What Is Fracture Toughness?

Fracture toughness is the resistance value that the material shows when there is a crack or crack-like defect in the sample or component that no longer breaks quickly with increasing load, which is the starting point. Such fracture toughness values can be expressed by a single parameter describing the mechanical state of the crack tip, such as energy release rate g, stress intensity factor K, crack tip opening displacement CTOD, and J integral. ^[1]

Fracture toughness characterizes a material's ability to prevent crack propagation and is

If the fracture toughness can be improved, the brittle fracture resistance of the material can be improved. It is therefore necessary to understand the factors controlling fracture toughness. There are external factors and internal factors that affect the fracture toughness.

external factors

External factors include the size of the section of the plate or component, temperature and strain rate under service conditions.

The fracture toughness of the material gradually decreases with the increase of the cross-sectional size of the plate or component, and finally tends to a stable minimum value, that is, the plane strain fracture toughness K _IC . This is a transformation process from a plane stress state to a plane strain state.

The relationship between fracture toughness and temperature is similar to that of impact toughness. As the temperature decreases, the fracture toughness can have a sharply reduced temperature range. Below this temperature range, the fracture toughness tends to a lower plateau with a very low value, and the temperature does not change even if the temperature is lowered.

Regarding the fracture toughness of the material at high temperature, Hahn and Rosenfied proposed the following empirical formula:

In the formula: nstrain hardening index of the material at high temperature; Eelastic modulus of the material at high temperature, MPa;

_s the yield stress of the material at high temperature, MPa;

_f true strain at break under unidirectional stretching at high temperature,

;

Section shrinkage when uniaxially stretched at high temperature.

The effect of strain rate is similar to the effect of temperature. The effects of increasing strain rate and decreasing temperature are consistent.

Internal factors

Internal factors include material composition and internal organization. As a combination of material composition and internal organizational factors, material strength is a macroscopic expression. From the point of view of mechanics rather than metallurgy, people always first discuss the level of fracture toughness from the strength change of the material. As long as the strength of the material is known, the fracture toughness of the material can be roughly estimated. The following figure shows the correlation between the fracture toughness of AISI 4340 (40CrNiMo) steel and the different yield strengths after quenching and tempering. It can be seen that the fracture toughness increases with the decrease of the yield strength of the material. This test result is representative, and most low alloy steels have this variation. This is true even for maraging steels (18Ni), but with higher fracture toughness at the same strength. ^[2]

In the measurement of fracture toughness, there are three stages. In the first stage, FPZ is gradually formed, and the value of the stress intensity factor K _I will increase monotonically. In the second stage, the cracks steadily expand; then in the third stage, they appear. The sudden decrease in the K _I value to the K _IC value. One possible explanation for this phenomenon is due to the inherent assumptions of numerical methods. An ideal linear elastic system is assumed in the finite element calibration, but as the experiment progresses, this assumption further loses its correctness. As the finite crack length increases, a large residual CMOD can be observed. This effect can be ignored at the beginning of the experiment, but in the later part of the experiment, the effect is quite large.

Generally, only the fracture toughness value at the second stage can be used for static analysis. Their averages are summarized in the table below. From the table, we can observe the following phenomena:

The relative deviations listed in the above table are all within the range of 20%, and the objective value of fracture toughness has nothing to do with the size of the test piece, and can be obtained;

Based on the relative error in the horizontal direction, it can also be found that the fracture toughness has nothing to do with the size of MSA;

These results were obtained in an unrestricted laboratory. The effect of limiting stress on fracture toughness can be seen in the research results of Saouma et al. (1990). ^[3]

Mechanical properties such as fracture toughness, crack growth rate, and crack growth threshold of metal materials have been understood by the majority of mechanical testing, material research, and metallographic experts, and have been used in the strength design of parts, the development of new materials, and materials. It has been widely used in application research, experimental research on material strength law, selection of heat treatment process and failure analysis. The purpose of this article is to provide more comprehensive and in-depth content for mechanics, materials and metallographic experts around the two aspects of "basic principles" and "engineering applications" of fracture toughness, for future experimental research and engineering applications. To play a greater benefit.

The plane strain fracture toughness K _{IC of} metallic materials is a new type of mechanical performance index refined after the formation of the discipline of fracture mechanics. The birth of early fracture mechanics was the result of research on preventing brittle failure. Therefore, we must first talk about the situation of brittle failure.

Brittle failure is one of the important ways of mechanical parts failure. It is a kind of damage that suddenly occurs during the loading process of the part without generating obvious macro plastic deformation. Since there are no obvious signs in advance, the danger of brittle destruction is high.

Traditional methods to prevent brittle failure of parts are:

The selected materials are required to have certain plasticity indexes and , and have a certain impact toughness _Ak value. This method of material selection is based entirely on the experience of the use of parts, it has neither sufficient theoretical basis, nor can it guarantee the safety of parts work. For example, in 1950, when the US Polaris missile solid fuel engine casing was experimentally launched, an explosion accident occurred, and the 1373MPa yield strength D6AC steel used was strictly tested: its plasticity and impact toughness indicators were fully qualified. For another example, the shaft of the 120T oxygen top-blown converter produced in China has also experienced a shaft break accident, and the strength, plasticity and impact toughness indicators of the 40Cr steel used have passed the inspection to meet the design requirements.

