Fatigue Strength of Welded Structures: Everything You Need to Know

Causes of fatigue failure of welded structures

The causes of fatigue failure of welded structures mainly include the following aspects:

① Objectively speaking, the static load bearing capacity of welded joints is generally not lower than that of base metal;

When bearing alternating dynamic load, its bearing capacity is much lower than that of base metal, and it is closely related to the type of welded joint and welded structure.

This is a major factor causing premature failure of some structures due to fatigue of welded joints;

② The early welding structure design was mainly static load strength design, without considering fatigue design;

Or the fatigue design specification of welded structures is not perfect, so that there are many welded joints whose design seems unreasonable now;

③ Engineering designers and technicians do not know enough about the characteristics of fatigue resistance of welded structures, and the designed welded structures often copy the fatigue design criteria and structural forms of other metal structures;

④ Welded structures are becoming more and more widely, and in the process of design and manufacturing, people blindly pursue the low cost and lightweight of the structure, resulting in the increasing design load of welded structures;

⑤ Welded structures tend to develop towards high speed and heavy load, and the requirements for dynamic load bearing capacity of welded structures are higher and higher;

However, the research level of fatigue strength of welded structures lags behind.

The Fatigue Strength Of Welded Structures

2. Causes of fatigue failure of welded structure

2.1 Effect of static load strength on fatigue strength of the welded structure

In the research of iron and steel materials, people always hope that the materials have high specific strength, that is, they can bear large load weight with light weight, because the structure with the same weight can have great bearing capacity;

Or the same bearing capacity can reduce its own weight.

Therefore, high strength steel came into being and has high fatigue strength;

The fatigue strength of base metals always increases with the increase of static load strength.

However, for the welded structure, the situation is different, because the fatigue strength of the welded joint has little relationship with the static strength of the base metal, the static strength of the weld metal, the microstructure and properties of the heat affected zone and the strength matching of the weld metal.

In other words, as long as the details of welded joints are the same, the fatigue strength of high-strength steel and low-carbon steel is the same, with the same S-N curve.

This rule is suitable for various joint types such as butt joint, corner joint and welded beam.

Maddox studied the fatigue crack growth of carbon manganese steel with yield point between 386-636MPa and weld metal and heat affected zone welded with six kinds of electrodes.

The results show that the mechanical properties of the material have a certain effect on the crack growth rate, but the effect is not great.

In the design of welded structures under alternating load, it is meaningless to try to meet the engineering needs by selecting high strength steel.

Only when the stress ratio is greater than + 0.5 and the static strength condition plays a major role, the base metal of welded joint shall be high-strength steel.

The reason for the above results is that there are slag wedge defects similar to undercut along the fusion line at the weld toe of the joint, with a thickness of 0.075mm-0.5mm and a tip radius of less than 0.015mm.

The sharp defect is the place where the fatigue crack begins, which is equivalent to the fatigue crack formation stage.

Therefore, the fatigue life of the joint under a certain stress amplitude is mainly determined by the fatigue crack propagation stage.

These defects make the same type of welded joints of all steels have the same fatigue strength, which has little to do with the static strength of base metal and welding materials.

The Fatigue Strength Of Welded Structures

2.2 Effect of stress concentration on fatigue strength

2.2.1 Influence of joint type

Welded joints mainly include butt joints, cross joints, T-joints and lap joints. Stress concentration occurs at the joint due to the interference of force transmission line.

The stress line interference of butt joint is small, so the stress concentration factor is small, and its fatigue strength will be higher than that of other joints.

However, experiments show that the fatigue strength of butt joints varies in a large range, because a series of factors affect the fatigue performance of butt joints.

Such as sample size, groove form, welding method, electrode type, welding position, weld shape, post welding weld processing, post welding heat treatment, etc.

For the butt joint with permanent backing plate, the fatigue strength of the joint is reduced due to the serious stress concentration at the backing plate.

The fatigue cracks of this joint are generated from the joint between the weld and the backing plate, not at the weld toe, and its fatigue strength is generally equal to that of the butt joint with the worst shape without backing plate.

Cross joint or T-joint has been widely used in welded structures.

