1. Basic Mechanisms of Fracture in Metals
transgranular
(in general)
intergranular

2. DUCTILE FRACTURE: VOID NUCLEATION, GROWTH, and COALESCENCE
Spherical void in a solid, subject to triaxial stress state
The limit load model for void instability. Failure is assummed to occur when the net section stress between voids reaches a critical value

3. Mechanism for ductile crack growth
Ductile growth of an edge crack. The shear lips are produced by the same mechanism as the cup and cone in uniaxial tension
Mechanism for ductile crack growth
Optical micrograph of ductile crack growth in a high strength-low alloy steel
Ductile crack growth in a 45° zig-zag pattern

4. CLEAVAGE FRACTURE SEM fractograph of cleavage in an A 508 steel
One model of cleavage fracture in steels: initiation of cleavage at a microcrack that forms in a second phase particle ahead of the macroscopic crack
CLEAVAGE FRACTURE
SEM fractograph of cleavage in an A 508 steel
Formation of river patterns, as a result of a cleavage crack crossing a twist boundary between grains
River patterns in an A 508 steel. Note the tearing lines (light areas) between parallel cleavage planes

5. MECHANISMS OF FRACTURE IN FATIGUE
5 mm
Fatigue striations
MECHANISMS OF FRACTURE IN FATIGUE
2 mm
Beach marking on a fatigue fracture surface in a thin walled pipe

6. d: onset of load reversal e: peak compressive load
Laird (1967) model of plastic blunting-re-sharpening wich leads to fatigue crack growth in fully reversed fatigue.
a: zero load
b: small tensile load
c: peak tensile load
d: onset of load reversal
e: peak compressive load
f: smal tensile load in the subsequent tensile cycle.
Arrows indicate slip direction
Fatigue Striations of Failure Surface in 2024-T3 Aluminium alloy. Arrow indicates growth direction
Region II of the da/dN vs. DK !!!

7. EXAMPLE: Striation width vs. da/dN
Fracture surface of high-strength Al T3 that failed by cycling fatigue.
Test specimen was a Centre Notch-panel 610 mm x 229 mm, 10 mm thickness with initial crack lenght 13 mm.
Arrow indicates direction of crack growth.
Image corresponds to a position 20 mm from de center of the plate.

8. (Da / DN)mean Block loading sequence Block A: 0.5 mm / cycle
eff
Block loading sequence
Block A
13 MPa m1/2
Block A: 0.5 mm / cycle
Block B: 0.34 mm / cycle
Block C: 0.05 mm / cycle
(Da / DN)mean
Block A, R = 0.5: DKeff = 0.75 DK
DK = 17 MPa m1/2, Ds?, smax?, smin?

9. INTERGRANULAR FRACTURE
Ductile metals usually fail by coalescence of voids formed at inclusions and second phase particles
Brittle metals typically fail by transgranular cleavage
Under special circumstances, HOWEVER, cracks can form and propagate along grain boundaries resulting in intergranular fracture
There is no single mechanism for intergranular fracture. Rather, there are a variety of situations that can lead to cracking on grain boundaries, including:
Precipitation of a brittle phase on the grain boundary
Hydrogen embrittlement and liquid metal embrittlement
Enviromental assisted cracking
Intergranular corrosion
Grain boundary cavitation and cracking at high temperatures

10. Examples:
(1): Brittle phases can be deposited on grain boundaries of steel as a result of improper tempering: tempered martensite embrittlement (tempering at 350 °C). Involves segregation of impurities (P, S) to prior austenite grain boundaries (blue brittleness!!!).
(2): Atomic hydrogen apparently bonds with the metal atoms reducing the cohesive energy strength at grain boundaries. Sources: H2S, hydrogen gas. Important problem in welding of steels: cracking in the Heat Affected Zone (HAZ). Hydrogen is a problem when welding high strength steels: special care!!!
(3): Intergranular fracture in a steel ammonia tank