As the use of composite materials continues to expand in the aerospace industry, test standards and corresponding customer requirements will continue to evolve. While qualification of these unique materials must continue with the stringency demanded by aerospace applications, the introduction of new solutions that streamline the process of testing is anticipated to support growing market demand.
A key factor contributing to the advancement of novel composite materials is the growing ability to tailor materials to achieve specific properties. Robust characterization of those properties through physical testing is necessary to ensure composite materials meet specifications.
To that end, Zwick Roell recently developed an electro-dynamic, linear-drive testing machine (LTM) for a customer that supplies glass fiber and composites. The customer was tasked with undergoing demanding quality-control fatigue tests required by its own customer. The dynamic test specification necessitated operating at a test frequency higher than 10-Hz maximum, which exceeded its existing testing facility capabilities.
The new quality-control procedure was introduced to assess material properties in glass-fiber composite aerospace specimens. Part of that process includes characterizing how stress bending affects the planes in the principle material axes. However, composite materials are not isotropic. Instead, they exhibit different material properties and failure modes in different planes.
Additionally, fiber orientation also makes material properties assessments for composites disproportionately more complex than for other materials, such as metals and plastics.
Assessing material properties of composites
Since fiber-reinforced composites consist of thin fibers that are either directionally or randomly oriented, the material requires different tests according to fiber orientation. Interaction of failure modes, such as in-plane failure combined with delamination through the thickness of the component, creates substantial complexity, so much so that typical uniaxial testing cannot capture these interactions. Test procedures must be designed to investigate them and assess their influence on product performance.
To accurately assess the glass-fiber composites, Zwick’s customer needed to perform high-cycle fatigue (HCF) testing of specimens in a 3-point bending mode, conducted at a frequency of 25 Hz with peak forces up to 1.4 kN (representing a peak-peak displacement of about 10 mm). The purpose of the test was to determine the number of cycles to specimen fracture. The fatigue life was anticipated to be up to three million cycles.
HCF refers to the effect of low-amplitude, high-frequency vibration within the elastic strain region of a material specimen. This is measured over the course of a number of load cycles (typically over 106). While the applied stress is within the material’s elastic region, plastic deformation can still take place on a microscopic level as the part ages. This can eventually lead to failure of the component.
A component or material’s fatigue characteristics can be quantified by generating the graph of stress vs. cycles at a given load, known as the Wöhler curve, where fatigue strength is determined from the maximum stress the component can withstand for a specified number of cycles. The endurance limit of the component can then be defined as the stress level below which failure does not occur, meaning the component has theoretically infinite life.
Fatigue failures can occur quickly if the endurance limit is exceeded. This is the result of cumulative stress cycles, which can be applied by thermal, mechanical, or vibratory effects. Performance must be guaranteed by demonstrating adequate fatigue strengths through cyclical testing that simulates installed conditions. The anisotropic nature of composites requires that materials testing initiatives take fiber orientation into account and apply different tests accordingly.
Finding a reliable testing solution
Best practices for reliable testing of composites are similar to those used for many other materials including the need for trained testing personnel, thorough procedures, and well-designed test equipment. To satisfy such demands, the oil-free LTM was installed with a 5-kN load cell with mass compensation, testControl ll electronics, testXpert R Sequencer testing software, and a 3-point bending fixture.
The LTM also utilized an open-cylindrical design actuator that provided space for an internal encoder to be mounted directly on the force axis, near the test specimen. This enabled high positioning repeatability and precise actuator piston travel measurement within ±2 µm . The LMT requires only an electrical supply for operation and the motor extracts sufficient amperage to perform a test, which lowers the energy and monetary costs of high-frequency testing.
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.
Dynamic forces due to aircraft maneuver loads create a constantly changing stress environment. The fatigue strength of composite materials in cyclic loading environments is a critical factor in the design process and in quality control of the manufactured part. Laboratory testing that simulates operational conditions is crucial to obtaining a thorough understanding of performance.
This article was written for Aerospace Engineering by Alexander Rank, Product Manager, LTM Systems, Zwick Roell.