Failure assessment of cracked uni-planar square hollow section T-, Y- and K-joints using the new BS 7910:2013+A1:2015

This paper covers the validation of standard safety assessment procedure in the new BS 7910:2013+A1:2015 for cracked uni-planar square hollow section (SHS) T-, Y- and K-joints using the finite element analyses. The procedure is based on the failure assessment diagram (FAD) method. A completely new and robust finite element mesh generator is developed, and it is validated using the full-scale experimental test results. FAD curves are constructed using the elastic J-integral (J e Je ), the elastic-plastic J-integral (J ep Jep ) and the plastic collapse load (P c Pc ) values calculated using the 3D cracked models of the joints. The results reveal that the standard Option 1 FAD curve of the new BS code is not always safe in assessing the safety and integrity of cracked uni-planar SHS joints. Therefore, a penalty factor of 1.2 for plastic collapse load is recommended to move all the constructed Option 3 FAD curves above the standard Option 1 curve. The new Option 3 FAD curves are found to generate optimal solutions for cracked uni-planar SHS T-, Y- and K-joints

Monitoring fatigue crack growth in welded joints using alternating current potential drop (ACPD) technique

To study the feasibility of using the Alternating Current Potential Drop (ACPD) technique to measure the rate of fatigue crack propagation in a plate-to-plate welded joints.RG 32/9

Fatigue strength of circular and rectangular hollow sections subjected to complex loading conditions

In order to understand the fatigue growth behaviors of tubular joints, large-scale tubular joint fatigue tests had been performed for a number of decades. But most of them were tested under basic loading, i.e. axial load (AX), in-plane-bending (IPB) and out-of-plane (OPB). In the current test program, full-scale circular hollow sections (CHS) and rectangular hollow sections (RHS) tubular joints were tested under the basic and combined loading so as to understand their fatigue crack growth behaviors as well as their fatigue strengths. The tests in this research project include experimental stress analyses under basic and combined loading and fatigue tests under basic and combined loading. The static tests were carried out first to determine the stress concentration factors (SCFs) of the joints, and the peak hot spot stress (HSS) locations where the cracks may initiate. The SCF results are also used to compare with some of the well-established existing SCF equations. In the past decades, many tests on tubular joints had been performed (van Delft et al., 1986; Ritchie et al., 1989; Myers, 1998 etc.), but the 3D crack shapes formed during propagation were scarcely captured in these fatigue tests. Therefore, it is difficult to model the accurate crack surface in finite element models which may produce inaccurate stress intensity factors solution. In order to validate the stress intensity factors along the crack front of the tubular joints for the next phase of the research project, fatigue tests were carried out whereby the 3D crack propagation and profiles were monitored using the Alternating Current Potential Drop (ACPD) technique, and hence the fatigue strengths of these specimens.RG 04/0

Analysis of non-load-carrying and load-carrying fillet welds using dual boundary element method

The main objective of the Applied Research Project is to study the parameters influencing the fatigue strength of non-load carrying and load-carrying cruciform filter welded joints. Dual boundary element method has been used extensively for the analysis of these welded joints due to its ability to perform analysis of crack problems in a single-region formulation.RP 54/9

Fracture analysis of load-carrying cruciform fillet welded joints with multiple cracks

Load-carrying cruciform fillet welded joints configuration is frequently encountered in many welded structures. Different from non-load-carrying joints, the crack in load-carrying joints can propagate from the weld root or from the weld toe. Hence, the crack propagation location is critical for a given weld size, transverse and main plates thickness. In this study, a finite element (FE) mesh generator developed previously is extended to generate 3-D models of load-carrying joints containing cracks at the weld root and the weld toe. Then, extensive parametric study is carried out to calculate the stress intensity factors (SIFs). Corresponding SIF equations are proposed according to multiple regression analysis. Based on these equations obtained for the weld root and weld toe cracks, the critical crack and weld sizes at which the crack propagates from the weld root to the weld toe are determined.Accepted versio

A nonlinear multi-spring tire model for dynamic analysis of vehicle-bridge interaction system considering separation and road roughness

This paper proposes a nonlinear multi-spring tire (NMST) model for dynamic analysis of vehicle-bridge interaction (VBI) system considering separation and surface roughness. First, a nonlinear single-spring tire (NSST) model is developed, which can only provide compression force rather than extension force. By introducing it into VBI system, the dynamic interaction problem considering separation and road roughness becomes a typical dynamic problem with material nonlinearity which can be easily solved by the combination of Newmark method and Newton method: the coupled governing equations never change whatever the tire separates from the bridge or not. It is more convenient because iterative process is required in conventional method to check the separation status and once separation occurs, the coupled governing equations have to change and decouple into independent equations corresponding to vehicle and bridge, respectively. It is then extended to NMST model which is more realistic because the contact surface of tire and road are of finite size instead of a point and the tire usually cannot touch the bottom of valleys in the road profile. In the numerical examples, it can be found that the dynamic responses of both bridge and vehicle obtained by NMST model are better than those obtained by NSST model because they have lower high frequency components, which is closer to the observation in the field testing. The proposed NMST model can be easily incorporated into various VBI models, which benefits for numerical study of dynamic responses of both vehicle and bridge and indirect methods to identify modal properties and local damage of bridge structures

