Time dependent elastic and plastic Time dependent creep Fracture Static loading Brittle run of cracks in a stressed material Ductile Environmental Fatigue cycling loading Point defects in metals Point defects in ceramics Vacancy and self interstitial usually in a strain state Vacancies for both cations and anions Interstitials frenkel schottky line defects Chapter 7 notes Deformation o o o o o o o o o o o o o imperfections in solids Dislocations appear with mech deformation burger s vector an extra plane of atoms are put into the crystal structure Strength of a material with no dislocations is 20 100x greater than a high dislocation density material o Materials with no dislocations may be very strong but they can t be deformed o o Grain boundaries serves and formation sites during deformation The dislocations weaken a material but make plastic deformation possible Imperfections in solids Shear deformation o o Dislocation motion produces plastic deformation Types of imperfections Vacancy atoms Interstitial atoms Substitutional atoms o o o All point defects 1 2 atoms o Dislocations o Grain boundaries Line defects 1 dimensional Area defects 2 dimensional Types of failure o Fracture Cracking that the material seprates into separate pieces Steps in fracture 1 crack formation 2 crack propagation o Ductile fracture most metals not too cold Occurs with plastic deformation Extensive plastic deformation ahead of crack Crack is stable doesn t expand any more unless applied stress is increases o Brittle fracture ceramic ice cold metals Little plastic deformation Chapter 7 notes Failure Crack is unstable Propagates rapidly without increase in applied stress o When looking at pics brittle fracture is rounded and a ductile like has some necking o Most failure occurs bc of the defects in materials Cracks or flaws stress concentrators Voids or inclusions o Presence of defects is best found beforehand and should be determined non destructively By x ray ultra sonic inspection surface inspection Impact fracture tests Fcc alloy generally ductile fracture mode o o Hcp alloy generally brittle fracture mode o o As seen in lab the temp of these mats is important Bcc alloy brittle mode at low temps and ductile mode at relatively high temp When they go from ductile to brittle at a certain temp and the value for ceramics is much larger than for metals Fracture mechanics designing materials to minimize the chance of fracture were cracks are difficult to avoid o Critical value of the stress intensity factor at a crack tip necessary to produce catastrophic failure under simple uniaxial load EQUATION o o o o o o I stands for mode uniaxial loading C stands for mode critical f is a dimensionless constant related to geometry of specimen and flaw a is the crack length or half the length of an internal crack Ki is a variable but NOT a materials property has unusual unit of Mpa m 1 2 or psi in 1 2 Fracture toughness EQUATION KIC is a measure of a materials resistance to crack propagation It is a material property KIC is dependent on temperature microstructure and strain rate KIC usually increases with a reduction in grain size o Highly brittle materials have low Kic value are high for catastrophic failure o High ductility alloys can undergo significant plastic deformation on both macroscopic and microscopic scale before fracture Effects of cracks on Strength o o o Value of ic decrease with increasing strain rate and decreasing temp Refining the grain increases Kic If load is too high the crack may suddently grow fracture in brittle manner with little plastic deformation Stress intensity factor K characterizes the severity of the crack situation as affected by the crack size stress and geometry Microstructure of facture in metals o o Ductile fracture occurs in transgranular manner goes through the grains o Brittle is intergranular along the grain broundaries