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The well-known Olson–Cohen model is modified by incorporating the effect of stress state on the transformation kinetics in metastable austenitic stainless steels. By assuming isothermal condition, the relationships between the parameters of Olson–Cohen model, stress triaxiality and absolute value of Lode angle parameter have been established. The proposed model predicts that the saturation level and slope of transformation curve increase with increasing the stress triaxiality in addition to the absolute value of Lode angle parameter. Uniaxial and plane strain tension tests have been conducted on the two types of austenitic stainless steels AISI 304 and AISI 201 to evaluate the validity of the model. The results show that the predicted transformation curves are in good agreement with the experimental data.

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The influence of strain state on the kinetics of martensitic phase transformation in stainless steel 304 is investigated. The methodology involves stretching various specimens with different widths over a hemispherical punch. The specimens are stretched to various depths by controlling the punch traveling in order to achieve different strain levels. The volume fraction of strain induced martensite is then measured using ferrite scope and plotted versus strain for different strain states. The results show that the martensitic transformation curve has the lowest value for plane strain condition among strain states used for determination of forming limit diagram. There are a close proximity between the two sets of the experimental data and those of the modified Olson-Cohen and Santacreu's models.

Foldover is a common, yet complex, transient boundary condition in several ‘metal forming processes’ including the barrelling compression test (BCT). The onset of foldover during BCT is comparable to that of necking in the tensile test. However, its role in flow curve identification has been ignored in the current literature. Methods to detect and measure its onset and growth during the processes are also not available. We propose three methods to identify the onset of foldover and measure its progress with deformation. These are conducted together with case studies of BCT to have a fundamental understanding of the onset and growth of foldover in the test and more general cases. Experimental and numerical methods are developed to accomplish this based on (1) direct measurement using tracing points, (2) energy-based and (3) geometry-based indirect measurements. These outline the required supplementary data and allow proper detection of the onset. In the first method, the migration of tracing points from the free surface to the platen-sample interface, simulated numerically, is monitored and measured visually. The second method employs the load-stroke data to calculate the total power input to the deforming system. In the third method, a criterion has been derived to detect the onset of foldover by monitoring the geometry of the sample profile during the test and comparing it to a reference quadratic profile. The observations and outputs of each method are compared and discussed.