Session: 03-03-01 Assessment Model Enhancements - Cracking

Paper Number: 86904

86904 - An Improved Axial Surface Crack Model to Predict Crack Growth and Burst Pressures Based on a Fe Analyses, Fad-Curve Fitting, and J-R Curve Methodology 

As part of a DOT/PHMSA project on Optimization of Hydrotesting for Vintage Liquid Lines, it was desired to have the best possible model to predict the start of ductile tearing, crack growth, and the burst pressure to predict sustained-load crack growth in a hydrotest, as well as determine the “Rupture Free Operation Time” after that hydrotest.  Empirical models like the Original Battelle Ln-Sec, Modified Ln-Sec, and CorLAS don’t allow for the crack growth analyses needed.  The PRCI MAT-8 model seemed to be a promising method, although on the fracture resistance side it used only the initiation toughness from a C(T) specimen for all flaw sizes.

During an in-depth review of the MAT-8 method, we liked the general approach, but there were assumptions about the shape of the FAD curve, and fitting using just a few points along FAD curves from FE anlayses to get the plastic contribution of rack-driving force (J_plastic).  Because the FAD-curve shapes from our FE analyses for difference crack sizes of interest to liquid line operation were quite different than the MAT-8 assumed FAD-curve shapes, we had to develop an alternative FAD-curve fitting procedure and run several hundred additional FE analyses for axial surface cracks in pipes.  (Many of the runs were for QA of mesh sensitivity analyses where over 400,000 elements were used in a quarter-symmetry FE mesh.)  The new FAD-curve fitting procedure used a nonlinear-regression analysis of all the FE-based results, not just two points.  Solutions for longer surface-crack lengths were obtained as well.  The fitted equation is quite complex due to the wide variety of FAD curve-shapes (using a Logistics equation).  Frequently for shorter cracks we obtained identical results with the MAT-8 analysis, but as the crack depth and length increased the difference in the FAD curve shape as well as J_plastic increased.  Additionally, we included the use of the J-R curve of the material using the a/t sensitivity of the J-R curve from SENT tests.  Trends between Charpy upper-shelf energy, C(T) specimen J_i vlaues, SENT J_i values (function of a/w) and predicting J-R curves in the SENT specimens will be shown.  Predicted burst pressures from the various methods will be compared to pipe tests.

Presenting Author: Gery Wilkowski Engineering Mechanics Corp. Columbus

Presenting Author Biography: Later.