Study Of The Crack Interactions And Equivalent Method Of The Crack Group

Existence of crack group in reality is objective and unavoidable.At present,most studies about crack groups mainly focuse on special crack group configurations,such as double parallel cracks.For general crack groups,however,as the problems are complicated,envelope or projection methods are usually employed to coalesce and simplify the crack groups.But these methods do not consider crack interactions,and the results are very conservative.In this dissertation,experiments and numerical simulations are performed to investigate some critical problems about the fatigue growth of the crack groups.Concentrations are paid on revealing crack interaction mechanisms,quantitatively characterizing crack interactions,and proposing a new crack group equivalent model.The main works are as follows.(1)Fatigue crack growth rate tests of specimens with single and double cracks were carried out to investigate the interactions of double cracks with different sizes,orientations,and locations on the crack growth directions and crack growth rates.It is found that the double co-centered cracks experience a shielding effect,and the crack growth directions do not change.The outer crack tips of the double parallel offset cracks experience an enhancement effect,and the crack growth directions do not change either;the inner crack tips experience a shielding effect,and the crack growth directions change to the adjacent crack.(2)Taking the stress intensity factors calculated by the numical simulation as the parameters,the interactions of double cracks with different sizes,orientations,and locations are investigated.Results showed that if the double cracks are co-centered and the total project length does not increase,compared with any single crack,the crack interaction can be considered as a shielding effect.On the other hand,if the two cracks are offset and the total project length increases,the crack usually experiences an enhancement effect.With sufficient simulation results,a dimensionless boundary for the shielding or enhancement effect between cracks was established;a so-called"correctional function of stress intensity factor" was defined and obtained as the function of the relative lengths and locations of cracks.The "correctional function of stress intensity factor" quantitatively represents the crack interactions,and lays the foundation of simplification and equivalence of crack groups.(3)Stress analysis of double cracks under a remote tensile load was conducted with the finite element method.Magnitude and distribution of the stress were investigated,and the mechanisms of the crack interactions were revealed.It was found that if the crack experiences a shielding effect,the intensity of the stress field around the crack tips decreases;if the crack experiences an enhancement effect,the intensity of the stress field around the crack tips increases.In addition,the crack interactions can change the hoop stress around the crack tips and the material parameters of the Paris equation during the crack growth process,leading to the change of the crack growth directions and crack growth rates.(4)A new crack group equivalent model named "one-by-one take-away of outer short cracks" was proposed.In this model,short cracks outside the crack group are one-by-one taken away,and their effects on other cracks are considered in terms of the "correctional function of stress intensity factor".As crack interactions are considered,this model is more rational and accurate than the existing envelope or projection methods.The software for performing the equivalent process based on the crack group equivalent model proposed in this dissertation was programed by Python,and the interfaces were well designed with Qt Designer,which would greatly facilitate the engineering application of the model.Clearly the proposed crack equivalent model has enriched the theory of fracture mechanics,and provided theoretical bases for the safety assessment of engineering structures with crack groups.(5)To investigate the interactions of three cracks,fatigue crack growth rate tests of specimens with three cracks were carried out,and the stress intensity factors were calculated by the finite element method.It is found that in the specimens with three cracks,the outer crack tip with a larger stress intensity factor("the dangerous crack tip")has a larger crack growth length,and the crack growth direction at the crack tip does not change,verifying the premise of the crack group equivalent model.Meanwhile,the software of the crack group equivalent model was used to calculated the stress intensity factors at "the dangerous crack tips",verifying the validity of the crack group equivalent model proposed in this dissertation.