Experimental damage tolerance evaluation of thick fabric carbon/epoxy laminates under low-velocity and high-velocity impact and compression-after-impact

Impact experiments of thick fabric carbon/epoxy laminate specimens, with small thickness ratio, are conducted at distinct energy levels and thicknesses to characterise the damage process. These specimens and loading conditions are representative of a new generation of critical structural components in aviation, such as wing spars, landing gear beams and fittings, that are increasingly being made entirely from composites. The tests address the need to better understand the damage process for specimens with a small thickness ratio since existing experimental impact data for large thickness ratio (thin laminates) may not be directly applicable. Two energy levels, two different fabric layups and two impact methods (drop-weight and gas-cannon) were used. Data from high-speed cameras were processed in a novel way, providing the force during impact. C-scans and micrographs were used to characterise damage. The results show that specimens with a thickness ratio of 5 (20 mm thick) experience more bending compared to specimens with a ratio 2.5 (40 mm thick). For gas-cannon impacts, this results in a higher delaminated area. The drop-weight impacts show almost no differences in damage size for the thickness range analysed. The influence of layup on the global impact response is negligible, but locally it can result in significant variations in dent depth. The dent depth scales linearly with the impact energy and the delaminated area linearly with the impact velocity. There is no clear correlation between the compression-after-impact failure mechanisms and the residual strength. Impact damage, at the current energy levels, showed a minimal reduction of residual strength.

Erosion Mechanism and Sensitivity Parameter Analysis of an innovative shaped Curved Pipeline

In the process of natural gas transportation, it is unavoidable for particles to collide with the wall, which will cause erosion of curved pipeline. Reasonable curved pipeline structure can effectively avoid the erosion failure. In this paper, an innovative shaped curved pipeline formed by extrusion of cylindrical indenter is presented. The erosion mechanism and sensitivity parameter analysis of the innovative shaped curved pipeline is studied by numerical simulation and compared with that of ordinary elbow. In addition, the effects of extrusion parameters and particle parameters on erosion of innovative shaped curved pipeline were also studied. The results show that the dent can effectively reduce the maximum erosion rate of elbow. With the increase of dent depth, the maximum erosion rate of elbow is decreasing. With the increase of indenter diameter, the ability to reduce the maximum erosion rate decreases. Under the harsh conditions of large particle diameter and high particle velocity, the dent has a better ability to reduce the maximum erosion wear rate, and the maximum erosion rate can be reduced by 26.8%.

Crack Propagation and Burst Pressure of Pipeline with Restrained and Unrestrained Concentric Dent-Crack Defects Using Extended Finite Element Method

Mechanical damage in form of dents, cracks, gouges, and scratches are common in pipelines. Sometimes, these damages form in proximity of each other and act as one defect in the pipe wall. The combined defects have been found to be more injurious than individual defects. One of the combined defects in pipeline comprises of a crack in a dent, also known as dent-crack defect. This paper discusses the development of finite element models using extended finite element criterion (XFEM) in Abaqus to predict burst pressure of specimens of API X70 pipeline with restrained and unrestrained concentric dent-crack defects. The models are calibrated and validated using results of full-scale burst tests. The effects of crack length, crack depth, dent depth, and denting pressure on burst pressure are investigated. The results show that restrained dent-crack defects with shallow cracks (depth less than 50% wall thickness) inside dents do not affect pipeline operations at maximum allowable operating pressure if crack lengths are less than 200 mm. Releasing restrained dent-cracks when the pressure is at maximum allowable operating pressure can cause propagation of deep cracks (depth of 50% wall thickness or more) longer than 60 mm. However, only very long cracks (200 mm and higher) propagate to burst the pipe. Cracks of depth less than 20% of wall thickness inside dents formed at zero pressure are not propagated by the maximum allowable operating pressure. Dent-crack defects having dents of depth less than 2% outside diameter of pipe behave as plain cracks if the dents are formed at zero denting pressure but are more injurious than plain cracks if the dents are formed in pressurized pipes.

