Accepted Manuscript

Influence of re-ultrasonic impact treatment on fatigue behaviors of S690QLwelded joints

Yang Liu, Dongpo Wang, Caiyan Deng, Liqian Xia, Lixing Huo, Lijun Wang,Baoming Gong

PII: S0142-1123(14)00114-5DOI: http://dx.doi.org/10.1016/j.ijfatigue.2014.03.024Reference: JIJF 3358

To appear in: International Journal of Fatigue

Received Date: 3 December 2013Revised Date: 6 March 2014Accepted Date: 27 March 2014

Please cite this article as: Liu, Y., Wang, D., Deng, C., Xia, L., Huo, L., Wang, L., Gong, B., Influence of re-ultrasonicimpact treatment on fatigue behaviors of S690QL welded joints, International Journal of Fatigue (2014), doi: http://dx.doi.org/10.1016/j.ijfatigue.2014.03.024

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1

Influence of re-ultrasonic impact treatment on fatigue

behaviors of S690QL welded joints

Yang Liu, Dongpo Wang, Caiyan Deng*, Liqian Xia, Lixing Huo, Lijun Wang,

Baoming Gong

Tianjin Key Laboratory of Advanced Joining Technology, School of Materials Science

and Engineering, Tianjin University, Tianjin 300072, PR China

Corresponding author: Dr. Caiyan Deng

E-mail: dengcaiyan@tju.edu.cn

Fax: +86-22-27405889

Address: Tianjin Key Laboratory of Advanced Joining Technology, School of

Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China

Abstract: The effects of re-ultrasonic impact treatment with different fatigue cycle

intervals between two UITs on the fatigue life of S690QL cruciform welded joints

were investigated. Fatigue life, residual stress, SEM micrographs of specimen cross

profile, and fatigue fracture were determined. Fatigue determination results indicated

that re-UIT increased the total fatigue life when the fatigue cycle interval between two

UITs was appropriate. The contribution of residual stress produced by re-UIT to

fatigue life was not significant at room temperature. The damages caused by UIT

were confirmed with optical microscope and scanning electron microscope.

Considering that UIT could close cracks, the effect of re-UIT on fatigue life strongly

depended on the competition between the role of closing cracks and the formation

2

process of new damage.

Keywords: Re-ultrasonic impact treatment; Welded joints; Fatigue; Residual stress;

Ultrasonic impact damage

1. Introduction

Fatigue fracture is one of main failure forms in practical engineering. For most of

structures and equipment, the weakest point in fabricated structures was the welded

joints during cyclic loading. Welds represent the global stress concentration zone and

normally possess high tensile residual stress. For these reasons, fatigue cracks in

welded structures normally propagate in the early service life of structures [1].

Mechanical surface treatments, such as deep rolling, shot peening and laser

shock peening, can induce several beneficial effects on metallic surfaces, and enhance

fatigue properties by improving the resistance against fatigue crack initiation and

propagation [2-5]. Ultrasonic impact treatment (UIT) originally developed in

shipbuilding industry in the former Soviet Union can reduce welding residual stresses

and deformations, introduce compressive residual stresses in fatigue critical locations,

increase corrosion fatigue strength of welded joints, and enhance the fatigue

resistance at subzero temperatures [6]. Mordyuk et al. [7] studied the effects of

ultrasonic impact process (UIP) and laser-shock peening (LSP) without protective and

confining media on the microstructure, phase composition, micro-hardness, and

residual stresses in near-surface layers of an austenitic stainless steel AISI 321 and

found that UIP produced a sub-surface layer which was 10 times thicker and about 1.4

3

times harder than that produced by LSP. Turski et al. [8] compared the merits of four

mechanical surface treatments (shot peening, laser shock peening, UIT, and water jet

cavitation peening) in terms of their effects on the surface roughness, microstructure,

plasticity processing level, residual stress distribution, and thickness. Berg-Pollack et

al. [9] found that UIT showed the high potential in improving the structural durability

and safety as well as saving material in the production of cast aluminum wheels

through significant changing the microstructure, porosity, and hardness of the

materials. In 2010, Yin et al. [10] studied the fracture mechanism of welded joints

treated by ultrasonic peening process. Zhao et al. [11] studied fatigue behaviors of

welded joints through S-N curves. In 2011, the fatigue properties of ultrasonically

peened friction stir welded parts were studied [12]. Abdulalah et al. [13] reported that

ultrasonic peening treatment led to the geometrical modification of weld toe and the

reduction of stress concentration in stress concentration zone and that ultrasonic

peening treatment closed cracks and voids, thus increasing fatigue strength.

