Current thermoplastic processing techniques involve high capital costs for moulds and significant use of energy to melt or soften the materials. Single-step process cold forming techniques, such as stretch forming, could be cost effective methods for manufacturing large parts with shallow cross-sections from plastic sheet. The present work is a preliminary investigation of a cold forming technique for polycarbonate. The objective of this work is to characterize the bulk deformation behaviour of polycarbonate using tensile tests and dome stretch forming tests. Two different molecular weight polycarbonate sheets with 1.6 mm thickness were studied: (i) one with Mw = 42,000 g/mol and (ii) the other with Mw = 52,000 g/mol. For the latter, 3.2 mm sheets were also studied. Tensile tests conducted at three different cross-head speeds, i.e., 2, 20 and 200 mm/min showed very similar elastic and plastic deformation properties for the two molecular weights. Correspondingly, the activation volumes at yield were almost identical. There was also negligible difference in the thermophysical properties between the two materials as found by differential scanning calorimetry. Dome stretch forming tests were conducted on a metal forming machine. Specimens of varying width were tested to give different strain states ranging from deep drawing to biaxial. The limiting dome height or the maximum level of stretch forming iv increases with specimen width. This is due to biaxial deformation which increases the maximum strain. Forming limit diagrams (FLDs) were also constructed from the local strains measured from printed fine circle grid patterns on the polycarbonate sheet surfaces. The FLDs showed common general characteristics with metals except for a few key differences. An area of very few data points was found to lie between the “safe zone” and the “necked zone”. This void was referred to as the “unstable neck formation zone”. It exists because of the large local increases in strain associated with the unstable nature of polymer neck formation. Much more study is required before polycarbonate can be cold formed at strains below the unstable neck deformation. However, the materials and techniques used in this work have demonstrated that the process can be viable for forming shallow large parts from relatively thin thermoplastic sheet a as long as the local biaxial strains are less than 20%.

Current thermoplastic processing techniques involve high capital costs for moulds and significant use of energy to melt or soften the materials. Single-step process cold forming techniques, such as stretch forming, could be cost effective methods for manufacturing large parts with shallow cross-sections from plastic sheet. The present work is a preliminary investigation of a cold forming technique for polycarbonate. The objective of this work is to characterize the bulk deformation behaviour of polycarbonate using tensile tests and dome stretch forming tests. Two different molecular weight polycarbonate sheets with 1.6 mm thickness were studied: (i) one with Mw = 42,000 g/mol and (ii) the other with Mw = 52,000 g/mol. For the latter, 3.2 mm sheets were also studied. Tensile tests conducted at three different cross-head speeds, i.e., 2, 20 and 200 mm/min showed very similar elastic and plastic deformation properties for the two molecular weights. Correspondingly, the activation volumes at yield were almost identical. There was also negligible difference in the thermophysical properties between the two materials as found by differential scanning calorimetry. Dome stretch forming tests were conducted on a metal forming machine. Specimens of varying width were tested to give different strain states ranging from deep drawing to biaxial. The limiting dome height or the maximum level of stretch forming iv increases with specimen width. This is due to biaxial deformation which increases the maximum strain. Forming limit diagrams (FLDs) were also constructed from the local strains measured from printed fine circle grid patterns on the polycarbonate sheet surfaces. The FLDs showed common general characteristics with metals except for a few key differences. An area of very few data points was found to lie between the “safe zone” and the “necked zone”. This void was referred to as the “unstable neck formation zone”. It exists because of the large local increases in strain associated with the unstable nature of polymer neck formation. Much more study is required before polycarbonate can be cold formed at strains below the unstable neck deformation. However, the materials and techniques used in this work have demonstrated that the process can be viable for forming shallow large parts from relatively thin thermoplastic sheet a as long as the local biaxial strains are less than 20%.

No description available.

The current study investigates the formability of ZEK100, a rare-earth containing magnesium alloy, using an in-house developed technique of pneumatic bulge forming. The thesis pursued innovation of sample preparation, testing, and experimental data analysis in order to create several forming limit diagrams (FLDs) of critical importance for determining a methodology for Mg formability. Samples were bulged through elliptical and circular dies at room temperature, 150 C, and 250 C, in two orientations, rolling direction (RD) and transverse direction (TD), in order to determine temperature dependence and orientation characteristics. The current research concluded ZEK100 is not a suitable alloy for room temperature forming processes used in automotive industries. Little difference between safe and marginal, as well as marginal and failure strain ratios was seen for RD orientation testing, while greater resolution is evident for TD orientation testing. ZEK100 exhibits a temperature dependence in relation to limiting strain between RD and TD.

