Fatigue crack initiation and propagation in sandwich structures

Abstract: The focus throughout this thesis is on the fatigue characteristics of core materials used insandwich structures. Three sandwich configurations are investigated, two with cellular foamsand one with honeycomb core material These corresponds to typical materials and dimensionsused in the marine and aeronautical industry.A modified four-point bending rig, which enables reversed loading, is successfully used forconstant amplitude fatigue tests of all material configurations. The core materials are tested asused in composite sandwich beams and through the design of the specimens the desiredfailure is in shear of the core. Analyses and inspections during and after the tests supports thetheory that the fracture initiation and fatigue failure occurs in a large zone of the core withwell distributed micro cracks rather than a single propagating crack. The fatigue test resultsare plotted in stress life diagrams including a Weibull type function which provides a goodaccuracy curve fit to the results. The fatigue life of the core materials is found to be reducedwith a increased load ratio, R.The influence on the strength and fatigue performance on sandwich beams with two types ofcore damages, an interfacial disbond and a flawed butt-joint, are experimentally investigated.The fatigue failure initiates at the stress intensity locations which are present due to the predamage.The specimens with flawed butt-joints display a fatigue crack propagation in theinterface between the core and face of the sandwich while the crack propagates through thethickness of the beams where an initial interface flaw is present. A fatigue failure predictionmodel is suggested which utilises the fatigue performance of undamaged beams and thestrength reduction due to the damages. The approach is correlated with results from fatiguetesting and satisfactory correlation is found.A uni-axial fatigue tests method is developed which simplifies the rig and specimenscompared to the four point bend method. A comparison between the results from uni-axialtension/compression fatigue tests and shear fatigue tests shows good correlation, although theR-dependency differs in some cases.The fatigue crack propagation rates are investigated for two configurations: crackspropagating in pure foam core material and cracks propagating in the core material near andalong a sandwich face/core interface. The rate at which a crack propagates stable in the socalled Paris’ regime is extracted for both Mode I and Mode II loading. The agreement betweenthe Mode I crack propagation rate in the pure foam and in the core/face sandwich interfacelayer supports the theory that the crack actually propagates in the sandwich core beneath astiffened resin rich layer present in the face/core interface. The stress intensity thresholds andthe limits at which the crack growth becomes unstable are further established.Acoustic Emission (AE) is used to monitor crack initiation and growth in the core, duringboth static and fatigue loading. It is found that the approximate location of AE-hits can bedetermined which demonstrates that AE has a potential both as an non destructive testing tooland to study the failure process of non-visible sub-surface damages in sandwich structures.