3-point dynamic HCF fatigue testing of aerospace composites

The utilization of composite materials offers great benefits to aircraft design with savings in weight, reduction in fuel consumption, increases in payload, extended flight range, and enhanced toughness, durability and safety.  The improvement in fatigue performance with glass fiber compared to aluminum is also a major advantage.

A leading, global supplier of glass fibre and composite materials was tasked with performing more demanding quality control fatigue tests, as prescribed by their aerospace sector customer. The dynamic test specification necessitated operating at a higher test frequency than the 10Hz maximum which had previously been achievable, in order to facilitate an increase in test specimen throughput. As the increase in specified test frequency was beyond the capability of their existing testing facility, it resulted in the company having to consider an alternative testing solution which would encompass both low operational and maintenance costs and deliver faster, reliable test results.

The newly introduced quality control procedure, involved high-cycle fatigue testing of glass-fibre specimens in a 3-point bending mode, to be conducted at a frequency of 25Hz with peak forces up to 1.4kN, representing a peak-peak displacement of ca. 10mm. The purpose of the test wasto determine the number of cycles to specimen fracture with an anticipated fatigue life of up to 3 million cycles.

To satisfy this demand, a Zwick Roell (5kN capacity) electro-dynamic, linear-drive testing machine (LTM) was installed to conduct the tests, which incorporated a 5kN load cell with mass compensation, testControl ll software, testXpert R Sequencer and a 3-point bending fixture.

The oil-free LTM incorporates a patented, open-cylindrical design actuator providing space for an internal encoder to be mounted directly on the force axis and in close proximity to the test specimen. This arrangement ensures high positioning repeatability and extremely precise actuator piston travel measurement within +/-2µm.

Testing at high frequencies, in conjunction with large displacements, induces a relatively large acceleration of the seismic mass component which is experienced by the load cell as an additional force. By employing mass compensation, testXpert R is able to subtract the additional force or error experienced at both peaks, allowing for more accurate force measurement and control.

A major benefit of the LTM machine is that it only requires an electrical supply for operation and the motor only extracts sufficient amperage to perform the test in hand. As standard, an integrated air-cooling heat exchanger is supplied with the LTM, avoiding the need for anexternal cooling water supply. The LTM requires minimal long-term maintenance, as the system incorporates a wear-free piston and braking system.

The Zwick Roell LTM is a durable and reliable oil-freetesting system which provides a versatile solution for any laboratory environment.