WO2001049139A1

WO2001049139A1 - Impact absorbing device

Info

Publication number: WO2001049139A1
Application number: PCT/GB2001/000052
Authority: WO
Inventors: Daniel William Davey; Colin Morrison; Alan Robert Sage Ponter
Original assignee: University Of Leicester
Priority date: 2000-01-07
Filing date: 2001-01-08
Publication date: 2001-07-12
Also published as: EP1246547A1; AU2386501A; GB0000227D0

Abstract

There is disclosed an impact absorbing device for protecting a human body part which is positionable against the body part and generally conforming thereto, the device comprising a shell layer and a compressible layer; in which the compressible layer has an average elastic modulus, as hereinbefore defined, of greater than 0.10MPa, preferably greater than 0.15MPa, most preferably greater than 0.25MPa, and less than 0.50MPa, preferably less than 0.40MPa.

Description

Impact Absorbing Device

This invention relates to impact absorbing devices for protecting body parts, with particular, but not exclusive, reference to devices for protecting a human shin.

Impact absorbing devices for protecting a human shin, often known as shin pads or shin guards, are well known. Such devices are commonly used in sports such as football, rugby and hockey in order to provide protection from an impact with objects such as balls, posts, sticks and players. The objective of such devices is to prevent or reduce the instances of injuries, which can take the form of soft tissue damage, such as cuts or bruising, cruciate ligamentous injury, and fracture of the tibia.

Many shin pads take the form of a relatively stiff outer shell, often fabricated from a plastic material, and compressible inner layer, such as a foam.

Whilst such shin pads do provide a degree of protection, there is an ongoing need to provide greater protection against impact induced injury. Furthermore, there is a need to provide lightweight shin guards, because lighter shin guards require the user to expend less energy in order to walk or run, and generally will be less cumbersome to wear.

The present invention address the above described needs, and provides improved absorbing devices for protecting a human shin. The devices are lightweight, and are particularly adapted to minimise tibia fractures and cruciate ligament injuries. At the heart of the present invention is a new design philosophy which i) considers the impact of an object with a human shin in terms of the dynamics of the forces applied during the impact and ii) uses the results obtained thereby in order to formulate improved designs.

According to the invention there is provided an impact absorbing device for protecting a human body part which is positionable against the body part and generally conforming thereto, the device comprising a shell layer and a compressible layer;

in which the compressible layer has an average elastic modulus, as hereinafter defined, of greater than O.lOMPa, preferably greater than 0.15MPa, most preferably greater than 0.25MPa, and less than 0.50MPa, preferably less than 0.40MPa.

The "average elastic modulus" is the gradient of an equivalent plot of the actual stress/strain characteristics of the compressible layer, in which the stress/strain characteristics from zero strain to a strain value of 0.8 are replotted so that the stress is directly proportional to the strain, but the area under the graph (i.e., the stress integrated between the strain limits of zero to 0.8) is equal to the area under the actual stress/strain graph. Thus, the "average elastic modulus" is the gradient of an equivalent, linearised stress/strain plot between zero and 0.8 strain. This description is necessitated because the stress/strain characteristics of a typical compressible material are not linear over a range of strains from 0.0 to 0.8, and therefore a single value for the elastic modulus cannot be obtained directly.

The device may be an impact absorbing device for protecting a human shin, which is positionable against the shin and generally conforming thereto. The compressible layer may have an average elastic modulus, and hereinbefore defined, of greater than or equal to 0.30MPa. As will be demonstrated below, values around 0.30MPa are preferred, because this allows the use of a shell layer having a stiffness similar to the stiffness of the human tibia. Devices of this type can absorb impact loads which approach or even exceed the accepted maximum tolerable load which the human tibia can withstand without fracture (ca. 5000 N).

The compressible layer may comprise a bubble wrap material having a plurality of compressible pockets. An example is a material termed "500 Solar Cover", manufactured by Plastica Limited of Brook Way, Ivyhouse Lane, Hastings, East Sussex, TN354NN, UK. This material is highly compressible, with an average elastic modulus of 0.359 MPa. In fact, 500 Solar Cover continues to absorb kinetic energy by compression up to a strain value of 0.95 (i.e., 95% compression). Furthermore, the material is lightweight. It is known to employ this material as a solar cover for swimming pools. However, its usefulness in the construction of impact absorbing devices is a surprising and unexpected aspect of present invention.

