Dent Depths Research Articles

Ultimate internal pressure bearing capacity of unconstrained and constrained X80 oil and gas pipelines with three typical dents

Three typical dents, denoted as types I, II, and III, encapsulate the prevailing manifestations of mechanical impairment encountered by oil and gas pipelines. Presently, scholarly attention is predominantly directed towards the scrutiny of load-bearing capabilities in pipelines afflicted by a solitary type of dent. To unravel the intricate impact of three typical dents on the performances of dented pipelines, three distinct models of pipeline dents under different constraints were studied in this work. These findings reveal that unconstrained dents consistently exhibit greater plastic strain and deformation zone dimensions compared to their constrained counterparts. The spring-back and rebound performances in unconstrained pipelines were fortified and the external load resistance in constrained pipelines was enhanced by augmenting the indenter diameter and introducing internal pressure. Both unconstrained dents and type I constrained dents failed consistently in the non-dented region, with their ultimate internal pressure resistance unaffected by the constraining factors. In contrast, type II and III constrained dents, featuring smaller indenters, greater dent depths, and reduced diameter-to-thickness ratios, failed within the dented region, which results in a weakened ultimate internal pressure-bearing capacity. Failure occurred in the non-dented region under opposite conditions, leaving the internal pressure resistance unaffected. Finally, dimensionless predictive formulas for the ultimate internal pressures of the type II constrained and type III X80 dented pipelines were obtained through nonlinear fitting. This comprehensive exploration revealed the variations in the ultimate internal pressure-bearing capacities induced by dents, thereby providing valuable insights for pipeline design and safety considerations.

The interplay of casing material and detonation wave shock parameters in steel witness plate dent formation

This study uses both experimentation and simulation to investigate how varying casing material around cylindrical Composition-B charges affects witness plate response and diagnoses the cause of the differences in the dents produced. Through experimentation, it was found that consistent dents are produced from repeated tests and characteristically different dents are produced by charges with different casing material. Charges cased in viscoelastic materials produced shallower dents than those without casing. Simulation was validated against the experimental dents, and the detonation wave parameters were measured for 25 differently cased charges: 15 metals and 10 polymers. Regression fit relationships were derived relating dent parameters to casing density, casing impedance, casing tensile strength, detonation pressure, detonation velocity, impulse, and time of arrival. Specifically, it was found that the dent volume was negatively correlated with the detonation velocity and the impulse of the detonation wave was negatively correlated with dent depths across the charge. The density of the casing material was shown to be linked to the width of the witness dents. Additional dent tests were simulated for trinitrotoluene (TNT) charges cased in a single polymer and a single metal. The relationships derived for Composition-B were adjusted to fit TNT using TNT equivalency and calculated the measured detonation wave values to within 10% accuracy. Finally, it was concluded that the measurable distinctions observed in the witness dents were not a result of the casing material itself, but of the changes in the detonation wave caused by its interaction with the casing material.

Dent-to-Stiffener Evaluation Concept for Thin-Walled Steel Cylinders

Abstract Defects/imperfections can occur during manufacturing, assembly, welding, and other processes, which can reduce the critical buckling load. However, the axial buckling load is beyond the scope of this work, and there are many studies on the stiffening effect of longitudinal dents. This concept combined the idea of the dent-to-stiffener evaluation concept for thin-walled steel cylinders. This study aims to transform the dents into artificial dents for a stiffening effect on the buckling phenomena. For this purpose, 37 thin-walled steel cylinder models, including the perfect model, were designed for varying dent shapes, dent widths, dent depths, dent lengths, and dent angles. The study also contributed to the effect of dent parameters on the critical buckling load of thin-walled steel cylinders. In particular, increasing the initial buckling will motivate the industry to convert dents into stiffeners with small artificial touches to enhance the longevity of the structure. The results showed that the introduction of certain artificial dents can significantly increase the critical buckling load of cylinders, thus improving their resistance against buckling, which has significant implications for various industries that use thin-walled steel cylinders in their structures. The proposed simulations for transforming dents into artificial stiffeners can be a valuable tool for enhancing the longevity and safety of thin-walled steel cylinders and other structures.

