Dent Depth Research Articles

An Assessment of the Residual Stress of Pipelines Subjected to Localized Large Deformations

Subsea pipelines subjected to impacts are prone to generating significant deformations and residual stresses, which could reduce their structural integrity and increase the risk of failure. This paper introduces analytical and numerical frameworks aimed at predicting residual stress behavior induced by subsea pipeline impacts. An empirical formula to regulate residual stress levels in extensively deformed submarine pipelines is derived through a parameter-fitting method. This formula enables the swift and accurate computation of the residual stress magnitudes in such pipelines. Abaqus is employed to simulate the residual stresses in large-deformation submarine pipelines. The results of the finite element analysis are validated through experimental work. A comprehensive database is constructed via the finite element method to fit an empirical formula for residual stresses in large-deformation submarine pipelines. The empirical formula places particular emphasis on the influence of the diameter-to-thickness ratio and dent depth on the residual stresses. It is crucial for pipeline design and maintenance.

Experimental and analytical investigations on single and repetitive impact failure responses of composite sandwich panels with orthogonal woven GFRP facesheets and PVC foam cores

Due to the unique design requirements for high load-bearing capacity of ship structures, composite materials used in the marine industry are gradually being developed with the characteristics of multi-layer sandwich structures with large thickness and cross-section, which are different from thin-walled composite materials widely applied in the aeronautical industry. However, sandwich composite materials used in marine structures have high susceptibility to impact event during in-service applications. The impact induced damage may seriously affect their mechanical performance and structural safety. Therefore, a comprehensive investigation in this paper on the failure mechanisms of composite sandwich panels with orthogonal woven GFRP facesheets and a PVC foam core layer is carried out with a series of single and repetitive low-velocity impact tests. The variations of impact force, dent depth, structural stiffness, failure modes, energy absorption and so on of composite sandwich panels against impact energy levels and impact numbers were explored. The results demonstrate that the delamination damage threshold at the first impact, the sudden drop of peak force and the slow descent process of impact force are the three typical characteristics of impact responses that corresponded to delamination initiation and fibre breakage of the upper panel, and compression damage of the core layer, respectively. The accumulated impact-induced damage has a significant negative effect on load-bearing and energy-absorption capabilities of composite sandwich panels. Moreover, an approximate theoretical analytical method is presented to solve the impact resistance of composite sandwich panels. The analytical results are compared well with experimental results. This research provides a detailed understanding of the damage mechanisms of composite sandwich panels under impact loadings and a guidance for impact resistant design of ship protective structures.

A phase field method for predicting hydrogen-induced cracking on pipelines

An accurate determination of the threshold conditions to initiate cracks on aged hydrogen pipelines is paramount for ensuring energy transport safety. In this work, a finite element-based phase field method was developed to assess the crack initiation on dented pipelines while considering the hydrogen (H) impact. Theoretical and multi-physics numerical formulas were derived for prediction of the elastic-plastic fracture behavior of H-contained steel. A critical phase field parameter, ϕ=0.69, is defined for predicting crack initiation at the dent on pipelines. The presence of H within the steel decreases the threshold dent depth for initiating H-induced cracks. When the initial H concentration increases from 0 to 0.5 wppm, the maximum dent depth for crack initiation reduces from 17.5 mm to 10.7 mm. The maximum dent depth required for crack initiation reduces from 17.5 mm to 7.8 mm when an internal pressure of 8 MPa is applied on the steel pipe. The site with the maximum phase field parameter changes during indentation, implying that the location initiating cracks depends on the dent dimension. The existing criteria in ASME B31.12 standard are not applicable for predicting H-induced crack initiation on dented pipelines. This study proposes a new method to predict hydrogen-induced cracking on aged pipelines when transporting hydrogen.

