Basic Mechanisms of Fracture in Metals

1. Basic Mechanisms of Fracture in Metals MECHANISMS OF FRACTURE IN METALS transgranular transgranular (in general) intergranular

2. 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 DUCTILE FRACTURE: VOID NUCLEATION, GROWTH, and COALESCENCE

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

4. 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 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

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

6. 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 2024 - 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. Block A 13 MPa m1/2 Block loading sequence eff Block A: 0.5 mm / cycle Block B: 0.34 mm / cycle Block C: 0.05 mm / cycle Block A, R = 0.5: DKeff = 0.75 DK (Da / DN)mean 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. (3): Intergranular fracture in a steel ammonia tank 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!!!