University of New Mexico

Civil Engineering Department

Civil Engineering Materials Laboratory, CE 305L

TENSILE TEST OF STEEL

General

The tensile test of a standard ASTM steel specimen to failure provides the important basic properties of steel, viz., the proportional (or elastic limit) of the material, the yield stress, the ultimate stress, the elongation (strain) and the material stiffness (Young's modulus of elasticity.

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.

Elastic modulus (Young's modulus): the ratio of stress to strain below the elastic limit.

Elongation: the strain at fracture expressed as a percentage; this is a measure of the ductility of the material.

Modulus of resilience: the amount of energy (or work) stored per unit volume at the elastic limit.

Modulus of toughness: the amount of energy stored per unit volume at fracture of the material; this is a measure of the ductility of the material.

Percent Area Reduction: reduction in area at fracture in necking region with respect to original cross-section area; this is a measure of the ductility of the material.

Strain (engineering): the unit deformation of the material under load. Strain is not normally measured. Deformation is typically measured using extensometers with strain subsequently computed by dividing the measured deformation by the original.

Strain hardening: portion of the stress-strain curve between the elastic limit and the ultimate stress.

Stress (engineering): load (force) per unit area; the normal (axial) stress is determined by dividing the load by the original cross-sectional area of the specimen.

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

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

Yield stress: the stress at which the material begins to “yield”; for mild steel there is a noticeable increase in deformation with little increase in load. For steel and 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 examine the behavior of mild steel tested to failure in tension. To determine certain elastic and plastic properties of mild steel.

Equipment

1. Universal Testing Machine (UTM) with applicable tensile grips

2. Mechanical extensometer

3. Calipers

•  Mechanical dividers

•  Machinist scale (6 inch)

•  Gage length punch & hammer

Specimen

1. Standard ASTM A36 mild steel specimen (nominal 0.505 inch)

Procedure

1. Determine the mean diameter of the nominal 0.505 inch specimen & record.

•  Mark a 2-inch gage length on the specimen using the gage punch & hammer.

•  Insert the specimen in the UTM and attach the extensometer. Carefully follow the manufacturer's directions for attachment of the extensometer. Select a load range for the UTM that will accommodate the maximum anticipated load during the test.

•  Apply the load slowly , obtaining simultaneous readings of load from the UTM and elongation from the extensometer. When the extensometer nears its range, remove. Then continue monitoring the elongation of the specimen using the mechanical dividers and machinist scale in 0.05 inch increment until fracture occurs. Attempt to obtain the load at fracture.

5. After failure, fit the broken halves together and measure the final “gage” length, and the smallest diameter.

View a Quicktime Movie of a tensile test of steel

Incomparison to the tensile test of steel, we ran a tensile test on Polyethylene Hi-Density plastic.

View a Quicktime Movie of a tensile test of plastic

Required

•  a) A complete stress vs. strain curve for the entire test to fracture.

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

•  A stress vs. strain curve just past the proportional limit stress

The following Figures 1 through 3 are typical results and presentations that are expected from your experimental results.

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

•  Proportional limit stress

•  Modulus of elasticity (Young's)

•  Yield stress (by 0.2% offset)

•  Ultimate stress

•  Percent elongation in 2-inch gage length

•  Percent reduction in area

•  Modulus of resilience

•  Modulus of toughness (use trapezoidal integration)

•  Compare your tabulated values to known theoretical values for A36 mild steel and report your experimental errors. Tabulate theoretical and experimental values along the % errors for all required values.

References

•  Hibbeler, R.C. Mechanics of Materials , 5 th Edition, Prentice Hall, 2002.

•  ASTM E8, Test Methods for Tension Testing of Metallic Materials , Vol. 3.01.

Figure 1. Stress vs. Strain to Fracture.

Figure 2. Stress vs. Strain to Yield (0.2%).

Figure 3. Stress vs. Strain to Proportional Limit.