A model-based framework to prevent excavator induced damage in operations on natural gas pipelines

Despite the presence of numerous safety precautions, the primary cause of damage to underground pipes is often attributed to external interference from excavators during their operations. To address this issue, this paper introduces a model-based approach that incorporates both static and dynamic contact in assessing the damage caused by excavators on pipes. The objective of this method is to offer a practical tool that can assist in determining the appropriate excavator size for buried gas pipes. By identifying the maximum safe excavator size, the aim is to minimize the potential risks of mechanical damage to the pipe, to prevent hazards for operators and to protect the integrity of the pipeline. For this purpose, a comprehensive excavator model is constructed. Following this, a suitable damage model for the pipe is selected and the interaction between the bucket tooth and the pipe is modelled. To enhance accuracy, the excavator and pipe are interconnected through the solution of damped contact equations, which take into account the stiffnesses of both the pipe and excavator, as well as the damping effect resulting from pipe deformation. Results provide valuable insight into the potential damage caused to the pipe, which can be attributed to either static or dynamic contacts, depending on which excavator is being used. Failure is addressed by plastic dent, that can be avoided by selecting the most suitable excavator size. Moreover, the analysis of dent depth caused by various excavators on different pipes for gas transmission, opens to the possibility of mitigating the risk of failures by limiting the excavator workspace.

Research on damage assessment of buried pipelines with circular dent defects subjected to blast loading

In order to evaluate the damage characteristics of buried natural gas pipelines with circular dent defects subjected to blast loading, based on pressure–impulse damage theory, explosion experiment and numerical simulations were implemented to evaluate the damage of a natural gas with circular dent defects buried in soil under blast loading in this research. The ALE method was used to develop a coupled pipeline-soil simulation model using LS-DYNA software, and the validity of the established model was verified correctly compared with experimental results and empirical theory equations calculations. Furthermore, according to regulations in the Assessment and Management of Pipeline Dents (American Petroleum Institute Recommended Practice 1183), the effect with different circular dent defect diameter on the mechanical properties of natural gas was also investigated and analysed. The results showed that, under the blast loading, plastic deformation happened on the surfaces of the pipeline facing the explosive, and the high-stress zone appeared in the circumferential direction with 32.27°. The range of the high-stress zone firstly expanded along the axial direction of the pipelines and then along the circumferential direction. The larger diameter of the circular dent defects had, which resulted in a failure at the defects under the condition of the depth of the defects keeping constant. The effect of the size of defect diameter on the amount of pipeline deformation was further investigated, and the mathematical formula described the maximum plastic deformation with different defect diameter was established. Meanwhile, a finite element model of pipeline with a diameter of 457 mm was established for numerical analysis, which implied that the larger diameter of pipeline with the same defect had, the lower risk of pipeline in failure subjected to blast loading had. And through mathematical analysis and considering the feasibility of the actual situation, the curve described the maximum plastic deformation of the natural gas pipeline with three different defects varying in diameter were established respectively. In addition, a formula to express the relationship between pipeline defect diameter and maximum plastic deformation was established, which can effectively predict the critical plastic dent deformation of pipelines with different defect diameter subjected to blast loading. Besides, based on the pressure-pulse damage theory and with the damage assessment criterion of dent depth–dent length ratio of 0.072, the pressure–impulse diagrams of buried natural gas pipelines with defect diameters of 30 mm, 40 mm and 50 mm were defined, which can be used to predict the damage of pipelines with different defect diameters. Moreover, with the pressure–impulse damage evaluation curves, mathematical formula for the buried natural gas pipeline with the circular defects were established. Furthermore, the critical circular defect diameters with 30 mm was confirmed and the unique formula was also established, which could be effectively used to evaluate the safety of buried natural gas pipelines with the critical circular defects.

Experimental investigation on the low-velocity impact responses of fibre metal laminates with various internal and external factors

This paper aims to investigate the low-velocity impact (LVI) failure mechanism of fibre metal laminates (FMLs), and systematically explore the effects of internal factors (layup sequence and laminate configuration) and external factors (impact energy and environmental temperature) on the LVI responses. The drop-weight impact tester was utilised to conduct LVI tests at -30 °C, 25 °C, and 80 °C on FML-2/1 and FML-3/2 laminates. These laminates were made of S-class high-strength glass fibre and 2024 aluminium-alloy sheet with unidirectional, angle-ply, cross-ply and quasi-isotropic layup sequences. The characteristic curves including contact force-time, contact force-deflection, absorbed energy-time, and strain-time near the dent on the impacted and non-impacted specimen surfaces were obtained respectively. Furthermore, the dent depth of the impacted surface of the FMLs was measured. FML's damage was detected by ultrasonic C-scan, and its longitudinal and transverse sections were scanned by X-ray computed tomography (CT). The findings suggest that the layup sequence has a significant effect on the LVI response of FML-2/1, but has no obvious influence on FML-3/2. Moreover, FML-3/2 exhibits greater impact resistance compared to FML-2/1. The severity of LVI damage increases with the increase of impact energy. Notably, compared to the cases at 25 °C, the LVI failure mechanisms of FMLs undergo significant changes at -30 °C. The elevated temperature of 80 °C significantly affects the LVI damage of FML-2/1, while it has no significant effect on FML-3/2.

