EFFECT OF REVERSE BENDING METHOD ON PRE-CRACK STRAIGHTNESS IN CTOD TEST OF WELDED THICK STEEL PLATES

Sehwan Jeong Structure Research Dep’t, Hyundai Heavy Industry

Ulsan, Korea

Hyun-su Kim Structure Research Dep’t, Hyundai Heavy Industry

Ulsan, Korea

Sang-beom Shin Structure Research Dep’t Hyundai Heavy Industry

Ulsan, Korea

Tae-jong Park Materials Research Dep’t Hyundai Heavy Industry

Ulsan, Korea

ABSTRACT

This study deals with the reverse bending method as a

residual stress modification technique for CTOD test of the thick weld. In the reverse bending method, the residual stress around the notch tip of the specimen is redistributed by compressive plastic deformation by reverse bending load. The reverse bending method requires a smaller level of applying load comparing to the local compression method. However, the excessive reverse load may introduce the excessive plastic deformation in way of notch tip which can affect the CTOD value of the weld. So, this study investigates how to establish the proper bending load for ultra-thick weld. In order to do it, stress intensity factor was determined by the plastic zone size around the notch tip by 3-dimensional finite element analysis. The validation of the reverse bending method was verified by examining the configuration of pre-crack at CTOD weld test specimen with various thickness and strength.

1. INTRODUCTION

Recently, application of the high-strength ultra-thick steel

plate is rapidly expanding as increase of ultra large container ship and large size off-shore structures. Safety assessment on the brittle fracture has been an important issue in the application of the ultra-thick steel plates. For the steel plates thicker than 65mm, it is known that it is difficult to suppress the crack growth and it becomes more critical to accurately estimate the creation of brittle crack [1-2].

Brittle fracture characteristics of steel plates are usually evaluated by means of Charpy impact energy, fracture

toughness(KIC) and CTOD(Crack Tip Opening Displacement) values. The CTOD value means the limit of the opening displacement at the crack tip and the CTOD test is considered as the most realistic brittle fracture evaluation because the fracture is induced from the preformed fatigue crack in full thickness of plate [3].

In the CTOD tests, unevenly induced pre-crack is one of the main causes which deteriorate reliability of the obtained results. The nonuniformity of pre-crack shape of thick welds is limited to 20% of the average depth in accordance with the test standard for the CTOD tests [4]. For the welds with thick plates, it is hard to create the pre-crack to be straight without any proper treatments due to unevenly distributed welding residual stresses. Even though the difference in pre-crack depth is controlled within the regulation of the test standard, it is needed to decrease the difference for improved accuracy of the result. Difference in the pre-crack length could be dominant factor to increase uncertainties of the measurement of CTOD.

In the test standard, the local compression, reverse bending and high-R ratio methods are introduced with brief application procedure. Among them, the local compression is most commonly used for the CTOD tests of thick steel plates and evaluated as the most effective method in previous researches [5-6]. In the local compression method, the specimen is compressed in the thickness direction by a flat indentor up to slightly higher than the yield strength. The required load to apply the local compression rapidly increases as specimen thickness increases because the minimum size of the indentor is determined as half of the specimen width. Therefore, higher capacity of testing equipment is needed as the specimen thickness increases. Meanwhile, the reverse bending method,

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Proceedings of the ASME 2014 International Mechanical Engineering Congress and Exposition IMECE2014

November 14-20, 2014, Montreal, Quebec, Canada

IMECE2014-37909

which uses bending load in the residual stress modification, has an advantage that requires smaller level of applying load comparing with the local compression method. But the procedures and effectiveness of the reverse bending method have not been sufficiently investigated yet.

In this study, effectiveness of the reverse bending method is evaluated on the straightness of pre-crack shape using single edge notched(SEN) bending specimen. Appropriate range of the reverse bending load was investigated by experiments with various thicknesses of specimens. Validation of the procedure was evaluated by analytic and numerical calculations for the plastic zone size around the notch tip and the reverse bending stress intensity factor, Krb.

