Analytical and numerical modelling of plasticity-induced fatigue crack closure near cold-expanded holes in aircraft structures

 

Paulo F. P. de Matos, David Nowell

 

Department of Engineering Science, University of Oxford, Parks Road, Oxford, OX1 3PJ, United Kingdom

 

ABSTRACT: Over the last few years considerable effort has been put into the improvement of the fatigue life of aircraft structures. Aircraft experience complex loading conditions during flight and safe operation requires understanding of the underlying mechanics of fatigue crack growth. One of the outcomes of the research carried out over the last thirty years is that fatigue cracks in metals are partially closed over part of a load cycle. This phenomenon is thought by many researchers to be the key to understanding the effect of non-uniform loading. Understanding crack closure is particularly challenging when initial residual stress fields (e.g. due to manufacturing or mechanical treatment) need to be taken into account. For example, rivet holes are a critical area for fatigue and they are usually cold expanded to create a beneficial residual stress field and to improve the fatigue performance.

This paper describes the development of a simple analytical model for plasticity-induced fatigue crack closure taking into account the residual stresses due to cold expansion of rivet holes. The model is compared to a more sophisticated finite element analysis of plasticity-induced crack closure. The results show that the residual stress field has a strong influence on the closure behaviour and therefore on fatigue crack propagation. The potential for the application of this model to real components is assessed by modelling some experiments taken from the literature. The model results agree with the experimental findings for the location of fatigue crack initiation. Residual stresses from FE analyses of cold expansion were used as an input to the analytical closure model, which successfully predicted crack propagation from cold-expanded holes. The results obtained show that this approach has potential for use as a life prediction technique in design.

Keywords: Cold-expanded holes, Residual stresses; Fatigue crack propagation, crack closure.

 

RESUMO: Ao longo dos últimos anos tem sido feito um esforço considerável para melhorar a resistência estrutural dos aviões a fenómenos de fadiga. Durante um vôo os aviões sofrem condições de carga complexas, nestas condições para que o seu funcionamento seja seguro é necessário que se percebam os mecanismos fundamentais que governam a propagação de fendas de fadiga. Um dos resultados da investigação efectuada ao longo dos últimos trinta anos indica que em metais as fendas estão parcialmente fechadas durante parte de um ciclo de carga. Muitos investigadores pensam que este fenómeno é a chave para se perceber o efeito de carregamentos não uniformes. O fenómeno de crack closure é particularmente interessante quando se tem em consideração a existência de tensões residuais (ex. devidas a processos de produção ou tratamentos mecânico). Como exemplo, os furos de rebites são zonas críticas propícias a fenómenos de fadiga e são usualmente expandidos para criar um campo de tensões residuais para melhorar a sua resistência à fadiga.

Este artigo descreve o desenvolvimento de um modelo analítico simples para quantificar o fenómeno de crack closure induzido por plasticidade tendo em consideração tensões residuais devidas à expansão de furos de rebites. O modelo é comparado com análises de elementos finitos mais sofisticadas. Os resultados mostram que o campo de tensões residuais tem uma influência sigificativa no fenémeno de crack closure e consequentemente na propagação de fendas de fadiga. A potencial aplicabilidade deste modelo em componentes reais é avaliado através da modelação de trabalhos experimentais publicados na literatura. Os resultados obtidos com o modelo analítico estão em concordância com as observações experimentais no que diz respeito às zonas de inciação das fendas. Os campos de tensões residuais obtidos através de análises por elementos finitos foram usados como dados de entrada no modelo analítico de crack closure, que previu com sucesso a propagação de fendas de fadiga a partir de furos expandidos. Os resultados mostram que esta técnica tem potencial para ser usado em gabinetes de projecto.

Palavras chave: Furos expandidos a frio, Tensões residuais, Propagação de fendas de fadiga, crack closure.

 

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