Material selection is a step in the process of designing any physical object. In the engineering design, Material Selection is an important aspect. Material of part depends on the requirements of the part, number of parts needed, and manufacturing process to be adopted to make the part. In the mechanics of materials, the strength of a material is its ability to withstand an applied load without failure or plastic deformation. The field of strength of materials deals with forces and deformations that result from their acting on a material.
Material failure is the loss of load carrying capacity of a material unit and is one of the most crucial problems to face for Mechanical Engineers.
Types of Material Failure:
Material failure can be distinguished in two broader categories depending on the scale in which the material is examined:
1. Microscopic failure
Microscopic material failure considers the initiation and propagation of a crack.
Failure criteria related to microscopic fracture: These criterion yield potential by introducing a scalar damage quantity, the void volume fraction of cavities and the porosity.
2. Macroscopic failure
Macroscopic material failure is defined in terms of load carrying capacity or energy storage capacity, equivalently.
Four categories of macroscopic failure criteria:
- Stress or strain failure
- Energy type failure (S-criterion, T-criterion)
- Damage failure
- Empirical failure
Brittle material failure criteria
Failure of brittle materials can be determined using following approaches:
Ductile material failure (yield) criteria
A yield criterion also expressed as yield surface, or yield locus. It is a hypothesis concerning the limit of elasticity under any combination of stresses.
Isotropic yield criteria
Isotropic yield criteria is based on the following theories:
- Maximum principal stress theory
- Maximum principal strain theory
- Maximum shear stress theory / the Tresca yield criterion
- Total strain energy
- Maximum distortion energy theory / octahedral shear stress theory
Anisotropic yield criteria
When a metal is subjected to large plastic deformations the grain sizes and orientations change in the direction of deformation, the isotropic yield criterion are unable to predict the yield behavior accurately.
Several anisotropic yield criteria have been developed to deal with such situations. Some of the more popular anisotropic yield criteria are:
Models for the evolution of the yield surface with increasing strain, temperature, and strain rate are used in conjunction with the above failure criteria for isotropic hardening, kinematic hardening, and viscoplasticity.
There is another important aspect to ductile materials – the prediction of the ultimate failure strength of a ductile material. Several models for predicting the ultimate strength have been used by the engineering community with varying levels of success. Such as, combination of porosity and strain to failure or a damage parameter.
Cost of materials plays a very significant role in their selection. The most straightforward way to weight cost against properties is to develop a monetary metric for properties of parts. Of course, cost per kg is not the only important factor in material selection. An important concept is ‘cost per unit of function’.
Optimizing complex combinations of technical and price properties is a hard process to achieve manually, so rational material selection software is an important tool.
The material selection primarily depends upon material’s properties and several other factors like availability, economic aspects, etc.
Properties of the Material
The properties, need to be considered as regards to material selection:
- Physical properties
- Mechanical properties
- Dimensional properties.
- Technological properties
- Acoustical properties
- Atomic properties
- Chemical properties
- Electrical properties
- Magnetic properties
- Manufacturing properties
- Optical properties
- Radiological properties
- Thermal properties
It is best to consider the alternatives from a wide gambit of materials, viz.
- Ferrous Materials as cast iron, cast steel, mild steel, alloy steel, carbon steel, tool steel etc.
- Non-Ferrous Materials and Their Alloys as plastics, ceramics, elastomers, composite materials, etc.
Availability of the Material
After selection of the material from consideration of properties, next step is to check availability:
- Whether material is readily available
- lead time for procurement
- Quantity required
- Nature of supplies
- Whether any special method required to manufacture.
The final step is to examine economics:
- The minimum cost
- Cost of a material consists of the 1) initial cost 2) processing cost and 3) maintenance cost.
- Reality often presents limitations, and the utilitarian factor must be taken in consideration. The cost of the ideal material, depending on shape, size and composition, may be prohibitive, and the demand, the commonality of frequently utilized and known items, its characteristics and even the region of the market dictate its availability.The appropriate material selection should be responsibility of the engineer to establish the performance requirements that the material must meet and the economic constraints.
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