in graphitized orientation, having a continuous or quasi-continuous carbon six-membered ring ordered structure. The graphitized hollow carbon sphere can be derived from phenolic resin, polyacrylonitrile, pitch, glucose, starch, core-shell polymer microspheres or other carbon-containing precursors through spheroidization, carbonization and graphitization heat treatment, or can be a hollow structure formed by etching, template method, extraction or vapor deposition taking carbon microspheres as the core. The above-mentioned graphitized hollow carbon sphere has the characteristics of low density, high thermochemical stability, controllable cavity size and adjustable interfacial interaction with the resin matrix, and can maintain the hollow structure without collapse during the resin curing process, thereby being used for simulating closed micro-pore type defects inside the composite material.

[0038] The first aspect of the present application relates to a comparative test block, which comprises a resin-based composite material matrix and a plurality of hollow particles dispersed in the matrix, the hollow particles being graphitized hollow carbon spheres modified by a silane coupling agent; the graphitized hollow carbon spheres have a particle size of 5-100 μm and a bulk density of 0.1-0.5 g / cm³, and are randomly dispersed in the composite material matrix.

[0039] The present application uses graphitized hollow carbon spheres modified by a silane coupling agent as a defect simulation unit, realizes a construction method with controllable morphology, controllable position and controllable signal response of the internal pore-like defects of the composite material, and has at least one of the following advantages:

[0040] Firstly, the defect simulation precision of the present application is high. The graphitized hollow carbon sphere has a stable hollow core-shell structure and a controllable particle size distribution, and the cavity size and spherical geometry can highly correspond to the morphology of the common small pores inside the composite material; by adjusting the particle size and volume fraction of the hollow carbon sphere, pore-like defects of different severity levels can be constructed. At the same time, a clear acoustic impedance interface is formed between the hollow structure and the resin matrix, and the corresponding echo signal boundary is clear and the amplitude is separable in ultrasonic testing, and compared with the traditional void-like test block formed by particle accumulation, the signal-to-noise ratio of defect identification can be significantly improved.

[0041] Secondly, this invention offers excellent process controllability. The silane coupling agent modification treatment forms a stable interfacial transition layer on the surface of the hollow carbon spheres, making them less prone to breakage, delamination, or agglomeration during resin curing, thus ensuring the complete preservation of the hollow structure after molding. The defect simulation layer exhibits high morphological stability across different batches of test blocks, providing repeatable and easily comparable defect characteristic signals, which is beneficial for establishing reliable non-destructive testing calibration standards.

[0042] Furthermore, this invention offers high cost-effectiveness. The graphitized hollow carbon spheres have low density and controllable dosage. By replacing traditional metal or high-density particulate fillers, the overall weight of the control block and resin consumption can be reduced, thereby decreasing processing and material costs. This makes it suitable for batch preparation and long-term testing system maintenance.

[0043] Finally, the present invention has a wide range of applications. The comparative test block prepared by the present invention can be applied to various non-destructive testing technologies such as ultrasound, phased array, X-ray and industrial CT. By adjusting the particle size, volume fraction and layup structure of the hollow carbon spheres, it can be adapted to the testing needs of composite material parts with different resin systems (including thermosetting and thermoplastic) and different thicknesses and component types, and has good method versatility and engineering promotion value.

[0044] In the aforementioned comparative test blocks, hollow carbon spheres are randomly dispersed in the resin matrix, forming uniformly distributed and controllable defect simulation regions. By controlling the particle size distribution, packing density, and content of the carbon spheres in the matrix, comparative test blocks with different defect sizes, volume fractions, and signal response characteristics can be obtained for defect signal identification, sensitivity calibration, and defect classification in nondestructive testing of composite materials.

[0045] In this invention, the particle size of the graphitized hollow carbon spheres can be controlled according to the simulated pore size in the composite material. Generally, the actual pores in the composite material are mostly distributed in the range of several micrometers to tens of micrometers. In some preferred embodiments, the particle size of the graphitized hollow carbon spheres is 10–50 μm. For example, hollow carbon spheres with an average particle size of approximately 15 μm, 25 μm, 35 μm, or 45 μm can be used. This particle size range can maintain the hollow structure from collapsing during resin curing and molding, while matching the acoustic wavelength scale inside the composite material, making the echo signal caused by the hollow defect identifiable and distinguishable.

