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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.