What are Fatigue Cracks?

Because the materials used to make parts have inclusions, segregation, or defects; or because the design is unreasonable; or because the manufacturing process is unreasonable, etc., stress concentrations tend to occur in some parts of the part, and cracks occur under repeated stress changes .

Because the materials used to make parts have inclusions, segregation, or defects; or because the design is unreasonable; or because the manufacturing process is unreasonable, etc., stress concentrations tend to occur in some parts of the part, and cracks occur under repeated stress changes .

Metal fatigue generally consists of three phases: fatigue crack initiation phase, fatigue crack growth phase, and fatigue fracture phase. Fatigue crack growth is the second stage of fatigue, which refers to the growth of fatigue cracks in the propagation zone.

Under continuous alternating stress, fatigue crack growth materials will gradually generate cracks on their surfaces, and gradually develop in depth with the action time. When cracks are allowed to spread, the mechanical properties of the test specimens are reduced, which eventually leads to complete fracture. It should be noted that some materials have good resistance to initial crack growth, but once developed, they develop rapidly. Other materials do the opposite.

The earliest recorded fatigue test was performed by the German WA Albert in 1829. He repeatedly loaded the welding chain for mine hoisting and broke after 100,000 cycles. In 1839, Frenchman JV Pensale first used the word "fatigue" in his writings. In 1843, the Scottish WJM Rankin discussed the damage of the locomotive axle, which was believed to be caused by the gradual deterioration of metal performance during operation. He analyzed the harmful effects of the sharp corners on the axle shoulders, and pointed out that increasing the radius of the shoulder radius can increase the fatigue strength. At the same time, a committee was established in the UK to investigate the applicability of iron as a material for railway bridge construction. After conducting several tests on beams, it was pointed out that the beams could withstand near-destructive loads for up to four years under static Failure, but if the beam is repeatedly bent with half of the static damage load, the beam will be destroyed as long as 1000 cycles. It is enough to show that the fatigue load is far below the ultimate load of the beam.

The first systematic test of fatigue strength was by German A. Waller, who completed various fatigue tests under cyclic stress during work on the Strasbourg Royal Railway from 1847 to 1889. In 1850, he designed a rotary fatigue tester for fatigue testing, recognizing that fatigue failure can occur when the stress is below the elastic limit, and there is a stress amplitude limit value that does not occur when the stress amplitude is less than this value Fatigue failure. And he first proposed the concept of SN curve and fatigue limit. In addition, he also studied the effects of heat treatment, stress concentration, and superimposed static loads on fatigue. For fatigue, the stress amplitude is more important than the average stress. In 1884, when J. Bauschinger verified the fatigue test of A. Waller, he found that the "cyclic softening" phenomenon did not attract much attention at that time, and it was not re-raised when Ke Yang did the fatigue test of copper rods in 1952 Come out and name it "Bauschinger Effect". Therefore, J. Bauschinger was the first to study the cyclic stress-strain relationship.

In 1874, according to A. Waller's test data, W. Gerber expressed the "limit" for fatigue failure with a non-zero average stress. At any given life, the corresponding fatigue limit line diagram can be drawn, that is, Gerber parabola. In 1930, the British J. Goodman proposed a simplified hypothesis on the fatigue limit line diagram, that is, a straight line connecting the symmetrical cyclic fatigue limit point on the vertical axis and the strength limit point on the horizontal axis to replace the Gerber parabola.

In 1903, JA Ewing and JCW Humphrey conducted a rotating bending fatigue test on iron. During the test, using an optical microscope to observe the pattern, it was found that a slip line was generated in the crystal under the effect of cyclic stress. As the number of cycles increased, the slip line gradually became deeper and wider, forming a slip band. Cracks first appeared on the crystal, and then a long continuous crack was formed between the crystals, which eventually led to damage.

In 1923, the British HJ Goff introduced another hypothesis to the theory of fatigue. He believes that when the stress amplitude is lower than the fatigue limit, no plastic deformation will occur; if the stress amplitude is higher than the fatigue limit, strain hardening will reach a limit value, thereby forming cracks.

In 1945, the American MA Mainer re-proposed on the basis of JV Palmgren's work: Damage is linearly related to the number of stress cycles, and later referred to as the MINER criterion. In 1974, American scholar JW Fisher proved through a large number of full-scale welding fatigue test results that the main factor affecting fatigue is the stress amplitude, not the maximum stress and stress ratio.

