Crack Growth Process Research Articles

On the slow crack growth process and associated structure–property relationships in polyamide 12 grades

AbstractThe influence of micro‐structural changes within polyamide 12 (PA12) grades on their resistance against crack initiation and subsequent Slow Crack Growth (SCG) was examined. Possible micro‐deformation mechanisms and processes guiding slow crack extensions in PA12 are discussed. Special focus was put on the formation of plastic zones as predecessors to microscopic cracks prior to crack initiation via crack freezing analysis. Simultaneously, variations in the pure SCG resistance after a recorded craze‐crack transition were determined via cyclic Cracked Round Bar (CRB) tests. In that context, a homologous series of PA12 grades of increasing molecular weight (MW), as well as a systematically developed series of PA12 grades incorporating a pigment and/or an impact modifier, were selected for this study. Results show a good correlation between the crack initiation resistance and the disentanglement resistance of PA12 chains as well as the size of the plastic zone. In that context, an increasing average MW, leads to increasing disentanglement resistances due to hydrogen bond effects. An impact modification promotes the development of notably larger plastic zones. Contrarily, colored grades exhibit a reduced plastic zone size, rendering a lower amount of dissipated energy before physical crack initiation. Pure SCG resistance follows a similar trend and is improved by high average MW as well as the use of impact modifier. Crack growth acceleration, however, is assumed to occur alongside the relatively weak interfaces between polymer matrix and pigments.

An improved small-time-scale crack growth rate model considering overloading and load-sustaining effects for deep-sea pressure hulls

The fatigue life assessment of deep-sea pressure hulls needs to consider its load characteristics reasonably, including short-term overloading and load-sustaining at a certain depth in a load cycle lasting several hours. In this case, the traditional reversal-based crack growth rate model is no longer applicable while the small-time-scale model which can consider the crack growth process in any small interval highlights its advantages. This paper presents an improved small-time-scale crack growth rate model with consideration of overloading and load-sustaining effects as well as the stress/strain constraints. The formula for crack tip opening angle is modified to more accurately demonstrate its changing tendency from crack opening to final fracture. A concept of plastic zone influence range is proposed to express the effect of the plastic zone size and the overload ratio. Creep effects at load-sustaining period at high stress level are included into calculation. Finally, model validation and parametric analysis are conducted. The improved model can be reasonably applied for fatigue life prediction of deep-sea pressure hulls.

Effect of the upper limit stress on the fatigue properties and crack propagation processes of sandstones containing pre‐existing crack under fatigue‐creep interaction

AbstractTo study the effect of the upper limit stress on the fatigue characteristics and crack propagation process of jointed rock masses under the interaction of fatigue‐creep, a series of fatigue‐creep tests with different cyclic stress levels were carried out on jointed sandstone samples. Acoustic emission, scanning electron microscopy, and digital image correlation technologies were used to analyze the fatigue characteristics and crack growth process of the specimen. The results show that as the stress level increases, the deformation of the sample increases, the strength decreases, the elasticity attenuates, and the damage increases. The splitting failure of grains is weakened, the area of fatigue folds increases, and the fracture roughness increases. Besides, the crack propagation process of the sample can be divided into three stages: crack initiation, stable crack growth, and unsteady crack growth. As the stress level increases, the failure mode of the sample gradually changes from brittle to ductile.

Crack propagation analysis of hydrogen embrittlement based on peridynamics

We introduced a coupled peridynamic hydrogen diffusion and fracture model to solve the hydrogen embrittlement fracture of low alloy steel AISI 4340. In this model, the influence of temperature on hydrogen diffusion coefficient is considered, and a new peridynamic constitutive analysis method is used to simulate the crack propagation of hydrogen embrittlement. We verified the model in 3D using the experimental test of the hydrogen embrittlement cracking process of AISI 4340 steel in 0.1 N H2SO4 solution from the literature. Considering different ambient temperatures, it is found that the crack propagation is highly similar to the experimental results. Based on the numerical analysis of peridynamics, the model can numerically simulate the hydrogen embrittlement fracture of AISI 4340 steel, and obtain a visual demonstration of the entire process of hydrogen atom diffusion and crack growth.

