The latest composite materials constantly create new challenges for testing specialists, such as ZwickRoell. Aerospace Manufacturing hears how the company deals with these complex testing demands.
The global composites market size is expected to reach $145 billion by 2028 and with it comes an increasing demand for quality product testing and analysis. The advantages of these materials have led to the rapid adoption of composites by the aerospace industry. However, before these materials are introduced, they need to be thoroughly tested and their performance validated.
Full characterisation of the properties of composite materials for use in demanding applications requires a wide range of mechanical tests to be conducted. Determination of mechanical properties requires tensile, compressive and shear tests. In qualification and materials development, other tests are used to determine more complex properties, such as open hole tension/compression, inter-laminar fracture toughness, compression after impact and fatigue. Tests need to be conducted over a range of temperatures on samples which may have been conditioned in a variety of environmental conditions, such as high humidity or immersion in fluids.
Testing composites for aerospace applications presents one of the most demanding areas of testing. Laboratories undertaking composites testing for aerospace applications face significant challenges which includes, ensuring that tests are conducted in compliance with the wide range of standards, being able to demonstrate accurate alignment of grips and fixtures and the ability to change test fixtures efficiently to cover a wide range of tests, and ensuring the correct test environment.
Composite test procedures have been standardised by various organisations. In addition to the internationally recognised standards, there are ‘in-house’ standards in common use including those from Airbus and Boeing. In many cases, the test methods described by different standards are fundamentally the same, but there are some significant differences in the specimen and fixture dimensions. In addition, auditing bodies such as Nadcap further define performance criteria, for example, alignment for the testing equipment.
In-plane tensile testing of plain composite laminates is probably the most common test, but tensile tests are also performed on resin impregnated bundles of fibres, through thickness specimens and sections of sandwich core materials.
The tensile strength and modulus of materials are crucial characteristics that engineers use to aid or improve component design. Tensile test specimens are parallel sided with bonded tabs to prevent the grip jaws from damaging the material and causing premature failure. Gripping arrangements include manual and hydraulic wedge grips and for demanding aerospace testing, hydraulic wedge grip solutions are generally preferred because of their controllability and repeatability. However, well-designed mechanical wedge grips can also provide good levels of alignment.
Alignment refinement
Accurate alignment of the grips and specimen is extremely important when testing composite materials since the anisotropic properties, such as the modulus and strength of the material differ depending on the direction of the applied stress and are often brittle in nature. Adjustable alignment fixtures ensure that testing systems meet the alignment criteria required for reliable composites testing and as required by specific audit programmes recognised by the aerospace industry.
Alignment fixtures need to allow adjustment of both concentricity and angularity while the machine load string is under load. The widely accepted method of checking for alignment under load is to use a strain gauged alignment specimen. The alignment specimen should have dimensions that are as close to the specimens being tested as possible and the alignment specimen will be fitted with groups of strain gauges. Purpose designed software is available to provide a display of both the bending, and concentricity and angularity errors.
Composite compression test methods need to provide a means of applying a compressive force into the material while importantly, preventing it from buckling.
There are several methods of introducing a compressive load into a test specimen which includes, end loading, where the load is introduced onto the end of the test specimen; shear loading, where the load is introduced into the wide faces of the test specimen and combined loading, which comprises a combination of shear and end loading.
All compression fixtures are required to have good axial alignment and a high lateral stiffness to provide and maintain accurate alignment under the lateral loads that can be generated by a compression test. Bending will have a significant effect on test results and most of the compression test methods include a value for the maximum allowed test specimen bending.
Damage tolerance is a major concern with composite laminates. Compression after impact testing provides a measure of damage tolerance and the test is usually performed on a rectangular laminate panel. The test consists of two separate parts; initially the panel is clamped around the periphery and then subjected to a controlled impact in the centre of the panel using a drop weight test machine. The impacted panel is then located in a special jig and subjected to an edgewise compressive load until it fails, and the failure load gives an indication of the residual strength of the panel after the impact damage.
Crack under the strain
A serious consideration with composite materials, particularly in the aerospace sector, is the ability to withstand crack propagation. Fracture toughness testing is becoming an area of increasing interest, as composite materials are notoriously weak in this area. Recent developments such as 3D weaving and ‘z pinning’ aim to address this weakness, however a testing solution must be available to assess their benefits.
ZwickRoell understand this importance and as such, has developed the most accurate, reliable, and repeatable system currently available. The system includes a camera which tracks the crack growth and provides the user with a side-by-side video of the test data and crack growth, which can be replayed in slow motion to ensure that the crack growth measured by the user is as accurate as possible.
For dynamic cyclic testing of composites, testing machines with different drive systems are used. Zwick’s LTM testing machines with linear motor technology are available with forces up to 10kN. The core of the system is the patented electrodynamic drive specifically developed for testing technology. It allows for test frequencies up to 100Hz and can precisely and reproducibly perform a wide variety of sequences such as sinusoidal, triangular, rectangular, and trapezoidal functions, as well as monotonic tests. A benefit of this system drive is precise test performance at low operating and service costs. Servo-hydraulic testing machines are used for dynamic cyclic tests as well as static tests. Typically, the standard frequency range is up to 100Hz depending on the test amplitude, and available forces up to 2,500kN.
Qualification of new composite materials demands a comprehensive testing campaign. ZwickRoell is a specialist for automation in testing technology and provides proven systems with exceptional performance to accommodate high-volume test specimen throughput.
The mechanical testing of composite materials is a complex topic, involving a range of test configurations, a wide selection of recognised testing standards and frequently the need to test in a variety of different environments. The availability of well-aligned test machines and gripping arrangements, advanced technology extensometers, interchangeable test fixtures and intuitive testing software, facilitates the mechanical testing requirements demanded by R&D, and routine quality assurance.
