us peening on weld 2

7/31/2019 US Peening on Weld 2

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1. INTRODUCTION

Welding was one of the most widely used connecting techniques in the modern industry. Every

year, 50% of the worlds steel output was used to make steel structures. However failures of theWelded structures occurred frequently with which 8090% was fatigue failure. This often

happened when used for bridges, pipelines, marine structures, bullet trains and water turbine

Blades, which resulted in mass economic loss. And it was found that many of the fractures in these

welded structures happened when they had been working for over 1x109 cycles. This

was regarded by the conventional theory as the infinite life region. Because of the conflicts

between the practical phenomena and traditional theory, studies on the behavior of the welded

structure especially the welded joints in the ultra-long life region had great theoretical and

application values.

Due to the discoveries mentioned, the fatigue limit was considered no longer appropriate. In the

1980s, Japanese researchers first found the disappearance of the fatigue limit in high-strength steel

under high fatigue load cycles (at about 108cycles). Hereafter, the researchers from Japanese

French and other countries had conducted many ultra-long life fatigue tests on base materials. Test

results confirmed that the fatigue limit does not exist in the one-billion-cycle zone, so traditional

structural strength design theory in the ultra-long life region no longer applied. The large number

of studies by the predecessors focused mainly on the ultra-long life fatigue behavior of the base

material, while adequate research was not performed on the welded joints which were considered

to be the weak links in the structure. Here, ultrasonic fatigue test technology was employed to

study the ultra-long life fatigue behavior of the welded joints.

We can use UPT to improve the fatigue properties of welded joints. The UP technique is based on

the combined effect of the high frequency impacts of the special strikers and ultrasonic oscillation

in treated material. The UP treatment introduces plastic deformation on the welded toe. During

plastic deformation of surface layers the density of defects is increased and compressive

macroscopic stresses are formed. As a result, engineering properties of materials, parts and welded

structures could be improved drastically.

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2. ULTRASONIC PEENING TECHNOLOGY

2.1 Technology

The surface plastic deformation is widely used as an effective method of strengthening of parts and

welded elements. During plastic deformation of surface layers the density of defects is increased

and compressive macroscopic stresses are formed. As a result, engineering properties of materials,

parts and welded structures could be improved drastically. Different improvement treatments and

techniques such as shot peening, hammer peening, laser shock processing, etc. are applied for this

purpose. One of the promising techniques for surface plastic deformation is the Ultrasonic Peening

(UP) of materials, parts and welded elements.

The UP technique is based on the combined effect of the high frequency impacts of the special

strikers and ultrasonic oscillation in treated material. High frequency of impacts is one of the main

differences and advantages of UP technology. The optimized equipment for UP consumes only 0.2

0.4 kW of the electric power. At the same time the quality of UP treatment allows providing the

highest fatigue characteristics of welded elements in comparison with the application of known

improvement treatments. The ultrasonic transducer oscillates at a high frequency, with 20-30 kHz

being typical. As was described earlier, the transducer may be based on either piezoelectric or

magnetostrictive technology. Whichever technology is used, the output end of the transducer will

be oscillating, typically at amplitude of 20 40 m. During the oscillations, the transducer tip will

impact the striker at different stages in the oscillation cycle.

The impact results in plastic deformation of the surface layers of the material. These high stress

impacts, repeated hundreds to thousands of times per second, and results in the ultrasonic peening

effect of this technique. The plastic deformation during UP is more intensive than under the action

of traditional surface treatment processes. The surface roughness is reduced and the wear

resistance is increased as a result of the action of UP. The UP is very effective for relieving of

harmful tensile residual stresses and introducing of the beneficial compressive residual stresses in

surface layer of parts and welded elements.

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The mechanism of residual stresses redistribution is connected mainly with two factors. At a high-

frequency impact loading the oscillation with complex frequency mode spectrum propagates in a

treated element. The nature of this spectrum depends on the frequency of ultrasonic transducer,

mass, quantity and form of impact units (spheres, rods etc.), and also on the geometry of the

treated element. These oscillations lead to lowering of residual welding stresses. The second and

the more important factor, at least for fatigue improvement, is surface plastic deformation, which

leads to introducing of the beneficial compressive residual stresses.

