Fractures- Failure Assessment Diagrams

Question 1

Plastic collapse

Answer 1

Is plastic deformation by slip. It can occur at scale before fracture. Strength is determined by whichever occurs first.

Question 2

Fracture vs plastic failure

Answer 2

Fracture: crack is driven forward to break something into 2 pieces.
Plastic failure: means permanent deformation of material which leads to it not bearing load the same way it used to.

Question 3

What are the mechanics of problems involving plasticity related to?

Answer 3

The yield stress σys

Question 4

Formula for Lr for plastic collapse when cracks are and aren’t present

Answer 4

No cracks: Lr=Applied stress/Collapse stress
Cracks: Lr=Applied load/Collapse load= P/Pc
Normally use one with cracks

Question 5

Where to find formulae for Pc for plates yielding by plastic deformation with centre crack or edge crack

Answer 5

Page 7 lecture 6 and datapack

Question 6

Where to find formula for Pc for pressurised cylinders

Answer 6

Page 8 lecture 6 and datapack. The σys t/R is the hoop stress. Extra diagrams in lecture notes clarify dimensions.

Question 7

Formula for Kr

Answer 7

Kr=Applied K1/K1c=Cσapp rt(πa)/K1c

K1c is fracture toughness

Question 8

General failure assessment diagram

Answer 8

Option 1. Geometry and material dependent. Kr vs Lr. Line curves down then straightish diagonal down then curves to be shallower gradient. safe case under line. Unsafe case above line. Critical case on line.
Equation on page 10 lecture 6 based on experimental data

Question 9

How to find safety factor on failure assessment diagram

Answer 9

Plot the point A(Lr, Kr) based on the conditions in the test. Draw straight line from origin to A and extend it until it meets the line at point B. Safety factor FL (subscript L) is OB/OA so minimum FL is 1.

Question 10

How do Kr and Lr change for a larger crack length?

Answer 10

Kr increases more rapidly than Lr with increasing crack length.