Crack Initiation Failure Research Articles

The high cycle fatigue behavior, failure characteristics, and fatigue life empirical relationship of 9% Cr steel under different stress ratios at 630 °C

During the startup and shutdown processes of the steam turbine rotor, the fatigue failure behavior caused by asymmetric stress is introduced. To study the effect of mean stress on the fatigue failure behavior of 9% Cr steel at high temperatures, the high cyclic fatigue tests are conducted under different stress ratios (R = −1, 0.1, 0.5, and 0.7) at 630 °C. The results indicate three fatigue failure modes: surface crack initiation failure, sub-surface crack initiation failure, and necking induced failure. With the increasing stress ratio, the surface and sub-surface crack initiation failure modes gradually change to necking induced failure mode. Moreover, the oxide intrusive-extrusion mechanism primarily determines the surface crack initiation failure and promotes surface defect formation. Meanwhile, the inhomogeneous microstructure induces sub-surface crack initiation failure, wherein the crack propagates along the martensitic lath boundary or through the martensitic lath. The attenuation of mechanical properties and the increase in mean stress level are the critical factors affecting the necking induced failure under 630 °C. In addition, the mean stress sensitivity factor (MR) is proposed to explain the effect of the stress ratio on fatigue performance. Furthermore, the mean stress sensitivity factor (MR) can be used to quantitatively analyze the effect of mean stress on fatigue properties under different stress ratios. Finally, the fatigue life empirical relationship considering the mean stress sensitivity factor (MR) is constituted.

Investigations on the macro-meso cumulative damage mechanism of the discontinuities in soft-hard interbedded rock mass under pre-peak cyclic shear loading

To investigate the macro-meso cumulative damage mechanism of the discontinuities in soft-hard interbedded rock mass, laboratory pre-peak cyclic shear tests and PFC 2D meso numerical calculations under constant normal load (CNL) were carried out, considering the effects of moisture content, normal stress, shear rate, shear amplitude, first-order asperity angle and number of cyclic shear. The curve of the shear stress with shear displacement can be divided into six stages, i.e., initial nonlinear compression-shear deformation, compression-shear deformation of approximate linear elasticity , cyclic shear cumulative damage deformation , compression-shear deformation with slowly rising stress, compression-shear deformation with sharply rising stress and compression-shear deformation of brittle stress drop. The peak (residual) shear strength and cumulative shear (normal) displacement decrease and increase, respectively, with number of cyclic shear increasing under the same moisture content, normal stress, shear rate, shear amplitude or first-order asperity angle; under the same number of cyclic shear, these increase and decrease, respectively, with normal stress or first-order asperity angle increasing, while these decrease and increase, respectively, with moisture content, shear rate or shear amplitude increasing. The curve of the meso cumulative damage crack number (energy) with shear displacement shows the characteristics of a slight increase-steep increase, a slow increase and a steep increase-slow increase-steep increase (crack number) or steep increase-slow increase-steep decrease (energy) in the early, middle and later stages, respectively, while the curve of that with number of cyclic shear shows the characteristics of a steep increase and slow increase in the early and later stages, respectively. The macro experimental observations and meso numerical results are in good agreement, and the typical failure modes of the rock discontinuities were summarized as compaction – crack initiation failure, cyclic dislocation – penetration failure and separation – gnawing failure; meanwhile, the macro-meso cumulative damage cracks are approximately an “inverted U-shaped” and densely distributed near the rock discontinuities.

Strength Characteristics and Failure Mechanism of Granite with Cross Cracks at Different Angles Based on DIC Method

The engineering rock mass is generally composed of the rock matrix and structural plane and is an anisotropic inhomogeneous geological body. Accidents such as roof collapse and well caving caused by joint and fissure expansion occur frequently during tunnel excavation and service, resulting in serious casualties and economic losses. It is of great theoretical significance and engineering value to study the fracture mechanism of the jointed rock mass to ensure the stability of the surrounding rock and the safe and efficient utilization of the urban underground space. To investigate the effects of crossed cracks on mechanical properties and failure characteristics of rock, wire cutting equipment is employed to make rock samples with different crossed cracks, and then acoustic emission system and digital image correlation technique are used to study the fracture process of rock samples under uniaxial compression. It has been found that the strength of rock samples with a single crack is generally larger than that of samples with cross cracks, and the strength changed with the angle of the crack in a “V” shape. When the angle of preexisting crack is 60°, the rock strength reaches the lowest. The primary crack has a more obvious influence on rock strength and is the main controlling factor of rock fracture. The initiation stress of rock samples with a single crack changes more significantly with angle. When the angle of the primary crack is 45°, the rock sample is most prone to crack initiation failure, and the crack initiation stress is only 1/4 to 1/2 of the strength. There are two types of cracks: wing and anti-wing, and the tensile cracks are the main ones. It is revealed that the fracture of cracked rock has significant directional characteristics. For the samples with cross cracks, the primary crack is the main control factor of crack initiation, and the secondary crack has a certain guiding effect on the crack.

