Dent Depth Research Articles

Ultimate External Pressure Capacity of Deepsea Dented Pipeline with Combined Axial Compressive Force

As a common defect of deepsea pipeline, dents will weaken the residual strength of the pipeline. In this paper, nonlinear finite element method is adopted to investigate the collapse failure of deepsea dented pipeline under the combined external pressure and axial compressive force. The influence of the axial compressive force on the collapse pressure of dented pipelines with different diameter-thickness ratio and dent depth are investigated based on numerical simulation. It is concluded that relative large axial compressive force will reduce the collapse pressure capacity significantly for deep dent case.

Investigating the effects of UV light and moisture ingression on low-impact resistance of three different carbon fiber–reinforced composites

In this article, the effects of ultraviolet light (UV) and moisture exposure on the low-velocity impact behavior of three different carbon fiber (CF) laminate composites (unidirectional fibers, plain weave woven fibers, and non-crimp fibers (NCF)) are reported based on the experimental observations. The composite laminate was fabricated by vacuum bagging method following the manufacturing specifications with symmetric and asymmetric stacking, and then the test coupons were extracted for impact testing, C-scan, and surface characterization studies before and after UV light and moisture exposures. A low-velocity impact test was carried out to evaluate the damage resistance and tolerance of the laminate specimens. The test outcomes uncovered that the NCF laminates were far predominant in load-carrying capacity than the woven and unidirectional laminates, with the NCF-asymmetric (NCF-NS) laminate exhibiting the greater load-carrying capacity of 3017.7 kN/m and impact energy of 7.07 kJ/m (NCF-NS). The NCF-symmetric (NCF-S) laminate showing impact energy of 7.0 kJ/m and load-carrying capacity of 2886.8 kN/m with some decrease after UV and moisture exposure for both cases. The ultrasonic C-scan revealed that NCF laminates, both NCF-S and NCF-NS, have the least penetration indicating greater out of plane fracture toughness and damage tolerance. The NCF-NS laminate has the least damage area (33.35 mm2) and dent depth (0.12 mm) as compared with other laminate studied here. The wettability of the panels was similar; however, the woven (baseline) panel showed the highest water contact angle (112°). After the UV and salt fog exposure, the contact angles of the composite panels were reduced between 66° and 58°. In addition, this study also reveals the effect of stacking sequence on the impact properties of the NCF composite laminate.

Experimental evaluation of surface damage relaxation effect in carbon-fiber reinforced epoxy panels impacted into stringer

Evaluation of the impact damage visual detectability threshold in composite structures requires not only determination of damage state right after the impact, but also at the actual moment of the visual inspection. Due to the specific features of aircraft maintenance programs the time between those two events may be considerable. During that time the damage size in the structure, subjected to operational and environmental factors, can be reduced (relaxed). To characterize the relaxation effect in typical aircraft stiffened panels, the long-term behavior of surface impact dents was investigated. It was demonstrated that among all the factors potentially reducing the dent depth, the combination of moisture and elevated temperature appears to be the critical case. Based on the results of the study the input data for damage tolerance analysis of commercial aircraft composite structures was obtained and recommendations on its applicability were formulated.

SUBSEA PIPELINE PROTECTION DESIGN SUBJECTED TO DROPPED ANCHOR USING CONCRETE MATTRESS

Out of the many potential hazards, certain critical possible hazards need to be assessed, and actions need to be taken to improve the probabilities and consequences to an acceptable degree. One of the major hazards commonly found in subsea pipeline operations is a dropped anchor, especially for pipelines located near jetties, where many ships are expected to move across the pipeline. One of the possible protection methods is the use of a concrete mattress. The dropped anchor hazard to a pipeline is characterized by the impact of energy, which will mostly be absorbed by the internal strain energy of the pipeline. The safety criteria for a pipeline subjected to a dropped anchor are given by DNV RP-F107 for the maximum dent depth and DNV OS-F101 for the maximum stress. A finite element analysis using ANSYS software was performed to obtain the maximum equivalent stress and dent depth for a pipeline. The effects of the length and height of a concrete mattress in redistributing the impact energy were obtained.

