Method for designing a cell-structured absorbent filler for a protective device such as, for example, a helmet

Abstract

The present invention has for its object a method for designing a cell-structured absorbent filler (1) for a protective device (100) - such as, for example, a helmet - which comprises: - a first step of finite element simulation in which, through a first numerical model (3) of the absorbent filler (1) in which the latter is approximated to a homogeneous or isotropic or orthotropic solid, one or more mechanical parameters (30) of said filler capable of satisfying a determined required protection level (2a) are determined, - a second step of finite element simulation in which, through a cell numerical model (4), one or more geometrical parameters (40) of a cell module (4a) usable to make the cell structure of the absorbent filler (1) are determined, - an experimental phase in which a physical model (5) of the cell-structured absorbent filler (1) is subjected to at least one test aimed at verifying that the absorbent filler (1) satisfies the required protection level (2a).

Description

[0001] Title: “Method for designing a cell-structured absorbent filler for a protective device such as, for example, a helmet”

[0002] DESCRIPTION

[0003] Technical Field

[0004] The present invention relates to a method for designing a cell-structured absorbent filler which finds useful application in the field of impact protection devices such as, for example, helmets for vehicles or for sports applications.

[0005] State of the Art

[0006] Known protective devices are not always able to protect a user adequately and / or sufficiently comfortably.

[0007] To satisfy increasingly stringent safety regulations, in the last decade various innovative solutions have been developed, including cell-structured absorbent fillers (or, more simply, cellular fillers) for protective devices.

[0008] Such cellular fillers are characterized by a reticular structure with alternating solids and voids, obtained by repetition of a module, commonly referred to as a "cell". It is worth noting that some cells making up the lattice of the cellular filler can be dilated, contracted or distorted according to a particular logic to create local variations in the mechanical properties of the filler or to adapt to the geometry of the protective device or the body region on which it is to be worn.

[0009] To satisfy high levels of safety and comfort, it is necessary to optimize the cellular lattice of the fillers according to the individual protective device and the level of protection to be achieved. Disadvantageously, the design of an optimized lattice requires long and complex experimental campaigns, the results of which are not directly correlated with the mechanical response of the final product. In the field of helmets for sports or motorcycling activities, these difficulties have led to the implementation of cellular fillers in combination with, and not as a replacement for, traditional EPS fillers. In other words, known helmets implement “add-on” solutions in which the cellular filler is added to the traditional EPS filler.

[0010] Disadvantageously, such “add-on” solutions are focused on the absorption of rotational or oblique impacts and not on linear ones.

[0011] Furthermore, to increase the comfort of the helmet, the “add-on” solutions require the introduction of intermediate layers configured to come into contact with the user's head.

[0012] From the foregoing, the need therefore emerges to provide accurate and simple methods for designing cellular fillers for protective devices, capable of optimizing the cellular lattice according to the degree of protection to be satisfied or any other geometric or mechanical constraint.

[0013] Scope of the invention

[0014] In this context, the task of the technician at the basis of the present invention is to propose a method for designing a cell-structured absorbent filler for a protective device that overcomes the drawbacks of the prior art mentioned above.

[0015] In particular, a scope of the present invention is to provide a simple method for designing a cell-structured absorbent filler capable of reducing the experimental load required by current design techniques.

[0016] It is also a scope of the present invention to provide a method for designing a cell-structured absorbent filler that is accurate in predicting the mechanical response of the filler in certification tests or any other test aimed at verifying its protection level.

[0017] It is also a scope of the present invention to provide a method for designing a cell-structured absorbent filler having high mechanical performance and, therefore, capable of satisfying high protection levels. In particular, it is a scope of the present invention to provide a method for designing a cell-structured absorbent filler that is potentially capable of responding effectively to both linear and rotational impacts.

[0018] It is also a scope of the present invention to provide comfortable protective devices - in particular, helmets for vehicles or for sports activities or construction site helmets - devoid of additional intermediate layers intended to come into contact with the user and capable of responding effectively to both rotational and linear impacts.

[0019] SUMMARY OF THE INVENTION

[0020] In accordance with the present invention, the indicated technical task and the specified scopes are achieved by a method for designing a cell-structured absorbent filler for a protective device according to one or more of the claims reported below.

[0021] In particular, the present invention proposes to implement a method articulated in finite element simulation phases and experimental phases which gradually allow determining the cell structure of the absorbent filler so that it is able to satisfy a predetermined safety level.

