Unit 6 - Fatigue Lifing Methods Flashcards
What does initiation refer to when discussing fatigue lifing?
The life to first crack, not the scientific definition of initiation.
What is meant by an “engineering sized” crack?
A crack which is larger than the lower limit of the inspection capability.
* For dye penetrate testing this is 0.7mm
* For ultrasonic testing this is ~0.3mm
What is meant by crack propagation life?
The cycles required to propagate a crack from initiation to failure.
What is meant by “safe life”?
An approach within which generous safety factors are given based on life to first crack data.
What is meant by “fail safe”?
The design of a component is based off of the failure being benign and not catastrophic.
When is a fail safe approach appropriate?
When the benign failure will not significantly impact continued operation e.g. engine casing structures.
When is the damage tolerance approach used?
When:
* There are known pre-existing defects
* An assumption is made that there are defects present that are below the inspection (NDT) threshold
How is the damage tolerance approach used?
- When pre-existing defects are known, it approximates the propagation life given the defect size
- When it is assumed that there are defects present, the initial defect size is assumed to be at the inspection threshold
What are the pros and cons of the safe life approach?
Pro:
* Very reliable for safe operation
Con:
* Many structurally sound components are removed from service
* It assumes no defects exist in the components
What are the pros and cons of the damage tolerance approach?
Pro:
* It can substantially increase the life of a component in service compared to safe life
* It takes into account defects in the structure
Con:
* It ignores initiation life which can comprise a large proportion of total data and so it may vastly underestimate the total life
What is the main issue with utilising the Basquin approach to stress life?
It doesn’t take into account mean stress effects (it assumes R = -1)
What does the Basquin method allow the calculation of?
- It allows for the calculation of applied stress through the Basquin formula.
- A modified version also takes into account the temperature range in a gas turbine.
What does the Morrow modification of the Basquin approach add?
It takes into account the mean stress effects by subtracting mean stress from the fatigue strength coefficient
What can the Goodman relationship accomplish and why is this useful?
It can determine the effective stress amplitude for any given R value. Replacing applied stress with effective stress in the Basquin equation take into acount mean stress effects and removes the issue with that approach.
What is the Coffin-Manson relation?
For very low cycle strain controlled fatigue, the elastic contribution is ignored.
