As we all know, the life of safety shoes can vary greatly depending on the materials and workmanship. Safety shoes may wear, crack or break throughout the life of the footwear due to strain caused by flexing of the foot, or sharp edges cutting notches and puncturing the sole. Extreme temperatures (especially below zero degrees) and contamination (such as motor oil) can also accelerate the cracking of the sole. Therefore, it is important to test the resistance of the sole to flex cracking, and with a variety of test methods to choose from, it can be difficult to choose the right method. Here are the three main flex test methods for rubber and plastic soles, their applications and the equipment SATRA provides to carry out these tests.
1. Testing Safety Shoes Using the Bennewart Method
For smooth sole designs and those with little or no tread, a Ross flex test machine (SATRA TM60) can be used. Three specimens measuring 150 mm x 25 mm are generally removed from the shoe, with the longer edge parallel to the heel seat. A notch is made in the specimen using a chisel with a 2 mm blade, and the specimen is then placed in the flex machine so that the notch is directly over the flex mandrel. The notch is measured before and after the test, and the increase in the notch can be calculated. The test is generally run at a temperature of -5°C for 150,000 cycles. This helps to develop a measurable increase in the notch within a reasonable time frame. The exception is testing thermoplastic rubber at +20°C, as this material performs better at lower temperatures. The SATRA Ross flex tester (STM 141) can accommodate up to 12 specimens at the same time. The machine flexes the specimen at a standard rate of 60 flexes per minute. However, machines with 100 flexures per minute are available for carrying out the ASTM version of this test. In addition to the standard machines, lower and higher temperature equipment are available. This allows temperature ranges from ambient temperature to -20°C and from ambient temperature to +40°C.
2. SATRA Ross Flex Tester (STM 141) Bata Belt Test
If the sole contains a large cleat or a complex design integrating different materials, then the SATRA method for testing resistance to flex cracking is the Bata Belt Tester. This is a generally recognized test that can produce repeatable results related to actual wear. The front section of the complete sole (minus the heel but still containing any cavity filler or midsole material) is bonded to a canvas belt. This section is driven through a driven spindle of different diameters, so that when the belt is driven onto the spindle, the sole is cyclically and repeatedly flexed. The driven spindle is generally 90 mm in diameter, but can be changed to 60 mm or 120 mm to increase or decrease the degree of flexing. Unlike the Ross Flex Test, the sole does not have to be cut. The test is run for 50,000 cycles, with continuous visual assessment during the run. The length and depth of any cracks are recorded. This test is generally carried out at room temperature, but at SATRA, the sole can be tested at temperatures as low as -15°C. The Bata Belt Flexometer (STM 459) is equipped with three replaceable driven spindles of 60 mm, 90 mm and 120 mm as standard, with a rotation speed of 90 deflections per minute on the smaller spindle. A lower temperature version of the machine is also available to perform tests at temperatures as low as -20°C.
3. Bennewart test
Like the old standard, the new standard for safety shoes (EN ISO 20344:2004) requires the use of a Bennewart machine to test the sole. More testers prefer the Bata tape test method, which allows the entire front of the sole to be tested. The insole is an important part of the sample, and a chisel similar to Ross is used to cut an open cut on the nominal deflection line. The sole is clamped at both ends and the roller pushes the insole to deflect the sole 90 degrees. The increase in the cut is measured after 30,000 cycles of operation at room temperature. If necessary, this test can be performed below zero degrees at SATRA. This is a demanding test, suitable only for durable footwear with a strong sole. For casual, fashionable and daily wear footwear, this test is considered too severe, especially when the sole is thick. SATRA's Rennewart full sole deflection tester (STM 465) is designed to ensure force balance, so it requires less force to perform the test, making the operation result smoother. The clamps of the instrument are rigid and strictly follow the standard. A modified Bennewart tester, using spring-actuated clamps, is not used for this test. However, a cryogenic version of this instrument is available to perform the test at temperatures as low as -20°C. A cutting fixture (STM 465) equipped with a chisel is available to assist in the precise cutting of the sole.