The stiffness of at least a portion of the shell layer, may be less than 1500 Nm, preferably less than 1200 Nm, most preferably less than 1000 Nm. The principal behind this is that, if the stiffness of the outer shell greatly exceeds the stiffness of the bone which it is protecting, ligamentous damage can be caused. In the case of shin protection, cruciate ligament damage could be caused if the stiffness of the shell layer at the point of impact exceeds the stiffness of the tibia at the point of impact.

The stifftiess of at least a portion of the shell layer may be greater than 400 Nm, preferably greater than 600 Nm, most preferably greater than 800 Nm. The principle behind this is that, if the stiffness of the outer shell is much less than the stiffness of the bone which it is protecting, then the impact load is highly localised, thereby increasing the likelihood of fracture. The shell layer may comprise a glass fibre laminate, such as Twintex (RTM). Such materials have a high stiffiiess to weight ratio which permits the production of devices of relatively light weight and minimal thickness. Twintex (RTM) is particularly advantageous because it is cost effective, and can be formed at low temperature and pressure thereby facilitating convenient mass production.

The shell layer may comprise a carbon fibre laminate. Such materials exhibit excellent stiffiiess to weight ratios.

The thickness of the shell layer may be between 1 and 3 mm.

Impact absorbing devices in accordance with the invention will not be described with reference to the accompanying drawings, in which:-

Figure 1 is a graph of w₀/w_inf against x;

Figure 2 shows (a) the impact of an impactor against a plate on an elastic foundation, (b) a graph of load against time during the impact, and (c) a graph of plate displacement against time during the impact;

Figure 3 is a graph of against w/1;

Figure 4 is a graph of against load P;

Figure 5 is a graph of v 2.65* ^~lM m against h/1 when h=l .25w_σ D Figure 6 shows a shin pad of the present invention viewed a) from the front and b) from above;

Figure 7 is a stress/strain plot for a typical foam;

Figure 8 is a stress/strain plot for 500 Solar Cover;

Figure 9 is a linearised stress/strain plot for 500 Solar Cover up to a strain value of 0.8; and

Figure 10 is a perspective view of a sample of 500 Solar Cover.

Consideration of the dynamics of an impact against a shin protected by such a device indicates that when the conditions imposed by this equation are met, loads up to the accepted maximum tolerable load for the tibia can be safely absorbed by the device.

Firstly, a dynamic model of an impact against a shin protected by a rigid outer shell on an elastic foundation will be developed.

The general solution for a circular plate on an infinite elastic foundation under a central point load ("the general solution") can be expressed as:

1 -

where A, B, C, D are constants w is the deflection of the plate divided by the characteristic length

E^p is the elastic modulus of the plate, t is the thickness of the plate and v is the Poisson's ratio k is the modulus of the foundation and is given by E/h for a finite foundation

E is the elastic modulus of the foam and h is the thickness of the foam.

The condition that the deflection of the plate must be finite at the centre i.e. when x=Q, dictates that C must be equal to zero and the equation may be reduced to:

⁶

This equation may then be written in the form

w/1 = A_gl(x) + Bg₂(x) + Dg₄(x)

where g,(x) =l-x⁴/64, g₂(x)=x -x⁶/576, g₄(x)=(x²-x⁶/576)ln(x)

By equating the reaction forces in the subgrade to the total load on the surface of the plate we may determine the constant D

X rdQ_r=a + P = 0 where Q is given by

which leads to a solution for the constant D

D = P/8πkl³

Using the boundary conditions at the outer edge (r=a) that M=Q=0, we obtain:

w 1 dw

= 0 dr² r dr (b)

From equations (a) (b) and (c) the constants A and B can be written in the form A=(P/8πkl³) fi(a/l) and B=(P/8πkl³) f₂(a/l)

where f(a/ ) and /(a/1) are linear functions of the size of the plate divided by the characteristic length.

The general solution may now be written as

The maximum deflection occurs at r=0, which gives g,(x)=l,g₂(x)⁼=0,g₄(x)=0

Using the term w₀ to define the deflection at the centre of the plate we obtain the equation:

P a w₀ =

8II&/² I

For the special case of an infinitely large plate (a=∞)

P w_ W_, =

8kl'

From this a non dimensional plot may be obtained, by dividing the experimentally obtained central deflection w₀ by the theoretical central displacement calculated for an infinite plate of otherwise identical properties to obtain a graphical solution for (f₁ (a/1)) which is shown in Figure 1. The solution is of the form:

The data points are shown as crosses in Figure 1, and the best fit is shown by the solid line. The best fit is of the form:

w„

= 2.65x -1.34

W inf

If we compare this equation d, we obtain

(/_; (a/l))= 2.65πχ-¹³⁴

The general solution relates to static problem, whereas an impact is dynamic in nature. Thus, we manipulate the solution by applying a dynamic solution with the conditions shown in Figure 2, in which an impactor 20 of mass M impacts against the centre of an outer shell 22 which is on an circular elastic plate 24. The impact causes a central deformation w₀ with a load P _max being exerted on the shell 22.

The impact is sinusoidal in nature with P _max and w₀ occurring when t=π/2.

From the non-dimensional plot we obtained

W -

= 2.65* -¹-³⁴ w inf

substituting w_inf = P/8kl² gives

2.65 P l²x ^'ΪM ^w SD From F=ma,

m d¹ = -P dt²

which leads to:

and ultimately gives

where

v = impact velocity

m = mass of impactor

P_max = maximum load at point of impact

We can validate these relationships using experimental data. The results are shown in Figure 3 (for w) and Figure 4 (for P_max). We now introduce criteria for the specific impact conditions. The first criterion is that the foam must not crush by more than 80% of its thickness. It is vitally important that the foam does not compress to an extent that it loses its impact absorbing properties causing the leg itself to become the impact decelerator, increasing the stresses on the tibia and allowing the possibility of fracture.

Substituting 0.8h for w₀ gives

expressing non-dimensionally

plotting this we obtain the solution shown in Figure 5.

The second criterion considers the peak load the tibia can withstand without fracture. The most comprehensive tibial impact test results are provided by Kramer et al (Kramer, M, Burrow, K and Hegar, A, Fracture Mechanisms of Lower Legs under Impact Load, 17th Stepp Car Crash Conference 1973 SAE730966, 81-150). Kramer et al addressed the problem of pedestrians in urban traffic accidents. It is particularly comparable to many sporting injuries to the lower leg as it simulates car bumper impact on the distal third of the lower leg. Kramer et al reports the results of tests on over 200 subject cadavers over an age range of 20 to 90 years. Analysis of the data of Kramer et al suggests that a value of about 5000N may be considered a tolerable load. Re-arranging our first criterion

From our dynamic solution

Rearranging

substituting (f) into (e) we obtain

h _ 0.414 P m Ix -1.34 ax

/ D (g)

We substitute the maximum tolerable load of 5000N of P_max as stipulated by our second criterion, and recall that:

* = £

a x = —

I Substituting these into equation (g) we are left with a relationship which defines the behaviour of an idealised shin pad to conform with the two criteria in terms of the thickness of the foam padding h, the stiffness of the outer shell D, the radius of the pad a, and the Elastic modulus of the foam padding E. The relationship is shown in equation (h).

_D 0.165 _A 0.165 _£ 0.835 _fl 1.34 _{= 2070} (^h)

As can be seen the behaviour is dependent in the radius of the plate and then in order of reducing influence, E, D and h. The radius of the plate is defined by the average dimensions of the shin, the stiffness of the outer shell is defined by the stiffness of the tibia (which is discussed in more detail below) and the thickness of the foam padding (which as can be seen in equation (h) is not particularly influential) is controlled by practicality. This leaves us with the dominant design factor of E.

If we now substitute exemplary values for h, D and a into this equation we arrive at an idealised E. The equations derived above are in the instance that the impacting load is borne by a plate of uniform radius. In practise, a shin guard is generally shaped as shown in Figure 6, with a breadth that is smaller than its longitudinal length, the guard being curved in a plane transverse to the longitudinal axis. Such a design enables the guard to conform to the shape of the shin. Typical dimensions are length 260mm and breadth 120mm. An "average" radius of a=0.095m can be derived from these dimensions by calculating the area associated with these dimensions and equating this area with π r², where r is an "effective" radius. A typical value of h is 6mm.

Preferably, the stiffness of the outer shell is equivalent to the stiffness of the tibia (El). The stiffness of the tibia is given in available literature as approximately 900 Nm (see below). Substituting these values into equation h so as to balance the LHS and the RHS we obtain an idealised E of 0.3 MPa when a = 0.095m.

Dealing with the non-linear behaviour of foams

The non-linearity of the stress strain curves for cellular materials provides a problem in defining a singular elastic modulus for the foams.

Figure 7 shows the compressive stress curve for HL34, a closed cell foam material. The characteristics may be broken down into three distinguishing sections.

• At small strains the foam deforms in a linear elastic manner due to a cell wall bending

• This is followed by a plateau of deformation at almost constant stress, caused by the elastic buckling of the cell walls

• Finally there is a region of densification where the cell walls crush together leading to a rapid increase in compressive stress.

To deal with this problem, we utilise the area beneath the curve up to a strain value of 80% (stipulated as one of the criteria) and re-plot this as a straight line, with the E value calculated as the gradient of this equivalent plot. Effectively, by using this method the two plots provide identical strain energies for the same deflection, but a definite elastic modulus is now defined.

Choice of Materials

Many differing possible materials were tested under compression to find one that fitted the required material properties. The material that came closest and is of the lightest weight is manufactured as "500 Solar Cover" by Plastica Limited (address provided above).

It should be emphasised that the intended end use of this product is as a solar cover for swimming pools. It is a surprising aspect of the present invention that a material intended for use as a swimming pool cover can be advantageously incorporated into impact absorbing devices of the present invention.

Figure 8 shows the conventional stress-strain curve for 500 Solar Cover, whereas Figure 9 shows the data replotted as a straight line in order to derive an average elastic modulus. The average elastic modulus E for 500 Solar Cover is 0.359 MPa which is only slightly larger than the calculated optimum of 0.3 MPa. 500 Solar Cover also has the added advantage that unlike foams it will continue to absorb energy up to approximately 95% compression as shown in Figure 8. Thus, even if the applied load is such that the 500 Solar Cover is crushed beyond a strain value of 0.8, the shin pad will continue to absorb the impact.

It should be noted that whilst 500 Solar Cover is a preferred material, other materials might be used providing that such materials sufficiently possess the required elasticity. Indeed, it is possible that other elastic materials having a sufficiently low elastic modulus might already exist, or might become available in the future. The present invention provides a design paradigm which encompasses and envisages the use of other suitable materials, even if they are not available at the time of writing. The skilled person readily utilise design parameters provided herein to determine if such other materials are suitable.

As shown in Figure 10, 500 Solar Cover is an example of a bubble wrap type material which comprises a plurality of compressible pockets 100. The pockets of 500 Solar Cover are air filled plastic bubbles (or "blisters") which are formed on a plastic substrate 102. The thickness of 500 Solar cover is 500 μm. This makes the material tougher than most bubble wrap products, and this is another advantage in the context of the present invention. However, other bubble wrap materials might be usefully employed in impact absorbing devices of the present invention. It might be possible to produce bubble wrap material in which the pockets are filled with a fluid, in order to improve or optimise compression properties.

For the shell material it is required that the stiffness to weight ratio is high to obtain the required stiffness while keeping the thickness and weight of the pad to a minimum. The optimum material to perform this task is a carbon fibre laminate. However, the cost and moulding time militates against the use of this material for mass production of pads. The "next best" material group is glass fibre laminates. The outstanding material is this group is called Twintex (RTM). This material is a fabric woven with commingled E glass and polypropylene rovings and is manufactured by Vetrotex International (Chambery Cedex, France) It is inexpensive and crucially will form to shape quickly at low temperature and pressure to allow ease of mass production.

It is important that the outer shell is of the same stiffness as the tibia itself as there is evidence that ligamentous damage may also be caused by impact where the guard is stiffer than the tibia. In a study by Viano et al (Viano, D, Culver, C, Haut, R, Melvin, J, Bender, M, Culver, R, and Levine, R, Bolster Impacts to the Knee and Tibia of Human Cadavers and an Anthropomorphic Dummy, Proceedings of the 22nd Stepp Car Crash Conference 1978, 347-357), in which a bolster impact was carried out on the lower leg of seven cadavers (4 male, 3 female), all female cadavers exhibited fracture of the tibia and fibula. However, none of the male legs were fractured. Two male subjects showed no serious injury, but the other two showed tears and avolutions of the posterior cruciate ligament. This is an even more serious injury than fracture of the tibia and the bolster impact test method may be considered very similar to a stiff shin guard being forced down on to the leg. At the other extreme a pad of lower stiffiiess than the tibia would lead to high localisation of the impact load and would increase the probability of fracture. Ideally, the pad deforms with the leg, maintaining contact so there are no regions of concentrated load.

Various literature gives the Elastic modulus of bone as 15 GPa. The required second moment of area for the prospective guard may be obtained by equating El_ti_bi_a P^er unit length to EI_pad at the six intervals along the tibia.

Table 1 Second Moment of Area for tibia at six intervals

Table 2 Required I_pad to maintain identical pad and tibial bending stiffnesses

Using the outer radius of the shin and the second moment of area for a hemispherical shell, the required thickness of the guard using Twintex (RTM) may be calculated.

