On the fatigue strength calculation of the welded shell structures from high-strength steels under low-cycle loading. Part 1: Estimation at the initial stage of fatigue failure

The normative methods for calculating the fatigue strength of welded joints are of limited use for low-cycle loads, as they do not take into account the possible variation in the asymmetry of the operating stress cycle, differences in the expected level of residual stresses, and the possible variety of joint geometry. Estimation procedures have been developed for shell structures made of high-strength steels subjected to external and internal pressure. They were based on experimental data on the resistance to fatigue fracture, physical modeling of individual stages of fatigue damage, and generalization of the results of numerical studies of the FEM of the stress-strain state.

Mechanical characterization of jute fiber based biocomposite to manufacture automotive components

In this research work, samples of the biocomposite were manufactured using the vacuum assisted resin transfer molding (VARTM) technique, whose matrix is a polyester resin and the reinforcement is a biaxial fabric (90°) made with jute fiber. Then, tensile and flexural tests were carried out on standardized specimens under ASTM standards, in order to mechanically characterize the jute-polyester biocomposite. In both destructive tests, the results showed a linear-elastic behavior with brittle fracture and greater strength of the jute-polyester biocomposite, with respect to the thermosetting matrix’s properties. Subsequently, a finite element based static analysis was performed, with the help of the ANSYS software, to determine the mechanical behavior of interior opening handle for a car door. In it, a model sensitivity study was run to determine the influence of the mesh type and identify the convergence of mesh. Later, the static analysis results were obtained: critical zone, maximum operating stress and safety factors. The results obtained computationally validate the use of jute-polyester biocomposite, as a substitute for the manufacture of an interior opening handle for a car door. Finally, a scale model of the piece made with jute-polyester biocomposite was manufactured.

Steam Turbine Overspeed Scenarios: Comparison Between API Energy Method and Dynamic Simulation

In turbomachinery applications the possibility to reduce size and costs of main flow-path components, by increasing shaft rotating speed, has always been appealing. The technological challenge in increasing this power density capability is typically related to performance prediction, to operating stress in blades and shafts, as well as to the need for a more accurate rotor-dynamic analysis. Yet another aspect, often reduced to standard assessments in less demanding applications, is related to the analysis of overspeed scenarios where, following a sudden loss of load and/or driven inertia, the turbomachine shall maintain its mechanical integrity. Especially in steam turbines applications, where the behavior of the machine is strongly affected by the plant conditions, valves intervention time and connected volumes, the reduction of the rotor inertia, against comparable power, may produce overspeed scenarios that can become a primary design constraint and, if overlooked, may have both availability and safety implications. In this paper several approaches to the analysis of overspeed scenarios are discussed, with increasing level of detail. The energy-based overspeed analysis method, as required by API612, is first discussed against practical design cases. A more accurate dynamic model is then presented, and its results compared with those of the energy-based approach. Finally, the sensitivity analysis of the overspeed peak value with respect to critical design parameters is discussed. With respect to previous works, mostly based on load rejection scenarios, the main focus is on the scenario of sudden coupling loss.

Health-Monitoring Methodology for High-Temperature Steam Pipes of Power Plants Using Real-Time Displacement Data

We developed a health-monitoring methodology for high-temperature steam pipes that estimated the life prediction of creep–fatigue interaction by directly measuring the displacement of hot parts. Three different methods (boiler code, design stress, and operating stress) were used to estimate the stress of the high-temperature pipe system. As a theoretical approach, the German boiler standard code calculates the stress according to the pipe shape, while design stress, which is also called allowable stress, was determined by a function of the operating temperature. The operating stress was immediately calculated using the surrogate model, with maximum displacement measured using the 3D displacement measurement system. To achieve the surrogate model, the stress was estimated by the pipe-stress analysis under the given displacements, and the surface-response model was developed to relate the stress and displacement. We showed that those methods are efficient methods to predict the stress and are applicable in health-monitoring methodology. Finally, the creep life and the low-cycle fatigue life were investigated using the Larson–Miller parameter equation, as well as the Smith, Hirschberg, and Manson equations. Our proposed monitoring system can be used to predict the fatigue and creep life of high-temperature steam pipes in real time, and we believe that the system can be applied to actual maintenance in thermal power plants.

Reliable Design: Basic Approach

The knowledge and experience learnt from product designing have resulted in development of their reliability theory. The classical concept of safe – life is based on product over dimensioned design that considers safety factor or safety margin for measure. However, practical engineering has found this concept in a manner inconvenient as design fault-resistance to determine the ultimate condition and operating stress are random values. A way out is in the concept of stochastic approach to reliability design resulting from the defect-production probability distribution law. That concept allows product designing with predetermined reliability, such as in the example contained in this paper.

