Dynamic Fracture Toughness of Polymer Composites
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Polymer composites are engineered materials widely being used and yet not completely understood for their dynamic response. It is important to fully characterize material properties before using them for applications in critical industries, like that of defense or transport. In this project, the focus is on determining dynamic fracture toughness property of fiber reinforced polymer composites by using a combined numerical- experimental methodology. Impact tests are conducted on Split-Hopkinson pressure bar with required instrumentation to obtain load-history and initiation of crack propagation parameters followed by finite element analysis to determine desired dynamic properties. Single edge notch bend(SENB) type geometry is used for Mode-I fracture testing and similarly end-notched flexure (ENF) type of geometry is proposed to test the samples for Mode-II type of fracture. Two different linear elastic fracture mechanics approaches are used- crack opening displacement and strain energy release rates. Dynamic fracture toughness values of around 50 MPa[square root of m] and 100 MPa[square root of m] in Mode-I, whereas, around 40 MPa[square root of m] and 6 MPa[square root of m] in Mode-II are observed for carbon-epoxy and fiberglass-epoxy composites respectively. To provide a better estimate of material response, Hashin damage model is employed which takes into account non-linear behavior of composites. As observed in previous studies, values estimated using a non-linear response of composite laminates are nearly three times as high, therefore, using a linear elastic material model could underestimate a material's capacity to sustain dynamic loads without failure. It is concluded that fracture initiation toughness property is rate dependent and is higher when subjected to dynamic loads. Microscopic examination of damaged samples and a higher value of dynamic fracture toughness for fiberglass-epoxy laminates as compared to carbon-epoxy laminates suggest that dynamic fracture toughness is also a function of many other variables like mode of fracture, dominant damage criteria, manufacturing process, constituent materials and their ratios.