Critical Crack Size Research Articles

Estimation of fatigue life parameters of an Alumino Thermic weld on UIC60 rail joint using LEFM

At wheel track contact point, the high stress concentration, poor weld quality, and heterogeneity of weld material are the main factors that cause fatigue crack on any rail weld. Railway network agencies are concerned about the safety of the railway track when it comes to detecting and fixing weld faults to avoid vehicle derailment and loss of lives. This study analysed a numerical simulation of fatigue crack and its evolution under loaded service condition. A 3-D CAD wheel rail weld assembly model was built to study an AT welded joint under fatigue, and for stress concentration factor (SIF) calculation. The results are found by inserting a semi elliptical crack on the rail weld head surface with ANSYS, and then numerical simulation has been performed to get the different three modes of SIF at rail weld crack. The analysis findings data was recorded with critical fracture parameters of SIFs and its number of cycles to failure using LEFM technique and respective results have been plotted. With ANSYS the stress intensity on a crack will be resulted. By using numerical method, the critical crack size and number of cycle load with fatigue life of rail would be determined. The numbers of rail weld inspection per year has been determine by using the maximum number of cycle. The aim of this paper is to develop an effective inspection and maintenance frequency based on rolling contact surfaces crack propagation analyse. This will help to prevent the occurrence of rail failure by taking the required action at the right time, and extend the rail life expectancy, reduce the rail maintenance work and its cost.

Safety assessment of the small welded pipe system by the test and FEA

Failures of small pipe systems continue to occur frequently at nuclear power plants and aerospace equipment resulting in degraded systems and unscheduled downtime. Currently, there is a lack of detailed technical requirements and evaluation criteria for the small pipe in the existing design codes. In the design code, if referring to the general fatigue evaluation method for metal parts, the results may be too conservative. It has been shown that fatigue tests based on structural parts can significantly extend the component life by recently studies. At the same time, for the small pipe, the influence of the crack propagation stage on the fatigue life (S–N curve) is usually ignored in the existing evaluation process. On the other hand, fatigue-related failures are generally detected as small cracks or leaks before major pressure boundary ruptures occur, so the “Leak-Before-Break” (LBB) can be also applied for the small pipe. This study is undertaken to improve the understanding of the fatigue life of the small pipe (comparing the pipe test results with the data in the code) and the characteristics of fatigue crack propagation. Also, the applicability of the LBB concept is discussed. The results show that the S–N curve obtained by the pipe test is higher than that gained by the standard specimen. The crack propagation life accounts about 3% of the fatigue life in the study case. The critical failure mode is dominated by the plastic collapse; it is therefore shown that the plastic collapse load or critical crack size can be increased by increasing the tensile strength of the materials. A safety factor of 9.25 for the LBB assessment is contained in the case; therefore, it is also useful to warn the final fracture of the pipe by installing a leak detection system.

Effects of flake-shape and content of nano-mullite on mechanical properties and fracture process of corundum composite ceramics

ABSTRACT Coal gangue is used to prepare flake nano-mullite with a molten salt method. In order to increase the utilization value, corundum composite ceramics were prepared from the nanomullite and alumina. The effect of particle size of the raw material size and the content of mullite on mechanical properties of the corundum composite ceramics was studied. It is found that the mullite with appropriate sizes can be filled in between the corundum grains, thus increasing the density of the samples. The mechanical properties were enhanced due to the variation in the crack propagation mode. The optimized sample exhibited flexural strength and fracture toughness of 263.7 MPa and 5.0 MPa·m1/2, respectively. Griffith fracture theory analysis results indicated that the critical crack size reached 173.9 μm and the fracture energy was 127.6 J/m2. Coal gangue has realized deep resource utilization, and the mechanical properties of composite ceramics are higher than alumina ceramics prepared under the same conditions. Flake nanomullite contributes to mechanical properties.