2. REVERSE BENDING METHOD

2.1 Residual stress modification by reverse bending method

The residual stresses are unevenly distributed as a consequence of the multi-pass welding for thick plates. There are several known factors which affect the residual stress after welding such as the welding heat input, the yield strength of weld metal, etc. Although the residual stress distribution shows various pattern with respect the welding conditions, generally the compressive residual stress appears at the central part of specimen and the tensile stress is located near the surfaces [7]. In the pre-crack generation procedure, the tensile stress is periodically applied at the crack tip, the compressive residual stress retards the crack growth, therefore the pre-crack has relatively narrow width at the central part of the specimen. Figure 1 shows an example of the fatigue pre-crack and the shortest crack propagation is located at the central region. In order to reduce the differences in the propagated crack length, the residual stress should be modified to have uniform distribution.

Figure 2 shows a schematic description to impose a reverse bending load to a specimen. In the reverse bending method, a bending moment is applied to the specimen in the opposite direction to the CTOD test and compressive stress is introduced around the notch tip. Due to the stress concentration around notch tip, compressive plastic deformation occurs in a certain region which varies with the level of applied load. After unloading, a tensile residual stress is induced as a consequence of the compressive plastic deformation and overall residual stress distribution is modified.

Transition of stress state around the notch tip is schematically represented in figure 3 during the reverse bending and unloading. The symbols which are designated the letters a and b denote stress levels at the regions of residual tension and compression respectively. When the reverse bending load is applied, a and b move to a’ and b’ by the imposed compressive stress and both a’ and b’ would go into the plastic deformation range. When the applying load is totally removed, tensile stress is introduced along the notch tip to satisfy the force equilibrium in the whole cross-section of the

specimen. The stress levels of a’ and b’ are transformed into a” and b” during the unloading stage. Although the stress level of b” does not become to the same as a”, the reverse bending method makes the residual stress more uniform by reducing the difference of stress level between the regions.

Figure 1 Nonuniform pre-crack shape of welded region

Figure 2 Schematic diagram of reverse bending method

Figure 3 Stress transition during reverse bending method

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2.2 Plastic zone size and stress intensity factor estimation In order to apply the reverse bending method, it is needed

to determine the appropriate level of load considering specimen dimension and the material strength. Excessive reverse bending load could induce global plastic deformation and affect the measurement result of CTOD test. Insufficient load, meanwhile, hardly improve the residual stress distribution.

The standard test method, BS 7448 part 2, specifies the limit of reverse bending load by Krb. In order to validate the reverse bending load in accordance with the test standard, Krb should be compared with the material toughness measured by stress intensity factor, Kmat. As Kmat could be obtained after performing the tests, it is necessary to find another way to estimate the appropriate range of the reverse bending load before the test.

In this study, a proper reverse bending load is investigated by introducing a reference load which can be determined by the specimen geometry and the yield strength. Krb values are also calculated with respect to the ratios of the reference load.

The reference load, Fy, is defined a load which causes the yield stress at the bottom surface of SEN bending specimen by the applied bending load. Using dimension and strength of the specimen, the reference load is defined as below:

20

y YS

B(W - a )F = σ

3(2W - 0.5d) (1)

where, σYS denotes the yield strength of the weld metal. B and W mean the thickness and width of the ligament of SEN bending specimen respectively. Distance between two contact points for the 4-point bending jig is indicated as d as shown in figure 2.

Krb can be analytically estimated in the assumption of the plain strain condition. Using specimen geometries and the yield strength, Krb is defined as below:

rbrb P0.2

8ωK = LR

π

(2)

where, L is the notch restrained factor and generally specified as 2.3 for rectangular cross-section [4]. RP0.2 and ωrb denote 0.2% proof strength and the plastic zone size respectively. ωrb is calculated under the plain strain assumption as below:

( )2

rb I P0.2

1ω = K / R

3π (3)

where Kt means the stress intensity factor for the opening mode. When a bending load is applied to a rectangular beam, KI is calculated as below [8]:

1/2 3/2 5/2 7/2 9/2I

4K = M πW(1.6α - 2.6α +12.3α - 21.2α + 21.8α )

B

,0α = a / W (4)

where M indicates applied bending moment and α is defined the ratio of the notch depth to the specimen width.

Figure 4 FE model and boundary condition for the reverse bending analysis

0.00 0.02 0.04 0.06 0.08 0.100

100

200

300

400

500

600

700

Tru

e st

rain

[M

Pa

]

Plastic strain Figure 5 Plastic stress-strain relation of weld metal

Figure 6 Plastic strain distribution and plastic zone size

The plastic zone size which is calculated above is also

validated by 3-dimensional finite element analyses. Figure 4 shows a 1/4 model of the SEN specimen and boundary conditions for the reverse bending method. The thickness and width of the specimen are 100mm and the initial notch depth is 45 mm. The reverse bending load is applied up to 0.5Fy. 8-node reduced integration elements are used to prohibit volumetric locking during plastic deformation. Element size was controlled smaller than 0.2 mm near the notch tip. For accurate estimation of the plastic zone size, the true stress-strain relation

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of the YP47 weld metal is applied to the analysis as shown in figure 5. The plastic zone sizes are defined as shown in figure 6. The maximum plastic zone is located near the surface and the minimum is shown at the center of the specimen.

The plastic zone size is shown as figure 7 with respect to the applied load. Analytically calculated result is located between the maximum and minimum values obtained by the finite element analyses.

2.3 Appropriate load range for the reverse bending method

As shown in figure 7, when the applying load increases up to 1.3 Fy, where Krb has the same value of Kmat, the maximum plastic zone size is still lower than 20% of specimen thickness. It means that the grown plastic zone does not get out of the admissible range of pre-crack length and the maximum load determined by Kmat in accordance with the test standard does not affect the result of CTOD test.

The minimum effective load level of the reverse bending method is investigated with pre-tests. The pre-tests were conducted with various applied load levels using a square section specimen of 100 mm thickness of YP47 steel plate. The specimens were welded with the single bevel groove and the distance between the loading points around the notch is adjusted 100 mm. Applying load was varied from 0.2 Fy to 0.5 Fy. The pre-crack is generated according to the CTOD standard method.

As the result of the pre-tests, ratio of the pre-crack length is represented in figure 8. It indicates ratio of the minimum crack length to the maximum value. amin and amax denote the minimum and maximum crack length respectively. The crack length is defined as the distance from the machined notch tip to the crack front. As the applied load increases up to 0.5 Fy, amin gradually has been closer value to amax. It means that when the reverse bending load higher than 0.5 Fy, maximum difference in the pre-crack length is expected to be controlled less than 10%. So the minimum effective load for reverse bending method is determined as 0.5 Fy.

For various thicknesses of specimens, 0.5 Fy was applied as the reverse bending load and the plastic zone size and krb are estimated. As summarized in table 1, analytically and numerically obtained Krb represent lower value than Kmat. Based on the evaluation, 0.5 Fy is not considered to cause any progressive fracture during the reverse bending procedure.

To examine the overall damage of the specimen during the reverse bending procedure, residual stress was measured on the specimen surface. The residual stress was measured at every 3 mm from the crack tip using the X-Ray diffraction(XRD). As shown in figure 9, significant change of residual stress is not observed after the reverse bending procedure with the applying load of 0.5 Fy.

3. APPLICATION OF REVERSE BENDING METHOD

The effectiveness of the reverse bending method is evaluated by experiments with various specimens. In the experiments, 0.5 Fy was applied to the specimens as the reverse bending load. Thickness of the specimens varies from 60 to 100 mm and the specimens were prepared with the single bevel grooves.

Figure 7 Plastic zone sizes with respect to applied load

0.0

0.2

0.4

0.6

0.8

1.0

As-Welded 0.5Fy0.33Fy

Rat

io o

f p

rec

rack

len

gth

Applied load

amin

amax

0.2Fy

Figure 8 amin/amax with respect to applied load

Figure 9 Measured residual stress distribution at the surface

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along the specimen center Table 1 Plastic zone size and stress intensity factor with various specimen thickness

Thickness [mm]

Load [kN]

Plastic zone size [mm] Stress Intensity Factor [MPa·mm0.5]

