Definitions and Long answer questions

Question 1

Define stress

Answer 1

A function of applied load and area, units Pa

Question 2

Define stress concentration factor

Answer 2

Dimensionless ratio of actual stress and theoretical calculated nominal stress

Question 3

Define stress intensity factor

Answer 3

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

Question 4

Define strength

Answer 4

A materials ability to withstand applied stress

Question 5

Define fracture toughness (Kic)

Answer 5

A components ability to resist extension of a crack or withstand applied stress intensity K

Question 6

What quantitative relationships does stress intensity factor give?

Answer 6

Crack length
Component and flaw geometry
Stress intensity factor
Applied stress

Question 7

What is the Linear elastic fracture mechanics LEFM approach

Answer 7

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

Question 8

Strength vs fracture toughness

Answer 8

As strength increases fracture toughness decreases

Question 9

Name some life limiting factors of materials

Answer 9

Material defects
Design
Fabrication methods
Operating conditions 
Fatigue failure from cyclic loading
Stress intensity factor
High strain rate impact

Question 10

Mode 1 failure is?

Answer 10

Opening ( most common on practise)

Question 11

Mode 2 failure is?

Answer 11

Sliding, materials with planes of weakness e.g. Laminates

Question 12

Mode 3 failure is?

Answer 12

Tearing mode. E.g. Torsional loading

Question 13

Why can’t inglis be used for cracks?

Answer 13

Cracks would have no volume so max stress would be infinite

Question 14

How is thickness related to K?

Answer 14

For thin specimens K can’t be predicted as it cannot be determined from plane stress dominated tests

Question 15

How can a Time vs Flaw Size graph be used?

Answer 15

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

Question 16

What problem remains when Inglis can’t be used in a crack?

Answer 16

Unknown stress for a singularity problem that depends on far field stress and defect size (STRESS INTENSITY FACTOR)

Question 17

What does the cut off line on a FAD represent?

Answer 17

It is used to prevent plastic collapse and its at the Lr max value