The impact‐induced deformation characteristics of polycarbonate panels

AbstractIn this study, the investigation of damage caused by a spherical steel projectile on the polycarbonate (PC) panels has been undertaken both numerically and experimentally. The material properties are obtained from both static and dynamic tests. High‐velocity impact experiments are performed using a pressure‐based projectile launching system. The effect of panel curvatures and impact locations on the deformation are investigated experimentally. The impact locations are selected as both at the plate center and near the edge of the plate. A validation study is performed by analyzing the impact response of a PC plate as previously presented in the literature and subsequently comparing it with the results obtained. An analysis of the impact behavior of PC panels is carried out through numerical simulations using finite element program (LS‐DYNA). The effect of boundary conditions on the impact response is also investigated numerically. Plastic strains, von Mises stresses, dent depth, and energy absorption are obtained. It is indicated by the results that the deformation behavior of PC panels exhibits a strain rate dependence. Additionally, the deformation behavior is influenced by panel curvatures and boundary conditions, emphasizing the necessity to consider these factors in the design of structures, such as canopies made of PC panels.Highlights Provides a comprehensive understanding of polycarbonate behavior. Emphasizes the influence of impact locations and panel curvatures. Dent depth and stress rise with panel curvature, crucial for design considerations. Emphasizes the significance of strain rate and boundary condition in design.

Low velocity impact and compression after impact of thin and thick laminated carbon fiber composite panels

This study explores the impact behavior of thin and thick composite panels, yielding insights into their behavior under increasing impact energies. Thin composite specimens demonstrated limited surface damage tolerance, while thicker panels remained visually intact but suffered internal damage, adversely affecting their compressive load capacity. The study shows a robust correlation between impact energy and key measurements, including dent depth, bottom surface matrix splitting, and internal delaminations. Moreover, the research identified a consistent pattern in damage initiation, showing that the onset of matrix splitting and delamination remained relatively constant regardless of increasing impact energy, emphasizing the predictability of damage initiation. Compression after impact study showed distinctive responses between thin and thick composites. Thin composites exhibited global buckling before final failure, with a gradual reduction in peak load-carrying capability as damage escalated. In contrast, thick composites suffered substantial damage and delamination at higher impact energies, leading to significant losses, up to 60%, in residual compressive load capacity. The study introduces a simplified quasi-static model to capture delamination effects on panel responses to low-velocity impacts. This study contributes significantly to understanding composite materials’ impact behavior, providing essential knowledge for practical applications and future research endeavors.

Effect of surface hemispherical dent depth on the microstructure evolution and stress corrosion cracking of heat transfer tubes in steam generator

The deformation mechanism and SCC behavior of alloy 690TT with dents are investigated. Dents lead to stress concentration and microstructure changes, increasing stress corrosion cracking (SCC) sensitivity. The deformation mechanism at dents is slip and twinning. The dent shoulder zone experiences oxidation due to the residual tensile stress, whereas the dent bottom zone induces SCC due to the stress concentration, exhibiting a symmetrical distribution of “splayed character”. There is a critical threshold exists for dent depth that induces SCC, and the length of SCC cracks is positively correlated with dent depth in a power function.

Model-experiment comparison of quasi static loading on a composite overwrap pressure vessel (Part 2)

A methodology to develop a 3D Finite Element (FE) model of a full metal-lined Composite Overwrapped Pressure Vessel (COPV) was developed and is presented in this paper. The model is intended for prediction of the metal-composite delamination and residual dent depth developed in the metal liner as a result of quasi-static indentation loading. Cohesive elements are used to model the composite and the metal-composite interfaces. Experimental and numerical comparisons of force–displacement curves, delamination area and residual dent depth are presented. Numerical results are in good agreement to the experimental data.