[0022] The finite element simulations with different levels of complexity, by predicting with increasing accuracy the mechanical behaviour of the cell-structured filler during design, allow reducing the load of experimental activities.

[0023] The reduction of the required experimental activities significantly simplifies the method for designing cellular fillers, also reducing its cost. In this regard, it is worth noting that each experimental activity requires the creation of a specific prototype of the cellular filler to be subjected to destructive tests (e.g. impact tests).

[0024] The implemented finite element simulations allow optimizing the geometrical parameters of the cellular lattice of the filler with increasing accuracy, thus being able to satisfy even the most demanding safety / protection levels. It is worth noting that the simplicity and accuracy of design achieved by the method object of the present invention allow providing protective devices - such as, for example, helmets for vehicles or for sports activities as well as construction site helmets - in which the traditional continuous EPS filler is completely replaced by the cellular filler. Indeed, the high control over the mechanical and geometrical properties of the filler allows providing protective devices devoid of the traditional EPS filler, capable of satisfying high safety levels in case of both rotational and linear impacts and, at the same time, being comfortable for the user without the need to introduce additional intermediate interface layers.

[0025] LIST OF FIGURES

[0026] Further characteristics and advantages of the present invention will become more apparent from the indicative, and therefore non-limiting, description of a preferred but not exclusive embodiment of a method for designing an absorbent filler, as illustrated in the accompanying drawings in which:

[0027] - Figure 1 shows a perspective sectional view of a protective device, in particular a helmet, provided with a cell-structured absorbent filler designed by means of the method according to the present invention,

[0028] - Figure 2a shows a perspective sectional view of a first numerical model of the absorbent filler in which the latter is approximated to a homogeneous isotropic or homogeneous orthotropic solid,

[0029] - Figure 2b shows a stress-strain graph in which the mechanical response of the first numerical model is reported, highlighting a plurality of mechanical parameters that describe it,

[0030] - Figure 3a shows a perspective view of a portion of a second numerical model of the cell-structured absorbent filler, - Figure 3b shows a perspective view of a cell of the second numerical model of Figure 3b with some of its geometrical parameters highlighted,

[0031] - Figure 3 c shows a perspective view of a further portion of the second numerical model characterized by a local variation of a geometrical parameter of the cell (graded model),

[0032] - Figure 3d shows a schematic perspective representation of a cell of the cellular filler dilated / contracted along a vertical direction (i.e. tapered);

[0033] - Figure 3e shows a schematic perspective representation of a cell of the cellular filler elongated along the vertical direction,

[0034] - Figure 4 shows a perspective view of a second numerical model of the cellular filler associated with a portion of the user's body to be protected in a virtual finite element simulation environment,

[0035] - Figure 5 shows an image of a physical model of the cell-structured absorbent filler to be subjected to experimental tests to verify its protection level,

[0036] - Figure 6 shows a block diagram of a method for designing a filler for protective devices according to the present invention.

[0037] DETAILED DESCRIPTION

[0038] The present description has as its object a method for designing a cell-structured absorbent filler 1 (or, more simply, "cellular filler") for protective devices 100 capable of at least partially absorbing (dissipating) the energy due to impact with external bodies and thus protecting a user from injuries and damage.

[0039] In the context of the present invention, the term "protective device" includes all those devices wearable by a user to protect a specific region of their body from impacts with external bodies. Examples of protective devices are both helmets for sports or work activities (ski, rugby, baseball helmets, or construction site helmets) and those for driving vehicles (motorcycle or bicycle helmets), as well as any other personal protective equipment (more synthetically, PPE) aimed at protecting the health and safety of the user against impacts or any other type of impact with external bodies.

[0040] The performance of the protective devices 100 (i.e. the protection level) is typically verified by means of specific certification tests (e.g. the tests required in Europe for the homologation of ski helmets EN1077, for bicycle helmets EN1078, for motorcycle helmets ECE 22.06) or significant tests not part of certifications implemented, for example, by the manufacturers of the protective devices.

[0041] It is well to specify that the protection level of the protective devices 100 can be determined either by means of binary -type criteria - i.e. the pass, or fail, of a specific test (e.g. certification / homologation test) - or by means of performance-type criteria -i.e. criteria aimed at assigning a merit score with respect to one or more parameters extractable from the test (e.g. probability of damage to a particular part of the body following a specific impact of less than N%).

[0042] In the design of protective devices 100 and, in particular, in that of their absorbent fillers 1 intended to absorb impact with external bodies, the protection level therefore represents a design constraint.

