Study On Surface Crack Propagation Behavior In Steel Plate Strengthened With CFL

Surface crack is a common defect in oil and gas pipeline, steel bridge, offshore oilplatform, aviation mechanical and other steel structures. It has little effect to load-bearingcapacity of steel structures when the surface crack size is small, but it is easy to propagationunder the repeated interaction of service load and environments, and results in structuredamage, especially leads to catastrophic results if it cannot be discovered timely. So surfacecrack is an important security risk and cannot be belittled. In order to reduce or completelyeradicate the security risk, adopting fiber reinforced polymer (FRP) to strengthen steel platecontaining surface crack is off course an advanced technology, and is also a forefront question.So a new type of FRP “carbon fiber laminate, CFL”, which was invented by this researchgroup, was adopted to strengthen steel plate containing3D surface crack, and numericalanalysis of the3D surface crack problems of CFL strengthened steel plate was conducted byusing the finite element method. And on this basis, propagation rule of3D surface crack inCFL strengthened steel plate was discussed. The main contents and conclusions of this paperare as follows:1) Numerical analysis method of3D surface crack in CFL strengthened steel plate. Finiteelement method for calculating stress intensity factors is put forward according to differentstress status in surface crack front. Based on the parameterized modeling using Python,Abaqus interface program was written, and stress intensity solver for3D surface crack isdeveloped. Adopting the above program and calculating method, the errors of surface cracksstress intensity factors calculated by using Abaqus can be reduced distinctly, and repeated andheavy work was avoided. Using the stress intensity solver, the finite element model can bemodified and reconstructed conveniently, and calculating efficiency was improved greatly.2) The impact rules of thickness of CFL on stress intensity factors (strengthened effects).stress intensity factors of3D surface crack in CFL strengthened steel plate under the action oftensile load, bending load and tensile-bending combined load in the condition of differentthickness of CFL, is analyzed by using above numerical calculating method and stressintensity factors solver. The results showed that, the stress intensity factors of3D surfacecrack in steel plate can be significantly reduced by strengthening with CFL, and theincreasing amplitude of strengthening effects (decreasing amplitude of stress intensity factors)decreased gradually with the increasing of thickness of CFL, especially after the thickness ofCFL great than0.6mm, strengthening effects almost no longer increase, so it is waste to use too more strengthening materials. In the condition of equal maximum tensile stress,strengthening effects of CFL on stress intensity factors under three kinds of loads from high tolow order was: bending load, tensile-bending combined load, tensile load.3) The impact rules of crack parameter on stress intensity factors. Finite elementanalysis was conducted using the stress intensity factors mentioned above to discuss theimpact rules of different crack parameter on stress intensity factors (strengthening effects)under the action of tensile load, bending load and tensile-bending combined load. The resultsshowed that, the increasing amplitude of CFL strengthening effects (decrease amplitude ofstress intensity factors) decreased gradually with increasing of the centrifugal angle θ and thecrack shape ratio a/c under the action of tensile load, bending load and tensile-bendingcombined load. Strengthening effects near the specimen surface (θ was small) was significantwith the increasing of crack relative depth a/B, but it decreased slightly when a/B<0.3andincreased when a/B>0.3. In the condition of equal maximum tensile stress, strengtheningeffects of CFL decreased slowly with the increasing of tensile-bending stress ration σt/σbunder tensile-bending combined load.4) Semi-empirical expression of stress intensity factors in steel plate strengthened withCFL. Using the stress intensity factors mentioned above, a great deal of finite elementcalculating was conducted to obtain the data of stress intensity factors of3D surface crack inCFL strengthened steel plate under different loads in the condition of different thickness ofCFL, different crack shape ratio a/c and different crack relative depth a/B. And then byanalysis and fitting of the calculating results, a series of semi-empirical expression of stressintensity factors of3D surface crack in CFL strengthened steel plate was put forward. Andcomparing finite element results, the average errors of these expressions was about2.5%.5) Experiment study on fatigue propagation rules of3D surface crack in CFLstrengthened steel plate. Combined with the research background of the project, this paperadopt high strength pipeline steel X80to design and make specimens to explore experimentalmethod of fatigue propagation of3D surface crack in CFL strengthened steel plate under theaction of bending load and in the condition of different crack shape ratio a/c and differentcrack relative depth a/B, and successfully applied beach-mark method to the crackpropagation experiment. Experiment results showed that, CFL strengthening can significantlyimprove fatigue crack propagation life of the specimen (more than2times).6) Fatigue propagation rule of3D surface crack in CFL strengthened steel plate. Byanalyzing the experiment data of fatigue crack propagation, a criteria, which adopted surfacefatigue crack propagation rate in the direction of length and depth as the crack propagation rule, was established. Combined with the finite element calculating method of stress intensityfactors of surface crack, this paper proposed the expression of the corresponding fatigue crackpropagation rate. The results showed that, under the action of three-point bending load,3Dsurface crack propagation rate in the direction of length and depth can be described by Parislaw. But for the non-strengthened specimens, surface crack propagation rate in the lengthdirection was more complex, and cannot be described by Paris law.