Evaluating safety footwear impact resistance

Describing the importance of the SATRA STM 609 test machine in assessing the effectiveness of protective shoes.

Many workers face potentially debilitating toe injuries if the safety footwear they are wearing is not fit for purpose. Safety footwear manufacturers also run a risk of litigation with hefty financial penalties if the toe caps they select do not live up to expectations. It is important to ensure that these components will actually provide the expected level of shielding.

Two vital aspects can be tested with the SATRA STM 609 safety footwear impact test machine: toe cap impact protection and metatarsal impact protection. The tests can be conducted in accordance with three sets of methods – European EN ISO 20344:2011, US ASTM F2412-18a and Canadian CSA Z195:14 (R2019). Analysis can be carried out on the toe cap before shoemaking (for example, under EN 12568:2010), or from a section cut from the complete footwear. In the EN ISO, ASTM and CSA test methods mentioned above, assessment of protection for the metatarsal bones of the foot is conducted only on complete samples of footwear.

Protective toe caps provide ‘defended space’, which must be maintained under impact. Two types of deformation can occur following an impact: ‘elastic’ and ‘plastic’. Elastic deformation is the deflection of the toe cap which is recovered following the impact, whereas plastic deformation is the permanent change of shape of the material after the impact. Both types of deflection are common. Not only the maximum amount of protection offered against impact intrusion into the toe space should be assessed; checking the condition of the toe cap after an impact is also required.

Key impact tests

Impact tests, as required as part of legislative requirements for safety footwear, typically specify the following:

• an impact energy to which the specimen on test is to be subjected
• impact striker geometry and associated mass (the EN ISO method uses a wedge striker whereas the ASTM and CSA methods require a cylindrical striker with a domed end)
• a location for where impacts are to strike the test specimen
• requirements for anchoring the footwear or component
• criteria for acceptance after the test.

These represent some key requirements which an impact test machine capable of conducting these tests must be able to accommodate.

To determine the maximum intrusion into the toe space during the test, a cylinder of calibrated modelling clay is placed inside the toe cap prior to the test. This material has negligible spring-back when compressed, so it will retain the depth to which it was compressed. After the test, the height of the compressed cylinder is measured and checked against the minimum heights specified in the relevant standards. For assessing the impact in the case of the metatarsal test, EN ISO and ASTM use a moulded wax impression which is fitted into the footwear and measurements are taken from this moulding before and after the test. In a similar test, CSA utilises a polyurethane (PU) foot form incorporating a section of modelling clay.

The SATRA STM 609 test machine imparts the specified impact energy by allowing an impact striker of accurate mass to fall from a specified height. This striker mass is guided in its fall by double-wheeled, low friction bearing units located between two vertical guides. An electric motor drive incorporated in the machine is used to hoist the impact striker to the correct height. The impact striker is then released via the control system and allowed to free-fall onto the target specimen. Different impact strikers can be readily fitted to conform to the requirements of the particular test method being used.

At the base of the machine, an adjustable mechanism catches the impact striker on rebound from the initial impact, and so does not impart a subsequent impact on the specimen. A speed sensor is incorporated immediately above the impact point. The machine’s computing system utilises the recorded speed and the known mass of the impact striker to calculate the impact energy imparted (to confirm that the specified impact energy has been applied). This displays on the operating screen, along with other relevant information including the drop height.

The display screen is also used to enter data and initiate the test. During set-up, the impact striker is lowered to a position just above the specimen, which can then be accurately positioned and clamped. A jog function is used to position the impact striker in contact with the specimen. The machine then raises the impact striker to the required height (depending on the particular test), before releasing it to impact the test specimen. The machine incorporates safety interlocked glazed security shields to provide protection from the falling impact striker or any flying debris resulting from a test.

STM 609 can hold sections of complete footwear, individual components or toe sections cut from footwear. Clamps allow the specimens to be locked in the correct position.

SATRA test machines are designed and manufactured by SATRA, and are in regular use in SATRA’s own laboratories. Test performance can therefore be monitored over extended periods, with information from user experience being fed back into the design.

The SATRA STM 609 test machine can be used for compliance testing and to verify component or footwear assembly performance for product development. It can also be used to assist with the quality auditing of production items.

Publishing Data

This article was originally published on page 28 of the December 2021 issue of SATRA Bulletin.

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