Plastic Collapse Load for Vessel With External Flaw Simultaneously Subjected to Internal Pressure and External Bending Moment: Experimental and FEA Results

Abstract

This paper is based on work done to establish the validity of a simple engineering approach to assess plastic collapse for a vessel with a local thin area (LTA). The approach is based on a recently developed p-M (internal pressure ratio and external bending moment ratio) diagram, which is an easy way to visualize the status of a vessel with a LTA simultaneously subjected to internal pressure, p and external bending moment, M due to earthquake, etc. If the assessment point (Mr,pr) lies inside the p-M line, the vessel with the LTA is judged to be safe. Numerous experiments and finite element analyses for a cylinder with an external flaw were conducted under (1) pure internal pressure, (2) pure external bending moment, and (3) subjected simultaneously to both internal pressure and external bending moment, in order to determine the plastic initiation load and plastic collapse load by applying the twice-elastic slope (TES) as recommended by ASME. It has been clarified that the collapse (TES) loads are similar to those calculated under the proposed p-M line based on the measured yield stress. The p-M line adopted in the Ibaraki fitness for service (FFS) rule based on the specified minimum yield stress with a safety factor of 1.5 indicates that the safety margin for the plastic initiation loads at LTA is about 1.0–3.0, about 1.5–4.0 for the TES loads at LTA, and 2.5–6.5 for the plastic instability (break) loads.

Proposal of acceptance criterion for plastic collapse assessment rule on local metal loss

“…The authors have already reported on the results of a comparative study between API 579-1/ASME FFS-1 and the p-M method in [6], which pointed out that the RSF approach in the level 1 and 2 assessments of part 5 of API 579-1/ASME FFS-1 may give either under-conservative or excessively conservative results, compared with the plastic limit loads obtained by the p-M diagram method, which are verified by both FEA and experiments [3].…”

“…The definition of the plastic limit loads, its explanations and verifications with experiments and FEA for the p-M diagram are detailed in [3]. The definition of the plastic limit loads, its explanations and verifications with experiments and FEA for the p-M diagram are detailed in [3].…”

Study on Sufficiency of Reference Stress Solutions for Crack-Like Flaws for Fitness-for-Service Evaluation

“…Reference stress represents the margin of the admissible stress state in the ligament against plastic collapse for cracked bodies in the unit of stress , and is given as a function of structure, crack size and shape, membrane stress (or axial load, internal pressure), and bending stress (or bending moment). In HPIS Z101-2, reference stress solutions derived from appropriate technical bases have been adopted based on the past studies [2][3][4], and the adequacy has been verified [5].…”

Numerical Study of Defect Free Elbows Subjected to In-Plane Bending Moment

“…So it is very important to know the plastic carrying capacity of elbow and the plastic limit load calculation of uncracked elbow holds an important position in engineering practice. Much work on plastic limit analysis has been performed for straight pipes, but much less for elbows [1][2][3][4][5][6][7][8]. Though Griffiths [4], Yahiaoui [5,11], J.chattopadhyay [6,10], and etc.…”

“…Though Griffiths [4], Yahiaoui [5,11], J.chattopadhyay [6,10], and etc. have studied the plastic load carrying capacities of elbows subjected to in-plane bending moment, and proposed the closed form equations of plastic limit loads, these researches have limitations: (1) The present studies and the experimental and analytical research by Yahiaoui K et al [11] have indicated that those formulas are excessively conservative when elbows subjected to in-plane bending moment, and on occasions, no applicable to the cases for which they intended ; (2) Do not differentiate in-plane opening bending moment from closing mode bending moment, which the responses of elbows are markedly different under these two different modes of bending moment; (3) Even though differentiate the bending mode of bending moment in analyses, the limit loads are defined by twice-elastic-slope (TES) method based on load vs. end displacement curves, instead of the true limit load that causes overall structural instability. Apparently, taking the load defined by TES method as limit load is excessively conservative for the elbow under in-plane opening bending moment.…”