It is interesting to determine whether re-UIT can extend the service life of

welded structures treated by UIT in service at room temperature. If re-UIT can extend

the service life, the welded components which are beyond original design life can be

used safely after re-UIT. This application of re-UIT will result in considerable

economic and social profits. The compressive residual stress caused by surface

mechanical enhancement technique plays an important role in enhancing the fatigue

life of welded components. Many scholars observed that fatigue cycle loading would

lead to the mechanical residual stress relaxation [14-16] and reduce the improvement

4

effect of surface mechanical enhancement technique on fatigue property [17,18].

Moreover, it is also necessary to explore whether there is another mechanism

that re-UIT affects the fatigue life of welded components. The paper aims to

investigate the effects of UIT and re-UIT on the residual stress, the axial fatigue

behaviors, and fracture morphologies of S690QL welded joints.

2. Testing materials and methods

The geometrical characteristics of fatigue specimens are shown in Fig. 1a. The

specimens were the cruciform joints prepared by FCAW with base material of

S690QL steel and welding material of FabCO XTREME 120 E121T5-GC H4. The

chemical composition and mechanical properties of the base and welding materials

are provided in Tables 1 and 2, respectively. Welding parameters are given in Table 3.

Fig. 1. (a) Geometrical characteristics of cruciform welds specimen (mm) and (b) UIT

equipment.

The half-wavelength UIT equipment is composed of two parts: ultrasonic

frequency generator and ultrasonic impact gun, as shown in Fig. 1b. The diameter and

length of the needle are 4 mm and 30 mm, respectively. The radius of the needle tip is

2 mm. Specific operation includes three steps. Firstly, ultrasonic impact gun was

pointed at the weld toe of specimen in the basically perpendicular direction. Secondly,

impact needles were arranged along the weld. Thirdly, the impact gun was slightly

pressed in order to direct the UIT. In order to obtain the smooth transition profile of

weld toes, treatment parameters were set as follows: frequency of 20 kHz, vibration

amplitude of 30 µm, treating speed of 1.0- 1.5 m/min, and weld treatment coverage of

5

300%.

The fatigue life of S690QL cruciform welded joints treated by UIT or re-UIT

was measured under the constant load range with the stress ratio R=0.5 and ∆σ=200

MPa at room temperature in air. Fatigue testing specimens were divided into two

groups. In Group I, specimens were treated by single UIT. The fatigue life was

calculated according to the survival fraction of 95% and the confidence level of 75%.

In Group II, specimens were treated by a re-UIT after every loading for 50% fatigue

life, which was calculated in Group I. Re-UIT and single UIT were performed under

the same conditions. Fatigue tests were completed on 300 kN high frequency fatigue

testing machine.

In order to study the mechanical residual stress relaxation of weld toe treated by

UIT, the residual stress of the following six specimens were measured in the depth

direction: (A) UIT specimen; (B) UIT specimen + 1 cycle loading; (C) UIT specimen

+ 5 cycles loading; (D) UIT specimen + 10 cycles loading; (E) UIT specimen + 74000

cycles loading; (F) UIT specimen + 74000 cycles loading + re-UIT. The cycle loading

above was completed under the conditions of stress ratio R=0.5 and ∆σ=200 MPa at

room temperature in air. All residual stress measurements were performed by a

standard X-ray diffraction technique according to the sin2ψ method. Depth profiles

were obtained through material removal by electrolytic polishing.

The microstructure of weld toe treated by UIT or re-UIT was observed with

optical microscopy and scanning electron microscope (SEM, HITACHI S4800).

Fatigue fracture surfaces were examined with SEM.

