Ratcheting, wrinkling and collapse of tubes under axial cycling

Abstract

a b s t r a c t Circular tubes compressed into the plastic range first buckle into axisymmetric wrinkling modes. Initially the wrinkle amplitude grows with increasing load. The wrinkles gradually induce a reduction in axial rigidity eventually leading to a limit load instability followed by collapse. The two instabilities can be separated by strain levels of a few percent. This work investigates whether a tube that develops small amplitude wrinkles can be subsequently collapsed by persistent cycling. The problem is first investigated experimentally using SAF 2507 super-duplex steel tubes with D/t of 28.5. The tubes are first compressed to strain levels high enough for mild wrinkles to form; they are then cycled axially under stress control about a compressive mean stress. This type of cycling usually results in material ratcheting or accumulation of compressive strain; here it is accompanied by accumulation of structural damage due to the growth of the amplitude of the initial wrinkles. The tube average strain initially grows nearly linearly with the number of cycles, but as a critical value of wrinkle amplitude is approached, wrinkling localizes, the rate of ratcheting grows exponentially and the tube collapses. The rate of ratcheting and the number of cycles to failure depend on the initial compressive pre-strain and on the amplitude of the stress cycles. However, collapse was found to occur when the accumulated average strain reaches the value at which the tube localizes under monotonic compression. A custom shell model of the tube with initial axisymmetric imperfections, coupled to a cyclic plasticity model, are presented and used to simulate the series of experiments performed successfully. A sensitivity study of the formulation to the imperfections and to key constitutive model parameters is then performed.

Strain ratcheting of steel tubulars with a rectangular defect under axial cycling: A numerical modeling

“…The cylindrical specimens tested were geometrically very similar to those used in the cyclic tests carried out by Limam et al [2], Jiao and Kyriakides [1] and monotonic tests conducted by Paquette and Kyriakides [4]. These researchers, however, did not include corrosion or geometrical defects in their experiments.…”

“…7). The element spacing has been selected based on the Jiao and Kyriakides [1] experimental results. They carried out wrinkling and ratcheting tests on intact steel tubes.…”

“…Empirical findings from other studies corroborate these results. Jiao and Kyriakides [1] reported results from some ratcheting tests carried out under axial cycling loads. The intact steel tubular specimens considered in their studies all had an average compressive initial strain of 2 to 3% but the cycle amplitudes were different.…”

“…During a lifetime of say 20-30 years, pipelines experience many start-up and shutdown cycles (of the order of hundreds). A question arises as to whether wrinkles formed as a result of such stress rises can grow (ratchet) and if so what are the consequences [1].…”

“…The problem of inelastic bending and collapse of tubes in the presence of internal pressure was experimentally and analytically investigated by Limam et al [2]. Plastic buckling of circular tubes under persistent axial cyclic loading, that leads to structural instability and collapse of pipelines, was studied by Jiao and Kyriakides [1]. Chang and Pan [3] reported the results from an experimental investigation on the degradation and buckling of circular tubes subjected to cyclic bending.…”