Low‐velocity impact response and damage tolerance of hybrid biaxial/triaxial braided composite laminates

AbstractThe structural properties of braided composite laminates are significantly affected by the low‐velocity impact (LVI). In this paper, the impact resistance and damage tolerance of triaxial braid structure laminates at different positions were mainly studied. Three hybrid biaxial (B)/triaxial (T) braided composite structures of BBTT, BTTB, and TTBB were designed, and LVI test and post‐impact compression tests were carried out. The results show that the triaxial braided fabric with quasi‐isotropic on the impact side caused more significant matrix cracks along the axis yarn direction, and the matrix damage area and delamination area were larger. For triaxial braided ply with high curl levels, severe fiber fracture and relatively concentrated damage occurred first on the non‐impact side. Interestingly, triaxial braided fabrics exhibited better impact resistance in terms of mechanical response when distributed across the specimen surface. However, a global damage pattern appears at high energy levels, which seriously reduced the residual strength of the structure and showed the worst damage tolerance. The damage quantification analysis of cracks, dent depths, delamination and residual compressive strength caused by the LVI of hybrid shaft number braided laminates was carried out in this work, which provided a valuable reference for engineering failure analysis and rational structure optimization of composite laminates.

A numerical perspective for CFRP‐wrapped thin‐walled steel cylinders

AbstractThe failure of steel cylindrical shells with carbon fiber‐reinforced polymers (CFRP) under hydrostatic pressure is studied in this article by experimental tests and the finite element method. Simulations were performed for 12 cylindrical shells with different dent numbers, dent depths, and two perfect models with and without CFRP. The finite element models are built to simulate the buckling behavior. The nonlinear stabilization method is preferred to simulate the buckling behavior. The finite element results agree well with the corresponding experimental tests and theories. A parametric study is conducted, and the number, depth, width of dents, and the effects of CFRPs on the critical buckling pressure are discussed. The dents are placed symmetrically around the cylinder with a thickness of tc (thickness of cylinder) and 2tc mm. The initial and overall buckling load decreased with increasing amplitude for dent numbers with different dent depths on all models. The results show that the number and depth of dents generally have a negative effect on the buckling strength, while CFRPs increase the critical buckling strength and are effective in repair as a retrofitting concept.

Modelling the impact of hailstones on flat steel roofing membranes for residential buildings

Metal roof panels are commonly used on residential and commercial buildings. Steel panels exposed to hail have not yet been adequately tested for dent resistance. A finite element model (FEM) was used to analyze the entire test setup. To compare artificial hailstones with natural hailstones, which remained intact after impact, different steel sheets were struck by different sizes of artificial hailstones at different terminal velocities. The simulation and the material properties are assessed by comparing the experimental results with the FE model. An equation to predict the dent depth based on kinetic energy and stress is also presented. The results of this study provide a better understanding of the failure modes of hail and roof panels and their effects on dent resistance. In this study, the results of observations and numerical simulations agreed well with those of analytical models. The result is that the proposed equation overestimates the dent depths compared to the dent depths obtained with finite element models, while the equation leads to an underestimation of the dent depths found in the steel sheets.

Assessment by finite element modeling of corrosion in dent on X52 steel pipelines

While corrosion in dent presents a realistic threat to integrity of pipelines, there have been limited methods available to assess the problem and its effect on fitness-for-service of the pipelines. In this work, a method was developed to assess the corrosion in dent on an X52 steel pipe by considering both mechanical and electrochemical corrosion factors and their interaction at an unconstrained dent through finite element modeling. Parametric effects including dent depth, dent size (i.e., open mouth diameter for spherical dents) and internal pressure on distributions of stress, anodic current density and net current density were determined. The stress concentration at the dent accelerates corrosion due to a mechano-electrochemical interaction. As the dent depth increases, the local stress concentration is enhanced, resulting in increased corrosion activity and corrosion rate. The most negative corrosion potential and the maximum anodic current are located at the center of the dent, where the anodic reaction occurs. The cathodic reaction mainly occurs at the dent sides. The internal pressure has a limited effect on the anodic current density. The dent with a smaller open diameter can cause greater local stress concentration and anodic current density. At specific dent depths, small dents tend to accelerate local corrosion at a more rapid speed than bigger dents.

Effects of midplane carbon nanotube sheet interleave on the strength and impact damage resistance of carbon fiber reinforced polymer composites

AbstractThe effects of embedding a midplane carbon nanotube (CNT) sheet on the impact damage resistance, tensile strength and propensity for interlaminar shear failure in a carbon fiber reinforced polymer matrix composite is experimentally evaluated. External and internal damage in the impacted composite laminates were characterized via surface profilometry and ultrasonic C‐scan, respectively. The external dent areas and dent depths were greater for impacted laminates without the CNT sheet interleave, while the planar internal damage areas were greater for the impacted laminates with the CNT sheet. Destructive evaluation of the impacted laminates showed significant delaminations and matrix cracks in the vicinity of the midplane for the laminates with the CNT sheet, while damage was more distributed through the thickness in the non‐CNT sheet ones. While a single CNT sheet interleave did not significantly affect the tensile strength of the laminate, residual strength after impact and the corresponding strain to failure was higher because of the CNT sheet interleave. The propensity for interlaminar shear failure was greater for laminates without the CNT sheet than those with the CNT sheet.

