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 30 27

Medium 13-30 39-90

Long 20-32 160-320

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>2.0friction is present at the contact surface.

(d) Barreling, when L/D < 2.0friction is present at the contact surfaces

e)Homogenous compression, when L/D <2.0no friction is present at the contact Surfaces

(f) Compressive instability due to work-softening

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: