MATSE 259 Exam 2

Grain Size Reduction

Grain Size Reduction

Improves Strength and Toughness

Dislocations and Plastic deformation

Linear crystalline defects around which there is atomic misalignment

slip systems

combination of slip plane and slip direction

Solid Solution

Crystal structure remains the same despite solute being added to host material

Recovery

Stored energy relieved by dislocation motion, typically annealing

Recrystallization

Formation of new strain free grains within a previously coldworked material. Ex: Annealing

Grain Growth

The increase in average grain size of a polycrystalline material; for most materials, an elevated-temperature heat treatment is necessary.

Slip

Plastic Deformation as a result of dislocation motion. Shear displacement of two adjacent planes of atoms

Slip Plane

the crystallographic plane along which the dislocation line traverses

Dislocation Density

The total dislocation length per unit volume of material; alternatively, the number of dislocations that intersect a unit area of a random surface section.

Resolved Shear Stress

An applied tensile or compressive stress resolved into a shear component along a specific plane and direction within that plane.

Hall-Petch Equation

The relationship between yield strength and grain size in a metallic material

Ductile Fracture

a mode of fracture attended by extensive gross plastic deformation

Brittle Fracture

fracture that occurs by rapid crack propagation and without appreciable macroscopic deformation

Transgranular Fracture

fracture of polycrystalline materials by crack propagation through the grains

Stress Concentration Factor

Factor of amplification of applied stress at the tip of a notch or small crack

Plane Strain

The condition, important in fracture mechanical analyses, in which, for tensile loading, there is zero strain in a direction perpendicular to both the stress axis and the direction of crack propagation; this condition is found in thick plates, and the zero-strain direction is perpendicular to the plate surface.

Maximum Allowable Flaw Length

largest flaw size before imminemt failure

Primary Creep

creep rate decreases with time due to strain hardening

Secondary Creep

steady-state creep where a nearly constant creep rate occurs because a balance exists between the competing processes of plastic deformation and strain hardening

Tertiary Creep

occurs when the creep life is nearly exhausted, and the material specimen begins to neck or develop internal voids which reduce load capacity

Coble Creep

The variant of diffusion creep operating at lower temperature where grain-boundary diffusion dominates

Steady State Creep Rate

slope of the secondary portion of the creep curve. it is the engineering design parameter that is considered for long-life applications

Range of Stress

σ(r) = σ(max) - σ(min)

stress amplitude

σ(a) = (σ(max) - σ(min))/2

Stress Ratio

R = σmin/σmax

S-N Curves

is a plot of the magnitude of an alternating stress versus the number of cycles to failure for a given material. Typically, both the stress and number of cycles are displayed on logarithmic scales

Fatigue Strength

The maximum stress level that a material can sustain without failing, for some specified number of cycles.

Fatigue Limit

maximum stress amplitude level below which a material can endure an essentially infinite number of stress cycles and not fail

Low-Cycle Fatigue

High loads, plastic and elastic deformation

High-Cycle Fatigue

fatigue lives that are relatively long; occurs at greater than about 10^4 to 10^5 cycles; when deformations are totally elastic at lower stress levels

Strain-Rate Effect

The behavior an increased rate of load application can cause in normally ductile material.

Charpy Test

impact test in which the standard specimen contains a notch where an impact occurs to complete a three-point bending

Izod Test

Test in which a specimen is supported at one end as a cantilever beam and broken by the impact of a falling pendulum.

Strengthening Mechanisms

1. reduce the grain size
2. solid solution alloying
3. precipitates of another phase
4. cold working

Strain Hardening

The increase in hardness and strength of a ductile metal as it is plastically deformed below its recrystallization temperature.

INtergranular Fracture

fracture of polycrystalline materials by crack propagation along grain boundaries

Fracture Toughness

The measure of a material's resistance to fracture when a crack is present.

Creep

Time dependent permenent deformation that occurs under stress

nabarro-Herring

The variant of diffusion creep operating at higher temperature where bulk lattice diffusion dominates

Fatigue

Failure where low stress is present during high cycle life

Mean Stress

σ(m) = (σ(max) + σ(min))/2

Fatigue Life

the total number of stress cycles that will cause a fatigue failure at some specified stress amplitude

Impact Energy/Impact Toughness

Measure of energy absorbed during Charpy or Izod test

Ductile to Brittle Transition

The transition from ductile to brittle behavior with a decrease in temperature exhibited by some low-strength steel (BCC) alloys; the temperature range over which the transition occurs is determined by Charpy and Izod impact tests.