Ortho Exam 2 - Lecture 13

Fatigue Analysis

Ductility

During tensile test to fracture, measure of how much the material elongates permanently form initial gauge length

Percent elongation

Ductile material

Will stretch and yield prior to actual fracture, causing noticeable decrease in cross sectional area of fractured region

Brittle material

Will fracture suddenly with little or no change in cross sectional area of fractured region

Ductile or brittle material more favorable for implants if they fail?

In general, more ductile fracture is desirable to give us a warning prior to fracture

What’s more common, fatigue failures or static over load? Why? Where does it occur?

Fatigue fractures due to multiple (cyclic) loading of material. Occurs at service loads at or below the maximum predicted stress levels expected during function. Always begins at flaw or crack

3 stages of fatigue fracture

Crack initiation

Crack propagation

Fracture - associated with crack growth instability

What is fatigue life drastically reduced by

The presence of flaws

(Crack initiation State is reduced or completely eliminated)

What primarily causes crack growth?

Cyclic tensile stresses

Describe Mode 1 of Basic Modes of Crack Displacement

Mode 1 - opening or tensile mode

crack surface move directly apart

Requires Mode 1 Stress Intensity Factor K(1)

Critical stress

Stress required for crack propagation in a brittle material

Fracture toughness

A property that is a measure of a material’s resistance to brittle fracture when a crack is present

Plain strain fracture toughness

Fracture toughness under Mode 1 type crack propagation

Fracture Strength

corresponds to stress, at fracture under a tensile load, that is “tensile strength”

fracture threshold

below stress. Kio corresponds to zero crack velocity. Slow of sub-critical crack growth

Total Life Approach

assumes the component is initially free of any flaws that are sufficiently sized for growth

fatigue failure due to crack nucleation and subsequent growth

majority of component life is spent in nucleation

Test Type for TLA: Stress-based test

measures conditions for failure over range of stress amplitudes and mean stresses

unnotched subject to cyclic loading, generates S-N fatigue plots, determines high cycle fatigue behavior

Design for fatigue resistance is centered on

stress-based testing

Stress Amplitude Cycle Curve

S-N curve, the relationship between applied stress and expected life, used to determine fatigue life

S = cyclic stress range

N = number of cycles to failure

Fatigue failure

failure at a point below the ultimate strength secondary to repetitive loading

Endurance Limit

maximal stress under which an object is immune to fatigue failure, regardless of number of cycles. Much lower than yield stress or ultimate tensile stress

Goodman Line

provides ability to predict life at any stress ratio

points below the relevant Goodman line represent longer life

Modified Goodman Line

endurance limit to yield stress

below the modified goodman line = safe zone