Low-Velocity Impact and Post-Impact Residual Flexural Properties of Kevlar/EP Three-Dimensional Angle-Interlock Composites.

In this study, five three-dimensional angle-interlock fabrics with different warp and weft densities were fabricated using 1000D Kevlar filaments. The Kevlar/EP composites were prepared by vacuum-assisted molding techniques. The low-velocity impact property of the composite was tested, focusing on the effects of the warp and weft densities, impact energy, impactor shape, and impactor diameter. The damage area, dent depth, and crack lengths in the warp and weft direction were used to evaluate the impact performance, and the specimens were compared with plain-weave composites with similar areal densities. The dominant failure mode of the conical impactor was fiber fracture, while the dominant failure mode of the hemispherical impactor was fiber-resin debonding. The cylindrical impactor showed only minor resin fragmentation. The residual flexural strength of the composite after impact was tested to provide insights into its mechanical properties. The study findings will provide a theoretical basis for the optimization of the design of impact-resistant structures using such materials and facilitate their engineering applications.

A novel approach to enhance the mechanical behavior of glass epoxy laminates with sustainable recycled milled fillers

AbstractThe aerospace, automotive, and energy industries can use fiber‐reinforced composites due to their high specific strength and stiffness. However, unexpected external impacts in work cause internal damage and residual strength loss in composite structures. Another ongoing issue is the sustainable management of composite materials' end‐of‐life. The quasi‐static indentation properties of glass epoxy composites are investigated in this study using different kinds of novel filler materials that are recycled from old composite materials: recycled milled carbon (rmCF), recycled milled Kevlar filler (rmKF), and an innovative hybrid filler (rmBF) composed of recycled milled carbon and Kevlar fillers (rmBF). Surface roughness and hardness studies assessed the quasi‐static indentation (QSI) response of neat glass epoxy and glass epoxy composites loaded with fillers using indentation force, absorbed energy, residual dent depth, and evolution of damage area. Compared to the baseline samples, the rmCF composites performed exceptionally well, showing a 16% increase in peak force, a 59% decrease in damaged area, and a 6.32% and 22.5% reduction in surface roughness, respectively (Ra and Sa). After QSI tests, rmC filler samples had 26.8% higher residual flexural strength than neat glass epoxy composites in a three‐point bending test. Through toughening mechanisms like filler interlocking and bridging, micrographs captured using a scanning electron microscope (SEM) showed that recycled fillers provide a higher level of stiffness, better energy dissipation, less damaged area, and overall improved hardness.Highlights Recycled milled carbon (rmCF) and Kevlar hybrid fillers (rmKF) have been integrated into glass/epoxy composites. The damage tolerance of glass epoxy composites was as a whole significantly improved by the rmCF integration. The Kevlar fillers' inadequate adhesion and interlaminar shear strength were compensated for by the Hybrid fillers (rmBF). Micrographs taken with a scanning electron microscope (SEM) clearly show the fillers' role in the resistance to damage propagation processes.

Experimental investigations into damage mechanism in the low‐velocity impact and tension‐after‐impact testing of z‐pin reinforced curved CFRP composite

AbstractThis paper studies the influence of z‐pins on the impact resistance and residual tensile properties of composite large curvature components through low‐velocity impact and tension‐after‐impact tests to explore the damage mechanism. Unpinned and z‐pinned specimens with different diameters (0.3 and 0.5 mm) and spacing (3, 4, and 5 mm) were impacted at energies of 28 J and 14 J + 14 J and were stretched until failure. The theoretical models were used to predict impact dent depth and tensile strength, and the effectiveness of z‐pins and damage patterns were characterized by force‐time/displacement curves and through visual observation. The agreement between the experiments and predictions can thus validate the presented approach. Through the low‐velocity impact test, it is found that z‐pins with higher density and smaller diameter make the impact dent shallower and the surface damage less, thus showing better performance. Later, the impacted specimens mainly show delamination, explosion, breakage, and splitting after tension. During the first stage of the force‐displacement curves, since z‐pins mitigate the delamination, tensile strength shows a 29.58%–59.17% increase while in the third stage, a decrease of 3.95%–15.12% occurs due to initial damage caused by z‐pins.Highlights Different impact energies are implemented on large curved CFRP laminates of z‐pins with different diameters and spacing to explore the damage mechanism. Low‐velocity impact and tension‐after‐impact (TAI) tests are used to simulate the centrifugal force of the blade subjected to impact and rotation after impact. Z‐pinned group with a small diameter and high density causes less damage and tensile strength in the first stage increases while decreasing in the third stage compared with the unpinned group.