[0043] In accordance with the above and with reference to what is shown in figures 4 and 5, the absorbent filler 1 designed through the method object of the present description has a cellular structure, i.e. a conformation comprising a plurality of interconnected cells defining an alternation of solids and voids. The cells can be, for example, constituted by interconnected beams and / or plates to create a cellular lattice defining said cellular structure.

[0044] The absorbent filler 1 designed by means of the method object of the present description defines a cellular solid that can have either a periodic or a stochastic structure. The absorbent filler 1 can be, for example, designed so as to have a geometry insertable in a rigid or semi-rigid outer shell and / or be wearable on a portion of the user's body to be protected. In the case of absorbent fillers 1 for helmets, these are designed so as to have an outer surface accommodatable in the outer shell defining a cap and an inner surface wearable on the user's head.

[0045] It is in any case well to specify that, in some embodiments, the absorbent filler 1 could itself define the protective device 100.

[0046] As will emerge from the following, the method object of the present description is articulated in a plurality of hybrid numerical-experimental phases which, according to one aspect, are conducted in parallel, for mutual verification and correction, gradually implementing different levels of complexity to increase the accuracy of design and, therefore, decreasing the degree of approximation.

[0047] According to one aspect, the ultimate objective of the method described herein is the construction of a meta-model capable of predicting the performance of the cell-structured absorbent filler 1 as a function of topological parameters and mechanical properties of the material used, cancelling, or in any case limiting, the workload required in the design phase. Preferably, this meta-model is constructed by working on three or more modules implemented or in any case associated with phases B), C) and D) described below.

[0048] With reference to phase A) shown in the block diagram of figure 6, the method object of the present description first provides for receiving - or otherwise determining - input data 2 comprising the protection level 2a that the absorbent filler must satisfy under one or more predetermined load conditions P, for example regulated by a specific certification test.

[0049] In a possible embodiment, at least one of these predetermined load conditions P is representative of an impact condition. In the case where the absorbent filler 1 is for helmets, the one or more predetermined load conditions P can for example be representative of an impact test at 0° or 45° with respect to the axis of the user's head and / or the direction of the impact velocity.

[0050] In a further possible embodiment, at least one of these predetermined load conditions P is representative of one of the following load conditions: shear load condition, static compression condition, static shear condition.

[0051] According to one aspect, the input data 2 also comprise further first data 2b relating to geometric constraints such as, for example, the external / intemal geometry and / or the thickness of the absorbent filler 1 to be designed.

[0052] According to a further aspect, in addition to or as an alternative to the further first data 2b, the input data 2 also comprise further second data 2c relating to characteristics (properties) of the absorbent filler 1 and / or of the protective device 100 including the stiffness value (global or local), the weight (parameter proportional to the cost of the raw materials used and related to user comfort), and other parameters indicative of the user comfort of the protective device 100 or the production cost of the absorbent filler 1.

[0053] The method object of the present description also comprises a first finite element simulation phase B) executable by means of a computer.

[0054] In detail, the first simulation phase B) first provides for a first sub-step Bl) in which a first numerical model 3 of the absorbent filler 1 is made.

[0055] With reference to figure 2a, in the first numerical model 3 the absorbent filler 1 is approximated to a homogeneous or isotropic or orthotropic solid.

[0056] Preferably, the choice of approximating the absorbent filler 1 as a homogeneous or isotropic or orthotropic solid is conducted as a function of the cell module 4 that is intended to be selected in phase Cl) described below. For example, the approximation to a homogeneous isotropic solid is preferable for cell modules 4 which describe cells characterized by mechanical properties independent of the direction of application of the load (for example stochastic cells, or periodic cells sufficiently small with respect to the thickness of the filler), while the approximation to a homogeneous orthotropic solid is for cell modules 4 which describe cells characterized by mechanical properties dependent on the direction of application of the load (for example periodic cells with strongly directional properties and / or dimensions of an order of magnitude comparable to those of the thickness of the filler).

[0057] According to one aspect, in the first numerical model 3 the absorbent filler 1 is approximated to a block homogeneous solid (i.e. the absorbent filler 1 is partitioned into a plurality of homogeneous blocks which can have different mechanical properties from each other). Advantageously, this allows modelling the effect of the cell gradation already in phase B). Further details regarding the gradation of the cells are reported in a subsequent part of the description.

