Cracking Phenomena Research Articles

Safe operation of gas pipelines based on the control of stress corrosion cracking

PurposeThe paper is devoted to the phenomenon of stress corrosion cracking (SCC), which is an urgent problem for major operator companies that use large-diameter gas transport systems built in the second half of the last century. The aim of the study is to predict the operability of gas pipelines with SCC and ensure their safe operation.Design/methodology/approachThe methodology of the article mainly consisted of strength calculations, mathematical and analytical approaches.FindingsThe paper describes practical methods of assessing the residual service life and operability of pipes with SCC defects as part of gas pipelines, the developed approaches to assessing the point of failure of pipes, provides recommendations for developing requirements to ILI smart tools in order to detect cracks and the methods of pipe repair depending on the SCC defect parameters.Originality/valueThe originality of the study consists in the analytical description of the point of destruction of gas pipelines with SCC, assessment of their performance, as well as the definition of modern requirements for the accuracy of in-line inspection to ensure the detection of potentially dangerous SCC defects.

Internal force analysis of the resistance unit of frame-truss composite wall

The frame-truss composite wall is a wall assembled by a certain number of resistance unit composed of outer frame and internal diagonal struts. As the primary energy dissipation unit of the system, the resistance unit should be studied in the perspective of internal force analysis. When the internal force of resistance unit is analyzed, the outer frame and the internal diagonal struts are regarded as composite members of reinforced concrete. According to the characteristics of resistance unit, the outer frame is regarded as a rigid frame and the internal diagonal struts are treated as struts hinging with the rigid frame and mainly subjected to axial force. So the simplified model is formed and it can be regarded as seven-degree statically indeterminate structure in mechanics. The internal forces of outer frame and internal diagonal struts corresponding to the constraint forces of simplified model can be calculated by solving the statically indeterminate problem. The ABAQUS model of resistance unit is established to simulated the performance so that the internal force can also be obtained in another way. The constrain force of simplified model and the internal force of ABAQUS model are comparatively analyzed and the results show a feasibility of simplified model. The phenomenon of cracks on diagonal struts and ultimate failure state of resistance unit are compared with the stress cloud of ABAQUS model to explains the reasonability of simplified model. The applicability of ABAQUS model is further stated by comparing the stress of longitudinal rebar on the most dangerous section between the simplified model and ABAQUS model for the resistance unit. This simplified model can provide a method to analyze the internal force of the resistance unit under external load, which also lays a foundation for the subsequent researches.

Numerical study on rock damage characteristics of multi hole blasting

In this study, a two-dimensional finite element numerical simulation method was used to study the damage characteristics of rock mass under multi-hole blasting load. In the analysis, Drucker-Prager constitutive model was used to characterize the strength of rock mass. The stress acting on the blasting hole was simulated by exponential functions, that the stress could reach the maximum value in a short time and then drop to zero in a relative long time. The damage characteristics of rock under different confining pressures, and the influence of free surface on the damage of rock were examined. The results show that under the same blasting load, increasing the confining pressure of surrounding rock can inhibit the development of rock cracks; The free surface made the explosion stress wave reflect, the rock mass between the blasting hole and the free surface produced a phenomenon of stratification crack which restrained the crack propagation below the blasting hole.

Influence of Minor Alloying Element Additions on the Crack Susceptibility of a Nickel Based Superalloy Manufactured by LPBF

Inconel 738LC (IN738LC) is a nickel-based superalloy specially used in the hot section components of turbine engines. One of its main drawbacks relies on the cracking susceptibility when it is manufactured by laser powder bed fusion (LPBF). This paper analyzes the influence of minor alloying element concentration on cracking tendency of IN738LC superalloy manufactured by LPBF. For that objective, samples were manufactured using two powders, which presented different minor alloying elements concentration (Si, Zr and B). It was shown that the samples crack tendency was very different depending on the powder used for their manufacturing. In fact, the measured crack density value was 2.73 mm/mm2 for the samples manufactured with the powder with higher minor alloying elements concentration, while 0.25 mm/mm2 for the others. Additionally, a special emphasis has been put on elemental composition characterization in cracked grain boundaries in order to quantify possible Si or Zr enrichment. It has been also studied the differences of solidification ranges and grain structures between both samples as a consequence of different minor alloying elements concentration in order to analyze their effect on crack susceptibility. In this sense, Scheil-Gulliver simulation results have shown that samples with higher Si and Zr contents presented higher solidification range temperature. This fact, as well as an increase of the presence of high angle grain boundaries (HAGB), leaded to an increment in the crack formation during solidification. Therefore, in this research work, an understanding of the factors affecting crack phenomenon in the LPBF manufactured IN738LC was accomplished.