Numerical Assessment of Dented Pipe Using Inline Inspection Data

The current finite element (FE) assessment methods of dented pipes are based on specific dent profiles, which are generally created based on the shape of indenters. However, the actual dent profile in real case scenarios is mostly irregular in shape, depending on the cause of damage. In this paper, FE analyses of dented pipes using inline inspection (ILI) data are presented. Based on the ILI data, the dent profile is generated by applying the nodal displacements to all the pipe nodes. The validation of this nodal displacement approach is discussed in this paper. Besides, a parametric study is carried out to study the behavior of dent for different dent depth, pipe geometry, and pipe grades. The significance of residual stresses generated during the dent formation on the behavior of dented pipe during the service life is also discussed. Finally, the remaining life estimation of dented pipes according to the API 579-1 is presented using FE analysis results.

Judge Me by My Size, Do You? How Reliable Are Dent Assessments Based on ILI Data?

Pipeline dents have historically been regulated and assessed using dent depth as the primary metric. Many of the earliest analytical models for dent remaining life are based upon depth. Current assessment guidelines from ASME and the Code of Federal Regulations utilize depth as a primary metric. Consequently, ILI geometry tool capabilities are stated in terms of dent depth. However, the best modern dent assessments, including both strain and fatigue assessments, are based on dent shape. At a minimum, these models require both axial and circumferential dent profiles, or the models may utilize the full three-dimensional shape of the dent. The utilization of advanced dent assessments is expected to grow in the future as the methods are incorporated into API Recommended Practices and US regulations. While operators may have confidence in the ability of an ILI tool to confidently capture the dent depth, the shape of a dent is a recent consideration that is not addressed by current tool specifications. Unlike depth alone, dent shape is often a function of sensor coverage, speed, and caliper technology. Unfortunately, there is virtually no information available on the reliability of these assessment methods when they are based on ILI data. To-date, there have been no published comparisons examining the variation in strain or fatigue life in identical dents between multiple inspections. The reliability of these dent assessment methods is critical when choosing safety factors or reinspection intervals. This study presents a first look at the repeatability of strain and remaining life assessments based on two separate geometry inspection using different technologies. The study examines dent strain according to ASME B31.8 and fatigue life calculated using shape factors and finite element methods for 257 dents. The paper examines the variation in each of the methods and provides guidance on how users should understand the results when they are based on a single geometry inspection.

Application of Noise Filtering Techniques for the Quantification of Uncertainty in Dent Strain Calculations

The integrity assessment of dents in pipelines is primarily driven by the dent depths as per the stipulations in current codes and standards. There is a provision for strain-based analysis to quantify the severity of dents based on their shapes in the ASME B31.8 non-mandatory Appendix R. In recent years, the pipeline industry has also started leveraging more advanced techniques such as Finite Element Analysis (FEA) for dent assessment. These assessments require the detailed deformation profile of dents, which are available from In-line Inspection (ILI) tools. The ILI tools use caliper arms that roll along the inside of the pipeline and scan the inner profile. The measurements recorded by each caliper arm are susceptible to noise due to the vibration of the ILI tool, and as a result, the dent shapes obtained from ILI are not smooth. Strain assessments of dents typically require the calculation of radius of curvature in the longitudinal and circumferential directions. This becomes a complex problem while the ILI data contains noise, particularly for relatively shallow dents, when the dent depth approaches the magnitude of the noise in the data. In these cases, the radius of curvature estimation can become highly inaccurate. Furthermore, the amount of noise in the data can vary between dents, and so the accuracy of the estimation varies as well. This paper presents several methods to resolve the above-mentioned issues. To address the issue of data noise itself, a combination of Fast Fourier Transform (FFT) and Gaussian filtering is used to produce a smooth profile that can be used to calculate the maximum radius of curvature of the dent. The smoothed profile also results in a better estimation of dent depth. To estimate the amount of uncertainty in the data, we apply many independent iterations of random noise to the smoothed curve. Characteristics required for further reliability analysis, such as dent depth or radius of curvature, are calculated for each iteration. This forms a distribution for each characteristic, and the properties of each distribution are used to quantify the uncertainty in the ILI data.

