The mechanical properties of materials affect how they behave as they are loaded. Every mechanical system is subjected to loads during operation, it is important to understand how the materials that make up those mechanical systems behave.
Stiffness
It is defined as the ability of a material to resist deformation under stress. The resistance of a material to elastic deformation or deflection is called stiffness or rigidity.
Stiffness relates to how a component bends under load while still returning to its original shape once the load is removed. Since the component dimensions are unchanged after load is removed, stiffness is associated with elastic deformation. Read More
Elasticity
When the material is deformed under the action of external force, it tries to regain its original shape after removal of that force. This mechanical property of the materials is called elasticity.
Elasticity is the tendency of solid objects and materials to return to their original shape after the external forces (load) causing a deformation are removed. An object is elastic when it comes back to its original size and shape when the load is no longer present. Physical reasons for elastic behavior vary among materials and depend on the microscopic structure of the material. For example, the elasticity of polymers and rubbers is caused by stretching polymer chains under an applied force. In contrast, the elasticity of metals is caused by resizing and reshaping the crystalline cells of the lattices (which are the material structures of metals) under the action of externally applied forces.
The two parameters that determine the elasticity of a material are its elastic modulus and its elastic limit. A high elastic modulus is typical for materials that are hard to deform; in other words, materials that require a high load to achieve a significant strain. An example is a steel band. A low elastic modulus is typical for materials that are easily deformed under a load; for example, a rubber band. If the stress under a load becomes too high, then when the load is removed, the material no longer comes back to its original shape and size, but relaxes to a different shape and size: The material becomes permanently deformed. The elastic limit is the stress value beyond which the material no longer behaves elastically but becomes permanently deformed.
Ductility
Ductility is a measure of a metal’s ability to withstand tensile stress—any force that pulls the two ends of an object away from each other. The game of tug-of-war provides a good example of tensile stress being applied to a rope. Ductility is the plastic deformation that occurs in metal as a result of such types of strain. The term “ductile” literally means that a metal substance is capable of being stretched into a thin wire without becoming weaker or more brittle in the process.
Ductility is the ability of a material to be drawn or plastically deformed without fracture. It is therefore an indication of how ‘soft’ or malleable the material is. The ductility of steels varies depending on the types and levels of alloying elements present. An increase in carbon, for example, will increase the strength but decrease the ductility.
Ductility in metals relates closely to work hardening. So, under conditions where twinning contributes to apparent work hardening – irrespective of the mechanism – extended uniform elongations can be expected.99 In circumstances where the effective work hardening is lowered by twinning, the deformation will be uniform over a reduced range of strain.
Hardness
The hardness of a material is a measure of its resistance to penetration by an indenter. Hardness is also a measure of strength and often has the units of stress. The indenter is often fabricated from a hard material such as diamond or hardened steel. The tips of the indenters may be conical, pyramidal, or spherical in shape. The indenter tips may also be relatively small (nano- or micro-indenters) or very large (macroindenters). Since indentation tests are relatively easy to perform (macroindentations require only limited specimen preparation), they are often used to obtain quick estimates of strength.
Micro- and nano-indenters have also been developed. These enable us to obtain estimates of moduli and relative estimates of the strengths of individual phases within a multiphase alloy. However, due to the nature of the constrained deformation around any indenter tip, great care is needed to relate hardness data to strength.
Nevertheless, some empirical and approximate theoretical ‘‘rules-of thumb’’ have been developed to estimate the yield strength from the hardness. One ‘‘rule-of-thumb’’ states that the yield strength (or tensile strength in materials that strain harden) is approximately equal to one-third of the measured hardness level.
Since hardness tests are relatively easy to perform (compared to tensile tests), estimates of the yield strengths (or ultimate strength) are often obtained from hardness measurements.
Creep
Creep is the property of a material which indicates the tendency of material to move slowly and deform permanently under the influence of external mechanical stress. It results due to long time exposure to large external mechanical stress with in limit of yielding. Creep is more severe in material that are subjected to heat for long time.
Under certain force-temperature conditions, solids deform under constant stresses that is termed as creep. Creep is observed for a wide class of materials: metals, plastic, mountain rocks, concrete, and glasses. The aim of creep theory is to describe strains responding to various stress states, magnitudes of stresses, and temperatures. Creep theory appeared not long ago as an independent division of engineering mechanics.
The creep deformation, to a considerable extent, depends on the regime of loading, by which we understand how the stress grows from zero to the value that remains constant in time. Under some conditions, the primary portion is absent and the strain-time diagram is linear from the very beginning. Under other conditions, there is no steady-state creep at all, i.e. the primary portion of creep diagram transits immediately into the portion with increasing strain rate (the tertiary creep).
Castability
Castability (Fluidity) is the ability of the molten metal to flow easily without premature solidification is a major factor in determining the proper filling of the mold cavity.
The higher the castability of a molten metal, the easier it is for that molten metal to fill thin grooves in the mold and exactly reproduce shape of mold cavity, there by successfully producing the castings with thinner sections. Poor castability leads to casting defects such as incomplete filling or misruns especially in thinner sections of a casting. Because castability is dependent mainly upon the viscosity of molten metal, it is clear that higher temperature improves castability of molten metal and alloys, where as presence of impurities and non metallic inclusions adverse it.
