Engineering Material Selection - CH 560

Engineering materials are categorized into metals, polymers, elastomers, ceramics, glasses, and composites.

Metals typically have high stiffness (modulus).

Metals can be shaped through deformation processes like rolling and forging.

Metals can weaken and fracture over time due to repeated stress.

Ceramics and glasses have high stiffness (modulus).

Ceramics are brittle and tend to break rather than bend.

Ceramics and glasses are hard and resist abrasion.

Ceramics retain their strength at high temperatures.

Materials that can undergo significant plastic deformation before fracture.

Materials that fracture with little or no plastic deformation.

Polymers have low elasticity, large elastic deformations, and creep even at room temperature with a large temperature dependence.

Polymer strength is limited above 200°C.

Polymers are utilized in designs requiring varied properties like strength and weight ratio and are easily shaped, suitable for complex parts with fast and cheap assembly possibilities, no finishing required.

Materials combining the desirable properties of various material types, often including polymers.

Composites with a polymer as the supporting material, often reinforced with fibers like glass, carbon, or Kevlar.

Polymer matrix composites are typically usable up to about 250°C, as the polymer softens beyond this.

Measures a material's stiffness under tension or compression.

Measures a material's resistance to shear deformation.

Measures a material's resistance to uniform compression (hydrostatic pressure).

Ratio of lateral strain to axial strain in a material under axial loading.

Stress at which a material starts to deform significantly.

Point in stress-strain curve where permanent deformation begins.

Maximum stress a material can withstand in tension before fracturing.

Maximum stress a material can withstand in compression before fracturing.

A type of failure that occurs due to repeated stress cycles, even if the maximum stress is below the material's yield strength.

The stress amplitude below which a material can withstand an infinite number of stress cycles without failing. This limit is not always present in all materials.

The ratio of the fatigue limit to the yield strength of a material. It indicates the material's ability to withstand repeated stress relative to its ability to withstand a single large stress.

The difference between the maximum and minimum stress values in a cyclic loading scenario.

Because fatigue failures are a major cause of mechanical failures in engineering applications, accounting for about 90% of mechanical engineering failures.

Engineering designs should account for fatigue by ensuring the stress amplitude is below the fatigue limit of the material.

Some materials possess a well-defined fatigue limit, while others experience fatigue failure at any stress level, even if it's very low.

The number of stress cycles a material can withstand before failure. This number is inversely proportional to the stress amplitude.

The maximum surface stress in a bent beam at the point of failure. It's a measure of a material's strength in bending, especially for brittle materials like ceramics.

The maximum stress a material can withstand before it starts to fracture. For brittle materials, it's the same as the tensile failure stress. For ductile materials, it's higher due to work hardening.

A measure of a material's resistance to indentation or scratching. Measured by pressing a diamond or steel ball into the surface.

Hardness is generally proportional to strength. A material with high hardness will also have high strength.

High hardness indicates resistance to plastic deformation (bending) or cracking in compression and better wear resistance.

A process where a metal becomes stronger and harder as it is deformed (e.g., by bending).

Work hardening increases the ultimate tensile strength (su) of ductile metals compared to their yield strength (sf).

In composites, the reinforcement (e.g., fibers) helps distribute the load and improves the overall strength.

A characteristic of a material that describes its behavior under specific conditions. This might be how strong it is, how well it conducts heat, or its resistance to wear.

The cost of a material compared to a benchmark material, often expressed as a ratio.

The mass per unit volume of a material. Heavier materials have higher density.

A measure of a material's stiffness or resistance to deformation under stress. Higher modulus = stiffer material.

The stress a material can withstand before it starts to permanently deform.

The ability of a material to absorb energy before fracturing. High toughness means it can withstand impacts.

A measure of a material's resistance to crack propagation. High fracture toughness means it is less likely to break from cracks.

The ability of a material to transfer heat. Higher thermal conductivity means it conducts heat well.

The amount of heat energy required to raise the temperature of a material by 1 degree Celsius.

The temperature at which a solid material transitions to a liquid state.