Theoretical analysis of fatigue failure in mechanically fastened Fibre Metal Laminate joints containing multiple cracks

Highlights

• Both pin bearing and MSD crack interaction accelerate the crack growth rate in FML joints.

• Load redistribution among fastener rows accelerate the crack growth in the cracked fastener row.

• The proposed model is particularly useful for symmetrically and mechanically fastened FML joints.

Abstract

Mechanically fastened joints are susceptible to the presence of multiple-site damage (MSD) cracks in the critical fastener row. Different from the MSD growth in joints consisting of metallic substrates, the two coupled metal crack growth and interfacial delamination propagation failure mechanisms in Fibre Metal Laminates (FMLs) make the prediction of fatigue behaviour in FML joints with MSD scenario burdensome and impractical when considering all factors influencing the fatigue performance. This paper presents a theoretical study on the MSD crack growth behaviour in mechanically fastened FML joints with a focus of modelling the effects of bearing and bypass loads. The proposed model in this paper is built upon analytical models dealing with MSD growth in flat FML panels and single crack growth in FML panels subjected to a combined tension-pin loading case. This model would be particularly useful for symmetric FML joints where no secondary bending effects present. A deliberately designed symmetric FML joint was tested to validate the proposed model. The model captures the rapid crack growth in the vicinity of fastener holes due to bearing stresses and crack acceleration due to the interaction of cracks. It is identified that the load redistribution between intact fastener rows and the cracked fastener row accelerates crack growth with crack length. The effects of secondary bending stresses in FML joints on the crack growth behaviour is extensively discussed. The performance of the proposed model for single lap FML joints is also examined using test data from open literature. It is found that the proposed model provides a conservative prediction for the tested single shear lap FML joint from open literature.

Introduction

Fibre Metal Laminate (FML) is a material concept comprising alternate metal sheets and composite layers. This material concept is evolved out of bonding thin metal sheets together to enhance the damage tolerance properties. The deliberately interleaved fatigue resistant fibre layers between the metal sheets in FMLs act as a second load path when fatigue cracks propagate in the metal layers, reducing the effective crack-driving force at the crack tip and prolonging fatigue life. The resultant slower and stabler crack growth behaviour in FML, in comparison with that in metals, is very desirable in the context of damage tolerance philosophy used in aerospace sector.

This fibre bridging mechanism is easy to understand but difficult to capture effectively in analysing the fatigue crack growth behaviour in FMLs. Great success of adequately capturing the bridging mechanism was only achieved until the composite nature of FML was embraced and the interplay between the fibre and metal layers was analytically described, resulting in an accurate fatigue crack growth prediction model for FMLs by Alderliesten [1,2]. The crack growth in the metal layers and delamination propagation at the metal/composite interface can be simultaneously predicted. Continued effort in extending the capabilities of predicting crack growth behaviour has also been achieved by building upon Alderliesten's analytical approach. These extensions have been made to account for residual strength [3], variable amplitude loading [[4], [5], [6]], generalized FML configurations [7], more recently MSD scenario [[8], [9], [10], [11]] and pin loading [12,13].

It is well understood that the mechanically fastened joints are potentially vulnerable structures as a result of secondary bending, stress concentration and pin bearing effects at fastener holes. The structural behaviour of mechanically fastened FML joints therefore has been the subject of extensive research, including the static behaviour and fatigue behaviour. J.J.M. De Rijk [14] has extended the neutral line model developed by Schijve [15] to calculate the load transfer and secondary bending stresses in FML joints. The bearing strength of FMLs [[16], [17], [18]] and the progressive damage behaviour of pin loaded FMLs [19,20] have been studied. The present authors have also studied the fatigue crack growth behaviour in FMLs subjected to a more representative case of a combined tension-pin loading for mechanically fastened joints [12,13].

In particular, mechanically fastened joints are susceptible to the simultaneous presence of multiple cracks at several fastener holes in the critical row of fasteners [[21], [22], [23], [24]]. In light of the introduction of Limit of Validity (LOV) concept to the airworthiness regulations that puts limitations on damage tolerance philosophy [8,25,26], it is crucial to examine the crack growth behaviour in mechanically fastened FML joints with MSD scenario. In this context, the present authors have developed and validated analytical models for predicting the MSD crack growth in FML plates under far-field tension [[8], [9], [10], [11]] and single crack growth in FMLs subjected to a combined tension-pin loading [12,13], with the intension of incorporating them into an analysis frame that can evaluate the MSD crack growth behaviour in FML joints. It has been identified from the previous research that both pin bearing loading and MSD crack scenario significantly accelerate the fatigue crack growth in FMLs, stimulating the motivation to investigate into the MSD crack growth behaviour in FML joints.

The crack in a mechanically fastened joint is subjected to a complex stress field. This stress field comprises stress components such as bearing, frictional forces, bypass, secondary bending stresses and residual stresses at fastener holes. Among them, pin bearing, bypass and secondary bending stresses dominate the MSD crack growth behaviour in FML joints. The aim of this paper is to propose an computationally efficient analysis frame for predicting MSD growth behaviour in mechanically fastened FML joints by systematically integrating the well established analytical models with a focus of analysing crack interaction under tensile bypass loading and pin bearing effects. This model is particularly suitable for analysing MSD growth behaviour in symmetric FML joints without secondary bending stresses. Experimental test has been carried out to validate the performance of the proposed model using the test data.

Secondary bending stresses present in most commonly used single lap joints; however, incorporating them into modelling crack growth in FMLs is highly complicated and significantly sacrifices computational efficiency. It was decided not to model secondary bending effects for the purpose of this paper. Nevertheless, the effects of secondary bending on crack growth in FML will be extensively discussed and the performance of the proposed model for a single lap FML joint will be examined using test data from open literature.