Fracture of brittle solid with an internal penny-shaped crack: Semi-circular bending test and numerical simulation

Semi-circular bending (SCB) tests are usually performed on samples with a bottom notch to investigate the mechanical properties of certain materials. Few studies have been conducted on the propagation of internal cracks in SCB tests despite the fact that the existence of internal defects is inevitable in most materials. When using the three-dimensional internal laser-engraved crack method, internal cracks are generated without causing any damage to the surfaces of the sample. Here, SCB tests are performed on samples with internal cracks at different depths. The crack growth process, failure mode, and macro- and micro-fractography are analyzed. Numerical simulations are carried out, revealing the distribution of the stress intensity factors around the crack tip and the internal crack growth path. The simulation results are consistent with those of the SCB tests. The results show that: (1) The propagation mode of internal cracks is mixed in the SCB tests; (2) The existence of internal cracks significantly reduces the failure load of the samples; (3) Dynamic fracture occurs in the intact samples of groups A and B1 (depth: 30 mm), while the failure of the samples of groups B2–B5 is due to the internal crack propagation. (4) Based on the J-integral and the maximum tensile stress criterion, the distribution of the stress intensity factors around the crack tip and the fracture path are in agreement with the test results.

Elasto-Plastic Fatigue Crack Growth Behavior of Extruded Mg Alloy with Deformation Anisotropy Due to Stress Ratio Fluctuation.

Fatigue crack growth (FCG) experiments were performed using a low-temperature extruded magnesium alloy AZ31 with texture. Under a constant maximum stress intensity factor (Kmax), the stress ratio R was changed from 0.1 to −1 during the fatigue crack growth process, and the FCG behavior before and after the R change was investigated. As a result, tensile twins were generated owing to the fatigue load on the compression side of R = −1, and the FCG velocity was accelerated. In addition, when the maximum compressive stress at R = −1 (|(σmin)R = −1|) exceeded the compressive yield strength of the material (σcy), the FCG velocity after R fluctuation greatly accelerated. On the other hand, under the condition |(σmin)R = −1| < σcy, the degree of acceleration of the FCG velocity due to R fluctuation was small. In either case, the degree of acceleration in the FCG increased as the Kmax value increased. The above FCG acceleration mechanism due to the R fluctuation was considered based on the observation of the deformation and twinning states of the fatigue crack tip, the fatigue crack closure behavior, and the cyclic stress–strain curve of the fatigue process. The FCG acceleration mechanism was as follows: First, the driving force of the FCG increased owing to the increase in crack opening displacement due to the generation of tensile twins. Second, the coalescence of the main crack and a plurality of microcracks were generated at the twin interface. The elasto-plastic FCG behavior after the stress ratio fluctuations is defined by the effective J-integral range ΔJeff.

XFEM crack growth virtual monitoring in self-sensing CNT reinforced polymer nanocomposite plates using ANSYS

This paper presents an eXtended Finite Element Method (XFEM)-based numerical scheme to compute electrical resistivity changes caused by the presence of cracks and the crack growth. Using the commercial finite element package ANSYS, the virtual continuous monitoring of the structure is solved in two steps. First, the strain response of the cracked composite domain is computed by means of the XFEM. In the second step, the electrical conductivity of the piezorresistive elements located in the domain are updated according to the strain state and the electric resistance between two electrodes of the damaged plate is computed. The comparison with the electric resistance measured for the undamaged plate allows us to detect the presence of a crack and its severity. Moreover, the crack growth process can be also monitored via the electric resistance increments. Several numerical studies are provided to show the capabilities of this computational framework.

Peculiarities of crack growth process under high-frequency action of grinding bodies on rocks

Peculiarities of crack growth process under high-frequency action of grinding bodies on rocks

Numerical investigation on the residual ultimate strength of cracked stiffened plates under extreme cyclic loads

This paper aims at gaining insights on the load-carrying behavior of stiffened plate under cyclic loads. Non-linear finite element method is employed to investigate the residual ultimate strength of cracked stiffened plate. Assuming that the precrack is a transverse and through-thickness crack and exists only in the plate or both in the stiffener and the plate, the corresponding four types of crack location models are analyzed. The crack propagation caused by extreme cyclic loads is considered in the present study. The crack growth process is simulated by adopting the element erosion approach. The effects of the amplitude of cyclic loads, crack location, the plate aspect ratio, the plate thickness as well as the stiffener size on the ultimate strength of cracked stiffened plate under cyclic loads are examined. The ultimate strength of cracked stiffened plate under monotonously increasing load is studied for comparative analysis. Based on the numerical results, a relationship between the number of cycles and non-dimensional ultimate strength is built to predict the residual ultimate strength of cracked stiffened plate under extreme cyclic loads.

