Testing dynamic shock absorption

How the SATRA STM 479 machine can help shoemakers to produce footwear with efficient shock absorption.

by Peter Allen

Although the human body can be very effective in absorbing shock when jumping, running or walking, injuries can still occur through impact loading to the foot (see ‘The biomechanics behind sports footwear design’). Such injuries could be mitigated by the provision of a shock absorption capability within footwear.

A number of health problems have been linked to one-off shock loading or repeated shock loading. These include lower back pain, joint disorders and heel bone fractures. Some conditions associated with poor health and advancing age can make the wearer more susceptible to the effects of impact loads when walking or running. For everyday footwear, shock absorbance within the heel construction can be a benefit to the user. Sports footwear and other high performance footwear (for example, for a military application) pose additional challenges for managing impact loading. Different sports place varying demands on footwear. This requires a consideration of the degree of shock absorbency to be provided, as well as its location within the footwear.

For all these types of footwear, being able to conduct a repeatable mechanised test to determine shock absorption capability provides a valuable contribution to product design, product verification or assurance of consistent quality. Claims made for an item of footwear’s shock absorption capability should be supported by verifiable relevant test results.

Static compression tests have been used to assess shock absorption, such as described in EN ISO 20344 for safety footwear. However, these static tests have limitations in the assessment of footwear’s ability to absorb impact loads. To overcome these limitations, SATRA has developed a test method to measure the dynamic shock absorption capability of footwear and materials (SATRA TM142 – ‘Falling mass shock absorption test’) – a method which can be conducted using SATRA’s STM 479 Dynamic Shock Absorption Test machine.

Output from research conducted using this test method and test machine has demonstrated that providing sufficient thickness of shock absorbing materials has a significant effect on improving footwear’s ability to accommodate impact loading. Insocks can significantly improve this characteristic for hard-soled shoes, but may offer only limited extra benefit when used in footwear with good inherent shock absorption capability.

The SATRA STM 479 Dynamic Shock Absorption Test machine enables a dynamic test to be carried out on footwear, cushion insocks or construction materials, and provides quantifiable test results. This test machine has been developed to conduct the SATRA TM142 test method. It was derived from fundamental research and developed to allow for an assessment of the shock absorption properties of footwear. Although primarily developed to test whole shoe heel impacts, it can also be used to assess forepart and cushioned insocks.

The machine operates by allowing a known mass to fall onto the footwear from a defined height. By the use of an accelerometer and displacement transducer, a value for the peak deceleration (‘g’ factor) for the falling mass is determined. A low ‘g’ value indicates good shock absorption characteristics. The test also assesses the energy return on rebound. Highly elastic materials give a high rebound height, while hard, low-resilience materials provide little energy return. The benefit to athletic performance of high-energy return is a matter of debate within the footwear industry. There is no direct relationship between shock absorption and energy return, and materials can be engineered to give different combinations of energy return and shock absorption properties. When SATRA conducts tests to TM142, the results are reported against expected performance levels specific to the footwear category. The SATRA STM 479 test machine is supplied with software which runs on customers’ own computer systems. This allows both graphical and numerical output of the test results and permits both energy return and shock absorption to be reported.

By combining the SATRA TM142 shock absorption test and the TM156 repeated compression test, an assessment can be made of the shock absorption and energy return characteristics for footwear when new, and also after a simulated period of use. Hence, these properties can be assessed over the anticipated lifetime of the footwear. Firstly, a shock absorption test (TM142 – using the SATRA STM 479 test machine) is conducted. This is followed by a repeated compression test (TM156 – using the SATRA STM 512 test machine, which simulates the effect over time of repeated heel strikes). The TM142 test is then repeated. By comparing the results for shock absorption and energy return from the TM142 tests before and after the repeated compression tests, an assessment can be made of any degradation of the absorption or energy return properties of the footwear over a simulated period of footwear use. More details of the SATRA STM 512 Repeated Compression Test machine can be found here.

Therefore, whether for benchmarking, product development or to monitor the quality of production footwear, the SATRA STM 479 Dynamic Shock Absorption Test machine supplies important data for the assessment of the shock absorption and energy return characteristics of footwear.

Click here for comprehensive information on SATRA's range of test equipment.

Publishing Data

This article was originally published on page 34 of the July/August 2011 issue of SATRA Bulletin.

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