[0046] In this invention, the volume fraction of graphitized hollow carbon spheres in the resin matrix has a significant impact on the defect simulation effect. When the volume fraction is low, the defect density is small, and the echo response signal is not obvious; when the volume fraction is too high, the defect region may not be consistent with the micropore morphology in the real material, affecting the quantitative nature of the signal. Those skilled in the art can adjust it as needed. In this invention, the volume fraction of graphitized hollow carbon spheres is preferably controlled within the range of 5% to 30%. For example, volume fractions of approximately 8%, 12%, 18%, 22%, or 28% can be used to construct different levels of defect simulation regions; a more preferred range is 10% to 25%. Hollow defects formed within this range achieve a relatively ideal balance between uniform distribution, molding stability, and the ability to classify ultrasonic signals. This parameter setting enables the comparative test block prepared by this invention to simulate different levels of porosity defects, meeting the practical application requirements for defect identification and sensitivity calibration in nondestructive testing of composite materials.

[0047] In this invention, the resin-based composite matrix used to construct the comparative test block can be a laminated composite system with resin as the continuous phase and reinforcing fibers as the reinforcing phase. The resin-based composite material can be selected from thermosetting or thermoplastic resin reinforcement systems, including but not limited to: epoxy resin, bismaleimide resin, phenolic resin, polyimide resin, vinyl ester resin, polyester resin, polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyphenylene sulfide (PPS), and other resin systems, in any combination with carbon fiber, glass fiber, basalt fiber, aramid fiber, polyphenylene sulfide fiber, UHMWPE fiber, and their hybrid fiber reinforcements. Furthermore, the layup can be plain weave, twill weave, unidirectional tape, multiaxial fabric, or prepreg laminate structure, without limitation on the specific layup method and lamination angle.

[0048] It should be noted that in the non-destructive testing application of the present invention, the comparison test block and the resin-based composite material to be tested are not required to be completely identical in resin formulation or reinforcing fiber type, but the two should be the same or similar in terms of acoustic impedance, ply structure and thickness, so as to ensure that the propagation path, interface reflection and scattering behavior of sound waves in the material are comparable, so that the echo signal exhibited by the defect simulation layer in the comparison test block can be used to determine the existence and classification of corresponding microscale defects in the material to be tested.

[0049] In a preferred embodiment, the resin-based composite matrix can be any one of a carbon fiber reinforced epoxy resin matrix, an aramid fiber reinforced resin matrix, or a glass fiber reinforced resin matrix. These systems are widely used in aerospace, rail transportation, and high-performance equipment manufacturing, are highly typical, and have good acoustic detection response characteristics, making it easy to establish the correspondence between echo signals and defect morphologies.

[0050] The graphitized hollow carbon spheres described in this invention undergo interface modification treatment with a silane coupling agent. The purpose of interface modification is to introduce functional groups on the surface of the graphitized hollow carbon spheres that can chemically bond, hydrogen bond, or interact polarly with the resin matrix, thereby improving the interfacial bonding stability between the hollow carbon spheres and the resin matrix and avoiding hollow structure damage, delamination, or abnormal acoustic scattering signals due to insufficient interfacial wetting during the curing process of the composite material.

[0051] The silane coupling agents that can be used in this invention are not limited to a specific type or structure, and may include a variety of silane coupling agents with different end groups and hydrolytic groups, for example:

[0052] • Aminosilanes: such as aminopropyltriethoxysilane (KH550), aminopropyltrimethoxysilane (APTMS), N-aminoethyl-γ-aminopropyltrimethoxysilane (DAMO), etc.;

[0053] • Epoxysilanes: such as γ-glycidyl etheroxypropyltrimethoxysilane (KH560 / GPTMS), etc.;

[0054] • Methacryloxysilanes: such as γ-methacryloxypropyltrimethoxysilane (KH570), etc.;

[0055] • Thiosilanes: such as γ-mercaptopropyltrimethoxysilane (MPTMS), etc.;

[0056] • Phenyl or alkylsilanes: such as phenyltrimethoxysilane, octyltrimethoxysilane, etc.