[0058] It is worth noting that the approximation of the absorbent filler 1 to a homogeneous isotropic or homogeneous orthotropic solid allows in the first numerical model 3 to describe the mechanical behaviour of the absorbent filler 1 - i.e. mechanical properties indicative of the response (deformation) of the absorbent filler 1 to mechanical stresses that induce a state of stress in the latter - by means of a finite number of mechanical parameters 30. Therefore, in the first numerical model 3, the mechanical behaviour of the absorbent filler 1 is described by one or more mechanical parameters 30.

[0059] With reference to figure 2b, according to one aspect, the mechanical parameters 30 used in the first numerical model 3 to describe the behaviour of the absorbent filler 1 comprise at least one of the following parameters: compressive collapse stress Yl, shear collapse stress Ys, compressive densification dl, shear densification ds, compressive elasticity El, and shear elasticity Es. It is well to specify that in the context of impact absorbers, densification is intended as the elongation value at which the constant force response deflects upwards (usually 60-70%). It therefore corresponds to the point at which the structure densifies, in fact it shortens and therefore the compression properties are in fact those of the solid material.

[0060] Preferably, the mechanical parameters 30 used in the first numerical model 3 to describe the behaviour of the absorbent filler 1 consist of the following six parameters: compressive collapse stress Yl, shear collapse stress Ys, compressive densification dl, shear deformability ds, compressive elasticity El, and shear elasticity Es.

[0061] According to one aspect, the mechanical parameters 30 also include the thickness of the absorbent filler 1 (or, if this is not constant, a function that describes its spatial trend). It is worth noting that, if necessary, the thickness of the absorbent filler 1 is provided in the input data 2 - in particular, in the further first data 2b -received as input in phase A).

[0062] Following sub-step Bl), phase B) provides for sub-step B2) of simulating the mechanical behaviour of the first numerical model 3 under the predetermined load condition P by varying the one or more mechanical parameters 30. In other words, substep B2) provides for conducting a series of finite element simulations in which the mechanical response of the first numerical model 3 to the predetermined load condition P is studied for different values of each mechanical parameter 30.

[0063] According to one aspect, sub-step B2) provides for conducting various simulations under predetermined load conditions P. In this case, the number and configuration of the simulations conducted in sub-step B2) depend on the type to which the protective device 100 to be designed belongs and, in particular, on the certification tests to which this type must be subjected to obtain homologation or consent for marketing. Furthermore, the number and configuration of the simulations conducted in substep B2) may also depend on the current state of complexity of the meta-model developed through the method object of the present description.

[0064] Phase B) then provides for sub-step B3) of analysing the results of the simulations conducted in sub-step B2) so as to determine a set of values 30a of the one or more mechanical parameters capable of satisfying the required protection level 2a. In other words, in sub-step B3) at least one combination of values 30a of the mechanical parameters is identified which is able to satisfy the required protection level 2a, i.e., for example, to pass a specific certification test.

[0065] In one embodiment, in sub-step B3), the set of values 30a of the one or more mechanical parameters 30 is determined by means of an optimization aimed at minimizing a target function related to the protection level of the protective device 100 and, therefore, of the absorbent filler 1 being designed.

[0066] Examples of target functions to be minimized to find the set of values 30a are: the cumulative probability of injury for a specific protection level such as the Abbreviated Injury Scale AIS4+

[0067]

[0068] Chttus: / / en.wjkj ZAbbreyiated jAU.qry_Scaje), or the Head Injury Criterion (HIC - https: / / en.wi ki pedi a . org / wiki / Head J nj ry__ cr i ten on). To determine the set of values 30a of the one or more mechanical parameters 30 it is also possible to minimize the target function weight of the absorbent filler 1 by placing the constraint of satisfying the required protection level 2a (e.g. passing a certification / homologation test).

[0069] Preferably, for the creation of the meta-model, the method object of the present invention provides for the creation of analytical relationships between the mechanical parameters 30 and the protection level 2a to be satisfied.

[0070] According to one aspect, before carrying out the simulations of sub-step B2), phase B) provides for the further sub-steps of: B 1.1) selecting a material law regulated by certain numerical parameters (stress-strain law that describes the behaviour of a material under the action of loads such as, for example, Hooke's law for linear elastic materials), and Bl.2) verifying by means of finite element simulations the mechanical behaviour of the first numerical model 3 through simple load conditions such as, for example, response to compression, shear and combined compression-shear.

[0071] Advantageously, sub-steps Bl.l) and Bl.2) allow linking (constructing analytical relationships between) the mechanical parameters 30, the physical variables of the problem (i.e. applied load state and imposed constraints), with the numerical parameters of the material law used.

