ter(6): Mechanical Properties of Engineering Materials Principal mechanical properties of materials To understand and describe materials behaviour under external loads or mechanical stresses,we need first to describe the mechanical Properties. The definitions of the most useful and important mechanical properties will be explain below: strain It is the ratio between the change of length to the original length dimensionless value) AL/Lo'Where AL, Change in length La Original length, Stress: Is the intensity of internal D L Stress units Force unites Area Uni Kg/cm2, lb/in2, Tim where 1 Kg, 2.205 lb. and 1 in 2.54 cm So l Kg./Cm 14.223 lb/in and l Kg/cm2 10.0 Tlm Normal Stress:Is the stress normal to the section, and could be tension or compression stress: PIA Where: o Normal stress, P Applied load A Cross sectional 4- Shear stress: Is the tangential stress. Q A Where: Shear stress, o shearing(tangential force, A Cross-sectional area.