WK15 - Failure Mechanisms Flashcards
Yield strength
Stress at which plastic deformation begins
Typically determined using 0.002 strain offset from stress-strain graph
Tensile strength
Max stress level on engineering stress-strain curve
Ductility
Degree to which material plastically deforms by the time fracture occurs
Modules of Resistance (U_r)
Strain energy per unit volume of a material required to stress a material to the point of yielding
> area under elastic portion of the stress-strain curve
Toughness
Energy absorbed during the fracture of a material
> Area under entire stress-strain curve
ductile materials typically tougher than brittle ones
What is true stress and true strain?
True stress - instantaneous load divided by the instantaneous cross-sectional area
True strain - natural log of ratio of instantaneous length over original length
Ductile vs brittle Failure
Ductile Failure - occurs at the onset of plastic deformation
Brittle Failure - failure occurs at fracture
State the stages of ductile failure - starting at necking
- Necking
- Void nucleation
> dislocations are pinned to obstacles (precipitates) - Coalescence of cavities
> build up of dislocations forms voids - Crack propagation
> voids join up to form microcracks - Fracture
>microcracks join up into large cracks
How to convert between true (St) and engineering stress and strain (3)?
St = Se*(1 + epi_e)
Epi_t = ln(1 + epi_e)
Only valid before necking occurs (before plastic deformation)
What is design stress, Sd
Calculated stress level, Sc, multiplied by a design factor N’ (> 1)
• for less critical static structures and tough materials
Typically favoured over safety stress as there is more uncertainty in the load
What is safe (working) stress, Sw
Yield strength of a material, Sy, divided by a factor of safety, N (1-2 - 4)
• depends on economics, experience, consequences of failure etx
Volumetric vs deviatoric stresses
V - corresponds to a change in the volume of the object, without any change in shape (also called hydrostatic strain)
D - the distortion and twisting of the material due to shear
> failure of ductile materials only depends on the deviatoric component
What is Von Mises stress criterion? How does it differ to Trstca
“Yielding occurs when the maximum distortion energy is equal to the distortion energy of yielding in an uniaxial tensile test”
> reflects that the material doesn’t always yield at the maximum yield stress, it is a better estimate of when a material will start to plastically deform
** not applicable to brittle materials that don’t plastically deform much before failure**
Tresca is based off maximum shear stress theory and is simpler to apply and more conservative
Inter granular vs transgranular fracture
IGF - occurs along grain boundaries, because GB weakened by chemical attack or elemental depletion
• more common in ductile materials
TGF - cleavage of particular atomic planes, ignoring the grain structure
• stronger bonds (than ductile) but less room for movement
• more common in brittle materials
What is the importance of a Charpy test?
Common method to measure toughness
Hammer pendulum creates a notch and energy required to to break is calculated
Also useful for determining ductile-to-brittle transition with decreasing temps
What is the critical stress, Sc
Stress required for crack propagation in brittle materials
Crack will expand locally when S > Sc
What is the stress concentration factor, K_I
A measure of the degree to which an external stress is amplified at the crack tip
• Sm/S0
> occurs at macroscopic internal discontinuities (voids, inclusions, notches, scratches etc)
> ductile materials can redistribute stress in the vicinity of a stress raiser leading to a lower KI value
What is fracture toughness, Kc
A materials resistance to brittle fracture when a crack is present
Thin specimens - Kc depends on specimen thickness
Thick specimens - plain strain exists
Plane strain - no strain component perpendicular to the front and back faces
What is plane strain fracture toughness?
Kic = fracture toughness for thick specimens
Brittle materials have relatively low KIC at crack tip and are vulnerable to catastrophic failure (relatively large for ductile materials)
Define fatigue limit and fatigue life on S-N curves
FLimit (endurance limit) - below this limit fatigue failure will never occur
Most non-ferrous alloys do not have a fatigue limit (defined by fatigue strength)
F life - stress level at which failure will occur for a specified number of cycles
What are beachmarks and Striations?
B - macroscopic dimensions and may be observed with the naked eye
• found when interruptions were experienced during the crack prop stage
S - microscopic in size and need an electron microscope
• each striation represents the advance distance of a crack front during a single loading cycle
Both represent the position of crack tip with concentric ridges that expand away from crack initiation sites
S and B confirm fatigue failure but absence of one or both does not rule out fatigue failure
