Rubber Flex, Fatigue & Crack Growth Testing Equipment

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Rubber Flex, Fatigue & Crack Growth Testing Equipment
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A shoe sole or a molded rubber part can pass every static test and still crack after thousands of bends in service. Flex testing reproduces that bending in the laboratory, so a crack starts and grows under controlled stroke and speed and compounds can be ranked by how long they last. NextGen works to ASTM D430, ASTM D813 and ASTM D1052 for these methods.

The range covers De Mattia testers for flex cracking and cut growth, Ross flex testers that bend soles and sheet materials through a right angle over a rod, and a Ross tester with its own low-temperature chamber for cold flex resistance. Testers are installed and calibrated on site, your team is trained on specimen preparation and cycle counts, and support continues for the life of the instrument.

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What Flex and Crack Growth Tests Measure

Many rubber parts fail from repeated bending long before they fail in a single pull. A flex test repeats a defined bend under fixed speed, stroke and angle, then records the cycles until a crack starts or follows the growth of a crack that was started on purpose. A compound that holds up in a tensile test can still split after a few hundred thousand flexes, which is why soles, hose walls and boot uppers are tested this way.

Two questions are asked. Crack initiation asks how many cycles pass before the first crack appears. Crack growth asks how fast a crack spreads from a pierced cut. A compound can resist initiation and still fail by cut growth, so the two are measured separately.

Standards for Rubber Flex Testing

ASTM D430 covers dynamic fatigue of soft rubber and rubber-fabric composites, ASTM D813 measures crack growth in a pierced vulcanized rubber specimen under repeated flexing, and ASTM D1052 measures cut growth in a pierced specimen on a Ross flexing apparatus. All three are comparative methods. They rank compounds and constructions and do not predict service life on their own.

What Affects Flex Results

Specimen geometry and preparation, the piercing of the starter cut, the bending angle, the speed and the endpoint chosen, such as the first crack or a crack length, all affect the cycle count. Fatigue results also depend on whether the specimen is strained in bending or in extension. Compound chemistry and cure state change the answer too, and rubber stiffens in the cold, so flex resistance can also be tested inside a low-temperature chamber. The Demattia tester fixes speed, mode and degree of bend so that results from one run can be compared with the next.

Flex Testing Equipment from NextGen

The Demattia flex cracking tester comes in two models, one for 154 mm strips and one for wider 108 × 70 mm pieces, with an automatic failure stop and a counter to 999,999 cycles. The Ross flex tester bends specimens through 90° over a 10 mm rod at 100 cycles per minute, in 6-grip and 12-grip versions. The GenRoss-CH places a Ross flexing mechanism in a chamber from -20 °C to +150 °C for six test pieces. The Ross tester has digital control, a memory function and vacuum holding of the specimen, and it ships with a standard die mold and a hole puncher for cutting and piercing specimens.

For Laboratories

One partner to equip your complete rubber laboratory

Flex testing is one station of an elastomer laboratory. NextGen equips the rest of it too, from tensile frames to cure testing, hardness, abrasion and specimen preparation, and stays responsible for it after delivery.

  • Equipment, fixtures and software
  • Installation, training, calibration
  • Lifetime technical support

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FAQs

The Demattia tester is listed under ASTM D813, ASTM D430, ISO 132 and EN ISO 20344. The Ross flex tester is listed under ASTM D1052, ISO 5423 and SATRA TM 60, and the GenRoss-CH adds SATRA TM 92 and HG/T 2411. Contact our team to match a callout to a model.

ASTM D430 covers this case. Its Type I methods create separation in rubber-fabric combinations through controlled bending, and its Type II methods create cracking on the surface of rubber through repeated bending or extension. NextGen's standard page lists tires, belts and rubber-textile composite constructions among its common applications, so confirm which type and endpoint your specification calls for before choosing a tester.

The GenRoss-CH flexes six test pieces of 152 × 25 mm inside a chamber with a range of -20 °C to +150 °C and a temperature fluctuation of ±0.5 °C or less. Cooling from +25 °C to -40 °C takes about 50 minutes without a load, and the counter runs to 9,999,999 cycles. It is used for the cold resistance and folding resistance of shoes and shoe materials.

One end of the specimen is clamped and the pierced end sits between two rollers, so each cycle bends the pierced area 90° over a 10 mm rod at 1.7 ±0.08 Hz. After a set number of cycles the operator measures the length of the notch and rates the material by how fast it grew. The NG-ROSS-6 holds six test films and the NG-ROSS-12 holds twelve films or six outsoles.

The choice follows the specimen. On the Demattia tester, GenFlex-B1 takes a long strip of 154 × 25 × 6.3 mm, runs a 57 mm stroke and cycles at 300 ±10 cpm. GenFlex-B2 takes a wider 108 × 70 mm piece, moves it 44 mm and cycles faster, at 340 to 400 ±10 cpm. Both stop automatically when the specimen gives way.