In this load-bearing joint, due to the obvious section change at the transition from weld to the base metal, the stress concentration factor is higher than that of the butt joint, so the fatigue strength of cross or T-joint is lower than that of butt joint.

For the non-beveled joint connected by fillet weld and the grooved joint with local penetration weld, when the weld transmits working stress, its fatigue fracture may occur in two weak links, that is, the junction between the base metal and weld toe or weld.

For the cross joint with groove penetration, the fracture generally occurs only at the weld toe, not at the weld.

The fatigue strength of T-shaped and cross joints where the weld does not bear working stress mainly depends on the stress concentration at the junction of the weld and the main stressed plate.

T-joints have higher fatigue strength, while cross joints have lower fatigue strength.

The fundamental measure to improve the fatigue strength of T-shaped or cross joint is groove welding and machining the weld transition to make it smooth transition;

Through this improvement measure, the fatigue strength can be greatly improved.

The fatigue strength of the lap joint is very low, which is due to the serious distortion of the force line.

It is extremely unreasonable to use the butt joint of the so-called “reinforced” cover plate.

Due to the increased effect of stress concentration, the butt joint with high fatigue strength is greatly weakened after using the cover plate.

For load-bearing cover plate joints, fatigue cracks can occur in the base metal or in the weld.

In addition, changing the width of the cover plate or the length of the weld will also change the distribution of stress in the base metal, so it will affect the fatigue strength of the joint, that is, the fatigue strength of the joint increases with the increase of the ratio of the weld length to the width of the cover plate;

This is because the stress distribution tends to be uniform in the base metal.

2.2.2 Influence of weld shape

Regardless of the joint form, they are connected by two kinds of welds, butt weld and fillet weld.

With different weld shapes, the stress concentration factor is also different, so the fatigue strength has great dispersion.

The shape of the butt weld has the greatest influence on the fatigue strength of the joint.

(1) Influence of transition angle

Yamaguchi et al. established the relationship between fatigue strength and transition angle (external obtuse angle) between the base metal and weld metal.

In the test, W (weld width) and H (height) change, but the H/W ratio remains unchanged.

This means that the included angle remains unchanged, and the test results show that the fatigue strength also remains unchanged.

However, if w remains unchanged and the parameter H changes, it is found that H increases and the fatigue strength of the joint decreases, which is obviously the result of the decrease of the external included angle.

(2) Influence of weld transition radius

The research results of sander et al. show that the weld transition radius also has an important impact on the fatigue strength of the joint, that is, the transition radius increases (the transition angle remains unchanged) and the fatigue strength increases.

The shape of the fillet weld also has a great influence on the fatigue strength of the joint.

When the ratio a/b of the calculated thickness a of a single weld to the plate thickness B is less than 0.6 ~ 0.7, it is generally broken in the weld;

When a / b > 0.7, it is generally broken from the base metal.

Because the increase of weld size can not change the strength of another weak section, that is, the base metal at the end of the weld toe, it can not exceed the fatigue strength at best.

Soete, Van Crombrugge used 15mm thick plate welded with different fillet welds.

The test under axial fatigue load found that when the weld leg was 13mm, the fracture occurred in the base metal or weld at the weld toe.

When the weld leg is less than this value, fatigue fracture occurs on the weld; When the leg size is 18mm, the fracture occurs in the base metal.

Based on this, they put forward the limit weld leg size: S = 0.85B. Where S is the weld leg size and B is the plate thickness.

It can be seen that even if the weld leg size reaches the plate thickness (15mm), the fracture result at the weld can still be obtained, which is in good agreement with the theoretical result.

2.2.3 influence of welding defects

There are a large number of different types of defects at the weld toe, which lead to the early cracking of fatigue cracks and the sharp decrease of the fatigue strength of the base metal (down to 80%).

The Fatigue Strength Of Welded Structures

Welding defects can be generally divided into two categories:

Planar defects (such as cracks, lack of fusion, etc.) and volumetric defects (pores, slag inclusion, etc.) have different influence degrees;

At the same time, the influence of welding defects on joint fatigue strength is related to the type, direction and location of defects.