Strain-based reliability analysis of dented pipelines using a response surface method

Dent defects can decrease the life span of oil and gas pipelines. Therefore, they need constant monitoring and maintenance to ensure the pipeline’s safety and integrity. Subsequently, this paper performs a strain-based reliability analysis on pipe dent defects using a response surface method (RSM) with a quadratic response surface (RS), including the interaction terms between the RS variables. The analyses are performed to determine the factors controlling the dent defects’ probability of failure (POF). Different pipe configurations, pipe lengths, indenter sizes, and dent depths are considered in this study. A suitable finite element (FE) model for the reliability analysis was developed for this study using the FE analysis software ABAQUS. The uncertainties in the pipe wall thickness, the dent depth, the yield strength of the pipe material, and the strain capacity are considered for the reliability analysis. The first-order reliability method (FORM) is used in the RSM as the reliability method to calculate the POF and the most probable point (MPP). The POFs of several dent defects were calculated. It has been found that the POF, which is highly related to the nominal value of the maximum equivalent plastic strains generated in the dent defect, is not only related to the indentation depth or the size of the indenter. Thus, the dent depth criterion used in the engineering practice can lead to inconsistent reliability levels in dented pipes.

Experimental investigation on the response and residual compressive property of honeycomb sandwich structures under single and repeated low velocity impacts

Single and repeated impact tests were carried out on honeycomb sandwich structures. The effect of varying impact energy levels and numbers on impact response, damage parameters, and energy absorption was studied. During these tests, the time histories of contact forces, deflections, and impact energies were recorded in real time. Three-dimensional digital image correlation (3D-DIC) was applied to obtain the dent depths, damage areas, and damage volumes of front and rear surfaces. The residual compressive properties of the above-mentioned structures were determined using compression after impact tests and 3D-DIC. Further, the onset of damage procession and failure modes were analyzed. A finite element model was performed to characterize the impact response of honeycomb sandwich structures. The simulation results was in good agreement with experimental data, which indicated the model could be used for the simulation on aluminum honeycomb sandwich panels subjected to single and repeated low-velocity impacts.

Strain evolution characteristics of X80 line pipes with plain dents

In the process of construction and service, high-grade line pipes will get defective, e.g. dents, which will change its stress and strain distribution characteristics and impact its service reliability. In this paper, a X80 line pipe was taken as the research object. The distribution characteristics of the strain field in the X80 line pipe with plain dents with the change of dent depth under external load were analyzed using the finite element analysis software ABAQUS. Then, the strain distribution and microstructure characteristics in the dent zone were explored by conducting prefabrication test on physical dent. Finally, combined with the finite element simulation results, the strain distribution laws of the X80 line pipe with plain dent were discussed. And the following research results were obtained. First, under the same internal pressure, the strain distribution characteristics in the dent zone at different dent depths are similar, i.e., the strain increases with the increase of the distance from the center of the dent, and decreases rapidly with the increase of the distance after the peak strain. Second, the strain increases with the increase of dent depth, and under the same internal pressure and dent depth, the axial strain is larger than the radial strain at the same location. Third, the greater the dent depth, the stronger the superposition effect of internal pressure and depth on the strain. Fourth, strain hardening occurs on the materials in the initial stage of the dent deformation. With the aggravation of deformation and the extension of dent radius, the strain response ability of materials increases, the grains at the bottom and side walls of the dent zone are elongated along the direction of maximum deformation, the lattice is distorted and strain hardening occurs. As a result, the dislocation density in this zone increases and the interaction occurs between dislocations, as a result, the strength of line steel is enhanced. In conclusion, the research results do well in predicting the stress–strain evolution laws in the process of dent, and provide a theoretical foundation and an experimental basis for studying the influence of mechanical damage on the service safety of pipelines.