Estimation of mode shapes of beam-like structures by a moving lumped mass

This paper presents a method to estimate mode shapes of beam like structures by using the acceleration of a moving lumped mass. In fact, the coupled frequencies of the vehicle-beam interaction system are time varying and the non-stationary instantaneous frequencies (IFs) contain information of mode shapes. The theoretical analysis in this paper shows that the mode shapes can be re-constructed by using the IFs; therefore, extracting mode shapes becomes a problem of IF estimation. A modified time-frequency analysis method based on weighted polynomial chirplet transform is developed to estimate the non-stationary IFs. Moreover, a new sampling algorithm based on accumulative measured energy is proposed to reconstruct the mode shapes, in which more data are sampled at the area with higher measured energy, making it more robust to noise. The proposed method is more convenient since only a lumped mass with a single accelerometer is required, and it is more practical because external exciter is not required, and the surface roughness can be the source of excitation. Numerical simulations and laboratory scale experiments have been carried out, which show that the proposed method performs well in extracting mode shapes, even if the travelling speed is high.Nanyang Technological UniversityThe authors would like to thank the School of Civil and Environmental Engineering at Nanyang Technological University, Singapore for kindly supporting this research topic

Fracture Response of Girth-Welded Pipeline With Canoe Shape Embedded Crack Subjected to Large Plastic Deformation

Long-distance offshore pipelines always suffer large plastic deformation during installation and operation. Accompanied by high internal pressure, potential flaws are found to initiate from the girth welds, and this brings a significant challenge to the structural integrity of the pipelines. The currently used procedures for fracture assessment of pipelines are usually stress based, which are unsuitable for application to cracked pipeline subjected to large plastic deformation. Therefore, the aim of this paper is to investigate the fracture assessment of pipeline subjected to large plastic deformation and identify and understand the critical parameters influencing the fracture responses under actual loading conditions. The evolution of crack tip opening displacement (CTOD) of a pipeline segment with an embedded canoe shape crack located in the middle of the girth weld is investigated under pure bending and biaxial loading through 3D elastic–plastic finite-element simulations. The effects of crack width, crack length, pipeline thickness, material hardening, and internal pressure on fracture response are discussed. Finally, a strain-based failure assessment diagram (FAD) is developed, and comparison between fracture assessment by BS7910:2013 and finite-element simulations concludes that the former produces conservative predictions for deep crack

Fracture behavior of clad pipeline containing a canoe shape surface crack subjected to large bending moment

This paper concerns the fracture assessment of a clad pipeline subjected to large bending moment, and identifies different parameters influencing the fracture behaviors. The evolution of Crack Tip Opening Displacement (CTOD) of a pipeline containing a canoe shape crack on the external surface of the girth weld is studied under pure bending and combined pure bending and internal pressure through 3D elastic-plastic Finite Element (FE) simulations. Various parameters affecting the evolution of CTOD like crack depth, crack length to perimeter diameter ratio, internal pressure and the weld geometrical configurations have been investigated. It is observed that only the average width of the weld has a significant influence on the fracture response under pure bending, while the weld reinforcement height and fusion line slope have a moderate influence on the fracture behavior under biaxial loading. The crack at the interface between the weld and back steel has a similar fracture response to that of a crack located in the weld with equivalent effective crack depth. Finally, a strain based Failure Assessment Diagram (FAD) is proposed and compared with the fracture assessments produced by BS7910:2013, and 3D elastic-plastic FE simulations suggest that the British Standard is more conservative especially for deep cracks

Detection of damaged supports under railway track based on frequency shift

In railway transportation systems, the tracks are usually fastened on sleepers which are supported by the ballast. A lot of research has been conducted to guarantee the safety of railway track because of its importance, and more concern is expressed about monitoring of track itself such as railway level and alignment. The ballast and fasteners which provide strong support to the railway track are important as well whereas the detection of loose or missing fasteners and damaged ballast mainly relies on visual inspection. Although it is reliable when the fastener is missing and the damaged ballast is on the surface, it provides less help if the fastener is only loose and the damaged ballast is under the sleepers, which are however frequently observed in practice. This paper proposes an approach based on frequency shift to identify the damaged supports including the loose or missing fasteners and damaged ballast. In this study, the rail-sleeper-ballast system is modeled as an Euler beam evenly supported by a series of springs, the stiffness of which are reduced when the fastener is loose or missing and the ballast under the sleepers is damaged. An auxiliary mass is utilized herein and when it is mounted on the beam, the natural frequencies of the whole system will change with respect to the location of the auxiliary mass. The auxiliary mass induced frequency shift is analyzed and it is found the natural frequencies change periodically when the supports are undamaged, whereas the periodicity will be broken due to damaged supports. In fact, the natural frequencies drop clearly when the auxiliary mass moves over the damaged support. A special damage index only using the information of the damaged states is proposed and both numerical and experimental examples are carried out to validate the proposed method.Accepted versio