Mechanical Properties and their Determination
Undoubtedly, the most commonly performed test on engineering materials is the static tension test. From this test many mechanical properties of material are evaluated and understanding about its behaviour is developed. At the outset the distinction between static and dynamic load must be understood carefully. A "static load" is the one which changes very slowly with respect to time during its application. Once applied, the static load remains unchanged.
Very common static tests which are performed for evaluation of mechanical properties are axial tension, compression loading, flexural loading or torsional loading. Axial load is applied along the axis of specimen. Flexural loading is achieved by application of load perpendicular to the axis while torsional loading result from eccentric transverse forces which do not cause bending or change in length. Direct shearing test is also performed sometimes but is not a very popular test. In all these the load is slowly increased from zero till failure occurs; rate of loading being such that definition of static load is not violated. Hardness is a very important mechanical property which is also determined under static load by indentation. Hardness and tensile strength are related. Fatigue is behaviour of material under load or stress which keeps changing with time and failure depends upon stress level and number of stress reversal. Creep is yet behaviour under statically applied load over a long period of time. Temperature plays an important role in deciding this behaviour.
Objectives
After studying this unit, you should be able to
- Understand the effect of force on solids qualitatively as well as quantitatively,
- Differentiate between the true stress and true strain,
- Analyse the compression test,
- Know universal testing machine,
- Understand tensile strengths and effect of strain rate,
- Know different impact tests,
- Explain temperature effects,
- Understand hardness and fatigue, and
- Identify material problems at elevated temperatures.