[0057] All of the above-mentioned silane coupling agents can form an interfacial transition layer containing Si–O–C or Si–O–Si structures on the surface of hollow carbon spheres through hydrolysis-condensation reaction, so that the hollow carbon spheres can maintain the integrity of the hollow structure during resin curing and obtain more stable and controllable acoustic interface properties.

[0058] In a preferred embodiment, the silane coupling agent may be aminopropyltriethoxysilane (KH550). This silane coupling agent has good hydrolytic reactivity and interfacial affinity with epoxy or other polar matrix resins, which can further improve the molding stability of the defect simulation layer structure and the clarity of non-destructive testing signals.

[0059] In a preferred embodiment of the present invention, the comparative test block further includes non-defective composite material layups disposed above and below the defect simulation layer. These non-defective layups can be formed from a resin-based composite prepreg that is the same as or similar to the matrix material, to construct a layup structure and thickness distribution similar to that in actual composite material components. By setting normal regions above and below the defect simulation layer, the defective region is in a "sandwich" state in the thickness direction, thereby more closely resembling the spatial location and stress environment of pore defects in real service structures of composite materials.

[0060] Non-defect layups are used to form complete laminated composite material structures. Their shape, material, and layup method are not limited, as long as they can form a continuous integral structure with the defect simulation layer. For example, the non-defect layup can be made of resin-based composite prepreg, and its reinforcing fibers can be carbon fiber, glass fiber, aramid fiber, basalt fiber, polyphenylene sulfide fiber, or ultra-high molecular weight polyethylene fiber, etc. The resin matrix can be epoxy resin, bismaleimide resin, polyimide resin, polyetheretherketone (PEEK), polyphenylene sulfide (PPS), or other thermosetting or thermoplastic resins that can be used for lamination and curing.

[0061] In terms of layup structure, non-defect layup can be constructed according to the structural characteristics of the material to be simulated, using unidirectional layup, plain weave fabric layup, twill fabric layup, multiaxial fabric layup or a combination thereof. The layup direction can be common angles such as 0°, ±45°, 90°, or the layup sequence consistent with the actual composite material component to be tested can be used.

[0062] Regarding thickness configuration, the non-defect layup can be configured as a symmetrical structure, an asymmetrical structure, a single-layer or multi-layer stacked structure, and its thickness can be selected according to simulation requirements, for example, it can be 0.1 mm to 10 mm, preferably close to the typical thickness of the composite material component to be tested. The non-defect layup can be located above, below, or on both sides of the defect simulation layer, forming a sandwich structure, a surface coating structure, or an embedded local structure.

[0063] Through the above-mentioned various possible compositions and layup forms, the non-defect composite material layup can be applied to non-destructive testing calibration and defect comparison analysis of different resin-based composite material systems without limiting the specific material system, fiber type, or molding process.

[0064] A second aspect of the present invention relates to a method for preparing a comparative test block as described above, comprising the following steps:

[0065] 1) Provide graphitized hollow carbon spheres modified with silane coupling agent, mix them with composite matrix resin to obtain a resin mixture of pore inclusion simulation layer;

[0066] 2) The resin mixture is coated or laid between the layers of the composite prepreg and then laminated;

[0067] 3) The laminated structure is cured and molded to obtain a comparative test block containing a simulated layer of pore inclusions.

[0068] In a general implementation, graphitized hollow carbon spheres modified with a silane coupling agent are mixed with a resin matrix at a desired volume fraction to obtain a resin mixture for constructing a defect simulation layer. This mixing process can be carried out using methods such as mechanical stirring, planetary mixing, shear dispersion, ultrasonic dispersion, vacuum degassing mixing, drum stirring, or continuous static mixing to achieve random dispersion of the hollow carbon spheres within the resin matrix.

[0069] The coating method can be scraping, anilox roller coating, dip coating, doctor blade spreading, or spraying; the laying method can be manual lamination, automatic tape laying (ATL), automatic fiber placement (AFP), cavity embedding, or local area patching. The defect simulation layer can form a single-layer structure or a multi-layer repeating structure, and its thickness can be selected according to the detection sensitivity requirements, for example, it can be 0.05 mm to 5 mm, preferably 0.1 mm to 3 mm, and even more preferably 0.1 mm to 1.5 mm.

