Fatigue crack growth in fibre metal laminates under selective variable-amplitude loading (original) (raw)
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This paper presents a study on the influence that load variations have on delamination shapes in Fibre Metal Laminates. Previously fatigue tested centre-crack tension specimens have been chemically etched to obtain the final delamination shapes. In addition, fatigue crack growth tests on similar specimens have been performed to investigate the formation of delamination shapes. For this purpose, digital image correlation has been used as strain measurement technique to record the delamination shapes in situ testing. An explanation is put-forward in order to understand the effects of variable amplitude loading on the formation of delamination shapes. A transition in delamination shapes was observed, but evaluating this observation using an analytical fatigue crack growth prediction model including the observed change in delamination shapes revealed no significant effect on subsequent crack growth.
Crack closure in fibre metal laminates
Fatigue & Fracture of Engineering Materials & Structures, 2007
GLARE is a fibre metal laminate (FML) built up of alternating layers of S2-glass/FM94 prepreg and aluminium 2024-T3. The excellent fatigue behaviour of GLARE can be described with a recently published analytical prediction model.This model is based on linear elastic fracture mechanics and the assumption that a similar stress state in the aluminium layers of GLARE and monolithic aluminium result in the same crack growth behaviour. It therefore describes the crack growth with an effective stress intensity factor (SIF) range at the crack tip in the aluminium layers, including the effect of internal residual stress as result of curing and the stiffness differences between the individual layers. In that model, an empirical relation is used to calculate the effective SIF range, which had been determined without sufficiently investigating the effect of crack closure.This paper presents the research performed on crack closure in GLARE. It is assumed that crack closure in FMLs is determined by the actual stress cycles in the metal layers and that it can be described with the available relations for monolithic aluminium published in the literature.Fatigue crack growth experiments have been performed on GLARE specimens in which crack growth rates and crack opening stresses have been recorded. The prediction model incorporating the crack closure relation for aluminium 2024-T3 obtained from the literature has been validated with the test results.It is concluded that crack growth in GLARE can be correlated with the effective SIF range at the crack tip in the aluminium layers, if it is determined with the crack closure relation for aluminium 2024-T3 based on actual stresses in the aluminium layers.
Composite Structures, 2017
Fibre Metal Laminates (FMLs) are a hybrid metal-composite laminate technology known for their superior resistance to fatigue crack growth compared to monolithic metals. This crack growth behaviour has been the subject of many studies, resulting in numerous empirical and analytical models to describe the complex damage growth phenomenon in the material. This study builds upon the analytical Alderliesten crack growth prediction methodology for FMLs, extending it from a tension loaded plate to a case of a combined tension-pin loaded plate. This new loading case is a more representative case to utilise for predicting fatigue crack growth behaviour in mechanically fastened joints. Development of the model extension and validation through experimental testing are detailed within this paper.
Finite Element Modeling of Fatigue in Fiber–Metal Laminates
AIAA Journal, 2015
Innovative hybrid materials developed at Delft University of Technology (TU Delft), e.g. ARALL and GLARE, dramatically reduce life-cycle costs and offer great opportunity for service life extension of legacy aircraft. Replacement or repair of damaged aircraft components requires high strength composite materials with high tailorability, fatigue and impact damage resistance, all of which are offered by the advanced hybrid materials. In addition, a reliable fatigue life evaluation methodology for hybrid structures of arbitrary layup, configuration, constituent materials and geometry is necessary. An efficient computational framework is presented for simulation of fatigue fracture in fiber metal laminates (FMLs) based on the homogenized laminate modeled with large shell elements and cohesive zone used to simulate crack propagation. The cohesive traction separation relationship is calibrated against the analytical solution for the strain energy release rate, which explicitly accounts for the effect of fiber bridging. Appropriate calibration of the cohesive energy results in approximately constant crack growth rate, characteristic for FMLs, as well as accurate distribution of bridging stresses for the considered crack and delamination configurations. The proposed methodology is illustrated by simulating an experimental test conducted on a large GLARE panel subjected to constant amplitude fatigue loading.
The effect of external stiffeners on the fatigue crack growth in fibre metal laminates
This paper presents the investigation of the fatigue crack propagation and delamination behaviour of Fibre Metal Laminates stiffened by externally bonded titanium straps. A previously developed prediction model for crack growth in FML is based on superposition of stress intensity factors accounting for crack opening by far field stresses and crack closing by intact fibre layers. This model has been extended to account for the presence of intact or broken externally bonded titanium straps. The approach has been validated with a selection of fatigue crack growth experiments. It is concluded that the developed prediction model predicts the crack propagation and delamination growth in line with observed data, but in addition provides information about the change in fibre bridging stresses in the wake of the crack induced by the presence of external stiffeners.
Analytical prediction model for non-symmetric fatigue crack growth in Fibre Metal Laminates
International Journal of Fatigue, 2017
This paper proposes an analytical model for predicting the non-symmetric crack growth and accompanying delamination growth in FMLs. The general approach of this model applies Linear Elastic Fracture Mechanics, the pricinple of superposition, and displacement compatibility based on the understanding of deformation behaviour in eccentrically cracked metal panels. The non-symmetric crack growth behaviour of two crack tips and accompanying asymmetric load transfer from the eccentrically cracked metal layers to the intact bridging fibres are successfully predicted with the model. The predicted crack growth rates and delamination evolution are compared to test data, good correlation is observed.
Procedia Structural Integrity, 2016
During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
International Journal of Fatigue, 2018
In general, a finite width correction to stress intensity factor (SIF) is required in the fatigue crack growth. The finite width correction factor can be explained physically from the energy point of view. It is assumed that the finite width correction factor primarily constitutes an energy correction factor, i.e. it corrects the applied load for the work applied. To evaluate the finite width correction for FMLs, constant amplitude load fatigue crack growth tests were performed on monolithic aluminium 2024-T3 and the Fibre Metal Laminate GLARE containing 2024-T3 aluminium layers. The loads and displacements were recorded to quantify the total amount of work applied throughout each fatigue test. The crack length and delamination size were monitored by using digital image correlation technique to evaluate the dissipative energy. It appears that the Feddersen's and all other standard finite width correction significantly overestimates the effect for FMLs. The finite width correction to SIF for FMLs is small but cannot be neglected, and it is also greatly related to the Glare grades, stress ratio and stress level.