1) Crack

Cracks in welding, such as cold and hot cracks, are serious sources of stress concentration in addition to brittle microstructure;

It can greatly reduce the fatigue strength of structures or joints.

Early studies have shown that in the low carbon steel butt joint sample with a width of 60mm and thickness of 12.7mm, when there are cracks with a length of 25mm and depth of 5.2mm in the weld (they account for about 10% of the cross-sectional area of the sample), the fatigue strength of its 2×106 cycle life is reduced by about 55% ~ 65% under alternating load.

2) Incomplete penetration

It should be noted that incomplete penetration is not necessarily regarded as a defect, because sometimes some joints are artificially required to be circumferential penetration.

A typical example is the design of some pressure vessel nozzles.

Incomplete penetration defects are sometimes surface defects (single-sided weld) and sometimes internal defects (double-sided weld). They can be local or overall.

Their main influence is to weaken the cross-sectional area and cause stress concentration.

Compared with the test results without such defects, the fatigue strength is reduced by 25%, which means that the effect is not as serious as the crack.

3) Incomplete fusion

Due to the difficulty of sample preparation, there is very little research so far.

However, there is no doubt that lack of fusion belongs to plane defects, so it can not be ignored. It is generally treated as a lack of penetration.

4) Undercut

The main parameters characterizing undercut are undercut length L, undercut depth h and undercut width W.

The main parameter affecting the fatigue strength is undercut depth h. at present, depth h or the ratio of depth to plate thickness (h/ B) can be used to evaluate the fatigue strength of joints.

5) Stomata

For volume defects, Harrison analyzed and summarized the relevant test results of predecessors.

The decrease of fatigue strength is mainly caused by the reduction of cross-sectional area of pores, and there is a certain linear relationship between them.

However, some studies show that when the surface of the sample is machined by machining method, so that the pores are on the surface or just below the surface, the adverse effect of pores will increase.

It will act as a source of stress concentration and become the starting point of fatigue cracks.

This shows that the location of pores has a greater impact on the fatigue strength of the joint than its size, and pores on the surface or under the surface have the most adverse effect.

6) Slag inclusion

The relevant research report of IIW indicates that as a volumetric defect, slag inclusion has a greater impact on the fatigue strength of the joint than porosity.

Through the above introduction, it can be seen that the influence of welding defects on the fatigue strength of joints is not only related to the defect size, but also determined by many other factors, such as the influence of surface defects is greater than that of internal defects, and the influence of planar defects perpendicular to the force direction is greater than that of other directions;

The influence of defects located in the residual tensile stress zone is greater than that in the residual compressive stress zone;

The effect of defects located in the stress concentration area (such as weld toe crack) is greater than that of the same defects in the uniform stress field.

2.3 Effect of welding residual stress on fatigue strength

Welding residual stress is a unique feature of welded structures.

Therefore, its influence on the fatigue strength of welded structures is widely concerned. Therefore, a lot of experimental research work has been carried out.

The fatigue test is often carried out by comparing the sample with welding residual stress and the sample after heat treatment to eliminate the residual stress.

Because the generation of welding residual stress is often accompanied by the change of material properties caused by the welding thermal cycle, and heat treatment not only eliminates the residual stress, but also restores or partially restores the material properties.

At the same time, due to the dispersion of test results, there are different explanations for the test results and different evaluations on the influence of welding residual stress.

Taking the early and recent research work carried out by some people as an example, this problem can be clearly explained.

Different researchers have reached different conclusions on the results of 2×106 cycle tests on butt joints with reinforcement.

It was found that the fatigue strength of the stress relief sample after heat treatment was 12.5% higher than that of the same sample as welded state;

Others found that the fatigue strength of as welded and heat-treated samples was the same, that is, there was little difference;

However, it was also found that the fatigue strength increased after heat treatment to eliminate the residual stress, but the increased value was much lower than 12.5%.

The same is true for the test results of butt joint samples with surface grinding, that is, some tests believe that the fatigue strength can be increased by 17% after heat treatment, but some test results show that the fatigue strength has not been improved after heat treatment.

This problem has been puzzling for a long time. It was not until some scholars in the former Soviet Union carried out a series of tests under alternating load that they gradually clarified this problem.

