1. Materials Engineering
Lecture 8:
Failure
Strengthening mechanisms

2. Failure by Fracture

3. Ductile fracture

4. Brittle fracture

5. Plain Strain Fracture Toughness
KIc is the fracture toughness; the lower KIc the easier is the brittle fracture
KIc, MPa∙m-1/2
Yield Strength, MPa
Material 24
495
Al alloy 7075 (5.6Zn, 2.5Mg, 1.6Cu, 0.23Cr) 44
345
Al alloy 2024 (4.4Cu, 1.2Mn, 0.1Zn) 55
910
Titanium alloy Ti-6Al-4V 50
1640
Alloy steel 4340 tempered at 260ºC
87.4
1420
Alloy steel 4340 tempered at 425ºC
Concrete
Soda-Lime Glass
Alumina
Polysterene
Polymethyl Methacrylate
2.2
62.1
Polycarbonate

6. Impact Fracture Testing

7. Ductile-to-Brittle Transition

8. Fatigue

9. S-N curves (Stress-life curves)
Material with infinite life (steels, titanium)
Materials with continuously decreasing S-N curves (Al, Mg, Cu alloys)
As a rough guide, the fatigue limit is usually about 40% of the tensile strength.

10. Case Hardening Case hardening: surface carburizing or nitriding.
Very hard carbides or nitrides are formed
Hardened steels have higher endurance limit

11. Strengthening mechanisms
The ability of a crystalline material to plastically deform largely depends on the ability for dislocation to move within a material.
Therefore, impeding the movement of dislocations will result in the strengthening of the material. There are a number of ways to impede dislocation movement, which include:
controlling the grain size (reducing continuity of atomic planes)
strain hardening (creating and tangling dislocations)
alloying (introducing point defects and more grains to pin dislocation)

12. Grain Size Strengthening
The smaller the grains in a metal, the more grain boundaries there are.
The more grain boundaries, the more likely that a dislocation will be stopped.
The more likely that a dislocation will be stopped, the less brittle the material is.

13. Strain Hardening Cold working produces more dislocations.
The dislocations “trap” each other, because their movement becomes difficult.

14. Solid Solutions Strengthening

15. Annealing: Recovery and Recrystallization
Cold working results in internal stress;
To release the stress annealing is made (heating to ~ 40% of m.p.)
First, recovery occurs;
Then recrystallization takes place.
Last, crystal growth happens.

16. Annealing (2)

17. Musts of this lecture Ductile and brittle fracture (mechanisms);
Fracture toughness (KIC);
Impact fracture testing;
Ductile-to-Brittle Transition;
Fatigue;
Stress-life curves;
Case hardening;
Strengthening mechanisms: general idea;
Grain Size Strengthening;
Strain Hardening;
Solid Solutions Strengthening;
Annealing: Recovery and Recrystallization.