Cracks In Metals Research Articles

Mechanisms of Cracking in Laser Welding of Magnesium Alloy AZ91D

Considerable research has been carried out to study the laser welding of magnesium alloys. However, the studies are mainly devoted to butt welding, and there has been limited information in the published literature concerning the bead-on-plate laser welding of AZ91D, even though bead-on-plate welding is required for the repair of cast AZ91D parts with surface defects. In the present investigation, surface cracking of the weld metal was observed when an AZ91D magnesium alloy was bead-on-plate welded using the laser welding method. This paper presents the experimental results and analyses to show that the cracking is “solidification cracking” initiated from the weld surface under high thermal stresses. This is in contrast to the “liquation cracking” observed in heat affected zones in tungsten inert gas welding of the same magnesium alloy. Laser power was found to be one of the main factors affecting the distance of the crack propagation. The higher laser power resulted in longer crack propagation distance into the weld metal. It is demonstrated that hot cracking could be avoided by lowering the laser power and welding speed.

Analysis of liquation and solidification cracks in the electron beam welding of GTD-111 nickel-base superalloy joint

The weldability of GTD-111 superalloy by electron beam welding was studied, and the cracking evolution of solidification and liquation, as well as microstructural characteristics, were also investigated. The weld metal microstructure consisted mainly of the inter-dendritic phases of MC, γ-γ′ and γ′ nanoparticles, primarily responsible for the solidification crack in the weld metal. In addition to solidification cracks, the formation of liquation cracks in HAZ was an important challenge with electron beam welding. It was found that M5B3, γ-γ′, MC and γ′ phases leave formation during the increase of non-equilibrium temperature in HAZ, with the formation of partial liquation in the phase-matrix interface and absorption of shrinkage stresses due to welding cooling. Both solidification and liquation cracks decreased as the heat input increased due to the increase in beam current. This was attributed to the reduction of cooling stress and the increase of backfilling capacity.

Fracture of Ni-Cr-Si-B thermal sprayed and fused reciprocating pump rods during straightening

A straightening fracture of an AISI 1045 pump rod with a fused coating by thermal spray welding was investigated. The fracture surface was surveyed for pinhole-related defects in the coating, debonding at the coating/substrate interface, and fracture initiation points. The base metal and coating were also examined using an optical microscope, scanning electron microscope with energy dispersive spectroscopy, tensile testing, and microhardness testing. The base metal rod experienced softening due to the thermal effect of spray welding and fusion. Large residual stress has been found in the outer coating layer using finite element analysis. The open-to-surface crack in the base metal was found to be partially filled by the hard coating. The brittle fracture is believed to be initiated at defects near the coating/rod interface and driven by a combined loading of thermal residual stresses and the applied straightening stress.

Neural network segmentation methods for fatigue crack images obtained with X-ray tomography

Synchrotron X-ray tomography allows to observe fatigue crack propagation during in situ tests. Accurately segmenting the 3D shape of the cracks from the tomography image is essential for quantitative analysis. Fatigue cracks have small openings which result in low contrast images making crack segmentation difficult. Phase contrast available at synchrotron sources improves crack detection but it also increases the complexity of the image and human intervention is generally used to help traditional segmentation methods. In this work, an image segmentation method based on a convolutional neural network is developed to replace the user interpretation of images. Combined with a ‘Hessian matrix’ filter, this method can successfully extract 3D shapes of internal fatigue cracks in metals.

Numerical and experimental investigation on the oblique successive impact behavior and accumulated damage characteristics of fiber metal laminates

This paper mainly investigates the dynamic response and accumulated damage characteristics of fiber metal laminate (FML) under successive impact loads through numerical and experimental methods. Firstly, the experimental tests are conducted to measure the successive impact response and explore the damage morphologies of FML under four times impacts of 10 J. Then, an integrated successive numerical model is developed to predict the impact behavior and characterize the failure mechanisms of FML, whose accuracy and reliability are well demonstrated. Subsequently, in virtue of the developed numerical model, different oblique successive impact loads are implemented on FMLs to characterize the interface delamination evolution and energy absorption. And the damage tolerances of FMLs under different impact angles are further simulated and compared. The conclusions indicate that the stiffness of FML and impact force experiences an evident enhancement after the first impact because of the strain hardening phenomenon of metal material. In addition, according to the number of impacts, the damage tolerance of FML can be improved at the greater impact angle. And for the last impact event, the metal crack propagation, the further fiber/matrix damage and even the penetration of metal layer will dissipate more successive impact energy.