FEA Plane strain FEA Plane strain

wrb_min wrb_max wrb_PE Krb_min Krb_max Krb_PE Kmat

60 72.0 1.03 1.97 1.30 1,724.6 2,385.1 1935.7 5,787.6

80 108.7 1.37 2.62 1.73 1,989.0 2,750.6 2235.2 6,102.1

100 155.6 1.71 3.28 2.16 2,222.2 3,077.6 2499.0 7,243.5

Table 2 Pre-crack shape of welded specimen after residual stress modification treatments

Thickness [mm]

Pre-crack shape Pre-crack length ratio

(amin/amax) As-welded Local compression Reverse bending

60

As-welded: 0.15

Local compression: 0.97

Reverse bending: 0.97

80

As-welded: 0.21

Local compression: 0.92

Reverse bending: 0.95

100

N. A.

As-welded: 0.48

Reverse bending: 0.91

In the cases the specimen thickness is less than 80mm, the local compression was also performed using separate specimens.

As the result, pre-crack shape and crack length ratio are summarized in table 2. When the specimen thickness is 60 mm, pre-cracks became almost straight after the local compression and reverse bending method. A severe curved shape which appeared in as-welded specimen is completely vanished. In the case 80 mm of specimen thickness, the curved shape still remains a little bit but the amount is abruptly decreased. The reverse bending method provided the similar level of improvement in the pre-crack shape. In the case of 100mm of specimen thickness, comparison with the local compression was not conducted due to the limitation of the loading equipment, a great improvement in the pre-crack straightness

was observed comparing with as-welded specimen. without a residual stress modification treatment.

4. CONCLUSION

In this study, the reverse bending was investigated as a

residual stress modification method to improve straightness of pre-crack shape for CTOD test of thick welds.

- For square section SEN bending specimens, difference in the pre-crack length decreases as reverse bending load increases.

- At the maximum level of applying load determined by Kmat in accordance with the test standard, the plastic zone size is still smaller than admissible pre-crack length and would not affect the CTOD test result.

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- Minimum effective level of reverse bending load was determined as 0.5Fy by several experiments. When the minimum reverse bending load applied, maximum difference in the pre-crack length is controlled to be less than 10%.

- Effectiveness of the reverse bending method was demonstrated using various thick welds which have single bevel grooves. In the evaluation, proposed reverse bending procedure provided similar level of improvement in the pre-crack shape to the local compression method.

REFERENCES [1] Takehiro Inoue, Tadashi Ishikawa, Shiro Imai, Tadashi Koseki, Kazuhiro Hirota, Masuo Tada, Hiroshige Kitada, Yoshiya Yamaguchi and Hiroshi Yajima, “Long crack arrestability of heavy-thick shipbuilding steels”, Proceedings of International Offshore and Polar Engineering Conference (2006), 132. [2] Joon Sik Park, Bo Young Jung, Gyu Baek An and Jong Bong Lee, “Crack Arrest Toughness of Thick Steel Plate Welds for Ship Building”, Journal of KWJS(Korean), Vol. 25, No. 4, pp.9-14 (2007) [3] sung-Min, Joo, Byung-Hyun Yoon, Woong-Seong Chang, Han-Sur Bang, Hee-Seon Bang and Chan-Seung Ro, “Redistributions of Welding Residual Stress for CTOD Specimen by Local Compression”, Journal of KWJS(Korean), Vol. 27, No. 6, pp.31-35 (2009) [4] BS 7448: Part2: 1997, Method for determinationof KIC, critical CTOD and Critical J values of welds in metallic materials. [5] H.S Reemsnyder et al, “Residual stesses and Fatigue Precracking Techniques for Weldment Fracture Toughness Specimens”, Journal of Testing and Evaluation, 1991 [6] M. KOCAK et al, “Comparision of Fatigue precracking methods for fracture toughness testing of weldments”, Proc. Int. Conf. Welding, 1990 [7] Sang Beom Shin, “A Study on Design Criteria of Hatch Coaming Weldment in Ultra-Large Containership on the Basis of Fracture Mechanics”, PhD thesis School of Naval Architecture & Ocean Engineering, University of Ulsan (2009) [8] A. F. Bower, Applied mechanics of solids. CRC Press (2009)

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