Tensile Test Overview

Introduction to Tensile Testing

• The tensile test is performed to evaluate the yield point phenomenon in materials.
• The test piece is a plain carbon steel specimen with:
  • Original diameter: 10 mm
  • Original gauge length: 100 mm
• Distance marks are drawn on the specimen to measure plastic behavior.

Setup of the Tensile Test

• The upper cros head of the testing machine is positioned correctly.
• The threaded ends of the test piece are placed in the grips.
• An extensometer is used to measure the gauge length throughout the test.
• Initial gauge length is displayed (100 mm) and force against elongation is recorded.

Force-Elongation Diagram

• Force (F) is plotted on the vertical axis; Elongation (Delta L) on the horizontal axis.
• The initial portion of the curve is steep, indicating elastic behavior:
  • If unloaded, the material returns to the original length.
• In materials with yield point phenomenon, there’s a sudden drop in force which indicates the start of plastic deformation:
  • Permanent elongation occurs (doesn't return to original length).

Key Phases of the Tensile Test

1. Elastic Region: Force increases rapidly, material behaves elastically.
2. Yield Point: Abrupt change indicates the start of plastic deformation.
3. Ludas Effect: A constant force level with slight fluctuations after initial yield.
4. Necking: At maximum force, a neck forms in the test piece leading to fracture.

Calculating Material Strength Properties

• Upper Yield Strength (Re): Calculated as:
  • ( Re = \frac{F}{S_0} ) (Force at upper yield point divided by original cross-section area)
• Lower Yield Strength (Rl): Similar calculation using force at lower yield point.
• Tensile Strength (Rm): Force at maximum divided by original cross-section area.

Post-test Measurements

• After fracture, the percentage elongation is measured (approx. 30% for carbon steel).
• Percentage Reduction of Area: Based on the smallest diameter at fracture relative to the original area.

Second Tensile Test - Non-Yield Point Material

• Material tested: Precipitation-strengthened aluminum alloy.
• Same dimensions as the first test (10 mm diameter, 100 mm length).
• Test setup is similar; the extensometer is used again.

Force-Elongation Behavior

• Initial linear curve indicates elastic behavior without a distinct drop in force.
• Transition from elastic to plastic deformation is smooth and gradual.
• Neck forms at maximum force, with subsequent deformation occurring at the neck until fracture.

Calculation of Proof Strength

• 0.2% Proof Strength (Rp0.2) is used to indicate yield strength:
  • Calculate elongation for 0.2% strain (0.2 mm for 100 mm gauge length).
  • Draw a line parallel to the elastic curve offset by 0.2 mm.
  • The intersection point gives the force FP0.2 for 0.2% plastic strain.
  • Proof strength calculated as: ( Rp0.2 = \frac{FP0.2}{S_0} )

Summary of Calculations

• Tensile strength, percentage elongation after fracture, and percentage reduction of area calculated similarly as in the first test.