Pipeline In-Line Inspection Method, Instrumentation and Data Management.

Pipelines play an important role in the national/international transportation of natural gas, petroleum products, and other energy resources. Pipelines are set up in different environments and consequently suffer various damage challenges, such as environmental electrochemical reaction, welding defects, and external force damage, etc. Defects like metal loss, pitting, and cracks destroy the pipeline’s integrity and cause serious safety issues. This should be prevented before it occurs to ensure the safe operation of the pipeline. In recent years, different non-destructive testing (NDT) methods have been developed for in-line pipeline inspection. These are magnetic flux leakage (MFL) testing, ultrasonic testing (UT), electromagnetic acoustic technology (EMAT), eddy current testing (EC). Single modality or different kinds of integrated NDT system named Pipeline Inspection Gauge (PIG) or un-piggable robotic inspection systems have been developed. Moreover, data management in conjunction with historic data for condition-based pipeline maintenance becomes important as well. In this study, various inspection methods in association with non-destructive testing are investigated. The state of the art of PIGs, un-piggable robots, as well as instrumental applications, are systematically compared. Furthermore, data models and management are utilized for defect quantification, classification, failure prediction and maintenance. Finally, the challenges, problems, and development trends of pipeline inspection as well as data management are derived and discussed.

Ensuring the reliability of metal smoke and veintilation pipes and their carrying towers

The problem of ensuring the reliability of the work of metal smoke and ventilation pipes and their bearing towers has recently gained special significance in connection with the large number of accidents at industrial enterprises.
 Methods of estimation of residual resource of bearing elements of high-rise buildings with various damage detected during diagnostics are developed. The formulas for determining the residual life of metal chimneys are given, the actual reserves of the residual resource are determined after long periods of operation for specific structures.
 In this work, variants of reinforcement of the elements of chimneys and towers with the most dangerous injuries (category A) – gaps, with cracks in the main metal and welds, with loss of the overall stability of the trunk of the pipe, have been developed and substantiated. On the basis of the analysis of the summation of internal and external stresses, we obtain formulas for calculating the elastic-deformation state of metal chimneys in the zone of through cracks and holes when performing construction reinforcement works by means of welds for overlays.
 The variants of amplification of smoke pipes after the destruction of stops of towers, which perceive wind loads, and as consequences of the loss of overall stability of the pipe are given.
 The research results make it possible to assess the reliability of metal smoke and ventilation pipes and their bearing towers after long operating periods, as well as used in the development of regulatory documents when assessing the technical condition and residual life of high-rise buildings.

Highly sensitive and stretchable strain sensor based on self-aligned and periodic cracking of wavy metal nanowire/elastomer composite film

In this work, we present a new class of stretchable strain sensor employing a wavy metal nanowire (mNW)/elastomer composite film. Stretching-induced strain is predominantly concentrated in the valley regions of the wavy composite film following thermal annealing, and self-aligned and periodic mechanical cracks are induced by the strain concentration effect therein. The strain-dependent opening-and-closing process of the cracks supported by the mNW percolation network allows the sensor to exhibit large stretchability (160%) and high mechanosensitivity (gauge factor ≈874.1 at 110%–160% strain) simultaneously. Significant hysteresis is not observed, even when subjected to repeated stretching cycles. The wavy strain sensor is also demonstrated as a motion detector to recognize finger joint motions of a wooden hand model in real time.

A metal/UHMWPE/SiC multi-layered composite armor against ballistic impact of flat-nosed projectile

A novel multi-layered composite armor system is proposed for ballistic protection in the present study. The composite armor is composed of a ductile metal front, followed by a ceramic/UHMWPE laminate composite as the intermediate layer, and a ductile metal back layer. The ballistic performance of the composite armor against flat-nosed projectile was investigated experimentally and numerically. Experimental results show that the proposed composite armor exhibited several failure modes, including ductile hole enlargement of metallic face sheets, fragmentations and cracks of the ceramic layer, delamination, fiber fracture and bulge deformation of UHMWPE laminates. Three-dimensional numerical model was established to analyze the evolution of the whole ballistic response, and to discuss the effect of the ceramic layer placement and the mass allocation between the ceramic layer and UHMWPE laminate layer on the ballistic performance. Simulation results reveal the evident reduction in residual velocity that results from the optimal placement of the ceramic layer. Good balance among the contributions of the target components can be achieved to maximize the total energy absorption of composite armor by optimizing the ceramic placement strategy. The projectile residual velocity and the total energy absorption are insensitive to the mass ratio of ceramic layer to UHMWPE laminate layer within a certain range. Under the condition of a higher mass ratio, the specific energy absorption of UHMWPE layer can be significantly improved due to the full development of its bulging deformation. Consequently, it would benefit the energy absorption capability of the composite armor.