The conventional fatigue strength design assumes that the material is a defect-free continuum. The fatigue failure process is divided into three stages: crack formation, crack propagation, and final fracture. But in fact, there are always flaws in the material. Therefore, the application of the theory of fracture damage mechanics to fatigue strength design is a future development trend. In 1920, the British AA Griffiths researched that the actual strength of glass is stronger than it. The molecular strength of the molecular structure is expected to be 1,000 times to 10,000 times lower. His research on the fracture of glass plates with cracks marks the beginning of modern fracture damage theory, but there has been no mathematics for quantitative treatment of fatigue failure models. The frame was proposed by American PC Paris until 1957. Under cyclic loading, the amplitude of the stress intensity factor at the crack tip is the basic parameter to control the fatigue crack growth rate of the component. In 1963, the exponential power law Paris formula was proposed.

In 1960, research on low-cycle strain fatigue performance was developed, and LF Cliff and SS Masson independently proposed the empirical relationship between plastic strain amplitude and fatigue life, and then formed the local stress-strain method. In 1968, the study of nonlinear fracture damage mechanics had a new starting point. R-CE proposed a path-independent J integral and published the singular solution of the famous HRR elastoplastic static crack tip, marking the beginning of this elastoplastic fracture mechanics theory. mature. Based on fracture mechanics, a damage tolerance method has been developed, and fatigue mechanics and fatigue mechanics methods may become the dominant method in the future. In 1977, the three design specifications for steel structures, railway steel bridges, and highway bridges in the United States were revised at the same time, and the stress amplitude was used as the resistance for fatigue check.

In the past 20 years, fatigue research has developed by leaps and bounds in China. Compared with foreign countries, fatigue research started late in China. It began in the early 1950s and developed in the 1980s. As far as research content is concerned, the focus of research is different from that of foreign countries. . Life estimation and random fatigue have become hot topics in China.

In short, the fatigue theory has been continuously improved by countless researchers, and has gradually formed a more systematic research theory. With the development of electronic computers, the use of software to analyze and simulate the development of fatigue cracks will become the leading research in the future. Computer simulation calculations To estimate the fatigue life. In order to find an effective method for fatigue assessment of steel bridges.

From a microscopic perspective, the most common explanation for metal crack formation is slip zone cracking. With the increase of the number of load cycles, the dislocation density of the crystal inside the metal welding structural material increases continuously. When the dislocation density increases to a certain value, dislocation entanglement is formed inside the crystal, thereby forming a high-density dislocation band. And low-density dislocation regions that hinder dislocation movement. Under the continued application of fatigue loads, dislocations interact and transform from high energy to low energy, gradually forming dislocation cells, and then developing into a sub-crystalline structure. In this way, the evolution and mutual movement of dislocations within the crystal causes slip zones in the metal.

The generation sequence of the slip zone is generally three parts: the appearance of the slip line, the formation of the slip zone, and the formation of the resident slip zone. Under the cyclic effect of fatigue load, dislocation movement first occurs on the weak grains inside the metal material, and this movement causes a trace on the metal surface, that is, a slip line. Under the effect of continuous cycle times, the slip line continuously accumulates and gradually forms a slip zone. When the slip zone is continuously squeezed into and out of the crystal interface by the cyclic load, the slip zone turns into a resident slip zone. Traces are left on the surface of the material by resident slip belts. When the traces are deep enough, an initial crack is formed. Therefore, resident slip bands are a key factor in crack formation.

After the resident slip zone was formed, it was gradually nucleated and controlled under high stress conditions. After nucleation, the resident resident slip zone is transformed into a permanent slump zone. Under the continuous action of fatigue alternating load, the resident resident slip zone is continuously expanding on the plane of maximum shear stress. Interconnected. As the crack area gradually expands, the micro-cracks converge and fuse together, eventually forming a main crack, and gradually expanding along the maximum shear stress surface.

As the crack grows, when the length of the main crack is larger than the critical crack size, the crack grows into an unstable fracture stage. At this stage, once the effective bearing capacity of the section is less than the cyclic load, fatigue instability occurs without any precursors. It can be seen that the life of the cracks at this stage is relatively complicated and varies in length. Therefore, the fatigue crack life is basically equal to the sum of the two stages of crack formation and propagation. ^[1]