Pre-stretching Process Research Articles

Influence of Different Pre-Stretching Modes on the Forming Limit Diagram of AA6014

In order to evaluate the formability of sheet materials forming limit diagrams (FLD) are recorded which represent the values of major and minor strain when necking occurs. FLDs are recorded based on the assumption that exclusively linear strain paths occur. In real forming parts, however, particularly in those with complex shapes, predominantly non-linear strain paths occur which reduce the accuracy of the failure prediction according to a conventional FLD. For this reason forming limits after loading with non-linear strain paths have to be investigated. In this contribution a systematic analysis of the forming limits of a conventional AA6014 alloy after loading with non-linear strain paths is presented. This material is pre-stretched in uniaxial, plane strain and biaxial direction up to several levels before performing Nakajima experiments in order to determine FLDs. During the pre-stretching process as well as during the Nakajima experiment the strain distribution can be measured online very precisely with the optical deformation measurement systems GOM Aramis or VIALUX. The gained curves are compared to the FLD of the as-received material. The results prove a significant influence of the pre-stretching condition on the forming limits of the used aluminum alloy. For a low pre-stretching in uniaxial as well as in biaxial direction the FLDs show a slightly reduced formability while after higher pre-stretching levels the forming limit can be improved such as for biaxial loading after uniaxial pre-stretching. The formability after pre-stretching in plane strain direction was changed. Also, a shift of the FLD depending on the direction of pre-stretching can be observed.

Parameter Simulation for Quenching and Pre-Stretching about Aluminum Alloy Thick Plates Based on MARC Finite Element

To predict the three-dimensional residual stress distribution of 7075 aluminum alloy thick plates, the quenching and pre-stretching process were simulated by using MARC finite element software. In order to simulate quickly according to different reality conditions, the parametric model was realized by using defining parameters and selecting objects by inputting coordinate value with the box mode. In the quenching process, a non-linear thermo-mechanical direct coupled analysis of the aluminum alloy thick plates was carried out, by taking into account of the temperature dependence of the thermo-physical properties. In the pre-stretching process, the clamp and release of the clamp of pre-stretch machine were simulated by using life and death element technology. The study demonstrates that the theoretical predictions agree well with the experimental values of residual stresses measured by the X-ray diffraction method. The simulation results provide the user a tool for analyzing and studying on quenching and pre-stretching technique using virtual reality method.

Static and Dynamic FT-IR Linear Dichroism Studies of Plasticization Effects in a Polyurethane Elastomer

By combining FTIR data of plasticized Estane (a polyester polyurethane copolymer) subjected to mechanical deformation with spectral differences as a function of added plasticizer, insight into the physical role of plasticization is gained. For Estane, the orientation functions and dichroic difference data show that, in static stretching, the soft domains reach orientation saturation before the hard domains. With added plasticizer, hydrogen bonding to the hard segments is disrupted and the glass transition of the soft domain decreases, indicating that plasticizer remains solubilized in the soft domain. On the time scale of the static experiment, plasticization diminishes the ability of both domains to reorient, while the plasticizer itself does not orient. With added plasticizer, neither domain reaches orientation saturation during the prestretching process, and dynamic distortion results in a similar response from both domains. Bimodal bands in the dynamic data indicate that the strain applied on a rapid ...