Lecture 8

Question 1

Dynamic loading

Answer 1

• implanted devices are usually exposed to variable loading conditions
• blood flow, for example, leads to periodic stresses acting on vascular devices
• repeated loading can drive a crack to grow (device will then fail at a lower stress than its reported yield stress)

Question 2

Fatigue-Life Methods

Answer 2

• stress-life method: stress levels only, least accurate for low cycle fatigue, traditional method
• strain-life method: detailed analysis of plastic deformation at localized regions, good for low cycle fatigue applications
• linear elastic fracture mechanics (LEFM) method: assumes a crack is already present, predicts growth with respect to stress intensity

Question 3

Stress-Life Method

Answer 3

• determines strength of materials under the action of fatigue loads, so specimens are subjected to repeating or varying forces of specified magnitudes
• cycles to destruction are counted
• fatigue strength (Sf) is the highest stress that a material can withstand for a given number of cycles

Question 4

S-N Diagram

Answer 4

• shows the varying Sf values for different cycle rates and number of cycles
• some materials have an endurance limit (line beneath which no fatigue failure will occur)

Question 5

Endurance Limit (Se)

Answer 5

• fatigue limit
• a “knee” in the graph
• beyond this strength, failure will not occur
• can take a long time to test, and is identified under carefully controlled conditions
• not all materials have an endurance limit

Question 6

Fatigue Strength (log relationships)

Answer 6

Sf = A * number of cycles^B

A = (f * ultimate tensile strength) ^2 / endurance strength
B = -1/3 * log(f * ultimate tensile strength / endurance strength)

f = fraction Sf/Sut at 1000 cycles

IF F NOT KNOWN:
A = Sut, B = -1/6log (Sut / Se)