Trufyakov’s research on the effect of welding residual stress on joint fatigue strength under different stress cycle characteristics is the most worthy to be proposed.

14Mn2 ordinary low alloy structural steel was used in the test. There was a transverse butt weld on the sample, and one longitudinal weld bead was overlaid on both sides.

One group of samples were heat treated to eliminate residual stress after welding, and the other group was not heat treated.

The fatigue strength comparison test adopts three stress cycle characteristic coefficients r = – 1, 0, + 0.3.

Under alternating load (r = – 1), the fatigue strength of the residual stress relieved sample is close to 130Mpa;

The residual stress without elimination is only 75mpa;

Under pulsating load (r = 0), the fatigue strength of the two groups of samples is the same, which is 185mpa.

When r = 0.3, the fatigue strength of the sample with residual stress eliminated by heat treatment is 260mpa, but it is slightly lower than that of the sample without heat treatment (270MPa).

The main reasons for this phenomenon are:

When the r value is high, such as under pulsating load (r = 0), the fatigue strength is high, and the residual stress is released quickly under the action of high tensile stress.

Therefore, the influence of residual stress on fatigue strength is weakened;

When r increases to 0.3, the residual stress further decreases under load, which actually has no effect on the fatigue strength.

The heat treatment not only eliminates the residual stress, but also softens the material, so the fatigue strength decreases after heat treatment.

This test shows the influence of residual stress and material change caused by welding thermal cycle on fatigue strength.

It can also be seen that the influence of welding residual stress on the fatigue strength of the joint is related to the stress cycle characteristics of fatigue load.

That is, when the cycle characteristic value is low, the influence is relatively large.

It was pointed out earlier that due to the residual stress reaching the material yield point in the structural weld, in the joint with constant amplitude stress cycle, the actual stress cycle near the weld will swing downward from the material yield point, regardless of the cycle characteristics of the original action.

For example, if the nominal stress cycle is + S1 to – S2, the stress range should be S1 + S2.

However, the actual stress cycle range in the joint will be from sy (stress amplitude at yield point) to SY – (S1 + S2).

This is very important in studying the fatigue strength of welded joints. It leads to some design codes to replace the cyclic characteristics r with the stress range.

In addition, the size of the specimen, loading mode, stress cycle ratio and load spectrum also have a great influence on the fatigue strength.

3. Process method for improving the fatigue strength of the welded structure

fatigue crack initiation position of welded joints

The fatigue crack initiation position of welded joints generally exists at the weld root and weld toe.

If the risk of fatigue crack initiation at the weld root is restrained, the dangerous points of welded joints are concentrated at the weld toe.

Many methods can be used to improve the fatigue strength of welded joints.

① Reduce or eliminate welding defects, especially opening defects;

② Improve the geometry of weld toe and reduce the stress concentration factor;

③ Adjust the welding residual stress field to produce the residual compressive stress field. These improved methods can be divided into two categories, as shown in Table 1.

The optimization method of welding process is not only considered to improve the fatigue strength of welded structure, but also has great benefits to the static load strength of welded structure and the metallurgical properties of welded joints.

There are a lot of data in this regard, which will not be repeated here.

Table 1 improvement methods of fatigue strength of welded structure

Improvement method of fatigue strength of welded structure Welding process optimization Local geometry Quality Control Control of welding defects 1
Improvement of geometry 2
Technological process Welding sequence 3
Residual stress(<0) Metallurgical treatment of weld toe 4
Weld bead modeling Weld toe geometry 5
Gold and metal state 6
Weld improvement Local geometry Machining Weld toe grinding 7
Water impact 8
Local remelting TG repair 9
Plasma repair 10
Residual stress Stress release method Heat treatment 11
Mechanical treatment 12
Local heating 13
Mechanical method Mechanical contact Shot peening 14
Hammering 15
Ultrasonic impact 16
welding Stamping 17
Local compression 18

The main methods to improve the fatigue strength of welded joints are discussed in detail in three parts from the perspective of process methods.

3.1 Methods to improve weld toe geometry and reduce stress concentration

1) TIG Dressing

TIG Dressing

Studies at home and abroad have shown that TIG repair can greatly improve the fatigue strength of welded joints.

