Dragiša Vilotić

The influence of torsional and tensile pre-straining on the validity of ductile fracture criteria

Continuous loading and smooth strain paths are usually used to obtain data for empirical ductile fracture criteria or to verify such criteria. It is of interest to understand whether or not these ductile fracture criteria are valid for discontinuous paths. To this end, an experimental program is carried out and its results are combined with two conventional empirical ductile fracture criteria. It is shown that the criteria are not applicable for discontinuous loading paths. Based on the experimental results obtained and experimental results available in the literature it is hypothesized that a measure of rotation of the principal stress (or principal strain rate) axes relative to the material affects the initiation of ductile fracture.

Reducibility of Stress-Based Workability Diagram to Strain-Based Workability Diagram

The strain-based and stress-based workability diagrams are often used to predict the initiation of ductile fracture in metal forming. The strain-based workability diagram is restricted to free surface fracture and postulates that the initiation of fracture is independent of the strain path. It is shown in the present paper that under these conditions the strain-based workability diagram is identical to the stress-based workability diagram. Using an available stress-based workability diagram the strain-based workability diagram is found in a much larger domain in the space of two in-surface principal strains as compared to the typical domain covered by standard tests used to determine strain-based workability diagrams. Two feasible tests are designed to determine the complete strain-based workability diagram.

Experimental Tooling for Contact Stress Measurement in Bulk Metal Forming Processes

Experimental Tooling for Contact Stress Measurement in Bulk Metal Forming Processes Knowledge of contact stresses in metal forming operations is essential not only for optimal die design, but also for theoretical and numerical analysis of the metal forming processes. Determination of contact stresses can be performed theoretically and experimentally. This paper presents a tool construction with built-in pin-load cells for experimental determination of contact stresses and friction coefficient at upsetting of prismatic specimen with cylindrical dies. This construction allows measurement of contact stresses in any point of the contact surface of specimen, due to adjustable location of pin-load cells. The adjustments are performed by turning cylindrical dies around its own axis in combination with the change of axial position of the specimen. Experimental determination of contact stresses in upsetting of Ck35 prismatic specimens using cylindrical tools was performed at the Laboratory for technology of plasticity, FTN Novi Sad. Alati Za Eksperimentalno Određivanje Kontaktnih Napona u Procesima Zapreminskog Plastičnog Deformisanja Određivanje relevantnih parametara procesa po fazama obrade plastičnim deformisanjem bazirano je na detaljnoj analizi napona i deformacija. Metode određivanja napona i deformacija dele se na teorijske, eksperimentalne i numeričke. U ovom radu prikazana je metoda eksperimentalnog određivanja kontaktnih napona pomoću merne čivije u procesu sabijanja prizmatičnog pripremka cilindričnim alatima. U radu je prikazana detaljna konstrukcija alata sa ugrađenim mernim čivijama za određivanje kontaktnih napona i koeficijenta trenja pri sabijanju cilindričnim alatima. Ovakva izvedba alata omogućuje merenje kontaktnih napona u bilo kojoj tački kontaktne površine, zbog mogućnosti promene početnog položaja merne čivije rotiranjem cilindričnih pritiskivača u kombinaciji sa aksijalnom promenom položaja pripremka. Eksperimentalna ispitivanja kontaktnih napona u procesu sabijanja prizmatičnog pripremka od Č1431 su izvedena su u Laboratoriji za tehnologije plastičnog deformisanja na Fakultetu tehničkih nauka u Novom Sadu. Provera tačnosti prikazane metode izvršena je na osnovu dijagrama deformacione sile određene direktnim merenjem tokom sabijanja i integracijom eksperimentalno određenih kontaktnih napona. Metoda merne čivije omogućuje precizno određivanje kontaktnih napona u procesima plastičnog deformisanja, uz uslov da postoji mogućnost ugradnje mernih elemenata u radne elemente alata.

Material formability at cold upsetting of cylinder by different plates as a base for metal forming technology design

Upsetting of cylinder by flat dies is one of the elementary bulk metal forming operation. But, upsetting of cylinder can also be performed by the dies different than flat, which influences the stress-strain state and material formability. This paper describes the formability analysis at cold upsetting of cylinder and hollow cylinder by different plates (flat plates, hollow plates and hollow plates whit concentric channels). The aim of this investigation is determination of influences of die and workpiece geometry on stress-strain state and material formability. The obtained results are integrated in formability limit diagram, which represents relationship between limit strain (ϕe1) and stress indicator (s). Experimental investigations have been performed on steel specimens C4146 (EN: 100Cr6). The determined diagram is a base for metal forming technology design, as it enables to predict damages of material structure during plastic deformation.

Using the Upper Bound Technique for Calculating the Strain Rate Intensity Factor

The strain rate intensity factor is the coefficient of the leading singular term in a series expansion of the equivalent strain rate in the vicinity of maximum friction surfaces. This factor can be used to describe the generation of fine grain layers in the vicinity of friction surfaces in metal forming processes. However, a difficulty is that the strain rate intensity factor follows from singular solutions and commercial finite element packages are not capable of finding this factor. In the present paper, the upper bound technique is used for this purpose. The kinematically admissible velocity field chosen accounts for the exact asymptotic expansion of the equivalent strain rate. Therefore, an approximate value of the strain rate intensity factor can be found from this field.

A Study on the Generation of a Fine-Grained Layer in Upsetting Between Flat and Conical Dies

A narrow fine-grained layer is generated in the vicinity of frictional interfaces in many manufacturing processes. The majority of previous studies on this topic is devoted to machining. The present is paper concerned with the generation of a fine-grained layer in upsetting of hollow cylinders between flat and conical dies. The layer occurs in the vicinity of the friction surface between the specimen and the conical die. The die angle varies to change the state of stress and strain near the friction surface. No lubricant is used to increase the friction stress. Based on a proposed criterion that defines the fine-grained layer, it is demonstrated that its thickness depends on the die angle. Moreover, the thickness varies along the friction surface. A general continuum mechanics model is proposed to connect the thickness of the fine-grained layer and the strain rate intensity factor. The model is based on dimensional analysis and a number of general assumptions. The strain rate intensity factor in the upsetting process is determined by an approximate method. Then, the experimental results are used to find some input parameters of the theoretical model.

Effect of a geometric singularity on a surface on ductile fracture

An experimental technique is proposed and applied to reveal a possible effect of a geometric surface singularity on ductile fracture during metal forming. The conditions of ductile fracture at the points of geometric singularity are shown not to be predicted by a standard workability diagram.

Effect of Intensive Plastic Deformation Near Frictional Interfaces on Ductile Fracture

A layer of intensive plastic deformation often appears in the vicinity of frictional interfaces in metal forming processes. The paper presents a study to reveal a possible effect of intensive plastic deformation in such a layer on ductile fracture. To this end, an upsetting test of special design is used to move the site of ductile fracture initiation to the friction surface independently of the effect of intensive plastic deformation on the occurrence of ductile fracture. Experimental results obtained are compared to the theoretical prediction based on a conventional empirical ductile fracture criterion. It is shown that there is some deviation of the fracture conditions predicted theoretically from the experimental results.