How to prevent brittle fracture accident of fabricated steel structure residence

How to prevent brittle fracture accident of fabricated steel structure residence

1、 Brittle fracture concept

Steel structure is a load-bearing structure composed of steel, although steel is an elastic-plastic material. Especially low carbon steel shows good plasticity. However, under certain conditions, brittle fracture will also occur in steel structures due to the combined influence of various factors. And it often occurs in the state of tensile stress. Brittle fracture refers to the sudden fracture of steel or steel structure under the condition of low nominal stress (lower than the yield strength or tensile strength of steel).

The brittle fracture of steel structures usually has the following characteristics:

Brittle failure is the most dangerous failure form in the limit state of steel structures. Because of the burst of brittle fracture, it often leads to disastrous consequences. Therefore, as a professional technician of steel structure, we should attach great importance to the severity of brittle failure and prevent it.

2、 Cause analysis of brittle fracture

Although the plasticity of steel structure is very good, brittle fracture will still occur, which is the result of the comprehensive influence or action of various adverse factors. The main reasons can be summarized as follows:

1. Material defect

When the content of carbon, sulfur, phosphorus, oxygen, nitrogen, hydrogen and other elements in steel is too high. It will seriously reduce its plasticity and toughness, and the brittleness will increase accordingly. Generally, carbon leads to poor weldability; Phosphorus and oxygen lead to "hot brittleness"; Phosphorus and nitrogen lead to "cold brittleness"; Hydrogen causes "hydrogen embrittlement". In addition, the metallurgical defects of steel, such as segregation, non-metallic inclusions, cracks and delamination, will also greatly reduce the ability of steel to resist brittle fracture.

2. Stress concentration

Steel structure is inevitable under load due to holes, notches and abrupt changes in section. These parts will produce local peak stress, while the stress in other parts is low and unevenly distributed, which is called stress concentration. We usually call the ratio of peak stress to average stress as stress concentration factor. To indicate the severity of stress concentration.

When there is stress concentration in a certain part of steel, there is a two-dimensional or three-dimensional stress field of the same number, which makes the material difficult to enter the plastic state, resulting in brittle failure. The more serious the stress concentration is, the more the plasticity of steel decreases, and the greater the risk of brittle fracture. The stress concentration of steel structure or member is mainly related to its structural details.

(1) In the design and fabrication of steel members, defects such as holes, grooves, concave corners, notches, cracks and abrupt changes in section are inevitable.

(2) As the main connection method of steel structure, welding has many advantages, but the disadvantage is that weld defects and residual stress often become the source of stress concentration. According to statistics, the brittle failure accidents of welded structures are far more than riveted structures and bolted structures. The main reasons are as follows:

① There are more or less some defects in the weld, such as cracks, slag inclusions, pores, biting, etc. these defects will become the source of fracture;

② The residual stress in the structure after welding can be divided into residual tensile stress and residual compressive stress. The combination of the former and other factors may lead to cracking;

③ The connection of welded structures is often rigid. When multiple welds meet, the plastic deformation of materials is difficult to develop and the brittleness increases;

④ Welding makes the structure form a continuous whole. Once the crack develops, it may crack to the end. Unlike riveting or bolting, cracks terminate when they meet screw holes.

3. Use environment

When the steel structure is subjected to large dynamic load or works at low ambient temperature, the possibility of brittle failure of steel structure increases.

It is well known that temperature has a significant effect on the properties of steel. Above 0 ℃, when the temperature rise is high, the strength and elastic modulus of steel change, generally the strength decreases and the plasticity increases. When the temperature is within 200 ℃, the properties of steel do not change much. However, at about 250 ℃, the tensile strength of the steel rebounds, the yield strength is greatly improved, and the plasticity and impact toughness decrease, resulting in the so-called "blue brittleness". At this time, the hot-working steel is prone to cracks. When the temperature reaches 600 ℃, the yield strength and elastic modulus are close to zero, we believe that the steel structure almost completely loses its bearing capacity.

When the temperature is below 0 ℃, the strength of steel increases slightly with the decrease of temperature. The plasticity and toughness decreased and the brittleness increased. Especially when the temperature drops to a certain temperature range, the impact toughness of steel decreases sharply and low-temperature brittle fracture occurs. Generally, the brittle failure of steel structure at low temperature is called "low temperature cold embrittlement" phenomenon, and the resulting cracks are called "cold cracks". Therefore, for the steel structure working at low temperature, especially the steel structure under dynamic load, the steel shall have the qualified guarantee of negative temperature impact toughness, so as to improve the ability to resist low temperature fusing.

4. Steel plate thickness

With the development of large-scale steel structure, especially the rise of high-rise structure, the thickness of component steel plate has a great increasing trend. The thickness of steel plate has a great influence on brittle fracture. Generally, the thicker the steel plate, the greater the tendency of brittle failure“ The problem of "lamellar tearing" should be paid great attention to.

To sum up, material defects, stress concentration, service environment and steel plate thickness are the main factors affecting brittle fracture. The influence of stress concentration is particularly important. It is worth mentioning that the stress concentration does not affect the static ultimate bearing capacity of steel structure. Its influence is usually not considered in design. However, under dynamic load, severe stress concentration, material defects, residual stress, cooling hardening and low temperature environment are often the root causes of brittle fracture.