Brittle Fracture Assessment and Failure Assessment Diagrams

A detailed fracture mechanics evaluation is the most accurate and reliable prediction of process equipment susceptibility to brittle fracture. This article provides an overview and discussion on brittle fracture. The discussion covers the purpose for evaluating, provides a brief summary of historical failures that were found to be a result of brittle fracture, and describes key components that drive susceptibility to a brittle fracture failure, namely stress, toughness/temperature, and flaw size. It also presents industry codes and standards that assess susceptibility to brittle fracture. Additionally, a series of case study examples are presented that demonstrate assessment procedures used to mitigate the risk of brittle fracture in process equipment.

Development of Design Criteria and NDE Method for HPHT Equipment

Subsea production systems have been using API-specified methods and detection criteria for nondestructive examination (NDE) for equipment up to 15,000-psi rated working pressure. With higher pressure requirements and use of methods for evaluation of the fatigue life of HPHT equipment, existing criteria cannot be sufficient to achieve the desired fatigue life. This paper demonstrates more stringent design criteria and a detection method that was developed to achieve HPHT fatigue life, yet is cost and schedule effective. When fracture mechanics was used for fatigue life estimation, an initial flaw size is stated as a starting point in determining design life. Using API-specified NDE criteria of 1/16-inch detection of surface flaw, project design life was not achieved for certain components without a major redesign and retest. Smaller flaw lengths were preferred in critical areas instead of standard flaw sizes. This created a need for a high-sensitivity penetrant inspection technique to effectively detect this new minimum flaw size in critical areas. Various methods of NDE were considered, and a conclusion was taken in selecting the best inspection method. Fracture mechanics and finite element analysis required a minimum detectable flaw size length of 1/32 inch to meet the project design life without changing equipment technology. By using surface NDE with fluorescent liquid dye penetrant of sensitivity levels 2 and 3, all seeded flaws of 1/32-inch were consistently detected in the validation test coupon, which enabled the use of this stricter criteria for the analysis. Detailed procedures were established, and validation testing results were documented with photographs of detected flaws. Seeded flaw coupons were manufactured for validation of procedures and to train the various facilities that will use these techniques for production equipment. Additionally, procedures and guidelines were provided to inspectors and inspection facilities to ensure proper implementation of the methods. Measurement system analysis for repeatability and reproducibility was conducted at the facilities. This enabled the fatigue design of the HPHT equipment to advance beyond the boundaries of traditional methods and acceptance criteria set by current industry standards. New and tighter acceptance criteria were developed to improve HPHT fatigue life. High-sensitivity penetrant inspection technique, capable of detecting flaw sizes as small as 1/32-inch, was established and implemented. This inspection technique is not common to the oil and gas industry because of the ability of standard methods to readily detect the API-required criteria. The method has improved detection capabilities and has the potential to move toward adopting advanced design methods to address HPHT requirements.

Failure modelling of glass plates in biaxial loading: using flaw-size based weakest-link systems

Experimental strength tests are performed on two series of nominally equal plate specimens of annealed soda-lime glass subjected to either ring-on-ring or ball-on-ring bending. The Weibull effective area which represents a fictitious surface area exposed to uniform tension is calculated using closed-form solutions. Finite-size weakest-link systems are implemented numerically in a computationally intensive procedure for random sampling of plates extracted from a virtual jumbo pane whose surface area contains a set of stochastic Griffith flaws. A non-linear finite element analysis is conducted to compute the bending stresses. The glass surface condition is represented in different flaw-size concepts that depend on a truncated exponentially decaying flaw-size distribution. Stress corrosion effects are modelled by implementation of subcritical crack growth. The effective ball contacting radius is determined in a numerical computation. The results show that surface size effects in glass are not only a matter of strength-scaling, as also the shape of the distribution changes. While the lowest strength value, as per the major in-plane principal stress at the recorded fracture origin, in the respective data sets is very similar, the strongest specimen observed in ball-on-ring testing is over 70% stronger than the correspondingly strongest specimen observed in ring-on-ring bending. The Shift function is used to make visual comparisons of the difference in quantiles in the observed data sets. Use of an ordinary Weibull distribution leads to non-conservative strength predictions on smaller effective areas, and to too low strength predictions than are viable for glass design on larger areas. The numerical implementation of finite-size weakest-link systems can produce better predictions for the strength-scaling compared to a Weibull distribution, in particular when the flaw-size concept is modified to include a doubly stochastic flaw-size distribution or a random noise added to each subdivided region of the discretized surface area. The simulated ball-on-ring fracture origins exhibit greater spread from the centre point than otherwise observed in laboratory tests. It is indicated that the chosen representation of surface condition may not be accurate enough for the modelling of all fracture origins in the ball-on-ring setup even though acceptable results are obtained with the ring-on-ring model. There is a need for more insight into the surface condition of glass which can be conducive to the development of flaw-size based weakest-link modelling.