Relationship between non-inclusion induced crack initiation and microstructure on fatigue behavior of bainite/martensite steel in high cycle fatigue/very high cycle (HCF/VHCF) regime

High cycle and very high cycle fatigue (HCF/VHCF) in a bainite/martensite (B/M) multiphase steel with varying inclusion size and microstructural features was studied using ultrasonic axial cycling test. The fatigue crack initiation was predominantly induced by inclusions in specimens with large inclusion size, whereas fatigue crack initiated from the sub-surface microstructure in specimens with coarse microstructure. The fatigue life from granular bright facet (GBF) to fish-eye and from fish-eye to the critical crack size was calculated to obtain an estimate of the contribution to fatigue life by GBF, for the two modes of crack initiation within the HCF/VHCF regime. The results demonstrated that the majority of fatigue life was consumed by the crack initiation process along with the formation of GBF irrespective of whether the crack initiated from inclusions or from sub-surface microstructure. In the case of crack initiation from sub-surface microstructure, the ratio of fatigue crack initiation life to total fatigue life (Ni/Nf) had a wide scatter, which is attributed to varying B/M hierarchical structure in individual prior austenite grains.

ABOUT DARSY’S LAW DURING FLUIDS MOTION IN THE MICRO-CRACKED CHANNELS

Firstly it has been experimentally revealed that during fluid motion in the micro-cracked channel and in the equivalent porous medium an unknown additional resistance arises in the scientific technical literature that is the “microcrack-fluid” effect. It has been demonstrated that the determined “microcrack-fluid” effect is the cause of linear Darcy’s law violation in the micro-cracked channels. It has been revealed in the work that during fluids moving in the microcracked channel there is a critical size of crack for the homogeneous fluid (water, viscous and anomalous fluids) and a hydrodynamic effect as so-called “microcrack-fluid” is manifested. So for the first time we determined the critical value of opening − hcr on the basis of experimental investigations in cracks. It was found that at h<hcr the anomalous properties are manifested for viscous fluids and rheological parameters are increased for anomalous fluids, and at h≥hcr these effects disappear. It has been established that the reason of the anomalous behavior of fluids in the microcrack with h<hcr opening is the effect occurred in the “microcrack-fluid” system. It is shown that microcrack with certain opening can be considered as a model but the ultra-low permeable porous medium is nature. It has been determined that the critical value of the Reynolds number calculated for viscous and abnormal fluids in the microcracked channel and in the equivalent porous medium in the microcrack is Re<1. The new fact about Darcy’s law violation during fluids flow in microcrack with h<hcr opening has been experimentally revealed i.e. micro-cracked effect of “microcrack-fluid” system is a cause of Darcy’s law violation. It is recommended to taking into consideration the microcracked effect in the “fluid-medium” system for regulation and creation of the new technical and technological processes in the different branches of industry

The effect of local residual stress on the fractal dimension of silicate glasses

Local residual stress caused by impacts, machining and indentation results in a decrease in strength in most materials that fail in a brittle manner. The ratio of the critical crack size, c, and the fracture mirror size, r, also is affected by the existence of local residual stress. The global fracture toughness of non-R curve materials is not affected by the local residual stress. The fractal dimension of the fracture surface as characterized by the fractal dimensional increment, D*, is directly related to the square of the fracture toughness. This paper addresses the question of the effect of the local residual stress on the fractal dimension of the fracture surface. We derive a relationship between the fractal dimensional increment and the c/r ratio for materials fractured with and without local residual stress. We then compare the prediction with two cases of experimental results. We show the fractal dimension remains constant with the change in the c/r ratio for local residual stress conditions.

