Cracks In Steel Pipes Research Articles

Numerical analysis on the SIF of internal surface cracks in steel pipes reinforced with CRS subjected to bending

ABSTRACT In this paper, Composite Repair System (CRS) is applied to repair the circumferential internal surface cracked steel pipes subjected to bending. The Stress Intensity Factor (SIF) is quantitatively analysed by means of numerical simulations, and the crack growth rate is predicted by using the Paris’ law. First, the three-dimensional finite element (FE) model is developed, and its reliability of evaluating SIF of internal surface crack in CRS reinforced pipe is validated. Then based on the FE method and combined with Paris’ law, a case study is deployed to predict the internal surface crack growth in steel pipes reinforced with CRS. The results show that CRS have significantly reduced the SIF of the internal surface crack and decrease the crack growth rate, while unchanging the variation trend of the crack aspect ratio. Afterwards, a parametric study is performed in order to guide the optimisation design of CRS reinforcement.

An investigation on the circumferential surface crack growth in steel pipes subjected to fatigue bending

In the present paper, we propose an analytical method to calculate the Stress Intensity Factor (SIF) of circumferential surface cracks in steel pipes subjected to bending. In light of pipe geometry and bending load case, the analytical formula is raised by introducing new bending correction factors and new geometry correction factors on the basis of the Newman-Raju’s method. The bending correction factors are deduced based on the bending stress gradient, while the geometry correction factors are determined by parametric studies for internal surface cracks and external surface cracks respectively. Owing to a large data set requirement by the parametric studies, three-dimensional finite element (FE) models of evaluating SIFs of circumferential surface cracks are developed. The FE method is validated to ensure that it could provide accurate SIF estimations. Analytical verification is conducted which shows that the SIF evaluated by the proposed analytical method match well with the results evaluated by the recommended analytical method. Then experimental investigations of external surface crack growth in offshore steel pipe subjected to fatigue bending are implemented to further validate the analytical method of predicting surface crack growth rate. The analytical results match well with the test results and the available experimental data from literature, indicating that the analytical method can be used for practical purposes and facilitate the crack growth evaluation and residual fatigue life prediction of cracked steel pipes.

Multisource Data Fusion for Classification of Surface Cracks in Steel Pipes

This paper focuses on the development and validation of a robust framework for surface crack detection and assessment in steel pipes based on measured vibration responses collected using a network of piezoelectric (PZT) wafers. The pipe structure considered in this study contained multiple progressive cracks occurring at different locations and with various orientations (along the circumference or length). The fusion of data collected from multiple PZT wafers was investigated based on two approaches: (a) combining the raw data from all sensors before establishing a statistical model for damage classification and (b) combining the features from each sensor after applying a multiclass support vector machine recursive feature elimination (MCSVM-RFE), for dimensionality reduction, and taking the union of discriminative features among the different sources of data. A MCSVM learning algorithm was employed to train the data and generate a statistical classifier. The dataset consisted of ten classes, consisting of nine damage cases and the healthy state. The accuracy of the prediction based on the two fusion approaches resulted in a high accuracy, exceeding 95%, but the number of features needed to enrich the accuracy (95%) differed between the two approaches. Furthermore, the performance and the precision in the prediction of the classifier were evaluated when the data from only a single sensor was used compared with the combined data from all the sensors within the network. Very promising results in the classification of damage were obtained, based on the case study that included multiple damage scenarios with different lengths and orientations.

Failure Analysis of Composite Repaired Pipelines with an Inclined Crack under Static Internal Pressure

The aim of the present work is to study the efficiency of the glass fiber reinforced polymer patch for repairing cracked steel pipe subjected to internal pressure. The effect of fiber orientation, [0°]8s, [90°]8s, and [0°/90°]4s, of bonded composite repair on reducing J-integral of stationary crack with different inclination angles (θ) is studied using the 3-D finite element method, FEM. Extended-FEM has been adopted to simulate the crack growth of different inclined stationary cracks in steel pipe subjected to internal pressure. It has been found that, the growing crack emanated from inclined stationary crack switched its path to be under pure mode I. The crack initiation pressure of inclined stationary crack in steel pipe with composite repair is higher than that of pipe without composite repair. The composite repair reduced the value of J-integral of stationary crack in steel pipe. This reduction is strongly affected by the crack length and θ of the stationary crack and it is fairly affected by the fiber orientation.

