Study Of Crack Propagation Mechanism And Numerical Simulation Of Tubesheet Cracking

Fracture is a major failure form of engineering materals which could lead to serious incidents with lots of loses especially in the chemical industries. In order to avoid or reduce fracture, crack initiation and propagation mechanisms must be understood. Three basic crack modes have been defined according to crack deformation. But most cracks in engineering are mixed-mode combined by the three basic crack modes. Crack mode transformation is a complicated problem which has to be addressed academically. In this dissertation, studies on crack propagation mechanisms have been carried out from macroscopic and microscopic point of view and tubesheet cracking in a practical heat-exchanger has been simulated to find the possible causes for the cracking.Firstly, edge dislocation emissions from a Modeâ… or Modeâ…¡crack tip along multiple inclined slip planes were simulated, and plastic zones as well as dislocation- free zones were obtained. It is found that the shape of the mode plastic zone is similar to that obtained by von Mises or Tresea yielding criterion but leaning forward from the crack tip. The shape of dislocation-free zone is similar to that of plastie zone. With dislocation emission, the plastic zone becomes larger and larger while the dislocation-free zone is getting smaller and smaller. Judged by the Strain energy density factor, the crack propagation potential decreases remarkably with dislocation emission. When a clear dislocation-free zone exists, the crack potential propagation direction will keep along the crack surface. However, when the plastic zone is fully developed or the dislocation-zone is vanished, the crack potential propagation direction may also be changed. The shape of plastic zone of Modeâ…¡is different from that based on von Mises. It is composed of three parts, and the most lies in front of the crack. There is a dislocation-free zone around the crack tip which has similar shape as the plastic zone. Dislocation emitting from the Modeâ…¡crack are more effective in the shielding of the crack from the external loadings while dislocations coming from dislocation dipoles have no obvious shielding effeects. With dislocation emission or the development of plastic zone in front of the Modeâ…¡crack tip, crack propagation becomes more difficult while the direction of the crack potential propagation may not be changed.Secondly, compact tension specimens with tilt cracks under monotonic fatigue loadings were tested to investigateâ… +â…¢mixed mode fatigue crack propagation in the material of #45 steel with the stress on the mode transformation and crack propagation rate. It is found that with the crack growth,â… +â…¢mixed mode changes to modeâ… and the larger the tilt angle or the larger the component of modeâ…¢. Crack mode transformation is governed by the Modeâ…¢component and the transformation rate is a function of the relative magnitude of the Modeâ…¢stress intensity factor. Even in the process of the crack mode transformation, the fatigue crack propagation is controlled by the Modeâ… deformation.Thirdly, Fracture surfaces of the fatigue test specimens were examined to investigate the fractography changes corresponding to the crack mode transformation. Results show that the fatigue cracks propagate with multi-sources initiated from micro-cracks on the line-cutting notch of the compact tension specimens. The tilted crack surface is rougher than that without tilting by presenting more tear ridges which actually contribute the tilt of the crack surface. Striations on the surface without tilting are more continuous while more secondary cracks are found on the tilted crack surfaces. With the crack mode transformation the fracture surfaces with different tilted cracks become more and more identical. So it can be reached that the Mode III component is responsible for the formation of the rough surface and secondary cracks.Finally, possible cracking causes for a tubesheet in a real heat exchanger have been analysed. A 3-D finite element model for a hydraulically expanded tube-to-tubesheet joint was established and the expansion process was simulated to obtain the residual stress in the tubesheet. Finite element model for a tubesheet with crack was also set up to investgate the possibility of crack propagation through thickness or tube-bridge. It is found that under the action of the residual stress induced by expansion, a crack along the tubesheet thickness or tube-bridge keeps open while it will close in the compressive region under the action of transversed pressure loading. So it seems that the residual expansion stress could be the driving force for the tubesheet cracking through the thickness.