# Strain Rate Dependencies and Competitive Effects of Dynamic Strength of Some Engineering Materials
## 1 Introduction  
The comparison of the mechanical parameters of materials usually begins with a comparison of stress-strain curve characteristics obtained by testing the materials for uniaxial tension or compression. These characteristics, for example, yield strength, tensile strength and modulus of elasticity are recorded in tables with the main characteristic properties of materials and used in practice to calculate the details of mechanisms and structures. However, such an approach for assessing the strength properties of materials can usually be applied only for static or quasi-static loads. In this case, stress-strain curves exhibit stable values of the strength characteristics of the material, so that they can be taken as a material parameter. In the case of high-speed or shock loads, the ultimate strength and the yield strength exhibit unstable values [1; 2; 3; 4; 5]. Experiments with dynamic loads show a number of effects that are not explained by traditional notions of material strength, for example, the strain rate dependencies of the strength characteristics. A changein the strain rate of a material by six orders of magnitude can lead to a doubling of these strength characteristics. This can occur for various types of structural materials, including rocks and concrete [2], metals and alloys [4], polymers [5] and so on. Thus, under dynamic loads, the strength of a material can no longer be characterized by a specific value of the critical stress. That is why, in view of the needs of modern technology, the problems of the behavior of materials and structures under conditions of fast high-intensity mechanical loads are fundamental for engineering applications. However, for the current state of dynamic fracture mechanics, there is a lack of common universal methods and approaches that could work effectively for a wide range of applied loads. One of the ways to characterize the strength of a material under dynamic loads is to construct a diagram of critical stresses in a wide range of strain rates [6; 7; 8; 9; 10]. Such rate dependencies are often considered as a function of a material [2; 11; 12]. At the moment, this way has disadvantages, including the lack of generally accepted approaches to both dynamic testing methods and methods for evaluating the results of such tests [2; 10; 13; 14; 15; 16; 17; 18]. This leads to dissimilar and sometimes contradictory results that make it difficult to develop a general theory of dynamic strength of materials. In addition to a wide variety of constructional materials, their properties can also be quite diverse, depending on, for example, heat treatment or moisture saturation. In the case of static and slow loads, this effect is well understood. The dynamic response of a particular material is usually investigated for a specific state or environmental condition. After that, it is assumed that the change in the obtained strain/stress dependence of critical stresses for other states of the material will take place analogously to quasi-static loads. For example, an increase in the strength in quasi-statics will lead to a similar increase in the dynamic strength over the entire range of loading rates. However, a number of studies show that this approach is not always justified [17; 18]. Materials can have different strain rate sensitivities, meaning their advantages can vary with the rate of loading. Due to the lack of parameters that characterize the response of a material to dynamic loads, the advantages of one material over another in the range of dynamic loads are difficult to assess at the design stage of structures. There is an obvious need to develop approaches that use new principles to predict dynamic strength characteristics. One promising approach is the structural-temporal approach based on the incubation time criterion [19; 20; 21]. The approach suggests the introduction of the characteristic of dynamic strength of a material with the dimension of time.