Solids

• A material undergoing elastic deformation will return to its original dimensions when the deforming force is removed. A plastic material will remain deformed
• A rubber band is often called an elastic band. It can be stretched to several times its normal length and will return to its starting length when the tension is removed.
• Some materials can behave in an elastic or plastic manner depending on the nature of the deforming forces
• hardness is a surface phenomenon. The harder the material, the more difficult it is to indent or scratch the surface. A simple method to compare the hardness of two materials involves finding out which scratches the surface of the other
• The mohs scale of hardness grades minerals from the softest which is rated as 1 on the scale to the hardest which has a rating of 10. R is the relative hardness o the minerals
• In engineering, the hardness of a metal is measured using the Brinell Hardness Number (BHN), which is the ratio of the load applied to a small steel sphere to the area of the indentation it makes in the surface of the metal being tested
• A stiff material exhibits very small deformations even when subjected to large forces. The stiffness of a material is measured in terms of its modulus of elasticity
• A tough material is able to absorb the energy from impacts and shocks without breaking. Tough metals usually undergo considerable plastic deformation in order to absorb the energy
• A brittle object will shatter or crack when subjected to dynamic shocks or impacts. brittle materials undergo little or no plastic deformation before breaking
• An object is strong if it can withstand a large force before it breaks. The strength of a material will depend on its size
• The strength of a material is therefore defined in terms of its breaking stress where stress is the force per unit cross-sectional area
• A malleable material can be hammered out into thin sheets
• Ductile materials can be drawn into wires. Although most ductile materials are also malleable, the reverse is not always true. Many malleable materials will shred or break when extended
• The elastic region is similar to that seen in the tensile tests. The bonds are 'squashed' as the atoms are pushed together and the particles move back to their original position when the force is removed.
• The energy density is the work done in stretching a specimen (the strain energy stored) per unit volume of the sample. For a wire that obeys Hooke's law: energy density = work done/volume = 0.5 x FΔl/Al = 1/2 stress x strain. This is represented on the stress-strain graph by the area between the line and the strain axis
• The hysteresis loop for a stress-strain graph represents the energy per unit volume transferred to internal energy during the load-unload cycle