[0072] With reference to the block diagram of figure 6, the method object of the present description also comprises a second finite element simulation phase C) which, according to one aspect, is aimed at characterizing the response of the absorbent filler 1 reduced to cells so as to determine one or more geometrical parameters of the latter that allow relating their behaviour to the set of values 30a of the one or more mechanical parameters 30 determined in sub-step B3.

[0073] In detail, with reference to figures 3a and 3b, phase C) first comprises sub-step Cl) of making or selecting from a database a cell numerical model 4 representative of a cell module 4a usable to make the cell structure of the absorbent filler 1 - for example, by means of a process of mapping the cell module 4a onto the geometry of the absorbent filler 1 executable through known codes.

[0074] As shown in figure 3b, the cell numerical model 4 is characterized by one or more geometrical parameters 40 that influence the mechanical behaviour of the respective cell module 4a. It is worth noting that the mechanical properties of the cell module 4a and, therefore, also those of the relative cell numerical model 4, are describable by the same mechanical parameters 30 used in phase A) to describe the mechanical behaviour of the first numerical model 3.

[0075] The geometrical parameters 40 preferably comprise at least one of the following parameters: diameter of one or more beams making the cell numerical model 4, thickness of one or more plates making the cell numerical model 4, distance between nodes N peripherally delimiting the cell numerical model 4.

[0076] According to one aspect, the cell numerical model 4 made or selected in substep Cl) is at least partially validated by means of numerical simulations and experimental tests.

[0077] In detail, in order to validate the cell numerical model 4 and / or to construct the database of already validated cell numerical models 4, the method provides for Cl.l) making a cell specimen PP representative of a respective cell module 4a (or more interconnected cell modules 4a) in a known material, and, by means of experimental activities, for Cl.2) studying its behaviour when subjected to one (or more) known load conditions CC such as, for example, static compression, static shear, perpendicular impact, or oblique impact.

[0078] Furthermore, still for the purpose of validating the cell numerical model 4 and / or constructing the database of already validated cell numerical models 4, the method provides for Cl.3) numerically simulating by finite elements the behaviour of the cell numerical model 4 when subjected to the known load condition CC, and for Cl.4) evaluating the correlation between the behaviour of the cell specimen PP obtained in sub-step Cl.2 with the behaviour of the cell model 4 in sub-step Cl.3).

[0079] Should a correlation value above a certain threshold value emerge from phase C.1.4), then the cell numerical model 4 is considered at least partially validated. By way of example, the correlation value can be calculated through a combined comparison between the total energies absorbed by the specimen and between the average resistant forces exerted by the specimen before densification. According to one embodiment, to obtain a complete validation of the cell numerical model 4, the characterization of the material of the cell specimen 4a is required. In detail, the latter preferably provides for C 1.5) performing a uniaxial tensile test on a dog-bone sample specimen 00 (i.e. standard specimen for tensile tests) made of a given material, and for Cl.6) simulating said tensile test by finite elements.

[0080] The simulation of sub-step Cl.6) is conducted on a model of the sample specimen MO to which a material property set is assigned which, in a sub-step Cl.7), is suitably calibrated so that the tensile mechanical behaviour of the sample specimen is correlated with the material characterization carried out in sub-step Cl.5). In other words, the material property set is calibrated so that the model of the sample specimen MO has a tensile mechanical behaviour equal to or in any case relatable / representative of that of the sample specimen 00 acquired during sub-step Cl.5).

[0081] The material property set calibrated in sub-step Cl.7) is in the following substep Cl.8) assigned to the cell numerical model 4 used in the finite element simulation described in sub-step C2) below.

[0082] According to one aspect, the given material in which the sample specimen used in the test of sub-step Cl.5) is made is the same material with which it is desired to make the absorbent filler being designed.

[0083] According to a more complex embodiment, since the cell module 4a undergoes a distortion when mapped onto the absorbent filler 1, the cell numerical model 4 is characterized by at least one distortion parameter representative of its geometrical compression / dilation distortion along at least one of its distortion directions (e.g. identified by its vertical axis).

[0084] Preferably, this distortion parameter is relatable to the Jacobian matrix of the cell numerical model 4 which, as is known, uniquely determines its state of deformation. Advantageously, the choice of a distortion parameter connected or relatable to the Jacobian matrix allows easily linking the state of deformation with an equivalent state of deformation and, therefore, using the latter to model the properties of the distorted cell, for example by imposing a suitably calibrated pre-stress.