[0070] After the laminated structure is formed, it is cured and molded. Curing methods may include autoclave curing, thermoforming, vacuum bag curing, resin transfer molding (RTM / RFI), or automatic tape-layout followed by thermosetting. The curing temperature can be determined according to the resin system, for example, 100℃ to 230℃; the curing pressure can be 0.2 to 0.8 MPa, or a pressureless curing system can be used; the curing time can be selected according to the resin reaction kinetics, for example, 0.5 to 5 h. After curing and molding, a comparative test block containing a defect simulation layer is obtained, with an overall laminate thickness of, for example, 2 mm to 20 mm, preferably 3 mm to 10 mm, to meet the non-destructive testing requirements of different composite material components.

[0071] A third aspect of the present invention provides a non-destructive testing method for resin-based composite materials, comprising the following steps:

[0072] Under the same non-destructive testing conditions, the comparative test block and the resin-based composite material to be tested were tested respectively to obtain the defect echo signal and the target echo signal.

[0073] The two echo signals are compared to determine whether there is a defect in the resin-based composite material to be tested that corresponds to the corresponding defect simulation unit in the comparison test block.

[0074] This invention does not limit the non-destructive testing methods and parameters, and can employ ultrasonic testing, phased array ultrasonic testing, acoustic holographic testing, guided wave testing, laser ultrasonic testing, resonance methods, or other non-destructive testing techniques suitable for resin-based composite materials. The testing frequency, beam angle, coupling method, and scanning path can be set according to different material thicknesses and structural characteristics. The key to this invention is that the comparison test block and the material under test should be the same or similar in terms of acoustic impedance, ply structure, and thickness, so that the propagation, reflection, and scattering behavior of sound waves in both are comparable, thereby establishing a mapping relationship between their echo signals.

[0075] In a preferred embodiment of the present invention, the non-destructive testing is ultrasonic non-destructive testing, preferably at least one of phased array ultrasonic testing, A-scan and C-scan testing.

[0076] The defect simulation unit described in this invention is a graphitized hollow carbon sphere with a hollow structure. A clear and stable acoustic impedance difference exists between its internal cavity and the outer resin matrix. This acoustic impedance interface generates controllable reflection and scattering signals when ultrasonic waves are incident. Compared to random porous structures formed by interparticle gaps, the hollow structure of this invention is a closed, dimensionally controllable, and clearly defined defect unit, thus its ultrasonic echo signal exhibits good repeatability and identifiability.

[0077] Meanwhile, because the graphitized hollow carbon spheres are randomly and dispersedly embedded within the resin matrix in this invention, forming defect scattering characteristics similar to those of real pores within the composite material, the energy reflection, attenuation, and wavefront distortion characteristics of ultrasound waves occurring within the defect region can be clearly demonstrated in phased array imaging, A-scan echo curves, and C-scan planar images. Specifically, phased array ultrasonic testing, through electronic control of the beam direction and focusing depth, can be used to observe the shape and depth characteristics of defects; A-scan testing can be used to compare echo amplitude and delay changes to identify the presence of defects; and C-scan imaging can display the planar contour and distribution range of the defect region.

[0078] In some embodiments, comparing the echo signals includes comparing at least one of the following: echo peak amplitude, signal-to-noise ratio, phase characteristics, echo attenuation trend, and imaging defect profile. It can be determined whether the composite material under test contains defect features corresponding to porosity-like defects in the comparison sample. When the two show consistency or high similarity in echo response mode, scattering pattern, or image indication, it can be determined that the material under test contains the corresponding type of microscale porosity or inclusion defect; conversely, it can be determined that the corresponding region does not contain the same type of defect or its defect level is significantly lower.

[0079] The embodiments of the present invention will be described in detail below with reference to examples. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. For experimental methods in the following embodiments where specific conditions are not specified, please refer to the guidelines given in this invention, or follow experimental manuals or conventional conditions in the art, or other experimental methods known in the art, or follow the conditions recommended by the manufacturer.

[0080] In the specific embodiments described below, the measurement parameters involving raw material components may have slight deviations within the weighing accuracy range unless otherwise specified. Temperature and time parameters are subject to acceptable deviations due to instrument testing accuracy or operational precision.

