Fracture behavior at high loading rates

The fracture behavior of materials at high loading rates is of particular interest to the automotive sector. For the required high-speed tensile tests and puncture tests on polymer materials the industry uses Zwick high-speed testing machines.

Polyurethane plastics are highly versatile, with a range of applications including the automotive sector (fenders, bumpers, side skirts and roof modules) ski-boards and snowboards, building panels and decorative moldings. Different properties are obtained according to process control and the amount of additives.

As well as static characteristic values, the behavior of plastics at high loading rates is of particular relevance for automotive applications, as stress and strain are dependent on the applied rate. For these tests the polymer industry uses Zwick HTM 2512 servo-hydraulic high-speed testing machines. They have a nominal force of 25kN and operate in a speed range of up to 12m/s (quasi-static). As the machines are suitable for tensile and compression tests, they can be used for determination of tensile properties at high strain-rates as per ISO 18872 and for puncture tests to ISO 6603-2.

Because force measurement occupies only a few milliseconds, sensors with a high natural frequency are required. Zwick's piezo-electric solution relies on lightweight construction throughout to minimize the connected masses of the clamped setup. The puncture test device has a measuring unit located directly in the tip of the die, while a digital non-contact travel measurement system incorporated into the testing actuator is used to measure piston travel. The advantages are high shock-resistance, high temperature-stability and high dynamic response. In the tensile test the operator can additionally use a laser-optic system for direct strain measurement on the specimen, enabling stress-strain curves to be displayed and evaluated. The testXpert testing software enables convenient, trouble-free integration of this system also.