BDCS Notes - Mechanical Testing of Steel

Tension Test: performed to determine the yield strength, yield point, ultimate (tensile) strength, elongation, and reduction of area.

Traditional way of calculating stress and strain uses the original cross-sectional area and gauge length. If the stress and strains are calculated based on the instantaneous cross-sectional area and gauge length, a true stress-strain curve is obtained (initial slope continuation), which is different than the engineering stress-strain curve (curve toward rupture point).

True Stress > Engineering Stress – Because of the reduced cross-sectional area at the neck. The specimen experiences the largest deformation at the regions closest to the neck.

Different carbon content steels have different stress-strain relations. Increasing the carbon content in steel increases the yield stress and reduces the ductility.

Steel is generally assumed to be a homogenous and isotropic material. However, in production of structural members, the final shape may be obtained by cold rolling. This causes the steel to undergo plastic deformations, with the degree of deformation varying throughout the member. Plastic deformation causes increase in yield strength and reduction in ductility. Therefore, it’s necessary to evaluate the sample of steel when it is collected for its material properties.

Torsion Test: Used to determine shear modulus of structural materials.

Shear Modulus: used in the design of members subjected to torsion, such as rotating shafts and helical compression springs.

In the test, a cylindrical, or tubular, specimen is loaded either incrementally or continually by applying an external torque to cause a uniform twist within the gauge length.

The amount of torque applied is measured against the responding angle of twist.

(tau)_max = Tr / J

(gamma) = (theta)r / L

&

G = (tau)_max / (gamma) = TL / J(theta)

Where: T = torque

r = radius

J = polar moment of inertia

(theta) = angle of twist in radians

L = gauge length

Charpy V Notch Impact Test: Used to measure the toughness of the material or the energy required to fracture a V-notched simply supported specimen.

Standard specimen is 55 X 10 X 10 mm with a V notch at the center. It is inserted into an impact-testing machine using centering tongs. A swinging arm of the machine has a striking tip that impacts the specimen on the side opposite the V notch. The striking head is released from the pretest position, striking and fracturing the specimen.

By measuring the height the strike head attains after striking the specimen, the energy required to fracture the specimen is computed (measured in ft-lb).

Fracture surface consists of a dull shear area (ductile) at the edges and a shiny cleavage area (brittle) at the center.

Bend Test: Used to check ductility needed to accommodate bending. Evaluates the ability of steel, or a weld, to resist cracking during bending.

Bend test is conducted by bending the specimen through a specified angle and to a specified inside radius of curvature. When fracture doesn’t occur, failure is measured by the number and size of cracks found on the tension surface.

Test is made by applying a transverse force to the specimen in the portion that is being bent, usually at midlength.

Three arrangements can be used:

1. Specimen fixed at one end and bent around a reaction pin or mandrel by applying a force near the free end.
2. Specimen held at one end and a rotating device is used to bend the specimen around the pin or mandrel.
3. Force applied in the middle of a specimen simply supported at both ends.

Hardness Test: Measured a material’s resistance to localized plastic deformation, such as a small dent or scratch on the surface of the material.

Tests include an indenter that is forced into the surface of the material with a specified load magnitude and rate of application. Depth, size and indentation is measured and related to hardness index number.

Rockwell Hardness Test: Depth of penetration of a diamond cone, or a steel ball, into the specimen is determined under fixed conditions. Preliminary load of 10 kg is applied first, followed by additional load. Rockwell number, which is proportional to difference in penetration between preliminary and total loads, is read from machine by means of a dial, display, pointer or other device.

For very thin steel, the Rockwell Superficial Hardness Test is used. Procedure is similar to test mentioned above, except that smaller preliminary and total loads are used.

The hardness number is reported as a number followed by the initials HR and another symbol representing the indenter and forces used. HRC, for instance, indicates a Rockwell hardness number of 68 on a Rockwell C scale.

Hardness tests are simple, inexpensive and nondestructive, and do not require special specimens.

Ultrasonic Testing: Nondestructive method for detecting flaws in materials. Useful for the evaluation of welds.

A sound wave is directed toward the weld joint and reflected back from a discontinuity. A sensor captures the energy of the reflected wave and the results are displayed on an oscilloscope.

Ultrasonic Testing is highly sensitive in detecting planar defects, such as incomplete weld fusion, delamination or cracks.