Elsevier

Materials & Design

Volume 37, May 2012, Pages 543-559
Materials & Design

Effects of temperature on the material characteristics of midsole and insole footwear foams subject to quasi-static compressive and shear force loading

https://doi.org/10.1016/j.matdes.2011.10.045Get rights and content

Abstract

The use of appropriate leisure, sports and orthotic footwear can help reduce the occurrence of foot problems, maintain correct gait and provide stability for the individual. The temperature of footwear components could play a key role in the development of foot problems in certain circumstances and can also affect gait and stability too.

For orthotic and sports applications, footwear should be designed to accommodate the effects of temperature to maximise their cushioning and stabilizing effect. Materials test data is therefore required for footwear foams at various temperatures to predict their performance in service and to aid materials development and product design.

The mechanical properties of eight commonly used elastomeric foams (midsole and insole) have been obtained from the current study and the effect of temperature quantified under displacement control test conditions. The foams were subject separately to quasi-static compression and shear force loading under varying temperature conditions (−10 °C to 40 °C) using a new testing protocol. All material test data were subject to a least squares fit procedure within the FEA software ABAQUS to determine the coefficients of the hyperfoam model used to describe the foams behaviour at different temperature, a validation exercise for these coefficients has been performed. Further numerical methods were employed to determine the foams energy absorption characteristics.

The present study indicated that all foams tested exhibit typical elastomer mechanical and energy absorption characteristics and that these characteristics are affected significantly as a function of temperature. All foams demonstrated some degree of softening with elevated temperature whereas lower temperatures resulted in a greater amount of energy absorption capability for a specific value of strain (as a result of increasing stiffness).

Highlights

► Mechanical properties of elastomeric footwear foams obtained over a range of temperatures. ► Mechanical and energy absorption characteristics affected as a function of temperature. ► Hyperfoam model coefficients determined for compressive, shear force and combined compressive and shear force loading. ► Footwear foams could be selected based on environmental temperature to reduce injury.

Introduction

The foot is subject to forces whilst walking, running and jumping and these can generate stresses in the foot that are potentially damaging. In fact, unnoticed, repeated bouts of excessive stresses, particularly on insensitive skin, constitute a primary contributing factor to skin breakdown. Understanding and managing the cause of skin breakdown is critical in reducing such incidences and the subsequent possible risk of amputation [1]. Forces impact the plantar foot during weight-bearing activities and generate ground reaction forces (GRF) which are distributed under the plantar surface of the foot. GRF’s can be perpendicular to the foot (known as pressure) or they can act parallel to the foot (known as shear force). These GRF’s can form ulcers, particularly in the feet of people with diabetes due to their inability to appreciate the damaging stresses that can occur [1], [2], [3], [4], [5]. Footwear and orthoses are seen as important factors in preventing ulcers and other injuries in the foot. Selection of the correct footwear foam components based on individual requirements can lower the resulting stresses.

The mechanical properties of most footwear foam materials are highly temperature dependent, the lower the temperature, the less elastic the material. As such, a shoe can have different cushioning characteristics under different temperature conditions. An increase in temperature and subsequent softening of the foam material can also result in less stability [6], [7], [8], [9]. The increase in footwear temperature can occur due to an individual’s body temperature, by repetitive friction, compression and stretching of the material due to the forces generated at heel strike, and due to general environmental conditions.

Biomechanists and sports medicine experts emphasise the importance of good shoe design for stability, off-loading of plantar stresses and impact force attenuation during activities such as running [8], [10], [11], [12], [13]. Footwear foam temperature can be a contributory factor when considering the effectiveness of a shoe to perform its function.

Elastomer foams are used extensively as outsoles, midsoles and insoles in a wide variety of footwear. The mechanical and energy absorption characteristics of several midsole and insole foams are investigated in this study under displacement control test conditions with particular attention focussed on the effect of temperature on their performance.

Elastomer foams are widely used in footwear [9], [14], [15] as they act as shock absorbers, are lightweight and have the ability to conform to complex contours and recover large deformations. These properties have led to the foams use particularly in sports and therapeutic footwear where the feet can experience damaging stresses.

Elastomer foam material properties are highly non-linear and sensitive to density [9], [15], strain rate [9] and temperature [6], [7], [8], [9]. They are best described by non-linear material models such as the Ogden model [16].

The physical properties of elastomeric foams can be greatly influenced by temperature variation and footwear foam temperature can have a direct effect on the forces transmitted to the foot. It has been shown that soft footwear foams result in a longer contact time between the foot and the ground resulting in a reduction in the peak value of the forces transmitted to the foot and skeletal system [17]. For many activities, such as running, the wearer must perform in hot and cold temperatures which will affect the attenuating properties of the footwear. Three main factors which play key roles in modifying the temperature of footwear are foot temperature; friction and compression between foot and footwear; and the temperature of the environment.

Repetitive friction and compression between foot and footwear induces a non-homogeneous elevation of temperature in the foot [18]. This elevation varies according to activity and to the ability of the footwear materials to evacuate heat. In general the heat is transferred to the shoe components (midsole and insole) resulting in a temperature rise.

Other studies [8] have found that midsole temperatures increase during the initial 15–20 min of a run followed by a relatively constant temperature period as the heat exchanges becomes balanced. They found that the average increase in midsole temperature was about 8 °C with a maximum value of 13 °C. This increase was believed to be due to the effect of internally generated heat resulting from repetitive compression of the air cells inside the footwear and from heat transfer from the foot. This heat transfer is significant as whilst running the maximum midsole temperature has been shown to exceed the higher of the exterior air temperature and the road temperature by around 10 °C.

