Kuang An Wang

Failure Modes of Composite Sandwich Beams

An investigation was conducted of failure modes and criteria for their occurrence in composite sandwich beams. The initiation of the various failure modes depends on the material properties of the constituents (facings and core), geometric dimensions and type of loading. The beams were made of unidirectional carbon/epoxy facings and aluminum honeycomb and PVC closed-cell foam cores. The constituent materials were fully characterized and in the case of the foam core, failure envelopes were developed for general two-dimensional states of stress. Sandwich beams were loaded under bending moment and shear and failure modes were observed and compared with analytical predictions. The failure modes investigated are face sheet compressive failure, adhesive bond failure, indentation failure, core failure andfacing wrinkling.

Compression facing wrinkling of composite sandwich structures

A thorough investigation was conducted of face wrinkling failures of sandwich columns under compression and beams in three- and four-point bending and cantilever beams under end loading. The beams were made of unidirectional carbon/epoxy facings and aluminum honeycomb and closed-cell PVC foam cores. The constituent materials were fully characterized. Face wrinkling failures were observed in sandwich columns and beams with foam cores, but not in those with honeycomb cores. Several wrinkling failure loads were measured and compared favorably with an early expression of Hoff and Mautner for the case of columns and long beams where the core is in the linear elastic range. However, for short span beams, core failure precedes face wrinkling. Core yielding and stiffness loss reduce core support of the facing and precipitate facing wrinkling at a lower stress. For this case a modified Hoff and Mautner expression, where the values of the reduced core Young’s and shear moduli in the through-the-thickness direction are introduced, appears more appropriate. It was concluded that failure by wrinkling is prevalent in the case of low through-the-thickness stiffness and long beam spans. In other cases, other failure modes including shear core failure, compressive facing failure, face sheet debonding or indentation failure may occur.

Nonlinear behavior of composite sandwich beams in three-point bending

The load-deflection behavior of a composite sandwich beam in three-point bending was investigated. The beam was made of unidirectional carbon/epoxy facings and a polyvinyl chloride closed-cell foam core. The load-deflection curves were plotted up to the point of failure initiation. They consist of an initial linear part followed by a nonlinear portion. A nonlinear mechanics of materials analysis that accounts for the combined effect of the nonlinear behavior of the facings and core materials (material nonlinearity) and the large deflections of the beam (geometric nonlinearity) was developed. The theoretical predictions were in good agreement with the experimental results. It was found that the effect of material nonlinearity on the deflection of the beam is more pronounced for shear-dominated core failures in the case of short span lengths. It is due to the nonlinear shear stress-strain behavior of the core. For long span lengths, the observed nonlinearity is small and is attributed to the combined effect of the facings nonlinear stress-strain behavior and the large deflections of the beam.

Indentation failure in composite sandwich structures

A combined analytical and experimental investigation of the indentation failure of a composite sandwich panel has been undertaken. Two cases have been studied: a sandwich panel with carbon/epoxy facing and a PVC foam layer supported on a rigid base and indented at the center with a cylindrical indentor; and a sandwich beam with symmetrical facing and core materials as in the sandwich panels. The load-deflection behavior of the loaded facing was monitored during the test. Strains were also measured near the load on both surfaces of the facing using embedded strain gages. A full-field analysis of the in-plane displacements in the foam was conducted using the moire method. The problem was modeled as an elastic beam resting on an elastic-plastic foundation. Initiation of indentation failure occurs when the foundation yields, while catastrophic failure takes place when the compression facing fractures. The experimental results are in good agreement with the results of the analytical modeling based on the Winkler foundation.

Multiaxial characterization and modeling of a PVC cellular foam

The failure behavior of a PVC closed-cell cellular foam under a multiaxial state of stress was investigated. The uniaxial compressive, tensile, and shear stress-strain curves along the in-plane and through-the-thickness directions were obtained. A series of multiaxial tests including strip, thin-wall ring and thin-wall tube specimens under combined tension/compression, torsion, and internal pressure were performed. From these tests biaxial strength results in the stress plane of the in-plane and through-the-thickness directions were obtained. The results are in good agreement with the Tsai-Wu failure criterion.

Deformation and Failure of Composite Sandwich Structures

An investigation was conducted of failure modes and criteria for their occurrence in composite sandwich columns and beams. The initiation of the various failure modes depends on the material properties of the constituents (facings and core), geometric dimensions and type of loading. The loading type or condition determines the state of stress throughout the sandwich structure, which controls the location and mode of failure. The appropriate failure criteria at any point of the structure account for the biaxiality or triaxiality of the state of stress. The specimens were made of unidirectional carbon-epoxy facings and aluminum honeycomb and PVC closed-cell foam cores. The constituent materials were fully characterized and, in the case of the foam core, failure envelopes were developed for general two-dimensional states of stress. Sandwich specimens were loaded under bending moment, shear and axial loading and failure modes were observed and compared with analytical predictions. The failure modes investigated are face sheet compressive failure, adhesive bond failure, indentation failure, core failure and facing wrinkling. The transition from one failure mode to another for varying loading or state of stress was discussed. Experimental results are compared with analytical predictions.

Core Failure of Sandwich Beams

Core failure modes were studied in composite sandwich beams under three-point bending and in cantilever beams under end loading. The beams consisted of unidirectional carbon/epoxy face sheets and a variety of core materials, including aluminum honeycomb, PVC closed-cell foams, polyurethane foam and balsa wood. The constituent materials were fully characterized and in the case of the core materials, failure envelopes were developed for biaxial states of stress. Deformation and failure mechanisms were studied experimentally by means of moire gratings and birefringent coatings. Results were obtained for stress (strain) distributions in the linear and nonlinear/plastic range of the core, critical failure loads and their dependence on geometrical dimensions, material parameters and loading conditions.