No description available.

Limiting dome height (LDH) tests were used to evaluate the formability of both base metal and laser butt welded blanks of AHSS (including High strength low alloy (HSLA), Dual phase (DP) steels of different grades). Mechanical properties of the base metal and welded blanks were assessed by uniaxial tensile and biaxial LDH tests, and related to measured microhardness distributions across the welds. The formability ratio of laser welded dual phase sheet steels generally decreases with increased base metal strength. A significant decrease of LDH was observed in the higher strength DP steel welded specimens due to the formation of a softened zone in the Heat Affected Zone(HAZ). Softened zone characteristics were correlated to the LDH. Larger softened zones led to a larger reduction in the LDH. HAZ softening has been shown to be a function of the base metal martensite content and the weld heat input. Formability also decreased with increased weld heat input. Both in experiment and numerical simulations strain is localized in the softened HAZ in the uniaxial tensile testing, indicating that strain localization decreases tensile strength and elongation of laser welds in DP980.

Limiting dome height (LDH) tests were used to evaluate the formability of both base metal and laser butt welded blanks of AHSS (including High strength low alloy (HSLA), Dual phase (DP) steels of different grades). Mechanical properties of the base metal and welded blanks were assessed by uniaxial tensile and biaxial LDH tests, and related to measured microhardness distributions across the welds. The formability ratio of laser welded dual phase sheet steels generally decreases with increased base metal strength. A significant decrease of LDH was observed in the higher strength DP steel welded specimens due to the formation of a softened zone in the Heat Affected Zone(HAZ). Softened zone characteristics were correlated to the LDH. Larger softened zones led to a larger reduction in the LDH. HAZ softening has been shown to be a function of the base metal martensite content and the weld heat input. Formability also decreased with increased weld heat input. Both in experiment and numerical simulations strain is localized in the softened HAZ in the uniaxial tensile testing, indicating that strain localization decreases tensile strength and elongation of laser welds in DP980.

Tailor welded blanks (TWBs) are used in the automotive industries as a method to meet economic, environmental and governmental demands. Conventionally, TWBs incorporated mild and low strength steels such as interstitial free and draw quality steels because of their excellent formability traits. However, due to their low strength they are unsuitable for energy absorption applications; thus, the interest of incorporating advanced high strength steels (AHSS) into the TWBs. Dual phase (DP) steel is a type of AHSS that is of interest because of its combination of high strength and good formability that is comparable to high strength low alloy (HSLA) steels. However, welding DP steel causes softening in the heat affected zone (HAZ), which leads to premature failure and reduces formability. The aim of this thesis was to study the effect of weld design on the formability of TWBs with DP steels and with HSLA steel. This thesis is divided into three parts; the first part examines TWBs with different weld line positions, weld line orientations and strain paths. The second part investigates bead-on plate curvilinear blanks and its effect on formability of the blanks. The last part examines the effects of multiple welds on the formability of TWBs.

published_or_final_version / Industrial Engineering / Master / Master of Philosophy

Tailor welded blanks (TWBs) are used in the automotive industries as a method to meet economic, environmental and governmental demands. Conventionally, TWBs incorporated mild and low strength steels such as interstitial free and draw quality steels because of their excellent formability traits. However, due to their low strength they are unsuitable for energy absorption applications; thus, the interest of incorporating advanced high strength steels (AHSS) into the TWBs. Dual phase (DP) steel is a type of AHSS that is of interest because of its combination of high strength and good formability that is comparable to high strength low alloy (HSLA) steels. However, welding DP steel causes softening in the heat affected zone (HAZ), which leads to premature failure and reduces formability. The aim of this thesis was to study the effect of weld design on the formability of TWBs with DP steels and with HSLA steel. This thesis is divided into three parts; the first part examines TWBs with different weld line positions, weld line orientations and strain paths. The second part investigates bead-on plate curvilinear blanks and its effect on formability of the blanks. The last part examines the effects of multiple welds on the formability of TWBs.