شبيه سازي عددي كمانش موضعي اعضاي لوله اي داراي خوردگي تحت بار گذاري هاي تناوبي كشش و فشار

Local Buckling of Corroded Tubular under Cyclic Compression & Tension Loadings

قطع و وصل پياپي جريان در خطوط لوله فولادي انتقال نفت و گاز فراساحلي، دوره هاي محوري كشش و فشار در خط ايجاد مي كند كه مي توانند سبب چين خوردگي، كمانش غير خطي موضعي يا خرابي خميري پيش رونده در اين خطوط شوند. اين نوع كاهش مقاومت موضعي، بيشتر به وسيله ناراستي هاي اوليه و خرابي هاي موجود در خط لوله تشديد مي شود. يكي ديگر از مسايل موثر بر روي مقاومت خطوط لوله دريايي، پديده خوردگي است. اين نوع آسيب به علت استقرار خط در محيط خورنده دريا و عبور سيال خورنده از داخل خط لوله ايجاد مي شود. به طور كلي آسيب خوردگي كه در عمق و اندازه از داخل و خارج سطح خطوط ايجاد مي شود، مي تواند بر مقاومت اين خطوط تاثير به سزايي داشته باشد. در اين پژوهش پديده خرابي پيش رونده در خطوط لوله هاي خورده شده تحت بارگذاري دوره اي محوري بررسي شده است. يك روش عددي براي بررسي اين پديده استفاده مي شود. خواص غير خطي مواد، به همراه رفتار سخت شوندگي غير خطي ايزوتروپيك/كينماتيك براي نمونه بررسي شده در نظر گرفته شده است. درستي داده هاي مربوط به پارامترهاي سخت شوندگي براي مدل سازي پديده خرابي پيش رونده اهميت زيادي دارد. اين نتايج با استفاده از يك سري آزمايش روي نمونه هايي كه در معرض دوره هاي زياد كرنش متقارن قرار گرفته به دست مي آيد. مدل اجزاي محدود استفاده شده در ابتدا با استفاده از نتايج به دست آمده از داده هاي آزمايشگاهي مربوط به آزمايش هاي يك سويه و دوره اي صحت سنجي شده است.در اين پژوهش، اثر دامنه و تنش ميانگين در بارگذاري دوره اي وتاثير خوردگي بر نرخ رشد كرنش غير خطي بررسي شده است.

Plastic axial strain, local buckling, wrinkling and plastic buckling of pipeline are caused by cyclic compression and tension loadings. This kind of local buckling is amplified by initial defect, heat affected zone and circular welding. Progressive plastic failure or ratcheting is caused by frequent periods of cyclic loading. On the other hand, life time of the offshore pipelines is decreased by the corrosion effect caused by fluids inside the pipeline and the sea Environment. This kind of corrosion can be found with variable size and depth in the inner or/and the outer surface of the pipeline. Corrosion can effect on the strength of pipeline. In the current study, an advanced finite element program has been used to simulate the ratcheting response of carbon steel tubes. The numerical model has been applied to reproduce a series of laboratory tests on small-scale tubes. These tests were carried out by the authors on intact and defected tubes, in which wrinkling and ratcheting behaviour of tubes under axial monotonic and cyclic loads were studied. A nonlinear isotropic/kinematic hardening model has been employed to represent the cyclic behaviour of the material. The verified model has then been used for a parametric study on ratcheting behaviour of the defected tubes under cyclic axial loading. Several stabilized cycles of specimens that are tested experimentally under symmetric strain cycles are used to obtain stress-strain data and hardening parameters of the material. The numerical model has then been used to investigate the effect of mean stress, stress amplitude and geometrical defects on the ratcheting response of steel tubes. It has been noticed that: a) The ratcheting strain rate was governed by (a) the initial non-linear strain in the tube, (b) by the stress amplitude and (3) by the mean stress, respectively. b) The ratcheting strains in the defected tubes had significantly higher rates in comparison to those in the intact tubes and very rapidly turned exponential. c) In defected tubes the local wrinkling first initiated from the damaged part. This local buckling then gradually proceeded to the entire circumference. The ratcheting strains in the defected area very rapidly turned exponential, while the ratcheting strains in the perfect zone still remained linear trajectory. d) It showed that surface corrosion imperfections had a very pronounced effect on the ratcheting response of the defected tubes, as compared to their monotonic response. e) The wrinkles in the defected tubes were non-axisymmetric and initiated from the damaged part of the tube