Testing children’s shoes for water resistance

The SATRA STM 505 dynamic footwear water resistance tester has now been reengineered and rescaled to perform these tests on a range of children’s footwear sizes.

by Harriet Mason

The SATRA STM 505 machine flexes the footwear in water, in order to assess its resistance to water ingress. It is also used to test all aspects of the footwear, including the materials, construction, seams and sole bond. The footwear is slipped over the pneumatically-driven foot mechanism, which has a hinged toe piece that flexes the footwear at a pre-determined rate of 60 flexes per minute in order to replicate a realistic walking rate (as specified in the test method). The mechanical foot has replaceable sections to allow different sized footwear to be tested. The toe, heel and cheek pieces can all be changed to suit the footwear size under test.

SATRA has the capability to test adult footwear (from UK sizes 3 to 13) on the SATRA STM 505, using the current toe and heel pieces (which give the machine a more realistic foot form). As the cheek, toe and heel have been reengineered into modular pieces, the size range has been widened by scaling down the modular pieces to now include children’s UK sizes 8 to 13 (figure 1). The test method defines the distance from the back of the heel piece to the pivot of the toe flexing section. Due to this measurement, a UK size 8 is the smallest child’s size of foot that can be fitted onto the current production SATRA STM 505 machine to achieve the correct distance from the back of the heel to the pivot point.

SATRA TM230:2017 – ‘Dynamic footwear water resistance test’ is the preferred laboratory assessment for whole shoe water resistance testing at SATRA. Depending on the intended use of the footwear, the water level in this test can be adjusted relative to the required performance. The water line for everyday footwear is normally up to the feather line at the joint position. The number of flexes used in a test can be varied, which allows the test to assess a wide range of footwear products – from everyday footwear to outdoor and military products, where performance abilities are more demanding. With more performance footwear, the product is immersed over the vamp and the number of flexes is increased (typically 300,000), as this a more realistic test for wet weather boots.

An increase in the number of flexes (and therefore the duration of the test), also allows assessment of the effects of wicking.  When wicking occurs along the seams, in the laces or in the upper material, water ingress can occur above the water immersion point. Such ingress in footwear can result in many performance failures, including wetting of the wearer’s foot, penetration into insulation layers (which can reduce the thermal efficiency), rotting of internal components, weakening of material strength, bacterial growth (and associated bad odours) and increased weight due to water retention.

A similar test to SATRA TM230 has also been developed to subject footwear to more extreme conditions. SATRA TM446:2015 – ‘Resistance to waterborne abrasive particulate’ evaluates the resistance of footwear by immersing and flexing the product in water that contains abrasive particles. This test has been developed to replicate the effect abrasive substances (such as sand, silt and mud) have on footwear that is submerged in water – for example, products for kayaking, off-road running and military applications.

The test uses abrasive aluminium oxide grit, which is commonly utilised in industry and metal finishing as an aggressive abrasive powder and is also used in some SATRA test methods for assessing a material’s resistance to abrasion. Aluminium oxide grit is an extremely hard, fine powder that can penetrate into small gaps in materials or products. This grit is diffused around the tank by a ‘baffle bed’, which produces a vigorous bubbling action through a supply of compressed air. This prevents the grit from settling at the bottom and so imitates running water as found in streams or waves. During the test, the aluminium oxide grit penetrates the test sample and becomes trapped between the component layers that rub together and so experience friction. The grit can also lodge between the lining of the boot and the foot form. This testing provides a realistic assessment of the footwear’s water resistance properties, and quickly highlights any potential problems in a product’s design or manufacture. This method of assessment is more subjective than mechanical tests, and it avoids the need to conduct field trials, which can be lengthy, harder to coordinate due to the need of suitable subjects which is an especially hard task with children. A number of issues must be considered when carrying out human trials – even more so when children are involved. This has contributed to SATRA taking the decision to re-engineer child’s size foot forms for use in the SATRA TM376:2009 ‘Advanced moisture management test’ (figure 2).

After the SATRA TM446 test is completed, the footwear is measured for mass gain, is the subject of a visual assessment (with photographs), and is then washed in running water to remove any grit that has accumulated inside the product. The footwear can be dissected to assess where the grit has settled (commonly between the upper and lining), but using an advanced X-ray machine at SATRA, it can be assessed as a whole (figure 3).

The new X-ray test can be added to both the SATRA TM446 test (to determine where the aluminium oxide grit has settled in the footwear) and the SATRA TM230 test, to determine if the footwear has remained watertight.

Publishing Data

This article was originally published on page 40 of the December 2017 issue of SATRA Bulletin.

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