Effects of hygrothermal and thermal aging on the low-velocity impact properties of carbon fiber composites

The effects of hygrothermal and thermal aging on the low-velocity impact properties of carbon fiber composites were systematically investigated by comparing the mass loss, morphologies, dynamic mechanical properties, and chemical structures of aged and non-aged specimens. The relationships between dent depth, damage area, residual compression strength, and impact energy were also discussed in detail. The obtained results showed that the non-aged composite featured good fiber-resin bonding, whereas hygrothermal and thermal aging led to fiber-matrix interface damage. For thermally aged samples, dent depth rapidly increased with impact energy above the inflection point, and the corresponding amplitude was greater than those observed for non-aged and hygrothermally aged specimens. Notably, hygrothermal and thermal aging did not affect the impact-induced damage morphology and the after-impact compression failure mode. At a constant impact energy, the damage area of thermally aged samples exceeded those of non-aged and hygrothermally aged ones; moreover, thermal aging exhibited greater influence on residual compressive strength than hygrothermal aging. Finally, whereas hygrothermal aging reduced the glass transition temperature (Tg) and did not induce the formation of new materials or the occurrence of chemical reactions, thermal aging resulted in increased Tg and was accompanied by the concomitant occurrence of oxidation reactions.

Remaining strength assessment of dented pipe based on depth and strain based criteria

There are currently two methods which are used in feasibility operation of dented pipeline systems, namely depth based and strain based. In this research, both methods will be compared to determine conservatism degree of each method and as a guideline and considerations in the integrity assessment of pipeline system. The finite element method with including nonlinearities of material properties and dimension will be used as a tool in the analysis. Static indentation was held with using half ball indenter. Variations of applied load are 600 N, 1000 N, 1500 N, 2000 N, and 2500 N. The results of indentation are dent depth ratio (d/D) with value 1,93 %, 3,78 %, 5,53 %, 7,08 %, 10,63 %, and plastic strain with value 4,24 %, 5,87 %, 8,36 %, 11,43 %, and 12,93 %, respectively. Based on the Code, depth based criteria use d/D<6 % and strain (ɛ) < 6 % as an acceptance criteria. Based on this research, it can be concluded that strain based method is more conservative rather than depth based method. Then, five model of defected pipe will be simulated in the condition of operation pressure applied to the pipe. Operation pressure was varied with (P/Py) 0.2, 0.4, 0.6, 0.8, and 1. The results show that depth of dent will decrease with variation of strain value.

Damage and Springback Analysis of Two Typical Dented Pipelines With Different Parameters

During pipeline construction, the pipeline may be impacted by sharp rocks or excavators. To study the failure mechanism of the pipeline, the damage degree and springback rate of the pipelines with two typical dents (transverse and longitudinal) were analyzed in terms of various factors (indenter size, pipeline size and internal pressure, and dent depth). The results reveal the following: (1) when pipeline size and internal pressure are unchanged and indenter size is changed, the integral value I used to measure the damage degree of the dented pipeline increases with increasing dent depth. When the dent depth reaches a certain value, at the same dent depth, the smaller the indenter size, the larger the damage integral value; (2) when other parameters remain unchanged, the larger the pipeline size is, the larger is the damage integral value, and the larger the internal pressure is, the smaller is the damage integral value. (3) The curves for damage and springback for the two kinds of dents are basically similar. Generally, the maximum damage of the longitudinal dent is larger than that of the transverse dent. (4) By a combination of an orthogonal experimental design and a gray correlation degree calculation, for the damage integral value of the two typical dented pipelines, the order of importance of the influential factors was obtained. (5) Formulas for the damage integral value and influence factors were fit using a nonlinear regression method, which provides a reference for calculation of pipeline damage.

Effect of Textured Dentated Surfaces on 30KhGSA Steel Damage and Life at Fatigue, Fretting Fatigue, and Fretting

Machines and constructions are noted to contain components, units, and assemblies, which are joined together, forming a large number of contacting pairs. At operating loads, the latter become sources of material damage as a result of fretting fatigue and fretting wear. The safety and long-term operation of those items would require studies on the nature of these processes and search for the methods of reducing their intensity. Investigation results for the effect of textured surfaces in the form of a dent (recess) network and electric-spark VK8 alloy embedding over 30KhGSA steel specimen surfaces on fatigue, fretting fatigue, and fretting wear characteristics are presented. The dent network is formed by plastic indentation using a specially developed device. This process initiates residual tensile stresses. Their optimization was carried out with computation-experimental correlation between the geometrical ratios of dent network parameters (spacing between dent rows, in-row spacing, dent depth). Discrete VK8 alloy-embedded surfaces were generated with the electric-spark method, providing optimum process parameters. Textured surfaces reduce fatigue and fretting fatigue resistance, while enhancing the fretting wear one in comparison with smooth surfaces. Additional treatment of the textured dentated surface with the ion-plasma thermocyclic nitriding method greatly increases the fatigue and fretting fatigue resistance of 30KhGSA steel. Electric-spark VK8 alloy embedding results in worse fatigue and fretting fatigue properties, which is determined by the surface formation technology and difference between embedded particle and base metal characteristics.