Preventive failure assessment of divertor armor blocks with a postulated flaw under slow transient conditions

As part of ITER research to devise suitable facilities withstanding extreme plasma, divertor modules have been designed in a way equipping tungsten armor blocks with optimized shapes at target regions. However, in spite of comprehensive considerations for the material and geometry, thermally damaged cracks were observed from recent limited mock-up tests. Objectives of the present numerical study are to address preventive failure assessment of mono- and multiple-armor blocks with a postulated flaw under slow transient conditions and to discuss engineering application. Firstly, systematic heat transfer and stress analyses were conducted without any defect according to typical heat fluxes and internal pressure. Subsequently, stress re-distributions due to a specific surface crack were taken into account and fracture mechanics analyses were carried out by changing their locations, orientations and sizes. As results, crack driving forces were quantified in terms of J-integral values and compared with each other to find out plausible cases threatening structural integrity of the armor block. Critical crack sizes were also estimated on the basis of a representative fracture toughness for future enhanced design and/or safe operation of the divertor.

Low-temperature degradation increases the cyclic fatigue resistance of 3Y-TZP in bending

ObjectiveDue to past failures of orthopedic 3Y-TZP femoral implants linked to accelerated tetragonal-to-monoclinic phase transformation (t → m), the susceptibility to ‘low-temperature degradation’ or ‘ageing’ of 3Y-TZP has been advertised as detrimental to its long-term structural stability. However, no systematic mechanistic experiments on the fatigue resistance of aged 3Y-TZP under cyclic loading can support such statement. In this study, we aim to clarify this issue. MethodsHere we evaluate the subcritical crack growth behavior of 3Y-TZP under cyclic loading after 0–50 h of accelerated ageing in an autoclave at 134 °C. The same 3Y-TZP sintered at two different temperatures (1450 °C or 1650 °C) allows for the comparison of materials containing grains with different susceptibilities to transformation. The volume fraction of surface transformed grains was measured using Raman spectroscopy, and the depth of the transformed surface layer from trenches milled with a Focus-Ion Beam. The fracture toughness before and after ageing was determined using the Chevron-notch Beam method. The quasi-static flexural strength was measured in dry conditions and the cyclic lifetime in water at 10 Hz and R-ratio = 0.3 in 4-point-bending at different applied stresses. The fatigue parameter n was derived from 3 different methods, namely SN curves, crack velocity plots and Weibull distributions. ResultsThe progression of transformation showed linear kinetics with higher rates for the 3Y-TZP sintered at 1650 °C. Accelerated transformation induced severe crack formation within the transformed layer with parallel orientation to the surface plane, which supposedly behaved as the critical crack size population governing fracture. The stress intensity factor within the transformed layer was increased due to compressive stresses. Consequently, the fatigue parameter n increased consistently from 5 to 50 h of ageing, regardless of the derivation method, suggesting an increased resistance against crack growth during cyclic loading in bending. SignificanceOur results do not support the long suggested negative clinical implications of LTD regarding mechanical performance, to the contrary, LTD seems to increase the resistance against subcritical crack growth in a humid environment in bending.

Modelling crack initiation in bituminous binders under a rotational shear fatigue load

This study aims to model fatigue crack initiation in bituminous binders. An energy-based crack initiation criterion is developed for bitumen under a rotational shear fatigue load. Based on a damage mechanics analysis of fatigue cracking process, the crack initiation is defined and local energy redistribution around crack tips due to ‘factory-roof’ cracking is quantified. A quantitative energy criterion is proposed for the fatigue crack initiation in the bitumen using viscoelastic Griffith’s theory. The crack initiation criterion is validated through comparing the predicted and measured surface energy of the bitumen. The results show that bitumen fatigue cracking under the rotational shear fatigue load can be divided into two stages: the edge flow damage and the ‘factory-roof’ cracking. The crack initiation is dependent of the shear modulus and surface energy of bituminous binders, critical crack size, and loading amplitude. The energy-based crack initiation criterion along with the DSR fatigue tests can be potentially used to determine the material surface energy.

