Thursday, 15 January 2009

mechanical properies and material testing

The mechanical properties of a material are used to determine its suitability for a particular application. A range of material tests is available to measure the properties.

The tensile test
The most comprehensive of all mechanical tests is the “tensile test”, which determines the strength of the material when subjected to a simple stretching operation. Typically, standard dimension test samples are pulled slowly and at uniform rate in a testing machine while the strain (elongation) and the engineering stress (applied force divided by the original cross-sectional area) are measured and plotted. A typical tensile test for a metal first stretches the material and then causes it to neck and fail.

The engineering stress-strain curve of a ductile material usually drops past the tensile strength point. This is because the cross-sectional area of the material decreases (necking). Because of the decreased area, a smaller amount of force is required to continue the material’s deformation. A plot of the true local stress vs. true strain, based on the changing specimen dimensions rather than the original dimensions, would continue to rise when necking occurs. However, these are rarely used because they are difficult to measure.
The stress-strain diagram
Plotting the applied stress versus the strain or elongation of the specimen shows the initial elastic response of the material, followed by yielding, plastic deformation and finally necking and failure. Several measurements are taken from the plot, called the engineering stress-strain diagram. These include:

Modulus of elasticity
The initial slope of the curve up to the elastic limit
A measure of the stiffness of the material
Related directly to the strength of the atomic bonds.

Yield strength, YS
The stress at which the specimen shows a consistent and measurable amount of permanent deformation.
For materials without a clear yield point, the YS is determined by constructing a line parallel to the initial portion of the stress-strain curve but offset by a strain of 0.002 from the origin. The intersection of this line and the measured stress-strain line is used as an approximation of the material’s yield strength. This is called the 0.02% offset yield.
Some materials do show an abrupt yield point. In plain carbon steels, the carbon steels, the carbon atoms may diffuse the dislocations in the material and pin them so that they cannot easily move. When the applied stress causes the dislocations to jump free of these points, they can move more easily. In many polymers a similar effect is produced when bonds between molecules break and they begin to move.

Tensile strength, TS(ultimate tensile strength, UTS)
The maximum stress applied to the specimen
YS or TS is a measure of the loading ability (strength) of the material.

The maximum amount of plastic deformation that the specimen can undergo without breaking. Ductile vs. brittle
Measured by the % elongation at failure (or the % Reduction in area)
%EL=(l_final-l_0)/l_0 ×100

Other properties
Modulus of resilience
The area under the linear part of the curve, measuring the stored elastic energy

Failure stress
The stress applied to the specimen at failure (usually less than the maximum tensile strength because necking reduces the cross-sectional area)

The total area under the curve, which measures the energy absorbed by the specimen in the process of breaking

Mechanical properties of some common meals:
Metal/alloy YS (MPa) TS(MPa) Ductility,(%EL)
Aluminum 35 90 40
Brass(Cu-Zn) 75 300 68
iron 130 262 45
steel 180 380 25
titanium 450 520 25

The hardness test
The hardness test offers the engineer a quick, inexpensive and nondestructive way to estimate the tensile strength of a specimen. Hardness tests all make a small (sometimes microscopic) indentation into the surface of a specimen, and then use the force applied and the size of the indentation to calculate a “hardness number”. The correlation between this value and the tensile strength allows this to be used as a quality control parameter. However, the relationship between hardness and tensile properties is unique for each class of materials. Two of the commonly used hardness tests are:

The Brinell hardness test
The Brinell hardness test utilizes a steel sphere which is usually 10mm in diameter. The sphere is forced into the surface of a material. Then, the diameter of the resulting impression is measured. The corresponding “Brinell Hardness number” is then calculated.

The Rockwell hardness test
The Rockwell hardness test utilizes two kinds of indentors. A small steel ball is used for soft materials and a diamond-shaped cone is used for hard materials. To perform the test, the indentor is pushed into the surface of the material being tested. The test machine measures the depth of penetration and automatically converts this data into a “Rockwell Hardness number”.
Empirical relationship between hardness and Ts for most steels:
TS (MPa)=3.45Xbrinell hardness number

Impact testing
It is important to examine a material’s reaction to short yet intense loads, because under such conditions the material may behave in a more brittle manner than is indicated from a simple tensile test. The charpy impact test is commonly used for this purpose. A notched bar is placed in the test machine, and then the hammer is allowed to fall and break it. The energy absorbed in fracturing the specimen is measured by the height to which the hammer rises.
This is a direct relationship between the energy absorbed in impact and the toughness measured as the area under the stress stain curve. Materials with a high toughness value absorb more energy in fracture, which may provide a margin of safety in real structures in the event of failure. Some newer cars use plastic body parts and bumpers which have a large range of elastic deformation so that fracture does not occur at all.

The mechanical properties of a material are used to determine its suitability for a particular application. It is convenient to break the properties, and the tests that measure them, into several types

Slow application of stress, as in the tensile test, allows dislocations time to move.
Rapid stress application, as in an impact test, measures the ability of the material to absorb energy as it fails
The materials response to the presence of cracks and flaws that act as stress concentrators is measured by fracture toughness
Repeated application of stresses below the failure stress determined in a tensile test can cause fatigue failure.
At high temperatures, materials deform continuously under an applied stress, as measured by the creep test.