Extension Of Surface Cracks Research Articles

Dynamic evolution of crack fractal of polypropylene fiber reinforced geopolymer during flexural process

Crack mouth opening displacement (CMOD) is the most widely used parameter to evaluate the crack width changing process in concrete fracture research. However, it is difficult to accurately and comprehensively describe the multi-crack cracking characteristics of high-toughness concrete with only crack width. This paper investigates the dynamic evolution of the fractal dimension (FD) of surface cracks on polypropylene fiber-reinforced geopolymer (PFRG) during flexural process. The box counting method was used to calculate the FD of cracks in the segmentation images that were generated by deep learning. Crack FD at the peak load and the fracture failure load could be used as indicators of deflection hardening behavior, with threshold values of 0.83 and 0.86, respectively. Crack development can be divided into four stages based on load, FD, CMOD, and energy capacity with time: elastic segment, internal crack development segment, surface crack extension segment, and crack destabilization segment. FD shows an obvious exponential increase in the surface crack extension segment and crack destabilization segment, and the changing trend is basically parallel with the load in the surface crack extension segment. The FD of peak load and fracture failure load showed a trend of increasing and then decreasing as the fiber factor was increased. The FD value peaks at 585 when W/B = 0.35 and at 252 when W/B = 0.38. To achieve the best stress diffusion and crack propagation process and achieve the toughening objective, it is necessary to reasonably match the fiber factor and matrix strength.

Influence of partial cement substitution by ground blast furnace slag on the mechanical properties of phosphogypsum cemented backfill.

Phosphogypsum (PG) stockpiles occupied a large amount of land resources, and serious environmental pollution problems have attracted the attention of countries around the world. Cemented backfill can reduce the environmental problems caused by tailings stockpiles and is an important development trend in green mine construction. To investigate the effect of binder type on the performance of PG cemented backfill, this paper used ground granulated blast furnace slag (GGBFS) to substitute part of Portland cement (PC) as binder and studied the effect of different ratios of binder on the uniaxial compressive strength (UCS), surface crack extension, acoustic emission (AE) characteristics, and microstructure of PG cemented backfill. The results show that substituting part of PC with GGBFS is beneficial to improve the mechanical properties of PG cemented backfill. When PC was substituted by 50% of GGBFS, the 28d UCS of the backfill was increased from 1.535 to 4.539 MPa. Furthermore, the UCS of the backfill gradually increased as the GGBFS substitution level increased, and more AE signals could be monitored during uniaxial compression. Compared with PC, the sulfate in PG participates in the hydration reaction of GGBFS, more hydrated calcium-aluminum-silicate-hydrate (C-A-S-H) gels and ettringite (AFt) are formed, and the microstructure of the backfill is denser, and the required strength can be obtained with less binder. Thus, substituting part PC with GGBFS as a binder can provide an economical and environmentally friendly alternative for the consumption and reuse of large quantities of PG.

Surface acoustic waves and Mach stem formation produced by lithotripter shock wave-stone interaction

Lithotripter shock wave (LSW)-stone interaction may vary significantly due to respiratory motion of the patient. In this work, we have investigated the variations of the LSW-stone interaction and associated stone fracture pattern using photoelastic imaging, phantom experiments, and three-dimensional fluid-solid interaction modeling at different lateral locations in a lithotripter field. In contrast to the T-shaped fracture pattern observed in a disk-shaped stone under symmetric loading, we have observed a gradual transition of the fracture pattern to a tilted L-shape under asymmetric loading. Importantly, our model simulations reveal the generation of surface acoustic waves (SAWs)—a leaky Rayleigh wave on the anterior and a Scholte wave on the posterior surface of the stone. The propagation of SAWs on the stone boundary is accompanied by a progressive transition of the LSW reflection pattern from regular to von Neumann and to weak von Neumann reflection near the glancing incidence and, concomitantly, the development of a Mach stem, swirling around the stone boundary. The maximum tensile stress and stress integral are produced by SAWs on the stone boundary under asymmetric loading, which drive the initiation and extension of surface cracks into the bulk of the stone that has been confirmed by microCT analysis.

Variations of stress field and stone fracture produced at different lateral locations in a shockwave lithotripter field

During clinical procedures, the lithotripter shock wave (LSW) that is incident on the stone and resultant stress field is often asymmetric due to the respiratory motion of the patient. The variations of the LSW-stone interaction and associated fracture pattern were investigated by photoelastic imaging, phantom experiments, and three-dimensional fluid-solid interaction modeling at different lateral locations in a lithotripter field. In contrast to a T-shaped fracture pattern often observed in the posterior region of the disk-shaped stone under symmetric loading, the fracture pattern gradually transitioned to a tilted L-shape under asymmetric loading conditions. Moreover, the model simulations revealed the generation of surface acoustic waves (SAWs), i.e., a leaky Rayleigh wave on the anterior boundary and Scholte wave on the posterior boundary of the stone. The propagation of SAWs on the stone boundary is accompanied by a progressive transition of the LSW reflection pattern from regular to von Neumann and to weak von Neumann reflection near the glancing incidence and, concomitantly, the development and growth of a Mach stem, swirling around the stone boundary. The maximum tensile stress and stress integral were produced by SAWs on the stone boundary under asymmetric loading conditions, which drove the initiation and extension of surface cracks into the bulk of the stone that is confirmed by micro-computed tomography analysis.

