- Email: [email protected]
Fatigue crack growth rates in arctic-grade line pipe steel at high ΔK
Scripta METALLURGICA et MATERIALIA
Vol.
26, pp. 1693-1694, 1992 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
FATIGUE CRACK GROWTH RATES IN ARCTIC-GRADE LINE PIPE STEEL AT HIGH AK C. R. Wake and J. G. Byrne Department of Metallurgical Engineering University of Utah Salt Lake City, Utah 84112-1183 (Received
M a r c h 24,
1992)
This work was concerned with the effect of roll direction on fatigue crack propagation in arctic-grade steel plate to be used for pipe fabrication. The steel was provided by Kaiser Steel Corporation and had a composition of 0.12 C, 1.51 Mn, 0.014 P, 0.009 S, 0.10 Si, 0.25 Cu, 0.02 Ni, 0.02 Cr, 0.25 Mo, 0.018 Co, and 0.065 Cb, all in wt. %. There was some alignment of nonmetallic inclusions and of grain shapes parallel to the roll direction. The ASTM plane strain criterion for fatigue testing of compact tension samples is B > 2.5 (K/yield stress) ~ where
(1)
B = compact tensile specimen thickness, and K = stress intensity factor.
The value of K is defined by the algorithm given in Equation (2). The stress at the crack tip is calculated based on the crack length, taking into account the relative location of the load to the crack tip and the reduced cross-sectional area as the crack progresses. K = P/(BW~'5)[29.6(a/W)°'5- 1.85.5(a/W) t'5 + 655.7(a/W) 2"5 - 1017(a/W) 3"5 + 638.9(a/W) ~~]
(2)
where K = stress intensity factor, P = applied load, B = thickness, W = width, and a = crack length. The R value, K , J K , ~ , used in the present work was 0.1. Loading frequency and wave form have been shown to have modest crack propagation effects (1). Light microscope inspection of longitudinal and transverse sections with respect to the rolling direction was performed in both unetehed and etched (2% Nital) conditions. Two tensile samples were machined from each of the directions parallel and perpendicular to the roll direction. The samples were two inches long in the gage length and 0.5 ×0.125 inches in cross-sectional area. These were tested in an Instron tensile tester at a strain rate of 2 x 104 sec~. Compact tension (CT) fatigue samples 7 mm thick with W = 57.5 mm and H -- 52.4 mm were machined in accordance with ASTM E399. The notch was sharpened with an electric discharge machine using an electrode of copper sheet 0.010 inches thick. The side surfaces of the CT samples were treated in a solution of hexa-methyldisilizane (HMDS) to promote photoresist adhesion. A grating with 100 lines per inch was placed perpendicular to each intended crack propagation direction. Fatigue cycling was performed at a frequency of 5 hertz in a closed-loop hydraulic machine with tension-tension load control at an R value of 0.1 and with a sinusoidal wave form. The crack position was followed with a travelling microscope with a resolution of about 20 ttm.
1693 0036-9748/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press Ltd.
1694
FATIGUE IN STEEL
Vol.
26, No.
Micrography indicated slight grain elongation and inclusion alignment parallel to the direction of hot rolling of the original plate. The yield strength of the material was 76,900 psi in the cross-roll direction and 77,850 psi in the roll direction. The ultimate tensile strength was 105,000 psi in the cross-roll direction and 111,500 psi in the roll direction. Fatigue crack growth rates were measured in the high AK region and are shown in Fig. 1 for two specimens in the LT orientation (crack moving in the transverse (T) direction with its normal parallel to the roll (L) direction) and one in the TL ori~tation (crack moving in the roll (L) direction with its normal parallel to the transverse (T) direction). The crack propagation rate was slightly more for sample 2 than for sample 3, and much larger than for sample 1, suggesting that this material offers slightly better resistance to cracks which propagate normal to the roll direction. The crack propagation rat~ are slightly lower than thosefor mild steels when tested in the AK range of 40 to 100 MPa m m (2).
1. M. Clavel, A. Pineau, "Frequency and Wave-Form Effects on the Fatigue Crack Growth Behavior of Alloy 718 at 298 K and 823 K," Metallurgical Transactions, 9A, 471-480 0978). 2. N. E. Frost, L. P. Ponk, and K. Denton, Eng. Fracture Mechanics, 3, 109 (1971).
5x 10.3
NO.2(T-L 4 xlO
-3
l
3xlo3
m
NO. 3(t" ¢J
Figure 1. da/dN versus AK for (T-L) and (L-T) oriented compact tension faligue specimens cycled at 5 hertz at room temperature and R = 0.1.
z
2xl0 "3
-
NO.
