Chapter 8 Flashcards
Things that make a material tough
Whatever blunts the crack tip
Whatever increases the amount of energy necessary for the crack to grow (most importantly, plastic deformation, hence the correlation between low strength and high toughness)
Mode 1 Fracture
Tensile Opening
Most important
Mode 2 Fracture
in-plane shear
Least important, most of the time the crack rotates to become a Mode 1 Fracture
Mode 3 Fracture
Out-of-plane shear
2nd most important
Applies in torsion of shafts
Design against fracture
K1 = K1c
where,
K1 = C * stress intensity of crack * square root (pi * length of crack)
K1c is a material property (critical mode 1 stress intensity factor called plane strain fracture toughness)
Prevent/Delay Fracture
Reducing stresses
Reducing stress concentrations near cracks
Choosing a tougher material (or heat treating your current material to be softer/more ductile)
Mechanism of crack initiation
Local plastic deformation at stress concentrations
Irregular slip on slip planes eventually creates a crack-like features
Prevent/delay crack initiation by reducing stress, reducing stress concentrations, making surface harder, making surface smoother.
Mechanisms of Crack Growth
Crack opening during tensile half-cycle blunts crack tip by plastic deformation and creates extra surface
Surface folds on itself during compressive half-cycle creating increment of crack length
Prevent/delay crack growth by reducing stresses, reducing stress concentrations, making material harder, introducing compressive residual stresses to prevent/reduce the tensile opening of the crack during tensile half-cycle
For initial defect free parts
Most of fatigue life is spent initiating the crack
a) Use S/N curve where S is stress amplitude for fully reversed cycles and N is fatigue life
b) Some BCC Materials like steel have an endurance limit. For stress amplitudes below endurance limit, fatigue life is infinite. FCC materials have no endurance limit
For objects with preexisting cracks
All of life is spent growing crack from initial size to critical size
a) Do initial nondestruction inspection
b) Find critical crack size ai using K1=KIC assuming worst possible crack location and orientation
c) Use crack growth equation to find number of cycles necessary to grow from ai to ac
d) If crack growth is small enough so the geometry factor C in KI does not change appreciably and can assume to be constant, closed form solution in handout may apply
Creep
Continuing elongation of a material under constant load or stress at high temperature
Applications include turbine blades, steam pipes
Three Stages of Creep
Primary Stage
Secondary Stage / Steady State
Tertiary Stage
Creep Temperatures
Temperatures above 0.4Tm - 0.5Tm
Creep and Grain Boundaries
Grain Boundaries are helpful mechanisms at low temp applications but are a liability at high temp operations where creep occurs
Creep Primary Stage
Dislocation density increases
Strain rate decreases