The features of high-frequency impact deformation explain the high efficiency of UP application

for fatigue life improvement. A rheological model explaining effect of essential lowering of the

deforming force in high-frequency impact loading was developed. It is known, that at the impulse

loading even with the level of stresses lower than the yield strength, the inelastic behavior of

material - mechanical hysteresis is observed. If the time of relaxation process exceeds the gap

between two subsequent impacts, the mechanical system is in non-equilibrium state and each

subsequent impact results in accumulation of deformation. It was shown experimentally that in

repeated static loading the size of the plastic impressions from a sphere does not practically change

after first loading. If the sphere vibrates with high frequency, the impression grows during repeated

impacts up to a certain extent. The dependence of accumulation of a plastic deformation depending

on the frequency of impacts could be analyzed by using the developed reological model.

During the UP process the strikers oscillate creating a small gap (~ 0.01 0.1 mm) between the

ultrasonic transducer, striker and the treated material. These oscillations have an aperiodic

character with the frequency lower than the frequency of ultrasonic transducer. It was found, that at

different oscillation amplitudes there is an optimum gap at which the highest plastic deformation is

observed. The deforming element oscillates with lower frequency than the tip of ultrasonic

transducer alternately hitting the tip of transformer and treated surface. The generation of the

intensive quazi-resonance vibrations of the strike in the gap between the ultrasonic transducer tip

and treated material is a specific feature of UP process.

In the fatigue improvement the beneficial effect is achieved mainly by introducing of the

compressive residual stresses into surface layers of metals and alloys, decrease in stress

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concentration of weld toe zones and the enhancement of the mechanical properties of the surface

layer of the material. The schematic view of the cross section of material/part improved by UP is

shown on Figure.1

Fig.1 Schematic view of the cross section/part improved by Ultrasonic Peening

Table 1

Zones of material/part improved by Ultrasonic Peening

2.2. Equipment for Ultrasonic Peening

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The Ultrasonic Peening equipment consists of a transducer, wave guide, indenter and a computer.

The transducer produces ultrasonic oscillations. These oscillations transmitted to the indenter

through wave guide. The indenter is the part which strikes the treating surface with ultrasonic

frequency. Computer controls the treating parameters such as frequency, amplitude of oscillations

etc.

Fig.2 Schematic diagram of esonix UPT (1- Magnetostrictive transducer, 2- Wave guide,

Indenter, 4 Treated surface, I- Oscillations, II- Impact pulses)

There are two general types of ultrasonic transducers which can be used for UP: magnetostrictive

and piezoelectric. Both accomplish the same task of converting alternating electrical energy to

oscillating mechanical energy but do it in a different way. In magnetostrictive transducer the

alternating electrical energy from the ultrasonic generator is first converted into an alternating

magnetic field through the use of a wire coil. The alternating magnetic field is then used to induce

mechanical vibrations at the ultrasonic frequency in resonant strips of magnetostrictive material.

Magnetostrictive transducers are generally less efficient than the piezoelectric ones. This is due

primarily to the fact that the magnetostrictive transducer requires a dual energy conversion from

electrical to magnetic and then from magnetic to mechanical. Some efficiency is lost in each

conversion.

Magnetic hysteresis effects also detract from the efficiency of the magnetostrictive transducer. In

addition, the magnetostrictive transducer for UP needs forced water-cooling. The equipment in this

case is relatively heavy and expensive. Piezoelectric transducers convert the alternating electrical

energy directly to mechanical energy through the piezoelectric effect. Today's piezoelectric

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transducers incorporate stronger, more efficient and highly stable ceramic piezoelectric materials,

which can operate under the temperature and stress condition.