The Effects of Carburization on the Fatigue Crack Growth Behaviors of Local Surface Cracks in Cylindrical Bars

In this study, a series of fatigue tests were conducted on C45 steel specimens with local surface cracks. The effects of carburizing treatment with different case depths were discussed under cyclic tensile and torsional loading. Moreover, the surface crack extension paths and crack propagation rates were discussed. In addition, the effects of additional static torsion under cyclic tensile loading and additional static tension under cyclic torsional loading were analyzed, and comparisons were made between the results of the carburized materials and those of the uncarburized materials. Some conclusions show that carburization can effectively retard crack extension and improve fatigue life. However, the improvement provided by the case depth on the fatigue life is limited to a certain range, and extensive case depths might decrease the fatigue strengths of materials. For the studied material, carburization had a more direct influence on the crack growth under cyclic tensile loading than cyclic torsional loading.

A two-parameter criterion for predicting the fracture location along a surface crack

The fracture prediction for materials containing surface cracks is not well established. In this paper, a new two-parameter fracture criterion which can predict the location of maximum crack extension along a surface crack front was presented. The surface-cracked specimens with different crack sizes and loads were selected, and the finite element method (FEM) was used to model the J-integral and the equivalent plastic strain (εp) distributions along the crack front for different specimens. And then, in succession, the areas APEEQ surrounded by the εp isoline ahead of crack tips under different J-integrals and the unified constraint parameter Ap were calculated. Based on the parameter Ap, J-integral and the fracture toughness Jref measured in a standard test, the J/(JrefAp), which considered the combined effects of crack driving force and material resistance force, was determined. The critical fracture location is the location for which the J/(JrefAp) is a maximum. The results show that for all the specimens, the prediction results obtained by two-parameter criterion are consistent with the measured results obtained by experiments. The J/(JrefAp) is a suitable prediction criterion to model the location of the maximum crack extension of surface crack. For comparison, similar predictions were also made using a single-parameter J-integral and Ap.

Influence of loading path on fatigue crack growth under multiaxial loading condition

ABSTRACTIn this study, the specimens made of carbon steel S45 with an initial surface straight edge notch were subjected to combined cyclic axial‐torsion loading at room temperature. The fatigue life, surface crack extension direction and crack length were experimentally investigated. The effects of loading path, stress amplitude ratio and phase angle on the crack growth behaviour were also discussed. The results showed that, under the combination of cyclic axial and torsion loading, the tension stress amplitude had more effect on the initial crack growth path than the latter. The shear stress amplitude contributed mainly to the latter crack extension. The crack extension path was mainly determined by the stress amplitudes and the ratio of the normal stress to the shear stress, and almost independent of the mean stresses. The increase of the tension stress amplitude and shear stress amplitude would both accelerate the crack growth rate.

Measurements of strain and humidity within massive concrete cylinders related to the formation of ASR surface cracks

This study aims at proving the validity of a notion that the formation of non-expansive near-surface layer is responsible for surface cracking in ASR-affected concretes by a laboratory experiment. Relationship between the progress rate of the front of non-expansive layer toward inner portions and the formation of the first surface cracks was scrutinized by measuring relative humidity (R.H.) values and strains within a massive concrete cylinder (ϕ450 mm × 900 mm) with reactive aggregates under a dry environment. It was presumed from the measurements that a non-expansive layer of about 40 mm had been formed at the first cracking. Thereafter, the environmental humidity was raised to > 95% R.H. Pursuit of the growth of surface cracks and subsequent measurements of strains and R.H. values within the concrete cylinder under the moist environment suggested that the re-saturation continuously gave rise to the generation of tensile stresses in near-surface regions leading to active extension of surface cracks.

Fatigue crack growth behavior for 3D inclined surface crack

In the present study, the fatigue life and surface crack extension direction in a cylindrical elastic bar with an inclined surface crack were experimentally investigated. The effects of loading type, stress ratio and crack inclination angle on the crack growth behavior were also determined. Carbon steel S45 was chosen as the test material due to its wide range of industrial applications. A surface crack was introduced at the center of the specimen with an inclination angle β with respect to the axial direction. The results showed that, even under pure fatigue tension or cyclic torsion loading, the crack extension direction deviated from the initial crack direction. Under biaxial fatigue loading conditions, the crack growth direction was mainly determined by the ratio of the normal stress to the shear stress.

