Mechanical properties are physical properties that a material(metal) exhibits upon the application of forces. The mechanical properties of metals determine the range of usefulness of the material and establish the service life that can be expected. Mechanical properties are also used to help classify and identify the material. The mechanical properties of a material are not constants and often change as a function of temperature, rate of loading, and other conditions.
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Some of the important mechanical properties of metals are:
- Fatigue strength
it is defined as the ability of a material to withstand an applied load. There are numerous types of strength, each dependent upon how the load is applied to the material :
- Tensile strength
- Shear strength
- Torsional strength
- Impact strength
- Fatigue strength
The tensile strength of a material is described as the ability of a metal to resist failure when subjected to a tensile or pulling load. The tensile strength is usually in two different ways: ultimate tensile strength and yield strength. Both refer to different aspects of material behavior.
UTS (ultimate tensile strength) sometimes simply referred to as tensile strength relates to the maximum load-carrying capacity of that metal or the strength of that metal at the exact point when the failure occurs.
To define yield strength it is necessary to understand what is meant when a metal behaves “elastically”. Elastic behavior refers to the deformation of metal under load which causes no permanent deformation when the load is removed. For example, a rubber band will stretch under a load but returns to its original shape when the load is removed. When a metal is loaded within its elastic region it responds with some amount of stretch or elongation. In this range, the stretch is directly proportional to the applied load, so elastic behavior is referred to as linear. If metal is stressed beyond its elastic limit, it’s no longer behaves elastically, its behavior is now referred to as plastic it also implies the stress-strain relationship is no longer linear. Once the plastic deformation occurs the material will not return to the original length upon removal of the applied load. The point at which the material behavior changes from elastic to plastic is referred to as its yield point.
Ductility is a term that relates to the ability of a material to deform or stretch under load without failing. The more ductile the metal is, the more it will stretch before it breaks. A ductile material will bend before breaking, which is a good indicator that the metal’s yield point is being exceeded. Metals having low ductility fail suddenly in a brittle manner, without any warning. A metal’s ductility is directly related to its temperature. As temperature increases metal ductility increases and as temperature decreases ductility decreases. A metal with high ductility is referred to as being ductile while metal with low ductility is referred to as being brittle.
It is the ability of a material to resist indentation or penetration. The term of hardness is also referred to as stiffness or temper or resistance to bending, scratching, abrasion, or cutting. It is the property of a material, which gives it the ability to resist being permanent, deformed when a load is applied. The greater the hardness of the material, the greater the resistance it has to deformation. Hardness has been variously defined as resistance to local penetration, scratching, machining, wear or abrasion, and yielding. In general, different materials differ in their hardness; for example, hard metals such as titanium and beryllium are harder than soft metals such as sodium and metallic tin, or wood and common plastics.
Toughness is the ability of the material to absorb energy. Another common term is notch toughness, it differs from toughness in that it refers to the material’s energy-absorbing ability when there are surface flaws present whereas toughness refers to the energy absorption capacity of a smooth unnotched sample. The difference between metals of low and high toughness is that low toughness values define brittle behavior while high values of toughness values are related to ductile fracture.
The toughness of metal will change as the temperature is changed. In general, as the temperature is reduced the toughness of the metal decreases as well. If a material exhibits a high amount of notch toughness, this means it will perform well whether or not there is a notch present. However, if the material is notch sensitive, meaning that it exhibits low notch toughness, it could easily fail during impact or repetitive loading. In general, a metal’s notch toughness decreases as its hardness increases and its temperature is reduced.
The fatigue strength of a metal is defined as that strength is necessary to resist failure under repeated load applications. The endurance limit is the maximum stress at which no failure will occur, no matter how many cycles the load is applied.
metal fatigue is caused by a cyclic or repeating mechanical action on a member. That is, the load alternatively changes between and some lower stress or a stress reversal. This action can occur quickly as in the case of a motor’s rotation or slowly where the cycles could be measured in days. An example of fatigue failure would be the repeated bending of a motor shaft to produce a break. This type of failure usually occurs below the tensile strength of the shaft. The fatigue strength of carbon steel is roughly equal to half its tensile strength.
Fatigue strength like impact strength is extremely dependent upon the surface geometry of the member. The presence of any notch or stress risers can increase the stress at that point to above the metal endurance limit. Upon the application of sufficient numbers of cycles, fatigue failure will result.