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Browsing by Author "Kalhor, Roozbeh"

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    Accelerated Testing Method to Estimate the Lifetime of Polyethylene Pipes
    Kalhor, Roozbeh (Virginia Tech, 2017-02-14)
    The ability to quickly develop predictions of the time-to-failure under different loading levels allows designers to choose the best polymeric material for a specific application. Additionally, it helps material producers to design, manufacture, test, and modify a polymeric material more rapidly. In the case of polymeric pipes, previous studies have shown that there are two possible time-dependent failure mechanisms corresponding to ductile and brittle failure. The ductile mechanism is evident at shorter times-to-failure and results from the stretching of the amorphous region under loading and the subsequent plastic deformation. Empirical results show that many high-performance polyethylene (PE) materials do not exhibit the brittle failure mechanism. Hence, it is critical to understand the ductile mechanism and find an approach to predict the corresponding times-to-failure using accelerated means. The aim of this study is to develop an innovative rupture lifetime acceleration protocol for PE pipes which is sensitive to the structure, orientation, and morphology changes introduced by changing processing conditions. To accomplish this task, custom fixtures are developed to admit tensile and hoop burst tests on PE pipes. A pressure modified Eyring flow equation is used to predict the rupture lifetime of PE pipes using the measured mechanical properties under axial tensile and hydrostatic pressure loading in different temperatures and strain rates. In total, the experimental method takes approximately one week to be completed and allows the prediction of pipe lifetimes for service lifetime in excess of 50 years.
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    Energy Absorption of Metal-FRP Hybrid Square Tubes
    Kalhor, Roozbeh (Virginia Tech, 2017-02-07)
    Lower-cost manufacturing methods have increased the anticipation for economical mass production of vehicles manufactured from composite materials. One of the potential applications of composite materials in vehicles is in energy-absorbing components such as hollow shells and struts (these components may be in the form of circular cylindrical shells, square and rectangular tubes, conical shells, and frusta). However, constructions which result in brittle fracture of the composite tubes in the form of circumferential or longitudinal corner crack propagation may lead to unstable collapse failure mode and concomitant very low energy absorption. As a result, metal-composite hollow tubes have been developed that combine the benefits of stable ductile collapse of the metal (which can absorb crushing energy in a controlled manner) and the high strength-to-weight ratio of the composites. The relative and absolute thicknesses of metal or FRP section has a substantial effect on energy absorption of the hybrid tubes. In particular, likelihood of delamination occurrence raises with increase in FRP thickness. This can reduce the energy absorption capability of the metal-FRP hybrid tubes. Additionally, adding a very thick FRP section may result in a global buckling failure mode (rather than local folding). Until now, there are no studies specifically addressing the effect of FRP thickness on energy absorption of hybrid tubes. In this study, the effects of fiber orientation and FRP thickness (the number of layers) on the energy absorption of S2-glass/epoxy-304 stainless steel square tubes were experimentally investigated. In addition, a new geometrical trigger was demonstrated which has positive effects on the collapse modes, delamination in the FRP, and the crush load efficiency of the hybrid tube. To complete this study, a new methodology including the combination of experimental results, numerical modeling, and a multi-objective optimization process was introduced to obtain the best combination of design variables for hybrid metal-composite tubes for crashworthiness applications. The experimental results for the S2 glass/epoxy-304 stainless steel square tubes with different configurations tested under quasi-static compression loading were used to validate numerical models implemented in LS-DYNA software. The models were able to capture progressive failure mechanisms of the hybrid tubes. In addition, the effects of the design variables on the energy absorption and failure modes of the hybrid tubes were explained. Subsequently, the results from the numerical models were used to obtain optimum crashworthiness functions. The load efficiency factor (the ratio of mean crushing load to maximum load) and ratio between the difference of mean crushing load of hybrid and metal tube and thickness of the FRP section were introduced as objective functions. To connect the variables and the functions, back-propagation artificial neural networks (ANN) were used. The Non-dominated Sorting Genetic Algorithm–II (NSGAII) was applied to the constructed ANNs to obtain optimal results. The results were presented in the form of Pareto frontiers to help designers choose optimized configurations based on their manufacturing limitations. Such restrictions may include, but are not limited to, cost (related to the number of layers), laminate architecture (fiber orientation and stacking sequence) which can be constrained by the manufacturing techniques (i.e. filament winding) and thickness (as an example of physical constraints).