PE Metallurgical flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

True stress at necking

By the Considère criterion, necking starts when the strain-hardening rate dσt/dεt equals the true stress σt, which corresponds to the true ultimate tensile strength of the metal.

Griffith fracture stress

For a brittle plate containing a through-thickness crack of half-length a, fracture stress is σf = sqrt(2Eγs/(πa)), showing that strength falls with the square root of crack size.

Poisson's ratio definition

Poisson's ratio is the negative ratio of lateral to axial strain under uniaxial elastic loading; typical metals are 0.25-0.35, ceramics near 0.2-0.3, and rubbery polymers approach 0.5.

Steady-state creep equation

The minimum creep rate is εdot = A·σ^n·exp(-Q/RT), where n is the stress exponent, typically 3-8, and Q is the activation energy of the dominant creep mechanism.

Fatigue endurance limit

An endurance limit is the stress amplitude below which a material survives a very large number of cycles; plain carbon steels show one near 0.4-0.5 of tensile strength, aluminium alloys do not.

Thermal shock resistance parameter

R = σf(1-ν)/(Eα) ranks materials by resistance to fracture under thermal gradients; high strength, low modulus and low thermal expansion give better thermal shock resistance.

Eutectoid reaction in steel

At 727 °C and 0.76 wt% carbon, austenite transforms into alternating lamellae of ferrite and cementite called pearlite; finer interlamellar spacing produces higher strength and hardness.

Hall-Petch equation

σy = σ0 + k·d^(-1/2) relates yield strength to grain size d; refining grains strengthens the metal because grain boundaries impede dislocation motion and deflect cracks.

Martensite in steels

Martensite is a body-centred tetragonal phase formed by a diffusionless shear transformation during rapid cooling; its hardness rises with carbon content and the Ms temperature falls.

Jominy hardenability test

A standard cylindrical bar is austenitized and end-quenched with water; hardness measured along the bar gives a curve that ranks alloys by hardenability, their ability to harden in depth.

Sensitization of stainless steel

Holding austenitic stainless steel at 425-815 °C precipitates chromium carbides at grain boundaries, depleting adjacent chromium and leaving the steel prone to intergranular corrosion.

Solution treatment and aging

In precipitation hardening, solution treatment dissolves alloying elements, quenching retains a supersaturated solid solution, and aging precipitates fine particles that harden until overaging causes coarsening and softening.

Weibull modulus

The Weibull modulus m describes scatter in the strength of brittle materials; a high m indicates narrow scatter and predictable strength, while a low m reflects wide variation in flaw size.

Glass transition temperature

Tg is the temperature at which an amorphous polymer changes from a rigid glassy state to a rubbery state, marking the onset of large-scale segmental chain mobility.

Rule of mixtures modulus

For continuous aligned fibres loaded parallel to their axis under isostrain, composite modulus is Ec = VfEf + VmEm, with V the volume fraction and subscripts f and m for fibre and matrix.

Critical fibre length

lc = σf·d/(2τi) is the shortest fibre length that can be loaded to its fracture stress in a composite; shorter fibres debond or pull out instead of fracturing.

Silicon band gap

Silicon has an indirect band gap of about 1.1 eV at room temperature, which sets its intrinsic carrier concentration and limits conventional junction devices to roughly 150 °C operation.

Chvorinov's rule

Solidification time follows t = B(V/A)^2, where V is casting volume, A is heat-extracting surface area and B is a mould constant; risers are sized to freeze after the casting.

Carbon equivalent formula

CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 estimates steel weldability from composition; values above about 0.4 indicate a tendency to form hard martensite and require preheat.

Recrystallization temperature

The temperature at which a deformed metal forms new strain-free grains, approximately 0.4-0.5 of the absolute melting temperature; it decreases with increasing prior deformation and higher purity.

Shot peening purpose

Shot peening bombards a surface with small media to create a compressive residual stress layer, which delays fatigue crack initiation and improves resistance to stress corrosion cracking.

Dendrite arm spacing

Secondary dendrite arm spacing decreases as cooling rate increases, following a power law; finer spacing reduces microsegregation and improves strength and feeding behaviour in castings.

Ashby performance index

A material index combines the properties controlling a design objective, for example M = E^(1/2)/ρ for a light stiff beam; maximizing the index identifies the best material class.

Galvanic series use

The galvanic series ranks metals by corrosion potential in a given environment; when two are coupled, the more active metal becomes the anode and corrodes faster, especially with a large cathode.

Leak-before-break criterion

A design philosophy in which the critical crack size computed from fracture toughness exceeds the wall thickness, so a crack leaks detectably before it can cause catastrophic fracture.

Specific stiffness definition

Specific stiffness is Young's modulus divided by density, E/ρ; it controls deflection-limited weight savings in stiffness-critical structures such as aircraft panels and beams.

Safety factor role

A safety factor divides the material's limiting stress or strength by a number greater than one to cover uncertainty in loads, properties, analysis and the consequences of failure.

Charpy V-notch test

A notched bar is broken by a falling pendulum and absorbed energy is recorded; plotting energy against temperature defines the ductile-to-brittle transition, sharp in body-centred cubic steels.

Dye penetrant inspection

A coloured or fluorescent liquid penetrates surface-breaking discontinuities and is drawn out by a developer for viewing; the method detects only defects open to the inspected surface.

Fatigue striations

Closely spaced parallel ridges on a fatigue fracture surface, each marking one stress cycle of crack advance; their spacing is used to estimate local crack growth rates.

X-ray diffraction phase analysis

XRD uses Bragg's law, nλ = 2d·sinθ, to measure lattice spacings; peak positions identify phases and lattice parameters, while broadening indicates microstrain or small crystallite size.

Cathodic protection

Cathodic protection makes a structure the cathode by supplying electrons, either from sacrificial anodes such as zinc or magnesium, or from an impressed current system.

Ductile-to-brittle transition

The temperature range where fracture mode changes from ductile to brittle, identified by a drop in Charpy impact energy; it is pronounced in BCC steels and minimal in FCC metals.