Solid material is one of the four states of aggregation in which the material surrounding us can be. In this article, we consider what mechanical properties are inherent in solids, given the features of their internal structure.
What is solid material?
Perhaps, every person can answer this question. A piece of iron, a computer, cutlery, cars, planes, stone, snow are all examples of solids. From a physical point of view, the solid state of aggregation of matter refers to its ability to maintain shape and volume under various mechanical influences. It is these mechanical properties of solids that distinguish them from gas, liquid, and plasma. Note that the fluid also retains volume (is incompressible).
The above examples of solid materials will help to more clearly imagine what important role they play for human life and the technological development of society.
There are several physical and chemical disciplines that study the considered aggregate state of a substance. We list only the most important of them:
- solid state physics;
- deformation mechanics;
- science of materials;
- solid chemistry.
Solid material structure
Before considering the mechanical properties of solids, one should get acquainted with their internal structure at the atomic level.
The variety of solid materials in their structure is great. Nevertheless, there is a universal classification based on a criterion for the periodicity of the arrangement of elements of the body of elements (atoms, molecules, atomic clusters). According to this classification, all solids are divided into the following:
Let's start with the second. An amorphous body does not have any ordered structure. Atoms or molecules in it are located randomly. This feature leads to isotropy of the properties of amorphous materials, that is, the properties are independent of direction. The most striking example of an amorphous body is glass.
Crystalline bodies or crystals, unlike amorphous materials, have a spatial arrangement of structural elements. On a microscale, they can distinguish between crystalline planes and parallel atomic rows. Due to this structure, crystals are anisotropic. Moreover, anisotropy is manifested not only on the mechanical properties of solids, but also on the properties of electrical, electromagnetic, and others. For example, a tourmaline crystal is capable of transmitting only oscillations of a light wave in one direction, which leads to polarization of electromagnetic radiation.
Examples of crystals are almost all metallic materials. They are most often found in three crystal lattices: face-centered and body-centered cubic (fcc and bcc, respectively) and densely packed hexagonal (hcp). Another example of crystals is common salt. Unlike metals, in its nodes are not atoms, but chlorine anions or sodium cations.
Elasticity is the main property of all solid materials.
Applying even the smallest stress to a solid, we cause it to deform. Sometimes the deformation can be so small that you may not notice it. However, all solid materials are deformed when an external load is applied. If, after removing this load, the deformation disappears, then we speak of the elasticity of the material.
A striking example of the phenomenon of elasticity is the compression of a metal spring, which is described by Hooke's law. Through the force F and absolute tension (compression) x, this law is written as follows:
F = -k * x.
Here k is a certain number.
In the case of bulk metals, Hooke's law is usually written through the applied external stress Ο, relative strain Ξ΅ and Young's modulus E:
Ο = E * Ξ΅.
Young's modulus is a constant for a particular material.
A feature of elastic deformation, which distinguishes it from plastic deformation, is reversibility. Relative changes in the size of solids during elastic deformation do not exceed 1%. Most often they lie in the region of 0.2%. The elastic properties of solids are characterized by the absence of a displacement of the positions of structural elements in the crystal lattice of the material after the termination of the external load.
If the external mechanical force is large enough, then after the termination of its action on the body, you can see the residual deformation. It is called plastic.
Solids ductility
We examined the elastic properties of solids. Now we turn to the characteristics of their plasticity. Many people know and have observed that if a hammer is hit on a nail, it becomes flattened. This is an example of plastic deformation. At the atomic level, it is a complex process. Plastic deformation cannot occur in amorphous bodies; therefore, glass does not deform when it is struck, but is destroyed.
Rigid bodies and their plastic deformation property depend on the crystal structure. The irreversible deformation under consideration occurs due to the displacement of special atomic complexes, called dislocations, in the bulk of the crystal. The latter can be of two types (edge ββand screw).
Of all the solid materials, metals have the greatest plasticity, since they provide a large number of slip planes directed at different angles in space for dislocations. Conversely, materials having covalent or ionic bonds will be brittle. These include gems or the mentioned salt.
Fragility and toughness
If you constantly exert external influence on any solid material, then it will sooner or later collapse. There are two types of destruction:
The first is characterized by the occurrence and rapid growth of cracks. Fragile fractures lead to catastrophic consequences in production, so they try to use materials and their operating conditions, under which the fracture of the material would be viscous. The latter is characterized by slow crack growth and the absorption of a large amount of energy before fracture.
For each material, there is a temperature that characterizes a brittle-viscous transition. In most cases, a decrease in temperature transfers fracture from a viscous region to a brittle one.
Cyclic and continuous loads
In engineering and physics, the properties of solids are also characterized by the type of load applied to them. So, the constant cyclic effect on the material (for example, tension-compression) is described by the so-called fatigue resistance. It shows how many cycles of application of a specific voltage value the material is guaranteed to withstand without collapsing.
The fatigue of the material is also studied at constant load by measuring the strain rate over time.
Hardness of materials
One of the important mechanical properties of solids is hardness. It determines the ability of a material to impede the introduction of a foreign body into it. Empirically, determining which of the two bodies is harder is very simple. It is only necessary to scratch one of them with the other. Diamond is the hardest crystal. It scratches any other material.
Other mechanical properties
Solid materials have some other mechanical properties besides those noted above. We list them briefly:
- malleability - the ability to acquire various forms;
- ductility - the ability to stretch into thin threads;
- ability to resist special types of deformation, for example, bending or torsion.
Thus, the microscopic structure of solids determines their properties in many ways.