UMT312T: Mechanical Testing Lecture 02-05

Study Notes

Focus on understanding mechanical testing of materials, specifically metals and their plastic deformation characteristics.
Key method: Constant strain-rate testing in tension, maintaining constant strain rate (𝜀̇) and temperature (T).

Engineering Stress (𝑠): Calculated as load (P) divided by the initial cross-sectional area (𝐴0).
- Formula: 𝑠 = 𝑃 / 𝐴0
Engineering Strain (𝑒): Extension divided by the original gage length (𝐿0).
- Formula: 𝑒 = ∆𝐿 / 𝐿0
- Can also be incrementally defined as: 𝑑𝑒 = 𝑑𝐿 / 𝐿0
Engineering Strain Rate (𝜀̇): Defined as the crosshead velocity divided by the initial gage length (𝐿0).
- Formula: 𝜀̇ = 𝑑𝑣 / 𝑑𝐿 = 𝑑𝑑 / 𝐿0

Essential for understanding the behavior of materials under load.
Different types of materials show distinct stress-strain characteristics:
- Brittle Materials: Typically exhibit a linear relationship up to fracture with little plastic deformation.
- Steels: Variety of stress-strain behaviors based on composition and processing.
- Polymers (e.g., PMMA): Show a different curve pattern compared to metals, indicating distinct deformation mechanisms.

Temperature: Alters the flow stress and strain behavior of metals.
Strain Rate: Increased strain rates can lead to more pronounced stiffening in materials, affecting the overall plastic behavior.

Stress and strain curves also provide insights into yielding, strain hardening, and ultimately failure mechanisms in metallic materials.
Understanding these curves allows for predicting material performance in real-world applications.

Plasticity refers to the ability of metals to undergo permanent deformation when subjected to stress.
Understanding stress-strain relationships is crucial for analyzing material behavior under load.

The test involves maintaining a constant strain rate (𝜀̇) and temperature (𝑇) to observe changes in stress (𝜎).
Engineering stress (𝑠) is calculated as the load (𝑃) divided by the initial cross-sectional area (𝐴₀).
Engineering strain (𝑒) is defined as the change in length (∆𝐿) divided by the original gage length (𝐿₀).
Engineering strain rate (𝜀̇) is determined by the crosshead velocity divided by the initial gage length.

Stress-strain curves visually represent the relationship between applied stress and resulting strain in materials.
Brittle materials typically exhibit sudden failure without significant plastic deformation.
Various steels and polymers demonstrate different stress-strain behaviors, impacting their applications in engineering.

Changes in temperature and strain rate significantly affect the material’s stress-strain characteristics.
Increased temperature can lead to reduced yield strength and increased ductility.
Strain rate influences the rate of plastic deformation, with higher rates usually resulting in increased strength and reduced ductility.

Engineering stress is crucial for understanding the load-bearing capacity of materials.
The area under the stress-strain curve provides insight into the energy absorbed by the material before failure.
Key features of the curve include yield point, ultimate tensile strength, and fracture point, indicating critical material properties.