Numerical investigation on crack propagation process of concrete gravity dams under static and dynamic loads with in-crack reservoir pressure

Proper consideration of external and internal loads acting on concrete gravity dams is of vital importance in concrete dam engineering, since it plays a significant role in the stability assessment of cracked dams under static/dynamic conditions. Several factors which may affect the mechanical response of concrete gravity dams, however, have not been comprehensively accounted for in most previous crack propagation analyses. To adequately evaluate the crack propagation response of concrete gravity dams, a finite element-based numerical analysis is carried out to estimate the crack propagation in a concrete gravity dam under static and seismic loads. In this work, the effects of the water pressure inside the stress-free crack and the fracture process zone (FPZ), the cohesive force within the FPZ, the hydrostatic water pressure, and the dam-reservoir interaction are incorporated via a series of reasonable assumptions. On the basis of a stress intensity factor (SIF)-based mixed mode I-II crack propagation criterion, a generic numerical algorithm is proposed to estimate the crack growth process of concrete gravity dams under static and seismic loads. Application of this algorithm to two case studies of concrete gravity dams under different loading conditions demonstrates the validity of the algorithm in estimating the static and seismic crack propagation response. It is found that when a concrete gravity dam is subjected to static loading, crack initiation does not occur before the reservoir water level exceeds the dam height, and that the water pressure inside stress-free cracks has a slight negative effect on the structural resistance against cracking while the water pressure within the FPZ has no effect on the dam capacity. The seismic crack growth analysis indicates that the water pressure inside cracks causes the crack to propagate at a larger downward angle towards the downstream face with a larger crack growth length.

Crack nucleation at forging flaws studied by non-local peridynamics simulations

We present a computational study and framework that allows us to study and understand the crack nucleation process from forging flaws. Forging flaws may be present in large steel rotor components commonly used for rotating power generation equipment including gas turbines, electrical generators, and steam turbines. The service life of these components is often limited by crack nucleation and subsequent growth from such forging flaws, which frequently exhibit themselves as non-metallic oxide inclusions. The fatigue crack growth process can be described by established engineering fracture mechanics methods. However, the initial crack nucleation process from a forging flaw is challenging for traditional engineering methods to quantify as it depends on the details of the flaw, including flaw morphology. We adopt the peridynamics method to describe and study this crack nucleation process. For a specific industrial gas turbine rotor steel, we present how we integrate and fit commonly known base material property data such as elastic properties, yield strength, and S-N curves, as well as fatigue crack growth data into a peridynamic model. The obtained model is then utilized in a series of high-performance two-dimensional peridynamic simulations to study the crack nucleation process from forging flaws for ambient and elevated temperatures in a rectangular simulation cell specimen. The simulations reveal an initial local nucleation at multiple small oxide inclusions followed by micro-crack propagation, arrest, coalescence, and eventual emergence of a dominant micro-crack that governs the crack nucleation process. The dependence on temperature and density of oxide inclusions of both the details of the microscopic processes and cycles to crack nucleation is also observed. The results are compared with fatigue experiments performed with specimens containing forging flaws of the same rotor steel.

Microstructurally small fatigue crack initiation behavior of fine and coarse grain simulated heat-affected zone microstructures in low carbon steel

Successive observation of fatigue crack initiation process was conducted on six types of thermally simulated HAZ microstructures to clarify the fundamental reason of fatigue life difference between coarse grain (CG) and fine grain (FG) heat-affected zone (HAZ) microstructures. As a result, regardless of microstructure, fatigue cracks initiated and propagated along shear stress plane when the crack is smaller than prior austenite grain size. In addition, this shear mode fatigue crack growth of CGHAZ were much faster than that of FGHAZ and categorized as microstructurally small fatigue cracks. Although prior austenite grain size of crack initiation life of the FG and CGHAZ were comparable, the remaining fatigue life after the grain size crack initiation of the CGHAZ become shorter than that of the FGHAZ as a function of grain size difference. As a result, total fatigue life of CGHAZ becomes shorter than that of FGHAZ. Therefore, relatively larger part of CGHAZ's fatigue life is consumed by shear mode microstructurally small fatigue crack growth process.

