L.-H. Lee

Constitutive Modeling and Rupture Predictions of Al-6061-T6 Tubes Under Biaxial Loading Paths

This brief note reports the results of a set of biaxial experiments on Al-6061-T6 tubes tested to rupture under radial stress paths of combined internal pressure and axial load. The experiments are then simulated with shell-type finite element models, in which several yield functions are calibrated and implemented and their performance evaluated against the experimental results.

Plastic Buckling and Collapse of Tubes Under Bending and Internal Pressure

This paper presents results from a recent combined experimental-analytical study of the inelastic response and the sequence of events that lead to collapse of pipes bent under internal pressure. Experimental results from stainless steel 321 seamless tubes with D/t of 52 are reported. The tubes were loaded by pure bending at fixed values of pressure ranging from zero to a value that corresponds to 0.75 times the yield pressure. The moment-curvature response is governed by the inelastic characteristics of the material. Bending induces some ovalization to the tube cross section while, simultaneously, the internal pressure causes the circumference to grow. Following some inelastic deformation, small amplitude axial wrinkles appear on the compressed side of the tube, and their amplitude grows stably as bending progresses. Eventually, wrinkling localizes, causing catastrophic failure in the form of an outward bulge. Pressure increases the wavelength of the wrinkles as well as the curvature at collapse. The onset of wrinkling is established by a custom bifurcation buckling formulation. The evolution of wrinkling and its eventual localization are simulated using a FE shell model. The material is represented as an anisotropic elastic-plastic solid using the flow theory, while the models are assigned initial geometric imperfections that correspond to the wrinkling bifurcation mode. It will be shown that all aspects of the observed behavior including the failure by localized bulging can be successful reproduced by the models developed.Copyright

Enhanced design criteria for offshore pipeline integral buckle arrestors

Integral buckle arrestors are relatively thick-wall rings periodically welded in an offshore pipeline at intervals of several hundred meters in order to safeguard the line in the event a propagating buckle initiates. They provide additional circumferential rigidity and thus impede downstream propagation of collapse, limiting the damage to the length of pipe separating two arrestors. The effectiveness of such devices was studied parametrically through experiment and numerical simulations in Park and Kyriakides [2]. The experiments involved quasi-static propagation of collapse towards an arrestor, engagement of the arrestor, temporary arrest, and the eventual crossing of collapse to the downstream pipe at a higher pressure. The same processes were simulated with finite element models that included finite deformation plasticity and contact. The experimental crossover pressures enriched with numerically generated values were used to develop an empirical design formula for the arresting efficiency of such devices. A recent experimental extension of this work revealed that for some combinations of arrestor and pipe yield stresses the design formula was overly conservative. Motivated by this finding, a new broader parametric study of the problem was undertaken which demonstrated that the difference between the pipe and arrestor yield stress affects significantly the arrestor performance. The original arrestor design formula was then modified to include the new experimental and numerical results producing an expression with a much wider applicability.Copyright