To determine the Yield Strength, Proportional Limit, and Ultimate Strength of the Steel Bar.
INTRODUCTION:
Yield Strength of steel:
It is the stress value that the steel can take while still showing elastic behavior. Any stress applied to steel above this value will take it into the plastic zone. The yield stress of steel is the point, generally, where a stress load imposed on the steel causes plastic deformation to begin.
Two types of deformation can occur in steel.
The first is elastic deformation. This is where the steel shape deforms because of the stress load imposed on the sample. When the load is reduced or removed, the steel returns to its original shape, like a spring. There is no permanent deformation.
Plastic deformation is where the steel deforms because of an imposed load and the resultant deformation is retained after the load is reduced or removed. The value of yield strength depends on the grade of steel. Mild steel (MS) has a yield strength of approx. 250 MPa in tension; Fe415 grade has a yield strength of 415 MPa.
Proportional Limit of Steel bar
Proportional Limit. Highest stress at which stress is directly proportional to strain. It is the highest stress at which the curve in a stress-strain diagram is a straight line. The proportional limit is equal to the elastic limit for many metals.
The limit of proportionality is the point beyond which Hooke’s law is no longer true when stretching a material. The elastic limit is the point beyond which the material you are stretching becomes permanently stretched so that the material does not return to its original length when the force is removed.
Ultimate Strength of steel bar:
Ultimate tensile strength is measured by the maximum stress that a material can withstand while being stretched or pulled before breaking.
Apparatus Required:
Universal Testing Machine Scale (6 inch)
Materials Required:
Two standard tensile specimens: choose steel, aluminum, brass, or cast iron
- Cast iron
- Choose steel, aluminum, or brass.
THEORY:
PROCEDURE:
- Measure the diameter and use the gage mark punch fixture to mark a 2-inch gage length on each specimen (2 sets of opposed marks). [The punch marks are used to locate the extensometer and to measure the elongation after the extensometer has been removed.]
- Install the specimen in the testing machine (bottom connection first, take care not to unscrew the top bracket). (Use the hand crank to apply an initial tensile load to the specimen.)
- Mount the extensometer on the specimen (also, attach the support wire from the extensometer to the top of the tester frame support). Set the extensometer dial to zero.
- Set the Range Indicator to 24,000 lbs.; set, the machine load indicator and the limit pointer to zero.
- Have the instructor check the setup before proceeding.
- Press the START button.
- Apply load to the specimen at a very slow rate (use the LOAD and UNLOAD hand wheels). Record load and elongation for constant increments of
elongation. - At the yield point, remove the extensometer. From this point, record the elongation between gage marks using the dividers and scale. (Increase the load rate slightly.)
- At the ultimate strength, stop recording the elongation strain.
- Continue loading the specimen to fracture (as a safety precaution, stand away from the loading area).
- Remove the specimen from the machine.
- Measure and record the diameter of the specimen at the fracture point.
- Have the datasheet signed by the instructor and the members of your
team?
REPORT:
Prepare a stress vs strain diagram for each specimen.
For each metal, use your test results to determine:
。 Yield strength Proportional limit
494.48 N/mm2
⚫ Ultimate strength
706-40 N/mm
157.
- % Elongation
% Reduction in cross-sectional area 77-529 mm2
Compare your results with published data for each material.
Based on your observations of the appearance of the fractures, identify the nature of the fracture for each specimen.
Prepare a stress vs strain diagram for each specimen.
For each metal, use your test results to determine:
- Yield strength
- Proportional limit
- Ultimate strength
- % Elongation
- % Reduction in cross-sectional area
Compare your results with published data for each material.
Based on your observations of the appearance of the fractures, identify the nature of the fracture for each specimen.
REVIEW QUESTIONS:
