MECHANICAL PROPERTIES

TENSION TESTS

One of the most common mechanical stress–strain tests is performed in tension. As will be seen, the tension test can be used to ascertain several mechanical properties of materials that are important in design. A specimen is deformed, usually to fracture, with a gradually increasing tensile load that is applied uniaxially along the long axis of a specimen.

ASTM Standards E 8 and E 8M, ‘‘Standard Test Methods for Tension Testing of Metallic Materials.

stress σ is defined by the relationship

in which F is the instantaneous load applied perpendicular to the specimen cross section, in units of newtons (N) or pounds force (lbf), and A0 is the original crosssectional area before any load is applied (m² or  in²).

 

FIGURE 1(a) Schematic illustration of how a tensile load produces an elongation and positive linear strain. Dashed lines represent the shape before deformation; solid lines, after deformation. (b) Schematic illustration of how a compressive load produces contraction and a negative linear strain. (c) Schematic representation of shear strain _, where γ = tan θ. (d) Schematic representation of torsional deformation (i.e., angle of twist Φ) produced by an applied torque T.

FIGURE 2 A standard tensile specimen with circular cross section.

Engineering strain  ε is defined according to

in which l0 is the original length before any load is applied, and li is the instantaneous length. Sometimes the quantity li -l0 is denoted as Δl, and is the deformation elongation or change in length at some instant, as referenced to the original length.  strain is expressed as a percentage, in which the strain value is multiplied by 100.

COMPRESSION TESTS

Compression stress–strain tests may be conducted if in-service forces are of this type. A compression test is conducted in a manner similar to the tensile test, except that the force is compressive and the specimen contracts along the direction of the stress.

ASTM Standard E 9, ‘‘Standard Test Methods of Compression Testing of Metallic Materials at Room Temperature

SHEAR AND TORSIONAL TESTS

the shear stress τ is computed according to

ASTM Standard E 143, ‘‘Standard Test for Shear Modulus.