Compression And Torsion Test

Compression and Torsion Test

Stress

Strain

Compression Test • Compression Behavior of metals is of interest in metallurgical forming industries. • Rolling, forging etc involves compressive deformation of metals.

Suitability of Test • Metals which have low rates of strain hardening. (This leads to early necking) • For metals that fracture under low stress in tension. • Where short specimen is required • For brittle materials

Disadvantage of Compression Test • Under compression material behavior is elastic/plastic. • Cross- sectional area and work hardening increases as the height decreases. • Therefore load requirement exceeds that available from even the largest equipment. • Friction causes barreling of the specimen

Shape and Size of Specimen • Square or rectangular section is often used. • Metallic materials circular specimen is always preferred over other shapes. • Dimension of metallic specimen ASTM(E-9) • Type dia (mm) Height (mm) Short Medium Long

30 13-30 20-32

27 39-90 160-320

Modes of Deformation in Compression Testing

• Buckling, when L/D > 5. (b) Shearing, when L/D > 2.5 (C) Double barreling, when L/D>2.0 friction is present at the contact surface. (d) Barreling, when L/D < 2.0 friction is present at the contact surfaces e)Homogenous compression, when L/D <2.0 no friction is present at the contact Surfaces (f) Compressive instability due to worksoftening

Bauschinger effect • Work piece is subjected to tension and then to compression or vice versa. e.g • Bending and unbending. • When a material is subjected to tensile stress up to plastic range and then unload and apply load in compression. • The yield stress in compression is lower than that in tension.

Bauschinger effect

• This effect is also observed when the load path is reversed i.e. compression followed by tension. • This phenomenon is exhibited in varying degree by all metals and alloys. • Lowered yield stress in the reverse direction of load application is also called strain softening or work softening.

• Modes of Deformation in Compression Testing • The figure to the right illustrates the modes of deformation in compression testing. (a) Buckling, when L/D > 5. (b) Shearing, when L/D > 2.5. (c) Double barreling, when L/D > 2.0 and friction is present at the contact surfaces. (d) Barreling, when L/D < 2.0 and friction is present at the contact surfaces. (e) Homogenous compression, when L/D < 2.0 and no friction is present at the contact surfaces. (f) Compressive instability due to work-softening material1

Torsion testing • Torsion is the stress associated with twisting (torque). • The torsion testing device has two sockets, one fixed and the other can rotate. • The fixed socket is attached to an instrument which senses torsional moment and displays this value on a graduated dial or digital torquemeter.

• Measure of the ability of a material to withstand a twisting load. It is the Ultimate Strength of a material subjected to torsional loading, and is the maximum torsional stress that a material sustains before rupture. Alternate terms are modulus of rupture and shear strength.

Ultimate Strength • Highest engineering stress developed in material before rupture. Normally, changes in area due to changing load and Necking are disregarded in determining ultimate strength.

• Select image to enlarge A torsion test can be conducted on most materials to determine the torsional properties of the material. These properties include but are not limited to:

• A device for measuring twist angle, which is called torsiometer, is mounted on the specimen before it is inserted into the sockets.The parts of the torsion testing machine are shown below: