Sustained Loading Bond Response and Post-Sustained Loading Behaviour of NSM CFRP-Concrete Elements under Different Service Temperatures

Nowadays, one of the foremost procedures for strengthening concrete structures is the Near-Surface Mounted (NSM) technique. This paper presents an experimental study on the effect sustained loading and different service temperatures (steady and cyclic) have on NSM Carbon Fibre-Reinforced Polymer (CFRP)-concrete bonded joints and their post-sustained loading load-slip behaviour. Four experimental campaigns using eight NSM CFRP-concrete specimens were performed by employing two different service load levels (15% and 30% of the ultimate load) and combining two groove thicknesses (7.5 and 10 mm) and two bonded lengths (150 and 225 mm). Two steady state temperatures (20 and 40 °C) and two cyclic service temperatures (ranging between 20 and 40 °C) were programmed. The slip obtained was proportional to the sustained load level. Furthermore, higher slips were registered for specimens under higher mean temperatures in the cycle. After 1000 h of sustained load testing, the specimens were tested under monotonic loading until failure (post-sustained loading tests). In general, the ratio between the post-sustained loading ultimate load and the instantaneous ultimate load was close to the unity, although some differences were perceived in series S2 (steady 37.7 °C) with a mean increase of 6.3%, and series S3-B (cyclic temperature ranging between 24.6 and 39.2 °C) with a mean reduction of 9%.

A Study of Recycling Process of Cobalt-based Conversion Coating Formed on Aluminium Alloys

In this study, the influence of different cyclic treatment and reuse process on the performance of the conversion film was systematically studied by electrochemical test, surface characterization and determination of cobalt ions in the conversion solution, and new technology of chemical conversion of cobalt salts in aluminum alloys were determined. Mode A indicated the practice of cyclical processing of the test object in the original conversion solution, in which components of the circulation liquid were consumed continuously. After the first cycle, the Ecorr and Rp were – 691.8 V and 63.5 kΩ. In the later time, Rp was just about 13.0 kΩ, and the corrosion resistance of conversion film degraded progressively. Mode B represented the practice of adding to the original solution before cyclical processing of the test object, so that volume of the solution remains unchanged, and components of the circulation liquid kept increasing. RP increased from 9.3 kΩ to the 69.9 kΩ at the beginning of the cycle, followed by a slow descent, and content of cobalt in the film was stable, also higher than that under model A. This demonstrates that mode A has high utilization of the cobalt in the conversion solution in the early recycling, whereas mode B can prolong the cyclic service life of the conversion solution in circulation. After adding oxidants, as NaClO3, NaBrO3, H2O2, NaClO, and KMnO4, to the circulation liquid before recycling, the reuse and recycle ratio of cobalt reaches up to 41.3 %, and Rp can be promoted to123.2 kΩ at most. The life of the recycling was prolonged, while the costs of chemical conversion of cobalt were reduced.

A BIOLOGICALLY INSPIRED FLUID MODEL OF THE CYCLIC SERVICE SYSTEM

A deterministic fluid model in the form of nonlinear ordinary differential equations is developed to provide the description for a multichannel service system with service-in-random-order queue discipline, abandonment and re-entry, where servers are treated like enzyme molecules. The parametric analysis of the model’s fixed point is given, particularly, how the arrival rate of new customers affects the steady-state demand. It is also shown that the model implies a saturating clearing function (yield vs. demand) of the Karmarkar type providing the mean service time is much shorter than the characteristic waiting time.

Continued Study on the Effect of Mean Stress on Ground Storage Vessels for Hydrogen Fueling

The use of high pressure vessels for the purpose of storing gaseous fuels for land based transportation application is becoming common. Fuels such as natural gas and hydrogen are currently being stored at high pressure for use in fueling stations. This paper will investigate the use of various levels of autofrettage in high pressure storage cylinders and its effects on the life of a vessel used for hydrogen storage. Unlike many high-pressure vessels, the life is controlled by fatigue when cycled between a high pressure near the design pressure and a lower pressure due to the emptying of the content of the vessels. There are many misunderstandings regarding the need for cyclic life assessment in storage vessels and the impact that hydrogen has on that life. Some manufacturers are currently producing vessels using ASME Section VIII Division 1 to avoid the requirements for evaluation of cylinders in cyclic service. There are currently rules being considered in all of ASME Section VIII Division 1 and Division 2, and even potentially for Appendix 8 of ASME Section X. Recommendations on updating the ASME codes will be considered in this report.

