Prediction of the mechanical behaviour of TRIP steel

TRIP steel typically contains four different phases, ferrite, bainite, austenite and martensite. During deformation the metastable retained austenite tends to transform to stable martensite. The accompanying transformation strain has a beneficial effect on the ductility of the steel during forming. By changing the alloy composition, the rolling procedure and the thermal processing of the steel, a wide range of different morphologies and microstructures can be obtained. Interesting parameters are the amount of retained austenite, the carbon content of the austenite, the stability of the austenite as well as its hardness. A constitutive model is developed for TRIP steel which contains four different phases. The transformation of the metastable austenite to martensite is taken into account. The phase transformation depends on the stress in the austenite. Due to the differences in hardness of the phases the austenite stress is not equal to the overall stress. An estimate of the local stress in the austenite is obtained by homogenization of the response of the phases using a self-consistent mean-field homogenization method. Overall stress-strain results as well as stress-strain results for individual phases are compared to measurements found in literature for some TRIP steels. The model is then used to explore the influence of some possible variations in microstructural composition on the mechanical response of the steel

Constitutive modeling of metastable austenitic stainless steel

A stress-update algorithm is developed for austenitic metastable steels which undergo phase evolution during deformation. The material initially comprises only the soft and ductile austenite phase which due to the phenomenon of mechanically induced martensitic transformation, transforms completely to the hard and brittle martensite. A mean-field homogenization algorithm is developed that can predict the mechanical response of the composite material during transformation. Furthermore, a physically based transformation model is developed that predicts the amount of transformation during deformation

A stress integration algorithm for phase transforming steels

A new stress integration algorithm for the constitutive models of materials that\ud undergo strain-induced phase transformation is presented. The most common materials that\ud fall into this category are metastable austenitic stainless (TRIP) steels. These materials can\ud be classi¯ed as metal-matrix composites which comprise a soft and a hard metallic phase. A\ud homogenization algorithm is presented that can estimate the evolution of state variables in each\ud phase for a given strain increment. The elastic-plastic behavior of the phases are calculated\ud individually using large deformation theory and the calculated algorithmic tangent moduli are\ud used in different homogenization schemes. Furthermore, the phase transformation process in\ud austenitic stainless steels involves a volumetric expansion and an inelastic shape change collinear\ud with the deviatoric stress. This transformation plasticity is approximated by a phenomenological\ud model and incorporated in the stress update algorithm

Physically based criterion for prediction of instability under stretchbending of sheet metal

This research focuses on the prediction of the forming limit of certain Advanced High Strength Steel grades under stretch-bending conditions. For these types of steels it is experimentally observed and shown that when there is a bending component added to the main membrane deformation the formability predicted by the regular FLCs underestimate the material behavior. Due to the added effects of thickness stress due to contact and small radius bending as well as bending stresses, a through-thickness stress gradient forms which gives additional stability to the material beyond the forming limits determined by tests that generate mostly uniform membrane deformation. It is observed experimentally and by the detailed simulations that the cross-sectional stability is not lost instantaneously but gradually. A surface dent forms first on the outer surface and progresses in a stable manner towards the contact side since on the contact side the material has still potential to harden. This process delays the localization of the strains and stabilizes the formation of the local neck. For the prediction of this phenomenon theoretically and using shell elements, a modified incremental form of the maximum tension stability criterion is proposed to be applied at integration points through thickness. It is shown that with this criterion the phenomenon of gradual loss of stability can be captured during stretch-bending with shell elements

Numerical and Experimental Validation of a New Damage Initiation Criterion

Most commercial finite element software packages, like Abaqus, have a built-in coupled damage model where a damage evolution needs to be defined in terms of a single fracture energy value for all stress states. The Johnson-Cook criterion has been modified to be Lode parameter dependent and this Modified Johnson-Cook (MJC) criterion is used as a Damage Initiation Surface (DIS) in combination with the built-in Abaqus ductile damage model. An exponential damage evolution law has been used with a single fracture energy value. Ultimately, the simulated force-displacement curves are compared with experiments to validate the MJC criterion. 7 out of 9 fracture experiments were predicted accurately. The limitations and accuracy of the failure predictions of the newly developed damage initiation criterion will be discussed shortly

Biaxial tests on Sandvik Nanoflex

Sandvik Nanoflex (ASTM-A 564) is a metastable steel that undergoes strain induced martensitic\ud transformation during a mechanical deformation process. The aim of the experiments is to observe the\ud transformation by recording the amount of martensite via a magnetic sensor. The physics underlying\ud the process suggests that although transformation proceeds with plastic straining, the stress-state\ud should have a big influence on its rate. Additionally, the Magee theory on martensitic variants states\ud that transformation proceeds as different variants become energetically favorable. Hence, it remains a\ud question how the material will behave upon a sudden change in the imposed stress-state since the\ud number of favoured variants is expected to change abruptly. Therefore, making use of the biaxial\ud setup a series of stress states and strain paths are imposed on the material and the effects on\ud transformation rate are observed