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Mechanical Properties and Testing Notes

CHAPTER 2: Mechanical Properties and Testing

2.1 Significance

  • Engineering applications necessitate an optimal interplay of materials, design, and processing for successful product development.

  • Mechanical Testing: Choosing the right mechanical tests is essential to gather relevant data which aids in material selection for engineering applications.

    • Key parameters that arise from testing dictate the suitability of materials for specific applications:

    • Crystal structure

    • Microstructure

    • Processing history

  • Purposes of Mechanical Testing:

    • Identify critical parameters for materials selection based on design integrity.

    • Interpret data with a focus on safety.

    • Simulate applicable service conditions such as tension, compression, torsion, and fatigue.

2.2 Concept of Stress and Strain

  • Stress (Engineering Stress): Defined as the load over the original area:
    \text{Stress} (S) = \frac{P}{A0} where P = Load, (A0) = Original area.

  • Strain (Engineering Strain): Defined as the change in length over the original length:
    \text{Strain} (e) = \frac{\Delta L}{L0} = \frac{L - L0}{L_0}

  • Flow Curves: Graphically demonstrate the relationship between stress and strain.

    • Toughness is signified by the area under the flow curve, indicating both strength and ductility.

2.3 Standard Test Methods and Interpretation

  • Tensile Testing (ASTM E8): Measures how materials respond to tension.

    • Parameters:

    • Load (P) in Newton (N)

    • Area (A) in m²

    • Stress (S) in Pascal (Pa)

    • Strain (e) Dimensionless

  • Key Stages in the Stress-Strain Curve:

    1. Initial Linear Portion (Elastic Region)

    2. Yield Strength (Point where permanent deformation begins)

    3. Ultimate Tensile Strength (Maximum load capacity)

    4. Necking and Fracture Percentage Extension and Reduction of Area (%E, %RA):

    • \%E = \frac{\Delta L}{L_0} \times 100

    • \%RA = \frac{A0 - Af}{A_0} \times 100

  • Hardness Testing Methods:

    • Brinell (ASTM E10), Rockwell (ASTM E18), and Vickers (ASTM E92).

    • These tests measure the resistance of a material to deformation and are vital for various applications that require precise hardness values.

2.4 Fatigue Testing

  • S-N Curve: Used to demonstrate the relationship between alternating stress and the number of cycles before failure for various materials.

    • Fatigue failures occur without noticeable plastic deformation and are primarily due to cyclic loading.

  • Parameters: Maximum Stress (\sigma{max}), Minimum Stress (\sigma{min}), Stress Range (\Delta\sigma) etc.

  • Factors Affecting Fatigue:

    • Nature of loading, surface quality, and metallurgical discontinuities.

2.5 Creep Testing

  • Creep: A time-dependent strain occurring under constant load and elevated temperature, typically evaluated under conditions where temperature exceeds 0.5 times the melting point (T).

  • Stages of Creep Curve: Primary, Secondary (steady rate), and Tertiary (rapid increase until failure). -

2.6 Impact Testing

  • Evaluates the toughness and ductile-to-brittle transition temperature using Charpy or Izod tests (ASTM E23).

    • Key Points:

    • Only BCC metals exhibit a noticeable transition at lower temperatures while FCC metals do not show this behavior.

Conclusion

  • Understanding mechanical properties is crucial for selecting materials suitable for engineering applications involving various stresses and types of loading conditions. This is an important aspect of material science and engineering design, affecting how components are created, tested, and utilized effectively in practice.