Enhanced Performance of Sandwich Structures by Improved Damage Tolerance

The PhD thesis investigates the enhancement of the damage tolerance of sandwich structures by the embedding of a new type of core inserts that act as face/core interface crack stopping elements. The thesis presents series of experimental investigations where the new crack stopping elements are embedded in both sandwich beam and panel specimens. The experimental observations form the basis for evaluating the efficiency of the proposed crack stopping inserts. For the experiments, Digital Image Correlation (DIC) was used to characterize the measure the local strain fields and overall deformation behaviour around the new crack stopper elements. In support for the experimental investigations, a Finite Element (FE) analysis based methodology, including fracture mechanics analysis and the so-called ‘cycle jump’ technique, was developed to predict the progression of damage in sandwich specimens with embedded crack stoppers.
The starting point for the research was is a new design for a crack stopper, referred to as a ‘peel stopper’, which is proposed for foam cored sandwich structures. Initially, the ability of the peel stopper to prolong the fatigue life of sandwich structures has been demonstrated through a series of three-point bending tests. During testing an initial crack front in the sandwich beams was arrested for a limited amount of cycles until a new crack initiated in the vicinity of the peel stopper. Subsequently, the study concentrated on investigating the main parameters that govern the performance of the proposed peel stopper, i.e. the crack deflection and crack arrest capability. The ability of the peel stopper to deflect a propagating face-sheet/core interface crack was investigated through a series of sandwich beam tests. Different configurations of the peel stopper were tested and the conditions for crack deflection for all configurations were identified by application of a fracture mechanics crack kinking criterion. From this research, the most promising peel stopper configurations were identified. Following this the crack arrest capacity of the peel stopper was investigated. Through the use of strain field measurements on the surface of sandwich beams with embedded peel stoppers using Digital Image Analysis (DIC), it was shown that the ability of the peel stopper to contain an arrested crack, or to prevent re-initiation of new cracks, is related to the inducement of strain concentrations in the foam core material on the back side of the peel stopper. By use of the developed numerical fracture mechanics based modelling tools, both fatigue crack growth and crack arrest in the specimens were simulated. It was shown that the strains responsible for crack re-initiation can be accurately calculated enabling the prediction of the fatigue life of the specimens.
To demonstrate the beneficial overall effect on the damage tolerance of realistic sandwich structures, the peel stoppers were also embedded in sandwich plates (or panels). It was shown that peel stoppers in all cases were capable of effectively capturing and containing a propagating interface debond crack. The lateral displacements of the debonded face-sheet were measured using DIC and used to identify the crack tip location inside the sandwich panel specimens. To support and further explain the experimental findings, a three-dimensional FE model was developed and used to simulate the behaviour of the debonded sandwich panel specimens. The FE model was able to predict both the fatigue crack growth and crack arrest behaviour accurately. Due to time constraints, the sandwich panel fatigue experiments were only conducted up to about 200,000 load cycles, and to assess the effect of high cycle fatigue damage propagation was simulated up to about 2,000,000 load cycles. It was demonstrated that the developed computational methodology is capable of modelling the fatigue behaviour of sandwich structures with embedded peel stoppers, and that the overall enhancement of the damage tolerance can be predicted accurately.