Microstructure-Based Model for Sharp Stress Raiser-Related Fatigue Stage Length Assessment

A model for evaluating the fatigue life of specimens/structure elements with sharp stress raisers/defects is presented. The model permits of computing the number of cycles to fatigue crack initiation and its growth from a sharp stress raiser to failure at a constant stress span with the only application of characteristics of static strength and microstructure of the initial material. The model can be used to assess the fatigue life of components that contain structural stress raisers and defects stemming from their manufacturing technique (surface roughness, surface cuts, scratches, and microcracks). The model reliability was verified with experimental results taken from the literature, calculations appeared to be in good agreement with experimental data. Fatigue curves to a grain-size crack initiation and to fracture of smooth specimens and those with a chemically-notched blunt raiser that simulates the casting defects in aircraft components were calculated. The two sets of specimens from a Ti–6Al–4V titanium alloy differing in the cross-section (rectangular and cylindrical) and in microstructure (different grain sizes). Smooth specimens exhibited the test surface roughness Rv = 10 and 19 μm (average dent depth), which was assumed to be a sharp raiser for calculations. The model need not long-term and labor-consuming high-cycle fatigue tests to construct the fatigue curve.

Experimental and numerical study of the response to various impact energy levels for composite sandwich plates with different face thicknesses

Composite sandwich structures find wide application in the aerospace sector thanks to their lightweight characteristics. However, composite structures are highly susceptible to low-velocity impact damage and therefore thorough characterization of the impact response and damage process for the used material configurations is necessary. The present study investigates the effect of face-sheet thickness on the impact response and damage mechanisms, experimentally and numerically. A uni-directional, non-crimp fabric is used as reinforcement in the face-sheets, and a closed cell Rohacell 200 Hero polymer foam is used as core material. Low-velocity impact tests are performed in a novel instrumented drop-weight rig that is able to capture the true impact response. A range of impact energies are initially utilized in order to identify when low level damage (LLD), barely visible impact damage (BVID) and visible impact damage (VID) occur. A thorough fractography investigation is performed to characterize the impact damage using both destructive and non-destructive testing. The damage from the impacts in terms of dent depth, peak contact force, deflection and absorbed energy is measured. The results show bilinear responses in dent depth vs. impact energy and absorbed energy vs. impact energy. It is found than the BVID energy works well as an indication for the onset of excessive damage. Fractography reveals that there is a failure mode shift between the LLD and the VID energy levels, and that delaminations predominantly grow along the fiber direction and rotate in a spiral pattern through the thickness, following the laminate ply orientations. Finally, a progressive damage finite element model is developed to simulate both the impact response and the delamination extent, incorporating both intra-laminar and inter-laminar damage modes. The simulation shows good agreement with the experiments.

Study of the strained state of steel plates of small thickness using coercimetry

The results of studying the strain state and depth of dents in K08Yu low-carbon steel plates of small thickness (3-5 mm) using the method of coercimetry are presented. It is known that mechanical stresses and strains that occur in steel rods and base metal of oil and gas pipes have a significant impact on the magnitude of the coercive force (CF) near the sample surface. The effect of plate deformation and geometric dimensions of the dents on the CF magnitude is analyzed using a KM-445.1 coercimeter and tools for stepwise determination of the depth of dents (calipers with a depth gauge, tape measures, metal strip, marker). The calculated differential dependences linking the curvature of the plate with the dent depth are obtained. The intensity of linear strains occurring in the plate is determined. The relations between the coercive force and strain characteristics of plates with different types of collapsing are determined, namely, it is shown that in a plate with a single dent, the value of the coercive force increases monotonically with the depth and intensity of deformations, whereas in a plate with a double dent, there is a complex wave-like dependence of the coercive force magnitude on the depth and intensity of deformations with a characteristic inflection point and the character of the dependence provides a possibility for determination of the size and type of the dent. The proposed analysis of the stress state using the method of coercimetry makes it possible to detect geometric imperfections on the surface of the plates and carry out their non-destructive diagnostics.

