1: Tensile Tests

Introduction to Tensile Tests

• Overview of the module on tensile tests and stress-strain curves.

• Importance of tensile tests in understanding mechanical properties of materials.

• The test provides insights into elastic, plastic, and failure properties crucial for material scientists and engineers.

Specimen Geometry

• Description of the dog bone specimen used in tensile tests.

• The dog bone shape allows for consistent stress distribution during testing.

• Focus on the cross-sectional area—particularly the gauge length—where maximum stress and deformation occur.

Stress and Strain Definitions

• Stress: Force per unit area (measured in Newtons or pounds force).

• Strain: Deformation of the material measured as a change in length relative to the original length.

• The gauge length is the region of interest where the majority of deformation is observed.

Measurement Techniques

• Use of extensometer to measure the strain accurately at the gauge length.

• Other techniques:

  • Digital image correlation: Involves marking specimen surfaces to track deformation.

  • Strain gauges: Track changes in resistance due to deformation of conductive materials applied to specimen surfaces.

Stress-Strain Curve Analysis

• The stress-strain curve is central to analyzing material behavior:

  • Linear Elastic Region: Initial response where stress is proportional to strain, characterized by Young's modulus.

  • Yield Stress (B3 y): Point where plastic deformation begins; permanent deformation follows.

  • Ultimate Tensile Strength (UTS): Maximum stress the material can withstand before failure.

  • Strain to Failure: Measures ductility/ability to deform before breaking.

  • Toughness: Area under the stress-strain curve, representing energy absorbed before failure.

Elastic vs. Plastic Behavior

• Elastic deformation: Temporary change; material returns to original shape upon unloading.

• Plastic deformation: Permanent change; material does not return to its original shape after unloading.

• The stress-relief behavior post-yield indicates the transition from elastic to plastic regions.

Factors Impacting Elastic Properties

• Impact of gauge length and cross-section on stress calculations.

• Different test setups can determine accurate strain measurements.

• Variability in stress distribution in regions outside the gauge length is acknowledged.

Influence of Microstructure on Properties

• The relationship between grain size and mechanical properties:

  • Annealed vs. Unannealed Samples: Change in grain structure affects plastic but not significantly elastic properties.

  • Larger grains from annealing might help preserve elastic modulus if the material remains isotropic.

• The significance of dislocation motion in relation to microstructure and yield strength.

Summary of Key Parameters Obtained from Tensile Tests

• Elastic Modulus (E): Measurement indicating stiffness of the material.

• Yield Strength (B3 y): Crucial for understanding elastic limits.

• Ultimate Tensile Strength (UTS): Defines maximum operational stress.

• Ductility: Assessing how much material can stretch before rupture helps in design applications.

• Toughness: Indicates how much energy the material can absorb before failure.

Conclusion

• Tensile tests are vital tools in the material evaluation process, providing comprehensive data critical for design and application in engineering fields.