PSI - Issue 14

Sachin Bandgar et al. / Procedia Structural Integrity 14 (2019) 330–336 Sachin V Bandgar/ Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction Ship hull structures and their integrity are of paramount importance for ship building industries as these materials are subjected to high fatigue loading during operational conditions. This is particularly true for high strength low alloy steels which are being used nowadays for hulls and superstructures due to their enhanced properties and lower- cost fabrication practices [Czyryca EJ. (1990)]. High Strength Low Alloy steel (HSLA) or mircoalloyed steel have properties of plain carbon steel as well as high tensile steels with better mechanical properties, higher load carrying capacity, lighter weight(High strength to load ratio) and good resistance to corrosion than plain carbon steels. Due to these factors, HSLA steels are used in heavy constructions like ship building, oil and gas transmission lines, and offshore drilling platforms. But, majority of HSLA steels are prone to fatigue failure in service [Htayaung (2007)]. Also, it is reported that 90% of mechanical failure is due to metal fatigue. The fatigue properties of a high strength low alloy steel are governed by its chemical composition, processing history, microstructural features, nature of loading and the test environment to which they are exposed to in service. Hence the fatigue crack growth rate (FCGR) behaviour in air of two different HSLA steels (with varying microstructures) at different load ratios are studied in this paper. Of the two HSLA steels, steel A has Ferrite- pearlite microstructure and steel B has Tempered martensite. The chemical composition of steel A and steel B is mentioned in table1 and 2.Mechanical properties are mentioned in table 3.

Table 1. Composition of Steel A

Composition

C

S

P

Mn

Si

Cr

Ni

Steel A

0.09

0.004

0.004

1.45

0.38

0.07

0.072

Table 2. Composition of Steel B

Composition

C

S

P

Mo

Si

Cr

Ni

Steel B

0.073

0.01

0.02

0.28

0.28

0.45

1.88

.

Fig 1. (a) Steel A-Ferrite+ pearlite ; (b) Steel B-Tempered martensite

Table 3. Mechanical properties of Steel A and Steel B

Material

YS (MPa)

UTS (MPa)

%EI

450 + 10

520 + 20

25 + 10

Steel A

655 + 10

720 + 20

15 + 10

Steel B