Mechanical Behavior and Evaluation of Dented Pipe Caused by Cylindrical Indenter

The defect of pipe dent has a great threat to the safety of oil and gas pipe transportation. In this paper, the numerical calculation model of pipe extruded by cylindrical indenter at vertical and transverse direction was established. Effects of indenter displacement, indenter radius and internal pressure on the dent behavior were discussed. The results show that different action forms of the indenter would cause different dent behaviors. A large degree of plastic deformation is formed at the edge of the cylinder end of the indenter when indenter made a vertical action. The larger the indenter radius is, the larger the plastic deformation is. But when the indenter extrudes transversely, the change in the indenter radius has little influence. For pressure pipeline, the strain in the dent fluctuates in a certain degree, and the plastic strain is larger than that in the non-pressure pipeline, but the dent depth would be lower in the end. Under different conditions of internal pressure, the laws of dent with “non-pressure” and “loading pressure” are the same, and the dented laws of “pressure” and “unloading pressure” are consistent. The strain and the depth of the dent achieve an extreme point or maximum near the edge of the dent.

A study of accidental impact scenarios for composite wing damage tolerance evaluation

ABSTRACTThe field data characterising aircraft accidental in-service damage was collected, sorted and processed. By means of probabilistic analysis, the wing damageability statistical parameters were determined. The scenarios of wing accidental impacts were described and the qualitative distribution of impact intensity over the wing surfaces was obtained. By means of original analytical method, the metal dent depth data were converted into impact energy data and energy probabilistic distributions were established. It was shown that the functional relationships generated on domestic data are generally consistent with similar foreign results obtained on other types of aircraft with serious differences in operating conditions. Along with realistic impact damage scenarios, the high energy impact events were considered. It was noted that in some cases severe damage events should not be addressed as extremely improbable and should be included into design and certification process.

Failure analysis and mechanical behavior of dented pipeline dent caused by indenter

Dented defects pose a great threat to the safety of oil and gas pipeline transportation. In this paper, dent behavior of a pipeline caused by a spherical indenter was investigated. Effects of thickness, indenter displacement, indenter radius and pipeline internal pressure on the stress, strain and depth of the pipeline dent are discussed. The results show that internal pressure has a great effect on the pipeline dent. When the internal pressure increases, folds appear on the pipeline wall. With the increasing of wall thickness and indenter displacement, the stress, plastic strain and dent depth increase. But the radius of the indenter has a little effect on the dent’s stress and strain. In general, the strain and the depth of the dent achieve an extreme point or maximum near the edge of the dent.

Determination of More Influencing Geometrical Parameter of Dent on Buckling Strength of Thin Plate Under Axial Load Using Response Surface Regression

Abstract The main aim of this paper is to determine the critical geometrical parameter of dent, out of two geometrical parameters namely extent of dent and Dent Depth (DD). Extent of dent can be of any shape but for simplicity it is taken as corner rounded square shape and the measure for this shape is taken as Extent of dent (DSL). In order to accommodate these two parameters simultaneously, FE models of thin plate with centrally located dent of thin plate are generated by varying the Extent of dent and Dent Depth .These generated FE models are analyzed using static nonlinear FE analysis module of ANSYS including both geometrical and material non-linearities to determine the ultimate strength of the dented thin plates. Non-linear response surface regression analysis is carried out to fit response surface for ultimate strengths obtained from FE numerical analysis results taking the DSL and DD as input variables. From the regression analysis, it is found that Dent Depth is more dominant factor than extent of dent

