Definitions and Long answer questions Flashcards
Define stress
A function of applied load and area, units Pa
Define stress concentration factor
Dimensionless ratio of actual stress and theoretical calculated nominal stress
Define stress intensity factor
Related to conditions in a cracked component under load. Due to singularity, actual stress cannot be estimated. Uses far field stress and crack size. Units Pa.m^1/2
Define strength
A materials ability to withstand applied stress
Define fracture toughness (Kic)
A components ability to resist extension of a crack or withstand applied stress intensity K
What quantitative relationships does stress intensity factor give?
Crack length
Component and flaw geometry
Stress intensity factor
Applied stress
What is the Linear elastic fracture mechanics LEFM approach
All materials contain flaws
K,may be calculated fro a particular loading and crack configuration
Failure is predicted if K exceeds Kc
Mode 1 plane strain fracture toughness is an experimentally determined property
Strength vs fracture toughness
As strength increases fracture toughness decreases
Name some life limiting factors of materials
Material defects Design Fabrication methods Operating conditions Fatigue failure from cyclic loading Stress intensity factor High strain rate impact
Mode 1 failure is?
Opening ( most common on practise)
Mode 2 failure is?
Sliding, materials with planes of weakness e.g. Laminates
Mode 3 failure is?
Tearing mode. E.g. Torsional loading
Why can’t inglis be used for cracks?
Cracks would have no volume so max stress would be infinite
How is thickness related to K?
For thin specimens K can’t be predicted as it cannot be determined from plane stress dominated tests
How can a Time vs Flaw Size graph be used?
Predict component life span, by using critical crack size cyclic loading to calculate the number of cycles it will take to grow a crack to critical length under normal operating conditions