Adhesively bonded joints under cyclic loading spectra

Abstract

Current designs which involve the use of composite materials in primary aircraft structures are often conservative. This, in turn, significantly lowers the weight advantage that composites have over established metallic airframe materials. Strain restrictions are often applied because the failure mechanism(s) in (fibre) composite joints and stiffener runouts where the stress state is often complex, are not fully understood. Nevertheless, from the airworthiness perspective it is essential that both the static strength and the fatigue behaviour of the components subjected to complex multiaxial stress conditions are both understood and predicted. This topic is extremely complex, and numerous criteria ranging from the purely empirical to the theoretical have been proposed. In both cases, it is necessary to know the localised stress–strain history. One common design methodology is to keep the stresses so low that fatigue will not be an issue. However, this can lead to an overly conservative design. On the other hand, while a detailed (nonlinear) finite element analysis can be performed it is often both resource‐intensive and time‐consuming. The present paper shows that Glinka's hypothesis can be used in order to calculate the localised stresses and strains for a bonded joint subjected to cyclic loading. This is a new result and has not previously been noted. It has the potential to extend the Hart‐Smith design methodology to the adhesively bonded joints in order to encompass durability considerations. This formulation also raises the possibility of enabling the degree of conservatism inherent in traditional joint design to be relaxed provided that failure occurs in the adhesive. This paper also addresses the problem of variable adhesive thickness. We show that while variable adhesive thickness can change the stress and the energy fields, the peak in the strain energy density is relatively insensitive to the stress–strain relationship for the adhesive and that Glinka's hypothesis still appears to be true. This means that, for the present class of problems, even if there is variability in the thickness of the adhesive bond the energy field and, hence, the strength of the joint can be estimated from a purely linear elastic analysis of the joint, provided that failure occurs in the adhesive.

Fatigue performance of adhesively bonded single lap joints with non‐flat sinusoid interfaces

“…According to the literature, crack propagation dominates the fatigue life of bonded SLJs, where the crack is initiated after 15% of the entire fatigue life. [49][50][51][52][53] After appearing and extending rapidly for a short distance, this crack grew in a stable manner until the final failure of the joint. When around 0.4-0.6 of the fatigue crack propagated along the bond length, sudden fracture occurred in all the specimens.…”

A model for predicting the mixed‐mode fatigue crack growth in a bonded joint

“…This is a function of the model geometry and of the characteristics of the material: As a consequence, this determination requires expensive experimental tests. [8][9][10] Schindler et al 1 suggested an analytical model that enables to obtain closed-form equations to estimate da/dN and threshold values for adhesive joints and for any engineering structures where thin ductile layers are bonded to stiff structures of materials with different mechanical properties. Indeed, the da/dN data combined with closed-form equations allows to quantify crack growth variables as well as residual stresses of jointed structures in real time during fatigue testing.…”

“…The MMELS model is discretized into four-node (PLANE182) isoparametric elements, and numerical analyses are conducted under plane-strain conditions. A structured mesh of elements is created, 8 and at least two elements are present in the adhesive layer. Numerical crack propagation is performed at the adhesive-adherent interface from the side of the loaded beam.…”

“…Studies of the delamination onset are based on a constitutive model that relates the interlaminar fracture to a defect dimension. This is a function of the model geometry and of the characteristics of the material: As a consequence, this determination requires expensive experimental tests …”

Fatigue Behaviour of Fastening Joints of Sheet Materials and Finite Element Analysis

“…The increasing complex joint geometry and its three-dimensional nature combine to increase the difficulty of obtaining an overall system of governing equations for predicting the fatigue properties of the fastening joints. To overcome these problems, the finite element analysis (FEA) is frequently used [16][17][18][19][20][21][22][23][24].…”