Dent Assessment and Management: API Recommended Practice 1183

Pipeline dents can be developed from the pipe resting on rock, a third-party machinery strike, rock strikes during backfilling, amongst other causes. The long-term integrity of a dented pipeline segment is a complex function of a variety of parameters including pipe geometry, indenter shape, dent depth, indenter support, secondary features, and pipeline operating pressure history at and following indentation. In order to estimate the safe remaining operating life of a dented pipeline, all of these factors must be considered and guidelines for this assessment are not available. US DOT regulations (49 CFR 192 and 195) include dent repair and remediation criteria broadly based upon dent depth, dent location (top or bottom side), pressure cycling (liquid or gas), and dent interaction with secondary features (weld, corrosion, cracks). The criteria defined above are simple to use, however, they may not direct maintenance to higher risk dent features and be overly conservative or, in some cases, unconservative. PRCI, USDOT, CEPA and other full-scale testing, finite element modelling and engineering model development research has been completed to evaluate the integrity of pipeline dents. These results have demonstrated trends and limits in dent behavior and life that can improve on existing codified and traditional treatment of dents. With these research results a guideline for dent management can be developed to support operators develop and implement their pipeline integrity management programs. This paper provides an overview of the newly developed API recommended practice for assessment and management of dents (RP 1183). The RP considers dent formation strain, failure pressure and fatigue limit states including the effects of coincident features (i.e. welds, corrosion, cracks and gouges). This paper will focus on how pipeline operators can derive value from this step change in integrity management for dents. The paper describes the basis for the dent screening and integrity assessment tools included in the RP. This RP provides well founded techniques for engineering assessment that may be used to determine the significance of dent features, if remedial actions are required and when these actions should be taken.

Experimental Investigation of Impactor Diameter Effect on Low-Velocity Impact Response of CFRP Laminates in a Drop-Weight Impact Event

The present study delved into the effect of impactor diameter on low velocity impact response and damage characteristics of CFRP. Moreover, the phased array ultrasonic technique (PAUT) was adopted to identify the impact damages based on double-sided scanning. Low-velocity impact tests were carried out using three hemispherical impactors with different diameters. The relationship of impact response and impactor diameters was analyzed by ultrasonic C-scans and S-scans, combined with impact response parameters. Subsequently, the damage characteristics were assessed in terms of dent depth, delamination area and extension shape via the thickness, and the relationships between absorbed energy, impactor displacement, dent depth and delamination area were elucidated. As revealed from experiment results, double-sided PAUT is capable of representing the internal damage characteristics more accurately. Moreover, the impactor diameter slightly affects the impact response under small impact energy, whereas it significantly affects the impact response under large impact energy.

Numerical studies on residual strength of dented tension leg platforms under compressive load

This paper focuses on numerical investigations and derived formulation to evaluate the residual strength of tension leg platforms (TLPs) with the local denting damage under axial compression loading. The damage generation scenarios in this research are represented the collision accidents of offshore stiffened cylinders TLPs with supply ships or floating subjects. The finite element model is performed using a commercial software package ABAQUS, which has been validated against the experiments from the authors and other researchers. Case studies are then performed on design examples of LTPs when considering both intact and damaged conditions. Based on the rigorous numerical results, the new simple design formulations to predict residual strength of dented TLPs are derived through a regression study as the function of a non-dimensional dent depth. The accuracy and reliability of the derived formulation are validated by comparing it with the available test results in the literature. A good agreement with existing test data for ship-offshore structure collisions is achieved. Keywords: dented stringer-stiffened cylinder; residual strength; tension leg platforms (LTPs); axial compression; residual strength formulation.