Fatigue Failure and Brittle Fracture of Welded Structures

1. Fatigue failure of welded structures

A large amount of statistical data shows that over 80% of structural failures in engineering are caused by fatigue. The research report submitted by the National Bureau of Standards of the US Department of Commerce to the US Congress states that the United States pays a cost of $119 billion annually for fracture and prevention, equivalent to 4% of the total national economic output. Statistics show that the vast majority of fractures are caused by fatigue.

Several bridges in the United States have experienced fatigue fracture cracks at the weld toe near the end of the weld, as shown in Figures 2-53. There is high stress concentration at the crack location shown in the diagram. Under load, the plane displacement of the web plate is concentrated at a relatively narrow and unsupported height of the web plate, that is, the height of the web plate from the wing plate to the bottom of the reinforcing rib (in the shaded area), causing the web plate to crack at that location.

Fatigue is defined as the damage to structural components caused by the initiation and slow propagation of cracks caused by repeated stresses. The fatigue fracture process usually goes through three stages: crack initiation, stable propagation, and unstable propagation.

(1) Characteristics of fatigue fracture surface

When conducting macroscopic analysis of fatigue fracture, the fracture surface is generally divided into three zones, which correspond to the formation, propagation, and instantaneous fracture stages of fatigue cracks, namely the fatigue source zone, fatigue propagation zone, and instantaneous propagation zone, as shown in Figures 2-54.

The fatigue source zone is the real record left by the formation process of fatigue cracks on the fracture surface. Due to the small size of the fatigue source area, it is difficult to distinguish the cross-sectional characteristics of the fatigue source area macroscopically. Fatigue sources generally occur on the surface, but if there are defects inside the component, such as brittle inclusions, they can also occur inside the component. Sometimes there is more than one fatigue source, but there are two or even more. For low cycle fatigue, due to its larger strain amplitude, there are often several fatigue sources located at different positions on the fracture surface.

(2) Factors affecting the fatigue strength of welded structures

The factors that affect the fatigue strength of the base material, such as stress concentration, cross-sectional size, surface condition, loading conditions, etc., also have an impact on the welded structure. In addition, some characteristics of the welding structure itself, such as changes in the performance of the joint near the seam area, welding residual stress, etc., may also have an impact on welding fatigue.

(1) The influence of stress concentration in welded structures. Due to different stress concentrations at the joint, they have varying degrees of adverse effects on the fatigue strength of the joint.

(2) Experimental research on the influence of changes in metal properties near the seam zone shows that welding of low-carbon steel under commonly used line energy. The fatigue strength of the heat affected zone is quite similar to that of the base metal, and the mechanical properties of the metal in the near seam zone have a relatively small impact on the fatigue strength of the joint.

(3) The influence of residual stress on structural fatigue strength depends on the distribution state of residual stress. In areas with high working stress, such as stress concentration areas and the outer edge of bent components, residual stress is tensile, which reduces fatigue strength; On the contrary, if there is compressive residual stress at that location, the fatigue strength will be increased. In addition, the influence of residual stress on fatigue strength is also related to factors such as stress concentration degree and stress cycle number, especially the higher the stress concentration coefficient, the more significant the influence of residual stress.

(4) The impact of welding defects on fatigue strength is related to the type, size, direction, and location of the defects. Flake defects (such as cracks, lack of fusion, and incomplete penetration) have a greater impact than defects with rounded corners (such as pores); Surface defects have a greater impact than internal defects; Defects located in stress concentration areas have a greater impact than the same defect in a uniform stress field; The influence of flaky defects perpendicular to the direction of the applied force is greater than in other directions; Defects located within the residual tensile stress field have a greater impact than those in the residual compressive stress zone.

(3) Measures to improve fatigue strength

1. Reduce stress concentration in components

The stress concentration in the structure is the main factor in reducing the fatigue strength of welded structures, and the following measures are generally taken.

(1) Reduce stress concentration with a reasonable component structure to improve fatigue strength.

(2) Reasonably choose the joint form and try to use butt joints with low stress concentration factors, with a smooth transition in the shape of the weld seam. Continuous welding is more advantageous than intermittent welding for vibration loads, and fillet welding should be used as little as possible.