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3. Results

3.1. Fatigue life

The load applied to specimens is the nominal stress range. Because angular

misalignment and axial misalignment exist in cruciform joints, misalignment in

axially loaded joints led to an increase of stress in the welded joint due to the

occurrence of secondary shell bending stress. Therefore, the structural stress range in

weld toe should be used to process the fatigue data. The structural stress range

strucσ∆ is expressed as

σσ ∆⋅=∆ mstruc k , (1)

where σ∆ is the nominal axial stress range in fatigue experiment; σ∆ =200 MPa; km

is the stress magnification factor.

In the case of the simultaneous occurrence of linear and angular misalignment,

stress magnification factor should be calculated according to Eq. (2) [19]:

)1()1(1 ,, −+−+= angularmaxialmm kkk . (2)

where km, axial is stress magnification factor of axial misalignment; km, angular is stress

magnification factor of angular misalignment. km, axial and km, angular are acquired from

ref. [19]. According to the calculation results, structural stress range of every

specimen is different (see Fig. 2), although nominal stress range of every specimen is

the same.

Fig. 2a shows the fatigue life of S690QL cruciform welded joints treated by

re-UIT and the S-N curve of the specimen treated by single UIT (re-UIT specimen).

The fatigue lives of most specimens treated by re-UIT (re-UIT specimen) are higher

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than those of specimens treated by single UIT. However, re-UIT does not result in the

beneficial effect in all specimens. In Fig. 2, the horizontal axis indicates the fatigue

life of a specimen measured in the experiments and the vertical axis indicates the

structural stress range, strucσ∆ , calculated according to Eq. (1).

Based on the S-N curve of the specimens treated by single UIT, the expected total

fatigue life (N) of every re-UIT specimen under single UIT can be calculated within

its actual structural stress range. Then the percentage of the fatigue cycle interval

between two UITs in total fatigue life (N) under corresponding stress was also

calculated. The percentage of fatigue life of re-UIT specimen in total fatigue life

under actual structural stress is shown in Fig. 2b. When the fatigue cycle interval

between two UITs exceeded 30% of N, the fatigue life of re-UIT specimen was longer

than the fatigue life of single UIT specimen. When the fatigue cycle interval between

two UITs reached 55% of N, the fatigue life of re-UIT specimen reached its maximum

value. When the fatigue life of re-UIT specimen was more than 77% of N, the fatigue

life of re-UIT specimen was significantly decreased.

Fig. 2. (a) S-N curve of specimens treated by UIT and fatigue lives of specimens

treated by re-UIT; (b) the percentage of the fatigue life of re-UIT specimen in the

expected total fatigue life under the structure stress range.

3.2. Residual stress

According to the residual stress profile of UIT specimen (Fig. 3), the

compressive stress zone caused by UIT is extended to the depth of about 1200 µm.

The maximum compressive residual stress (about -400 MPa), while the stress

8

recorded at the surface is around -350 MPa. The actual stress ( actualσ ) of welded

structures is the combination of the applied stress ( appliedσ ) and welding residual

stresses ( residualσ ), which is independent of the load. The tensile part that contributed

to the development of fatigue crack was defined as the effective stress range ( effr ,σ ).

When compressive residual stress was induced by post-weld treatment, the tensile

residual stress and the applied tensile stresses might be eliminated or reduced [20].

Therefore, maximum tensile stress was extended to the depth about 1600 µm beneath

the surface or the deeper position. It is more difficult to initiate fatigue cracks. Thus,

the compressive residual stresses induced by UIT enhanced the fatigue life of welded

joints.

Fig. 3 shows the residual stress depth-profiles of the UIT specimen after different

cycles and re-UIT under the constant load range with the stress ratio R=0.5 and

∆σ=200 MPa at room temperature. Considering the measurement error, the difference

of residual stresses under different testing conditions can be ignored. Residual stress

relaxation and redistribution were not found in UIT specimen of S690QL cruciform

welded joints after fatigue loading and re-UIT at room temperature.

Fig. 3. Residual in-plane stress profiles of UIT specimen under different testing

conditions.

4. Discussion

4.1. Redistribution of residual stress

The residual stress relaxation starts when the combination of external stress and

residual stress exceeds local yield strength of materials [16, 20] at room temperature.

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For the residual stress inside specimens is in the equilibrium state, the high

compressive residual stress on the surface caused by UIT led to the tensile residual

stress inside specimen. When the surface compressive residual stress field area is the

same, the peak value of interior tensile residual stress is decreased with the increase in

the area of interior tensile residual stress. In this paper, the thickness of specimen is 12

mm, and the thickness of compressive residual stress field is less than 1.5 mm.

