Fracture Characterisation by Butterfly-Tests and Damage Modelling of Advanced High Strength Steels

2021 ◽  
Vol 883 ◽  
pp. 294-302
Author(s):  
Bernd Arno Behrens ◽  
Kai Brunotte ◽  
Hendrik Wester ◽  
Matthäus Dykiert
Keyword(s):  
Stress State ◽  
Shear Fracture ◽  
Pure Shear ◽  
Testing Machine ◽  
High Strength Steels ◽  
High Strength ◽  
Fracture Initiation ◽  
Forming Limit ◽  
Advanced High Strength Steels ◽  
Damage Models

Advanced High Strength Steels (AHSS) are widely used in today's automotive structures for lightweight design purposes. FE simulation is commonly used for the design of forming processes in automotive industry. Therefore, besides the description of the plastic flow behaviour, also the definition of forming limits in order to efficiently exploit the forming potential of a material is required. AHSS are prone for crack appearances without prior indication by thinning, like exemplary shear fracture on tight radii and edge-fracture, which can not be predicted by conventional Forming Limit Curve (FLC). Stress based damage models are able to do this. However, the parameterisation of such models has not yet been standardised. In this study a butterfly specimen geometry, which was developed at the Institute for Forming Technology and Machines (IFUM), was used for a stress state dependent fracture characterisation. The fracture behaviour of two AHSS, CP800 and DP1000, at varied stress states between pure shear and uniaxial loading was characterised by an experimental-numerical approach. For variation of the stress state, the specimen orientation relative to the force direction of the uniaxial testing machine was orientated at different angles. In this way, the relevant displacement until fracture initiation was determined experimentally. Subsequently, the experimental tests have been numerically reproduced giving information about the strain and stress evolution in the crack impact area of the specimen for the experimentally identified fracture initiation. With the help of this testing procedure, two different stress-based damage models, Modified Mohr-Coulomb (MMC) and CrachFEM, were parameterised and compared.

Experimental Study on Shear Fracture of Advanced High Strength Steels

2008 ◽  
Author(s):  
Hua-Chu Shih ◽  
Ming F. Shi
Keyword(s):  
Computer Simulations ◽  
Shear Fracture ◽  
High Strength Steels ◽  
High Strength ◽  
Thickness Ratio ◽  
Forming Limit ◽  
Forming Limit Curve ◽  
Advanced High Strength Steels ◽  
Stretch Bending ◽  
Type Of Fracture

Fracturing in a tight radius during stretch bending has become one of the major manufacturing issues in stamping advanced high strength steels (AHSS), particularly for those AHSS with a tensile strength of 780 MPa or higher. Computer simulations often fail to predict this type of fracture, since the predicted strains are usually below the conventional forming limit curve. In this study, a laboratory stretch-forming simulator (SFS) is used to simulate the stretch bending of AHSS in stamping to develop a possible failure criterion for use in computer simulations. The SFS simulates the stamping process when sheet metal is drawn over a die radius with tension applied. Various sizes of die radius are used during the experiment, and the shear fracture phenomenon can be recreated using this test for a given material and gauge. It is found that shear fracture depends not only on the radius-to-thickness ratio but also on the tension/stretch level applied to the sheet. The experimental data show that a critical radius-to-thickness ratio for shear fracture exists for any given material and gauge, but this ratio is not unique and it depends upon the amount of tension imposed during the bending.

Experimental Study on Shear Fracture of Advanced High Strength Steels: Part II

2009 ◽  
Author(s):  
Hua-Chu Shih ◽  
Ming F. Shi ◽  
Z. Cedric Xia ◽  
Danielle Zeng
Keyword(s):  
Sheet Metal ◽  
Failure Criterion ◽  
Shear Fracture ◽  
High Strength Steels ◽  
High Strength ◽  
Forming Limit ◽  
Tangent Point ◽  
Advanced High Strength Steels ◽  
Tension Level ◽  
Data Points

Developing a proper local formability failure criterion is the key to the successful prediction of the local formability of Advanced High Strength Steels (AHSS) in computer simulations. Shear fracture, which refers to the fracture occurred in the die radius when a sheet metal is drawn over a small die radius, often occurs earlier than predicted by the conventional forming limit curve (FLC). As shown in a previous study using a laboratory Stretch-Forming Simulator (SFS), shear fracture depends not only on the radius-to-thickness (R/T) ratio but also on the tension/stretch level applied to the sheet during stretching or drawing. In the SFS test, a flat sheet is first clamped at the both ends then gradually is wrapped around the die radius as the punch moves downward. This process simulates the early stage of stamping when a sheet metal is initially stretched or drawn over a die/punch radius. However, shear fracture may not occur in this stage if the stretch/tension level is not high enough. In this study, the Bending under Tension (BUT) tester is used to evaluate shear fracture occurring in the later stage of stamping, after the sheet metal is totally wrapped around the die radius. It is demonstrated that shear fracture does occur in this deformation mode when a sufficient tension level is applied. Effects of forming conditions, such as forming speeds and lubrication on shear fracture, are also investigated. When compared to the results from the SFS, the data points failing at the die radius tangent point agree very well. It is observed that all data points above the tangent point failure line show shear fracture, while data points below this line show tensile failure (localized necking) regardless of the test methods used. This indicates that the tangent point fracture line can be used as the shear fracture failure limit. This failure criterion can be used in a computer simulation to simulate the shear fracture phenomenon in the entire deformation process involved in a sheet metal stretching or drawing over a die radius.