[0081] Example 1: Preparation of a carbon fiber composite comparative specimen for ultrasonic testing

[0082] 1. Raw material preparation

[0083] Matrix material: T800 carbon fiber / epoxy resin prepreg, resin content 33%, prepreg single layer 0.2mm, 10 layers, total thickness 2.0mm.

[0084] Graphitized hollow carbon spheres: average particle size 20 μm, density 0.25 g / cm³, placed in γ-aminopropyltriethoxysilane (KH550), ultrasonically dispersed for 30 min, and dried at 80 °C for 3 h.

[0085] 2. Defect layer construction

[0086] Pore ​​inclusion simulation layer: Modified microspheres and epoxy resin E51 were mixed at a volume ratio of 1:5, and 30% by mass of 4,4'DDS diaminodiphenyl sulfone was added. The mixture was stirred in a planetary mixer at a speed of 1000 rpm for 40 minutes. After stirring evenly, the mixture was poured into 9 positions in the mold with different numbers of prepreg layers to form circular inclusion simulation areas with a diameter of 3 mm.

[0087] 3. Composite molding: Alternately lay prepreg and defect layer on a flat plate, place in an autoclave, evacuate at room temperature with a vacuum degree of not less than 0.095MPa, pressurize to 0.6MPa at a rate of 0.06MPa / min; heat to 130℃±3℃ at a rate of 1.5℃ / min, hold for 0.5h, heat to 180℃±3℃ at a rate of 1.5℃ / min, hold for 2h, maintain pressure, and cool to below 60℃ at a cooling rate not exceeding 2℃ / min before removing from the autoclave.

[0088] 4. Post-processing: Cut three test blocks (300mm×300mm×2.0mm each) and polish the test surfaces to complete the test block preparation.

[0089] 5. Testing and Verification: The non-destructive testing equipment used was a portable ultrasonic flaw detector from Krautkramer, Germany, model USM100, with a probe frequency of 5MHz and water as the coupling agent. A full-range scan of the test block was performed. The results showed that all defects were detected, with high echo intensity (approximately 15.9dB). The location and size of pores and inclusions were largely consistent with the design. Figures 2-5 It can be effectively used for the calibration and evaluation of industrial ultrasonic testing.

[0090] Example 2: Preparation of comparative test blocks of aramid fiber / bismaleimide resin composite material for ultrasonic testing

[0091] 1. Raw material preparation

[0092] Matrix material: aramid fiber / bismaleimide resin prepreg, resin content 55%.

[0093] Graphitized hollow carbon spheres: average particle size 50 μm, density 0.38 g / cm³, placed in γ-aminopropyltriethoxysilane (KH550), ultrasonically dispersed for 60 min, and dried at 80 °C for 3 h.

[0094] 2. Defect layer construction

[0095] Pore ​​inclusion defect simulation layer: Modified microspheres and diallyl bisphenol A are mixed at a volume ratio of 1:5, and 100% by mass of bismaleimide resin is added. The mixture is stirred in a planetary mixer at a speed of 2000 rpm for 60 minutes. After stirring evenly, the mixture is poured into the middle area of ​​the mold to form a circular inclusion simulation area with a diameter of 3 mm.

[0096] 3. Composite molding: Alternately lay prepreg and defect layer on a flat plate, place in an autoclave, evacuate at room temperature with a vacuum degree of not less than 0.095MPa, pressurize to 0.6MPa at a rate of 0.06MPa / min; heat to 150℃±3℃ at a rate of 2℃ / min, hold for 0.5h, heat to 180℃±3℃ at a rate of 1℃ / min, hold for 2h, heat to 220℃±3℃ at a rate of 1℃ / min, hold for 4h, maintain pressure, cool to below 60℃ at a cooling rate not exceeding 2℃ / min, and remove from the autoclave.

[0097] 4. Post-processing: Cut the test blocks to 300mm×300mm×4mm in total, polish the test surfaces, and complete the test block preparation.

[0098] 5. Testing and Verification: The non-destructive testing equipment used was a portable ultrasonic flaw detector from Krautkramer, Germany, model USM100, with a probe frequency of 5MHz and water as the coupling agent. The test block was scanned from all directions. The results showed that the location and size of pores and inclusions were in good agreement with the design, and it can be effectively used for the calibration and evaluation of industrial ultrasonic testing.

