Lecture 15

Question 1

Experimental Fracture Mechanics - History

• Irwin (1957)

concept of …….. ………. …….. (SIF)
characterised stress field around a crack

• Westergaard

Demonstrated how SIF could be derived using classical elasticity theory

• Forms the basis of ……… ………… ………… mechanics (LEFM)

Answer 1

Stress intensity factor

linear elastic fracture mechanics

Question 2

Experimental Fracture Mechanics - 3 modes

Sketch the 3 modes of cracking ?

Answer 2

Question 3

Theoretical background

What does this show us?

Answer 3

Can show stress field round crack subject to mode I loading

Question 4

Theoretical background 2

what does this show us?

Answer 4

Stress field for mode II loading

Question 5

Experimental Fracture Mechanics - Practical Application

• Techniques based on mode I & II equations (1) & (2) - . . . . must apply
• But, theoretical ……… stress at crack tip (zone .. ) - imposes non-linear effects
• However, stress singularity at crack tip dscribed by equation (1) only applies near crack tip (outside zone ..)
• Measurements taken in zone .. only

Answer 5

LEFM

infinite

III

I

II

Question 6

Experimental Fracture Mechanics - Photoelasticity

Stress optic law relates fringe order N to maximum in-plane stress

• what is this equation (4) ? where f and t are fringe constants and specimen thickness.

what is Mohrs circle equation (5) for this?

Answer 6

Question 7

Experimental Fracture Mechanics - PE 2

what does this tell us ?

Answer 7

This expression used in many ways to evaluate K_I and K_II

Question 8

Experimental Fracture Mechanics - classical approach (fill in the gaps)

with this , what is it possible to find?

Answer 8

Using (6), (7) with K_II =0, possible to find K_I and sigma_ox in terms of r_m, q_m and N_m

Question 9

PE Pattern

what does this show ?

Answer 9

pure mode I crack

Question 10

PE Pattern 2

what type of fringe pattern is it ? What does this show ?

Answer 10

Isochromatic fringe pattern around
a crack loaded in mixed-mode tension

Question 11

Line approach

how are and K_I sigma_ox found ?

Answer 11

K_I and sigma_ox found using two points (r₁, 90) and (r₂, 90)

Question 12

Experimental fracture mechanics - other approaches

what are 2 other approaches?

• ……..-……. ………………
• expression (6) contains three unknowns K_I, K_II and sigma _ox
• select three arbitrary points
• solve using Newton-Raphson iteration
• …….-…………….
• select more than three points
• surface fitting process used

Answer 12

Three-point deterministic

Over-deterministic

Question 13

Practical use of PE

• ……. made with blade or thin saw
• ………-…………. PE
• slices taken …………. to crack front
• LEFM requires plane sections remain ……
• surface slices tend to give large errors
• Reflection PE
• Examine real cracks in ……. and ……… tests
• Coatings must be ….., not bridging crack or plastic zone
• Transmission PE
• Used to examine propagation paths

Answer 13

crack

Three dimensional

perpendicular

plane

static , fatigue

thin