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Informal Seminar by James Ratcliffe |
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Date: October 19, 2005
Time: 12:00pm
Location: NIA, Rm 137
Predicting the Residual Compression Strength of Impact-Damaged Composite Sandwich Panels
James Ratcliffe, NIA
Sandwich construction is an efficient means of providing structural components with high bending stiffness for a given overall weight. However, the interaction between the stiff facesheet and low-density core results in numerous possible failure modes such as core crushing, facesheet wrinkling and facesheet/core debonding. Characterizing the performance of these structures is further complicated when damage is introduced into the facesheet material. One particular concern is the residual compression strength of composite sandwich structure containing low-velocity impact damage on the external facesheet. Two types of compression failure of impact-damaged sandwich structure have been observed in the literature. The first failure mechanism, named indentation growth, involves the progressive inward buckling around the original impact dent until catastrophic panel failure. In the second failure mechanism, local microbuckles of the load-direction fibers in the facesheet are formed at the regions of peak strain in the impact dent region. The microbuckles, known as kink bands, propagate in a stable manner perpendicular to the applied load until a critical length is reached. At this instant, kink-band growth becomes unstable yielding catastrophic panel failure. An analysis method was developed to predict the residual compression strength of sandwich panels that exhibit the kink-band growth failure mechanism. The analysis was used to predict the residual compression strength of five different sandwich panel configurations. All of the panels consisted of Nomex honeycomb core. The facesheets consisted of different types of carbon/epoxy fabric. The analysis was conducted in two stages. First, the impact-damaged facesheet was treated as an orthotropic plate with an open hole. Stable kink-band growth was represented by cracks emanating from the open hole. Plate theory was used to determine the decay in normal stress along the kink-band growth path. The average stress criterion was then used to estimate the far-field stress required for stable kink-band growth. This was repeated for several kink-band lengths, and the stable growth stress was plotted as a function of kink-band length. In the second step of the analysis, linear elastic fracture mechanics was used to calculate the far-field stress for unstable kink-band growth. To this end, the facesheet fracture toughness was measured using a compact compression test (The test is similar to the compact tension test used to evaluate fracture toughness of metals). The resulting plot of unstable growth stress versus kink-band length was superimposed onto the stable growth stress plot. The intersection of the two plots was used to determine the residual compression strength and kink-band length at panel failure. This method was used on the basis that panel failure occurs during the transition from stable to unstable kink-band growth. A comparison of the predicted residual compression strength of all the sandwich configurations showed good agreement with corresponding experimental data.
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