Mechanical Properties of Materials

Of the many properties of materials the mechanical ones are of great significance, since they deal with the principal phenomena regarding stability under force. Deformation under applied forces and the fracture of materials depend on their structure. The macroscopic responses of materials to the acting forces may result in their changing shape or even disintegrating, if these forces are sufficiently large. Interatomic forces must be overcome by external forces in order to cause shape changes in a material, which may eventually lead to its separation into two or more parts, depending on the atomic forces which resist any structural change, either in shape or dimension. The overall macroscopic behavior and the changes occurring in materials are inspected, tested experimentally and described in terms of the acting force per unit area, namely stress and the displacement per unit distance or strain. In a perfectly ideal material, free of lattice defects (which, in reality, does not exist, except in the form of whiskers), tremendous forces are required to cause the above changes. Real crystals contain various defects; lattice defects, such as dislocations, are responsible for the ease of deformation, which may often be observed in functioning machine elements. Mechanical engineers are expected to prevent this from occurring. To this end, laboratory tests must be performed in order to realize the practical potential of a given material of interest. In this chapter, a discussion of the mechanical behavior of materials on a macroscopic scale is described as observed during laboratory experiments intended to forecast the actual performance in real service. (An understanding of their real behavior will be the subject of Chap.​ 2.) The observed behavior, as revealed by the various laboratory tests, will be considered in terms of dislocations, providing a basic conceptual framework for the mechanical properties of materials.

When exposed for the first time to materials engineering, students often ask: “Why study dislocations? Why can’t we get an engineering diploma without making this effort?” Such questions did not cross the minds of Taylor, Orowan and Polanyi, the fathers of modern dislocation theory in 1934, when they independently suggested their novel theory of dislocations, following concepts developed by Volterra in 1905. Their basic insights were crucial for the development of the modern science of structure, for the understanding of structural properties and, in particular, for the essential concept of the deformation of materials. A dislocation, due to its extent, is considered to be a line defect and is one of the various types of defects found in materials which determine each and every property of a crystal. Briefly, materials are not perfect and contain a variety of defects. These defects, that determine the properties of a material, are:

On the basis of the first two chapters, it is clear that deformation in materials encompasses the following:

The strengthening of materials is of great importance for engineering applications. Construction parts are designed not only to endure the anticipated forces, which are intentionally applied (those they are expected to withstand while in service), but also any sudden, short-duration forces that might cause catastrophic failure, if not taken into account. In order to avoid the probability of such failure, liberal safety factors are generally adopted by designers. Their approach is to strengthen materials beyond the magnitude which would be sufficient to prevent failure, even if a steady force was exerted during the entire period of their use. This extra strength value constitutes the safety factor required for ensuring the safe use of a construction part, even in the event that a sudden force of larger magnitude appeared during service. There are several mechanisms by which materials may be strengthened, listed below:

In the previous section, the time element was not considered in the determination of the stability of materials exposed to a continuous force. However, this aspect is of considerable importance in designing structural components that are required to maintain dimensional stability over a long period of time while in service.

The most common failure that occurs in materials, such as metals, is caused by fatigue. The simplest way of looking at fatigue is by considering a specimen which is being repeatedly stressed under tension and compression. Not only tensile stresses that are repeatedly applied can cause fatigue failure, but any force which is acting in a reverse direction may ultimately result in such a failure. Loading a test specimen repeatedly by applying a force acting axially, torsionally or flexurally can induce fatigue failure.

Atomic cohesion is the bond between atoms, holding them together to form an aggregate that does not disintegrate under the normal conditions characteristic of that specific material. Hence, a short look at the essentials of cohesion aids in understanding fracture, which occurs when a force of a certain magnitude is applied against the atomic bonding of the atoms to cause the disintegration of a material. Those forces that hold the groups of atoms or molecules of a substance together are called ‘bonds’. The formation of bonds between atoms is mainly due to their tendency to attain minimum potential energy, thus reaching a stable state. In solid material, it is usually assumed that two types of forces act between the atoms: (a) an attractive force, which keeps the atoms together, forcing them to form solids and (b) a repulsive force, which comes into play when a solid is compressed. Figure 7.1a shows the concept of cohesion, based on the relation below, graphically:

Throughout this book, there has been frequent discussion about the effect of size on the mechanical properties of materials. Usually, strength properties increase with decreasing dimensions, while ductility decreases. Decreasing the dimensions of a material may decrease the size of the grains in polycrystalline materials. The size of single crystals depends on their growth conditions, but, also in this case, decreased size has the same influence on the mechanical properties. The expectation of improved mechanical characteristics, especially in the submicron/nanometer range, however, must be supported by experimental evidence. Experimental evidence has, indeed, indicated the outstanding mechanical properties of nanocrystalline (NC) materials that often show: superstrength, superhardness, improved specific strength and tribological performance (as attested in the literature). This pattern of reduced ductility with increased strength is also indicated in materials having small dimensions; however recently, some cases of substantial ductility were reported in superstrong NC materials undergoing 100% elongation or more without failure. These reported properties, the unique combination of high strength and good ductility, make such materials ideal for applications in a wide range of fields, such as the aviation, automotive and electronics industries, to name just a few. The aim of this chapter is to provide an overview of some of the mechanical properties discussed thus far regarding materials with small dimensions and to characterize their observed behavior.