Steel Tube Flying Lead Accumulated Plastic Strain Calculation

Abstract

Abstract This paper documents a study to determine the accumulated plastic strain (APS) developed in steel tube flying leads (STFLs) assembled in loose-tube bundle configurations throughout their product lifecycle. This lifecycle consists of the various processes involved in the manufacturing of STFLs,including initial tube manufacturing, load-out after tube manufacturing, subsequent shipping, potential over-sheathing, delivery and unspooling for flying lead manufacturing, pressure testing, and the flying lead final load-out. Throughout the product lifecycle, the steel tube undergoes the repetitive processoperations of coiling, uncoiling, straightening, and pressurizing. These operations cause plastic strains to be developed in the tube, which means that the tube experienceshardening,and its mechanical properties are modified throughout its lifecycle. Therefore, it is necessary to account for this accumulation of plastic strain and to ensure that the total APS at the end of the lifecycle is below APS acceptance criteria. A kinematic hardening material model was used to simulate the nonlinear material behavior. The APS in super duplex tubing within loose STFLs for any number of bend cycles was calculated. The total strain developed in the tubing was calculated on outer fibers in a wrapped configuration. With the total strain (ε) known, the corresponding true stress (σ) can be determined using the tubing material’s true stress-true strain curve. The elastic strain is calculated by dividing the true stress by the tube’s elastic modulus (E). The plastic strain can then be calculated by subtracting the elastic strain from the total strain. Accumulation of plastic strain due to tubing bent about a radius and applied with internal pressure was difficult to obtain, as there were no analytical equations available to calculate it. To approximate the plastic strain developed due to pressure applied, a finite element analysis (FEA) study was completedfor plastic strain variation with respect to different bend radii. Results of the APS calculation were verified with the FEA results for each of the product lifecycle processes. Using this kinematic material model, the difference between the FEA and the APS calculation was within 3.5%.