The Influence of Diameter-to-thickness Ratios on the Response and Failure for Locally-dented Circular Tubes Submitted to Cyclic Bending

This paper describes the cyclic bending response and failure of 6061-T6 aluminum alloy locally-dented tubes (abbreviation: 6061-T6 LDT) with different diameter-to-thickness ratios (abbreviation: Do/t ratio) of 16.5, 31.0 and 60.0. A small lateral local dent was created on the tube using a simple technique. A tube having a dent depth from very shallow to about 2.4 times the thickness of the tube wall is considered. From the first bending cycle, the moment-curvature relationship depicts a closed and stable loop. Due to small and localized dents, dent depth shows very small influence on its relationship. But, as the number of bending cycle increases, the ovalization-curvature relationship shows an increase and a ratchet-like distribution. However, the dent depth shows a dramatic effect to this relationship. Furthermore, for a certain Do/t ratio, five non-paralleled straight lines of five different dent depths were found for the controlled curvature-number of cycles required to ignite failure relationship on a log-log scale. Finally, an empirical formula was introduced to describe the aforementioned relationship. As a result, the experimental and analytical data were found to agree well.

CFRP Effect on the Buckling Behavior of Dented Cylindrical Shells

Considering that the use of thin-walled shells is expanding every day, it is important to examine the problem of instability in this form of structure. Many steel structures such as high-water tanks, water and oil reservoirs, marine structures, and pressure vessels, including shell elements, are under stress tension. In addition, shell elements are subject to instability owing to the loads applied. Ten thin-walled cylindrical shell specimens in two groups with different dent depths of tc and 2tc, and the different dent number subject to uniform external pressure were tested in the present research (tc is the thickness of cylindrical shell). The samples were modified to include either one or two dent line with amplitudes of h/3 in height (h the height of cylinder shell). Moreover, CFRP Strips on the dent depth was used in one of the groups. The results of testing under different theories and codes are compared.

Experimental analysis of the effect of dent variation on the buckling capacity of thin-walled cylindrical shells

Tanks, silos, and most large steel-shell structures consist of smaller pieces connected together during the manufacturing process. This causes several types of malformations on the shell walls. Furthermore, thin-walled members can be easily deformed in wall surfaces owing to the thickness of the structure. Fourteen thin-walled cylindrical shell specimens in two groups with different dent depths and various longitudinal dent numbers subject to hydrostatic pressure were tested in the present work. The models were designed to demonstrate how repairing dents by using carbon-fibre-reinforced polymer can recover lost capacity. The results of testing under different theories and codes were compared. This study shows the decreasing effects of the longitudinal dent number on the buckling capacity of the shells. Using carbon-fiber-reinforced polymer strips resulted in softening or stiffening behaviour in the models. Furthermore, to obtain the initial and overall buckling according to theoretical formulas, coefficients were predicted to obtain the initial, overall, and collapse buckling without an experiment for the models that were beyond the scope of the theories.

Buckling and post-buckling behavior of various dented cylindrical shells using CFRP strips subjected to uniform external pressure: Comparison of theoretical and experimental data

Thin-walled circular cylindrical shells are used in many civil engineering applications. Any initial geometric imperfections and the type of applied load determine the buckling and the post buckling of cylindrical thin-walled shell structures. Fourteen thin-walled cylindrical shell models in two groups (labelled without CFRP and with CFRP) with different dent directions (H= horizontal, V= vertical and D= 45° diagonal direction) and dent depths (2tc and 4tc) subjected to external pressure were tested in the present research (tc is the thickness of the cylindrical shell). The results of testing under different theories and codes are compared. Moreover, the results are identified and investigated in the present experimental research.

Exploration of energy absorption and viscoelastic behavior of CFRPs subjected to low velocity impact

The attention of the present work is focused on the energy absorption and transformation of carbon fiber reinforced polymer (CFRP) composites under low velocity impact. Moreover, the viscoelastic behavior of composites was explored by observing the changes of dent depths via a micrometer and 3D microscope. In addition, the residual strengths of CFRP specimens were investigated by the compression after impact (CAI) test. The results show that more than 88% of impact energy was absorbed by the CFRPs through reversible deformation, irreversible damage and viscoelastic behavior. The impact damage threshold value and the maximum allowable deformation of the CFRP laminates were 9 J and about half of its thickness respectively. The energy is mainly absorbed by deformation when the impact energy was not larger than the threshold value. The energy dissipated by viscoelastic behavior increased with the increase in the impact energy and even exceeded 1.4 J in the case of 21 J, which was much higher than the kinetic energy when the impact energy exceeded the threshold value. The residual strength of the CFRP laminates was related to the energy absorbed by irreversible damage.

