Material Failure and Alloys

Vocabulary flashcards covering material failure types, fracture mechanics, corrosion, and alloy types.

Elastic Failure

Material returns to its original shape after stress is removed; a reversible process.

Plastic Failure

Material undergoes permanent deformation beyond the elastic limit, resulting in long-lasting changes; an irreversible process.

Brittle Failure

Failure without significant plastic deformation.

Ductile Failure

Failure that occurs with significant plastic deformation.

Stress Concentration

Amplification of loads at defects, especially crack tips, leading to material failure at lower average stresses than theoretical strength.

Fracture Toughness

A material's resistance to crack propagation, dependent on material thickness (plane stress in thin materials and plane strain in thick materials).

What is the Izod impact test?

Measures materials resistance to sudden force

What is fatigue?

Gradual and permanent structural damage that occurs when a material is subjected to repeated unloading and loading.

What are the stages of fatigue?

Crack initiation, crack propogation - gradual growth of cracks, and final failure.

Reversed Stress Cycle

Stress alternations from maximum tensile to maximum compression of equal magnitude.

Repeated Stress Cycle

Maximum and minimum stresses are asymmetrical relative to the zero-stress level.

Random Stress Cycle

Stress cycles with no pattern.

Fatigue Limit

The stress level below which a material can theoretically endure an infinite number of cycles without failure.

Creep

Time-dependent plastic deformation of materials subjected to a constant load/stress at elevated temperatures

What are the stages for creep?

Increase creep resistant due to strain hardening, plot becomes linear, tertiary creep and ultimate failure.

What is thermal expansion?

Most solid materials expand when heated and contracted when cooled, reflects an increase in the average distance between atoms.

Thermal Stress

Stress induced in a body as a result of changes in temperature.

What are bimetallic strips?

Two metals bonded together with different coefficients of expansion, when heated the metal with the higher coefficient expands more, causing the strip to bend.

Ductile to Brittle Transition Temperature

The temperature at which a material's failure mode changes from ductile to brittle.

• Low strength (FCC AND HCP) = do not exhibit ductile to brittle transition

• High strength (BCC) = exhibit ductile to brittle transition.

• High-strength materials: the impact of energy on high-strength metals is relatively sensitive to the temperature.

What is impact energy?

A measure of a material’s resistance to fracture under sudden impact

Corrosion

Destructive and unintentional electrochemical attack of a material, starting at the surface.

Oxidation

Loss of electrons at the anode during corrosion.

Reduction

Gain of electrons at the cathode during corrosion.

Passivity

Formation of a thin, adherent oxide film on a metal surface that protects it from further corrosion like aluminium and chromium

Uniform corrosion

occurs evenly over the entire surface of a material.

Galvanic Corrosion

Corrosion that occurs when two dissimilar metals are in contact in the presence of an electrolyte.

Pitting Corrosion

Localized corrosion that results in the formation of small pits or holes.

Crevice Corrosion

Corrosion that occurs in small crevices or gaps where stagnant electrolyte can accumulate.

Erosion Corrosion

Corrosion accelerated by the relative motion of a corrosive fluid and a metal surface.

Intergranular Corrosion

Corrosion that occurs preferentially along grain boundaries.

Stress Corrosion Cracking

Cracking that occurs due to the combined action of tensile stress and a corrosive environment.

Dealloying

Removal of one element from an alloy by corrosion.

How can we prevent corrosion?

material selection, remove corrosive species, adding chemicals to environment, design changed, coatings and cathodic protection

Metal Alloy

A mixture of two or more metals to improve its properties - stronger, more durable, better corrosion resistance

Substitutional Alloy

An alloy where atoms of the alloying element replace the host metal atoms - similar atomic sizes, brass - copper and zinc

Interstitial Alloy

An alloy where atoms of the alloying element fit into the spaces between the host atoms - small alloying atoms, steel - iron and carbon

Ferrous Alloy

A metal alloy that contains iron as the primary element - strong, prone to rust if not protected - stainless steel.

Nonferrous Alloy

A metal alloy that does not contain significant iron - corrosion resistance, lighter - aluminium alloys and copper alloys.

Steel

A ferrous alloy made primarily of iron and carbon, with properties varying based on carbon content and other added elements.

Different types of steel

• Low carbon steel - ductile and weldable, less than 0.25% carbon

• Medium carbon steel - strong, tough, 0.25-0.6% carbon

• Hard carbon steel - hard and brittle, 0.6-1.4% carbon

• Stainless steel - corrosion resistant, iron

Ceramic

Non-metallic solids made from compounds of metallic and non-metallic elements with resistance to high temperatures and corrosion, like oxides and nitrides

Polymer

Long chains of molecules that link together to form large structures; includes plastics and rubbers

Hybrid Material

A material that combines two or more distinct materials to achieve enhanced properties of the individual components.

Composite Material

A hybrid material with polymer, metal and/or ceramic reinforced with fibers or particles to increase strength and toughness.

Nanocomposite

A hybrid material using nanoparticles, such as nanotubes, to enhance properties like strength and thermal conductivity.

Concrete

A hybrid material composed of cement paste (the matrix) and aggregates (particulates).

Wood

natural composite material made of strong and flexible cellulose fibres held together by a stiffer material called lignin. Cellulose causes the strength, stiffness, and toughness. Efficient in bending and resistance to buckling.