IMPACT MITIGATING STRUCTURE

Abstract

A method of designing an impact mitigating structure. The method determines the force exerted by an object on the structure (11) as a function of the distance by which the object displaces a surface of the structure during impact. The method calculates a ratio of the integral of the force exerted by the object on the structure with respect to the distance by which the object displaces the surface of the structure during impact to the product of the maximum force exerted by the object on the structure during the impact and the total distance by which the object displaces the surface of the structure during the impact. The method also determines the respective values of characteristic variables of the structure that maximise the ratio for use in designing the structure.

Claims

1. A method of designing an impact mitigating structure; the method comprising: determining the force exerted by an object on the impact mitigating structure as a function of the distance by which the object displaces an outer surface of the impact mitigating structure during an impact of the object onto the outer surface of the impact mitigating structure; calculating a ratio of the integral of the force exerted by the object on the impact mitigating structure with respect to the distance by which the object displaces the outer surface of the impact mitigating structure during the impact to the product of the maximum force exerted by the object on the impact mitigating structure during the impact and the total distance by which the object displaces the outer surface of the impact mitigating structure during the impact; and determining the respective values of one or more characteristic variables of the impact mitigating structure that maximise the ratio for use in designing the impact mitigating structure.

2. The method as claimed in claim 1, wherein the impact mitigating structure comprises a cellular structure or a lattice structure.

3. The method as claimed in claim 2, wherein the cellular structure comprises a plurality of tessellating cells, wherein the plurality of cells each have a plurality of side walls that are shared with adjacent cells, and the plurality of side walls extend perpendiclarly to the surface of the impact mitigating structure and the cells each have a polygon shaped cross-section in a direction substantially perpendicular to the direction in which the side walls extend.

4-5. (canceled)

6. The method as claimed in claim 3, wherein the one or more characteristic variables comprise one or more of: the thickness of the side walls, the characteristic width of the cells, the height of the cells and the shape of the cells.

7. The method as claimed in claim 6, wherein the characteristic width of the cells is between 10 mm and 50 mm, and thickness of the side walls of the cells is between 0.4 mm and 5 mm and the height of the cells is between 10 mm and 30 mm.

8-9. (canceled)

10. The method as claimed in claim 6, wherein the shape of the cells has a hexagonal cross-section.

11. The method as claimed in claim 6, wherein a relative density of the cells is between 0.025 and 0.07, wherein the relative density is approximately 2t/w, where t is the thickness of the side walls of the cells and w is the characteristic width of the cells.

12. The method as claimed in claim 2, wherein the lattice structure comprises a plurality of struts extending between a plurality of vertices, and wherein the one or more characteristic variables comprise one or more of: the length of the struts, the thickness of the struts and the geometry of the struts.

13. (canceled)

14. The method as claimed in claim 1, wherein the impact mitigating structure comprises a curved outer and/or inner surface, and wherein the radius of curvature of the impact mitigating structure is between 60 mm and 140 mm.

15. (canceled)

16. The method as claimed in claim 1, the method further comprising impacting an object on the impact mitigating structure to determine the force exerted by the object on the impact mitigating structure as a function of the distance by which the object displaces the outer surface of the impact mitigating structure during an impact of the object onto the outer surface of the impact mitigating structure.

17. The method as claimed in claim 1, wherein the step of determining the respective values of one or more characteristic variables of the impact mitigating structure that maximise the ratio comprises repeating the steps of determining the force exerted by an object on the impact mitigating structure and calculating the ratio for a plurality of different respective values of the one or more characteristic variables.

18. The method as claimed in claim 1, the method further comprising setting one or more constraints and determining the respective values of one or more characteristic variables of the impact mitigating structure that maximise the ratio within the one or more constraints.

19. The method as claimed in claim 18, wherein the one or more constraints comprises a maximum allowed deceleration of 250 g when using the impact mitigating structure.

20. A method of designing an impact mitigating structure; the method comprising: determining the acceleration of the impact mitigating structure during an impact of an object onto the outer surface of the impact mitigating structure; calculating, using the acceleration, an objective measure of the ability of the impact mitigating structure to mitigate the impact of the object on the impact mitigating structure; and determining the respective values of one or more characteristic variables of the impact mitigating structure that optimise the objective measure for use in designing the impact mitigating structure.

21-22. (canceled)

23. The method as claimed in claim 20, the method further comprising manufacturing the impact mitigating structure using the respective values of the one or more characteristic variables of the impact mitigating structure that have been determined.

24. The method as claimed in claim 23, wherein the impact mitigating structure is manufactured using Additive Manufacturing.

25. The method as claimed in claim 23, the method further comprising generating a set of Additive Manufacturing instructions using the respective values of the one or more characteristic variables of the impact mitigating structure that have been determined; and manufacturing the impact mitigating structure according to the Additive Manufacturing instructions.

26-30. (canceled)

31. The method as claimed in claim 1, the method further comprising manufacturing the impact mitigating structure using the respective values of the one or more characteristic variables of the impact mitigating structure that have been determined.