Surface Glass Transition Temperature Region of Diarylethene Films Determined by Nano-Marangoni Effect.

For the last two decades, research has addressed whether the glass transition temperature and the molecular motions on the surface of organic films are significantly different from those inside the bulk glasses. It is reported that the surface of the photochromic diarylethene film prepared by vacuum deposition has fluidity and the vacuum deposition of small amount of rubrene molecules induces surface tension fluctuations, generating dents due to the Marangoni flow in nanoscale. The depth of the dents increases in proportion to these radii for the colorless diarylethene film with a bulk glass transition temperature (Tg) close to room temperature. On the other hand, in the colored diarylethene obtained by UV irradiation to the colorless film, the depth becomes constant at a certain level. The Tg distribution in the depth direction is clarified based on an analysis of the dent depth. By approximating the obtained Tg depth distribution with an exponential function, the outermost surface Tg is about 100 K lower than the bulk Tg in the case of photoisomerized diarylethene.

Influence of various damage mechanisms on the low‐velocity impact response of composite laminates

AbstractThis study exhibits the effect of different damage mechanisms on the mechanical behavior of composite laminates subjected to low‐velocity impact load. To achieve this, seven low‐velocity impact energies were applied to laminates with five different thicknesses and four unique stacking sequences. The extent of damage was quantitatively evaluated through contact measurements and ultrasonic C‐scan techniques. Numerical simulations, showing good agreement with experimental results, characterized the distribution and evolution of damage within the laminates. The findings reveal that when matrix and delamination damage are predominant, the dent depth increases slowly and is visually invisible, while the delamination area expands almost linearly. Conversely, When fiber damage prevails, the laminate absorbs more energy, leading to rapid increases in dent depth, the appearance of visible cracks, and the occurrence of sharp fluctuations with multiple peaks in contact force and displacement curves. Nonetheless, the delamination area and projection tend to be stable and unchanged. Furthermore, alterations in thickness and stacking sequence directly affect the damage characteristics and energy absorption properties. This research offers valuable theoretical insights and serves as a reference for enhancing the comprehension of the failure mechanism of laminates and optimizing their performance.Highlights Demonstrate the utility of dent depth in identifying changes in damage mechanisms. Evaluate the delamination damage of laminates after impacts by ultrasonic C‐scan techniques. Reveal the regulation of impact response parameters for different damage mechanisms. Investigate the effect of variation in thickness on damage characteristics and energy absorption. Evaluate the impact resistance of laminates with different stacking sequences.

Numerical analysis of hydrogen atom diffusion and trapping at an unconstrained dent on pipelines

In this work, a three-dimensional finite element-based mechanics-diffusion coupling model is developed to determine the stress/strain distribution and hydrogen (H) atom concentration at an unconstrained dent on an X52 steel pipe which is to be used for transport of high-pressure hydrogen gas. Both von Mises stress and equivalent plastic strain concentrations and a high hydrostatic stress gradient exist at the dent. The distribution of H atom concentration coincides with that of the hydrostatic stress. The maximum H atom concentration in the steel lattice locates at the dent bottom, while the distribution of the H atom concentration at metallurgical traps is consistent with that of the equivalent plastic strain. A competitive effect exists between the hydrostatic stress and the plastic strain on the H atom distribution. At low initial H atom concentrations (e.g., 0.001 mol/m3), the equivalent plastic strain dominates the distribution of H atoms. At high initial H atom concentrations (e.g., 0.1 and 1 mol/m3), the hydrostatic stress is more important to determine the distribution of H atom concentration. As the dent depth increases, both the von Mises stress and plastic strain at the dent increases, while the total H atom concentration decreases due to decreased hydrostatic stress. When the internal pressure increases from 5 MPa to 15 MPa, there are slightly increases in von Mises stress, equivalent plastic strain and the total H atom concentration.