Probabilistic Analysis of Fatigue Behavior of Single Lap Riveted Joints

This research deals with the fatigue behavior of 200 small single lap multiple-riveted joint specimens, widely used for aeronautic structures. The tests were performed with three different levels of stress with stress ratio R = 0.05; three levels were set: 90 MPa, 120 MPa and 160 MPa. The fatigue life and critical crack size for all tested specimens were analyzed. According to the results’ analysis, two types of fracture, through-hole and in proximity of the hole, were observed, depending on the level of stress: the higher the applied stress, the more through-hole cracking. Indeed, under the fatigue load with a stress level of 90 MPa, less than 30% of specimens showed cracks propagating through the hole, while, at the stress level of 120 MPa, the percentage reaches 36.3%. At the stress level of 160 MPa, 100% of specimens failed through the hole. Moreover, aimed to use experimental data for probabilistic methods, a statistical analysis was performed according to the Anderson–Darling test. This method allowed the analysis of the datasets, in terms of both fatigue life and critical crack size, providing information about the best distribution function able to fit experimental results.

Engineering critical assessment of RPV with nozzle corner cracks under pressurized thermal shocks

Nozzle corner cracks present at the intersection of reactor pressure vessels (RPVs) and inlet or outlet nozzles have been a persistent problem for a number of years. The fracture analysis of such nozzle corner cracks is very important and critical for the efficient design and assessment of the structural integrity of RPVs. This paper aims to perform an engineering critical assessment of RPVs with nozzle corner cracks subjected to several transients accompanied by pressurized thermal shocks. The critical crack size of the RPV model with nozzle corner cracks under transient loading is evaluated on failure assessment curve. In particular, the influence of cladding on the crack initiation of nozzle corner crack under thermal transients is studied. The influence of primary internal pressure and secondary thermal stress on the stress field at nozzle corner and SIF at crack front is analyzed. Finally, the influence of different crack size and crack shape on the final critical crack size is analyzed.

Fatigue Life Assessment and Reliability Analysis of Cope-Hole Details in Steel Bridges

Cope-hole details are widely applied to steel bridges. However, the safety of steel bridges is influenced by the fatigue performance of welded details. So, cope-hole details with flange and web subjected to axial loads were selected as the research object. Based on the basic theory of linear elastic fracture mechanics and the Finite Element Method, the stress intensity factors of cope-holes details were calculated. The influences of geometry size and crack size of the detail on the stress intensity factors were then investigated. The Paris model of fatigue crack propagation predicted the crack propagation life of cope-hole details. Besides, the fatigue limit-state equation was also established to analyse the effect of random variables (such as initial crack size, critical crack size, crack propagation parameter) on the fatigue reliability index. Finally, the recommended value of the detection period was present. The results show that the stress intensity factor gradually increases with the increase of the cope-hole radius, the weld size, the flange plate thickness, the crack length and the web thickness. However, it gradually decreases with the increase of the ratio of the long and short axle to the crack. The predicted number of fatigue cyclic loading required by the fatigue crack depth propagating from 0.5 mm to 16 mm under nominal stress amplitude of 63 MPa is 122.22 million times. The fatigue reliability index decreases with the fatigue growth parameter, the crack shape ratio and the mean of initial crack size increasing, which is relatively sensitive. However, the variation coefficient of the initial crack size has little effect on it. The detection period of cope-hole details is the service time corresponding to the fatigue accumulated cyclic loading of 198.3 million times.

Brief destructions of basic nodes of excavators and critical size of cracks

The article studies brittle fractures of metal structures of mine and hydraulic excavators basic units. Based on the data analyzed and physics of the brittle fracture process, recommendations are given to improve the reliability of machines. For the analysis of brittle fractures, statistical data on machine failures under the conditions of the Korshunovsky GOK and ALROSA quarries were used. The approach to the control of nucleation and formation of brittle fractures by non-destructive methods, which substantially reduces the costs of repair works is described. The analysis of brittle fractures of mining excavators basic units revealed general patterns of destruction causes. Accurate identification of brittle fractures requires simplification of repair works. The method of nondestructive control of machines should be applied. The non-destructive control method is used to control the a brittle cracking in advance and eliminate during scheduled repairs. The scheduled identification and elimination of cracks reduces laboriousness of repair and recovery operations of large-sized units and aggregates of excavators.