Repair of small damages in steel pipes with composite patches

The temperature change, internal pressure and corrosion are main reason of cracks in pipes. Removal of damaged pipes with new ones gives rise to major disruption during operation. This causes loss of time and cost. This study aims to prevent crack propagation and to obtain fast repair in damaged area, at the beginning of the damage. Repair of small cracks in steel pipes using composite patches has been studied in this work. The aim of the study is to repair small cracks less than 8 mm length. For this purpose, holes with 8 mm diameters were drilled in 170 mm long pipes and composite patches with 2, 2.6 and 3.2 mm thickness and 35, 40 and 45 mm overlap lengths with 60, 90 and 120° overlap angles were attached with an adhesive of 0.2 mm thickness. The repaired pipes were subjected to internal pressure until damage occurred. A finite element model was developed and the numerical results were verified based on experiments. The repair of steel pipe with adhesive bonding was successfully achieved under high pressure with composite patches.The results show that the increase in overlap angle results in an increased surface area, and consequently an increase in failure load. Furthermore, the failure load increased with an increase in patch thickness. Additionally, it is seen that the failure load does not change significantly with increasing patch thickness.

Analytical model of elastic fracture toughness for steel pipes with internal cracks

Corrosion or manufacture defects can cause internal cracks in steel pipes. For ductile materials, the crack front can yield before the stress intensity reaches its fracture toughness. The yielding of the crack front could ease the stress concentration at the crack front. Therefore, to predict the failure of cracked steel pipes using linear elastic fracture mechanics it is necessary to quantify the part of fracture toughness that withstands the elastic stress field, namely, elastic fracture toughness. This paper intends to propose an analytical model of the elastic fracture toughness for steel pipes with internal surface cracks.

Fracture Assessment of Axial Crack in Steel Pipe under Internal Pressure

Fracture assessment of axial crack in steel pipe subjected to internal pressure is reviewed. Assessment results were obtained using different methods FC, GS and FAD in order to compare between their results and facilitate their use in any maintenance program. Material type and crack geometry are considered as the main two parameters controlling the failure of the pipe. The constitutive equation of the material is represented by the Ramberg – Osgood model. Six different half crack lengths (40, 60, 80, 100, 120,and 140mm) and four values of the ratio between the crack depth and the pipe thickness are considered (0.2, 0.4, 0.6 and 0.8). The relationship between the fracture parameters (i.e., stress intensity factor K and the J-integral) and the crack length for each assessment method is used. The material properties, plane strain fracture toughness KIc as well as the critical value of J-integral Jcr, are used to determine the critical crack size under specified internal pressure. The assessment method that gives smaller critical crack size is the one which is more conservative than the others. The results obtained by using GS and FAD assessment methods show this type of conservation.

Effect of material plasticity on fatigue crack propagation under complex stress state

The maximum energy release rate criterion, i.e., G max criterion, is commonly used for crack propagation analysis. However, this fracture criterion is based on the elastic macroscopic strength of materials. In the present investigation, a modification has been made to G max criterion to implement the consideration of the plastic strain energy. This criterion is extended to study the fatigue crack growth characteristics of mixed mode cracks in steel pipes. To predict crack propagation due to fatigue loads, a new elasto-plastic energy model is presented. This new model includes the effects of material properties like strain hardening exponent n, yield strength σy and fracture toughness and stress intensity factor ranges. The results obtained are compared with those obtained using the commonly employed crack growth law and the experimental data.

Scattering of Guided Waves by Circumferential Cracks in Steel Pipes

A novel numerical procedure is presented in this paper to study wave scattering problem by circumferential cracks in steel pipes. The study is motivated by the need to develop a quantitative ultrasonic technique to characterize properties of cracks in pipes. By employing wave function expansion in axial direction and decomposing the problem into a symmetry problem and an antisymmetry problem, a three-dimensional wave scattering problem is then reduced into two quasi-one-dimensional problems. This simplification greatly reduces the computational time. Numerical results for reflection and transmission coefficients of different incident wave modes are presented here for a steel pipe with cracks (may have arbitrary circumferential crack length and radial crack depth) and they are shown to agree quite closely with available but limited experimental data.

Vibro-Acoustic Modulation Nondestructive Evaluation Technique

Nonlinear acoustic technique has been recently introduced as a new tool for nondestructive inspection and evaluation of fatigued, defective, and fractured materials. Various defects such as cracks, debonding, fatigue, etc. lead to anomalous high levels of nonlinearity as compared with flawless structures. One of the acoustic manifestations of such nonlinearity is the modulation of ultrasound by low frequency vibration. Two methods employing the nonlinear interaction of ultrasound and vibration were developed, namely vibromodulation (VM) and impactmodulation (IM) methods. VM method employs forced harmonic vibration of a structure tested, while IM method uses impact excitation of structure natural modes of vibration. The feasibility tests were carried out for different objects and demonstrated high sensitivity of the methods for detection of cracks in steel pipes and pins, bonding quality in titanium and thermoplastic plates used for airspace applications, cracks in combustion engine, adhesion flaws in bonded composite structures, and cracks and corrosion in reinforced concrete. The model of the crack allowing to describe the modulation of sound by vibration is discussed. The developed nonlinear technique demonstrated certain advantages as compared with the conventional linear acoustic technique, specifically discrimination capabilities, sensitivity, and applicability to highly inhomogeneous structures.