Determination of True Breaking Elongation of a Tensile Specimen

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Project Description

A reliable and accurate estimate of the critical loading, that causes the crack and failure of the engineering structure, is a difficult challenge for the engineering design. For this purpose, multiaxial analysis should be performed instead of uniaxial fracture analysis. Stress triaxiality is the relative degree of hydrostatic stress in a defined stress state and the triaxiality factor, which is the ratio of the hydrostatic stress to equivalent stress (von Mises in most cases), is used to predict the type of fracture in the structure subjected to multiaxial load.

To this aim, a fracture locus should be obtained for the material to define a relationship between fracture strain and triaxiality factor. A sample fracture (triaxiality) locus is presented in Figure 1. For the different regions in this locus, different experiments should be performed and the corresponding mathematical (fitting) models should be developed.

Graph with a blue curve for a triaxiality

Figure 1: Triaxiality locus

Main Goals

The main aims of this project are understanding the triaxiality behavior of the materials, searching and deciding the details of experiments to define triaxiality locus, collecting of triaxiality locus of different materials in the literature, selecting and use of a suitable, simple, practical, and applicable fitting model, and the estimation of the triaxiality locus by using very limited test results.


The methods, which are searched in the literature, are applied for sample cases. The parameters and variables, which have effects on the breakage mechanism, are determined and their effects are evaluated. With the knowledge gained in the previous steps, a suitable method to detect true elongation calculation are selected and improved. For a sample case, different fitting models are studied, and the hyperbolic-quadratic-hyperbolic fitting model (HQH3) is chosen. According to this fitting model, only three tensile tests (for different specimen geometries) are sufficient to obtain a reasonable triaxiality locus for the case of the very limited number of test results and correct definition of the mathematical constraints. The results are presented in Figure 2.

Graph with two curves as a result of a simulation

Figure 2: Test data and developed model results

Cooperation Partner


  • December 2020 - February 2021


Privatdoz. Dr. Eray Arslan

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