Basic Mechanisms of

Fracture in Metals

MECHANISMS OF FRACTURE IN METALS

transgranular

(in general)

intergranular

transgranular

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

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

Ductile crack growth in a 45° zig-zag pattern Optical micrograph of ductile crack growth in a high strength-low alloy steel

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

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

MECHANISMS OF FRACTURE IN FATIGUE

2 mm

Beach marking on a fatigue fracture surface in a thin walled pipe

5 m

Fatigue striations

Fatigue Striations of Failure Surface in 2024-T3 Aluminium alloy. Arrow indicates growth direction

Region II of the da/dN vs. K !!!

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

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.

Block loading sequence

Block A: 0.5 m / cycleBlock B: 0.34 m / cycleBlock C: 0.05 m / cycle

a / N)mean

eff

Block A, R = 0.5: Keff = 0.75 K

K = 17 MPa m1/2, ?, max?, min?

Block A

13 MPa m1/2

INTERGRANULAR FRACTURE

Ductile metals usually fail by coalescence of voids formed at inclusions and second phase particles

Brittle metals typically fail by transgranular cleavage

There is no single mechanism for intergranular fracture. Rather, there are a variety of situations that can lead to cracking on grain boundaries, including:

1. Precipitation of a brittle phase on the grain boundary

2. Hydrogen embrittlement and liquid metal embrittlement

3. Enviromental assisted cracking

4. Intergranular corrosion

5. Grain boundary cavitation and cracking at high temperatures

Under special circumstances, HOWEVER, cracks can form and propagate along grain boundaries resulting in intergranular fracture

(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!!!

Examples:

(3): Intergranular fracture in a steel ammonia tank