[0085] As a distortion parameter, for example, the known Jacobian ratio - i.e. the minimum ratio of the Jacobian calculated at the integration points (Gauss points) - can be used to describe the vertical tapering of the cell numerical model 4, or the known elongation parameter ("stretch") to describe the dilation / contraction. In this regard, it is evident that the Jacobian ratio is independent of dilation or contraction transformations of the element itself and therefore isolates the tapering with respect to the contribution of dilation and other non-significant contributions, while the stretch isolates the contribution of dilation / contraction with respect to the contributions of tapering and other non-significant contributions.

[0086] According to a further more complex embodiment, to take into account the variation of relative density of the cellular structure of the absorbent filler (figure 3c), the cell numerical model 4 is characterized by a gradation parameter that allows taking into account the variation of the mechanical behaviour of the cell module when the parameters are scaled. Preferably, direct relationships are derived between the geometrical parameters 40 of the cell numerical model 4, a gradation function that describes how the gradation parameter varies when moving within the cellular structure of the filler, and the mechanical performance of the absorbent filler 1.

[0087] As shown in the block diagram of figure 6, phase C) then provides for sub-step C2) of simulating the mechanical behaviour of the cell numerical model 4, preferably in the determined load condition P, so as to determine an optimal value 40a for each geometrical parameter 40 such that the mechanical behaviour of the cell numerical model 4 is relatable to the set of values 30a of the one or more mechanical parameters 30. The behaviour of the cell numerical model 4 with the one or more geometrical parameters 40 set to their respective optimal values 40 is therefore describable by means of the set of values 30a of the one or more mechanical parameters 30 identified in phase B3). In other words, the mechanical properties of the cell numerical model 4 with the one or more geometrical parameters 40 set to their respective optimal values 40 are assimilable to those of the first numerical model 3.

[0088] According to one aspect, in sub-step C2), the method provides for simulating the mechanical behaviour of the cell numerical model 4 by varying each geometrical parameter 40 and thus identifying the value of at least one geometrical parameter 40 (or the combination of values of the geometrical parameters) that makes the mechanical response of the cell numerical model 4 relatable / assimilable to that of the first numerical model 3, i.e. describable by means of the set of values 30a of the one or more mechanical parameters 30 identified in phase B3).

[0089] Preferably, the mechanical behaviour of the cell numerical model 4 is evaluated by extracting its impact response along a first direction (impact response at 0°) and / or an impact response along a second direction oriented at 45° with respect to the first direction (impact response at 45°) and / or the shear response. In this regard, it is worth noting that: the impact response at 0° allows evaluating the mechanical parameters of compressive collapse stress Yl, compressive elasticity El, and compressive densification dl of the cell numerical model 4; the impact response at 45° allows evaluating the mechanical parameters of compressive collapse stress Yl and shear deformability ds of the cell numerical model 4; and the shear response allows evaluating the mechanical parameters of shear elasticity Es, shear collapse stress Ys and shear deformability ds.

[0090] According to one embodiment, in the second finite element simulation phase C) the level of simulation complexity - and, therefore, the accuracy of the results - is increased by moving from the simulation of the behaviour of the cell numerical model 4 to that of the entire cell-structured absorbent filler.

[0091] In detail, the embodiment with increased computational complexity provides for C3) making a second numerical model 6 of the absorbent filler 1 having a cell structure made by repetition of the cell numerical model 4 with the geometrical parameters set to their optimal values 40a. The second numerical model 6 is therefore obtained by mapping the cell numerical model 4 having the characteristics determined in sub-step C2) onto the geometry of the cellular filler 1. In this regard, figure 4 shows an example of a second numerical model 6 of a cellular filler for helmets worn on a numerical model of a head T.

[0092] Once the second numerical model 6 is made, the embodiment with increased computational complexity provides for C4) simulating the mechanical behaviour of the second numerical model 6 under the predetermined load condition P and verifying that the modelled absorbent filler 1 satisfies the required protection level 2a. If the verifications give a positive outcome, it will then be possible to proceed to the experimental phase D) described below, otherwise sub-step C2) is conducted again to identify a further optimal value 40a' for at least one geometrical parameter 40 and, then, sub-steps C3) and C4) considering this further optimal value 40a' in place of the corresponding one previously identified.

[0093] It is worth noting that the second numerical model 6, in the face of more complex and computationally more expensive numerical analyses, provides a more realistic modelling of the actual load condition and implicitly considers the effects of the distortion and gradation of the cell modules 4a due to their mapping onto the geometry of the cellular filler 1.

[0094] According to one aspect, for the purpose of creating the meta-model, phase C) comprises a further sub-phase in which mathematical relationships are developed between the one or more geometrical parameters 40 of the cell numerical model 4 and the protection level achievable through its use.