The shape evolution and application studies of electrodeposited bumps in electroforming

Deposition around the microvias opening is crucial for desirable quality of metallic micro components and micro bumps in electroforming. This paper presents a numerical and experimental study of shape evolution of electrodeposition front on the extent of over-plating. Computational models based on kinetics of the electrode reaction are used to simulate the entire deposition process from start of deposition until the formation of bumps. The effects of the current density and microstructure geometries are discussed through analyzing the current density distribution along the transient cathode surface. When the current density is 4 A dm−2, the significant Ni craters are observed for microvias with diameters of 200 µm in the over-plating stage. The dent depths decrease as the current density and diameter decrease. For the 30 µm diameters, spherical structures in the over-plating stage form upon the photolithographic patterns for an applied current density of 1 A dm−2. Furthermore, the spherical surfaces enable the fabrication of hemispherical lenses with various curvature radii. Spherical structures are obtained by electrodeposition into microcavities. A numerical and experimental study is carried out to study the mechanics of shape evolution. The polymeric microlenses are replicated through a combination of PDMS molding and UV curing. This process demonstrates the potential of this method in the fabrication of microlenses.

Surface damage evaluation of honeycomb sandwich aircraft panels using 3D scanning technology

A 3D scanning method is proposed for the measurement of surface damage on aircraft structural panels. Dent depth measurements were shown to be within 0.04 ± 0.06 mm (95%) of those taken using a Starrett 643J dial depth gauge based on 54 flat panel dents, and 0.04 ± 0.05 mm (95%) based on 74 curved panel dents. Dent depths were quantified by the difference between a point cloud rendering of the damaged surface and a surface fit approximating the original, undamaged surface. Convergence studies were used to evaluate the accuracy of the surface fit, enabling this technique to be used as a stand-alone inspection method. Image processing was used to measure dent length and area, and the results showed that this method is more efficient and reliable compared to manual methods. This novel non-destructive evaluation technique thus demonstrates potential to enable the timely extraction of surface dent measurements during on-site aircraft inspections.

Impact Modelling and A Posteriori Non-destructive Evaluation of Homogeneous Particleboards of Sugarcane Bagasse

With a view to gaining an in-depth assessment of the response of particleboards (PBs) to different in-service loading conditions, samples of high-density homogeneous PBs of sugarcane bagasse and castor oil polyurethane resin were manufactured and subjected to low velocity impacts using an instrumented drop weight impact tower and four different energy levels, namely 5, 10, 20 and 30 J. The prediction of the damage modes was assessed using Comsol Multiphysics $$^\circledR .$$ In particular, the random distribution of the fibres and their lengths were reproduced through a robust model. The experimentally obtained dent depths due to the impactor were compared with the ones numerically simulated showing good agreement. The post-impact damage was evaluated by a simultaneous system of image acquisitions coming from two different sensors. In particular, thermograms were recorded during the heating up and cooling down phases, while the specklegrams were gathered one at room temperature (as reference) and the remaining during the cooling down phase. On one hand, the specklegrams were processed via a new software package named Ncorr v.1.2, which is an open-source subset-based 2D digital image correlation (DIC) package that combines modern DIC algorithms proposed in the literature with additional enhancements. On the other hand, the thermographic results linked to a square pulse were compared with those coming from the laser line thermography technique that heats a line-region on the surface of the sample instead of a spot. Surprisingly, both the vibrothermography and the line scanning thermography methods coupled with a robotized system show substantial advantages in the defect detection around the impacted zone.

Fatigue Life Analysis of Dented Pipes Subjected to Internal Pressure

Steel pipelines that transport pressurized fluids can contain a dent defect which is a permanent deformation of the outer wall due to an impact with a foreign body. This defect induces a high local stress concentration. This paper presents an optimized numerical model based on finite elements to assess the stress concentration factor around constrained and unconstrained dents in an API X52 steel pipe subjected to internal pressure. The simulation of the finite element model is conducted in two phases; the first phase concerns the realization of the dent, while the second one takes as initial geometry the dented pipe and subjects it to an internal pressure. The validation results of this finite element model show that it can precisely provide Von Mises stress around the defect needed to calculate the stress concentration factor. The fatigue life is then estimated based on S-N curves with the Gerber criterion to account for the mean stress effect for various pipe geometries, dent depths and dent types. These dents are generated using spherical and rectangular indenters in longitudinal and transverse orientations under constrained and unconstrained configurations. A parametric study conducted on 150 cases of dented pipes subjected to internal pressure between 0% and 50% MOP (maximum operating pressure) enabled us to analyze the effect of the various types of dents on the integrity of the structure.