3-Dimensional Multiphysics Modeling of Behaviors of Pressurized Water Reactor Fuel Rods With Missing Pellet Surface

A 3-dimensional (3D) fuel performance analysis program, able to simulate normal operating conditions and accident conditions for PWR fuel behaviors, was developed based on the Multiphysics Object-Oriented Simulation Environment (MOOSE) finite-element framework. By taking fission products swelling, densification and expansion of pellet, thermal and irradiation creep, gap heat transfer, fission gas release, and cladding crack propagation into consideration, detailed fuel behaviors have been simulated in a multiphysics coupling way. Local defects in fuel pellet caused during manufacturing and filling processes known as the missing pellet surface (MPS) can cause abnormal stress distribution of the cladding and it could even lead to cladding failure. Taking Stress Corrosion Cracking (SCC) phenomenon into consideration, a simulation of PWR fuel rodlet that consists of a pellet with an MPS defect and an intact pellet was conducted. The fuel rod has experienced with sorts of events, including normal operating conditions and a high-power ramp event. The simulation results indicated that: 1) The MPS defect affects the temperature and displacement distribution in the vicinity of the MPS defect. When the pellets are in contact with the cladding, the inner surface of the cladding presents a large tensile hoop stress, which accelerates the crack propagation. 2) During the ramp event, the crack propagation rate was higher than that under normal condition and crack length expanded by about 0.1 µm.

Influence of Fibers on High Performance Concretes: Fibers Fabricated in Cameroon

In this article, steel fibers (sf) and Alcohol polyvinyl fibers (apvf) are used as addition with the aim to obtain high performances concrete (hpc). Some mechanical properties of high performances concrete containing these materials have been studied through bending and compressive tests on prismatic and cylindrical test tubes. The results showed that the addition of fibers in a formulation of high performances concrete gives results that varies from one solicitation to another and from a type of fiber to another: in the case of the simple compressive strength test, strengths are rather reduced, despite the fact that pieces of test- tube remain sewn after rupture. This is due to the phenomena of multidirectional cracking that appear at the time of the solicitation. The compressive strength decreases more when apvf is used; ii) concerning the bending test, steel fibers improve considerably the strength and the ductility of concretes thanks to their nature and their geometry. This improvement is favored by the compactness of the matrix; iii) there is no interest using apvf to improve the ductility or the strength of the hpc in bending. They rather decrease the resistance to the cracking and the rigidity of the material.

Experimental Study on Angular Flexural Performance of Multiaxis Three Dimensional (3D) Polymeric Carbon Fiber/Cementitious Concretes

Multiaxis three-dimensional (3D) continuous polymeric carbon fiber/cementitious concretes were introduced. Their angular (off-axis) flexural properties were experimentally studied. It was found that the placement of the continuous carbon fibers and their in-plane angular orientations in the pristine concrete noticeably influenced the angular flexural strength and the energy absorption behavior of the multiaxis 3D concrete composite. The off-axis flexural strength of the uniaxial (C-1D-(0°)), biaxial (C-2D-(0°), and C-2D-(90°)), and multiaxial (C-4D-(0°), C-4D-(+45°) and C-4D-(−45°)) concrete composites were outstandingly higher (from 36.84 to 272.43%) than the neat concrete. Their energy absorption capacities were superior compared to the neat concrete. Fractured four directional polymeric carbon fiber/cementitious matrix concretes limited brittle matrix failure and a broom-like fracture phenomenon on the filament bundles, filament-matrix debonding and splitting, and minor filament entanglement. Multiaxis 3D polymeric carbon fiber concrete, especially the C-4D structure, controlled the crack phenomena and was considered a damage-tolerant material compared to the neat concrete.

Effect of increased length fraction of Ʃ3n special boundaries on OAIC response of cold rolled Ni-based alloy 718 thin sheets

