Untitled Document

University of New Mexico

Civil Engineering Department

Civil Engineering Materials Laboratory, CE 305L

TORSION TEST OF ALUMINUM

GENERAL:

The most notable test that demonstrates the effects of shearing forces and resulting stresses is the torsion test of a solid circular bar or rod. As a matter of fact, this test generates a state of pure shear stress in the torsionally loaded rod. Such a test is used to ascertain all the major shear properties of metal materials, i.e., the ultimate shear stress, the yield shear stress and the modulus of rigidity or shear modulus.

The applied torque ( T ) to the specimen and resulting deformation (angle of twist, Φ ) are measured during the torsion test. These measurands are converted to shear stress ( τ ) and shear strain( γ) by the following respective equations:

(1)

(2)

where c is the radius of the solid circular rod, L_o is the length over which the relative angle of twist is measured ( this angle must be in radians ) and J is the polar moment of inertia defined as follows:

(3)

The shear modulus of elasticity is defined as the linear slope, of the shear stress-shear strain relation, between zero shear stress and the proportional limit shear stress (defined below), i.e.,

(4)

This equation clearly states that the shear modulus, like Young's modulus, is only valid for the linear elastic range of the material.

DEFINITIONS (see Hibbeler, Chapter 3):

Elastic Limit (proportional limit): the highest magnitude of stress for which the stress and strain are proportional to each other.

Shear modulus of elasticity (modulus of rigidity): the ratio of shear stress to sheare strain below the elastic limit.

Shear stress-strain curve: an x-y plot of shear stress vs. shear strain through the entire range of loading of the specimen until specimen failure.

Ultimate shear stress: the maximum observed shear stress that the specimen will withstand.

Yield stress: the stress at which the material begins to “yield”. For most metals, a 0.2% offset is used to define the yield stress. A strain value of 0.002 is selected and a line parallel to the elastic portion of the stress-strain curve is constructed. The intersection of this line with the stress-strain curve defines the value of the yield stress.

OBJECTIVES:

To observe the behavior of aluminum subjected to failure in torsion. To determine mechanical properties of aluminum in shear.

EQUIPMENT:

• Torsional testing machine w/ compass & pointer plates (see Figure 1)

• Compass plate & pointer plate attaachments (see Figure 2)

• Calipers

• Tape Measure

SPECIMEN:

• 6061-T6 aluminum torsion specimen, nominal 0.75 inch (see Figure 3)

PROCEDURE:

Click here to download the torsion test of aluminum procedures.

1. Determine the mean diameter of the nominal 0.75 inch specimen using the calipers and record. Calculate the polar moment of inertia of the section and record.

2. Select an appropriate torque range for the specimen being tested. Place the specimen in the torsion machine and tighten the grips. Note any initial torque offset and record.

3. Clamp the compass and pointer on the specimen securely using the setscrews provided. Zero the pointer on the “zero” degree point on the compass. Measure the gauge length between the “compass plate” and the “pointer plate” using the tape measure (the nominal gauge length will be approximately 10.25 inch).

4. Apply the torque slowly , obtaining simultaneous readings of torque (in•lb) and angle of twist (degrees) from the compass. Determine the torque for every degree from 0-30°, then every 5° from 30-60°, then every 30° thereafter until failure (or 720° total degrees, whichever comes first). Attempt to obtain the angle of twist if fracture should occur.

REQUIRED:

1. a) A complete shears stress vs. shear strain curve for the entire test to fracture.

b) A shear stress vs. shear strain curve to the yield point (by the 0.2% offset method)

c) A shear stress vs. shear strain curve just past the proportional limit stress

Figures 4 through 6 are typical results and presentations that are expected from your experimental results.

2. a) Tabulate the following values and clearly show them on the above stress vs. strain curves:

b) Proportional limit shear stress in torsion

c) Shear modulus of elasticity (modulus of rigidity)

d) Yield stress in torsion (by 0.2% offset)

e) Ultimate shear stress

3. Compare your tabulated values to known theoretical values for 6061-T6 aluminum and report your experimental errors. Tabulate theoretical and % error along with the experimental values.

Torsional testing machine