Table 3 Twintex pad thickness

Considering the above results shown in table 3 we can see that to ensure identical bending stiffiiess both for the pad and the tibia would require the construction of a pad that had varying thickness along its length. This is within the scope of the invention, although it is envisaged that, in practise, a constant or near constant thickness would be used. If a thickness of 2 mm is selected for the shell, the bending stiffness for the tibia and leg are adequately close. This is particularly true lower on the tibia where impact is most likely to occur and the tibia is correspondingly most vulnerable. Thus, a shell thickness of approximately 2 mm manufactured from woven balanced Twintex (RTM) would be suitable. Figure 6 shows an example of a shin pad according to the invention having a outer shell layer 60 and a compressible layer 62. As stated above, the preferred material for the outer shell layer 60 is Twintex (RTM), and the preferred material for the compressible layer 62 is 500 Solar Cover. However, this choice of materials is non- limiting. A layer of breathable cloth 64 can be provided to eliminate irritation which can be caused by direct contact of the compressible layer 62 with the skin.

The foregoing discussion is based on the assumption that the shin pad is supported by a rigid medium. In reality, the shin is not fully rigid and thus the dynamic response of the leg itself is not considered by the model discussed above. Consideration of the non-rigidity of the shin results in equation (h) being replaced by the rather more complicated equation (i). Note that equation (i) specifically relates to the instance in which 80%) compression of the compressible layer occurs.

(i)

Using the values previously used to solve equation (h) (i.e., with an outer shell stiffness equivalent to the stiffness of the tibia) and iterating equation (i) to solve for E produces an "optimum" E value of 0.16MPa. Thus, it may be possible to improve upon the shin design described above by using a material having a lower average elastic modulus than 500 Solar Cover.

It will be appreciated by the skilled reader that the exact shape of the shin pad shown in Figure 6 might be altered in numerous ways: for example, the breadth of the shin pad may taper from top to bottom. Such variations in overall shape are within the scope of the invention. Also, it is possible to use two layers of compressible material.

Although the foregoing discussion is primarily with respect to the protection of the human shin, it will be appreciated that the principles described above can be extended to the protection of body parts. In particular, equation (h) can be utilised to predict parameters for the design of impact absorbing devices for protecting other body parts. For example, the maximum tolerable load for the human femur is about 8000N. This value, together with "sensible" values relating inter alia to the dimensions of the human thigh, can be used to indicate required compression properties of the compressible layer. By equating the stiffness of the shell layer with the stiffiiess of the bone being protected, or, more accurately, with the stiffiiess of the bone in the most important region of contact, further constraints on the design parameters can be imposed. Other body parts such as the upper and lower arm, hand and fingers might be protected in this way.

Claims

C AIMS

1. An impact absorbing device for protecting a human body part which is positionable against the body part and generally conforming thereto, the device comprising a shell layer and a compressible layer;

in which the compressible layer has an average elastic modulus, as hereinbefore defined, of greater than O.lOMPa, preferably greater than 0.15MPa, most preferably greater than 0.25MPa, and less than 0.50MPa, preferably less than 0.40MPa.

2. An impact absorbing device according to claim 1 for protecting a human shin, the device being positionable against the shin and generally conforming thereto.

3. An impact absorbing device according to claim 2 in which the compressible layer has an average elastic modulus, as hereinbefore defined, of greater than or equal to 0.30MPa.

4. An impact absorbing device according to any previous claim in which the compressible layer comprises a bubble wrap material having a plurality of compressible pockets.

5. An impact absorbing device according to any of claims 1 to 4 in which the stiffness of at least a portion of the shell layer may be less than 1500Nm, preferably less than 1200Nm, most preferably less than lOOONm.

6. An impact absorbing device according to any one claims 1 to 5 in which the stiffiiess of at least a portion of the shell layer may be greater than 400 Nm, preferably greater than 600 Nm, most preferably greater than 800 Nm.

7. An impact absorbing device according to any previous claims in which the shell layer comprises a glass fibre laminate.

8. An impact absorbing device according to claim 7 in which the glass fibre laminate is Twintex (RTM).

9. An impact absorbing device according to any of claims 1 to 6 in which the shell layer comprises a carbon fibre laminate.

10. An impact absorbing device according to any previous claims in which the thickness of the shell layer is between 1 and 3 mm.

11. An impact absorbing device substantially as hereinbefore described with reference to the accompanying drawings.