Among the well-established Ni-based superalloys, alloy 718 is one of the most widely used in the industry. However, its application is limited up to 650°C due to the occurrence of the oxidation assisted intergranular cracking (OAIC) phenomenon. As a general rule, an increased fraction of special boundaries (3 < Ʃ<29) along the grain boundary network, in replacement of the high energy random high angle boundaries (RHABs), is usually associated with enhanced resistance to several intergranular failure mechanisms. In the present investigation, thin sheets of alloy 718 were subjected to three different thermomechanical processing (TMP) routes, in order to manipulate the grain boundary character distribution (GBCD), aiming to achieve an increased fraction of low-Ʃ special boundaries and, consequently, to reduce the alloy's OAIC susceptibility. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were used to characterize the different microstructures and GBCD resulting from TMP. Thin sheet specimens underwent hot tensile tests at 650 °C under secondary vacuum at a strain rate of 3.2 × 10−4 s−1. Fractography analyses were performed to quantify the fraction of brittle areas on the fracture surface. The results indicate that the sample processed through iterative steps of cold rolling and solution annealing, followed by a double aging heat treatment, resulted in a microstructure resistant to OAIC, with 100% of ductile fracture. Such good resistance to OAIC was attributed to the higher percentage of special Ʃ3n boundaries, as well as to the best configuration of grain boundaries network, presenting increased fraction of special triple junctions 3CSL and 2CSL as well as a favorable triple junction ratio (TJR).

Investigation of concrete cracking phenomena by using cohesive fracture-based techniques: A comparison between an embedded crack model and a refined diffuse interface model

Cracking simulation in concrete-like structures is the subject of numerous investigations of computational kind, in which different modeling techniques based on either smeared or discrete fracture approaches have been proposed. In this work, two different finite element-based cohesive fracture models, proposed by some of the authors and belonging to the discrete approaches, have been compared: (i) a novel refined diffuse interface model and (ii) a well-established embedded crack model, which are based on an inter- and intra- element fracture approach, respectively. In particular, the first one is based on an intrinsic cohesive zone formulation, according to which nonlinear interface elements, equipped with a softening constitutive law, are inserted among all the finite elements of the computational domain. The second one, which has been successfully used to simulate concrete cracking along nonprescribed paths and is here used a reference model, combines the cohesive crack concept with the embedded strong discontinuity approach, allowing the cracks to be inserted across finite elements as discontinuities in the corresponding displacement field once a certain stress criterion is satisfied. These two models have been compared by analyzing the cracking behavior in concrete specimens subjected to general loading conditions. A critical discussion of the obtained computational outcomes, in terms of computational efficiency and numerical accuracy for both predicted loading curve and crack pattern, shows the strengths and limits of the investigated discrete fracture models.

Progress and opportunities in modelling environmentally assisted cracking

Environmentally assisted cracking phenomena are widespread across the transport, defence, energy and construction sectors. However, predicting environmentally assisted fractures is a highly cross-disciplinary endeavour that requires resolving the multiple material-environment interactions taking place. In this manuscript, an overview is given of recent breakthroughs in the modelling of environmentally assisted cracking. The focus is on the opportunities created by two recent developments: phase field and multi-physics modelling. The possibilities enabled by the confluence of phase field methods and electro-chemo-mechanics modelling are discussed in the context of three environmental assisted cracking phenomena of particular engineering interest: hydrogen embrittlement, localised corrosion and corrosion fatigue. Mechanical processes such as deformation and fracture can be coupled with chemical phenomena like local reactions, ionic transport and hydrogen uptake and diffusion. Moreover, these can be combined with the prediction of an evolving interface, such as a growing pit or a crack, as dictated by a phase field variable that evolves based on thermodynamics and local kinetics. Suitable for both microstructural and continuum length scales, this new generation of simulation-based, multi-physics phase field models can open new modelling horizons and enable Virtual Testing in harmful environments.

Numerical Simulation of Mechanical Characteristics of a Gas Generator Grain under Temperature Load

In view of the crack phenomenon in the low temperature storage process of a certain type of gas generator, the viscoelastic finite element method is used to study a certain type of adherent cast solid gas generator grain, and the numerical simulation study is completed. The simulation results show that the stress concentration on the inner surface of the star shaped grain is located at the B point of the arc of the star root during the process of cooling and low temperature. At the initial moment of curing stage the rising trend of stress and strain of B point is not obvious, before the end of cooling stage the linear increases with the increase of time. During the low temperature storage, the stress relaxation and creep behavior are not obvious. The maximum stress of the B point is 0.289MPa, and the maximum strain value is 16.881%. The simulation process assumes that more simplification is made, and the actual maximum strain value may have more than 20%, which may lead to the crack phenomenon. It lays a foundation for the optimization of the propellant formula and the structure of the grain.

