National Physical Laboratory

Fatigue

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.

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.

Continuous monitoring of specimen degradation during fatigue

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 loads have now been extended to continuous fatigue loading. 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.

Contact

For more information, please contact Richard Shaw

Last Updated: 16 Jun 2016
Created: 15 Jun 2010

Registration

Please note that the information will not be divulged to third parties, or used without your permission

Login