| Crack is the most dangerous defect for pressure vessels in service.Reasonable assessment of cracking is the basis for the safety evaluationof pressure vessels containing cracks. In this dissertation, numericalanalysis and experiments were performed together with the fracturetheory to study some cracking problems. TOFD inspection standardswere also preliminarily addressed. The main contributions are as follows.Firstly, an embedded elliptical crack in pressure vessels under tensionwas taken into consideration, and stress intensity factors at the crack tipwere calculated numerically with the emphasis on the influences of thestructure surface. It is found that when the depth is2times larger than thehalf short length of the ellipse, the structure surface effects on the crackcan be neglected and the numerical solution for the stress intensity factorswell agrees with the analytical solution. This result can be used todistinguish a shallow embedded crack from a deep embedded crack. It isalso found that the point with maximum stress intensity factor is alwaysat the end of minor axis of the ellipse whatever are the shape of the ellipse and the depth it was embedded.Secondly, the coplanar elliptical cracks in pressure vessels undertension were studied, and stress intensity factors at the crack tip werecalculated with the emphasis on the interaction between cracks. It isfound that:(1) for subsurface coplanar cracks, when the distance between thedouble embedded elliptical cracks is larger than the half long length ofthe ellipse, influence between the cracks can be neglected. Unlike thesingle embedded crack, owing to the crack interactions, the point withmaximum stress intensity factor is not always at the end of minor axis ofthe ellipse, it may swift to the end of the major axis, especially when theellipse is close to a circle.(2) for elliptical surface cracks, the interactions between the coplanarcracks is similar to the coplanar subsurface cracks. A little difference isthat the maximal SIF always locates at a fixed point and is not affected bythe distance between cracks when the shape ratio is a specific value.(3) regarding the effect of the thickness and mutual influence betweencracks, ratios of the stress intensity factor for double cracks with respectto a single crack as well as between different points on the single crackfront are regressed using least square root method with the confidencezone of95%.Thirdly, T-shape specimens with different declined crack were studied under the tension load. Stress and displacement field around the crack tipwere calculated numerically and measured by digital image correlation(DIC). The microstructure of fracture surface was observed and the effectof declined angle was analyzed.Fourthly, the envelope method for covering multiple coplanar crackswhich is used in ASME-2235and GB/T19624for the safety assessmentof equipment with cracks was studied. It is found that in some cases theenvelope method could not reveal the crack interactions correctly. Basedon the present research, a new method, namely the method themagnification factor on the stress intensity factor, for effectivelycoalescencing coplanar cracks is proposed which is more accurate toreflect the crack mutual influence and very convenient in engineeringapplication.Fifthly, based on the strain energy intensity factor theory, thecoalescence process of surface coplanar crack propagation under fatigueloading was simulated numerically. Experiments were conducted forvalidation and the experimental measurements agree well with thenumerical results. Special attentions were paid to the changes of the crackshape and crack propagation rates. By performing fractographicobservation, it was found that the cleavage fracture was the main featuresof fatigue crack propagation.Finally, a data table of the allowable crack size for single crack as well as coplanar cracks was given which could be used in engineeringand could be a basis for working out the TOFD inspection standards.
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