Study on failure mechanism of the coated carbide tool

During metal cutting process, cutting tools have to resist extreme heat, high pressure, abrasion, thermal shock and mechanical shock. Chemical stability and hardness of chemical vapor deposition (CVD) coated carbide tools are much better than that of the uncoated carbide substrate. However, the fracture toughness of CVD coated carbide tools is not as good as that of the uncoated carbide substrate. This paper analyses the failure mechanism of CVD coated carbide tools. Experimental results show that the failure of coated carbide tools is mainly caused by the formation and extension of surface crack. Coating thickness, coating materials, and substrate materials significantly affect the toughness of coated carbide tools. X-ray diffraction technique is applied to measure the residual stress in coating surface, and equations model are introduced to calculate the surface residual stress induced during coating process.

Fatigue crack growth for a surface crack in a round bar under multi‐axial loading condition

ABSTRACTIn this paper, the fatigue life, surface crack extension direction and crack growth rate in an elastic bar with a circular cross section are determined through experiments under cyclic torsion with axial static and cyclic tension/compression loading. The effects of the loading type, loading value and stress ratio on the crack growth behaviour are discussed. The results show that, under pure fatigue torsion loading, the crack extension direction is almost the same whatever the value of torsion loading. Under fatigue torsion with cyclic tension loading, it is found that the crack extension direction is mainly determined by the alternating parts of the stresses and is almost independent of the average parts of the stresses, whereas the fatigue life is obviously dependent on the average stress.

Probability of cone crack initiation due to spherical contact loading

The probability of the extension of surface cracks due to spherical indentation is analyzed. Contact loading problems typically result in multi-axle and strongly inhomogeneous stress fields near the contact surface. This property which strongly affects the stress intensity factors is taken into account in the failure analysis. Edge cracks were selected to model the actually occurring surface flaws. The stress intensity factors were integrated with the weight function method, and propagation of pre-existing cracks was assessed by using an energy release rate failure criterion. For the statistical analysis the Weibull theory was extended in some points. By applying the method to the cone crack formation during a spherical indentation, it was found that the probability for cone crack formation increases abruptly, when a certain yield load is exceeded. Therefore, the loading range can be divided into a critical and a less critical section, which were found to correlate to separated spatial regions of an occurring local risk of fracture. Because of the effects discovered, it seems to be inevitable to consider the strongly inhomogeneous stress fields in a failure analysis of a contact loading problem.

Ductile tearing in thin aluminum panels: experiments and analyses using large-displacement, 3-D surface cohesive elements

This paper describes a large-displacement formulation for a 3-D, interface-cohesive finite element model and its application to predict ductile tearing in thin aluminum panels. A nonlinear traction–separation relationship defines the constitutive response of the initially zero thickness interface elements. Applications of the model simulate crack extension in C(T) and M(T) panels made of a 2.3 mm thick, Al 2024-T3 alloy tested as part of the NASA-Langley Aging Aircraft program. Tests of the M(T) specimens without guide plates exhibit significant out-of-plane (buckling) displacements during crack growth which necessitates the large-displacement, cohesive formulation. The measured load vs. outside surface crack extension behavior of high constraint ( T-stress>0) C(T) specimen drives the calibration process of the cohesive fracture model. Analyses of low constraint M(T) specimens, having widths of 300 and 600 mm and various a/ W ratios, demonstrate the capabilities of the calibrated model to predict measured loads and measured outside surface crack extensions. The models capture accurately the strong 3-D effects leading to out-of-plane buckling and various degrees of crack front tunneling in the C(T) and M(T) specimens. Previous analyses of these specimens using a crack tip opening angle (CTOA) criterion for growth show good agreement with measured peak loads. However, without the ability of the interface-cohesive model to predict tunneling behavior, the CTOA approach overestimates crack extensions early in the loading when tunneling behavior dominates the response.

Effect of Stacking Sequence on Fatigue Damage Process in CFRP Laminates after Lateral Impact.