[0099] Summarize:

[0100] In Examples 1 and 2, graphitized hollow carbon spheres can simulate porosity inclusions in different composite material systems, and the non-destructive testing A-scan echoes are clearly visible, demonstrating significant effects.

[0101] To verify the feasibility and effectiveness of using graphitized hollow carbon spheres modified with silane coupling agent to construct pore-like defects in composite materials, this application sets up Comparative Example 1 and Comparative Example 2 as controls.

[0102] In Comparative Example 1, a defect simulation layer was constructed using graphitized hollow carbon spheres that had not undergone interface modification.

[0103] In Comparative Example 2, glass microspheres were stacked to create interparticle voids to simulate porosity defects, referencing existing technologies. The specific implementation method is as follows:

[0104] Comparative Example 1

[0105] 1. Raw material preparation

[0106] Matrix material: T800 carbon fiber / epoxy resin prepreg, resin content 33%, prepreg single layer 0.2mm, 10 layers, total thickness 2.0mm.

[0107] Untreated graphitized hollow carbon spheres: average particle size 20 μm, density 0.25 g / cm³.

[0108] 2. Defect layer construction

[0109] Carbon balls were filled into nine different locations in the mold with different numbers of prepreg layers to form a circular defect area with a diameter of 3 mm.

[0110] 3. Composite molding: Alternately lay prepreg and defect layer on a flat plate, place in an autoclave, evacuate at room temperature with a vacuum degree of not less than 0.095MPa, pressurize to 0.6MPa at a rate of 0.06MPa / min; heat to 130℃±3℃ at a rate of 1.5℃ / min, hold for 0.5h, heat to 180℃±3℃ at a rate of 1.5℃ / min, hold for 2h, maintain pressure, and cool to below 60℃ at a cooling rate not exceeding 2℃ / min before removing from the autoclave.

[0111] 4. Post-processing: Cut the test blocks to 300mm×300mm×2.0mm in total, polish the test surfaces, and complete the test block preparation.

[0112] 5. Testing and Verification: The non-destructive testing equipment used was a portable ultrasonic flaw detector from Krautkramer, Germany, model USM100, with a probe frequency of 5MHz and water as the coupling agent. The test block was scanned from all directions. The results showed that the defect echo was low, around 12.6dB, and two defects were not detected.

[0113] Comparative Example 2

[0114] 1. Raw material preparation

[0115] Matrix material: T800 carbon fiber / epoxy resin prepreg, resin content 33%, prepreg single layer 0.2mm, 10 layers, total thickness 2.0mm.

[0116] Glass microspheres: average particle size 20μm, density 2.4g / cm³.

[0117] 2. Defect layer construction

[0118] Glass microspheres were filled into nine locations in the mold with different numbers of prepreg layers to form circular defect areas with a diameter of 3 mm.

[0119] 3. Composite molding: Alternately lay prepreg and defect layer on a flat plate, place in an autoclave, evacuate at room temperature with a vacuum degree of not less than 0.095MPa, pressurize to 0.6MPa at a rate of 0.06MPa / min; heat to 130℃±3℃ at a rate of 1.5℃ / min, hold for 0.5h, heat to 180℃±3℃ at a rate of 1.5℃ / min, hold for 2h, maintain pressure, and cool to below 60℃ at a cooling rate not exceeding 2℃ / min before removing from the autoclave.

[0120] 4. Post-processing: Cut the test blocks to a total of 300mm×300mm×2.0mm, polish the test surfaces, and complete the test block preparation.

[0121] 5. Testing and Verification: The non-destructive testing equipment used was a portable ultrasonic flaw detector from Krautkramer, Germany, model USM100, with a probe frequency of 5MHz and water as the coupling agent. The test block was scanned from all directions. The results showed that the defect echo was weak, only about 9.8dB, and 5 defects were not detected, resulting in a high missed detection rate.

[0122] Based on the above comparative examples, the test results are as follows:

[0123]

[0124] Based on the comparison results of Comparative Example 1 and Example 1, although these hollow carbon spheres can disperse in the resin matrix during the initial molding stage, the lack of effective interfacial bonding between their surface and the resin leads to resin flow and shrinkage during curing, causing the hollow carbon spheres to break, collapse, or partially delaminate, resulting in an unstable internal cavity structure. In ultrasonic testing, this unstable interface causes scattered scattering signals, irregular echo amplitudes, and unclear defect boundaries, making it difficult to establish a stable and comparable echo response. Therefore, Comparative Example 1 shows that using only hollow carbon spheres without interface modification cannot form a repeatable and identifiable pore-like defect structure; interface control is a necessary condition.

