Key Material Properties and Mechanical Testing Methods
Material Structure and Properties
Vitreous Structure Characteristics
An amorphous structure characterized by short-range order. Its key characteristics include being fragile and isotropic.
Fibrous Structure Properties
A structure with specific and dominant directional order. Its primary characteristic is slenderness. Examples include plastics and wood.
Thermal Properties of Materials
Understanding Thermal Conductivity
Thermal conductivity (q) is an empirical coefficient representing the quantity of heat that passes per unit time through a wall of infinite extension and unit thickness when a temperature difference of 1°C is established across its parallel flat faces.
Thermal Transmittance (U) Explained
Thermal transmittance (U) indicates the amount of heat (q) transmitted through a material or composite element of a particular thickness.
Mechanical Behavior: Stress, Strain & Fracture
Shear Strains and Shear Force
Shear strains are those experienced by a material subjected to a shear force. A shear force is produced by a torque; this torque should not cause any bending moment or significant displacement and deformation of the materials.
Poisson’s Ratio: Longitudinal vs. Lateral Strain
Poisson’s ratio measures the relationship between longitudinal strain and lateral strain. For a solid subjected to tension or compression during its elastic phase, it is defined as:
u = - (Lateral Strain) / (Longitudinal Strain)- Where:
Lateral Strain (ε_lateral) = (l_f - l_i) / l_iLongitudinal Strain (ε_longitudinal) = (e_f - e_i) / e_i
This ratio works very well with highly homogeneous materials such as plastics and metals, where it is generally consistent for both tension and compression. However, for heterogeneous materials like wood, it does not perform as well, yielding significantly different information depending on whether the material is under tension or compression, and on the specific type of wood.
Understanding Ideal Fracture
Ideal fracture occurs in ideal bodies: materials without discontinuities where their particles occupy their rightful place according to their crystalline organization. The fracture surface is typically very smooth, bright, and perpendicular to the applied stress. This characteristic surface is often used to measure the degree of perfection of the crystalline structure.
Ductile Fracture: Process and Characteristics
Ductile fracture occurs in materials with a prolonged plastic phase. It is generally considered a safer type of fracture due to the significant deformation before complete failure.
Process:
- Formation of micro-cavities (voids) in the central region.
- These cavities begin to cluster along a line roughly perpendicular to the direction of the applied stress, forming a crack.
- The final break occurs along two planes at approximately 45° to the applied stress, often resulting in a cone-and-cup fracture surface.
The failure surface is typically fibrous and opaque. Common materials exhibiting ductile fracture include plastics and some metals (though not all metals exhibit this behavior).
Essential Material Testing Methods
The Tensile Test: Material Strength Analysis
The tensile test is one of the most fundamental and important material characterization tests. It involves subjecting a specimen to a controlled tensile force along its axis until it breaks, measuring its elongation.
Specimen Characteristics:
- A specimen is a precisely prepared sample of material, often with a standardized shape.
- For tensile tests, it is typically a longitudinal piece with widened ends to ensure a secure grip in the testing machine.
- The transition between the central gauge section and the wider ends is rounded and smooth to prevent stress concentrations.
- The gauge length may sometimes have a graduation for precise measurement.
- It is crucial to know the cross-sectional area of the specimen.
- Test data is considered valid only if the specimen fractures within the middle third of its gauge length (LC).
Data Obtained from the Test:
- Tensile Strength (Resistance to fracture)
- Elongation (Total deformation before fracture)
- Necking (Reduction of the specimen’s cross-section at the point of fracture)
- Yield Strength (The stress value at which material deformation becomes irreversible, marking the onset of plastic deformation)
Impact Resistance Testing: Measuring Toughness
The impact resistance test aims to measure the energy absorbed by a specimen during impact fracture, relative to its cross-section. These tests typically use a pendulum impact tester (e.g., Charpy or Izod).
The process involves dropping a mass (m) from an initial height (H) to strike the specimen, and then measuring the final height (h) after impact.
Key Formulas:
- Potential Energy (Ep) =
mgh - Absorbed Energy (E_abs) =
mg(H - h) - Impact Resistance =
E_abs / (Cross-sectional area of the specimen at the notch)
This test is often required for structural steels, and the results can vary significantly with temperature. If the initial impact does not break the specimen, a different test or specimen might be used.