Improving Safety Through Engineering Assessments for Change in Location Class

In Canada, when location class changes on a gas pipeline CSA Z662-15 requires operators to comply with design requirements of the new location class or perform an Engineering Assessment (EA). The compliance option is often perceived by regulators and the public as the better option compared to the EA option. This paper demonstrates that a well-executed EA that accounts for relevant threats and consequences, and provides explicit levels of reliability, can deliver improved pipeline safety. To comply with design requirements with respect to location factor, the two compliance options are to de-rate or replace the pipeline to achieve the lower operating stress level dictated by the new location factor. However, lower operating stress levels do not always address the higher risk levels or safety concerns caused by the change in class and ensuing potential increase in mechanical damage. For gas pipelines, where class location is applicable, ensuring human safety is the primary objective of pipeline integrity management. In this context, safety is defined as the control of recognized hazards to achieve an acceptable level of risk. To provide site-specific safety, an acceptable level of risk needs to be achieved by ensuring sufficiently low enough probabilities of failure for given site-specific consequence levels. Increased wall thickness via pipe replacement, can lead to lower probability of failure for a pipeline. However, as pipelines are subjected to many different combinations of threats, which depend on site specific conditions, the pipelines that are designed with thicker walled pipes for higher location classes do not always provide lower probabilities of failure. As the general design considerations do not account for the site specific threats and mitigation actions, complying with design requirements alone do not consistently provide lower probabilities of failure, especially in areas of potentially higher third-party activities. In TransCanada’s site-specific EAs, quantitative risk or reliability assessments consider verified population estimates, actual lethality zones and site-specific threats. Appropriate and site-specific mitigation actions address the actual risk. This enables providing an appropriate site specific reliability level. Case studies and comparison between methodologies are used to illustrate the importance of performing site-specific EAs using site-specific information to achieve safety levels that are greater than those achieved by strictly complying with the standard design requirements. Accounting for actual-site specific threats and the actual consequences ensures accurate assessment of risk and consequent appropriate mitigation and efficient risk reduction.

Probability-Based Sentencing Criteria for Volumetric In-Line Inspection Data

ASME B31.8S, Figure 7.2.1-1 (referred to as Figure 4 in earlier editions of the Standard) is used by many operators of natural gas transmission pipelines to schedule the remediation of corrosion features found via in-line inspection (ILI). The underlying philosophy of this approach is that wall loss features should be repaired before the calculated failure pressure falls below 110% of the maximum allowable operating pressure (MAOP). ASME B31.8S Figure 7.2.1-1 provides a basis for establishing maximum response times as a function of pipeline operating stress level, based in part on assumed corrosion growth rates. The corrosion rates assumed in the derivation of ASME B31.8S Figure 7.2.1-1 depend on the wall thickness of the pipe and the operating stress level as a percent of SMYS. As documented in PHMSA’s March 17, 2016 Notice of Proposed Rulemaking, the 1.1xMAOP repair criterion that forms the basis of Figure 7.2.2-1 has a demonstrated successful history of use in response management for wall loss ILI data. Despite this successful record, some potential exists for the underlying corrosion growth rate assumptions that are incorporated within that criterion to be non-conservative. Under some circumstances, the underlying corrosion growth rate assumption that is incorporated in Figure 7.2.1-1 can be significantly less than that which is provided in the guidance provided in NACE SP0502 (referenced in Appendix B of ASME B31.8S). Therefore, operators should ideally take measures to verify that the growth rate assumptions incorporated within Figure 7.2.1-1 are appropriate for their circumstances before adopting the scheduled response criteria from that Figure. On the other hand, for the majority of circumstances, it could be demonstrated that the Figure 7.2.1-1 criteria may represent overly-conservative response times, particularly where feature-specific information related to corrosion rates are available, and/or can be inferred from ILI data. A desirable solution would be to employ a response time threshold that utilizes the 1.1xMAOP repair criterion that has been demonstrated to be successful through industry’s widespread adoption of the Figure 7.2.2-1 criteria, along with some basis for incorporating feature-specific corrosion growth rates (from ILI data), and additionally, some basis for accounting for tool measurement error. Techniques for estimating the relative probability of failure (Pf) exist that employ ILI data and account for tool measurement error, model error, and tolerances in pipe dimensions and material properties. The problem to date is that probability targets have not been available for use in conjunction with a Pf analysis. Building on previous work done by Kiefner and Kolovich, this paper derives an approach for expressing Pf targets in terms of the 1.1xMAOP repair criterion adopted by ASME B31.8S, Figure 7.2.1-1. The Pf targets are derived using stochastic modeling, and incorporate probability density functions on tool error for feature depth and length, wall thickness, yield strength, and model error. Using a wide range of pipeline material and design parameters, a relationship for establishing lower-bound Pf targets is developed for broad application.