Fig.3 Schematic view of transducer for ultrasonic peening

Piezoelectric transducers are reliable today and can reduce the energy costs for operation by as

much as 60%. Due to the high energy efficiency of piezoelectric transducers the effect in fatigue

life improvement by UP is practically the same by using of the magnetostrictive transducer with

power consumption of 1000 Watts and peiezoceramic transducers with power consumption of only

250 Watts .

Fig.4 Computerized complex for ultrasonic peening of parts and

welded joints

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3. ENDURANCE LIMIT FOR ULTRA-LONG LIFE

The failures of the welded structures occurred frequently with which 8090% was fatigue failure.

This often happened when used for bridges, pipelines, marine structures, bullet trains and water

turbine blades, which resulted in mass economic loss. And it was found that many of the fractures

in these welded structures happened when they had been working for over 1x109 cycles. This was

regarded by the conventional theory as the infinite life region. Because of the conflicts between the

practical phenomena and traditional theory, studies on the behavior of the welded structure

especially the welded joints in the ultra-long life region had great theoretical and application

values.

Fig.5 Endurance limit of steel

Due to the discoveries mentioned, the fatigue limit was considered no longer appropriate. In the

1980s, Japanese researchers first found the disappearance of the fatigue limit in high-strength steel

under high fatigue load cycles (at about 108cycles). Hereafter, the researchers from Japanese

French and other countries had conducted many ultra-long life fatigue tests on base materials. Test

results confirmed that the fatigue limit does not exist in the one-billion-cycle zone, so traditional

structural strength design theory in the ultra-long life region no longer applied.

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4. TEST SETUP

4.1. Specimens preparation

As the ultrasonic peening technology enhanced the fatigue strength and prolonged fatigue life of

welded joints, the treatment of the welded joints was conducted and the performance of the as-

welded and UPT joints were compared in this experiment. Carbon steel Q235B and low alloy

structural steel Q345 has been selected as the test material of the welded joints. The specimens

of the two kinds of materials were divided into the base material group and the ultrasonic impact

treatment group using the cruciform welded joints. The chemical constituents and mechanical

properties of the two steels were listed in Tables 2 and 3

Fig.6 Geometry shapes and dimensions of the cruciform weld joints (mm)

Table 2

Chemical constituents of steel Q235 and Steel Q 345

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Table 3

Mechanical properties of steel Q235 and Steel Q 345

UPT should be guided by the following methods:

1. The operation for the UPT Specimens should be carried out with the tool held at

approximately 450to the plate surface with the ends of the needles in contact with the weld

toe.

2. Sufficient force should be applied to the tool to prevent unsteady movement and to ensure

even treatment.

3. The toe should be needle peened four times to achieve optimum benefit and adequate

coverage.

4. The resulting surface should be bright in appearance and contain a uniform distribution of

small indentations.

5. All the specimens in this test were cruciform welded joints. The geometry and dimensionsof each group of specimens are shown

4.2. Fatigue test methods

All the tests were conducted on the TJU-HJ-I (Tianjin University Hanjie Model 1) ultrasonic

fatigue test equipment. The load in this test was axial and cyclic with the stress ratio R = -1,

and the load frequency is about 20 kHz. Water cooling was used to control the temperature of thespecimens.

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4.3. Fatigue test data processing method

The fatigue data of were computed according to the statistical method recommended by the

International Institute of Welding (IIW). It was assumed that the results of the fatigue tests

consisted with the Gaussian log-normal distribution, and then a dispersion zone was formed by the

two nominal SN curves with K (K as the eigenvalue) standard deviation. The survival probability

was at a value of 95% with a two-side 75% confident level of mean. The nominal value could be

calculated as below: All data of the stress ranges () and the number of load cycles (N) should be

convert to denary logarithms. Calculate m and constant log C by linear regression analysis taking

stress as the independent variable:

m log + log N = log C

Calculate the values Ci being log C from the (N, ) test results. Then, calculate the mean Ci and

standard deviation of log C using m

Then, calculate the characteristic values Ck

5. RESULTS AND ANALYSIS

5.1 Fatigue test results analysis

Two kinds of materials were divided into the as-welded and the UPT specimens group according

to the treatment states of the samples, then the corresponding SN curves were plotted based on

the test data obtained (50% survival probability, data points in ultra-long life range were mainly

considered). The SN curves are shown

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Fig.7 S-N curve of steel Q235B using ultrasonic fatigue test method

