Petr Kubík

Calibration of Selected Ductile Fracture Criteria Using Two Types of Specimens

This paper presents the calibration of three universal ductile fracture criteria with the stress triaxiality and the normalized third invariant of deviatoric stress dependence. Xue-Wierzbicki, Bai-Wierzbicki and Extended Mohr-Coulomb criteria are calibrated using butterfly specimen and newly designed notched tube specimen for an austenitic stainless steel. Different stress states necessary for successful calibration are generated by suitable combination of tension/compression-shear or tension/compression-torsion loading of both specimens, respectively. Suitability of the specimens for ductile fracture criteria calibration is evaluated comparing the range of reached stress states, the homogeneity of stress in process zone and appropriateness for the computational simulation of tests. Possible combination of above mentioned results with tensile testing of standard and notched cylindrical specimens is discussed, too.

Ductile Fracture Criteria in Prediction of Chevron Cracks

Selected ductile fracture criteria are introduced and applied to prediction of chevron crack initiation and development during forward extrusion of long shafts. We present the calibration of selected criteria for carbon steel, simulate the process of forward extrusion and compare the simulation results with real experiments realized in a cooperating industrial company.

Application of Coupled Ductile Fracture Criterion to Plane Strain Plates

The paper presents a complex material model which covers the elastic-plastic behavior, material deterioration and ductile fracture. The calibration of such model was conducted for Aluminum Alloy (AA) 2024-T351 using specimens with various geometries and loading which covers various stress states. The model was then applied to the simulations of tensile test of plates. The computations were carried out in Abaqus/Explicit using the user subroutine Vectorized User MATerial (VUMAT), where the crack initiation and subsequent propagation was realized using the element deletion technique. The results were compared to the experimental observation in the end.

Behavior of Lode Dependent Plasticity at Plane Strain Condition and its Implication to Ductile Fracture

Variety of metals are complex materials exhibiting various behavior under different loading. Many metallic materials exhibit Tresca-like behavior rather than von Mises. It means different behavior in tension under plane strain and uniaxial stress conditions. This might be described by Lode dependent plasticity which should result in better prediction in force or torque responses of material tests. Good agreement between computation and experiment is also very important when calibrating the ductile fracture criteria. Several tests under plane strain and uniaxial stress states were carried out on aluminum alloy 2024-T351 where the Lode dependency was significant. The Lode dependent plasticity was implemented along with von Mises and Tresca-like yield criteria, which resulted in improvement of force–displacement responses of plane strain tests simulations. But it also caused significant change in the stress state of tensile flat and grooved plates which wrongly approached uniaxial tension condition. This inconvenience prevents plane strain experiments from using for calibration of ductile fracture criteria under these circumstances.

Extremely Low-Stress Triaxiality Tests in Calibration of Fracture Models in Metal-Cutting Simulation

The cutting process is now combined with machining, milling, or drilling as one of the widespread manufacturing operations. It is used across various fields of engineering. From an economical point of view, it is desirable to maintain the process in the most effective way in terms of the fracture surface quality or minimizing the burr. It is not possible to manage this experimentally in mass production. Therefore, it is convenient to use numerical computation. To include the crack initiation and propagation in the computations, it is necessary to implement a suitable ductile fracture criterion. Uncoupled ductile fracture models need to be calibrated first from fracture tests when the test selection is crucial. In the present article, there were selected widespread uncoupled ductile fracture models calibrated with, among others, an extremely low-stress triaxiality test realized through the compression of a cylinder with a specific recess. The whole experimental program together with the cutting process experiment were carried out on AISI 1045 carbon steel. After the fracture models were calibrated and the cutting process was simulated with their use, fracture surfaces and force responses from computations were compared with those experimentally obtained and concluding remarks were made.

Prediction of central bursting in the process of forward extrusion using the uncoupled ductile failure models

Abstract Forward extrusion is a manufacturing process in which the cross section of product is reduced by forcing it through the die under high pressure. Cold extrusion is most often used in the mass production of cylindrical rods or hollow tubes due to its economic advantages, although it can be used for more complex shapes of cross section. The process is accompanied by excessive amount of plastic deformation and central burst defects, the so-called chevron cracks can occur. These defects are dangerous because of their invisibility on the product’s surface. Therefore, it is important to carry out computational simulations to successfully predict and prevent the central bursting. Solution of this problem is especially influenced by the die cone angle, friction and reduction ratio. In this study, the process is analysed through numerical simulations using the uncoupled ductile failure models which were calibrated and implemented into Abaqus/Explicit by user subroutine VUMAT. Obtained results are compared to experiments conducted on the carbon steel AISI 1045 and predictive potential of the criteria is discussed.

Verification of Ductile Fracture Criteria Based on Selected Calibration Tests

Phenomenological ductile fracture criteria represent, among others, one of powerful tools for prediction of ductile fracture. These criteria are based on evaluating damage throughout the solid body as a response to straining. The damage is influenced by plasticity but not vice versa. Therefore, these criteria are often called uncoupled as they do not mutually couple the damage and plasticity. One of immense advantages of such criteria is a possibility not only to predict the crack initiation but also to follow the propagation based on the damage. Moreover, it is not restricted for one specific locus but the damage is evaluated in the entire solid body and one or more cracks can be tracked simultaneously or sequentially. Ductile fracture criteria are calibrated on the basis of several independent calibration tests under various stress states. One way how to verify calibrated model is to simulate numerically an experimental tests and follow the crack initiation and propagation. In the present study, selected phenomenological criteria were calibrated using various calibration tests. Then, selected calibration tests were simulated together with implemented ductile fracture criteria. In our case, the verification is carried out on tensile cylindrical specimens. Finally, computationally obtained results were compared to the experimentally observed ones and the prediction ability and reliability of selected phenomenological criteria is discussed.

DUCTILE FRACTURE CRITERIA IN PREDICTION OF SLANT FRACTURE

The ductile fracture of metallic materials is consequence of damage accumulation after straining accompanied by large plastic deformations. It is significantly dependent on the microstructure of particular material and micromechanical defects as vacancies or second phase particles. The nucleation, growth and coalescence of voids is the fracture mechanism which applies in high values of stress triaxiality. The shear mechanism appears in the region of negative stress triaxialities. Finally, the combination of both fracture mechanisms occurs in cases of moderate stress triaxialities. The crack or fracture surface in specimens or real components is often tilted approximately 45 degrees to applied load. This slant fracture is driven by shear mechanism and occurs in the plane of maximum shear stress. It was shown that coupled ductile fracture criteria are convenient for the prediction of slant fracture in finite element simulations. There is conducted analysis of slant fracture prediction ability of two chosen coupled and uncoupled criteria in the present paper. Those criteria were calibrated and applied to two different metallic materials. The prediction is validated on those calibration fracture tests at which the slant fracture was observed.

Ductile Fracture Criteria Calibration and their Application

This paper deals with ductile fracture criteria calibration process. Choice of calibration tests and their relation to corresponding type of failure is discussed. Further, some of the calibrated criteria are used to simulation of selected technological processes and results are compared with experiments.