by Butcher, Cliff; Chen, Zengtao; Bardelcik, Alexander; Worswick, Michael

Numerical simulations of straight tube hydroforming of a dual phase (DP600) advanced high strength steel were performed using a variant of the Gurson–Tvergaard–Needleman (GTN) constitutive model to account for the influence of void shape and shear on coalescence. The effect of axial-feed (end-feed) on damage development and formability is investigated for end-feed loads of zero and 133 kN. A parametric study was conducted to determine an appropriate void nucleation stress and strain and the numerical values compared with the experimental data. The calibrated GTN damage model gives good agreement with the experimentally determined burst pressure, formability and failure location with the best performance occurring for the high end-feed load.

DOI: 10.1007/s10704-009-9323-x
Online Date: 3/5/2009
Print publication date: 1/1/2009
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by Nguyen, C. Thang; Vu-Khanh, Toan; Dolez, Patricia I.; Lara, Jaime

Resistance to puncture is a critical property for several applications, in particular for elastomer materials used in protective clothing. To evaluate the puncture resistance of membranes, some methods have been proposed as standard tests. However, the rounded puncture probes used in these tests are very different from real pointed objects like medical needles, and may not measure the level of material resistance that corresponds to them. In fact, puncture by medical needles is shown to proceed gradually as the needle cuts into the membrane. This behavior is highly different from puncture by rounded probes which occurs suddenly when the strain at the probe tip reaches the failure value. In addition, maximum force values are observed to be much smaller with medical needles. A method has been developed based on the change in strain energy with the puncture depth to evaluate the fracture energy associated to puncture. The results show that the phenomenon of puncture by medical needles involves contributions both from friction and fracture energy, in a similar way as for cutting. A lubricant was tentatively used to reduce the friction contribution for the computation of the material fracture energy.

DOI: 10.1007/s10704-009-9326-7
Online Date: 3/4/2009
Print publication date: 1/1/2009
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by Nguyen, C. Thang; Vu-Khanh, Toan; Dolez, Patricia I.; Lara, Jaime

Resistance to puncture by medical needles is becoming one of the most critical mechanical properties of rubber membranes, which are heavily used in protective gloves. Yet the intrinsic material parameters controlling the process of puncture by medical needles are still unknown. In a first paper presenting this two-part study, it has been shown that puncture by medical needles proceeds gradually as the needle cuts through the rubber membrane. The phenomenon of puncture by medical needles was revealed to involve contributions both from friction and fracture energy, in a similar way as for cutting. The use of a lubricant was not successful for removing the friction contribution for the determination of the material fracture energy corresponding to puncture by medical needles. This paper describes an alternative approach based on the application of a prestrain to the sample in a similar way as the work of Lake and Yeoh on cutting. A theoretical formulation for the tearing energy is derived from the theory of Rivlin and Thomas on the rupture of rubber. It is validated with a model extending expressions provided by the linear elastic fracture mechanics (LEFM) to include the non-linear stress–strain behavior displayed by rubber. For low values of the tearing energy, the total fracture energy, i.e. the sum of the puncture and tearing energies, is constant; the material fracture energy is obtained by extrapolation at zero tearing energy. This prestrain method allowed a complete removal of the friction contribution. The value obtained for the fracture energy corresponding to puncture by medical needles is found to be larger than the energy associated to cutting and smaller than that obtained for tearing. This can be related to the value of the crack tip diameter, which is, in that case, given by the needle cutting edge diameter.

DOI: 10.1007/s10704-009-9325-8
Online Date: 3/3/2009
Print publication date: 1/1/2009
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by Rupnowski, Przemyslaw; Sopori, Bhushan

This paper describes a model to predict mechanical strength distribution of silicon wafers. A generalized expression, based on a multimodal Weibull distribution, is proposed to describe the strength of a brittle material with surface, edge, and bulk flaws. The specific case of a cast, unpolished photovoltaic (PV) wafer is further analyzed. Assuming that surface microcracks constitute the dominant mechanism of wafer breakage, this model predicts the strength distribution of PV silicon that matches well the experimental results available in the literature.

DOI: 10.1007/s10704-009-9324-9
Online Date: 3/3/2009
Print publication date: 1/1/2009
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