According to the recommended fatigue design code by IIW on steel structures treated by UPT the

fatigue strength from the SN curves show that the fatigue strength of both kinds of steel

specimens of the UPT groups increased in varying degrees at 1x107 cycles relative to that of the as-

welded group. When the cycle number exceeded 1x107regardless of whether it was UPT or as-

welded state the fatigue limit cannot be seen in the SN curves, in other words the curves kept

declining.

Table 4

Comparison of fatigue strength of steel Q235 and steel Q345

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Although the fatigue strength of UPT joints improved relatively, the curve still kept a downward

trend. For the constant amplitude fatigue test the traditional sense of fatigue limit did not exist

within the107109 cycles region and the samples still fractured after 1x107fatigue cycles. Results in

Table 4 show that compared with the fatigue strength at 1x107 cycles the value at 1x109still

decreased, but UPT groups had a much slower decreasing rate. In addition, Fig 7 had revealed that

fatigue fracture still occurred up to 1x108 load cycles. With the load going down the SN curve

still continued to decline even after 1x109 load cycles (i.e. if the lower alternate stress provided the

fatigue fracture probably could occur with long enough fatigue cycles). This meant that the fatigue

limit was not yet reached even after 1x109load cycles. Therefore, it could be dangerous using

the fatigue strength at1x107cycles to design structures serviced in the ultra-long life conditions

which was recommended in document, even when the structures only withstand the constant

amplitude load. Since there were differences between the conditions in the recommendations for

fatigue design from IIW and that in this experiment the test results should be modified with stress

ratio, wall thickness and misalignment. The specific correction method was described as follows.

For modification of the stress ratio an enhance factor should be considered here following the

formulas:

F(R) = 1.6 R < -1

F(R) = -0.4R + 1.2 -1 < R < 0.5

F(R) = 1 R > 0.5

Where F(R) is the stress ratio enhance factor and R is the stress ratio. The enhance factor in this

test should be as 1.6. Misalignment in axially loaded joints leads to an increase of stress in the

welded joint due to the occurrence of secondary shell bending stresses. The resulting stress is

calculated by stress analysis or by using the formulae for the stress magnification factor given in

the recommendation. So the factor was set as 1.45 for cruciform joints. The influence of the wall

thickness should also be taken into account with the equation below:

Where f(t) is the thickness reduction factor, t is the plate thickness, teffis the effective thickness and

n is the thickness correction exponent.

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Fig.8 Residual stress at weld toes of steel Q235 and Q345 in thickness direction

5.2. Fatigue strength

During the UPT process, the surface geometry was changed by the plastic deformation which will

lead to the compressive stress in the surface and sub-surface layer. Fig. 8 shows the results of the

UPT welded joints of steel Q235B and Q345. The figure revealed that the values of the residual

compressive stress in the surface and sub-surface layer (the depth of the sub-surface layer: Q235B

to 0.075 mm, Q345 to 0.050 mm) were higher than the yield strength of base metals. As the depth

increased, the residual stress changed from compression to tension gradually, and the thickness of

the residual compressive stress layer was about 1.5 mm or so similar trend showed in both kinds of

materials. Most fatigue cracks originated from the surface or sub-surface of the structures and

components. Their expansions were caused by the tensile stress. Due to the high-value

compression residual stress in a certain depth employed by the UPT, the applied tensile stress and

the compressive residual stress were superimposed which led to the reduction of the total stress

level, thereby increasing the fatigue strength of materials and extending the fatigue life.