In this method, TIG welding is used to remelt the transition part of the welded joint once, so as to form a smooth transition between the weld and the base metal.

The stress concentration is reduced, and the small non-metallic slag inclusion in this part is also reduced, so the fatigue strength of the joint is improved.

The welding gun is generally located 0.5 ~ 1.5mm away from the welding toe in the welding repair process, and the remelting part shall be kept clean.

If it is equipped with slight grinding in advance, the effect will be better.

The important thing is how to deal with the method of re arcing when arc extinguishing occurs in remelting.

Because this will inevitably affect the quality of remelting weld bead, it is generally recommended that the best position for re arcing is 6mm in front of the weld bead crater.

Recently, the international welding society organized some welding research institutes in some European countries and Japan to conduct a unified study on the effectiveness of some methods to improve the fatigue strength of the joint with the samples prepared by the British Welding Research Institute.

It is confirmed that the nominal fatigue strength of the joint under the 2×106 cycles is increased by 58% after being treated by this method.

If the nominal value of 211MPa fatigue strength is converted into the corresponding characteristic value (K index), it is 144MPa.

It has been higher than the highest FAT value in the joint detail fatigue strength of the international welding society.

2) Machining

If the weld surface is machined, the stress concentration will be greatly reduced and the fatigue strength of the butt joint will be improved accordingly.

When there are no defects in the weld, the fatigue strength of the joint can be higher than that of the base metal.

However, the cost of this surface machining is very high, so it is suitable for this machining only where it is really beneficial and can be machined.

However, for welds with serious defects and without bottom welding, the stress concentration at the defect or weld root is much more serious than that on the weld surface.

Therefore, in this case, the machining of weld surface is meaningless.

If there is a lack of penetration defect, because the fatigue crack will not start at the reinforcement and weld toe, but transfer to the lack of penetration at the root of the weld.

In the case of incomplete penetration defects, machining tends to reduce the fatigue strength of the joint.

Sometimes, the whole weld metal does not need to be machined, but only the weld toe needs to be machined and ground.

This method can also greatly improve the fatigue strength of the joint.

The research shows that in this case, the crack initiation point is not at the weld toe, but transferred to the weld defect.

Makorov of the former Soviet Union on high strength steel (tensile strength σb = 1080MPa) the fatigue strength test of transverse butt weld under alternating load shows that the fatigue strength is ± 150MPa under the condition of 2×106 cycles as welded.

If the weld is machined and the reinforcement is removed, the fatigue strength is increased to ± 275MPa, which is equivalent to the fatigue strength of the base metal.

However, if local grinding is carried out at the butt weld toe, the fatigue strength is ± 245MPa.

It is 83% of the machining effect, and the fatigue strength is increased by 65% compared with the welded state.

Of course, whether machining method or grinding method is adopted, if it can not be carried out carefully according to the requirements in order to ensure the machining effect, the improvement of fatigue strength is limited.

3) Grinding wheel grinding

Grinding wheel grinding

Grinding with grinding wheel is not as effective as machining, but it is also an effective method to improve the fatigue strength of welded joints.

The International Welding Society recommends the use of high-speed electric or hydraulic driven grinding wheel with speed of (15000 ~ 40000) / min. the grinding wheel is made of carbon tungsten material, and its diameter shall ensure that the grinding depth and radius shall be equal to or greater than 1 / 4 of the plate thickness.

The recent research of the international welding society shows that the nominal fatigue strength of the sample under 2 cycles is increased by 45% after grinding.

If the nominal value of 199mpa fatigue strength obtained is converted into the corresponding characteristic value (135Mpa), it is also higher than the highest fat value in the joint detail fatigue strength of the international welding society.

It should be noted that the grinding direction should be consistent with the direction of the stress line, otherwise a notch perpendicular to the stress line will be left in the weld, which is equivalent to a stress concentration source and plays a role in reducing the fatigue strength of the joint.

4) Special electrode method

This method is to develop a new type of electrode. Its liquid metal and liquid slag have high wetting capacity, which can improve the transition radius of the weld, reduce the angle of the weld toe, reduce the stress concentration at the weld toe, and improve the fatigue strength of the welded joint.