Therefore, the large area of interior tensile residual stress reduces the peak value of

tensile residual stress. For the combination of the residual stress and external stress

does not exceed local yield strength of materials, no obvious relaxation of residual

stress is observed. The residual stress is not significantly changed after fatigue loading

and re-UIT. Therefore, the effect of residual stress on the fatigue performance of

welded components is not changed.

In the case of residual stress relaxation, mechanical residual stress relaxation of

shot-peened medium-carbon steel has been observed and described in detail [16]. The

macroscopic compressive residual stresses are largely reduced in the first cycle.

Although macroscopic compressive residual stresses at the weld toe are redistributed

by re-UIT, the mechanical relaxation of residual stress also occurs during subsequent

fatigue cycles. The macroscopic compressive residual stresses are largely reduced in

the first cycle of the next fatigue cycles. Compared with the fatigue cycle interval

between two UITs, the life of the first cycle with high macroscopic compressive

residual stresses can be ignored.

It should be noted that the residual stress after relaxation affects the fatigue life.

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Therefore, the contribution of high macroscopic compressive residual stresses induced

by re-UIT to enhancing the fatigue life of welded joints at room temperature is not

significant.

4.2. Closure of crack

As shown in Fig. 2b, the total fatigue life of specimens treated by re-UIT is

longer than the total expected life of the single UIT specimens in corresponding actual

stress range when the fatigue cycle interval between two UITs is appropriate. The

possible explanation was that the re-UIT closed crack within a certain length limit at

weld toe. The closure of crack and the modification of curvature were found at weld

toe for the peened specimens [13]. We only close one end of crack on the surface by

disorganizing the crystalline grain on the surface and near-surface. Therefore, it is

inferred that after a long fatigue cycle interval, the surface crack has propagated so

deep that the re-UIT can not close the crack completely.

4.3. Formation of new UIT damage

Re-UIT produced the probabilistic damage during re-UIT. Fig. 4a indicates that

plastic flow is formed in the depth of about 100 µm beneath the surface. Because

grain refinement is mainly determined by large plastic deformation, material structure

presents the graded distribution in depth [21]. Furthermore, a gap divides the metal

treated by UIT into two independent parts and the gap is defined as the folded defect

(Fig. 4a). The two parts on either side of the gap are extruded and contact each other

due to UIT. Especially, the outer part is the half-stripped hardened layer. There is no

metallurgical bonding on the two parts. The stress concentration factor of tip is very

11

large. The tip is the preferred fatigue crack initiation site, as shown in Fig. 4b.

Fig. 4. SEM micrographs: (a) cross-section of the specimen with UIT damage and (b)

magnification of the tip.

According to the SEM observation of the typical fatigue fracture surface of UIT

specimen (Fig. 5a), there is a smooth area which is about 46 µm deep from the treated

surface by UIT, and the smooth surface is not parallel to the fracture surface. This

depth is consistent with that of gap in Fig. 5b, so the smooth surface is formed not by

fatigue propagation but by the extrusion and plastic deformation during UIT. The

boundary line between the smooth area and the rough area in Fig. 5a corresponds to

the tip in Fig. 5b. The fatigue crack starts from the boundary line (tip, as shown in Fig.

5b) between smooth area and rough area probably. In addition, the burr can be

observed on the surface of specimen due to UIT (see Fig. 4a and Fig. 5a).

Fig. 5. SEM micrographs: (a) the fatigue fracture surface of UIT specimen and (b)

cross-section of the specimen with UIT damage.

During UIT, the folded defect is formed by large plastic deformation, which is

the surface damage caused by UIT. The main process consists of the following steps

(see Fig. 6). Firstly, when the impact needle impacts specimen surface, crater in the

center and burr in the outboard of impact zone are formed. The plastic damage of

AISI 304 stainless steel induced by UIT was studied with the finite element model

[22]. The results showed that the damage area in the process of UIT was annular and

that the indent center was not affected. Then, when impact needle impacts the next

position, the surface adhering burr occurs in the indent center, and the gap between

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burr and base metal is pressed into the hardened layer. The gap tip is formed in the

hardened layer and is most probable fatigue crack initiation area.