32. The method as claimed in claim 31, wherein the impact mitigating structure is manufactured using Additive Manufacturing.

33. The method as claimed in claim 31, the method further comprising generating a set of Additive Manufacturing instructions using the respective values of the one or more characteristic variables of the impact mitigating structure that have been determined; and manufacturing the impact mitigating structure according to the Additive Manufacturing instructions.

Description

[0098] Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0099] FIG. 1 shows a flow chart detailing the steps of designing and manufacturing a helmet, according to an embodiment of the present invention;

[0100] FIG. 2 shows a cellular honeycomb structure designed according to an embodiment of the present invention;

[0101] FIG. 3 shows the force-displacement response of an object impacting against a cellular honeycomb structure;

[0102] FIG. 4 shows a plot of the cell wall thickness against the cell width for a cellular honeycomb structure;

[0103] FIGS. 5a, 5b, 6a, 6b, 7a, 7b, 8a, 8b, 9a and 9b show a various objective measures as a function of characteristic variables of the structure being designed according to an embodiment of the present invention.

[0104] In order to design and manufacture a (e.g. cycling) helmet that is optimised for its load mitigation, embodiments of the present invention will now be described that provide a method of designing and manufacturing such a helmet.

[0105] FIG. 1 shows a flow chart detailing the steps of designing and manufacturing a helmet for mitigating an impact, according to an embodiment of the present invention. First, the dimensions of the head of a user that is to wear the helmet are measured, e.g. using 3D scanning (step 1, FIG. 1).

[0106] Using these dimensions, a helmet having a cellular honeycomb structure is designed (step 2, FIG. 1). FIG. 2 shows such a cellular honeycomb structure 11 having a curved surface.

[0107] The cellular honeycomb structure 11 is formed with a curved surface and has a plurality of columnar hexagonal tessellating cells 12. The cells 12 are hollow and have flat side walls 13 that each extends in a direction along the surface normal (thus the side walls 13 of any one cell 12 are not parallel to each other as they would be for a cellular structure formed on a flat surface).

[0108] The initial design of the cellular honeycomb structure 11 is made with a set of nominal values for a number of characteristic variables. This may depend on the size and shape of the head that has been measured for wearing the helmet. For example, the cell height, the cell width and the cell wall thickness may be chosen as the characteristic variables to be varied. A nominal set of values of a cell height of 20 mm, a cell width of 22 mm and a cell wall thickness of 1.5 mm may be chosen, for example.

[0109] For the nominal helmet design and using appropriate material properties, e.g. those of polyamide 11, the impact of an object (a circular disk of radius 100 mm, a mass of 5 kg and a velocity of 5.4 ms.sup.1) onto the outer surface of the cellular honeycomb structure of the helmet is simulated (step 3, FIG. 1) using finite element analysis. The finite element model is calibrated using experimental data from actual impacts of objects onto cellular honeycomb impact mitigating structures.

[0110] For the simulated impact, the force exerted by the impacting object on the cellular honeycomb structure as a function of the distance by which the impacting object displaces the outer surface of the cellular honeycomb structure is determined (step 4, FIG. 1). FIG. 3 shows a graph of the force, F, exerted by an object against a cellular honeycomb structure as a function of the displacement, d, of the outer surface of the cellular honeycomb structure by the object (the cellular honeycomb structure for the graph of FIG. 3 has a cell height of 25 mm, a cell width of 30.6 mm, a cell wall thickness of 1.1 mm and a radius of curvature of 100 mm). (In other embodiments, e.g. when determining a different objective measure, the acceleration of the structure, owing to the impact from the impacting object, may instead or additionally be determined, with the acceleration then being used to determine the objective measure (step 5, FIG. 1).)

[0111] Using the force-displacement response of the impact (i.e. as shown in FIG. 3), the integral of the force with respect to the displacement is calculated (i.e. the area under the curve shown in FIG. 3). This is the actual work done by the cellular honeycomb structure during the impact. The product of the maximum force and the total displacement is also calculated. This is the ideal work for the cellular honeycomb structure. These two areas are shown in FIG. 3.

[0112] The chosen objective measure, e.g. the ratio, of the actual work done by to the ideal work for the cellular honeycomb structure is calculated (step 5, FIG. 1). In this embodiment, this ratio gives a measure of the load mitigation effectiveness of the cellular honeycomb structure. It will be appreciated that the higher the ratio, the closer the actual work done is to the ideal work done and thus the more efficient the cellular honeycomb structure is at mitigating impacts.

[0113] The nominal values for the characteristic variables of the cellular honeycomb structure are then changed (step 6, FIG. 1) and the simulation of the object impacting into the cellular honeycomb structure having this new set of values is repeated (step 3, FIG. 1). This enables another force-displacement relationship to be determined (step 4, FIG. 1) and the ratio of the actual work done by to the ideal work for the cellular honeycomb structure to be calculated (step 5, FIG. 1).