[0095] As shown in the block diagram of figure 6, the method object of the present description also comprises an experimental phase D) aimed at verifying the performance of the numerically simulated absorbent filler 1.

[0096] In detail, with reference to figure 5, phase D) provides for making a physical model 5 of the absorbent filler 1 having a cell structure made by repetition of the cell module 4a associated with the cell numerical model 4 with the geometrical parameters 40 set to their optimal values 40a. The physical model 5 therefore has a cellular structure obtained by mapping the cell module 4a having the characteristics determined in sub-step C2) onto the geometry of the cellular filler 1.

[0097] According toone aspect, phase D) provides for making the physical model of the second numerical model 6 which has been shown to satisfy the required protection level 2a in sub-step C4).

[0098] Phase D) therefore provides for subjecting the made physical model 5 to at least one experimental test adapted to verify that the absorbent filler satisfies the required protection level 2a.

[0099] Such an experimental test in the case of absorbent fillers 1 for helmets is for example an impact test, in particular, at 0° (i.e. directed along the axis of the head of the user wearing the helmet) or at 45° (i.e. directed at 45° with respect to the axis of the head of the user wearing the helmet).

[0100] Depending on the definition of the required protection level 2a, the experimental test can have a binary -type outcome (pass, fail) or can lead to the assignment of a merit score with respect to one or more parameters extractable from the test.

[0101] The at least one experimental test can also be codified in a certification standard such as, for example, that for helmet homologation. According to one aspect, the experimental test is conducted at points which are preferably chosen randomly. Advantageously, the random choice allows not only validating the optimal solution for the specific case but also the meta-model in its entirety.

[0102] It is worth noting that the validation of the entire meta-model allows searching for new optimal solutions as a function of the variation of its own objectives (e.g. protection level and geometric constraints). Indeed, given a validated meta-model, an analysis performed on a geometry (e.g. BCC), optimized to minimize the weight of the helmet and at the same time pass the certification test for bicycle helmets (EN1078 standard, resulting acceleration < 250g), can be exploited, without further analysis, to search for an optimal solution by choosing a different objective function, for example the minimization of the probability of injury.

[0103] Preferably, in phase D) the method can also comprise sensitivity tests aimed, for example, at evaluating the deviation of the results as a function of the variation of the impact angle in the case of experimental impact tests, the effect of the coupling of the absorbent filler 1 with the rest of the protective device 100 (e.g. rigid shell of the helmets), the effect of aging by exposing the absorbent filler 1 (or the entire protective device 100) to particular humidity and temperature conditions to artificially induce an aging process.

[0104] The present invention also relates to a protective device 100, in particular a helmet 100a for sports activities or for driving, comprising an outer shell 101 and a cell-structured absorbent filler designed through the method described above.

[0105] According to one aspect, during use, the filler has an upper surface in direct contact with the outer shell 101 and a lower surface in direct contact with the user's head. Therefore, preferably, the helmet 100a does not comprise the commonly used EPS fillers. Furthermore, preferably, the helmet 100a does not have intermediate interface layers between the absorbent filler 1 and the user's head.

[0106] Obviously, a person skilled in the art may make numerous equivalent modifications to the variants described above, without thereby departing from the scope of protection defined by the appended claims.

Claims

CLAIMS1. A method for designing a cell-structured absorbent filler (1) for a protective device (100), comprising the steps of:A) receiving input data (2) comprising a required protection level (2a) under a predetermined load condition (P),B) performing a first step of finite element simulation comprising the sub-steps of:Bl) making a first numerical model (3) of the absorbent filler (1), in the first numerical model (3) the absorbent filler (1) being approximated to a homogeneous solid or isotropic solid, or orthotropic solid, in the first numerical model (3) the mechanical behaviour of the absorbent filler (1) being described by one or more mechanical parameters (30);B2) simulating the mechanical behaviour of the first numerical model (3) under the predetermined load condition (P) by varying said one or more mechanical parameters (30),B3) determining a set of values (30a) of the one or more mechanical parameters (30) able to meet the required protection level (2a);C) performing a second step of finite element simulation comprising the sub-steps of:Cl) making or selecting from a database a cell numerical model (4) representative of a cell module (4a) usable to make the cell structure of the absorbent filler (1), said cell numerical model (4) being characterized by one or more geometrical parameters (40) affecting the mechanical behaviour of said cell module (4a), C2) simulating the mechanical behaviour of the cell numerical model (4) and determining an optimal value (40a) for each geometrical parameter (40) so that the mechanical behaviour of the cell numerical model (4) is relatable to the set of values (30a) of the one or more mechanical parameters (30) identified in step B3);D) making a physical model (5) of the absorbent filler (1) having a cell structure madeby repetition of the cell module (4a) associated with the cell numerical model (4) with the geometrical parameters (40) set to the optimal values (40a), carrying out with said physical model (5) at least one experimental test adapted to assess that the absorbent filler (1) meets the required protection level (2a).