Fabrication, mechanical properties and thermal shock resistance of a dense SiC NWs/α-Si3N4 composite coating for protecting porous Si3N4 ceramics

To prevent porous Si3N4 ceramics from absorbing moisture in practical application, a dense SiC NWs/α-Si3N4 composite coating was fabricated using the two-step technology involving slurry and liquid infiltration method. The results revealed that the SiC NWs not only can promote densification sintering of the coating but also can reduce thickness of the coating effectively due to the block effect of the SiC NWs. Furthermore, the SiC NWs in the coating had a good interfacial bonding with α-Si3N4 matrix, meanwhile the toughening effects of the SiC NWs involving crack deflection, crack branching, SiC NWs bridging and pullout, also presented during mechanical damage. The strengthening and toughening mechanisms of the SiC NWs not only enhanced mechanical properties of the coating but also raised significantly critical crack size (L), thereby which made the coating have a good ability for thermal shock resistance. After thermal shock at ΔT = 1000 °C for 15 times, water absorption of the coated porous Si3N4 ceramics increased only to 7.21% from 4.54%, and its residual strength ratio was up to 83.4%. More importantly, the coating exhibited an ability for crack healing during thermal shock testing. All those above results indicated that as-prepared SiC NWs/α-Si3N4 composite coating can meet the applied requirements in long-term extreme conditions.

Case Studies of Aircraft Fuselage Cracking

Fatigue plays a significant role in the crack growth of the fuselage skin structures. In addition, the fuselage may suffer also from the corrosion damage, and the wear defects. The proper maintenance and scheduled test intervals can avoid the sudden skin failure. Therefore, the inspection interval has to be shortened. Nevertheless, the young machines may be also suffering from the unexpected skin rupture. The cracks are emanating from the rivets and the holes under cyclic loading. The stress concentration around the notch has an effective role under the effect of cyclic loading. The cracks propagate toward the high stressed area such as the notches or other crack locations. The propagation into a critical crack size is rather fast and causes a sudden aircraft fuselage cracking. Hence, the number of cycles to failure will be decreased dramatically. During the last decades, the fracture toughness, design, and the new alloying element have been enhanced. The previous fuselage failures show that the inspections against the cracking are recommended even after a few thousand of cycles. To prevent the crack extending, the crack arresting is recommended to use around the fuselage.

Fracture analysis of a total hip prosthesis based on reverse engineering

A reverse engineering procedure based on optical 3D scanning was implemented to generate a total hip prosthesis CAD model needed to create the finite element model of the femoral component cemented into cortical bone. Finite element fracture analyses of a loosened femoral component shaft of the total hip prosthesis damaged with a crack was performed assuming loading and boundary conditions given by the standard ISO 7206-4. In the femoral component shaft a damage crack was modelled, where several crack depths were considered. Mode I stress intensity factor KI values were calculated for nodes along the crack front for each of the considered crack depths. It was demonstrated that for the given loading conditions a critical crack size associated with fracture onset can be estimated.

FATIGUE LIFE ANALYSIS OF RAIL-WELDS USING LINEAR ELASTIC FRACTURE MECHANICS

The initiation and growth of fatigue cracking is mainly due to high stress concentration, heterogeneity and poor quality of weld. The detection and rectification of such weld defects are major concerns of rail network managers to reduce potential risk of rail breaks and derailments. To estimate the fatigue life of welded joints and to analyze the progress of fatigue cracks, a fracture mechanics-based analysis and fatigue models were developed using Finite Element Analysis. The initial flaw is obtained from a sample weld using ultrasonic flaw detecting machine test. Linear Elastic Fracture Mechanics (LEFM) approach based on the Paris law was applied to determine critical crack size and the number of cycles to failure using FRANC3D software. The inspection interval of rail welds before fracture (failure) was suggested based on reliability and life cycle analysis that correspond with minimum overall cost and frequency interval. It is recommended that fracture-based models in combination with reliability analyses can be a sustainable infrastructure decision-making algorithm.

