Materials Selection- Fatigue Intro Flashcards
What can fluctuating or cyclic stresses lead to?
Failure at a stress level lower than the yield or ultimate tensile strength
Stress range, amplitude, ratio and mean stress
Stress range = σmax-σmin=Δ σ
Stress amplitude =Δ σ/2=σa
Stress ratio = σmin/σmax =R
Mean stress = 1/2(σmax+σmin)
3 factors to cause fatigue failure
Maximum tensile stress of sufficiently high value
Large enough fluctuation (range/ amplitude) of applied stress
Sufficiently large number of cycles N of the applied stress
Diagram for how fatigue testing works
See page 6 lecture 4
Fatigue testing in lab
Material subjected to relatively large stress amplitude (about 2/3 UTS). Number of cycles at this fluctuating stress required to cause failure is measured. Process repeated at progressively smaller stress amplitudes.
Describe S-N curve
Stress amplitude on y axis and number of cycles to failure on log scale x axis. Generally curves down and sometimes reaches minimum. If know stress amplitude, read off number of cycles to failure from line.
Fatigue endurance limit σe or Se
Stress below which the lifetime of the part is infinite
High cycle fatigue
Nf >10^3-10^4
Deformation largely elastic with some localised (or microscopic) plasticity.
Low cycle fatigue
Higher levels of stress amplitude means fatigue life decreases such that Nf<10^3-10^4
More plastic deformation occurs
Stages of high cycle fatigue failure
Occurs by initiation and growth of a crack in a component. Stable crack growth occurs with successive cyclic tensile stresses until it reaches a critical crack length. Then fast fracture takes place.
What does high cycle fatigue fracture look like?
See slide 11 and 12 lecture 4
Graph of crack length against number of cycles for HCF
Crack length on log y axis. N/Nf on normal x axis. Region 1 is crack initiation and early crack growth, line steep and gets shallower. Region 2 is stable crack growth with very shallow gradient over about 0.17 to 0.7 on x axis. Region 3 is final fast fracture and line curves up like exponential. See slide 13
Crack initiation and early crack growth
Depend on microstructure and plastic flow properties of the material. Small fatigue crack initially grows along shear planes which are 45° to tensile stress axis. Finer grains mean closer spacing between grain boundaries which crack has to break through so increased yield strength. Then tension driven crack propagation perpendicular to tensile stress axis for stable crack growth.
Stable crack growth
Fatigue crack growth normal to tensile load. Growth rate fairly insensitive to microstructure (grain size has only small effect). Crack growth rate can be modelled using Paris law equation
Paris law equation
da/dN=C(ΔK)^m=C(YΔ σrt(πa))^m C and m constants different for each material K is stress intensity parameter a is crack length N is number of cycles Y is geometry factor
Final fast fracture
Behaviour rather sensitive to microstructure and flow properties of the material. Basically a brittle or ductile failure so either fast crack propagation or tensile overload.
What metallurgical considerations affect fatigue life?
Inclusions nucleation cracks so cleanliness during manufacture improves the fatigue life. Internal pores also sites for nucleation of fatigue cracks. So castings have lower fatigue performance than wrought materials. Increasing hardness or strength tends to raise fatigue endurance limit for HCF largely due to increased resistance to fatigue crack initiation.
How does fatigue endurance limit relate to UTS?
Is about 1/3 of it
How do notches affect fatigue life?
They decrease fatigue life through introduction of a stress concentrator. Should incorporate rounded fillet on a sharp corner.
How does surface roughness affect fatigue life?
Increased surface roughness lower fatigue life and so do corrosive environments. Rough surface goes up and down and the bottom corners are stress concentration points.
How does shot peening work?
Introduce moderate compressive residual stresses at the surface to increase the fatigue life. Small ball-bearings shot at component. See slide 21 for diagram
Fatigue S-N probability of failure curves
Multiple curves for same material. Each one has a certain chance of survival at each point along it Ps. Can be 99%, 90, 75, 50
Procedure for determining an expected fatigue life
Perform a number of texts at fixed stress amplitude and determine mean and standard deviation of sample (formulae on slide 24). Can find expected life for given probability of survival and confidence level using Xbar-qS
S is standard deviation and q is read from tables
Statistical look up table for determining q
Slide 25. Use given confidence level and Ps and number of tests done to locate number in table and this is q.
