Jindřich Petruška

On the evaluation of strain inhomogeneity by hardness measurement of formed products

Abstract Large plastic deformation of cold formed metal is followed by its hardening, which can be experimentally evaluated by measuring hardness. To convert the measured hardness values into strain, the strain–hardness reference curves are used. Their evaluation is traditionally based on compression tests of cylindrical specimens. New procedure of reference curves evaluation is suggested here, based on an FE simulation of a forming process and direct relation of computed strain and measured hardness. The procedure is illustrated on the example of forward extrusion process and reference curves, obtained by classical and newly suggested procedure are compared.

Computationally-experimental workability determination of compressed cylindrical specimen with surface defect

Abstract Fracture of cylindrical specimens with longitudinal surface notch under compression test were experimentally studied. Geometry of the specimens (height to diameter ratio, notch depth) and frictional conditions (grooved, dry smooth and lubricated smooth platens) were changed. Subsequent finite element numerical simulation of the tests produced local stress–strain history at the notch tip. This was used for evaluation of the ability of various ductile fracture criteria to predict correctly the fracture initiation. Results for individual criteria are presented and discussed in the paper.

Cold forming of bolts without thermal treatment

Abstract In this paper, we describe the experience with cold forming of bolts, made of micro-alloyed steel, specially designed to enable elimination of thermal treatment. First, the flow stress curve and hardening of material are evaluated by compression testing. Next, cold forming of various geometrical versions of the bolts is numerically simulated to obtain and compare their strain distribution and formability exhaustion. The results are verified by hardness measurement of produced specimens. Final products are then subjected to tensile testing to evaluate basic mechanical characteristics of the bolts—yield and tensile strength and elongation at rupture.

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.

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.

Optimal intermeshing of multi roller cross roll straightening machine

The paper deals with optimal setting of intermeshing parameters in multi roller cross roll straightening machine. The procedure is based on a fast algorithm of the straightening process simulation, which leads to an efficient analysis of the process for many combinations of intermeshing parameters, selected at random. Multi-objective optimization is then realized over the obtained response variables. Presented example shows the potential of the suggested solution strategy.

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.