Crack initiation and propagation

Crack propagation

Stage 1: crack is initially propagate slowly along the plane of crystal where shearing stress is high. It involves just a few grain and has flat fracture surface.

Stage 2: The crack propagates faster perpendicular to applied stress. It grows rapidly due to repeatation of stress and sharpening of crack at crack tip.

Stage 3: Finally the crack reaches its critical dimension and propagation to fast.

Factor that affect fatigue life:

1. Magnitude of stress
2. Quality of surface
3. Effect of temperature
4. Effect of chemicals
5. Effect of geometry I.e. dimension

Q.What is creep?

It is one of the type of failure of metals or materials. It is time dependent and permanent deformation of material when subjected to constant load at high temperature. Approximately 0.4timed the melting point of substance. Generally creep testing is required for the furnace. Creep testing is required for the materials like turbine blades, stem generator etc.

The different stages of creep are:-

1. Instantaneous deformation which is elastic in nature.
2. Primary creep in which slope of strain versus time decreases with time.
3. Secondary or steady-state, creep rate of straining is constant
4. Rapidly acceleration strain rate up to failure due to formation of internal crack, voids, grain boundary and separation.

A creep test made of putting a material to a constant load with keeping the temperature constant; deformation or strain which is produced from this mechanism plotted and measured as a function of total time. Almost all of the tests are the constant-load type, which give information of constant stress-strain tests. Which are used to provide a way to understanding the process of creep. By the applied of the load there is an instant deformation, which is shown in the figure is totally elastic. The creep curve made of three regions, each of these region have own strain–time feature. At first primary creep is occurred, represented by a continuously decreasing creep rate i.e. the slope of the curve decreases with time. Which implies that the material is feeling an extend in creep resistance as the material is strained. In the case of secondary creep, it is constant state creep, the rate is constant i.e. the plot becomes linear. Which is often the condition of creep in which the plot occurred in the longer duration.

By the process of balance between the competing processes of strain hardening and recovery the constancy of creep behavior can be explained. Due to this constancy of creep condition a material becomes softer and retains its ability to experience deformation.

At the last for third creep condition, there is an increase up to fracture. The term fracture is mostly termed rupture and results from which the microstructure of material changes. For example,formation of voids, scratches, crack, necking etc . Again for some tensile loads, in the deformation region the necking can occur. These all example shows to a decrease in the effective cross-sectional area and an increase in strain rate.

In the case orf metallic materials, most creep tests are applied in uniaxial tension using a material having the same structure as for tensile tests. For the next method, uniaxial compression tests are more suitable for brittle materials. By these method or process we can measure the all properties like intrinsic without the stress amplification and the crack propagation, as with tensile loads. We are mostly used Compressive test for the materials which have the criteria for length to diameter ratios ranging from about 2 to 4 for the right cylinders or parallelepiped.

For the most cases loading direction is independent with the creep properties for most materials. For creep test the most appropriate parameter may be the slope of secondary part of the creep curve; which is called the steady-state creep rate. It is considered as an important parameter which is used for the various sector such that in the long life application(power plant component that is scheduled to operate for several decades) without failure.

References:

Callister, W.D and D.G Rethwisch. Material Science and Engineering. 2nd. New Delhi: Wiley India, 2014.

Lindsay, S.M. Introduction of Nanoscience . New York : Oxford University Press, 2010.

Patton, W.J. Materials in industry . New Delhi : Prentice hall of India, 1975.

Poole, C.P. and F.J. Owens. Introduction To Nanotechnology. New Delhi: Wiley India , 2006.

Raghavan, V. Material Science and Engineering. 4th . New Delhi: Pretence-Hall of India, 2003.

Tiley, R.J.D. Understanding solids: The science of Materials. Engalnd : John wiley & Sons , 2004.