10"3 I.--. i
60
"tO
80
AK(MPo/m 112)
90 I00
11
Vol.
26, pp. 1693-1694, 1992 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
FATIGUE CRACK GROWTH RATES IN ARCTIC-GRADE LINE PIPE STEEL AT HIGH AK C. R. Wake and J. G. Byrne Department of Metallurgical Engineering University of Utah Salt Lake City, Utah 84112-1183 (Received
M a r c h 24,
1992)
This work was concerned with the effect of roll direction on fatigue crack propagation in arctic-grade steel plate to be used for pipe fabrication. The steel was provided by Kaiser Steel Corporation and had a composition of 0.12 C, 1.51 Mn, 0.014 P, 0.009 S, 0.10 Si, 0.25 Cu, 0.02 Ni, 0.02 Cr, 0.25 Mo, 0.018 Co, and 0.065 Cb, all in wt. %. There was some alignment of nonmetallic inclusions and of grain shapes parallel to the roll direction. The ASTM plane strain criterion for fatigue testing of compact tension samples is B > 2.5 (K/yield stress) ~ where
(1)
B = compact tensile specimen thickness, and K = stress intensity factor.
The value of K is defined by the algorithm given in Equation (2). The stress at the crack tip is calculated based on the crack length, taking into account the relative location of the load to the crack tip and the reduced cross-sectional area as the crack progresses. K = P/(BW~'5)[29.6(a/W)°'5- 1.85.5(a/W) t'5 + 655.7(a/W) 2"5 - 1017(a/W) 3"5 + 638.9(a/W) ~~]
(2)
where K = stress intensity factor, P = applied load, B = thickness, W = width, and a = crack length. The R value, K , J K , ~ , used in the present work was 0.1. Loading frequency and wave form have been shown to have modest crack propagation effects (1). Light microscope inspection of longitudinal and transverse sections with respect to the rolling direction was performed in both unetehed and etched (2% Nital) conditions. Two tensile samples were machined from each of the directions parallel and perpendicular to the roll direction. The samples were two inches long in the gage length and 0.5 ×0.125 inches in cross-sectional area. These were tested in an Instron tensile tester at a strain rate of 2 x 104 sec~. Compact tension (CT) fatigue samples 7 mm thick with W = 57.5 mm and H -- 52.4 mm were machined in accordance with ASTM E399. The notch was sharpened with an electric discharge machine using an electrode of copper sheet 0.010 inches thick. The side surfaces of the CT samples were treated in a solution of hexa-methyldisilizane (HMDS) to promote photoresist adhesion. A grating with 100 lines per inch was placed perpendicular to each intended crack propagation direction. Fatigue cycling was performed at a frequency of 5 hertz in a closed-loop hydraulic machine with tension-tension load control at an R value of 0.1 and with a sinusoidal wave form. The crack position was followed with a travelling microscope with a resolution of about 20 ttm.
1693 0036-9748/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press Ltd.
1694
FATIGUE IN STEEL
Vol.
26, No.
Micrography indicated slight grain elongation and inclusion alignment parallel to the direction of hot rolling of the original plate. The yield strength of the material was 76,900 psi in the cross-roll direction and 77,850 psi in the roll direction. The ultimate tensile strength was 105,000 psi in the cross-roll direction and 111,500 psi in the roll direction. Fatigue crack growth rates were measured in the high AK region and are shown in Fig. 1 for two specimens in the LT orientation (crack moving in the transverse (T) direction with its normal parallel to the roll (L) direction) and one in the TL ori~tation (crack moving in the roll (L) direction with its normal parallel to the transverse (T) direction). The crack propagation rate was slightly more for sample 2 than for sample 3, and much larger than for sample 1, suggesting that this material offers slightly better resistance to cracks which propagate normal to the roll direction. The crack propagation rat~ are slightly lower than thosefor mild steels when tested in the AK range of 40 to 100 MPa m m (2).
1. M. Clavel, A. Pineau, "Frequency and Wave-Form Effects on the Fatigue Crack Growth Behavior of Alloy 718 at 298 K and 823 K," Metallurgical Transactions, 9A, 471-480 0978). 2. N. E. Frost, L. P. Ponk, and K. Denton, Eng. Fracture Mechanics, 3, 109 (1971).
5x 10.3
NO.2(T-L 4 xlO
-3
l
3xlo3
m
NO. 3(t" ¢J
Figure 1. da/dN versus AK for (T-L) and (L-T) oriented compact tension faligue specimens cycled at 5 hertz at room temperature and R = 0.1.
z
2xl0 "3
-
NO.
10"3 I.--. i
60
"tO
80
AK(MPo/m 112)
90 I00
11