[0114] These steps are repeated for multiple different sets of values of the characteristic variables, to calculate multiple different ratios in order to ascertain an optimum set of values of the characteristic variables that maximise the ratio (or otherwise optimise the objective measure, as appropriate). The values of the characteristic variables are chosen within a set of boundary conditions, which are shown in FIG. 4.

[0115] FIG. 4 shows a plot of acceptable ranges of cell wall thickness against the cell width for a cellular honeycomb structure, on which lines of constant relative density (here approximately 2t/w, where t is the cell wall thickness and w is the cell width) are indicated. FIG. 4 shows the boundaries imposed for the characteristic variables whose values are to be explored during the optimisation of the ratio.

[0116] It can be seen from FIG. 4 that the cell wall thickness is varied from 0.4 mm upwards, the cell width is varied up to 50 mm (above this size there is too much variability in load response with impact location owing to the large size of the cells), and the cell wall thickness and the cell width are varied such that the relative density is between 0.025 and 0.07 (this is such that the density is less than foam but large enough to prevent densification).

[0117] Once the ratios corresponding to the values of the characteristic variables within the boundary conditions have been calculated, e.g. for a particular cell height of the cellular honeycomb structure, the set of values of the characteristic variables are chosen (e.g. numerically) that maximise the target objective measure, e.g. the ratio, of the actual work done by to the ideal work for the cellular honeycomb structure are output as an optimised design for the helmet (step 7, FIG. 1).

[0118] FIGS. 5a and 5b show the ratio (CJS) as a function of the cell width and the wall thickness, with the boundaries shown in FIG. 4 imposed. In FIG. 5a the ratio has been fit using bilinear interpolation; in FIG. 5b the ratio has been fit using multiple linear regression. In these figures, the Data points each represent data from a single simulation.

[0119] FIGS. 6a and 6b show a different objective measure, the peak acceleration (a peak), as a function of the cell width and the wall thickness, with the boundaries shown in FIG. 4 imposed. In FIG. 6a the peak acceleration has been fit using bilinear interpolation; in FIG. 6b the peak acceleration has been fit using multiple linear regression. In these figures, the Data points each represent data from a single simulation.

[0120] FIGS. 7a and 7b show a different objective measure, the Head Injury Criterion (HIC), as a function of the cell width and the wall thickness, with the boundaries shown in FIG. 4 imposed. In FIG. 7a the HIC has been fit using bilinear interpolation; in FIG. 7b the HIC has been fit using multiple linear regression. In these figures, the Data points each represent data from a single simulation.

[0121] FIGS. 8a and 8b show a different objective measure, the normalised displacement (D.sub.max) as a function of the cell width and the wall thickness, with the boundaries shown in FIG. 4 imposed. In FIG. 8a the normalised displacement has been fit using bilinear interpolation; in FIG. 8b the normalised displacement has been fit using multiple linear regression. In these figures, the Data points each represent data from a single simulation.

[0122] FIGS. 9a and 9b show a different objective measure, the peak stress (strength) as a function of the cell width and the wall thickness, with the boundaries shown in FIG. 4 imposed. In FIG. 9a the strength has been fit using bilinear interpolation; in FIG. 9b the strength has been fit using multiple linear regression. In these figures, the Data points each represent data from a single simulation.

[0123] After the set of values of the characteristic variables that optimise the objective measure has been determined, a helmet can then be manufactured according to the optimised design (step 8, FIG. 1), e.g. using polyamide 11. The Applicant has found that when the ratio of the actual work done to the ideal work for a cellular honeycomb structure is chosen as the objective measure, the maximum value obtained is 0.73, which is greater than twice the ratio (approximately 0.35) found for expanded polystyrene used in conventional bicycle helmets.

[0124] It will be seen from the above, that at least preferred embodiments of the invention provide a method for designing (and, e.g., manufacturing) an impact mitigating structure, as well as the impact mitigating structure itself. Designing an impact mitigating structure in this way helps to optimise the load mitigation of the structure and thus improve the safety and/or performance able to be provided by the structure. The design method may also be able to provide a customised design for a particular shape of impact mitigating structure, e.g. to fit a user. This may help to select the optimal impact mitigating structure for a particular surface curvature of the structure and/or a particular impact scenario (e.g. a maximum force that needs to be protected against).

[0125] The Applicant has found that structures designed using at least preferred embodiments of the method of the present invention may provide approximately a two-fold improvement in load mitigation compared to conventional foams that are conventionally used in crash helmets, for example.

[0126] While the above embodiments have been described primarily with reference to helmets, it will be appreciated that the load mitigation structure may be used in any suitable and desired type of structure that may be subject to an impact. This includes shields, body armour and (e.g. soles of) shoes, for example. During an impact, the load mitigation structure aims to make the load (on the structure) and, e.g., also the deceleration (of the object impacting the structure or the structure itself, depending on the inertial frame of reference) consistent. In a load mitigation structure such as a helmet which may experience a blow to the user's head, this helps to protect the user's head. In a load mitigation structure in a shoe, for example, this may help to improve the wearer's running efficiency.