(3) When using fillet welds, comprehensive measures must be taken, such as machining the end of the weld, selecting the shape of the fillet joint plate reasonably, and ensuring that the root of the weld is fully penetrated.

(4) Using surface machining methods to eliminate various grooves near the weld seam and reduce stress concentration in the joint

2. Process measures to improve the fatigue strength of welded structures

(1) The correct welding specifications should be selected in the process to ensure that the weld seam is well formed and there are no defects inside or outside.

(2) TIG welding arc shaping can significantly improve the fatigue strength of welded joints.

(3) Adjust residual stress. There are two types of methods: overall treatment of structures and components, including overall annealing or overload pre stretching method; Local treatment of the joint area involves using methods such as heating, rolling, and local explosion to generate residual stress at the stress concentration point of the joint.

(4) Improving the mechanical properties of materials through surface strengthening treatment can increase the fatigue strength of joints by using small wheel extrusion or lightly tapping the weld surface and transition zone with a hammer, or spraying the weld area with small steel balls.

3. Adopting special protective measures

The use of special plastic coatings to improve the fatigue performance of welded joints is a new technology with significant effects.

2. Brittle fracture of welded structures

Since the widespread application of welded structures, many countries have experienced brittle fracture accidents of welded structures, with serious and even catastrophic consequences. The results of a joint investigation by the UK Atomic Energy Agency and the United Nations Technical Committee indicate that the majority of the catastrophic accidents that occurred in 12700 pressure vessels under manufacturing were brittle fractures, with an accident rate of 2.3 × 10~4; Among 100300 in-service pressure vessels, the catastrophic accident rate is 0.7 × 10~4, injury accident rate 12.5 × 10~4 Total 13.2 × 1 to 4. The most typical example of many serious accidents is the collapse of the Hesselt Bridge on the Albert Canal in Belgium on March 14, 1938.

(1) Characteristics of brittle fracture

(1) Brittle fracture generally occurs when the stress is not higher than the structural design stress and there is no significant plastic deformation, and it extends to the entire structure, resulting in severe losses.

(2) Brittle fracture often starts from the point of stress concentration, such as the presence of defects and welds within the component.

(3) At low temperatures, thick sections and high strain rates are prone to brittle fracture under dynamic loading. A large number of studies on brittle fracture accidents have shown that the reasons for welding brittle fracture are multifaceted, but the main ones are improper material selection, unreasonable design, imperfect manufacturing processes, and inspection techniques.

(2) Factors affecting brittle fracture of metals

1. The influence of temperature on the mode of damage

Lowering the temperature will transform the failure mode from plastic failure to brittle failure. This is because as the temperature decreases, the risk of cleavage fracture increases, and the material will undergo a transition from ductile to brittle fracture, that is, the brittle transition temperature of the material increases.

2. The influence of stress state

Objects generate different normal stresses on different cross-sections when subjected to external loads б And shear stress т， Among them, there is a maximum normal stress б Max and maximum shear stress т Max. б Max and т Max and its ratio б Max/ т Max is related to the loading method. A= б Max/ т Max is called the stress state coefficient, which is related to the loading method and the shape of the part. б The increased stress state is conducive to ductile fracture of plastic deformation shear stress, while б Reducing it is beneficial for brittle fracture under normal stress.

3. The impact of loading speed

Research has shown that increasing loading speed can promote brittle failure of materials, which is equivalent to reducing temperature. It should also be pointed out that under the same loading rate, when there are defects in the structure, the strain rate can have a negative effect of doubling. Because at this point, stress concentration greatly reduces the local plasticity of the material.

4. The influence of material status

(1) The influence of plate thickness is first that thick plates are prone to forming a plane strain state of three-dimensional stress at the defect location. In addition, thick plates have fewer rolling cycles, loose microstructure, and uneven internal and external properties.

(2) The influence of grain size has a significant impact on the brittle transition temperature. The finer the grain, the lower its transition temperature.

(3) The influence of chemical composition on elements such as C, N, O, H, S, P in steel can increase its brittleness.