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You are in: Measurement Advice > Introduction to testing composites > Fatigue Test Methods
As reinforced plastics are increasingly used in more critical applications or in applications with higher applied stresses and strains, there is a greater need for reliable data, both for initial material selection and for detailed engineering design. Recent production or prototype applications include helicopter blades, van and lorry leaf springs and flywheels. The fatigue strength is one of the important mechanical properties that must be evaluated for these applications.
The ability of composites to be formulated to yield a wide range of materials can create costly evaluation programmes. The magnitude of the test programme could be reduced by implementing reliable predictions based on the properties and macro-arrangement of the constituent materials, or by the development of a framework for the behaviour under different loading conditions. Whilst laminate analysis is well established for stiffness properties and is available in simple computer programs; the situation is not so well developed for strengths and for fatigue strength in particular. For example, great care would be required in predicting the performance of a new composite consisting of a combination of a new fibre with a new matrix.
NPL is involved in research programmes aimed at understanding the effect of test conditions and the mode of degradation, using continuous monitoring. It has also coordinated an international round-robin test exercise under the VAMAS scheme, aimed at establishing the scientific framework necessary for writing standards for the measurement of material properties.
A framework has been developed for both material and test parameters based on normalisation of the fatigue strength data with respect to the ultimate strength of identically conditioned specimens measured at the fatigue test loading rate (see below 1, 2, 3). For a glass-fibre fabric reinforced epoxy resin this normalisation yields a single, normalised S-N curve (stress versus number of cycles to failure). The principal parameters studied include: loading rate (equivalent to test frequency, 0.002 Hz to 50 Hz), test temperature (-150 °C to 150°C), fabric orientation, holed and notched specimens and different exposures to water. Other parameters studied were the type of waveform, the stress range and the stress mode. The benefits of an extended framework should be visible in the selection of minimal evaluation programmes and in the provision of guidelines on the interpretation of data for conditions other than those under which they were obtained (e.g. in failure analysis).
In the majority of cases fibre reinforced plastics degrade throughout the stressed volume until sufficient damage has accumulated to cause ultimate failure. Only in a few cases, such as propagation of a delamination crack between plies or a compressive fatigue crack initiated by a loading point stress concentration, is a macro-crack propagation stage present. Several techniques can be used to assess the uniform micro-damage occurring such as radiography, thermography, microscopy, dynamic mechanical analysis measurements (DMA) and stress wave (or acoustic) emission. The last three techniques are in use at NPL and detailed correlations have been undertaken between the DMA data and direct observations of micro-damage in the non-aligned plies. Initial experiments using stepped static (see below 4) loads have now been extended to continuous fatigue loading (see below 5). A straight line relationship was obtained between the observed number of transverse resin cracks and a hysteresis term, based on the current specimen modulus and the increase in damping.
3. VAMAS (Versailles Agreement on Advanced Materials and Standards) international round-robin
Researchers in Europe, Japan and USA have cooperated on a round-robin fatigue test programme. Both tensile and flexural tests were carried out for a range of unidirectionally reinforced polymer matrix systems6. In spite of some scatter in the ultimate properties, the fatigue data showed reasonable consistency (see below 7). Further tests may be conducted with particular emphasis on specimen and machine alignment, and grip pressure for the tensile tests.
The VAMAS initiative is an international agreement covering pre-standards research. The overall objective is to help promote trade in high technology products through international collaboration in pre-standards research, in order to generate the technical basis from which agreed standards and specifications for advanced materials can be developed. The initiative is supported by Canada, France, Germany, Italy, Japan, UK, USA and the EEC.
The experience from previous NPL work and the VAMAS round-robin are contributing to a new ISO standard for fatigue testing, related to the equivalent short-term strength specimen.
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