Complex anisotropic fracture behaviors of 3D-printed fiber-reinforced composites based on multi-scale hierarchical microstructure

3D printing allows large flexibility to control the alignment of fiber orientations engineered to the closely matched design requirements, making it advantageous for manufacturing fiber-reinforced composites with customized characteristics. Quantitative three-dimensional observation of inherent fracture behavior of 3D-printed composites has so far been a challenge due to its printing direction dependence. This article aims to clarify the structural hierarchy and anisotropic fracture behaviors of 3D-printed fiber-reinforced composites. We prepared two unidirectional and two alternate layup composite specimens for comparison and explored the resultant microstructures and fracture behaviors. In-situ X-ray tomography provided successive 3D images of fracture initiation and propagation at each tensile load increment to clearly define the progressive fracture mechanisms. The color-coded segmentation images described in detail the crack growth process during the uniaxial tension and highlighted the different fracture modes between specimens. The 3D-printed composites showed the complex anisotropic fracture behaviors based on the multi-scale hierarchical microstructure where the intermediate filaments embedded with micron-size reinforcing fibers form a macro-scale integrated single matrix. The complex fracture process occurred simultaneously at the micro-scale fiber-matrix interface as well as the macro-scale inter-filament interface. Eventually, 3D-printed composites reached the final failure following various fracture mechanisms represented by localized deformation, interfacial failure, heterogeneous cracking behavior, and structural evolution according to the localized orientation of printed materials to the tensile loading direction. Our effort will be the first step in finding the origins of the structural hierarchy and anisotropic fracture behaviors of 3D-printed fiber-reinforced composites.

External surface cracked offshore pipes reinforced with composite repair system: A numerical analysis

This paper conducts a numerical analysis on the external surface cracked steel pipes reinforced with Composite Repair System. A three-dimensional finite element (FE) model is developed to calculate the Stress Intensity Factor (SIF) of the surface crack, and the crack growth process is evaluated by the Paris’ law. The effect of FRP-to-steel interfacial bond condition on the SIF evaluation has been considered by incorporating the cohesive zone modelling. Then the FE model is validated by the experimental results. Thereafter, major issues including “interfacial bond condition” and “reinforcement effectiveness and influential parameters” have been discussed. The results indicate the reinforcement effectiveness on reducing the SIF owes to the decreasing of stress magnitude and the crack-bridging effect. Because of the crack-bridging effect, composite reinforcement performs more efficiently on reducing the SIF at the surface point than at the deepest point of the surface crack. The negative influence of the FRP-to-steel bond condition on the surface crack growth is not as significant as on reinforcing through-thickness cracks. However, since the interfacial stiffness is sensitive to the adhesive thickness, choosing an ideal adhesive thickness to acquire a good reinforcement effectiveness and to avoid potential interfacial bond failures is recommended.

Effect of Layer Thickness on the Physical and Mechanical Properties of Sand Powder 3D Printing Specimens

The application of sand powder three-dimensional (3D) printing technology in the field of rock mechanics and mining engineering has tremendous potential, but it is still in the preliminary exploration stage. This study investigated the effect of printing layer thickness on the physical and mechanical properties of rock-like specimens with sand powder 3D printing. Quartz sand powder was used as the printing material, and the specimens were prepared with three different layer thicknesses of 0.2, 0.3, and 0.4 mm. Uniaxial compression tests with a combination of digital image correlation (DIC), acoustic emission (AE) and 3D microscope observations were performed to analyze the mechanical properties and failure patterns of the specimens during loading. Experimental findings showed that increasing the layer thickness from 0.2 to 0.4 mm would result in a decrease in the weight, density, uniaxial compression strength, and elastic modulus of the specimens. The stress-strain curve, deformation and failure patterns, crack growth process, and AE characteristics of the specimens with a layer thickness of 0.2 mm are similar to the AE characteristics of rock-like material, whereas the specimens with layer thicknesses of 0.3 and 0.4 mm deform like a ductile material, which is not appropriate for simulation of coal or rock mass. In future studies, rock-like specimens should be prepared with a small layer thickness.

