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Mechanical Material Properties
Tensile Strength
This is the ability of a material to withstand tensile (stretching) loads without rupture occurring. The material is in tension.
Explanation Tensile Strength
Compressive strength
This is the ability of a material to withstand compressive (squeezing) loads without being crushed or broken. The materials is in compression.
Explanation Compressive Strength
Shear Strength
This is the ability of a material to withstand offset or transverse loads without rupture occurring. The rivet connecting the two bars shown is in shear whilst the bars themselves are in tension. Note that the rivet would still be in shear if the bars were in compression.
Rivet connecting the two bars in resisting shear
Rivet connecting the two bars has failed in shear
Toughness: impact resistance
This is the ability of a material to resist shatter. If a material shatters it is brittle (e.g. glass). If it fails to shatter when subject to an impact load it is tough (e.g. rubber). Toughness should not be confused with strength. Any material in which the spread of surface cracks does not occur or only occurs to a limited extent is said to be tough.
Explanation Tough Material
Explanation Plasticity
Ductility
This is the term used when plastic deformation occurs as the result of applying a tensile load. A ductile material combines the properties of a plasticity and tenacity (tensile strength) so that it can be stretched or drawn to shape and will retain that shape when the deforming force is removed. For example, in wire drawing the wire is reduced in diameter by drawing it through a die.
Explanation Ductility
Malleability
This is a term used when plastic deformation occurs as the result of applying a compressive load. A malleable material combines the properties of plasticity and compressibility, so that it can be squeezed to shape by such processes as forging, rolling and rivet heading.
Explanation Malleability
Hardness
This is the ability of a material to withstand scratching (abrasion) or indentation by another hadrd body. It is an indication of the wear resistance of a material.
The Ductile – Brittle Transition
The increase in yield stress associated with low temperature or high strain rates can results in a material changing its mode of fracture from ductile to brittle and this is very important when selecting materials for engineering purposes.
- The plot of brittle fracture stress ( ) and the yield stress ( y ) as a function of temperature or strain rate can explain the ductile to brittle tranisition.
- The curve for brittle fracture stress rises slightly because surface energy increases as temperature decreases.
- The yield stress curve shows the strong temperature dependence.
- The brittle fracture stress and the yield stress curves are intersect with each other then a vertical line is drawn at the point of intersection.
- This is called the ductile brittle transition temperature.
Factors Affecting Mechanical Properties The mechanical properties of materials are affected by various factors
- Grain size
- Heat treatment
- Atmospherics exposure
- Low and high temperature
Effect of Grain size
The metals are composed of crystals (or) grains. If the grain size of a metal is small, it is called a fine grained metal, on the other hand, when the grain size is comparatively large, then it is called a coarse grained metal. A fine grained metal has a greater tensile and fatigue strength. It can be easily work hardened. A coarse grain causes surface roughness. Coarse grain metal is difficult to polish. Course grained metal is less tough and has a greater tendency to cause distortion than the fine grained metal. Coarse grained metal has a better workability, hardenability and forgeability. At Room Temperature the grain boundary is more for fine grained metals. Therefore it has higher strength and hardness than the coarse grained metal. At higher temperature coarse grained materials have better creep resistance than the fine grained ones.
The strength of the metal is inversely proportional to the square root of the grain size
Effect of low temperature
- Decrease in temperature there is an increase in the tensile strength and yield strength of all metals.
- Alloys of nickel, copper and aluminium retain most of their ductility and toughness at low temperature.
- For mild steel, the elongation and reduction in cross – sectional area is satisfactory upto - 180°c but after that it goes down to a large extent.
- Near absolute zero temperature many metals exhibit the phenomenon of super conductivity
- Below - 100°c non-ferrous metals show better properties than ferro metals.
Low temperature causes low thermal vibrations and lattice parameters are stabilized.
Effect of high temperature
- Field stress and ultimate tensile strength decrease with rise in temperature
- Stiffness and fracture stress of many metals also decrease with increasing temperature
- At high temperatures, the toughness of steel is reduced.
- At high temperature, creep takes place and the material fails even at a very small stress.
- Due to rise in temperature, there is a corresponding rise in thermal vibration of atoms causing changes in structural properties.
