Axial Fatigue Characterization of Core Joints in Sandwich Composite Structures

Abstract

Composite sandwich structures are currently being employed in a variety of structural applications where high strength/stiffness to weight ratio is critical. Applications include space structures, bridge decks, marine, train and truck flooring, wind turbine blades etc. Core joints exist in all these composite large structures and special attention with regards to the effect of core joint on the fatigue performance of sandwich structure is of interest to composite engineers. When core joints in composite sandwich structures loaded in-plane, mismatched Poisson’s ratio between the joint and the core generates out-of-plane stresses (interlaminar) at the core joint/facesheet interface, creating a stress riser at this location that will eventually affect the fatigue life of the structure. Quantifying this effect and its consequence on the service life and reliability of the composite structures are not well developed. This experimental study characterizes the effect of different core joint designs in composite sandwich structures. Static, tension-tension and tension-compression fatigue tests with constant load amplitudes at varied frequencies were applied to describe the fatigue life prediction of composite sandwich structures with core joints. Post-Fatigue tensile tests were also performed to predict the residual strength of such structures. Nondestructive Evaluation Techniques (DIC, SEM and AE) were used to locate the stress concentrations and justify the damage mechanism. The experimental results show that the H100 and H200 scarf core joint designs have higher fatigue strength than the H100 and H200 butt core joint designs at low and high cycle fatigue regimes. The suggested scarf core joint design increased the axial stiffness of H200 sandwich panels by 21%. The traditional butt core joint design showed rapid decrease in stiffness causing an instant failure to the H100 and H200 specimens. A logical blend of experimental and analytical prediction of the service life of composite sandwich structures is carried out. This research study will benefit composite engineers and joint designers in both academia and industry to better apprehend the influence of core joints and its consequence on the functionality of sandwich structures.