H. W. Liu

A dislocation barrier model for fatigue limit : as determined by crack non-initiation and crack non-propagation

Fatigue damage is caused by cyclic slip, and cyclic slip is driven by dislocation glide force. In order to cause fatigue damage, the cyclic glide force has to overcome the resistance of the primary and secondary dislocation barriers. Based on this cyclic damage process, the following diverse fatigue phenomena are synthesized into an integral and self-consistent analysis: Fatigue damage occurs in persistent slip bands (Hempel, 1956; Smith, 1957; Forsyth, 1957, 1961, 1963), and a nucleated fatigue micro crack is a shear crack (Forsyth, 1961). Pre-cracking fatigue damage is confined to the surface layer of a stressed metal (Wood et al., 1963). Fatigue limit is inversely related to grain size as shown in brass (Sinclair and Craig, 1952), in mild steels (Klesnil, Holzmann, Lukáš and P. Ryš 1965; Yoshikawa and Sugeno, 1965; and Taira, Tanaka, and Hoshina, 1979), and in ferritic-martensitic steel, (Kunio, Shimizu, and Yamada, 1969). Forrest and Tate (1964) found fatigue cracks in fine-grained brass at an alternating stress even below the fatigue limit. The cracks were within the boundaries of single grains. But they found no cracks in coarse-grained brass below the fatigue limit. The analysis synthesizes all of these experimental observations. The analysis is based on a realistic physical model. With a better understanding of the model and an improved calculation of the glide force, quantitative evaluations of the resistance of the dislocation barriers would eventually be possible. The needs for additional research are pointed out. A number of means of improving fatigue strength, based on the analysis, are suggested or explained.

Fracture toughness of thin and tough plates

Abstract : Wells has proposed using crack tip opening displacement as a measure of the fracture toughness of cracked plates in general yielding. The concept is applicable to thin plates loaded to extensive plastic deformation. The method is used to determine the fracture toughnesses of HY-80 steel plates. (Author)

A dislocation barrier model for fatigue crack growth threshold

The primary mechanism of fatigue crack growth is crack-tip dislocation emission followed by the glide of the emitted dislocations. Both of these two processes are controlled by the crack-tip resolved shear stress field, which is characterized by the resolved shear stress intensity factor, \(K_{R\tau } \). A dislocation barrier model for fatigue crack growth threshold is constructed. The model assumes that a fatigue crack stops growing when crack-tip slip bands are incapable of penetrating the primary dislocation barrier. The derived and deduced threshold behaviors agree with the observed constant threshold Kmax,th in the low R region and constant threshold ΔKth in the high R region. Kmax,th is the Kmax at the threshold. The constant Kmax,th is related to the resistance of the primary dislocation barrier, which in most of cases is grain boundary; and the constant ΔKth is related to the resistance of secondary barriers. Furthermore, the analysis shows that Kmax,th is proportional to √d, where d is the grain size. The relation has been observed in steels. The model also helps to explain the characteristics of, and the transition from, microstructure-sensitive to microstructure-insensitive growth.

Catalytic dissociation, hydrogen embrittlement, and stress corrosion cracking

Fatigue cracked 4340 steel specimens were austenized and oil quenched to hardness 42 R . All specimens are WOL type [9]; thickness is 1/4 in, and overall c~ck length is 2.03 ± 0.02 in. All the specimens are of the same geometry and were heat treated identically. The fracture load of the specimen in air was 7820 lb. This value provides a reference for experiments subsequently performed in a hydrogen atmosphere.

An Experimental and FEM Study on Crack Opening Displacement

Publisher Summary Crack opening displacements (COD) are calculated with the finite element method (FEM) under the conditions of plane strain and small scale yielding. In the case of small scale yielding, a small plastic zone is embedded in the elastic crack tip stress field. This chapter presents an experimental and FEM study on COD. CODs between the upper and lower crack surfaces under static tensile load were measured with the moire method. The moire measurements fell into the range where the FEM calculations and the elastic results coincide.