Strength reduction of foam-core sandwich structures with core mismatches

Discontinuities in foam-core composite sandwich panels are challenging to assess, in part due to shortcomings in the fundamental understanding of failure processes occurring in the foam. Extensive testing and analysis are frequently required to overcome this lack of understanding when evaluating large-scale structures. This integrated experimental and analytical study investigates the effect of core thickness mismatches on the load carrying capabilities of composite sandwich structures. These mismatches are commonly found in large composite components in aerospace, marine, automotive and other industries and can cause major reductions in sandwich shear strength. Mismatches result from joints between separate pieces of foam core within the sandwich, becoming potential failure initiation sites. Quantifying the reduction in shear strength is difficult due to the complex interaction between the facesheets, butt-joints and core. Sandwich structures containing butt joints are investigated through a research effort that included fabrication, testing, analysis, and nondestructive evaluations. The strength reduction of sandwich beams due to the presence of butt joints is quantified via three-point bend (3PTB) testing for various types of mismatches such as height and thickness of the butt joint. Key mechanical properties of the foam and the lamina are measured through testing and incorporated into analytical models. A stress area approach is proposed as a failure criterion, as test results show this to be promising in predicting the strength reductions measured in the 3PTB testing and the failure initiation site.