Similar to the disadvantages of TIG welding repair, it has strong selectivity for welding position, especially suitable for flat welding position and fillet welding, while its advantages are significantly reduced for vertical welding, horizontal welding and overhead welding.

3.2 Method of adjusting residual stress field to produce compressive stress

1) Pre overload method

If the tensile load is applied on the specimen containing stress concentration until yield occurs at the notch, accompanied by a certain tensile plastic deformation, after unloading, the compressive stress will be generated at the tensile plastic deformation at and near the load notch. The tensile stress below the yield point will be balanced in other sections of the sample.

In the subsequent fatigue test, the stress range of the specimen subjected to this treatment will be different from that of the original specimen without preload, that is, it will be significantly reduced.

Therefore, it can improve the fatigue strength of welded joints.

The results show that a certain pre overload test is required before large welded structures (such as bridges, pressure vessels, etc.) are put into operation, which is beneficial to improve the fatigue performance.

2) Local heating

Local heating can adjust the welding residual stress field, that is, compressive residual stress can be generated at the stress concentration, which is beneficial to improve the fatigue strength of the joint.

This method is currently limited to longitudinal discontinuous welds or joints with longitudinal stiffened plates.

For single-sided fillet plates, the heating position is generally about 1 / 3 of the plate width from the weld. For double-sided fillet plates, the heating position is the center of the plate.

In this way, compressive stress can be generated in the weld, which can improve the fatigue strength of the joint.

Different researchers obtained different results by using this method. For single-sided gusset plate, the fatigue strength was increased by 145-150%, and for double-sided gusset plate, the fatigue strength was increased by 70-187%.

The local heating position has an important influence on the fatigue strength of the joint. When the spot heating is carried out at the end of the weld, the compressive residual stress is caused at the notch at the end of the weld, and the fatigue strength is increased by 53%;

However, when the spot heating is carried out in the center of the sample at the weld end, the distance from the weld end is the same.

Although this has the same metallographic effect, the measured fatigue strength of the joint is the same as that of the untreated specimen because the residual stress is tensile residual stress.

3) Extrusion method

The local extrusion mechanism is the same as the point heating method, that is, the fatigue strength of the joint is improved by compressive residual stress.

However, its action point is different, and the extrusion position should be located at the position where residual compressive stress needs to be generated.

The effect of extrusion method for high-strength steel sample is more significant than that of low-carbon steel.

4) Gurnnert’s method

Because it is sometimes difficult to accurately determine the heating position and heating temperature of local heating method, gunnert proposed a method in order to obtain satisfactory results.

The key point of this method is to heat directly to the notch rather than the nearby part to a temperature that can produce plastic deformation but lower than the phase transformation temperature of 55 ℃ or 550 ℃, and then spray it sharply for cooling.

Due to the late cooling of the metal under the surface and the metal around it that is not sprayed, the shrinkage will produce compressive stress on the cooled surface when it is cooled.

Thus, the compressive stress can improve the fatigue strength of the member.

It should be noted that in order to heat the bottom layer, the heating process should be slow. Gunnert recommends that the heating time be 3min, while Harrison recommends that the heating time be 5min.

Ohta successfully prevented fatigue cracks in the butt pipe by using this method.

The specific method is that the outside of the pipeline is heated by induction method and the inside is cooled by circulating water.

Therefore, compressive stress is generated in the pipeline, which effectively prevents fatigue cracks from occurring in the pipeline.

After treatment, the fatigue crack growth rate of butt weld pipe is greatly reduced and reaches the same crack growth rate as the base metal.

3.3 Methods of reducing stress concentration and generating compressive stress

1) Hammering method

Hammering is a cold working method. Its function is to cause compressive stress on the surface of the weld toe of the joint.

Therefore, the effectiveness of this method is related to the plastic deformation on the surface of the weld toe;

At the same time, hammering can also reduce the notch sharpness, thus reducing the stress concentration, which is also the reason for greatly improving the fatigue strength of the joint.

The air hammer pressure recommended by the international welding society shall be 5 ~ 6Pa.