The method of transition temperature is used to put forward certain requirements for the transition temperature of materials. Because of a current punch test, only the two factors of stress concentration and increased strain rate have been considered, and the effect of temperature reduction on brittle failure of the material has not been considered. To this end, a series of impact tests were designed, that is, the impact tests were performed at a series of different temperatures to obtain the _Ak- T curve and the brittle fracture percentage-temperature T curve, thereby determining the brittle fracture transition temperature. FATT ₅₀ is commonly used. It is generally believed that as long as the actual working temperature of the component is greater than the brittle transition FAATT _{50 of the} material, brittle failure will not occur.

However, both of the above methods are empirical. They cannot find the conversion relationship between the transition temperature in the laboratory and the transition temperature of the actual component. Therefore, the design and selection of materials by this method are either very conservative or still produce brittle failure. A large number of brittle failure accidents in shafts, rotors, containers and pipes, and welded structures at home and abroad show that the traditional anti-breaking methods must be changed.

Experimental research shows that the occurrence of a large number of low-stress brittle failures is related to the existence of macroscopic defects in the parts. Some of these defects are generated during the production process, such as inclusions, porosity, porosity, white spots, folds, cracks, and incomplete penetration during smelting, casting, forging, heat treatment, and welding; and some are generated during use. Such as fatigue crack, stress corrosion crack and creep crack. All these macro defects are assumed (abstracted) as cracks in fracture mechanics, and stress concentration occurs at the crack tip when the component is subjected to an external load. If the plasticity of the material is good, it can sufficiently relax the concentrated stress at the crack tip, which may avoid brittle cracking. but. If for some reason: either the plasticity of the material is poor; or the size of the part is too large to constrain the deformation of the material; or the operating temperature is reduced to make the material work below the transition temperature; or the loading rate is increased, The plastic deformation of the material can not keep up and become brittle; or the embrittlement of the material caused by the effect of the corrosive medium or ray irradiation, etc., may cause brittle cracking at the crack tip, resulting in brittle failure of the part.

When a defect-bearing body is under stress, the internal defects, such as the near stress and strain field of the crack attachment, and their changing laws, the conditions for crack cracking, and the propagation law of the crack under alternating loads are studied to form a content. A new discipline

With the gradual deepening of the application of probabilistic fracture mechanics engineering, the problem of dispersion of fracture toughness of materials has become one of the key factors affecting the probabilistic safety assessment of structures with defects. Reasonably solving the fracture toughness dispersion of materials is a very complicated problem. On the one hand, due to deviations in metallurgical processes, etc., the dispersion of the fracture toughness of the material is caused; on the other hand, due to test errors such as sample geometry and crack length measurement, it will also lead to uncertainty in test results, as well as different test specifications and The processing of test data by standards also results in uncertainty in test results. If the defect is located at the weld, the influencing factors will be more complicated. In addition to the above reasons, factors such as welding process, welding materials, and different operators and post-weld heat treatments cause the dispersion of fracture toughness test results to be more serious. Although analyzing and solving such dispersion problems is so complicated and difficult, the safety assessment of welded structures with defects (especially industrial boilers, pressure vessels and piping) focuses on the welded joint area rather than the base metal. How to deal with the problem of dispersion of fracture toughness has become an unavoidable problem in the engineering community, and it is also one of the basic problems to be solved in probabilistic safety assessment.

The research on the fracture toughness dispersion law has made great progress both in theory and practice.

Hauge and Thualow used Weibull distribution, LogNormal distribution, Slather model, and Neville model to statistically analyze two sets of CTOD data (86 base metals and 16 welding materials). The main conclusions are as follows:

The two sets of CTOD data do not obey the Weibull distribution (or Slather model) with a shape parameter of 2; the two-parameter Weibull distribution, LogNormal distribution, and Neville distribution are all suitable for fitting these data.

The median expected value of the 90% confidence limit can be better obtained from the LogNormal distribution; for only three subsamples, it is better equivalent to the method of taking the minimum value of the three values; for large subsamples, LogNormal agrees better .

For the small sample, the LogNormal distribution provides the most reliable estimation. The Weibull distribution and the Neville model are difficult to estimate the distribution parameter values because the data is not enough when the samples are 3 and 5.

Both the numerical simulation results and the fitting results show that the LogNormal distribution has sufficient fitting accuracy, not particularly conservative, for the prince-like or smaller samples.

Mimura et al. Analyzed and experimentally studied the dispersibility of fracture toughness caused by non-uniform materials. Based on the analysis of CharpyV test blocks sampled from the same board, a method to distinguish the dispersion caused by the non-uniformity of the material and the dispersion caused by the test is proposed.