S.B. Lambert

Stress intensity factors for low aspect ratio semi-elliptical surface cracks in finite-thickness plates subjected to nonuniform stresses

Abstract Three-dimensional finite element analyses have been conducted to calculate the stress intensity factors for low aspect ratio semi-elliptic cracks. The stress intensity factors are presented for the deepest and surface points on semi-elliptic cracks with low aspect ratios of 0.05 and 0.1, and a/t values of 0.2, 0.4, 0.6 and 0.8. Uniform, linear, parabolic or cubic stress distributions were applied to the crack face. The results for uniform and linear stress distributions were combined with corresponding results for higher aspect ratio surface cracks and edge cracks to derive the weight function over the entire range of 0 ⩽ a/c ⩽ 1 and 0 ⩽ a/t ⩽ 0.8. The weight function is then verified against finite element data for the parabolic or cubic stress distributions. Differences were less than 3% for the surface point and 5% for the deepest point.

Transgranular Stress Corrosion Cracking of X-60 Pipeline Steel in Simulated Ground Water

Abstract An investigation was carried out on API X-60 steel specimens with multiple edge cracks to determine transgranular stress corrosion crack (TGSCC) growth rates in simulated ground water (NS-...

Approximate weight functions for embedded elliptical cracks

Abstract An approximate weight function for embedded elliptical cracks was deduced from the properties of weight functions and available analytical weight functions for penny shape and half plane cracks. The weight function was then validated against available exact stress intensity factor solutions for embedded elliptical cracks for several linear and non-linear stress distributions. The proposed weight function is suitable for the calculation of stress intensity factors for embedded elliptical cracks under any stress distribution.

Cyclic Crack Growth Rates of X-60 Pipeline Steel in a Neutral Dilute Solution

Abstract An experimental investigation was carried out to measure transgranular stress corrosion crack growth rates (CGR) in precracked X-60 steel specimens exposed to neutral (pH 7) dilute simulated ground water (designated NS4) while being cyclically loaded in cantilever bending. Testing was carried out over short-term (generally < 40 days) and long-term periods (∼ 1 y). Loading and environmental conditions in the long-term tests were similar to those for buried natural gas pipelines, with the R ratio (minimum/maximum load) varying from 0.82 to 0.98 and frequencies ranging from 1 to 400 cycles/day. Cyclic loading conditions for specimens in the short-term tests were more severe, with R ratios ranging from 0.5 to 0.9 and frequencies from 40 cycles/day to 5,000 cycles/day. Post-mortem metallographic examination by optical (OM) and scanning electron microscopy (SEM) was used to evaluate crack growth. Average CGR from 4.50 × 10−8 mm/cycle to 1.25 × 10−4 mm/cycle (equivalent to 1.4 × 10−9 mm/s to 7 × 10−7 mm...

Stress intensity factors and weight functions for high aspect ratio semi-elliptical surface cracks in finite-thickness plates

Abstract Three-dimensional finite element analyses have been conducted to calculate the stress intensity factors for high aspect ratio semi-elliptical cracks. The stress intensity factors are presented for the deepest and surface points on semi-elliptical cracks with aspect ratios of 1.5 and 2, and a t values of 0.2, 0.4, 0.6 and 0.8. Uniform, linear, parabolic or cubic stress distributions were applied to the crack face. The results for uniform and linear stress distributions were combined with corresponding results for lower aspect ratio surface cracks to derive weight functions over the range 0.6 ≤ a c ≤ 2.0 and 0.0 ≤ a t ≤ 0.8 . The weight functions were then verified against finite element data for parabolic or cubic stress distributions. Differences were less than 3% for the surface point and 6% for the deepest point. The present results complement weight functions for low aspect ratio cracks, 0.0 ≤ a c ≤ 1.0 , developed previously by the authors.

Semi-elliptical surface cracks in finite-thickness plates with built-in ends. II. Weight function solutions

Abstract The methods of deriving weight functions from reference stress intensity factors developed for traction type boundary conditions are extended to derive weight functions with displacement boundary conditions. The weight functions for the flat plates with built-in ends were derived from reference stress intensity factors obtained from Part I of the current paper. Validation of the weight functions was conducted by comparing stress intensity factors for several non-linear stress distributions calculated using finite element method. The weight functions are given in closed form, suitable for computer numerical integration. They are suitable for fatigue and fracture analysis of semi-elliptical cracks in complex stress fields with displacement controlled boundary conditions.

Environmental Crack Growth under Variable Amplitude Loading of Pipeline Steel

Abstract An investigation was carried out on API X60 steel specimens with surface cracks to study environmentally assisted crack growth rates in simulated groundwater (NS-4 solution). The environmental conditions were similar to those for buried natural gas pipelines in service, and loading was aggressive. The maximum stress intensity factor at the tip of the crack on the surface ranged from 20.6 MPa√m to 31.4 MPa√m. A periodic underload was applied to pre-cracked cantilever bending specimens. Tests were carried out for a period of 40 days at a frequency of either 1 cycle or 40 cycles per day. Crack growth rates were obtained for two stress ratios (R = minimum/maximum load) of 0 or 0.5 under both constant and variable amplitude loading conditions. Depending on loading conditions, the crack growth varied from 0 μm to 2,640 μm. However, the majority of crack growth measured was <100 μm. The experimental results showed that there was an acceleration in growth following an underload. The growth was higher whe...

Semi-elliptical surface cracks in finite-thickness plates with built-in ends. I. Stress intensity factor solutions

Abstract A crack compliance analysis approach was used to calculate stress intensity factors for surface cracks in flat plates with built-in ends. Stress intensity factor solutions are presented for both constant and linear stress distributions acting on the crack face. The results were verified using finite element analysis and good agreement was obtained. The solutions are suitable for the analysis of surface cracked plates with fixed displacement boundary conditions.

SCC growth in pipeline steel

Publisher Summary Stress corrosion cracking (SCC) occurs on the soil-side of a buried pipeline when the coating breaks down and groundwater comes in contact with the steel. This chapter examines the growth of single cracks, and proposes a superposition model incorporating the environmental, and mechanical effects to describe the later stages of near-neutral SCC growth using laboratory testing that duplicates pipeline conditions. Cantilever bending loading is used for the SCC tests on both the edge and surface crack specimens. Crack tip loading is characterized using the linear elastic stress intensity factor; K. SCC tests are performed under anaerobic conditions to simulate the conditions of a buried pipeline by enclosing the specimen and electrolyte chamber in a larger outer environmental chamber. This, in turn, is sealed and continuously flushed with pure N2 gas. Microscopic examination of the edge crack specimens reveals that the cracks are transgranular, traversing both pearlite, and ferrite grains. For the short-term tests of less than 80 days, the cracks remain narrow and sharp. This is in contrast with the long-term test results, where the cracks are blunt and wide. As the exposure time in NS4 solution increases, all the cracks experience general corrosion causing widening, despite the anaerobic environment.