Fatigue Consideration of Layered Vessels

Layered pressure vessels provide a cost-effective solution for high pressure gas storage. Several types of designs and constructions of layered pressure vessels are included in ASME BPV Section VIII Division 1, Division 2 and Division 3. Compared with conventional pressure vessels, there are two unique features in layered construction that may affect the structural integrity of the layered vessels especially in cyclic service: (1) Gaps may exist between the layers due to fabrication tolerances and an excessive gap height introduces additional stresses in the shell that need to be considered in design. The ASME Codes provide rules on the maximum permissible number and size of these gaps. The fatigue life of the vessel may be governed by the gap height due to the additional bending stress. The rules on gap height requirements have been updated recently in Section VIII Division 2. (2) ASME code rules require vent holes in the layers to detect leaks from inner shell and to prevent pressure buildup between the layers. The fatigue life may be limited by the presence of stress concentration at vent holes. This paper reviews the background of the recent code update and presents the technical basis of the fatigue design and maximum permissible gap height calculations. Discussions are made in design and fabrication to improve the fatigue life of layered pressure vessels in cyclic service.

Method B Fatigue Screening in ASME BPV Code, Section VIII, Division 2, Part 5

ASME Boiler and Pressure Vessel Code (BPVC), Section VIII, Division 2, Part 5 Method B fatigue screening is intended to be a quick and simple method that is sufficiently conservative to screen components in cyclic service thus not requiring detailed fatigue analysis. The method assesses pressure, thermal, and mechanical loads separately. The basis for each portion of the method is discussed along with an alternative bases for the assessments. Each assessment is reformulated as a fatigue damage factor and all variables are provided so that the intent of each equation is clearly identifiable. A penalty factor will be included in each equation rather than assuming one penalty for all designs, the reformulation creates penalty for non-fatigue resistant designs and reduces the penalty for fatigue resistant designs. Examples are given showing the potentially non-conservative results if a summed damage is not used.

Further Considerations for the Determination of Service Life for Delayed Coker Drums

Delayed coker drums are unique in hydrocarbon processing facilities in that estimating their true design and service life has been problematic. Generally, pressure containing equipment in these facilities is designed using the notion of design life based on required pressure thickness and corrosion allowance considerations. Hence, pressure containing equipment is routinely monitored by facility inspectors for wall thickness. Although many analysts have ascribed coke drum failure to “thermal stress cycling”, the difficulty posed by the operation of coke drums results in an inability to measure or calculate the magnitude of the thermo-mechanical “stresses” and the actual number of significant exposures, that is, cycles causing fatigue damage. As well, the use of Code construction practices has been generally misapplied, for this specific equipment, as the practices are intended to define a safe design life rather than a service life. Indirect measures of service life based on shell bulge severity have fallen from favor by being ineffective. A trend to use a strain index method is somewhat more appealing but is based on static load and monotonic material property considerations rather than those properties indicative of thermal cyclic operation. Recent work has shown that thermo-mechanical strain cycling can be characterized quantitatively and used to determine a cyclic service life for both undamaged and damaged coke drums. This paper discusses some of the engineering specifics to generate a high probability estimate of coke drum fatigue service life for a new drum, a damaged-stable drum, drums with weld overlay and for drums exhibiting incremental damage.

Verification of the EPP Code Case for Strain Limits Evaluations by Inelastic Analysis Method

An ASME Section III Division 5 code case, N-861, for the evaluation of strain limits based on the elastic-perfectly plastic (EPP) methodology has recently been published. A key feature of the EPP methodology is the application of the EPP finite element analysis method with a pseudo yield stress to bound component response under elevated temperature cyclic service. The simplified inelastic approach in Division 5, Appendix HBB-T that is based on the elastic analysis results is not applicable at the elevated temperature range where creep and plasticity cannot be distinguished and unified viscoplastic model is required to describe the deformation behavior. The EPP strain limits code case overcomes such limitations. It also has the distinct advantage that stress classification, which is required by the simplified inelastic approach, is not needed. Thus, it is ideally suited for modern-day finite element technology. The conservatism of the EPP strain limits code case was verified for some simple geometries. In this paper, a viscoplastic constitutive model calibrated to the experimental data for 316H stainless steel is used to conduct a full inelastic analysis. The calculated strain accumulation is compared with that obtained from using the EPP code case approach.

Integrity of Small Angle Mitered Joints

Miter bends are permitted for use in the oil and gas industry in a variety of configurations. This paper addresses small angle miter joints used to correct minor misalignments. The CSA Z662-15 standard (Oil and Gas Pipeline Systems) states in the construction section (Clause 6.2.3(g)) that “mitred bends shall not be used”. However, a note accompanying the clause states that “deflections up to 3 degrees caused by misalignment are not considered to be mitred bends.” Nonetheless, concerns continue to be raised that stresses introduced due to this misalignment can affect safety and operation of pipelines. This paper reviews literature of failures associated with mitered joints and the theoretical development of stresses in miters, and presents results from a linearized sensitivity analysis of buried mitered joints under pressure and thermal loading based on ASME B31.1 and B31.3 criteria. The paper contains an analysis of the origins of CSA-Z662 Clause 6.2.3(g). Recognizing that the stresses introduced by miters are discontinuity stresses, the paper discusses the effects of such stresses, including the use of miters in cyclic service. Recognizing also the strong dependence of D/t ratio on the discontinuity stress, the paper suggests a modification to the Z662 approach to account for this effect. This modification would provide guidance to the use of miters to effect small deflections both during design and construction of piping.