Inspection interval optimization of aircraft landing gear component based on risk assessment using equivalent initial flaw size distribution method

The nondestructive inspection interval is highly related with both system reliability and maintenance burden. Conventional inspection interval decision criteria based on the deterministic crack propagation analysis could require too much frequent inspection or sometimes occur structural failure owing to the rapid crack propagation than expected. The stochastic crack growth analysis method was proposed to compensate for the shortcomings of the deterministic analysis. This research studied the crack growth of aircraft landing gear components based on the equivalent initial flaw size distribution algorithm, and then we assessed failure risk. The calculated risk was validated using Monte-Carlo simulation, and finally, the optimum inspection interval was proposed to satisfy the US Airforce risk management criteria.

Modeling Crack Growth in ASME Section XI Flaw Evaluations

Rules for assessment of flaws found as a result of periodic in-service examinations of nuclear components, first appeared in Section XI of the ASME Boiler and Pressure Vessel Code in 1974. Since that time, literally hundreds of evaluations have been completed, which have allowed flaws that met the required margins to be accepted for continued service without repair. The flaw evaluation process of Section XI involves prediction of the future growth of such flaws, and then comparison with a calculated allowable flaw size for the location of interest. The crack growth models to be used for such evaluations are generally contained in the ASME Code itself, in Section XI Nonmandatory Appendices A or C, or the new Nonmandatory Appendix Y for Crack Growth Rate Curves (2021 Code Edition), depending on the application and material type. Occasionally, Code Cases are also used to provide recommended crack growth models. The philosophy that has been followed by the Code since its inception has been to model crack growth as accurately as possible, so as to make an accurate prediction of future flaw growth, and then compare the predicted final flaw size with the allowable flaw size for the area of interest, after a margin has been applied. Therefore, the margin is applied only once, so it is easily identified, and is not “double-counted”. The goal of this paper is to summarize the background and basis behind the ASME Section XI general philosophy of the use of a best estimate, or mean, treatment of the crack growth models in Section XI of the ASME Code. This paper will discuss the various approaches that are available to characterize crack growth, and then discuss the approaches which have been used in ASME Code Section XI flaw evaluation rules over the years.

Fatigue and Crack Growth under Constant- and Variable-Amplitude Loading in 9310 Steel Using “Rainflow-on-the-Fly” Methodology

Fatigue of materials, like alloys, is basically fatigue-crack growth in small cracks nucleating and growing from micro-structural features, such as inclusions and voids, or at micro-machining marks, and large cracks growing to failure. Thus, the traditional fatigue-crack nucleation stage (Ni) is basically the growth in microcracks (initial flaw sizes of 1 to 30 μm growing to about 250 μm) in metal alloys. Fatigue and crack-growth tests were conducted on a 9310 steel under laboratory air and room temperature conditions. Large-crack-growth-rate data were obtained from compact, C(T), specimens over a wide range in rates from threshold to fracture for load ratios (R) of 0.1 to 0.95. New test procedures based on compression pre-cracking were used in the near-threshold regime because the current ASTM test method (load shedding) has been shown to cause load-history effects with elevated thresholds and slower rates than steady-state behavior under constant-amplitude loading. High load-ratio (R) data were used to approximate small-crack-growth-rate behavior. A crack-closure model, FASTRAN, was used to develop the baseline crack-growth-rate curve. Fatigue tests were conducted on single-edge-notch-bend, SEN(B), specimens under both constant-amplitude and a Cold-Turbistan+ spectrum loading. Under spectrum loading, the model used a “Rainflow-on-the-Fly” subroutine to account for crack-growth damage. Test results were compared to fatigue-life calculations made under constant-amplitude loading to establish the initial microstructural flaw size and predictions made under spectrum loading from the FASTRAN code using the same micro-structural, semi-circular, surface-flaw size (6-μm). Thus, the model is a unified fatigue approach, from crack nucleation (small-crack growth) and large-crack growth to failure using fracture mechanics principles. The model was validated for both fatigue and crack-growth predictions. In general, predictions agreed well with the test data.