CourseWiki/Materials and Manufacturing Processes/Foundations

Material Properties

Distinguish strength, stiffness, hardness and toughness, then compare physical and chemical behavior, cost and workability.

State the requirement before choosing the material

“Strong,” “light” and “durable” are starting points. A useful comparison says what load, temperature, environment and lifetime the part must handle.

Mechanical properties describe behavior under force. Physical properties describe features such as density and heat or electrical behavior. Chemical properties describe interaction with the environment. Cost and workability determine whether a suitable material can become a practical product.

Start with the four material families, then compare actual materials within them. Processing and condition matter: a material name alone does not tell you its final properties.

Strength resists failure; stiffness limits elastic deformation

Strength describes the stress a material can withstand before a specified limit, such as permanent deformation or fracture. Stress relates force to the area carrying it. A thicker part may carry more load without being made of a stronger material.

Tensile strength: Resistance to a pulling load. Think of a tie rod being pulled along its length.
Compressive strength: Resistance to a pushing or crushing load. Think of a block squeezed between two surfaces. A material good in compression may be much less capable in tension.
Stiffness: Resistance to elastic deformation, the change that recovers when the load is removed. A shelf that sags too far may be strong enough to avoid failure but insufficiently stiff.

A part's stiffness depends on both its material and its geometry. Compare similarly shaped specimens when comparing materials; do not infer material stiffness by comparing a thin wire with a thick bar.

Stress, strain and modulus give the comparison units

For a simple axial test, engineering stress is force divided by original cross-sectional area. Engineering strain is change in length divided by original length. Stress has units such as MPa; strain is a ratio with no unit.

In a linear elastic region, the slope of stress against strain is Young's modulus, a measure of material stiffness. Yield strength concerns permanent deformation; ultimate tensile strength is the highest engineering tensile stress reached in the test. A nonlinear curve does not, by itself, prove that a material has no yield behavior.

See Measurement Language for writing quantities and units consistently.

Hardness, toughness, brittleness and ductility answer different questions

Hardness: Resistance to local indentation or scratching, depending on the test. It can help resist wear, but hardness alone does not determine wear performance.
Toughness: Ability to absorb energy before fracturing. A part that must survive a blow needs more than a hard surface.
Brittleness: A tendency to fracture with little permanent deformation. Glass may resist scratching but crack instead of bending permanently.
Ductility: Ability to deform permanently in tension before fracture. A ductile metal can be drawn into wire. Elastic stretch that disappears when unloaded is not the same as ductility.

Do not treat these as synonyms. A hard ceramic tile can be brittle; a ductile metal can bend before it breaks. Toughness depends on both the stress sustained and the deformation before fracture, as well as test conditions.

Why a single “toughness number” needs its test method

The area under a tensile stress–strain curve through fracture measures energy absorbed per original volume in that test. Impact tests and fracture-toughness tests describe related but different behavior. Temperature, loading rate and flaws can change the result, so compare like tests.

Density and thermal behavior decide weight and temperature limits

Density: Mass per unit volume. At equal volume, lower density gives lower mass. A large hollow metal part can weigh less than a small solid plastic part, so compare the whole design too.
Melting point or melting range: The temperature or range where a material melts. It helps choose processing conditions, but it is not the allowable service temperature: a loaded part may lose useful stiffness or strength earlier.
Thermal conductivity: How readily heat travels through a material. A heat spreader benefits from high conductivity; an insulating handle usually benefits from low conductivity.
Electrical conductivity: How readily electric current passes through a material. A wire needs a conductor; its surrounding insulation needs low conductivity under the intended conditions.

Many common metals conduct heat and electricity well. Many unfilled plastics, dry woods and familiar ceramics are poor electrical conductors. Additives, moisture and temperature can change those tendencies.

Not every material has one useful melting-point number

Pure crystalline substances can have a sharp melting point; many alloys melt over a range. Glass and amorphous thermoplastics soften over a range rather than showing a sharp crystalline melting transition. Thermosets and wood decompose rather than melting into a reusable processing liquid. Use material-specific processing and service data.

Corrosion resistance is resistance to a particular environment

Corrosion resistance is the ability to resist chemical or electrochemical deterioration in service. A metal that works well indoors may perform differently in salt water or against another metal.

Rust is one form of corrosion involving iron. Aluminum and stainless steel can resist many exposures, but neither is immune to every environment. Coatings can protect a surface, and their condition matters.

For plastics, wood and composites, also consider chemical attack, sunlight, moisture and decay as appropriate. “Does not rust” is not the same as “will last outdoors.” Name the exposure before selecting a family or grade.

Cost and workability belong in the same comparison

Workability means how readily a material can be processed into an acceptable part. It is specific to the operation: a material that molds well may be difficult to bond, and one that cuts readily may be unsuitable for bending.

Cost includes more than price per mass. Consider how much material the design needs, available supply forms, tooling, processing time, waste, inspection and any protective finish.

A family profile narrows the search: common metals often offer stiffness and conductivity; plastics often offer low mass and moldability; wood and composites offer useful directional performance; ceramics and glass often offer hardness and chemical resistance. The particular material decides whether those benefits meet the job.

Use Manufacturing Process Categories to describe the required operations. Detailed manufacturing economics and design for manufacturing extend the cost comparison.

Choose a candidate, give a reason, and name what still needs checking; Checklist; Practice

Choose a candidate, give a reason, and name what still needs checking

Write the required function and service conditions.
Name the properties that control success.
Screen the four families, then identify a specific candidate material.
Check direction, temperature, moisture, processing condition and data source.
Check workability and total cost, then verify the finished part against its requirements.

For a heat-spreading plate, Metals is a sensible starting family because common choices such as aluminum and copper conduct heat well. The next checks include temperature, dimensions, corrosion exposure and cost. That is a justified candidate, not a complete design specification.

Practice selecting a family Open requirements practice

Appears in

DET 130 DET Materials and Manufacturing Foundations