Papers by Keyword: Crack

Abstract: In this study, in order to evaluate the progress of internal cracks in PPS thrust bearings under rolling contact fatigue in water, cracks were observed by a full-cross-section observation method using a lathe machining. “Main subsurface crack” initiated at the surface toward the inside, then grew in a direction parallel to the surface. They connected with many “Semi-circular cracks” initiating at the surface from the opposite side to the inside, to from a semi-ellipsoidal flaking damage. It was found that the “Semi-circular cracks” and the “Main subsurface crack” dominated the flaking destruction.

Abstract: It is possible to explain the phenomenon of fatigue destruction and the patterns that are observed only in the deep study of the processes taking place in the material under conditions of repeated-variable loading, i.e. in the development of the physical theory of metal fatigue. Despite the large number of work on this issue, there is currently no single interpretation of the process of fatigue destruction of metals, which is primarily due to the exceptional complexity of the problem. The purpose of the study is to study low-cycle corrosion fatigue of steel using the example of A 414 Grade A steel. The work solved problems, such as research of kinetics of crack development in conditions of low-cycle loading of metal structure made of carbon structural steel A 414 Grade A. Regression analysis has also been applied to predict a change in the thermodynamic stability of the metal during cyclic loading. Analysis of fatigue crack development at alternating loading cycle was carried out. The results of calculations based on the proposed model of elastoplastic deformation near the top of the crack at the sign-alternating loading cycle can be described by non-linear dependence. The regression analysis revealed that the correlation coefficient of the selected model is-0.93, which indicates a relatively strong relationship between the variables. In experimental way it has been proved that reduction of thermodynamic stability of metal in corrosive medium is connected with increase of number of loading cycles, which leads to accumulation of fatigue damages.

Abstract: This paper presents an experimental investigation with the objective to determine the root causes for the cracking of a large size bar made of a medium carbon low alloy steel after open die forging and heat treatments operations. The cracks were observed below the surface during the machining step. In order to understand the mechanisms of crack initiation and propagation, micro-CT tomography and scanning electron microscope (SEM) were employed. Microstructural damage analysis revealed oxidation of different alloying elements, more specifically manganese, chromium and silicon. The presence of defects in the form of cavities and porosities were also observed at the grain boundaries. Some of the above defects were observed along the crack path, while others were on both sides of the cracks without any connection to them and finally, a third group completely isolated from any crack. The characteristics of the defects were thoroughly analyzed and it was found that the crack initiation could be attributed principally to the porosities/cavities formed during solidification. The analysis also showed that crack propagation occurred during solidification and/or forging and heat treatment steps.

Abstract: In this study, crack propagation in PPS thrust bearings under rolling contact fatigue (RCF) in water was observed in order to investigate the relation between cracks and flakings. RCF tests in water under loads of 700 N and 900 N were performed. The semi-circular cracks propagated in a direction perpendicular to the rolling direction were observed under a load of 700 N. The line cracks propagated in a direction parallel to the rolling direction at periphery of contact area and the semi-circular cracks were observed under a load of 900 N. To study the subsurface cracks, full section of rolling contact area was observed. It is concluded that the flaking mechanism in PPS thrust bearing has three features as follows: Initiation and propagation of surface cracks depend on the load; When load is 700N, the semi-circular cracks growing from the surface and the cracks branching from the main subsurface cracks join to form the flaking; When load is 900N the line cracks and the semi-circular cracks growing from the surface join to form the flaking.

Abstract: Problems lived with fatigue fracture for the safe design of the members and structures. In this study, rotating bending tests were performed to investigate the fatigue crack propagation behavior of induction quenched and tempered JIS S45C low carbon steel. Hardness distribution was checked by the Vickers hardness test machine and the microstructure in cross section and fracture surface were observed with an optical microscope and a scanning electron microscope. The depth of the hardened boundary was approximately 1 mm from the surface and formation into martensite occurred at the surface of the specimen. It was ascertained that fracture surface of notched specimens consisted of the five fracture types. In addition, the maximum stress intensity factor of fatigue cracks increased during rotating bending test on notched specimen. The relation between SIF and the fracture surface is discussed.