The top of the hammer head shall be solid material with a diameter of 8 ~ 12mm. It is recommended to use 4 times of impact to ensure that the hammering depth reaches 0.6mm.

The recent work of the international welding society shows that for non load-bearing T-joints, the fatigue strength of the joints under 2×106 cycles after hammering is increased by 54%.

2) Shot peening

Shot peening

Shot peening is another form of hammering, which also belongs to the method of impact machining.

The effect of shot peening depends on the shot peening diameter. The shot peening size should not be too large to deal with small defects.

At the same time, the size of shot peening shall not be too small to ensure a certain cold work hardening performance. Shot peening can generally act at the depth of a few thousandths of a millimeter on the surface.

The results show that shot peening can significantly improve the fatigue strength of high-strength steel joints. Shot peening has an outstanding effect on argon arc welding high-strength steel materials, and its degree is even higher than that of TIG repair.

At the same time, the effect of TIG fusion repair is more significant if shot peening is used.

4. The latest technology to improve the fatigue strength of welded joints

4.1 Ultrasonic impact treatment method

In recent years, ultrasonic impact has been developed to improve the fatigue strength of welded joints and structures. Its mechanism is basically consistent with hammering and shot peening.

However, this method has the advantages of light actuator, good controllability, flexible and convenient use, minimal noise, high efficiency, less restrictions in application, low cost and energy saving.

It is suitable for all kinds of joints. It is an ideal method to improve the fatigue performance of welded joints after welding.

Ultrasonic impact treatment was applied to butt joints and non bearing longitudinal corner joints of several typical welded structural steels;

Then the comparative fatigue tests of as welded and impact treatment were carried out;

The practical effect of ultrasonic impact method on improving the fatigue strength of welded joints was studied;

See Table 2 for comparison results.

It can be seen that the fatigue strength of welded joints is increased by 50 ~ 170% after ultrasonic impact treatment.

Table 2 Comparison of fatigue strength before and after ultrasonic impact treatment

Material and joint form Fatigue strength Ds / MPa Increase degree(%)
As welded Shock treated state
Q235B (R= 0.1) – butt joint 152 230 51
SS800 (R= 0.05) – butt joint 306 101
16Mn (R= 0.1) – butt joint 285 88
Q235B (R=0.1) – longitudinal corner joint 104 200 92
SS800 (R=0.05) – longitudinal corner joint 279 168
16Mn (R=0.1) – longitudinal corner joint 212 104

4.2 Low phase change spot welding strip method

4.2.1 Principle and development of improving fatigue strength of welded joints

Compressive stress can improve the fatigue strength of welded joints, which has been discussed in a large number of literatures. However, the problem is how to introduce compressive stress into welded joints conveniently.

It is well known that due to different chemical composition, alloy content and cooling rate.

During the cooling process of iron and steel materials, different or multiple microstructure changes will occur.

This structural transformation is accompanied by volume expansion, which will produce phase transformation stress under restrained conditions, which belongs to compressive stress.

For weld metal, this will be conducive to the reduction of residual tensile stress and even residual compressive stress, so as to improve the mechanical properties of welded joints.

Low transformation temperature welding electrode (LTTE) is a new welding material that uses phase transformation stress to produce compressive stress in welded joints and improve the fatigue strength of welded joints.

As early as the 1960s, welding experts in the former Soviet Union put forward the method of low phase transformation spot welding strip, which can improve the fatigue strength of welded structures.

However, at that time, the concept of “low phase transformation spot welding strip” was not put forward, and it was only called a special electrode.

The surfacing metal composition mainly depends on 3-4% Mn content to reduce the phase transformation point and realize metallurgical phase transformation.

The literature points out that when these special electrodes are selected for fatigue test on small specimens, the fatigue strength after surfacing with these electrodes is 75% higher than that without surfacing test.

In recent years, relying on Cr and Ni to reduce the martensitic transformation point of deposited metal of welding materials, and due to the development of ultra-low carbon steel, low transformation spot welding strip has been developed rapidly.

Japan and China have done a lot of research in this field, but it is still in the laboratory stage.

4.2.2 Effect of LTTE electrode on improving the fatigue strength

The school of materials of Tianjin university designed and optimized the low phase transformation spot welding strip, and carried out a large number of fatigue tests and process performance tests on various welded joints.