scholarly journals Experimental Characterization and Deterministic Prediction of In-Plane Formability of 3rd Generation Advanced High Strength Steels

Metals ◽  
2020 ◽  
Vol 10 (7) ◽  
pp. 902
Author(s):  
Jon Edward Gutierrez ◽  
Jacqueline Noder ◽  
Clifford Butcher
Keyword(s):  
Shear Fracture ◽  
High Strength Steels ◽  
High Strength ◽  
Forming Limit ◽  
Large Strains ◽  
Third Generation ◽  
Plane Shear ◽  
Stress Criterion ◽  
Advanced High Strength Steels ◽  
Biaxial Stretching

The objective of the current study is to develop a practical, deterministic approach to the prediction of the in-plane formability of two third generation advanced high-strength steels (AHSS) of 980 and 1180 MPa ultimate tensile strength using only quasi-static mechanical property data. The hardening response to large strains was experimentally measured with the use of simple shear and tensile tests and validated in tensile simulations. The process-corrected limit strains in the Nakazima and Marciniak tests were compared to various analytical Forming Limit Curve (FLC) models for in-plane stretching. It was observed that the widely-used Marciniak–Kuczynski model can adequately predict the experimental FLC in biaxial stretching but significantly underestimated the limit strains in uniaxial stretching for both third generation AHSS. The observed through-thickness shear fracture mode in biaxial stretching was reasonably well-captured by the Bressan–Williams (BW) instability model for the 1180 MPa steel. A proposed extension of the BW model to uniaxial tension by adoption of the maximum in-plane shear stress criterion (BWx model) provided superior experimental correlation relative to the zero-extension model of Hill that was too conservative. Finally, a linearized version of the modified maximum force criterion (MMFC) was proposed that markedly improved the correlation with the process-corrected FLC for in-plane stretching of AHSS. The developed framework for FLC prediction was then applied to a DP980 AHSS and an AA5182 aluminum alloy from the literature. The DP980 corroborated the observed trend for the two third generation AHSS whereas the MK and the BWx models performed best for the AA5182 with its saturation-type hardening behavior and non-quadratic yield surface.

Determination of the Shear Modulus of Orthotropic Thin Sheets With the Anticlastic-Plate-Bending Experiment

2018 ◽  
Vol 140 (4) ◽  
Author(s):  
Nengxiu Deng ◽  
Yannis P. Korkolis
Keyword(s):  
Shear Modulus ◽  
Numerical Study ◽  
Pure Shear ◽  
High Strength Steels ◽  
High Strength ◽  
Elastoplastic Material ◽  
Plate Bending ◽  
Thin Sheets ◽  
Principal Stresses ◽  
Advanced High Strength Steels

The shear modulus of orthotropic thin sheets from three advanced high-strength steels (AHSS) is measured using the anticlastic-plate-bending (APB) experiment. In APB, a thin square plate is loaded by point forces at its four corners, paired in opposite directions. It thus assumes the shape of a hyperbolic paraboloid, at least initially. The principal stress directions coincide with the plate diagonals, and the principal stresses are equal and opposite. Hence, at 45 deg to these, a state of pure shear exists. A finite element (FE) study of APB is reported first, using both elastic and elastoplastic material models. This study confirms the theoretical predictions of the stress field that develops in APB. The numerical model is then treated as a virtual experiment. The input shear modulus is recovered through this procedure, thus validating this approach. A major conclusion from this numerical study is that the shear modulus for these three AHSS should be determined before the shear strain exceeds 2 × 10−4 (or 200 με). Subsequently, APB experiments are performed on the three AHSS (DP 980, DP 1180 and MS 1700). The responses recorded in these experiments confirm that over 3 × 10−4 strain (or 300 με) the response differs from the theoretically expected one, due to excessive deflections, yielding, changing contact conditions with the loading rollers and, in general, the breaking of symmetry. But under that limit, the responses recorded are linear, and can be used to determine the shear modulus.

Effect of Strain Paths on Formability Evaluation of TRIP Steels

2010 ◽  
Vol 89-91 ◽  
pp. 214-219 ◽  
Author(s):  
David Gutiérrez ◽  
A. Lara ◽  
Daniel Casellas ◽  
Jose Manuel Prado
Keyword(s):  
Strain Path ◽  
High Strength Steels ◽  
High Strength ◽  
Test Method ◽  
Forming Limit ◽  
Trip Steels ◽  
Advanced High Strength Steels ◽  
Strain Paths ◽  
Analysis System

The Forming Limit Diagrams (FLD) are widely used in the formability analysis of sheet metal to determine the maximum strain, which gives the Forming Limit Curve (FLC). It is well known that these curves depend on the strain path during forming and hence on the test method used to calculate them. In this paper, different stretching tests such as the Nakajima and the Marciniak tests were performed, with different sample geometries to obtain points in different areas of the FLD. An optical analysis system was used, which allows following the strain path during the test. The increasing use of advanced high-strength steels (AHSS) has created an interest in determining the mechanical properties of these materials. In this work, FLCs for a TRIP steel were determined using Nakajima and Marciniak tests, which revealed different strain paths depending on the type of test. Determination of the FLCs was carried out following the mathematical calculations indicated in the ISO 12004 standard and was also compared with an alternative mathematical method, which showed different FLCs. Finally, the tests were verified by comparing the strain paths of the Nakajima and Marciniak tests with a well-known mild steel.

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