Columnar grain-driven plasticity and cracking in nanotwinned FCC metals

The mechanisms of strengthening and plasticity in columnar-grained metals with preferentially oriented nano-sized twins have been examined traditionally by considering dislocation processes, but rarely from the perspective of grain boundary (GB) deformation. Here, the effects of GB strain accommodation on plastic deformation in four different columnar-grained nanocrystalline nanotwinned (nt) face-centered-cubic metals (Cu, Ag, Al, and Ni) were studied by large-scale molecular dynamics simulations. It is observed that in tensile deformation parallel to coherent twin boundaries (CTBs), the dislocation mechanisms in each metal are identical and associated with GB emissions of jog and threading dislocations at small and large CTB spacings, respectively. However, CTB strengthening effects are increasingly more pronounced in columnar-grained nt metals as their shear modulus increases, which is rationalized by the dependence of GB stress concentrations on twin size, metal type and strain rate. Also, while flow stresses in nt-Cu, nt-Ag, and nt-Al metals increase linearly with decreasing CTB spacing, a maximum strength limit is reached in nt-Ni below a critical CTB spacing of 6 nm. The strength limit in nt-Ni results from columnar GB cracking induced by prominent GB sliding. For columnar-grained microstructures, GB sliding is equivalent in nt-Ag and nt-Al and slightly lower in nt-Cu but markedly higher in nt-Ni. These findings underscore the importance of new GB deformation mechanisms on plasticity and fracture in columnar-grained nt metals and enrich our understanding of CTB strengthening in fcc metals synthesized in the literature.

Investigations on the weld metal composition and associated weld metal cracking in laser beam welded steel copper dissimilar joints

Cracking as one of the most common weld defects for joining dissimilar metals can lead to challenges as lack of mechanical properties, lack of joint tightness, lowered electrical conductivity or can be a point of attack for corrosion. Joints between stainless steel and copper also suffer from weld metal cracking which is caused by the effect of liquid copper on the microstructure of solidifying steel matrices. The crack formation is dependent on the dilution of copper in the weld metal, the overall metal intermixing and thermal stresses while cooling down. To investigate the crack susceptibility of laser beam welds with that material combination, lap joints with stainless steel on the top side were welded with a wide range of parameter settings to create different weld metal compositions. The joints were evaluated regarding metal intermixing, cracking, micro hardness and shear tensile strength by metallographic cross sections, x-ray spectroscopy line scans, x-ray spectroscopy element mapping and shear tensile tests to create sufficient datasets for analyses. The results show a correlation between the mixing ratio of copper and the formation of hot cracks with a critical amount of approximately 10 w co .% copper in the fusion zone. Based on that, a hypothesis in order to reduce cracks by decreasing the amount of copper in Fe-rich weld metals or to create copper dominated weld metals was put forward. The crack formation has no influence on the tensile shear strength, which is dependent on the connection width reached between the sheets with a maximum shear strength of 9.6 kN at full penetration of both sheets and shows ductile fraction behavior. Consequently, the crack formation in this welding configuration at copper contents above 10 w co .% has to be weighed against the better mechanical properties at higher connection width and mixing ratios for joining stainless steel and copper. Hardness measurements of the weld metal show results mostly independent of the overall mixing ratio with median values of 170 HV0.1.

Orthogonally polarised shear waves for evaluating anisotropy and cracks in metals

The detection of crack-like defects using ultrasound is a problem widely studied by the non-destructive evaluation community. In this work, a technique of how to detect the presence of a crack-like defect and estimate its orientation in an anisotropic material by means of two orthogonal shear waves is presented. A detailed study of shear wave propagating and splitting at any incident angle, and its interaction with crack-like defects in anisotropic materials, is also presented. In this paper it is shown that by the use of two orthogonal shear waves with linear polarisation the authors could: 1) estimate the material level of anisotropy within approximately 0.5% of the measured value; 2) estimate the angular position of the principal shear directions of the material with respect to the orthogonal shear wave sources within ±5∘ accuracy; 3) excite a shear wave purely polarised along one of the principal shear directions with two orthogonal shear waves placed at an arbitrary angular position; 4) detect the presence of a crack-like defect in an anisotropic material based on the back-wall echoes of two orthogonal shear waves, 5) estimate the orientation of a crack-like defect in an anisotropic material within ~±5∘ of the theoretical value. The simulation and experimental results obtained show that the technique presented in this paper has potential to become an effective tool for crack-like defect detection and material characterisation.

