McCartney, L N (2001) Model of composite degradation due to environmentally assisted fatigue damage. NPL Report. MATC(A)26

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Abstract

This report describes an extension, to account for the effects of fatigue loading, of a model that is able to predict the time-dependent axial residual strength of a unidirectional fibre reinforced composite arising from environmental exposure. The model assumes that environmental exposure leads to interfaces that are very weak so that a parallel two bar model can be developed where one bar represents the behaviour of the fibres which are regarded as a loose bundle, and where the second bar represents the matrix. The principal effect of the environment is assumed to be its effect on the time dependent strength of individual fibres arising from stress corrosion cracking of small defects within the fibres when the composite is subject to fatigue loading. The initial strength of the fibres is assumed to be governed by the Weibull distribution which, when used in conjunction with fracture mechanics, defines an initial statistical size distribution of fibre defects. Defect growth arising from environmentally assisted fatigue loading is assumed to be governed by a fracture mechanics based growth law where the defect growth rate is controlled by the stress intensity factor. The model has been extended to the case of cross-ply laminates.
The model developed for the prediction of the degradation of a unidirectional composite arising from stress corrosion cracking in the fibres under general time dependent loading is not suitable for application to high frequency fatigue loading. For the case of uniform amplitude fatigue loading the model can, however, be used to develop a very convenient method of predicting composite degradation. The methodology applies a factor F (= 1) to the time variable in order that degradation data for fixed applied loads can be used to predict degradation during uniform amplitude fatigue loading. The value of the factor F depends on the fatigue R-ratio, an exponent n appearing in the defect growth law, and on the nature of the fatigue cycle where sinusoidal and saw-tooth cycles (with dwell periods at maximum and/or minimum load) have been considered.
As the use of the factor F in conjunction with degradation data for the special case of fixed loading avoids having to consider fatigue loading, the methodology described once validated has potential for assessing the degradation of unidirectional composites without having to conduct difficult time-consuming and fatigue expensive tests. In addition, it is clear that static fatigue tests could be the basis of a method of accelerating dynamic fatigue tests. Model predictions indicate that there are good prospects for accelerating lifetime tests by carrying out tests at high loads (short lives) and then predicting long term behaviour using the linearity of the load vs log tf measured relationship. The results of model predictions indicate that this could be the basis of a conservative approach to design.

Item Type:	Report/Guide (NPL Report)
NPL Report No.:	MATC(A)26
Subjects:	Advanced Materials Advanced Materials > Composites
Last Modified:	02 Feb 2018 13:17
URI:	http://eprintspublications.npl.co.uk/id/eprint/2008

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