Analysis on the Fatigue Properties of Shot-Peened Al-Si-Mg Alloy and Its Fatigue Life Prediction

Ceramic micro-shot peening (CMSP) and steel micro-shot peening (SMSP) were utilized to investigate the effect of micro-shot peening (MSP) on the high-cycle fatigue properties of Al-7Si-0.3Mg casting aluminum alloy in a previous study. However, the improvement effects of CMSP and SMSP on the fatigue strength (at 5 × 107 cycles) were only 33% because the depth of harden layers was only 20 and 55 μm while the depth of compressive residual stress affected layers was only 37 and 68 μm. In this study, conventional shot peening (CSP) was utilized, and the results were compared with those of MSP, with the expectation that CSP would provide a greater improvement in the fatigue strength. The affected surface layers of the shot-peened specimens were characterized using surface morphology, microhardness, and residual stress analyses. In addition, the effect of CSP on the fatigue strength at 5 × 107 cycles was investigated using a rotating bending fatigue test (R = − 1). An investigation of the extensive surface compressive residual stress relaxation process for the three different shot-peened specimens during cyclic loading was conducted using x-ray diffraction. In addition, the initiation sites for fatigue cracks on the fracture surface were observed using scanning electron microscopy. Furthermore, the fatigue life of the samples with the internal casting defect failure mode was predicted using linear elastic fracture mechanics, while that for samples with the surface crack initiation failure mode was predicted using the modified Morrow model considering the residual stress.

The analysis of failure in concrete and reinforced concrete beams with different reinforcement ratio

In this paper the analysis of failure and crack development in beams made of concrete is presented. The analysis was carried out on the basis of the performed experimental investigation and numerical simulations. A fictitious crack model based on nonlinear fracture mechanics was applied to investigate the development of strain softening of tensile concrete in plain concrete and slightly reinforced concrete beams. The role of strain softening was also discussed according to the inclined crack propagation in highly reinforced concrete beams. The analysis has brought the evidence that the mode of failure in flexural beams varies according to a longitudinal reinforcement ratio. A brittle failure due to the formation of a flexural crack takes place in plain and slightly reinforced concrete beams, and strain softening of tensile concrete is of paramount importance at failure crack initiation and propagation. A stable growth of numerous flexural cracks is possible in moderately reinforced concrete beams, and then the load carrying capacity is connected with reaching the yield stress of reinforcing steel or concrete crushing in the compression zone. In higher reinforced concrete beams without transverse reinforcement, brittle failure can take place due to shear forces and the development of diagonal cracks. However, strain softening of tensile concrete is not the only mechanism influencing the propagation of an inclined crack. Such mechanisms as aggregate interlock and dowel action of steel bars contribute more importantly to the development of failure crack.

Fatigue resistance of a hot corrosion exposed disk superalloy at varied test temperatures

The fatigue resistance of the hot corrosion pitted ME3 disk superalloy was investigated. Low cycle fatigue specimens were subjected to hot corrosion exposures that produced pits on the gage sections. These specimens were tested at varied temperatures and strain ranges. Corrosion pitting influenced fatigue life and failure mode by varying degrees, depending on temperature and strain range. As observed through interrupted tests, fatigue cracks initiated at a smaller fraction of life for high-temperature tests, in comparison to that at low temperatures. Correspondingly, the crack initiation failure mode changed significantly with test temperature. While cracks initiated from the hot corrosion pits for all test conditions, at 704 °C the intergranular initiation failure mode was dominant, whereas at the lower temperatures cracks initiated within the pits from crystallographic facets. Finite element analyses were performed to quantify the effect of varying pit dimensions and spacing on elastic stress concentration. The highest stress concentration was calculated to occur at the narrow ligaments between overlapping hot corrosion pits. Increasing the number of overlapping pits did not further add to the stress concentration. There was good qualitative agreement between the calculated stress concentrations and the location of crack initiations for tests conducted at 704 °C but not for tests conducted at 204 °C.

Vibration-Based Bending Fatigue of a Hybrid Insert-Plate System

A hybrid bending specimen system was developed to reduce material waste when using the vibration-based fatigue method to gather high cycle fatigue (HCF) data. In the vibration-based fatigue method, a base-excited plate specimen is driven into a high frequency resonant mode until failure. Compared to the axial testing methods, vibration-based failure better represents the conditions and surface-initiated cracks experienced by structural components subject to high frequency vibrations, like blades and vanes in gas turbine engines. In addition, the method produces data over 40 times faster than conventional axial testing methods. Only a small portion of the vibration-based bending specimen is needed to determine crack initiation failure. In order to reduce material waste, a hybrid bending system was designed to test a small specimen held in a reusable carrier plate, thereby reducing material costs to gather HCF data. The final optimized design, the Bruns-Zearley plate specimen, produced fatigue data using 95 % less material. The results are comparable tofatigue data acquired from standard vibration-based plate specimen methods within a 95 % confidence interval.

Material characterizations of mild steels, stainless steels, and both steel mixed joints under resistance spot welding (2-mm sheets)

Resistance spot welding is an essential welding mechanism for joining two or more metal sheets together in automotive industries. The mechanical assemblies are easily joined at discrete spots using high current flow through the area of concentration. It is easily achieved by compressing the base metals together to a certain pressure using copper electrode caps and allow the high current to flow through the concerned areas. The heat generation due to the current flow causes the metal sheets to be fused and consequently form bonds between base metals. The molten areas of base metals undergo solidification process by itself when the current flow is utterly stopped. Basically, the weld growth in any joint is determined by its process-controlling parameters, particularly the welding current, weld time, electrode force, and electrode tip. In these experiments, the welding current and weld time variations were carried out to characterize the weld growth for three types of joints mainly for stainless steel, mild steel, and both steel mixed joint. A pneumatically driven 75-kVA spot welder was used to accomplish the entire welding processes, and the welded samples were later subjected to tensile, hardness, and metallurgical tests to relate the diameter growth, loading force during tensile test, failure crack initiation, post-failure crack propagation direction, macro and microstructural changes, and also the hardness changes due to solidification process. Assessing the experimental results of 2-mm thickness of materials revealed that the welded areas’ characteristics have been varied from its original states to enriched states, in terms of shearing strength and hardness distribution as well as the microstructural orientation.