Abstract: In order to partially improve the wear resistance for dies and jigs, layer-forming technologies for adding cemented tungsten carbide is effective way as an additive manufacturing technique. As one of the layer-forming technologies, directed energy deposition (DED) has attracted much attention from industry. However, many cracks are usually observed inside a formed layer processed by the DED due to the difference of material properties between an iron-based substrate and a cladded cemented tungsten carbide layer. In terms of durability of the formed layer, the crack formation is not preferable as well. In this study, as an attempt to suppress the crack initiation and propagation in cladded layers processed by the DED, formation of compositionally graded cemented tungsten carbide layer was performed by inserting a layer with low to medium tungsten carbide content between the substrate and cemented tungsten carbide layer. At first, single layers of cemented tungsten carbide having various tungsten carbide content were formed on iron-based substrates by the DED processing to understand the relationships between the tungsten carbide content and the number of formed cracks. By considering these experimental results, the compositionally graded cemented tungsten carbide layers were optimized. It was revealed that the crack initiation and propagation could be suppressed by compositionally graded cemented tungsten carbide layers.

Abstract: On the thermomechanical treatments of Cu-Ni-Si alloy, cold-rolling (CR) before solution heat treatment (SHT) is commonly conducted to eliminate defects in a casting slab. In addition, a rolling is applied to reduce/adjust the thickness of casting slab before SHT. In a heavily deformed microstructure by CR, on the other hand, grain growth during a heating in SHT is likely to occur as the result of recrystallization. In general, tensile strength and fatigue strength tend to decrease with an increase in the grain size. However, the effect of difference in grain sizes produced by with and without CR before SHT on the fatigue strength is unclear. In the present study, fatigue tests of Cu-6Ni-Si alloy smooth specimens with a grain fabricated through different thermomechanical processes were conducted. The fatigue behavior of Cu-Ni-Si alloy was discussed.

Abstract: The historical building which can reflect the local history and culture of a city is the wealth of all mankind, but after the baptism of time, the historical building has been damaged by nature or man. In order to preserve the historical buildings and the culture contained therein, the damaged historical buildings need to be reinforced and restored. In this study, ABAQUS finite element software was used to simulate and analyze the reinforcement and repair effects of four kinds of damage states (mild damage, moderate damage, severe damage, and nearly collapse) of historical building walls caused by reinforced mesh cement. The results showed that in the case of mild damage, moderate damage and severe damage, the ultimate load displacement of the wall was raised in different degrees (the ultimate load and displacement were 200 kN and 4 mm respectively in case of mild damage and were 400 kN and 8 mm respectively after reinforcement; the ultimate load and displacement were 210 kN and 4 mm respectively in case of moderate damage and were 400 kN and 6 mm respectively after reinforcement; the ultimate load and displacement were 150 kN and 4 mm respectively in case of severe damage and were 375 kN and 7 mm respectively after reinforcement) and restored or even exceeded the performance of the wall without damage by using the reinforced mesh cement layer. In the nearly collapse wall, the ultimate load and displacement of the wall could be improved, but the performance of the wall could not be recovered as it was before being damaged (40 kN and 3 mm before reinforcement, and 165 kN and 6 mm after reinforcement). Reinforced reticulated concrete can be used to reinforce and repair historic buildings with mild, moderate and severe wall cracking, but it is not suitable for the reinforcement and restoration of cracked historical buildings which are nearly collapsing.

Abstract: This work consists of the study is to analysis by the finite element method the effect of the ageing of the adhesive exposed simultaneously to the temperature and water on the degradation of its mechanical properties and consequently on the transfer of loads from the plate to patch. The stress intensity factor was evaluated according to immersion time and temperature. Several parameters have been taken into consideration, namely the properties of the composite patch, temperature, water absorption, rate and length of crack, the distribution of maximum shear stresses in the adhesive and peel in the patch were also analyzed.

Abstract: The sustainable development of industry and society requires new approaches to the building structures design. The article presents the indices of strength, crack resistance and width of crack opening obtained as a result of experimental testing of beams with hybrid reinforcement with basalt plastic and metal armature. The following beams were examined for comparison purposes: the ferroconcrete beams of the control-series, and the twin beams reinforced only with basalt-plastic reinforcement. It was found that the replacement of the metal armature with basalt plastics led to an increase in strength, on average, by 40%. Similar strength indices were obtained for hybrid reinforcement beams. Crack resistance indices of hybrid reinforcement beams were found to be close to ferroconcrete beams of the control series. Crack resistance indices for these beams were also by 84... 89% higher in comparison with beams reinforced with basalt-plastics. The width of crack openings in hybrid reinforced beams did not exceed the maximum permissible norms at the operational level of loads (70% of destructive) and were smaller than in beams reinforced with basalt plastic reinforcement. Hybrid reinforcement efficiency has been established to improve the performance criteria of beams reinforced with composite armature.