Test machines for heel assessment

How SATRA’s machines and test methods are used to assess the strength of heels and their attachment.

by Peter Allen

Deficiencies in footwear design or production which result in a heel breaking or completely detaching can have significant consequences. These include the potential for injury and litigation, the high costs associated with product recalls and the loss of brand image.

When designing a heel or heel attachment, the capability of the heel structure and its associated attachment to withstand a range of loads encountered in use are important considerations. This article outlines some SATRA test machines and devices which can be used to assess these capabilities.

Heel integrity

SATRA has developed test methods which can be used to validate the integrity of the heel itself; that is, its capability to withstand loads encountered in use. SATRA test method TM21 is used to determine the resistance of the heels of women’s shoes to the repeated impacts that occur in normal walking. The method is applicable to all types of slender heels fitted to women’s shoes, but is particularly useful for moulded plastic heels incorporating a tension pin or steel dowel for reinforcement. SATRA produces a test machine to carry out this test – the STD 156 heel fatigue tester (figure 1). The SATRA TM21 test method is carried out on a heel component – either a new one or one that has been removed from footwear. It is important when conducting this test that the heel is securely mounted, so that the impact loads are imparted to the heel rather than dissipated at the heel attachment. To achieve a secure heel mounting, the heel is held rigidly at the seat in a low melting-point alloy in one of the special holders provided. SATRA can supply a suitable melting pot along with the low melting-point alloy.

Adjustments are provided within the machine so that the heel can be set to receive forward impacts at the back of the stem, 6mm from the tip and perpendicular to the stem direction. Impacts are applied with an impact energy of 0.68J and delivered at the rate of one per second by a free-falling pendulum. The pendulum is lifted mechanically by the machine before being released, thus giving a repetitive rate of strike. After each impact, the pendulum is caught by the machine on rebound. The number of impact strikes can be set on the machine counter and inspections carried out at predetermined intervals. The machine is designed to switch off automatically in the event of a heel failure, and the counter will indicate the number of impacts which have been applied up to this point.

Another test method applicable to testing the integrity of the heel is SATRA TM20 – ‘Lateral impact tests for shoe heels’. This method is intended to determine the resistance of the heels of women’s shoes to the occasional heavy blows that occur in wear. The method is applicable to all types of high heels (women’s shoes with heights greater than 38mm), but it is particularly useful for moulded plastic heels incorporating a steel dowel for reinforcement. The SATRA test device used to conduct this test is SATRA STD 155 (figure 2). As with SATRA TM21, SATRA TM20 specifies a low melting-point alloy to be used to hold the heel specimen securely into the mounting block. The heel, mounted in its block, is positioned within the frame of the device so that the pendulum impacts the heel in a forward direction, and 6mm from the tip of the heel. The device provides the means to clamp the heel unit securely in place so that it is not dislodged by the impacts.

The test is conducted manually, with the pendulum being raised to a position marked against a scale. The scale marks correspond to increments of 0.5J of energy, from 0J to a maximum of 19J. The initial pendulum swing is released from the 0.5J mark, and after the impact the pendulum is caught on the rebound by the operator to prevent a second impact. The process is repeated, with the impact energy increasing 0.5 joules per impact. This is achieved by releasing the pendulum at the next higher scale point compared to the previous impact. The process continues, with rising impact loads, until the heel fails. The test method requires the reason for failure and the maximum impact energy at failure to be reported.

Heel attachment

In addition to ensuring the heel integrity itself, it is critical to ensure that adequate heel attachment is obtained both by design and in the consistency of manufactured products. SATRA test method TM113 – ‘Measurement of the strength of attachment of heels to footwear and backpart rigidity of such footwear’ is intended to determine the strength of heel attachment in completed footwear. The method is applicable to all footwear with separate attached heels. The test is conducted with a tensile tester using appropriate jaws. The SATRA STM 566 tensile tester (figure 3) fitted with STM 566HA and STM 566F jaws is suitable for the SATRA TM113 test. To conduct the test, the footwear forepart is securely clamped. The specialist jaws supplied then allow load to be applied to the heel. The test method identifies the measurements to be taken to ensure that the correct position is obtained for the attachment to the heel.

An additional fixture can be supplied for clamping to slender heels. The test continues until failure is obtained, and the load at failure, failure mode and, if required, the backpart deformation are reported. This relatively simple test can be used to assess the vulnerability for heel detachment from completed footwear and, hence, the risk of serious failures in footwear during use.

Another aspect of heel attachment can be assessed using another SATRA test method, TM96 – ‘Security of heel pins’. SATRA TM96 can be used to determine the heel pin holding strength of a material, or the heel pin pull-out force on a completed shoe. The method is mainly applicable to plastic heels for women’s footwear, but it can also be used with wooden heels. The method is not suitable for testing heels made of several layers of fibreboard or leather, or low plastic heels for men’s footwear. This test is also conducted with the STM 566 tensile tester – in this case, fitted with a STM 566HP jaws set.

Before conducting a test, the method sets out the process for inserting nails into the heel specimen using a commercial heel nailing machine, and through board material, which is subsequently removed before conducting the test. The method describes the details of conducting tests and associated measurements in which the tensile tester is used to pull out the pins from the heel, and so determine the heel pin holding strength and the heel pin pull-out force.

SATRA TM113 and TM96 as described above are examples of the use of the SATRA STM 566 tensile tester which, together with a range of specialist jaws, is capable of conducting a wide range of tests for both footwear and textiles.

The test methods and associated machines and devices listed in this article are important means of assessing the capability of heels to resist loads encountered in use, in addition to the attachment of the heel to the footwear – including the ‘grip’ provided by heel pins. These tests can be used to assess new footwear designs, as well as for the auditing of production batches of footwear to ensure the quality of manufacturing is maintained with respect to heel strength and heel attachment.

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Publishing Data

This article was originally published on page 40 of the September 2013 issue of SATRA Bulletin.

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