Castability is nothing but producing a casting with minimum cost, defects and time. This can be done by the high compatibility between the product requirement and process castability.
Castability is the ease of forming a casting. Castability can be thought of as how easy is it to cast a quality part. A very castable part design is easily developed, incurs minimal tooling costs, requires minimal energy, and has few rejections.
Strength
Strength is the property of a material which enables it to resist fracture under load. This is most important property as it helps us to identify load value under which material will fail. The stronger the materials the greater the load it can withstand. Read More
Plasticity
Plasticity is a property of a material to undergo a non-reversible change of shape in response to an applied force. The theory of plasticity, being the section of continuum mechanics, is concerned with the analysis of stresses and elasto-plastic strains in a body.
The material undergoes inelastic strain when the external force is applied. Plastic deformation will take place only after the elastic range has been exceeded.
The material deforms irreversibly and does not return to its original shape and size, even when the load is removed. When stress is gradually increased beyond the elastic limit, the material undergoes plastic deformation. Rubber-like materials show an increase in stress with the increasing strain, which means they become more difficult to stretch and, eventually, they reach a fracture point where they break. Ductile materials such as metals show a gradual decrease in stress with the increasing strain, which means they become easier to deform as stress-strain values approach the breaking point.
Microscopic mechanisms responsible for plasticity of materials are different for different materials.
Malleability
Malleability is a physical property of metals that defines their ability to be hammered, pressed, or rolled into thin sheets without breaking. In other words, it is the property of a metal to deform under compression and take on a new shape.
A metal’s malleability can be measured by how much pressure (compressive stress) it can withstand without breaking. Differences in malleability among different metals are due to variances in their crystal structures.
Malleability in metals is an important physical property to study and adapted to, as per the desired result of malleability. It is defined as the ability of the metal to be hammered, rolled, pressed or drawn into sheets for the required applications. A tested and approved amount of pressure is applied to the metal at the required temperature settings in order to attain a certain physical shape or type. Under pressure, the metal deforms to take different shapes or forms without breaking.
Gold is regarded as a flowing metal and is the most malleable due to its particle placement in the liquid state. Gold is used in intricate, delicate and expensive jewelry and therefore the malleability allows great flexibility in designing a variety of accessories. Here, very small amounts of fine gold metal are usually used for malleability.
Tin and lead are soft solids that can be extensively treated to be spread out as various types of sheets or rolled into any kind of pipes owing to its existing physical state. Common use of tin-foil (extremely thin sheet of beaten tin) is seen in electrical systems. Lead in its malleable form is extensively used in power and communication cables and electrical batteries, it is one of the very few metals that is malleable but not ductile.
Wrought Iron and Steel are extremely malleable and allow great flexibility with acquiring different shapes as per the variety of applications. Iron in its original form or its alloy as Steel is deformed with the applied pressure to acquire and fit their usage. Thin wires, pipes, sheets of steel can be attained with the proper pressures and temperature environment. These malleable forms are also used as raw materials for a variety of other applications as well.
Copper and its alloys are readily malleable and coupled with their germicidal and corrosion-resistance properties allows their ease of usage in the plumbing and furniture industry. A wide range of copper fittings can be molded for the desired requirements due to the malleability of the metal. Also, vast variety of Copper knobs and handles are created with great ease and flexibility.
Brittleness
Brittleness identifies material easily broken, damaged, disrupted, cracked, and/or snapped. Brittleness can result from different conditions such as from drying, plasticizer migration, etc.
A material is brittle if, when subjected to stress, it breaks without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses (which do not deform plastically) and some polymers, such as PMMA and polystyrene.
Many steels become brittle at low temperatures (see ductile-brittle transition temperature), depending on their composition and processing.
When used in materials science, it is generally applied to materials that fail when there is little or no plastic deformation before failure. One proof is to match the broken halves, which should fit exactly since no plastic deformation has occurred.
Formability
Formability refers to the ease with which a material can be formed while satisfying quality requirements. In effect, it refers to a material’s ability to undergo plastic deformation. Therefore, as already mentioned, ductile materials tend to be more suited for forming processes. However, two major factors affect the formability of a material. As already mentioned, heat is often used in forming processes, that is, hot rolling. This is because many materials have good formability at elevated temperatures, but poor formability at room temperatures. Therefore, the first major influence on formability is the temperature at which forming is undertaken.
Similarly, many materials have good formability when the rate of deformation is low, that is, slow loading. However, these same materials may react in a brittle fashion when undergoing a high rate of deformation, that is, impact or sudden loading. Therefore, the other major influence on the formability of a material is the rate at which it is deformed. Thus, there is no hard and fast rule that guides the formability of material due to the variations in forming processes and therefore each combination of material and process must be considered individually.
Weldability
The term weldability has been coined to describe the ease with which a metal can be welded properly.
In terms of weldability, commonly used materials can be divided into the following types:
- Steels
- Stainless steels
- Aluminium and its alloys
- Nickel and its alloys
- Copper and its alloys
- Titanium and its alloys
- Cast iron
Several factors influence the weldability of metals. Below is a list of the most common factors.