Study by finite element modeling of hydrogen atom diffusion and distribution at a dent on existing pipelines for hydrogen transport

Pipelines, especially existing natural gas pipelines, provide an efficient and economical means for hydrogen transport, accelerating realization of a full-scale hydrogen economy. However, the existing aged pipelines usually contain dents, which serve as effective traps to accumulate hydrogen atoms, potentially causing hydrogen embrittlement and pipeline failures. To date, there has been a strong interest in development techniques for assessment of hydrogen atom distribution at a dent on pipelines. In this work, a three-dimensional nonlinear finite element-based model was developed to investigate the hydrogen atom diffusion and distribution at a constrained dent on an X52 steel pipe. Parametric effects, including dent depth and internal pressure, were determined. The presence of a dent on pipelines drastically changes the local stress and strain distributions, facilitating diffusion and accumulation of hydrogen atoms at the dent. The minimum hydrogen atom concentration at the crystalline lattice sites, (i.e., 3.8 mol/m3) is located on the outer surface of the dent center, and the maximum hydrogen atom concentration (i.e., 12.1 mol/m3) is found on both sides of the dent with 103 mm from the dent center in the circumferential direction. The hydrogen atom concentration at trap sites at the dent is much lower than that at the lattice sites. The diffusion and accumulation of hydrogen atoms at the dent area is affected by two competitive mechanisms, i.e., hydrostatic stress and plastic strain, depending on the initial concentration of hydrogen atoms entering the interior of the pipe steel. At a low initial concentration such as 0.001 mol/m3, the equivalent plastic strain dominates the distribution of hydrogen atoms at the dent, and the hydrogen atom concentration at the lattice sites is much lower than the concentration at the trap sites. At high initial hydrogen atom concentrations such as 0.1 and 1 mol/m3, the hydrostatic stress is the dominant factor to determine the distribution of hydrogen atoms, which mainly accumulate at the lattice sites. With an increased depth of the dent, both the von Mises stress and plastic strain concentrations increase, and so is the hydrogen atom concentration. The increase of internal pressure from 5 MPa to 15 MPa only slightly elevates the von Mises stress, equivalent plastic strain and the hydrogen atom concentration.

Local buckling and method for predicting ultimate compressive strain of pipeline with kinked dent

ABSTRACT Bending loads can lead to local buckling in pipelines with kinked dents. The Finite Element Method is adopted to establish the numerical model of the pipeline with kinked dent. The local buckling failure mechanism of pipeline with kinked dent under bending load is investigated. In addition, the influence of parameters such as the dent depth, dent angle and pipe properties on the failure mode and ultimate compressive strain is analysed. It is found that the depth of the dent depth significantly affects the buckling failure mode and ultimate compressive strain. In addition, the larger the dent angle and the yield ratio, the smaller the ultimate compressive strain capacity. The larger the internal pressure and pipeline diameter thickness ratio, the larger the ultimate compressive strain. Based on the numerical results, the method for predicting the ultimate compressive strain of pipeline with kinked dent is proposed.

Research and analysis on low-velocity impact of composite materials

Abstract In this article, the concept of low-velocity impact resistance in composite materials is presented. Three existing expressions (e.g., bilinear, parabola, and exponential) that show the relationship between the dent depth and impact energy of composite laminates subjected to low-velocity impact are presented. Because of some flaws in these expressions, new supposed functions between impact energy and impact dent have been established. By means of existing ASTM D 7136 tests and simulation analysis, the new expressions are verified, and the nature of dent formation in composite materials is well explained. The conclusion is reflected in the ending part.

A dynamic constitutive model for high-density rigid polyurethane foam subjected to impact loading

This study experimentally and numerically investigated the dynamic response of high-density rigid polyurethane foam (RPUF). Firstly, drop-weight tests were conducted under three impact velocities, 2.42 m/s, 3.43 m/s, and 4.85 m/s. Test results showed that the dent depth and absorbed energy rise as impact velocity increases. Then the constitutive model for high-density RPUF based on an ideal elastoplastic constitutive model is proposed based on the measured mechanical properties, including quasi-static uniaxial compression tests, quasi-static uniaxial tensile tests, triaxial confining pressure tests, and split-Hopkinson pressure bar tests. Finally, the numerical simulation of the drop-weight tests was conducted. It was shown that the proposed dynamic constitutive model can accurately describe the dynamic performance of a high-density RPUF subjected to impact loading.