Material Properties and Testing Techniques
1. Material Properties
Material properties depend on internal structure and determine behavior during manufacturing and applications. Modifying the internal structure, such as alloying metals or applying thermal treatments, can alter these properties. When choosing a material, consider its chemical, physical, mechanical, economic, and aesthetic properties. Scientists design materials with surprising properties by studying the physical and chemical properties of atoms, molecules, and compounds, using computer-aided design tools.
1.1 Chemical Properties
Interaction between a material and its environment can cause loss or damage to its properties. Effects vary depending on the material type: metal, ceramic, or polymer.
a) Oxidation
Oxidation occurs when atoms move from their elemental state to form cations, combining the material with oxygen. Ag, Au, and Pt do not rust but are expensive for industrial use. Al self-protects. Alloying steel with Cr or Ni makes it stainless. Protective coatings like galvanization or cremation can also be used.
b) Corrosion
Corrosion is the slow deterioration of a material due to external agents (e.g., O2) in the presence of water. It’s a non-uniform process, with rust spots appearing on different parts of the material.
1.2 Physical Properties
Physical properties arise from the arrangement of atoms in space.
a) Density
Density is the ratio between the weight of a quantity of material and the volume it occupies. If the density is less than 1 kg/m³, the material will float in distilled water.
b) Electrical Properties
Electrical properties describe a material’s resistance to electric current when subjected to a potential difference. Electrical resistance depends on factors like the presence of mobile electrons and their mobility under an electric field. This substance-specific property is called resistivity.
- Insulators: High resistance to electrical current.
- Conductors: Low resistance to electrical current.
- Semiconductors: Intermediate resistance to electrical current.
- Superconductors: Resistivity drops to zero, allowing current to flow without resistance indefinitely (many medical applications). The transition to superconductivity occurs at the Curie temperature (Tc), related to the material’s magnetic properties.
c) Thermal Properties
Heating a solid can cause absorption, heat transfer, and expansion or dilation.
- Linear Thermal Expansion Coefficient: Materials expand when heated (without phase changes) due to increased atomic vibration and separation. ΔL = L₀(1 + αΔT), where α is the linear expansion coefficient.
- Specific Heat: Energy absorbed to raise the temperature of a material by 1°C without phase changes.
- Fusion: When heating a solid, increased particle vibration causes dilation. If the temperature continues to rise, the vibration can break the material’s structure, leading to fusion (melting). The melting point is accompanied by an increase in volume. Higher melting points indicate stronger interparticle forces.
- Thermal Conductivity: Heat transfer by conduction occurs through bodies from higher to lower temperature points due to collisions between atoms and particles. Thermal conductivity indicates a body’s ability to transfer heat by conduction.
d) Magnetic Properties
Magnetic properties represent the interaction of a material’s atomic structure with a magnetic field. Magnetization occurs when permanent or induced dipoles align with the magnetic field (B). cm = M/H.
- Diamagnetism: The induced magnetic dipole opposes the applied field, weakening the magnetic field inside the material (cm < 0).
- Paramagnetism: The magnetic field inside the material is slightly higher than the applied field, with dipoles aligning with B (cm > 0).
- Ferromagnetism: Permanent dipoles align with the applied field (cm > 0). Magnetic domains are present.
e) Optical Properties
Optical properties describe the interaction between a material and visible light. When light hits a surface, it can be reflected, transmitted, or absorbed (increasing internal energy). The color of a body is determined by the reflected light. Opaque materials absorb or reflect all light. Transparent materials transmit light, allowing objects to be seen through them. Translucent materials let light in but block objects from being seen clearly. The refractive index (n = c/v) determines a material’s optical properties.
2. Mechanical Properties
Mechanical properties describe how a material withstands applied forces, including tension, compression, impact, cyclic or fatigue loads, and high temperatures. Many materials in service are subjected to forces or loads. Understanding material properties is crucial for designing instruments that can withstand these stresses without fracturing. Mechanical behavior reflects the relationship between a material’s response (deformation) and an applied force or load.
a) Elastic-Plastic
Elasticity is the ability of a material to regain its original shape after the deforming force is removed. In a solid, each atom occupies an equilibrium position due to cohesive forces. When an external force is applied, deformation occurs. If the material returns to its original shape, the deformation is considered elastic. Otherwise, it’s considered plastic. Tensile strength is the resistance to axial stress. Immediate elastic deformations are reversible. Tension and compression tests are used to determine a material’s elasticity and plasticity. In many materials, including metals and minerals, strain is directly proportional to stress.
b) Plasticity
Plasticity is the ability of a material to retain its new shape after deformation. It’s important in shaping processes like deformation (especially in metals), such as hot rolling steel and cold forming sheets for automobiles.
c) Ductility
Ductility is the ability of a material to be stretched into a wire.
d) Malleability
Malleability is the ability of a material to be spread into sheets without breaking.
e) Hardness
Hardness is the resistance of a material to scratching or penetration (wear resistance).
f) Tenacity
Tenacity is the resistance of a material to breaking under slow strain.
g) Fragility
Fragility is the tendency of a material to break upon impact (Resilience).
h) Fatigue
Fatigue is the deformation of a material under variable loads, below the breaking point, when acted upon for a certain time or number of cycles.
i) Miscellaneous
Other mechanical properties include machinability (ease of metal removal), toughness (increased hardness, brittleness, and resistance in certain metals due to cold deformation), and castability (ability of a material to fill a mold when cast).
3. Test and Measurement Techniques for Properties
Tests can be destructive or non-destructive (X-rays, gamma rays, ultrasound, magnetic particle, liquid penetrant, eddy current, magnetic, sonic). Methods include:
- Chemical tests (chemical composition and behavior towards chemical agents)
- Metallographic tests (study of internal structure to determine thermal and mechanical treatments: homogeneity, cracks, grain, surface rolling, forging, etc.)
- Physical tests (density, melting point, specific heat, thermal and electrical conductivity)
- Mechanical tests
3.2 Hardness Test
3.2.1 Scratch Hardness Testing
- a) Martens Test: Measures the width of a line produced by a pyramidal diamond tip.
- b) File Test: For mild steel.
3.2.2 Penetration Testing
Quantitative techniques based on forcing penetrators into the surface under controlled load and measuring depth or size of the indentation.
- a) Brinell Test: Uses a hardened steel ball.
- b) Vickers Test: Uses a regular pyramid.
- c) Rockwell Test:
3.3 Dynamic Test (Impact or Impact Test)
Determines the energy absorbed by a specimen to cause rupture in a single blow. Uses a Charpy pendulum.
3.4 Fatigue Testing
Tests parts subjected to variable stresses (rotation, flexion, or vibration) in magnitude and direction that are repeated. Loads may cause breakage below the breaking point.
3.5 Creep Test
Creep is the deformation of a material over time and temperature when subjected to a constant load or stress (e.g., turbine blades).