6 Flashcards
What does a notch do to stress?
- Increase average stress by reducing x-section
- Concentrates Stress (Kt)
What is Nominal Stress?
The average stress in the net-section higher that the applied stress
Stress Concentration factor, Kt=
σpeak/σnominal
For a circular hole, Kt=
3
For an elliptical hole, Kt=
1+2(a/r)½
What is Fracture Mechanics?
The general analysis of the failure of structural materials with existing flaws. The main outcome is fracture toughness
Why is there such a variation between theoretical and experimental bond strength?
Stress concentration at microscopic flaws. It is amplified at tips of microscratches, voids, notches, surface scratches, corners etc.
What are Stress Raisers?
- Microcracks
- Voids
- Notches
- Surface Scratches
- Corners
What does the magnitude of ampification of Stress depend on?
Microcrack orientation, geometry and dimensions
Define Fracture toughness?
Critical value of stress intensity factor at a crack-tip necessary to produce catastrophic failure under simple uniaxial load
Stress Intensity Factor, KI=
fσ(πa)½
a= crack length
KI is a variable
Fracture Toughness, KIC=
fσmax(πamax)½
This is a property and increases with refinement of grain size
Design Criterion for KIC
KI <=KIC
Brittle materials have
Low KIC
KIC decreases when
- Strain rate increases
- Temperature decreases
KIC increases when
Grain size decreases
Cracks will suddenly grow if
Load is too high, leading to fracture in a brittle manner
Ductile fracture occurs in a
Transgranular manner- Through the grains
Brittle fracture occurs in a
Intergranular manner- along grain boundaries. Enhanced by impurities
How does ductile fracture occur in a simple tensile test?
- Nucleation, growth and coalescence of microvoids at necked region
- The stress then causes seperation of grain boundaries
- As local stress increases, microvoids grow and coalesce into larger cavities
- Eventually metal-metal contact is too small to support load thus fracture
Which is more notch-sensitive under tensile loading; Brittle or Ductile?
Brittle
What is Fatigue?
The lowering of strength or failure of a material due to repetitive stress
What are the 3 stages of fatigue?
- A small crack nucleates at the surface and can include scratches, pits, sharp corners, inclusions, grain boundaries or dislocation concentrations
- Crack gradually propagates as the load continues to cycle
- Sudden fracture when remaining x-section is too small to support applied load
For Fatigue to occurs at least part of the load must be
Tensile
Fatigue is common in
Plastics and metals. Not common in ceramics due to low fracture toughness
What is Striation?
The cyclic build up of crack growth from a surface intrusion. These are on a finer scale and show the position of crack tips after each cycle
What is Granular portion of Fracture surface?
Rapid crack propagtion at the time of catastrophic failure
How are the theoretical infinite stresses relieved at tip ends in practice ?
- Ductile materials- Blunted crack tips
- Some polymers- Elongated voids with bridging structure
- Brittle materials- regions of high density tiny cracks
What is thermal fatigue?
Repeated heating and cooling can cause cyclic stresses due to differential thermal expansion and contraction
Mean Stress, σmean=
(σmax+σmin)/2
Stress amplitude, σa=
(σmax-σmin)/2
Stress range, σr=
σmax-σmin
Stress ratio, R=
σmin/σmax
Stress concentration decreases the
Fatigue limit
Low cycle fatigue
High loads, plastic and elastic deformation
High cycle fatigue
Low loads, elastic deformation
Define fatigue limit
When the S-N curve becomes flat when stress amplitude decreases to a certain level
What is fatigue strength?
For materials that don’t show fatigue limit, the stress level at which the failure will occur for a specified nuber of cycles
Endurance Ratio=
Fatigue limit/Tensile Strength ≈ 0.25-0.5
Factors affecting Fatigue Life?
- Magnitude of stress
- Qualities of surface
- Thermal fatigue
- Corrosion fatigue
Ways to improve fatigue life
- Polishing
- Introducing compressive stress by “shot peening”
- Case hardening- Introduces compressive stress
- Optimize geometry- no internal corners etc.
Stress intensity factor, delta K=
f(σmax-σmin)(πa)½
Selection criteria for fatigue?
- Initiation and growth of small cracks= 80-90% of total life. Remainder of life has fast groth to failure
- Experimental strength justification based on S-N fatigue (safe life)
- Experimental life 3-4 times required life
- Thickness of fatigue critical parts is greater than required for static strength
Means to increase life?
- Damage tolerance approach
- Avoid stress concentrations
- Less fatigue senstive materials