According to the Goodman relation:

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Where n is the mean stress, na is the fatigue limit when n exits and

0a is the fatigue limit

Known as the mean stress sensitivity coefficient. (In order to avoid repeating the all parameter m

in the classic formula was replaced with the parameter n).When the residual stress was considered

to be equivalent to the mean stress, the equation above became:

The parameter r in Eq.was the residual stress which played a role of the mean stress, therefore

a known as the change of the fatigue strength caused by the residual stress was:

This shows that the influence of the mean stress on the fatigue strength increased as the mean

stress sensitive factor became higher, and the higher the value of the compressive residual stress

became the more the fatigue strength increased. In this test, for example, when the alternative load

reached its tensile maximum value the compressive residual stress could play a role of lowering the

value of the total stress so that the surface layer could keep at a low tensile stress state. Fig.9 shows

this situation.

Fig.9 Cyclic loading and residual stress before and after superimposition

5.3. Work hardening

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The plastic deformation in the surface layer of the specimens which led to work-hardening was

generated because of UPT. The hardness and strength in this layer had increased significantly.

It can be seen that the largest value of the hardness located in top layer, and then it gradually

reduced with depth. Metal plastic deformation was achieved by dislocation motion, and in the

process of plastic deformation the density of dislocations increased due to the interaction between

them. The left image in Fig.10 shows the original microstructure in the welded toe where the

grains and sub-grains were relatively large and clear. The middle image showed the transformation

microstructures turned from source material to the UPT material where due to the effect of

ultrasonic energy the dislocations rearranged and new sub-boundaries were formed. As the plastic

deformation continued, the density increased which made the grain crushing, refining and the grain

boundary reforming, as shown in the right image. Therefore the surface hardness and strength of

the weld enhanced.

Fig.10 Metallurgical structure and grain size of the cruciform welded joint of steel Q235:

Base material to UPT treated from left image to right image

5.4. Fractography

The fatigue cracks in the original welded joints often initiated from the weld toe slag, inclusions

and small surface machining trace while the fatigue fractures in the UPT welded joints might have

different type of crack sources. The fractures images of the specimens were analyzed by the

scanning electron microscopy (SEM). The types of the crack sources are shown in Figs.11 and 12.

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Fig.11 A slag type crack initiation point in specimens of steel Q235B.

Fig.12 Micro-crack caused by UPT in specimens from steel Q235B

It can be seen from the former two images in Figs. 11 and 12 that the crack origins in specimens of

as-welded joints were initiated mostly from the surface or sub-surface slag inclusions. It can also

be noticed in the image of the as-welded joints of steel Q235B (Fig.11) that the fatigue striation

was not found at the source area neither did the fatigue beaches. As around the initial point of

fatigue crack in the specimens as-welded of steel Q345 (Fig. 12), there was no obvious smooth

halo observed, and even the crack propagation area was not evident.

In UPT welded joints groups, apart from the types mentioned above, a new type of crack origin

was found on the fractography: the micro-crack (MC) probably produced by the UPT power.

5.5. Fracture modes

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5.5.1. Fracture modes in as-welded specimens

The crack source generally had several different types. It often originated from the slag inclusions,

machining location and other defects. The granular bright facet (GBF) in the high strength steel

was observed to be the internal fracture initial pointwhile in the welded joints it frequently started

from the inclusions and second phase particles near the surface or interface. For carbon and low

alloy structural steel, the domain fracture mode was still the surface layer slag inclusions induced

cracking. Fig. 11 shows the surface-type fatigue crack sources in the as welded sample from

Q235B, which initiated at the sub-surface inclusions with a fatigue life of 3.68 x 10 8 and 4.98 x 105

cycles. It can be seen that, in the as-welded joints, whether in the normal or ultra-long life range

the slag inclusions initiation point was the primary reason for the fatigue fracture. Although a large

number of observations had been done other types of crack sources were not found.

