Failure of high strength steel sheets: Experiments and modelling

Abstract

Failure in sheet metal structures of ductile material is usually caused by one of, or a combination of, ductile fracture, shear fracture or localised instability. In this paper the failure of the high strength steel Docol 600DP and the ultra high strength steel Docol 1200M is explored. The constitutive model used in this study includes plastic anisotropy and mixed isotropic-kinematic hardening. For modelling of the ductile and shear fracture the models presented by Cockroft-Latham and Bressan-Williams have been used. The instability phenomenon is described by the constitutive law and the finite element (FE) models. For calibration of the failure models and validation of the results, an extensive experimental series has been conducted including shear tests, plane strain tests and Nakajima tests. The geometries of the Nakajima tests have been chosen so that the first quadrant of the forming limit diagram (FLD) were covered. The results are presented both in an FLD and using prediction of force-displacement response of the Nakajima test employing element erosion during the FE simulations. The classical approach for failure prediction is to compare the principal plastic strains obtained from FE simulations with experimental determined forming limit curves (FLC). It is well known that the experimental FLC requires proportional strains to be useful. In this work failure criteria, both of the instability and fracture, are proposed which can be used also for non-proportional strain paths.

Predicting Steel Tensile Responses and Fracture Using the Phenomenological Ductile Shear Fracture Model

“…Fracture initiation in the shear 24 mode occurs under intense shear stresses resulting either from extensive slip on the activated slip 25 planes and/or microcracking without any void nucleation, or from void nucleation in the slip 26 bands (French and Weinrich 1979). In any case, the shear mode is less pressure-dependent than 27 the tensile mode (Björklund et al 2013). 28…”

“…Either mode or a combination of both may take place in 23 structural steel (Hooputra et al 2004, Björklund et al 2013. Fracture initiation in the shear 24 mode occurs under intense shear stresses resulting either from extensive slip on the activated slip 25 planes and/or microcracking without any void nucleation, or from void nucleation in the slip 26 bands (French and Weinrich 1979).…”

“…In the tensile mode, fracture initiation is always associated with void nucleation under 29 predominantly tensile stresses at high stress triaxiality (Björklund et al 2013 In any case, all the aforementioned models require a number of parameters (up to nine) that are 50 to be calibrated from elaborate laboratory tests, with some requiring fractographic analysis. 51…”

“…Since fracture of structural steel could be by either the shear or the tensile mode, or a 62 combination of both, it is important to employ models that are capable of capturing the two 63 modes (Björklund et al 2013). To the best of the authors' knowledge, the prediction of tensile 64 responses of civil engineering structural steel using the phenomenological shear fracture model 65 that simulates ductile tensile fracture under high stress triaxiality and employs the shear stress 66 ratio rather than the Lode angle parameter to simulate ductile shear fracture under low/negative 67 triaxiality has not been published.…”

Failure characteristics of a dual-phase steel sheet

“…The ductile shear fracture can be either a through-thickness shear fracture, see e.g. Björklund et al (2013), or an in-plane shear fracture, see e.g. Li et al (2010).…”