Characterisation of structural and damage features

Several aspects of characterisation are relevant to the study and use of composites. These include:

a) Establishing the initial structure (fibre type, lengths, orientations and voids), the presence of damage and the analysis of failure surfaces. The definition of structure is necessary if analytical models are to be used to predict the properties for the full range of potential materials. These models form part of the current "CoDA" PC software for generating material properties for a wide range of composite materials. The quality of these predictions depends on the accuracy of both the models and the input data. Techniques used include: optical and electron microscopy, ultrasonics, microwave digestion and resin burn-off.

b) Quality control to ensure the in-coming materials and the production materials meet the requirements as defined in qualification procedures etc. There is increased interest in physico-chemical test methods. Industry would prefer to use these test methods instead of mechanical test methods of doubtful validity, such as flexure and interlaminar shear. Relevant test methods are: differential scanning calorimetry (for cure-state etc.), dynamic mechanical analysis, high performance liquid chromatography, infra-red spectroscopy, rheological properties, dielectric analysis properties etc. NDE techniques for assessing the initial condition, such as radiography and ultrasonics, are also used for damage assessment. A programme on developing standardised procedures and reference samples for the use of ultrasonic C-scan is underway in response to the needs identified in a study of UK industry.

c) Damage assessment, either for continuously monitoring the damage development during loading (acoustic emission, thermography and dynamic mechanical analysis) or as a post-service assessment. NPL is engaged as part of a working group on a fractographic procedure 'manual', highlighting the use of many techniques. In particular, the work will lead to recommended procedures and preferential ordering of the methods used, in order that the maximum amount of information is obtained.

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