Change of Specimen Temperature during the Monotonic Tensile Test and Correlation between the Yield Strength and Thermoelasto-Plastic Limit Stress on the Example of Aluminum Alloys.

Adam Lipski

doi:10.3390/ma14010013

Abstract

This paper presents an attempt to generalize the description of the course of specimen temperature changes during the tensile test and to connect the value of the thermoelasto-plastic limit stress with the value of a clear (physical) or proof strength (offset yield strength) on the example of tests of the following aluminum alloy sheets used in Poland for airplane structures: 2024-T3 and D16 in three grades: D16ATV, D16CzATV, and D16UTV. A thermographic camera was used for specimen surface temperature measurement during the tensile test. The Portevine–Le Chatelier effect (the so-called PLC effect) was observed for tests of specimens cut from sheet plates, which was strongly reflected in the temperature fluctuations. The course of temperature change during tensile tests was divided into four characteristic stages related to the occurrence of a clear or offset yield strength. It was found that if there is a clear yield strength, the value of the thermoelasto-plastic limit stress was greater than this yield strength. If there was an offset yield strength, the value of the thermoelasto-plastic limit stress was lower than this yield strength. The differences in the aforementioned values of individual yield strengths did not exceed several percent. Thus, it can be concluded that the thermoelasto-plastic limit stress determined on the basis of the course of specimen temperature changes during the tensile test is well correlated with the value of the yield strength of the material.

Highlights

Received: 15 November 2020Accepted: 17 December 2020Published: 22 December 2020Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.license
Comparison of the presented courses shows, first of all, that the tested sheet plate demonstrates a clear yield strength in the sheet plate rolling direction, unlike in the direction perpendicular to the sheet plate rolling direction, where it does not show such yield strength. This fact is reflected in the temperature course at the moment of specimens macroscopic plastic strain occurrence: there is a smooth transition through the minimum temperature for specimens with no clear yield strength and abrupt transition for samples characterized by a clear yield strength
On the basis of the above examples of monotonic tensile of specimens made of airplane aluminum alloy sheets, the temperature change during loading can be divided into several phases (Figure 14): Phase I

Summary

Introduction

Received: 15 November 2020Accepted: 17 December 2020Published: 22 December 2020Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.license Different mechanisms of microscopic material deformation, associated mainly with its dislocation structure, manifest in the macroscopically perceptible strain [1]. The relationship between mechanical properties and the material structure, dislocation as well as their effect on the material behavior in engineering structures is described in more detail, e.g., in the study [2]. This study focuses mainly on the macroscopic relationship between strain and stress and the associated change of the tested object temperature. An increase in load and strain in the case of material in an elastic state results in a temperature drop in line with the thermoelastic effect, while a drop in load increases the temperature up to the initial value

Methods

Discussion

Conclusion