(i) Types of stress. Stress is the internal resisting force per unit area set up in a material when it is loaded, i.e. load divided by the original cross-sectional area (tensile or compressive stress = F/A, shear stress = F/A). Types:

• Tensile stress: pulls the material apart, tending to elongate it; set up by forces acting away from each other.
• Compressive stress: pushes the material together, tending to shorten it; set up by forces acting toward each other.
• Shear stress: acts parallel to a section, tending to slide one part over another (differential/bending shear).
• Bending stress: a combination, with one face in tension and the opposite in compression, producing internal moments (e.g. in a beam/shaft).
• Torsional (twisting) stress: a shear stress set up when a member is twisted, e.g. in a shaft; shear stress varies from zero at the axis to a maximum at the surface.
• Combined/complex stress arises when several simple stresses act together, e.g. a shaft under both torsion and bending.

(ii) Types of strain. Strain is the ratio of change in dimension to the original dimension, a dimensionless quantity describing deformation.

• Tensile strain: extension / original length (elongation) of a bar pulled.
• Compressive strain: contraction / original length of a bar compressed.
• Shear strain: transverse deformation angle (change in right angle) when a cube is sheared.
• Volumetric strain: change in volume / original volume.
• Elastic strain: the recoverable deformation - given by Hooke's law, strain proportional to stress below the proportional limit, and it disappears when the load is removed.
• Plastic (permanent) strain: deformation that remains after the load is removed, occurring above the elastic limit.
• Linear strain versus lateral strain; the negative ratio of lateral to longitudinal strain is Poisson's ratio.

• Tensile test: determine yield stress (0.2 per cent proof stress) and ultimate tensile strength and percentage elongation, confirming the grade (e.g. high-tensile or mild steel per classification requirement).
• Bend (transverse) test: a specimen is bent cold through a set angle without cracking, demonstrating ductility and freedom from inclusions/lamination.
• Impact (Charpy) test: toughness of the material at the service temperature.
• Hardness test: resistance to indentation/abrasion.
• Chemical analysis of carbon, manganese, sulphur, phosphorus (and alloying additions) to meet the specification and welding grade.
• Metallurgical/microscopic examination and ultrasonic/magnetic-particle non-destructive testing for lamination, cracks, inclusions or plate defects.
• Dimensional and surface checks against the mill/grade certificate, and verification of mechanical test certificates supplied with the steel.

The results verify the plate has adequate strength, ductility, toughness and weldability for use in the hull side shell.