Introduction
The tensile dynamic curve is a graphical representation of the interplay between the strain, stress and strain rate or strain rate sensitivity of a material in the plastic working state. It is widely used to evaluate the dynamic behaviour of various metals and contribute to the establishment of effective reforming procedures, such as hot and cold working, which are usually employed to strengthen the material structure and improve the mechanical properties. The production of a tensile dynamic curve requires the evaluation of the strain rate history of the specimen under investigation during tensile deformation in conditions of constant temperature. In addition, the strain rate or strain rate sensitivity parameters of the material under investigation must be measured in order to construct the tensile dynamic curve.
Theory and Principles
The dynamic behaviour of a material is a manifestation of its strain rate sensitivity. The evaluation of the strain rate sensitivity of the material is based on the analysis of the tensile dynamic curve. This curve is obtained by plotting the dynamic stress (σd) along the strain rate (εd) on a graph. In order to construct this curve, the stress has to be measured when the strain rate is held constant. The rate of change of the stress is proportional to the strain rate and the strain rate sensitivity parameters.
When the rate of loading is high, the dynamic behaviour of the material is known as strain hardening or strain rate hardening and the tensile dynamic curve exhibits a curved shape. In this instance, the dynamic stress increases with an increase in the strain rate. In contrast, if the rate of loading is low, the dynamic behaviour of the material is considered strain-softening and the dynamic curve consists of a more linear form. In this case the dynamic stress decreases with the increase in the strain rate.
Dynamic curve
The structural and mechanical properties of materials are highly dependent on the shape and form of the dynamic curve. In this case, the dynamic curve for a material can be divided into three distinct regions, namely: i) straight line, ii) S-shaped or curved, and iii) plateau.
In the straight-line region, the material exhibits a linear relationship between the dynamic stress and strain rate. This section is usually encountered when the loading process is carried out at low strain rates, in temperatur. However, when the strain rate increases, the dynamic curve can take on a curved shape, which is associated with strain hardening. In this case, the dynamic curve is S-shaped or curved and the dynamic stress increases with increasing strain rate. This region is usually encountered when the loading process is performed at higher strain rates, often in the presence of elevated temperatures. The plateau region is typically encountered when the strain rate is further increased, often in conditions of elevated temperatures. Here, the dynamic stress remains nearly constant and the strain rate becomes very small.
Conclusion
The tensile dynamic curve plays an important role in evaluating the strain rate sensitivity of materials as well as determining the strain rate history of a specimen during a loading process. By analyzing the shape and form of the tensile dynamic curve, it is possible to determine whether a material is strain hardening or strain-softening and determine the effect of strain rate increase on the dynamic stress. Furthermore, understanding the dynamic behaviour of the material during tensile deformation can greatly improve the performance of materials in various applications, such as the production of automotive components, aircraft constructions and energy storage devices.
