Abstract
Pipes are widely utilized in many industries such as petroleum,chemical,nuclear power and pharmaceutical.The flaws of pipes in the process of manufacture and service are inevitable.Crack is a common type of defects in pipe which is also the main cause of fracture.As a result of the occurrence of leakage and fracture accidents caused by crack,the stress intensity factor(SIF)of the cracked pipe is a vital part of the design and evaluation of the pipeline system.To avoid destructive failures of cracked pipes,it is essential to accurately examine the SIF for these cracks.The SIF solutions were used in analysis for a circumferential semielliptical surface crack in a plain pipe.
Raju and Newman made a great contribution on surface cracks.They presented an empirical stress intensity factor equation[1]for a surface crack as a function of parametric angle,crack depth,crack length,plate thickness and plate width for tension and bending.The SIF were obtained from a finite-element analysis of semielliptical surface cracks in finite elastic plates subjected to tension or bending loads.Then they presented stress intensity factors[2]for a wide range of nearly semi-elliptical surface cracks in pipes and rods.The configurations were subjected to either remote tension or bending loads.
Zhao et al.[3]provided an engineering assessment of an oval-shaped clad pipes with a circumferential part-through surface crack subjected to bending moment based upon equivalent stressstrain relationship method(ESSRM)in association with EPRI J-estimation procedure.Plastic limit load equations were developed particularly to identify the equivalent stress-strain relationship of the welded clad pipe.
Hoh et al.[4]developed a methodology to simulate and calculate the stress intensity factors and weld toe magnification factors for semi-elliptical surface cracks in a circumferentially welded pipe.They considered several parameters including depth-to-thickness(a=t)ratios and crack shape aspect ratios(a=c).
Carpinteri et al.[5]investigated the influence of a circular-arc circumferential notch in a pipe.He identified the stress concentration factor and the stressintensity factor(SIF)along the surface crack front.
On the other hand,the superior mechanical,fatigue,high strength to density ratio and in-service properties of carbon fibre reinforced polymer(CFRP)composites made them remarkable candidates for strengthening and retrofitting of steel structures.CFRP reinforcement had broad prospects for cracked pipes.
However,little research had been undertaken on SIF for circumferential semielliptical surface cracks in pipes under axial tension repaired by carbon fiber reinforced plastics(CFRP).This article focused on the numerical simulation of circumferential semielliptical surface cracks in a pipe under tension.Due to analytical results available for only limited problems with simple configurations,ABAQUS was used in this article to investigate.A meshing technique with mixed types of tetrahedron and hexahedron elements had been used to achieve simplification in modelling complex geometry of crack region.The finite element model was shown in Figure 1.Circumferential surface crack was shown in Figure 2.By applying the finite element method(FEM),the effects of different relative crack depth,tensile stress,and applying CFRP to reinforce the pipe were considered.
Figure 1 Finite element model
Figure 2 Circumferential surface crack
This research concentrated on the numerical simulation of circumferential surface cracks in a pipe under tension with carbon fiber reinforced plastics(CFRP)reinforcement.Reductions of SIF were surveyed with various parameters including different relative crack depth c/t,tensile stress S,CFRP length Lc and CFRP thickness Tc.The CFRP reinforcement seriously reduced SIF,and the reduction effect became more incredible with increased relative crack depth.Reinforcing the cracked region of the pipe reveals that laminates with larger thickness could be more advantageous to reinforcing a defected structure.