Multiple-site Damage in Fiber Metal Laminates

(Title: Bookman old style 66pt)

IntroductionFibre Metal Laminates(FMLs) are a class of hybrid materials, which are composed of boned thin metallic layers and fibres embedded in an adhesive system. One variant of FMLs is Glare which is known for its excellent fatigue and damage tolerance behaviour. As a result, Glare was optimized for aircraft fuselage skins used on Airbus A380.In spite of the low crack growth rates in FMLs, a more dangerous damage scenario is Multiple-site Damage (MSD) due to the presence of many rivet joints. Multiple-site damage is generally characterized by the simultaneous presence of multiple fatigue cracks in one structure element and usually appears at collinear fastener hols.After the Aloha accident in April 1988, the significance of MSD has been widely recognized. Although MSD in metal alloy structure is extensively studied the phenomenon of MSD in FMLs is still not fully studied and there is no physically sound analytical model to analyse and predict crack growth in case of MSD in FMLs. The purpose of this PhD research is to develop such an analytical prediction model that could predict how cracks will propagate with presence of adjacent cracks.

Research HypothesisIn case of MSD in FMLs, the presence of additional cracks could be treated as ‘negative stiffeners’ similar to the stiffened panel approach. The linear elastic fracture mechanics, superposition and displacement compatibility approach will be implemented to develop this analytical model.

The stress intensity factor of the lead crack is given as:

Objectives The objective of this PhD research project is to develop a analytical methodology with the capability to predict the damage progression of multiple cracks in Fibre Metal Laminates. If there is still enough time left at last phase, the corresponding residual strength would also be addressed. Research PlanIn order to achieve the objective of my research, the big goal is divided into three small goals. 1) The one lead central crack

with two notches symmetrically located on both side of central crack in the aluminium layer of FLMs. In this case, the central crack will propagate under cyclic load, while there would be no crack at the notch edges

Literature ReviewCalvin Rans et al. developed an analytical method for stress intensity factors in cracked panels with stiffening element based on linear elastic fracture mechanics, superposition and displacement compatibility approach. The influence of stiffener on crack growth is classified by three conditions: 1) intact stiffeners ahead of crack tip; 2) Broken stiffeners behind crack tip; 3) Bridging stiffeners over the crack.

Decomposition of stiffened panel into three stiffener condition (Calvin Rans et al. 2012)

Fatigue crack growth in Fibre Metal Laminates with multiple holes was studied by P.-Y. CHANG et al.. Fatigue test was carried out using Glare 3-3/2 specimens. From the experimental results, it is illustrated that crack growth rate increases in the last phase of crack propagation for the sake of link-up of two approaching cracks and finally they bypass each other leading to an eye-shaped region.

Crack bridging of intact fibres and delamination of the layers (Alderliesten,

R.C., 2000)

Fatigue crack propagation in Glare3-3/2 specimens ( P.-Y. CHANG et al. 2011)

Average crack length as a function of cycles for Glare3-3/2 laminates at the applied stress level of

120 Mpa (P.-Y. CHANG et al. 2011)

MSD fatigue delamination pattern in the fibre/adhesive layer (P.-Y. CHANG et al. 2011)

MSD fatigue delamination linkup in the fibre/adhesive layer (P.-Y. CHANG et al. 2011)

The delamination shapes in the fibre/adhesive layers appeared to avoid link-up while they were growing toward each other. Ultimately, the two propagating delaminations emerged to have approximately a rectangular shape.

PhD Candidate: Wandong WangDepartment: ASMSection: Structural Integrity & CompositesSupervisor: René AlderliestenPromoter: Rinze BenedictusStart date: 24-9-2012Funding: Chinese Scholarship Council

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Application of Glare in A380

Aloha accident due to MSD

2) The interaction of cracks could be approached. The effect of MSD cracks on SIF of lead crack can be determined based on the job of goal 1. The influence of lead crack on MSD cracks could be analysed by regarding lead crack as negative stiffener. Then the process is iterated to predict the crack growth3) The third goal is the analysis of the MSD in butt joints. The neutral line model will be utilized to analyse the secondary bending in the butt joints, the bending stress is then determined and the bending stress may result in slant cracks.