Fig. 6. The schematic diagram of the surface damaging process during UIT.

4.4. Crack propagation path

Fig. 7 shows the crack propagation path in fatigue cycles. The fatigue crack

grows along the gap line firstly. Then, the fatigue crack propagates along another line,

which is perpendicular to the loading direction. It is proved that the tip of the folded

defect caused by UIT is the initiation area of fatigue crack.

Fig. 7. The cross-sectional metallograph of weld toe treated by UIT.

The effects of re-UIT on fatigue life are the competition consequence between

the closure of tiny crack and the formation of UIT damage. When the fatigue cycle

interval between two UITs is less than 30% N, the shorter fatigue cycle interval

between two UITs is, the greater the probability of closing tiny crack by UIT is.

Meanwhile, the more re-UITs lead to the deformation of weld toe and more UIT

damages, thus decreasing the area of thrust surface of weld toe and increasing the

structural stress concentration factor. When the fatigue cycle interval between two

UITs was so long that the fatigue crack has propagated over a critical depth, re-UIT

has no effect on the fatigue crack tip any more.

5. Conclusions

The effect of re-UIT on fatigue life of S690QL cruciform welded joints with

different fatigue cycle interval between two UITs was investigated at room

temperature. The following conclusions can be drawn from this work:

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1) Re-UIT increased the total fatigue life of S690QL cruciform welded joints when

the fatigue cycle interval between two UITs was appropriate.

2) The contribution of macroscopic compressive residual stresses caused by re-UIT

to enhancing fatigue life of welded joints was not significant.

3) Re-UIT produced the probabilistic damage, such as folded defect. The tip of

folded defect had a very large stress concentration factor. Therefore, it was the

preferred fatigue crack initiation site.

4) The effect of re-UIT on fatigue life strongly depended on the competition

between closing tiny cracks which had propagated and bringing out new damages

by re-UIT.

Acknowledgements

The authors are grateful for the support by Program for New Century Excellent

Talents in University (Grant No. NCET-07-0601) and National Natural Science

Foundation of China (Grant No. 51375331).

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peening treatment, Mater. Design. 32 (2011) 88-96.

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Figure Captions

Fig. 1. (a) Geometrical characteristics of cruciform welds specimen (mm) and (b) UIT

equipment.

Fig. 2. (a) S-N curve of specimens treated by UIT and fatigue lives of specimens

treated by re-UIT; (b) the percentage of the fatigue life of re-UIT specimen in the

expected total fatigue life under the structure stress range.

Fig. 3. Residual in-plane stress profiles of UIT specimen under different testing

conditions.

Fig. 4. SEM micrographs: (a) cross-section of the specimen with UIT damage and (b)

magnification of the tip.

Fig. 5. SEM micrographs: (a) the fatigue fracture surface of UIT specimen and (b)

cross-section of the specimen with UIT damage.

Fig. 6. The schematic diagram of the surface damaging process during UIT.

Fig. 7. The cross-sectional metallograph of weld toe treated by UIT.

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Fig. 1

19

Fig. 2

20

Fig. 3

21

Fig. 4

22

Fig. 5

23

Fig. 6

24

Fig. 7

25

Table 1

Chemical composition (wt. %).

Material C Si Mn P S Cr Mo Ni Al

S690QL 0.14 0.42 1.43 0.020 0.002 0.02 0.004 0.02 0.047

E121T5-GC H4 0.07 0.14 1.35 0.008 0.005 0.33 0.22 3.90 0.45

Table 2

Mechanical properties.

Material Yield strength (MPa) Tensile strength (MPa) Elongation rate (%)

S690QL 790 829 20

E121T5-GC H4 763 866 17.8

Table 3

Welding parameters.

Welding

voltage (V)

Welding

current (A)

Welding speed

(mm/min)

Gas flow rate

(L/min)

Interpass

temperature (°C)

27 300 200 20 <200

26

27

Highlights

� Re-UIT increased the fatigue life if interval fatigue cycles were

appropriate.

� The contribution of residual stress due to re-UIT to fatigue life was

small.

� The folded defects due to UIT were found.

� Re-UIT effect relies on the competition between closing cracks and

inducing damage.