Runners generally seem to use the same or similar shoes throughout the year regardless of environmental temperature. Footwear foams have a melting temperature of about 70 °C. In colder temperature conditions, foams tend to become firmer depending on their formulation and cold ambient temperatures significantly reduce the shock attenuation of commonly used running shoes [7]. These findings suggest that although the footwear foam temperature is not identical to the environmental temperature during an outdoor activity, it is still heavily influenced by environmental temperature. Considering the wide range of extreme weather conditions in which today’s athletes train, the performance of their running shoes could vary significantly.

There are three common phases of deformation observed during compression of footwear foams [9] as seen in Fig. 1. The first phase is a linear elastic response, where stress increases linearly with deformation and the strain is recoverable. The second phase is characterised by continued deformation at relatively constant stress, known as the stress or collapse plateau and provides the bulk of the energy absorption capabilities for the material. The final phase of deformation is densification where the foam begins to respond as a compacted solid. At this point the cellular structure within the material has collapsed and further deformation requires compression of the solid foam material. Fig. 1 shows that temperature elevation results in decreased elastic modulus and plateau stress levels and an increased densification strain.

In Fig. 2, the shear stress–strain behaviour of the foams can be seen to be divided into two regions characterized up to 25% strain; the first is of linear elasticity where stress increases linearly with deformation and the strain is recoverable; the second is characterised by continued deformation at relatively constant stress [19], [20]. Again, Fig. 2 shows that temperature elevation results in decreased elastic modulus and plateau stress.

The energy absorption of elastomeric foams is affected by temperature variation [7]. When an elastomeric foam is compressed, energy is absorbed in the bending and buckling of the cell walls. Some of this energy is returned on decompression of the material. The energy and shock absorption ability of footwear foams can affect oxygen consumption and the peak forces transmitted to the skeletal system during activities such as running [7], [8], [21], [22]. Firm foams tend to exhibit less shock absorbency in comparison to softer foams. As such a firm foam will result in more energy being transferred to the body requiring greater muscle effort to absorb it and thus a greater oxygen demand. Stiffening of a shoe foam in cold weather is likely to increase the energy demands of running which would lead to earlier fatigue and hence a higher risk of injury. Conversely, high foam temperatures can soften foams to such an extent that they could ‘bottom-out’ providing no shock absorption for the wearer at all.

The total energy absorption (EA) for the foams can be calculated using the following equation,EA=EI-ER

The input energy (EI) and returned energy (ER) of a footwear foam is illustrated in Fig. 3, Fig. 4 which show typical elastomer compressive and shear loading/unloading stress–strain response. The EI and ER can be obtained numerically by estimating the area under the loading and unloading curves respectively [23].

The aim of this study was to investigate the effects of temperature on the material and energy absorption characteristics of midsole and insole footwear foams subject to quasi-static compressive and shear force loadings and to provide a library of non-linear hyperfoam material parameters for these foams. To achieve this, some of the most commonly used closed cell polymeric footwear foams [24] such as EVA, Nora Plastazote and Poron were tested to obtain their mechanical and energy absorption characteristics using displacement control test conditions. Subsequently, non-linear hyperfoam material coefficients were determined using ABAQUS for use in the appropriate strain energy function for each set of data.

Section snippets

Experimental work

The footwear foams were tested in compression and shear to obtain their stress–strain characteristics at temperatures ranging from −10 °C to 40 °C. The testing was undertaken using a Tinius Olsen (H1kS Model) tensile testing machine with a 5 kN load cell in conjunction with an integrated environmental chamber and appropriate loading fixtures as shown in Fig. 5.

Ogden strain energy formulation

A material model normally used to describe a foams response to loading is the non-linear hyperfoam model described by the Ogden strain energy formulation, Eq. (2) [15], [16].U=i=1N2μiαi2λˆ1αi+λˆ2αi+λˆ3αi-3+1βi(Jel)-αiβi-1where N is a polynomial order, μi, αi and βi are temperature dependent parameters. λi are the principal extension ratios, as shown in the following equation.λˆi=(Jth)-13λiλˆ1λˆ2λˆ3=Jel

The coefficients μi, are related to the initial shear modulus, μ0, by the expression shown

Discussion

A testing method for subjecting footwear foam materials to quasi-static uniaxial compressive and shear loading based on the two standard test methods, ASTM D575-91 [27] (standard test methods for rubber properties in compression) and [28] (principle of testing elastomeric materials under shear loading), under varying temperatures and using displacement control has been presented and used to investigate the effect of temperature on footwear foam material characteristics.

Conclusions

It was shown during testing in both compression and shear that it is necessary to pre-condition the foams to stabilize their stress–strain characteristics. The material response to loading for all foams tested was seen to stabilize after between two and three pre-conditioning repetitions of the loading cycle depending on the foam.

Four midsole and four insole polymeric footwear foam materials were evaluated in compression and shear at varying temperatures of −10 °C to 40 °C. The shape of the

Future work

The current study indicates that temperature can significantly affect the behaviour of footwear foams, this is a clinically important topic and warrants further investigation. Future work should involve: further experimental and analytical analysis under load control test conditions to quantify the effect of temperature on the shock attenuation properties of the footwear foams tested here; to extend testing over a wider range of temperature; incorporation of temperature dependent material

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