Crack closure in Glare

Crack closure in Glare

Plokker, H.M.

Benedictus, R. (mentor)
Schijve, J. (mentor)
Alderliesten, R.C. (mentor)
Homan, J.J. (mentor)

Aerospace Engineering

Mechanics, Aerospace Structures & Materials

2005-10-30

The fatigue life of an aircraft is dependent on the loads of the aircraft service spectrum. Due to the variable nature of the load spectrum, fatigue crack growth is accelerated and decelerated by interaction effects. Interaction effects, like crack closure, make an accurate prediction of the fatigue life challenging. The material GLARE, a laminate built up of aluminium and glass fibre layers, is used in the fuselage of the Airbus A380. Fatigue crack propagation is the dominant part of the fatigue life of GLARE structures. For GLARE, crack closure is thought to be a significant interaction effect and is not yet investigated for GLARE. In this thesis crack closure in GLARE is investigated. The study limits itself to crack growth in simple coupon specimens with through cracks, loaded in the longitudinal direction in a lab air environment at room temperature. The investigation started with a literature study on fatigue, interaction effects, crack closure, and load variations in both metals and GLARE. A test plan is proposed and executed from which the test results are analysed and conclusions drawn. Crack closure is the result of crack tip plastic deformation. As the crack propagates, plastic deformation comes in the wake and closes the crack. This reduces the effective stress range and consequently the crack growth rate. Load variations in CA cycles lead to a crack growth acceleration or decelerations that can be partly explained by crack closure. For two stress level sequences, the crack growth rate acceleration or deceleration can be explained by crack closure and compatibility of the crack front geometry. Crack growth in GLARE is a self-balancing mechanism with delamination growth between the aluminium- and fibre layers. The fibres “bridge” the crack by leading the stresses through it. This results in an approximately constant stress intensity factor and crack growth rate for long ranges of crack lengths. Crack closure in GLARE is investigated with two tests series: • CA fatigue tests with determination of the crack opening stress for various stress ratios. These tests are duplicated for different aluminium sheet thickness, lay-up and maximum stress levels. • CA fatigue tests with simple load variations and two stress level sequences. The course of the crack opening stress and crack growth rate are monitored for (multiple) overloads, underloads, combinations of these and for low-high and low-high stress level sequences. The load variation tests are duplicated for different magnitudes of the maximum/minimum stress. Also the delaminations are investigated at different stages in the load history. The tests results of the CA fatigue tests proved that crack closure in standard GLARE can be described with the crack closure correction proposed by Schijve for aluminium 2024-T3. Crack growth in GLARE loaded under CA, can be calculated with the relation of Schijve with the stress ratio in the aluminium layers (excl. curing stresses) multiplied with a stress ratio correction and new constants C and n for the Paris region. In the load variations tests with overloads, the behaviour of the crack growth rate after a load variation is comparable to aluminium. An underload does not cause crack growth acceleration, different to what is seen aluminium. The crack growth retardation after an under-/overload combination was comparable to the crack growth retardation after an over-/underload combination and a single overload. Low-high and high-low stress level sequences in GLARE gave similar effects on the crack growth rate as what can be seen aluminium. This is the result of a temporal change of the effective stress range.

fatigue
crack growth
interaction effects
crack closure
Fiber Metal Laminates
GLARE

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Student theses

master thesis

(c) 2005 Plokker, H.M.