A series of tensile and fatigue tests were conducted on CFRP angle ply laminate specimens with different atacking sequence, i. e. three-layered, five-layered and seven-layered laminates, which were damaged from lateral impact by a free fall rod. The extension of surface cracks and the development of delaminations were observed by using a replicating method and a scanning acoustic microscope (SAM), respectively. In all stacking sequences under fatigue loading, the crack growth process consisted of several stages, and crack growth rates gradually decreased. In three-layered specimen, a single crack was nucleated in the opposite layer to the surface which was given lateral impact. After the crack tips attained to the edge of specimen, inter laminar delamination began to extend. On the other hand, several cracks were observed in the five-layered and seven-layered specimen. In addition to the extension of cracks in the surface layer introduced by impact load, several new matrix cracks initiated at the edge of specimen. Crack growth was followed by the development of interlaminar delamination. The delamination in each stacking sequence developed along the preceding matrix cracks, and the effect of the preceding matrix cracks in the middle layer was notable in threelayered specimen. In five-layered specimen, the delaminations in the failured specimen was not influenced by the matrix cracks in the middle layer, but were affected by the cracks emanating from the free edge in the outer layer.

Observations of Damage Propagation Process in CFRP Laminates after Impact.

A series of fatigue tests have been performed on CFRP angle-ply laminate specimens. Prior to fatigue tests, all specimens were given the impact of a free fall rod. The extension of surface cracks and of delaminations were observed using a replicated method and a scanning acoustic microscope (SAM), respectively. The crack growth process consisted of four stages, and crack growth rates gradually decreased over these stages. Although the area of delamination did not increase with the surface crack growth, once either of crack tip reached the edge of specimen, the delamination area began to extend with the internal matrix cracks. After surface cracks were generated on the upper surface given impact, fatigue damage propagated rapidly. Specimens were ruptured when the delamination area of the upper surface extended sufficiently.

Stress Intensity Evaluation for Surface Crack with Use of Boundary Element Method and Influence Function Method and the Surface Crack Extension Analysis

Stress Intensity Evaluation for Surface Crack with Use of Boundary Element Method and Influence Function Method and the Surface Crack Extension Analysis

Stress intensity evaluation for surface cracks by means of boundary element method and influence function method and the surface crack extension analysis.

Stress intensity evaluation for surface cracks by means of boundary element method and influence function method and the surface crack extension analysis.

Crack Stability and Strength Distribution of Ceramics That Exhibit Rising Crack‐Growth‐Resistance (R‐Curve) Behavior

Crack stability and strength distribution are examined for ceramics that exhibit R‐curve behavior. An empirical power‐law description for the R curve leads to simple analytical formulations for the Weibull modulus and the scale parameter; however, the power law is inconsistent with the stability of surface cracks recently observed in MgO‐partially‐stabilized zirconia and coarse‐grained alumina. It is shown that alternate empirical fits to the R curve are compatible with the stable extension of surface cracks, even in the absence of any localized residual stresses.

Some advances in fracture studies under the heavy-section steel technology program

Recent results are summarized from HSST studies in three major areas that relate to assessing nuclear reactor pressure vessel integrity under pressurized-thermal-shock (PTS) conditions. These areas are irradiation effects on the fracture properties of stainless steel cladding, crack run-arrest behavior under non-isothermal conditions, and fracture behavior of a thick-wall vessel under combined thermal and pressure loadings. Since a layer of tough stainless steel weld overlay cladding on the interior of a pressure vessel could assist in limiting surface crack extension under PTS conditions, its resistance to radiation embrittlement was examined. A stainless steel overlay cladding, applied by a submerged arc, single-wire, oscillating-electrode method, was irradiated to 2 × 10 23 neutrons/m 2 (> 1 MeV) at 288°C. Yield strength increases up to 27% and a slight increase in ductility were observed. Charpy V-Notch data showed a ductile-to-brittle transition behavior caused by temperature-dependent failure of the 8-ferrite phase. The type 308 cladding, microstructurally typical of that in reactor pressure vessels, showed very little degradation in either upper-shelf energy or transition temperature due to irradiation. Crack-arrest behavior of A533 grade B class 1 steel was examined for temperatures extending above the onset of Charpy upper-shelf. Crack-arrest experiments that use wide-plate specimens have shown crack arrest occurring prior to transition to tearing or tensile instability. High values of crack-arrest toughness have been recorded (static values above 400 MPa m that are well above the maximum value that safety assessment criteria assume such materials can exhibit. A validation experiment was performed by exposing an intentionally flawed HSST intermediate test vessel to combined pressure and thermal transients. The experiment addressed warm-prestressing phenomena, crack propagation from brittle to ductile regions, and crack stabilization in ductile regions. Test and analysis results are summarized.

Determination of Residual Surface Stresses Caused by Grinding in Polycrystalline Al2O3

Residual surface stresses introduced into polycrystalline Al2O3 during diamond grinding (320 grit) were examined by an X‐ray direction technique commonly used for metals. Compressive stresses, estimated to be in the range 135 to 170 MPa and to extend 15 μm deep, were observed at the surface. It is believed that the compressive surface layer coincides with the plastic layer produced by the elastie/plastic interaction of the abrasive grains with the ceramic. The results are discussed with regard to the effect of the compressive layer on the extension of surface cracks.