Hsing-Lu Huang

A study of dislocation evolution in polycrystalline copper during low cycle fatigue at low strain amplitudes

Abstract The dislocation structures evolution on the polycrystalline copper at constant strain amplitude during low cycle fatigue is well understood. However, the dislocation structures developments at variable strains, which change from high to low strain amplitude, are seldom reported. In order to realize the dislocation morphology evolution at reduced strain amplitude, the polycrystalline copper is used in this study. The results show that. (1) The S–N curve at reduced strain amplitude from high to low reveals softening, and it is lower than S–N curve which is fatigue in constant strain amplitude at low strain amplitude. (2) The dislocation structures of walls, labyrinth walls, cells or misorientation cells transfer into scattering walls, loop patches or cells. (3) The dislocation morphology development at decreased strain amplitude is determined by the magnitude of reduction in strain amplitude.

The study of fatigue in polycrystalline copper under various strain amplitude at stage I: crack initiation and propagation

Abstract During the past two decades, several studies on the crack initiation have shown that the crack initiation at high plastic strain amplitude is mainly started at the grain boundaries(GB), and at low plastic strain amplitude crack initiation is mainly started at the persistent slip band(PSB). However, the GB is still considered to be the preferred locations for crack initiation. It has also been pointed out that crack initiation started at GB results from the interactions between grain boundaries and persistent slip bands (PSB–GB), but only few studies has been devoted to finding the starting places for the following crack propagation, which may start at either the PSB or the GB. Moreover, most of the observations on the PSB–GB are done by scanning electron microscopy (SEM) — yet, observation by transmission electron microscopy (TEM) is seldom reported. In this paper, pure copper square specimens are employed in low cycle fatigue testing under strain-controlled condition. Optical microscopy (OM), SEM and TEM are used to investigate the initiation, propagation and the accompanied dislocation structures of the cracks. The results reveal that the mechanisms of crack initiation are the same as those in earlier studies. The dislocation structures, for crack initiated at the PSB, are ladder-like dislocation wall; and for those initiated at the GB, are dislocation cell. With regard to the crack propagation, crack initiated at the GB is the majority — no matter the crack was initiated at the PSB or the GB at the beginning.

The model of crack propagation in polycrystalline copper at various propagating rates

Abstract In this work, the fracture surface of the crack paths in specimens of pure copper have been studied by secondary electron imaging and the dislocation structures in front of crack tip were observed by backscattered electron imaging. It was observed that the propagation of the cracks is mainly transgranular, but the behavior models change in accordance with the variation of dislocation structures located at the front of the crack tip. With a 10 −6 mm/cycle rate of cracking, the dislocation structures are in full development with a vast range of area. Therefore, a model for the propagation of cracks is transgranular along the sides of grain boundaries (GB). However, with a propagation rate of 10 −7 mm/cycle, the dislocation structures evolve incompletely, and the range of cell or ladder-like walls of persistent slip band is limited to a small area. The sequence of crack propagation is initiation, propagation in strain localization in front of the crack tip, and then coalescence with the crack tip. The crack propagation is transgranular, but the side of the GB is still the preferred path. A model is proposed to explain these behaviors. The major factors of the model are the evolution of the dislocation structures at the crack tips, and the interactions between the grain boundaries and persistent slip bands.

The observation and analysis of the dislocation morphology of fatigue crack tips at steady state propagation rates subject to a single peak load

Abstract During fatigue crack propagation there is large interaction effects of the fatigue cycle for different loading amplitudes. Two examples are the retarding effect of overload and the accelerating effect of underload on the crack propagation. In the former, the result is explained in terms of residual stress effects associated with the overload, and in the latter, the underload partially annihilates the residual stress built up by the positive load. However, at the microstructure level of material under fatigue, the crack propagation is caused by dislocation action. Assuming this, this paper reports studies of the crack propagation interaction effect by using microstructural examination of crack tips. Observations were made with the Back Electron Images (BEI) of the Scanning Electron Microscope (SEM). The results are: (1) the microstructure of cells close to the crack tips formed into vein or loop patch structures upon the overload, so that the crack propagation was reduced. (2) The region of cells s close to the crack tips become enlarged on the underload and the scale of the other microstructure (such as PSB, vein and loop patch) were also enhanced too, so that the crack propagation was accelerated.