Retracted] Examining Dent Formation Caused by Hailstone Impact

Hailstorms pose significant risk for exposed building cladding materials. Steel sheeting is the most important cladding material used. The understanding of steel sheets behavior under hail impact loading is not sufficient for the manufacturing of hail‐resistant sheets. With the purpose‐built equipment, artificial hailstones of different sizes were launched to impact at steel sheets of different thicknesses and yield stresses as targets. A theoretical approach for the problem of predicting the dent size due to hailstone impact was developed and compared to the test results. The expressions developed in the theory can predict the dent depth before the impact, assuming the ratio between the dent depth and dent diameter is constant. The expression is not able to predict the depth of dents smaller than 0.75 mm and cannot predict whether the denting will occur or not. All hailstone sizes lead to visible dent on steel sheet of thicknesses 0.35 mm, 0.42 mm, and 0.55 mm. Visible denting was also obtained for the 0.75 mm steel samples with 45 mm and 55 mm hailstones; however, no denting occurred using 40 mm hailstones. It was found that the dent depth was inversely proportional with thickness and yield stress, while the dent diameter was found to be proportional to yield stress. As the yield stress of the steel sheet increased, the dent depth decreased for G300 and G550 steel. The dent diameter however increased as the yield stress increased. When the artificial hailstone shatters on impact, significant energy is lost and less energy is available to cause plastic deformation of the impacted material.

Fatigue strength predictions of FOD dents using ΔK threshold methods considering residual stresses

Fatigue strength predictions of TC4 FOD-dent specimens using ΔK threshold methods considering residual stresses were the main emphasis in this paper. Both the experimental simulation and the fatigue test were conducted to evaluate the HCF performance of the real FOD dent. The geometrical models of FOD dents were established for the elastic stress concentration analysis. Residual stresses around the dent were numerical simulated or reservedly solved from the laboratory test data and subsequently established as a function of the dent depth. With numerical simulated residual stresses, the prediction results fall into an error interval of about ±30 %. With the reserved residual stress, the method has a smaller error interval of about ±20 %. Both the two procedures can fortunately meet the exit criteria which is probably because the potential micro-damages like micro crack and adiabatic shear band etc. could be counted as sharp cracks.

An analytical study of the plasticity of sandwich honeycomb panels subjected to low-velocity impact

A new analytical modelling method for the study of low-velocity impact response in honeycomb sandwich panels with metallic face-sheets is proposed by using Hamilton's principle. A modified Lagrange's function for this model is then proposed by extending the application of Hamilton's principle from elastic bodies to face-sheets with plasticity. The internal physical mechanism in the system is proved to be reasonable by comparing the energy converting history with published data. This study advances the understanding of the role of face-sheet plasticity in low-velocity impact response of honeycomb sandwich panels by having the ability to incorporate either the energy absorption from elastic deformation or the energy dissipation from plastic deformation of the face-sheet depending on the severity of the damage state in the dented region. The predicted maximum deflection and impact force history are compared with published experimental and finite element results, and lower and upper bounds of the maximum dent depth are obtained.

Influence of the Fatigue Load Level and the Hole Diameter on the Laminate Structure’s Fatigue Performance

Abstract Damage tolerance of composite aircraft structure is one of the main areas of research, important when a new product is being developed. There are a number of variables, such as damage characteristics (dent depth, delamination area) and loading parameters (load type, amplitude of cyclic loading, load sequence) that need to be investigated experimentally [1]. These tests of composite materials are usually performed at an element level and are carried out in order to validate the analytical model, developed to predict the full-scale component’s behaviour. The paper presents the results of compression testing of the [36/55/9] carbon fibre/epoxy laminate, manufactured with the Automated Fibre Placement technology (AFP) and subjected to static and fatigue loads. The laminate compression loading mode was achieved through sandwich 4-point flexure. At the stage of fatigue testing, two parameters were investigated: the damage size, simulated by the hole diameter and the fatigue load level. Based on the test results, the laminate fatigue load limit equal to 75% of the OHC failure load was evaluated. By collating the static and fatigue tests results, the damage tolerance characteristic of the considered laminate was created.

Study on the Impact Damage of Submarine Pipelines by Anchor under Emergency Anchorage

In this article finite element modeling of anchor and submarine pipeline under the condition of emergency anchorage were built up by using ABAQUS software, then numerical simulations were made. Security of the pipeline was checked according to the ratio of the dent depth and pipe wall diameter after that. At the same time the relationships between towing anchor collision damage and pipe wall thickness as well as pipeline slenderness ratio were got by horizontal comparison. It was concluded that the damage degree of the towing anchor is reduced with the increase of thickness and slenderness ratio of the pipelines. Then vertical impact between dropping anchor and pipeline was analyzed by ABAQUS also. Finally, values of DNV specification rules were compared with the numerical simulation results. The research conclusions have some practical significance for the analysis of deformation regularity of the submarine pipelines damaged by the anchor, the design and repair of the pipelines.