(1) LTTE method

The transverse butt joint, non load bearing cross joint, longitudinal circumferential fillet joint, longitudinal parallel fillet weld joint and longitudinal butt joint were welded with low phase change spot welding rod LTTE and ordinary electrode E5015 respectively, and the fatigue comparative test was carried out.

The results show that the fatigue strength of LTTE joint of phase change spot welding rod is 11%, 23%, 42%, 46% and 59% higher than that of ordinary electrode E5015, and the fatigue life is increased from several times to hundreds of times.

Table 3 Improvement effect of fatigue strength of different types of welded joints

Electrode type Transverse butt joint Non load bearing cross joint Longitudinal circumferential fillet weld joint Longitudinal parallel fillet weld joint Longitudinal butt joint
E5015 welding rod 176.9 202.1 167.0 182.7 179.4
LTTE electrode 157.8 164.8 118.3 124.9 113.0
Degree of improvement 11% 23% 41% 47% 58%
Stress concentration Mild K1 Medium K2 Strong K3 Particularly strong N4 Particularly strong K4
Degree of restraint Small                                      large

Because the low transformation spot welding strip is the residual compressive stress obtained by the volume expansion of martensitic transformation at lower temperature, the magnitude of the residual compressive stress is closely related to the restraint of the welded joint.

The greater the degree of restraint, the greater the residual compressive stress and the greater the effect of improving the fatigue strength.

(2) LTTE dressing method for low phase transformation spot welding

However, in order to make the weld metal undergo martensitic transformation at a normal cooling rate and low temperature, more alloy elements are added to the welding materials, which greatly increases the cost of low transformation spot welding materials.

If all welds of a welded structure are welded with low phase change welding materials, the cost of the welded structure will also increase significantly, which is very uneconomical.

It is well known that the fatigue fracture of welded joints mainly cracks at the weld toe.

If the residual compressive stress is generated at the weld toe of the welded joint, the fatigue strength of the welded joint can be improved without using all low phase change spot welding strips, which can reduce the use cost.

Considering this idea, Tianjin University proposed a method to improve the fatigue strength of welded joints by low phase transformation spot welding rod toe dressing (LTTE dressing) on the basis of experiments.

The fatigue strength of LTTE dressing and ordinary electrode welded joints is compared by using two types of non load-bearing cross joint and longitudinal circumferential fillet weld joint. The fatigue strength of the former is 19.9% and 41.7% higher than that of the latter respectively, which proves the feasibility and practicability of this idea.

The preliminary experimental research is carried out for the more reasonable application of low phase change spot welding strip LTTE in engineering practice.

At the same time, the toe dressing joint of low phase change spot welding strip can also reflect the application of low phase change spot welding strip in cover weld and near toe cover weld bead.

4.2.3 Advantages and disadvantages of low phase change spot welding strip

advantage:

(1) The welding method of low phase change spot welding strip is carried out simultaneously with the welding process, which avoids the inconvenience of post welding processing;

(2) The low phase change spot welding strip method does not need special operation requirements, so the operation is simple and convenient;

(3) Low phase change spot welding materials are used to improve the fatigue strength of welded joints.

Because it is not affected by the thermal effect of subsequent weld beads, it is more suitable for improving the fatigue strength of hidden welds, covered welds, back welds of single-sided welding and other welds that cannot be processed after welding;

(4) LTTE electrode can also be used to repair fatigue cracks in welded structures.

Disadvantages:

More alloy elements are added to the welding materials, which increases the cost of low phase change point welding materials, but it can be made up by LTTE dressing and other methods.

5. Conclusion

To sum up, it can be seen that in recent years, due to the development trend of welded structures towards high-speed and heavy load, the requirements for their dynamic load bearing capacity are becoming higher and higher.

Therefore, the development and application of new technologies to improve the fatigue performance of welded joints is of great significance to promote the application of welded structures.

Relatively speaking, the newly developed ultrasonic impact technology at home and abroad and the method of using low phase change spot welding materials to improve the fatigue strength of welded joints are important research directions of fatigue performance improvement technology and process of welded structures.

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