[0125] Based on the comparison results of Comparative Example 2 and Example 1: During the curing and heating phase, the viscosity of the thermosetting resin decreases significantly, increasing its fluidity and penetrating into the gaps between particle packings, completely displacing and filling the air within. After curing, the originally intended pore structure cannot be retained or is significantly weakened, inconsistent with the true micropore morphology. In ultrasonic testing, such test blocks exhibit weak or indistinguishable echo signals, failing to provide an identification benchmark for defect grading and sensitivity setting. Therefore, Comparative Example 2 demonstrates that the method relying on particle packing to form "voids" cannot stably retain the pore structure during curing, and the hollow structure construction method of the present invention is irreplaceable.

[0126] In summary, effective simulation of pore-like defects inside composite materials cannot rely on the accumulation of voids between particles, but must depend on the stable closed cavity inherent in the hollow structure itself. Furthermore, the interfacial stability between the hollow carbon spheres and the resin matrix is ​​a key factor determining whether the hollow structure can maintain its complete morphology during curing. By using graphitized hollow carbon spheres modified with silane coupling agents, this invention can construct hollow inclusion structures with controllable morphology, clear acoustic impedance interfaces, and repeatability within composite materials. This results in ultrasonic echo signals exhibiting good identifiability and gradability, thus making the prepared comparative test blocks engineering-applicable as standard parts for non-destructive testing calibration and defect level assessment.

[0127] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims, and the specification and drawings can be used to interpret the content of the claims.

Claims

1. A comparative test block comprising a resin-based composite matrix and a plurality of hollow particles dispersed in the matrix, characterized in that, The hollow particles are graphitized hollow carbon spheres modified with a silane coupling agent; the graphitized hollow carbon spheres have a particle size of 5 to 100 μm and a bulk density of 0.1 to 0.5 g / cm³, and are randomly dispersed in the composite matrix.

2. The comparative test block according to claim 1, characterized in that, The particle size of the graphitized hollow carbon spheres is 10–50 μm.

3. The comparative test block according to claim 1, characterized in that, The volume fraction of the graphitized hollow carbon spheres is 5% to 30%, preferably 10% to 25%.

4. The comparative test block according to claim 1, characterized in that, The resin-based composite matrix is ​​any one of carbon fiber reinforced epoxy resin matrix, aramid fiber reinforced resin matrix, or glass fiber reinforced resin matrix.

5. The comparative test block according to claim 1, characterized in that, The silane coupling agent is aminopropyltriethoxysilane.

6. The comparative test block according to any one of claims 1-5, characterized in that, The comparative test block also includes non-defect composite material lay-ups located above and below the defect simulation layer.

7. The method for preparing the comparative test block according to any one of claims 1-6, characterized in that, Includes the following steps: 1) Provide graphitized hollow carbon spheres modified with silane coupling agent, mix them with composite matrix resin to obtain a resin mixture of pore inclusion simulation layer; 2) The resin mixture is coated or laid between the layers of the composite prepreg and then laminated; 3) The laminated structure is cured and molded to obtain a comparative test block containing a simulated layer of pore inclusions.

8. A non-destructive testing method for resin-based composite materials, characterized in that, Includes the following steps: Under the same non-destructive testing conditions, the comparative test block and the resin-based composite material to be tested according to any one of claims 1-6 were tested respectively to obtain defect echo signals and target echo signals. The two echo signals are compared to determine whether there is a defect in the resin-based composite material to be tested that corresponds to the corresponding defect simulation unit in the comparison test block.

9. The method according to claim 8, characterized in that, The non-destructive testing is ultrasonic non-destructive testing, preferably at least one of phased array ultrasonic testing, A-scan, and C-scan testing.

10. The method according to claim 8 or 9, characterized in that, The comparison of the echo signals includes a comparison of at least one of the echo peak amplitude, signal-to-noise ratio, and defect indication image profile.

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