A unified phase-field model of fracture in viscoelastic materials

The phase-field approach has proven to be a powerful tool for the prediction of crack phenomena. When it is applied to inelastic materials, it is crucial to adequately account for the coupling between dissipative mechanisms present in the bulk and fracture. In this contribution, we propose a unified phase-field model for fracture of viscoelastic materials. The formulation is characterized by the pseudo-energy functional which consists of free energy and dissipation due to fracture. The free energy includes a contribution which is related to viscous dissipation that plays an essential role in coupling the phase-field and the viscous internal variables. The governing equations for the phase-field and the viscous internal variables are deduced in a consistent thermodynamic manner from the pseudo-energy functional. The resulting model establishes a two-way coupling between crack phase-field and relaxation mechanisms, i.e. viscous internal variables explicitly enter the evolution of phase-field and vice versa. Depending on the specific choice of the model parameters, it has flexibility in capturing the possible coupled responses, and the approaches of recently published formulations are obtained as limiting cases. By means of a numerical study of monotonically increasing load, creep and relaxation phenomena, rate-dependency of failure in viscoelastic materials is analysed and modelling assumptions of the present formulation are discussed.

Off-Axis Flexural Properties of Multiaxis 3D Basalt Fiber Preform/Cementitious Concretes: Experimental Study.

Multiaxis three-dimensional (3D) continuous basalt fiber/cementitious concretes were manufactured. The novelty of the study was that the non-interlace preform structures were multiaxially created by placing all continious filamentary bundles in the in-plane direction of the preform via developed flat winding-molding method to improve the fracture toughness of the concrete composite. Principle and off-axis flexural properties of multiaxis three-dimensional (3D) continuous basalt fiber/cementitious concretes were experimentally studied. It was identified that the principle and off-axis flexural load-bearing, flexural strength and the toughness properties of the multiaxis 3D basalt concrete were extraordinarily affected by the continuous basalt filament bundle orientations and placement in the pristine concrete. The principle and off-axis flexural strength and energy absorption performance of the uniaxial (B-1D-(0°)), biaxial ((B-2D-(0°), B-2D-(90°) and B-2D-(+45°)), and multiaxial (B-4D-(0°), B-4D-(+45°) and B-4D-(−45°)) concrete composites were considerably greater compared to those of pristine concrete. Fractured four directional basalt concretes had regional breakages of the brittle cementitious matrix and broom-like damage features on the filaments, fiber-matrix debonding, intrafilament bundle splitting, and minor filament entanglement. Multiaxis 3D basalt concrete, particularly in the B-4D structure, controlled the crack phenomena and it was recognized as a more damage-tolerant material than the neat concrete.

Experimental and Numerical Investigations on Conductive Drying of Phosphate Washing Waste Clay

Morocco is the leading producer of phosphate and its derivatives in the world with a total production of 35 Mt. However, the extraction and the valorization of this mine generate huge quantities of phosphate washing waste clay (PHWWC) that constitute a main environmental and economic concern. To facilitate this waste clay storage and handling, it is necessary to decrease its moisture content that represents 80% of PHWWC. The present paper is devoted to studying the conductive drying of PHWWC. Drying experiments were conducted in a laboratory pilot. Afterwards, the experiment results were implemented in a one-dimensional numerical model of heat and mass transfer in a porous media to identify the drying parameters and performances. It was found that most of the water contained in PHWWC is free water that is removed with a constant drying rate. The volume reduction with a marked cracks phenomenon attained 65% without any significant effect of drying temperature and sample thickness. The effective moisture diffusivity of the PHWWC for a conductive drying process is ranged between 10−9 and 1.1 × 10−8 m2·s−1. The thermal efficiency of the drying system is up to 86%. The results could be used for the purpose of design and scale-up of the industrial dryer based on laboratory-scale experiments.

Finite Element Analysis Of Thermal Fatigue Loading Of Nano Materials Coated Turbine Blade For Critical Applications

Thermal barrier coatings (TBC) have structures which are very complex and are employed in high temperature condition. The phenomenon of cracking is usually noticed at both the Top Coat (TC) and the interface between the bond coat and the thermally grown oxide layer under the Thermal Cyclic Fatigue (TCF) loading. In this study, the cracking behavior in Bond Coat (BC) and interface for TBC were thoroughly determined by using the commercially available ABAQUS/CAE 6.14-3 FEA software. This study also analyzed the key aspects causing the cracking behaviors in TBC. Various crack propagation path in the Bond Coat (BC) could be obtained on changing the thermal wave forms. The beginning location of TC (Top Coat) was independent of the roughness in the interface. The thermally grown oxide layer (TGO) which is thinner at the beginning could increase the service life of TBC. From the results it is evident that in order to characterize the service environment, the thermal fatigue load and the working temperatures have to be designed

Optimization for Connected Steel Plates of Single-Layer Shaft Wall under Blasting Excavation by Numerical Simulation Method

