Fractures- Fracture Toughness Testing Flashcards
Basic approach to measuring fracture toughness
Prepare sample with crack of known size. Test until failure. Use failure stress to calculate K1 at crack tip at failure.
Which region is K1 measured in?
Plane strain region where the K1 at failure will be lower than plane stress or transition regions. Plastic zone is smaller here.
Differences between toughness obtained in plane strain and stress regions
Plane strain: minimum toughness here. Is a material property. This is what is conventionally measured and referred to as K1c.
Plane stress: may significantly underestimate toughness in thin sections for through thickness cracks. Sometimes measure toughness on relevant section thickness. Is not fracture toughness and is not material property.
Where to find formulae for single edge notched beam (SENB) test and conditions
Page 7 lecture 5 or datapack page 11
Where to find formulae for compact tension test and conditions
Page 8 lecture 5 or datapack page 11
Range of values of f(a/W) for SENB and compact tension
SENB: 2.3 to 3.1
Compact tension: 8.3 to 11.4
Empirical rules to ensure you are in plane strain fracture toughness regime
a>=2.5(K1/σys)^2
B>=2.5(K1/σys)^2
W>=5(K1/σys)^2
Problem with using rules to determine if you are in plane strain region
Rules require you know result of test before making measurement
Solutions to problem with using rules to determine if you are in plane strain region
Could overestimate likely toughness based on similar materials.
Use largest possible specimen.
Estimate specimen size based on E and σys.
Make specimen and test, assess whether rules were met, repeat with large specimen if not.
Recommended minimum specimen dimension table
Page 10 lecture 5. Uses σys/E (E is YM).
Gives min values of a and b
Method of determining onset of crack growth
Plot load against displacement of sample. There is a splinters region. Plot line from origin with 5% lower gradient (5% secant offset) than this. This corresponds to the effective crack length being 2% greater than at the beginning of the test.
Type 1 load vs displacement graph
Linear then gradient decreases to a point where the line drops off at failure. Intersection of 5 secant offset line with curved part is Ps. This is equal to Pq. Pmax is at maximum load just before failure.
Type 2 (pop-in) load vs displacement graph
Linear region then line comes back down a bit then goes back up to slightly higher peak then drop off at failure. Ps at first intersection. Pq at first peak before goes down. Pmax at highest peak.
Type 3 (brittle fracture) load vs displacement graph
Linear region then line straight diagonal at failure. Ps at intersection. Pq is Pmax at peak load just before failure.
How to tell if empirical rules for being in plane strain are met
Use Pq to obtain conditional K value which is Kq. Check Kq meets empirical conditions for a, b and W. If it does, K1c=Kq. If not test should be done with larger test piece.
q subscript is capital.