2. The method according to claim 1, wherein the second step of finite element simulation C) comprises the further sub-steps of:C3) making a second numerical model (6) of the absorbent filler (1), said second numerical model (6) having a cell structure made by repetition of the cell numerical model (4) with the geometrical parameters (40) set to the optimal values (40a),C4) simulating the mechanical behaviour of the second numerical model (6) under the predetermined load condition (P) and assessing that the absorbent filler (1) meets the required protection level (2a), if the second numerical model (6) meets the required protection level (2a) proceeding to step (D), otherwise carrying out again substep C2) to identify a further optimal value (40a’) for at least one geometrical parameter (40) and then conducting again sub-steps C3) and C4).

3. The method according to any one of the preceding claims, wherein the cell numerical model (4) is at least partly validated by conducting the following sub-steps:Cl.l) making a cell specimen (PP) of the cell module (4a) from a known material,Cl.2) studying the behaviour of the cell specimen (PP) when subjected to a known load condition (CC),Cl.3) numerically simulating the behaviour of the cell numerical model (4) when subjected to said known load condition (CC),Cl.4) evaluating the correlation between the behaviour of the cell specimen (PP) obtained in sub-step Cl.2) and the behaviour of the cell model (4) in sub-step C1.3).

4. The method according to claim 3, wherein the known load condition (CC) is at least one of the following: static compression, static shear, perpendicular impact, oblique impact.

5. The method according to claim 3 or 4, wherein the validation of the cell model (4) involves characterizing the material of the cell specimen (4a) by conducting the substeps of:Cl.5) performing a uniaxial tensile test on a sample specimen with dog bone geometry,Cl.6) finite element simulating the tensile test conducted in sub-step Cl.5) on a sample specimen model with a material property set,Cl.7) calibrating the material property set of the sample specimen model so that the tensile mechanical behaviour of the sample specimen is related to the material characterization carried out in sub-step Cl.5),Cl.8) the material property set calibrated in sub-step Cl.7) is assigned to the cell numerical model (4) in the simulations conducted in sub-step C2).

6. The method according to any one of the preceding claims, wherein step C) comprises the sub-step of developing mathematical relationships between the one or more geometrical parameters (40) of the cell numerical model (4) and the protection level of the protective device (100).

7. The method according to any one of the preceding claims, wherein said one or more mechanical parameters (30) comprise at least one of the following parameters: compressive collapse stress (Yl), shear collapse stress (Ys), compressive densification (dl), shear deformability (ds), compressive elasticity (El), and shear elasticity (Es).

8. The method according to any one of the preceding claims, wherein said geometrical parameters (40) comprise at least one of the following parameters: diameter of one or more beams making the cell numerical model (4), thickness of one or more plates making the cell numerical model (4), and distance between nodes peripherally delimiting the cell numerical model (4).

9. The method according to any one of the preceding claims, wherein the experimental test conducted in step D) and / or the predetermined load condition (P) simulate an impact.

10. The method according to any one of the preceding claims, wherein in sub-step B3) the set of values (30a) of the one or more mechanical parameters (30) is determined by means of an optimization intended to minimize a target function related to the protection level of the protective device (100).

11. The method according to claim 10, wherein said target function is one of the following functions: cumulative probability of injury for a specific protection level, and Head Injury Criterion (HIC).

12. The method according to any one of the preceding claims, wherein in step C2) the mechanical behaviour of the cell numerical model (4) is evaluated by extracting theimpact response along a first direction and / or the impact response along a second direction oriented at 45° with respect to the first direction and / or the shear response.

13. The method according to any one of the preceding claims, wherein the cell numerical model (4) is characterized by at least one distortion parameter representative of a compressive / dilatational geometrical distortion thereof along at least one direction of extension thereof, said at least one distortion parameter being connected or relatable to the Jacobian matrix of the cell numerical model (4).

14. A helmet for the protection of a user’s head featuring an outer shell and a cell-structured absorbent filler designed through the method according to any one of the preceding claims.

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