The observation of dislocation reversal in front of crack tips of polycrystalline copper after reducing maximum load

The loading at a crack tip varies during fatigue crack propagation. As a result, overloading causes retardation of crack propagation, and underloading causes the acceleration of crack propagation. In addition, reducing the load range by changing either the minimum load or maximum load can cause a reduction or retardation of crack propagation to occur. Examining a fatigue cracked specimen made of polycrystalline copper with back scattered electron images (BEI) in a scanning electron microscope (SEM) revealed that (1) the dislocation structures close to the crack tips gradually evolved from a cell structure into a new loop patch structure during the crack retardation period which follows after reducing the maximum load; (2) restoring the crack propagation rate is a result of re-establishing the cell structure from new loop patches or PSBs; and (3) the evolution of the dislocation structure at the crack tip due to the maximum loading reduction is affected by residual active slip systems.

The study of dislocation structures at fatigue crack tips in polycrystalline copper under various crack propagation rates at stage II : crack propagation

Abstract The purpose of this study is to investigate the dislocation structures in front of fatigue crack tips embedded at various propagation rates in copper. Back electron images with the scanning electron microscope were used for the observation. The results are, at a rate of 10−6 mm per cycle, that the dislocation morphologies are distinguishable and can completely to evolve into various dislocation structures. At a rate of 10−7 mm per cycle, the dislocation morphologies are the same as those of the 10−6 mm per cycle rate, but the ranges of the dislocation structures are too small to be distinguished. Regardless of the plastic strain amplitudes, the average diameter of the dislocation cell in front of the crack tip is about 0.7 μm. Therefore, once the low-energy dislocation cell was formed, the crack initiates and/or propagates. At a 10−8 mm per cycle rate, the crack tip is propagated toward the region of true strain localization. The dislocation structure threshold for propagating the crack tip is cell with an average diameter of less than 0.7 μm.

The microstructure of the fatigue crack tip in Fe–Al–Mn–0.4% C alloy near the stress intensity threshold

Abstract This work is an investigation of the dislocation structures close to the crack tip near the stress intensity threshold, and the dislocation morphology of surface crack initiation. In these experiments, a material (Fe–Al–Mn–0.4% C alloy) with the planar dislocation structure at early stages in fatigue and wavy type at the later step in fatigue was used. The crack propagation was produced by the Instron hydraulic testing machine to obtain the near threshold of the crack propagation. The microstructure was observed by the JEOL 200CX transmission electron microscope. The results are (1) the dislocation structure in front of the crack tip at the stress intensity threshold is the cell with an average diameter about 0.5 μm and with a range of about 2–3 μm. Out of this range, the dislocation is similar to the dislocation of a planar type. (2) The surface crack initiates when the multiple slip system is created and the accomplishment of dislocation is triggering multiple ladder-like wall in persistent slipband (wavy dislocation) embedded in a matrix–planar dislocation (persistent slip Luder band).

Dislocation evolution in interstitial-free steel during fatigue near the endurance limit

In order to clear the relationship between dislocation development and endurance limit in fatigued body-centered cubic (BCC) metals, the automotive grade interstitial-free steel (IF steel) was fatigued near the endurance limit in this study. When cycling just below the endurance limit, the dislocation structures are mainly composed of loop patches, moreover, a few large dislocation cells and dislocation walls can also be found, and thus these structures have no significant effect on fatigue failure. However, once cyclic strain slightly exceeds the endurance limit, the small dislocation cells tend to develop near grain boundaries and triple junction of the grains, and which provide a more appropriate structure for crack growth than do large dislocation cells.

The observation of dislocation morphology in front of fatigue crack tips of as-rolled 2205 duplex stainless steel at various propagation rates

Abstract The purpose of this work is to investigate the dislocation structure in front of crack tips of a duplex (ferrite–austenite) steel at various crack propagating rates, and to investigate the influence of the dislocation structure on the threshold stress intensity. The experimental results show that the result in difference of threshold stress intensity factors for as-rolled 2205 duplex (ferrite–austenite) stainless steel along different crack directions is very slight, which is because the influence of grain boundary is greater than that of grain orientation for small grains. In front of crack tips, no dislocation cells were observed. Instead, persistent Luder bands embedded in a highly dense dislocation matrix were found, and the dislocation density increased with the increase of the crack propagation rates. Nucleation of fine CrN/CrC particles from many small amorphous domains, which evolved from a very high-density dislocation in the matrix, were also observed along the crack path close to the crack surface.