MatSci Ch 8 Concepts

failure

• occurs when a material can no longer perform its intended function safely or effectively

  • defined on whether a component can still satisfy design requirements

• fracturing, permanently deforming, or losing its dimensional accuracy

fracture

• separation of a material into two or more pieces due to stress

• occurs through crack initiation and crack propagation

  • microscopic defects often control when fracture occurs

ductile vs. brittle fracture

• ductile:

  • substantial plastic deformation before failure

  • usually provides warning because visible deformation (bend, stretch, etc.) develops before failure

• brittle:

  • occurs with little or no plastic deformation

  • often occurs suddenly and catastrophically without obvious warning signs

ductile fracture

three stages:

1. Void nucleation: tiny cavities in material

2. Void growth: cavities grow as deformation continues

3. Void coalescence: large cavities merge into a crack

brittle fracture

• occurs with very little plastic deformation

• Once a crack begins propagating, it may travel rapidly through the material because little energy is absorbed through plastic deformation

• often fail suddenly and without warning

transgranular fracture

• occurs when a crack propagates through grains rather than around them

• fracture surface cuts directly across the crystal structure of multiple grains

intergranular fracture

• occurs when a crack propagates along grain boundaries

• grain boundaries are energetically different from grain interiors

  • preferred paths for crack growth under certain conditions

cup-and-cone fracture

• characteristic fracture surface observed in many ductile metals after tensile testing

• center region fails primarily through tension while the outer region fails through shear

• fracture surface: cup-shaped section and a cone-shaped section

measured vs. theoretical fracture strengths

• theoretical:

  • often assume a perfect crystal with no defects

• measured:

  • materials contain pores, inclusions, scratches, machining marks, and microscopic cracks

    • imperfections concentrate stress and dramatically reduce the load required for fracture

actual fracture strengths are often far lower than theoretical values

cracks

• create extremely large local stresses at their tips

  • stress near a crack tip may become high enough to initiate fracture

stress concentration

• localized increase in stress caused by discontinuities in geometry

• Holes, grooves, keyways, threads, sharp corners, and cracks all create stress concentrations

• control failure because damage tends to initiate where local stress is greatest

sharp corners

• create severe stress concentrations because force must flow around an abrupt change in geometry

  • sharper the corner, the higher the local stress

fracture mechanics

• branch of materials science that analyzes crack behavior

  • accounts for crack size, applied stress, geometry, and material properties

Griffith's contribution to fracture theory

• fracture occurs when the energy released by crack growth exceeds the energy required to create new surfaces

• microscopic flaws strongly influence fracture behavior

• explain why defects dramatically reduce strength

• focused on brittle materials

Orowan's modification

• recognized that ductile materials absorb energy through plastic deformation near the crack tip

• including plastic work in the energy balance

fracture toughness

• material's resistance to crack growth and brittle fracture

  • strength, ductility, and crack-tip behavior

  • depends on:

    • Temperature

    • Strain rate

    • Microstructure

    • Material composition

    • Heat treatment

• high fracture toughness = tolerate larger cracks before failing

  • more damage tolerant and safer in critical applications

stress intensity factor

• severity of the stress field near a crack tip

• Larger cracks or higher applied stresses produce larger values of K

• fracture becomes unstable and catastrophic crack propagation occurs when K = critical

KIC

• plane-strain fracture toughness of a material and represents its resistance to brittle fracture under the most restrictive conditions

• cracks are often evaluated by comparing the applied stress intensity factor to KIC

plane stress vs. plane strain

plane stress:

• occurs mainly in thin components where stresses are relatively free to redistribute through deformation

plane strain:

• occurs mainly in thick components where deformation is more constrained

• generally more severe because it promotes higher crack-tip stresses and lower apparent toughness

Charpy impact test

• measures the energy absorbed during fracture under impact loading

• provides a practical measure of toughness and impact resistance

ductile-to-brittle transition temp. (DBTT)

• materials become significantly more brittle as temperature decreases

• high temp.: substantial plastic deformation occurs before fracture

• low temp.: crack growth becomes easier and plastic deformation becomes more difficult

fatigue

• failure caused by repeated or fluctuating loading

• maximum stress may be below the yield strength and even below the tensile strength

• responsible for a large percentage of engineering failures

  • damage develops slowly and often remains invisible for much of a component's life

• three stages:

  1. crack initiation

  2. crack propagation

  3. final rapid fracture

S-N curve

• relates cyclic stress amplitude to the number of cycles required for failure

• higher stress amplitudes = shorter fatigue lives

• lower stress amplitudes = allow components to survive more cycles

fatigue limit, fatigue strength, and fatigue life

fatigue limit:

• stress below which certain materials can theoretically survive an infinite number of cycles

fatigue strength:

• stress required to cause failure at a specified number of cycles

fatigue life:

• number of cycles to failure under specific loading conditions

low-cycle fatigue

• occurs when stresses are high enough to cause plastic deformation during each cycle

• failure occurs after a relatively small number of cycles

• Components experiencing frequent overloads often fail through low-cycle fatigue

high-cycle fatigue

• occurs primarily within the elastic range

• enormous number of cycles eventually produces crack initiation and growth

improve fatigue life

• Removing stress concentrators

• Polishing surfaces

• Introducing compressive residual stresses

• Reducing mean stress

• Improving geometry

shot peening improves fatigue resistance

• introduces compressive residual stresses near the surface

  • compressive stresses oppose crack opening

• delays crack initiation and slows crack growth, significantly improving fatigue life

creep

• time-dependent permanent deformation that occurs under constant load at elevated temperature

  • high temperatures and constant loads (applied stress)

• occur even when stress remains constant

• slow creep deformation can eventually produce failure

• three stages:

  1. Primary creep: creep rate decreases with time.

  2. Secondary creep: constant creep rate

  3. Tertiary creep: deformation accelerates; internal damage weakens material. creep rate increases until rupture.

Larson-Miller parameter

• method used to estimate long-term creep rupture behavior

• combines stress, temperature, and rupture time into a single parameter

  • predict service life over extremely long periods without waiting decades for experiments to finish