EFFECTS OF OVERLOAD AND SUPPLYING OIL ON CRACK GROWTH BEHAVIOR IN MG ALLOYS

A single overload was applied during the crack growth process under constant stress amplitude, and retardation of crack growth was observed in the case of magnesium alloys as well as carbon steel, aluminum alloys, etc. The retardation of crack growth was related to crack closure, the fracture surface roughness, and crack tip deformation. In addition, the effects of supplying oil into the crack on crack growth behavior of an overloaded specimen were investigated in this study. The crack growth rate in the case of supplying oil became lower than in the case without supplying oil. In the case of the magnesium alloy AZ31, powder of oxide magnesium appeared from the crack after overloading. It is one of the typical behaviors of AZ31. In the case of AZ31 and AZX912, the crack growth behavior after overloading was slightly different due to the deformation of the crack tip.

Assessment of integrity and remaining working life of welded steel structures

Analyzing the period of exploitation of welded steel structures it can be concluded that they are predominantly exposed to the action of variable load. The welded joint as the largest stress concentrator due to the heterogeneity of structural, mechanical and operational properties is a key problem that is further complicated by the possible and realistically probable presence of crack-type faults. The assessment of integrity largely depends on a comprehensive analysis of the welded joint as the most critical place of welded steel structures. Integrity assessment is a necessary obligation for extending the working life, as well as revitalization, as a way to keep the structures in operation, despite the long period of exploitation. This paper presented an analysis of the process of fatigue crack initiation and growth, i.e. an assessment of the of the welded steel structures’ integrity and remaining service life under the influence of variable load.

Crack Propagation Process and Seismogenic Mechanisms of Rock Due to the Influence of Freezing and Thawing

A common problem in cold regions is the influence of freeze–thaw damage on the stability of rock engineering. Therefore, a series of uniaxial compression tests of sandstone after freeze–thaw treatment was carried out in this study. The purpose was to express the initial damage of rock after freezing and thawing treatment more quantitatively and to study the influence of freeze–thaw damage on the rock crack growth process to provide some reference for the stability evaluation of rock engineering in cold regions. The results showed that the number of freeze–thaw cycles and the lowest freeze–thaw temperature had a significant effect on the rock damage and characteristic stress value, and the number of cycles had a more obvious effect: with an increase in the number of freeze–thaw cycles and a decrease in the freeze–thaw temperature, the initial damage DNT had an increasing trend. In addition, the intrinsic mechanism of the freeze–thaw effect on the rock characteristic stress was explained from the perspective of a meso-mechanical mechanism. From the inversion results of the acoustic emission (AE) moment tensor, it was found that the proportion of tensile cracks gradually increased with the increase in the initial damage of the rock in the stable and unsteady stages of the rock crack propagation.

Numerical simulation of crack growth process in Brazilian disk test using finite element method

Accurately simulating the compressive and tensile mechanical behavior of rock is one of the challenges faced by numerical simulation. In this paper, the rock’s triaxial compression experiment and the Brazilian disk simulation were simulated, and the propagation process of the rock crack was simulated in Brazilian disk test in the finite element method. According to the experimental data, the KCC model was completely modified and the triaxial compression experiment was carried out with the modified model. The modified model was applied to the Brazilian disk test simulation, and the crack propagation in Brazilian disk test was simulated by inducing the erosion algorithm, which verifies the applicability of the erosion algorithm in the finite element method of material continuity loss.

Orthogonal sampling moiré method and its application in microscale deformation field measurement

Sampling moiré (SM) method is one of the modern deformation measurement methods in photomechanics. This method realizes the visualization measurement of deformation field by a sparse sampling of grating images, showing the advantages of high efficiency and strong robustness. However, in the traditional sampling moiré method, the unidirectional sampling mode is used with the result that the two-dimensional deformation cannot be acquired at the same time. In this paper, by combining the orthogonal sampling and Fourier phase analysis techniques, an orthogonal sampling moiré (OSM) method is proposed with which the two-directional moiré phase of specimen grating can be separated synchronously. The accuracy of the OSM method for the deformation measurement is verified by numerical experiments and PMMA uniaxial tensile tests. As an application, in a scanning electron microscope (SEM), the deformation field near the crack tip of small fatigue cracks is measured, based on which the crack opening displacement (COD) and the stress intensity factors (K) are obtained. Furthermore, the crack closure effects at different temperatures are characterized. The experimental results show that the crack closure significantly affects the process of small crack growth at both room temperature and high temperature, and the crack opening load at high temperature is greater than that at room temperature, which indicates that small cracks are easier to close at high temperature.