Materials are subjected to force or loads during their services. In such situations, it is necessary to know the characteristics of materials and to design the member from which it is made of such that any resulting deformation will not be excessive and fracture will not occur.
One of the most common mechanical properties is tensile property.
Elastic and plastic Deformation
When a piece of metal is subjected to a uni-axial tensile force, deformation of the metal occurs. If the metal returns to its original dimensions when the force is removed, the metal is said to have undergone elastic deformation.
If the metal deformed to such an extent that it cannot fully recover its original dimensions, it is said to have undergone plastic deformation.
Engineering Stress = F/A
Engineering Strain = Change in length/original length
Anelastic Behavior of Materials
Elastic Recovery is the time dependent. After load release some finite time is required for complete recovery of elastic strain with reversal of relaxation process. This time dependent elastic behavior is known as Anelasticity.
A viscoelastic material can be thought of as a material whose response is between that of a viscous material and an elastic material.
Several relaxation processes take place within a material in response to an externally applied stress. If the time scale of a relaxation process is too fast or very slow as compared to the time interval over which the stress is applied, the stress-strain relationship is essentially independent of time.
Conditions in which stress taken place:
(A) The C-atoms along the two contracting axes jump to occupy vacant positions along the elongated axis, as there is more space available there. This reduces the total distortional energy around the interstitial atoms. This jumping results in an additional stretching in the direction of the applied stress.
(B) If Fe with small quantity of C in solution be subjected to a number of alternating cycles of loading and unloading within the elastic region. If the time taken for each cycle is very small as compared to the relaxation time given above, the C-atoms will not have enough time to jump from the contracted axis to the elongated axis before the stress reversal takes place. Under such conditions, the C-atoms do not jump at all. The stress-strain curve simply corresponds to bond stretching.
(C) Finally consider the situation where the time taken for each cycle of loading and unloading is about the same as the relaxation time. The c jumping will continue to occur, as loading is done. The strain due to C Jumping will somewhat lag behind the strain due to bond stretching, which is instantaneous. Even after the maximum load has reached, the strain due to C jumping will continue to occur, resulting in further strain as function of time. Due to this effect, the streess-strain curve during loading does not coincide with the curve during unloading.
Fatigue Behavior of Materials
Fatigue is the lowering of strength of failure of a material due to repetitive stress, which may be above or below the yield strength. In many types of service applications, metal parts are subjected constantly to repetitive of cyclic stresses in the form of tension, compression, bending, vibration, thermal expansion and contraction, etc which will fail at a much lower stress than that which the part can withstand under the application of single static stress. These failures, which occure under repeated or cyclic stress, are called fatigue failures.
Types of Fatigue Failure
1. A tiny crack initiates or nucleates typically at surface, often at a time well after loading begins.
2. The crack gradually propagates as the load continues to cycle. During this stage of fatigue process, branch marks or striations are created as shown in.
3. A sudden fracture of the material occurs when the remaining cross section of the material is too small the applied load.
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