Influence of absorptivity of the material surface in crack detection using laser spot thermography

Laser thermography (LT) is one of the novel NDT methods for detecting surface-breaking cracks in metals. LT technique is well known for its non-contact nature and relatively fast-detection of surface defects for objects that are at room temperature. In LT, a continuous-wave (CW) laser is used to make a laser spot/line/area, which excites the sample under inspection to create local heating. Using optical mirrors, the spot is then rapidly made to move over the surface of the test sample. The temperature distribution over the sample due to the laser excitation is then monitored using an infrared (IR) thermal camera. This paper investigates the influence of the absorptivity of the material surface during defect detection using LT and its influence in detecting surface-breaking cracks. Six different materials were used under this study, i.e., Aluminum, Brass, Copper, mild Steel, Stainless Steel, and Titanium and with two surface conditions i.e., with and without black paint. 3D FEM numerical models were developed and validated with experiments for the different case studies for surface crack detection. The term Crack Thermal Contrast (CTC) is introduced to indicate the detectability of the surface crack and found to increase linearly with absorptivity of the material.

Fatigue experimental characterization of preloaded injection bolts in a metallic bridge strengthening scenario

All over the world, the number of civil engineering structures, particularly bridges with long service periods, has been increasing. The most common evidences of damages are the presence of corroded metallic elements and cracks in structural details due to the fatigue phenomenon. A large number of cases were found in which fatigue cracks were detected in structural details, such as riveted connections. Different strategies can be implemented for repairing and strengthening operations of old metallic riveted bridges. However, the use of injection bolts has been considered as an alternative with important advantages. In this regard, it is essential to study their performance under fatigue loading. In this investigation, an experimental campaign has been performed to assess the fatigue strength of injection bolts by means of comparison with standard bolts. These fatigue tests are defined to be representative of a structural strengthening scenario of an old metallic bridge. Experimental results show that injection bolts contribute to significantly reduce the scatter in the data related to fatigue resistance. For double shear specimens, the characteristic curve proposed for connections with injection bolts presents a detail category with 15% higher value when compared to non-injected specimens. The beneficial effect is also verified in terms of slip deformation. For single shear specimens, the overall effect of the adhesive is not clear.

The Study of Eddy Current Sensors Signals over a Group of Cracks in Metal

We studied corrosion fatigue cracks of operational origin in a steel gas pipeline and obtained a number of statistical data on their distribution on the metal surface.This information is important for eddy current flaw detection of steel gas pipelines with stress-corrosion cracks. These data were used for making specimens of crack groups in special steel samples. Experimental studies of the signals of eddy current sensors over a group of cracks were carried out using these specimens.Results of experimental studies of the signals of eddy current sensors of various types (absolute, differential) with different diameters of ferrite cores over specimens of crack group in special steel samples that are located at different intervals are presented. The research results show that the waveform of eddy current sensors undergoes significant transformation when changing the parameters of the group of continuity defects in the metal.In the case of an absolute surface eddy current sensor additional local maxima signals, which are located over the center of continuity defects in a group of cracks, appear. In the case of differential surface eddy current sensor additional local extrema (maxima and minima) signals appear. These extrema are associated with the number and location of continuity defects in a group of cracks. It is shown that the waveform of the surface eddy current sensor over a group of cracks is determined by its type and parameters of a group of cracks in the metal. Mathematical conditions that make possible resolution or non-resolution of the signal of the surface eddy current sensor in the space of a group of cracks in the metal are found.It is shown that when the defects in a group of cracks approach each other, a significant increase in the amplitude and half-width of the signal of the surface eddy current sensor occurs. The half-width of the signal of the surface eddy current sensor depends on the interval between the defects of continuity and their number in the group of cracks.

Analysis of Cracking Causes of P91 Welding Seam of High Pressure Steam Guide Pipe of Supercritical Unit

A 600MW supercritical unit in a thermal power plant has been in operation for 63,000 hours, and a P91 weld leak occurred between the high-pressure steam guide pipe and the high-pressure governor valve. After macroscopic morphology analysis, alloy composition analysis, metallographic structure observation, and microhardness test analysis, it is found that the cracking of the base metal and weld on the high-pressure speed control valve is caused by structural stress and welding stress, which leads to cracking due to creep. The crack originated in the fine-grained area of the heat-affected zone, and it has been operating in a high-temperature operating environment for a long time. The P91 weld on the high-pressure steam guide tube produces creep holes in the fine-grained heat-affected zone, which gradually grow to form creep cracks, and eventually lead to cracks and leakage. Therefore, the failure of pipeline cracking is caused by the joint action of tensile stress and type IV crack.

