Crack Paths 2009

uncoated condition, reflecting increased surface protection and hardness rather than

yield strength (which is reduced in both these conditions compared with the uncoated

case).

C R A CPKA T H S

Table 4 summarises the observed crack paths in all these alloys grades. It allows

conclusions to be drawn regarding the mechanisms of crack initiation and growth and

provides a rationale to explain and support the fatigue life data given in Figures 3 and 4.

On the basis of the E L CH D Gand IF (TiNb) G Adata in Table 4, increasing the kt value

from 1 to 3 has no influence on crack initiation mechanism.

Table 4. Crack path observations for these steel grades

Alloy

Surface Condition Stress ktConc. Pre-S%train

Crack Path

Lowcycle

High cycle

ELC

H D G 1 & 3

0

Ductile

Ductile

ELC

H D G

1

10

Ductile

Infrequent IG

facets near

initiation and in

interior

IF (TiNb)

G A

1 & 3

0

IG facets ~100- 200 μ m from surface, ductile

Extensive IG

faceting

interior

IF (TiNb)

G A

1

10

IG facets ~100- 200 μ m from surface, ductile

Extensive IG

faceting

interior

IF (TiNb)

H D G

3

0

Infrequent IG facets ~10-30 μ m

Mixed IG/ductile

from surface

IF (TiNb)

U C

3

0

Infrequent IG facets ~10-30 μ m

Mixed IG/ductile

from surface

IF (TiNbP)

G A

1

0

Infrequent IG facets ~10-50 μ m from surface, ductile interior

Infrequent IG

facets ~10-50 μ m

from surface,

ductile interior

IF (TiNbP)

H D G

1

0

Ductile

Mixed IG/ductile

IF (TiNbPB)

G A

1

0

Ductile

Ductile

37