In order to increase the cross-section utilization rate of the deep shaft, a new type single-layer wall with connected steel plates has been invented. The numerical simulation method is used to analyze the coupling interaction between the steel plate and the concrete. The results show that: there are four vibration strengthening regions in each formwork wall. The order from strength to weakness for the regions is as follows: the concrete outside of vertical steel plate > the concrete at the interface between shaft wall and rock wall > the concrete on the upper side of each formwork > the concrete on the lower side of each formwork. There are two potential leakage channels in the vicinity of the connected steel plate, and the channel under the inclined steel plate is a controlled channel. The thickness of steel plate has an important influence on the damage degree and range of concrete. The concrete damage is affected by vertical steel plate, inclined steel plate and thickness combination of themselves. When the project takes the inclined steel plate 10 mm and the vertical steel plate 6 mm, the phenomenon of crack and leakage of the concrete wall is greatly reduced, and the quality of the project is good.

Phase-field modelling for fatigue crack growth under laser shock peening-induced residual stresses

For the fatigue life of thin-walled components, not only fatigue crack initiation, but also crack growth is decisive. The phase-field method for fracture is a powerful tool to simulate arbitrary crack phenomena. Recently, it has been applied to fatigue fracture. Those models pose an alternative to classical fracture-mechanical approaches for fatigue life estimation. In the first part of this paper, the parameters of a phase-field fatigue model are calibrated and its predictions are compared to results of fatigue crack growth experiments of aluminium sheet material. In the second part, compressive residual stresses are introduced into the components with the help of laser shock peening. It is shown that those residual stresses influence the crack growth rate by retarding and accelerating the crack. In order to study these fatigue mechanisms numerically, a simple strategy to incorporate residual stresses in the phase-field fatigue model is presented and tested with experiments. The study shows that the approach can reproduce the effects of the residual stresses on the crack growth rate.

Experimental study on the phenomenon of cracking of a reinforced element in sand concrete

Experimental study on the phenomenon of cracking of a reinforced element in sand concrete

A multilateral study on the FRP Composite’s matrix strength and damage growth resistance

In the paper, fiber reinforced polymer (FRP) composite’s matrix damage resistance was evaluated based on a mechanical behavior of selected materials. Four materials were selected to scientific investigation. Epidian 5 epoxy resin and Epidian 53 epoxy resin were cured using the curing agents with various chemical composition: Z1 triethylenetetramine curing agent and PAC polyaminoamide curing agent. Tensile tests were conducted to obtain strain–stress characterizations, as well as Charpy dynamic impact tests and quasi-statical three-point bending tests were performed to calculate both dynamic and static fracture toughness (KID and KIC, respectively). The relation between the values of fracture toughness and other mechanical properties was discussed. The literature review in the field of polymer materials and adhesives reveals that there is a lack of scientific publications considering the testing results of that type. Therefore, the results obtained from the conducted research brings far-reaching benefits for the practical use of them in these branches of industry where technology based on adhesion phenomenon is dominant or significant. Aerospace and space industry, as well as automotive are the branches of engineering directly interested in the obtained results. The specific motivation of this multilateral matrix-characterization research framework is a need to calibrate the acoustic emission-based elastic wave frequency analysis used in damage identification in composite laminates (FRPs), where matrix cracking and delamination phenomena take place.

Multi-sensing investigation of crack problems for concrete dams based on detection and monitoring data: A case study

Cracks, a major issue in concrete structures, will reduce structural integrity, stability and impermeability. In this paper, a comprehensive site crack status monitoring system was constructed in a roller compacted concrete (RCC) gravity dam during the construction period, by combining acoustic emission (AE), micro-seismic (MS), cross-hole acoustic wave detection (CAWD), crack gauge and strain gauge techniques; this system was used to investigate crack development problems that may occur under varying construction conditions. Through knocking location tests, the acoustic wave transmission rate of RCC was determined by the AE and MS system, to monitor the real-time status of micro-cracks. Moreover, the crack tip displacement, adjacent concrete strain and concrete ultrasonic velocity were captured, to monitor the propagation of macroscopic cracks. By combining the information obtained at both the micro- and macro- levels, the development trend of a crack was analyzed. Results show that micro-crack signals were captured, but the sensitivity of the AE approach was relatively smaller than that of MS method, and the crack status was in the structural internal stress adjustment stage. On the macro-level, the macroscopic expansion phenomenon of the original crack was not captured by the crack gauge, strain gauge and acoustic penetration. Based on the size, the concentration of micro-cracks and the development trend of the macro-cracks monitored by the multi-sensing system, the crack behavior can be evaluated in huge concrete infrastructures.