Creep Rupture Behavior for Welded Pipe of Ni-Based Alloy Using Full Thickness Specimen and Application of Nondestructive Damage Detection

Creep rupture behavior of HR6W weldment using full thickness specimen cut from the circumferentially welded pipe was investigated for evaluating its applicability to advanced USC (A-USC) power plant. Creep tests were conducted at 750°C for durations up to 8,600 hours, and damage morphology of weldment during creep was characterized. The applicability of several nondestructive detection methods to the creep damage evaluation was discussed. It was found that full thickness specimen was broken at the base metal and main crack was inclined approximately at 45 degrees to the axial direction of the specimen. Based on stress at the ruptured location, times to creep rupture of full thickness specimen were comparable with those of the smooth bar specimen of base metal. Among several nondestructive detection methods, a small crack in base metal on the outer surface was first observed at life fraction of 35% by replication. PT can detect the crack up to 0.5mm in length in about half of the life. The crack whose length is longer than 3mm can be detected by UT in latter half of the life.

Correlation between Microstructure and Hydrogen Degradation of 690 MPa Grade Marine Engineering Steel.

Electrochemical H charging, hydrogen permeation, and hydrogen-induced cracking (HIC) behavior of 690 MPa grade steel substrate and different heat-treatment states (annealed, quenched, normalized, tempered) are investigated by cyclic voltammetry (CV), hydrogen permeation, electrochemical H charging, and slow strain rate tensile test (SSRT). The results show that hydrogen diffuses through the steel with the highest rate in base metal and the lowest rate in annealed steel. The hydrogen-induced cracks in base metal show obvious step shape with tiny cracks near the main crack. The cracks of annealed steel are mainly distributed along pearlite. The crack propagation of quenched steel is mainly transgranular, while the hydrogen-induced crack propagation of tempered steel is along the prior austenite grain boundary. HIC sensitivity of base metal is the lowest due to its fine homogeneous grain structure, small hydrogen diffusion coefficient, and small hydrogen diffusion rate. There are many hydrogen traps in annealed steel, such as the two-phase interface which provides accommodation sites for H atoms and increases the HIC susceptibility.

Hydrogen enhanced cracking via dynamic formation of grain boundary inside aluminium crystal

By in situ bending tests inside an environmental transmission electron microscope, we found that from the pre-crack tip of single crystalline aluminium cantilevers, dislocations are emitted and self-organized to build a low angle grain boundary (LAGB), along which the crack propagates more easily in hydrogen atmosphere than in vacuum. Atomistic modelling suggests that the LAGB formed can strongly trap hydrogen atoms, and its cohesive strength can be substantially reduced. These results imply that the hydrogen-induced transgranular crack in a polycrystalline metal can proceed by repeating a process of dynamically forming a hydrogenated LAGB and its subsequent separation in grain.

Interface characteristics and reaction mechanism of steel/Al welds produced by magnetic field assisted laser welding-brazing

In this paper, we describe experimental laser welding-brazing aluminum alloy to galvanized steel using an external magnetic field. The morphology, microstructure and phase transition at interface was carefully investigated using scanning electron microscope, energy diffraction spectrum and X-Ray Diffractometer. The reaction mechanism of steel/Al welds was also discussed. The results showed that the banded structures with zigzag pattern morphology were produced near the steel matrix. The thickness difference at interface was closely associated with characteristics of laser heat source. The introduction of Zn element in filler metals induces the presence of η-Al5Fe2Zn0.4 phase and δ-FeZn10 phase at the steel interface. These non-brittle phases have good ductility, which can effectively reduce the susceptibility of hot cracking of weld metal and improve the behavior of interface bonding. Meanwhile, fine columnar crystals and eutectic structures formed near the Al matrix as the result of dissolution of Al matrix and regulation of the ampere force. In addition, it has been also found that the metallurgical reaction at interface can be mainly divided into three stages: the solute transfer process, the initiation and the growth of non-brittle phases. The driven effect of magnetic field on molten pool can effectively control the size and distribution of non-brittle phases such as Zn-Al eutectic phases, greatly improving the interfacial performance.