5.5.2. Fracture modes in UPT specimens

Fracture modes in UPT specimens Due to the ultrasonic peening treatment plastic deformation was

introduced into certain depth (about 1500 lm deep) of the weld toe area where high value

beneficial compressive stress was produced in the same time. Therefore, the fracture modes there

could probably be different from that of the as-welded joints. Slag inclusions type surface crack

source was also observed in the UPT welded joints from steel Q345. The small fractures were

formed on the very top layer at weld toe due to the peeling of the slag from the base material under

external loads and because of this continuous interaction the cracks propagated until it finally

broke, as shown in Fig. 14. But it was not the predominant crack source in the UPT joints, a new

type of alternative was observed more often: the micro-crack-type surface or sub-surface crack

source. As the mechanism of the fracture behavior in UPT welded joints was not very clear the

following section will make an explanation of it. In addition to slag inclusions surface crack

source, MC was also observed where the cracks initiated there and propagated till it finally failed.

No slag or inclusions were found in that area. This phenomenon was observed in two UPT welded

joint groups, therefore the MC caused fatigue fracture should be a unique mode of fatigue failure in

the UPT joints. It can be pointed out from the two figures above that the MC was under a certain

depth from the surface (generally no more than 100 m) where the fatigue cracks expanded

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eventually to fracture. However, the fatigue life of these defects containing joints compared to

those of the as-welded ones had increased significantly.

6. CONCLUSION

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Traditional fatigue design codes with fatigue limit were no longer applicable for welded structures

working in the long life range. For welded joints, whether they were bearing the constant

amplitude load or variable amplitude load, the SN curve continuously declined. Even if the cyclic

load exceed 1 x 107 or 1 x 109 cycles fatigue limit still cannot be observed, in other words, the SN

curve kept going down. Therefore, if the fatigue life of the welded joints were designed according

to the fatigue limit theory it could be very dangerous in the ultra-long life region. Fatigue life of the

welded joints should be designed based on different post-weld treatment states and modifications

should be conducted correspondingly. For the as-welded joints, the slope value m should be set as

3 when the cycle number was less than 1 x 107 and set as 5 when it was more than 1 x 107; for the

UPT joints, the m value should always be set as 10 in the whole life zone for fatigue design.

The weld toe area plastic deformation layer was formed as a result of UPT which led to strongly

refined grains in a certain depth (not less than 200 m deep). Thus the strength and hardness was

improved in that area. The crack nucleation formed at the slag inclusions and then propagated

under the external load until it finally fractured. This still predominated in the fracture modes of

the as-welded joints. As for some of the UPT specimens, the crack did not originate from the slag

inclusions but from the surface or sub-surface microcracks created by the ultrasonic peening

energy. Because the microcracks were located in the compression residual stress zone, the crack

propagation rate was lower than that of the as-welded specimen. The fatigue lives of the UPT

groups were longer than those of the as-welded groups. The theoretical analysis above was also

proved by fatigue test data and the metallographic photos.

REFERENCE

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1. Danqing Yin,Dongpo Wang,Hongyang jing,Lixing Huo.The effect of ultrasonic peening

treatment on the ultra-long life fatigue behavior of welded joints. Materials and Design

31(2010);3299-330

2. Statnikov ES, Korolkov Oleg V, Vladimir Vityazev N. Physics and mechanism of ultrasonic

impact. Ultrasonics 2006;44:e5338.

3. Y.Kudryavtsev,J.Kleiman,L.Lobanov,V.Knysh,G.prokopenko.Fatigue life improvement of

welded elements by ultrasonic peening.IIW Document XIII-2010-04

4. Xiaohui Zhao,Dongpo Wang, Lixing Huo.Analysis of the S-N curves of welded joints

enhanced by ultrasonic peening treatment. Materials and Design 32 (2011) ; 88-95

5. Xiaohua Cheng, John W. Fisher,Herny J. Prask, Thomas Gnaupel-Herold, Ben T. Yen,

Sougata Roy. Residual stress modification by post weld treatment and its beneficial effect on

fatigue strength of welded structures. International Journal on fatigue 25 (2003) ; 1259-1269

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