Crack Morphology

The incubation period varies widely depending on such factors as crack morphology, water chemistry, and temperature. However, experience in a wide variety of cooling water environments has shown that many stainless alloys develop noticeable attack within 6 months of first being exposed to water. It is rare to see attack initiating many years after equipment commissioning unless service conditions change in the interim. 

H. Kitagawa, R. Yuuki, and T. Ohira, Crack Morphological Aspects in Fracture Mechanics, Eng. Fract. Mech., 7, 515-529 (1975). 

Fig. 25. Crack morphology and failure modes of A508C1.2 CERT specimens as a function of temperature and oxygen concenPation [58]. Reproduced from Corrosion J, 38, 136 (1982) by permission.

The indent size, the surface crack morphology, and the extent of subsurface damage are all influenced by the crystal structure, chemical composition, and micro-structural morphology of the material. 

On the basis of the observation of fracture and debonding for quasi-static indentation, crack morphologies in an atmospheric pressure plasma spray specimen were reproduced with 4 mm indenter. In the case of pressurized plasma spray specimen, crack morphologies were also reproduced with 4 mm indenter except debonding from cone crack tip. 

Figure 1.19 shows typical secondary cracks on the specimen gage length. Only half of the fractured specimen is shown since the other half exhibited similar cracking morphology. These cracks are typically developed in ductile materials susceptible to SCC. This type of specimen fracture is attributable to SCC, which is corroborated by the drop-off in ductility and the formation of secondary cracks. Therefore, the combination of an applied stress and applied potential in a corrosive environment degrade the material mechanical properties. 

Corrosion damage that is time- and cycle-dependent it becomes important or dominant at an intermediate load and/or frequency. In such conditions, cracking may be transgranular or intergranular, and its morphology may become very similar to SCC crack morphology. 

Fig. 9.24 Regioregular HT PDDT film cast from xylene. Film shows brittle cracked morphology.

Ni alloys coatings formed in ILs showed a moderate corrosion pa-otection, materialized in polarization resistances of 30-40 kQ, probably due to the cracked morphology. However, after 240 hours of continuous immersion in chloride containing aggresive aqueous medium the exposed specimens did not exhibited any major surface modification and no pits have been evidenced. 

Crack morphology Transgranular Intergranular Branched Transgranular Unbranched Blunt tip Transgranular Intergranular Unbranched Sharp tip... 

Tyfour, W R and Beynon, J H, 1994, "The effect of rolling direction reversal on fatigue crack morphology and propagation". Tribology International, Vol 27, pp 273-282. 

On each type of coupon, a monotonic tensile test interrupted at 150 MPa was also performed, in order to polish the entire face of the coupon and to compare crack morphology to predicted stress fields on coupons that were not subject to tensile rupture. 

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