Predictive maintenance of wind turbine low-speed shafts based on an autonomous ultrasonic system

Low-speed shafts breakage in wind turbines (WT) supposes, besides an elevated repair cost, an unattainable risk for workers integrity due to the induced rotor fall. Based on a Root Cause Analysis (RCA) an autonomous ultrasonic monitoring system has been developed with the aim of extending the shaft useful life safely beyond the 50% obtained when manual ultrasonic inspections are used.This system consists of ad-hoc electronics, a specific firmware that includes the detection and assessment algorithm, and an annular array of transducers attached to the shaft by a specific mechanical holder. To estimate the transducers type, quantity and optimum location and to establish references, healthy shafts were analyzed and tests specimens with artificial defects were manufactured and studied reproducing the critical crack sizes estimated in the RCA. The firmware controls the entire system and is the responsible for autonomous diagnostics that launches preliminary alerts when detecting non-critical cracks. The system creates an “imminent failure” alert when the shaft has consumed up to 96% of its useful life. The system has been tested in real WTs with positive results, omitting no-relevant cracks and even detecting certain type of cracks not detected by manual inspections.

Fracture mechanics assessment of divertor vertical target under thermal loading conditions

Since divertor modules undergo extreme thermal loads, lots of stress analyses and several accompanying tests have been carried out for tungsten armored mono- and multi-blocks. However, while noteworthy cracks were found at surface of mono-block during recent high heat flux experiments, there was a little fracture mechanics related research. The objective of present study is to investigate structural integrity of flawed ITER outer vertical target (OVT) based on elastic-plastic fracture mechanics assessment. At first, heat transfer and thermal stress analyses were performed on the mono-block and OVT. Especially, for which two disjunctive heat fluxes were applied at plate and baffle of the OVT as transient condition. Subsequently, parametric fracture mechanics analyses were carried out for the OVT with a postulated surface crack after mapping pre-determined stress profile. In addition, a typical fracture mechanics parameter as well as temperature and stress distributions were also calculated from similar analyses of a mono-block. As results, dependency of J-integral values on crack positions, sizes and locations was discussed, and critical crack sizes were decided by comparing with a representative fracture toughness of plasma facing material.

Subcritical and critical states of a crack with failure zones

• A new semi-analytical technique is developed to solve the problems of crack mechanics within the cohesive zone model . • Proposed technique implicitly takes into account the finite stress condition. • New dependencies for critical crack lengths are obtained for the non-uniform traction-separation laws. The problem on the stress–strain state near a mode I crack in an infinite plate is solved in the frame of a cohesive zone model. The complex variable method of Muskhelishvili is used to obtain the crack opening displacements caused by the cohesive traction , which models the failure zone at the crack tip, as well as by the external load. The finite stress condition and logarithmic singularity of the derivative of the separation with respect to the coordinate at the tip of a physical crack are taken into account. The cohesive traction distribution is sought in a piecewise linear form, nodal values of which are being numerically chosen to satisfy the traction-separation law. According to this law, the cohesive traction is coupled with the corresponding separation and fracture toughness . The tips of the physical crack and cohesive zone (geometric variables) along with the discrete cohesive traction are used as the problem parameters determining the stress-strain state. If the crack length is included in the set, then the critical crack size can be found for the given loading intensity. The obtained determining system of equations is solved numerically. To find the initial point for a standard numerical algorithm, the asymptotic determining system is derived. In this system, the geometric variables can be easily eliminated, which make it possible to linearize the system. In the numerical examples, the one-parameter traction-separation laws are used. Influence of the shape parameters of the law on the critical crack size and the corresponding cohesive length is studied. The possibility of using asymptotic solutions for determining the critical parameters is analysed. It is established that the critical crack length slightly depends on the shape parameter, while the cohesive length shows a strong dependence on the shape of cohesive laws .