Properties Of Materials
(toc)
1. Introduction
Selection of a material for a given job depends upon its physical and mechanical properties. Most structural materials are subject to external forces, which generate internal mechanical stresses. The reaction of the part to these stresses can be critical to its continued functioning. hence, it is important for the NDT personnel to know the normal material properties, as well as effect of discontinuity upon the material serviceability.
2. Types of Properties
The application for which a
material is used, determines which property is most important.
2.1: Chemical Properties
Chemical properties (reaction with other materials) are of interest, mainly because of the need for resistance to corrosion.
2.2: Physical Properties
Physical properties of materials are associated with their atomic structures - eg.. density, crystalline type, atomic spacing, specific heat, melting point, etc.
2.3: Mechanical Properties
Mechanical properties of materials like strength, hardness, are most important in manufacturing processes and for determining sizes and shapes necessary for carrying loads.
2.3.1: Tensile Strength
A stress strain diagram is used to describe many of the mechanical properties important in the strength of a material. It shows the stress-strain behavior of the material under gradually applied and increasing tensile stress. It indicates three regions:
a. Elastic Region at Low Stresses - indicates that the longitudinal strain produced by stresses is quite small and is proportional to the applied stress.
b. Plastic Region at Medium Stresses - that at a cert-am stress level an abrupt increase strum occurs and the material is said to yield.
c. Necking Region High Stresses - wherein, when the ultimate strength is reached, the material starts to neck into larger strains, until the material ruptures and breaks into parts.
Stains beyond the elastic limits, which result un residual strains on unloading are called inelastic or plastic strains. Materials undergo relatively large strain to rupture are referred to as “ductile”. Those which undergo little or no plastic strain, prior to rupture, are referred to as "brittle'.
2.3.2: Toughness and Notch-Toughness
The toughness of a
material is defined as the ability of an unnoticed member (e.g. a smooth round
bar) to absorb energy, when loaded slowly Notch toughness of a material. is
defined as the ability of a material to absorb energy in the presence of a
sharp notch, when loaded very rapidly with an impact load.
2.3.3 : Creep
Creep is the now of
material over a period of time, when under a load too small to produce any
measurable plastic deformation at the time of application. The simplest type of
creep test is made by just hanging a weight on the test specimen and observing
its elongation, as a function of time by using a microscope or other sensitive
detector of strain.
2.3.4: Fatigue
Fatigue testing determines the ability of a material
to withstand repeated applications of stress which in itself is too small to
produce appreciable plastic deformation. Fatigue, usually is a more critical
design criterion than any other, for the structural safety and reliability of
machinery or structural components.
2.3.5: Hardness
The hardness of a material is measured by hardness tester.
Three types of hardness test are the scratch, rebound and penetration tests.
Hardness measurements are extremely useful as a quick and rough indication of the
mechanical properties of a metal.