Handling Uncertainty in the Remnant Fatigue Life Assessment of Offshore Process Pipework

A typical procedure for a remnant fatigue life (RFL) assessment is stated in the BS-7910 standard. The aforementioned standard provides two different methodologies for estimating RFL; these are: the S-N curve approach and the crack growth laws (i.e. using Linear Elastic Fracture Mechanics (LEFM) principles) approach. Due to its higher accuracy, the latter approach is more commonly used for RFL assessment in the offshore industry. Nevertheless, accurate prediction of RFL using the deterministic LEFM approach (stated in BS-7910) is a challenging task, as RFL prediction is afflicted with a high number of uncertainties. Furthermore, BS-7910 does not provide any recommendation in regard to handling the uncertainty in the deterministic RFL assessment process. The most common way of dealing with the aforementioned uncertainty is to employ Probabilistic Crack Growth (PCG) models for estimating the RFL. This manuscript explains the procedure for addressing the uncertainty in the RFL assessment of process piping with the help of a numerical example. The numerically obtained RFL estimate is used to demonstrate a calculation of inspection interval.

The Influence of Laser Irradiation Parameters on Tribological Behaviour of Commercially Pure Titanium for Dental Prostheses

Laser shock peening (LSP) is one of the innovative technique that produces a compressive residual stress on the surface of metallic materials, thereby significantly increasing its fatigue life in applications where failure is caused by surface-initiated cracks. The specimens were treated with laser shock waves with different processing parameters, and characterization studies were made on treated specimens. The purpose of the present study was to investigate the influence of Nd:YAG laser on commercially pure titanium (CP-Ti) used in prosthetic dental restorations. The treatment influenced change in microstructure, micro hardness, surface roughness, and wear resistance characteristics. Though CP-Ti is considered as an excellent material for dental applications due to its outstanding biocompatibility, it is not suitable when high mastication forces are applied. In the present study, pulsed Nd:YAG laser surface treatment technique was adopted to improve the wear resistance of CP-Ti. The wear test pin specimens of CP-Ti were investment cast with centrifugal titanium casting machine. The wear properties of specimens were evaluated after LSP on a “pin-on-disc” wear testing tribometer, as per ASTM G99-05 standards. The results of the wear experiment showed that the treated laser surface has higher wear resistance, micro hardness, and surface roughness compared to as-cast samples. The improvement of wear resistance may be attributed due to grain refinement imparted by LSP processes. The microstructure, wear surfaces, wear debris, and morphology of the specimen were analyzed by using optical electron microscope, scanning electron microscope, and X-ray diffraction (XRD). The data were compared using ANOVA and post-hoc Tukey tests. The characteristic change resulted in increase in wear resistance and decrease in wear rate. Hence, it is evident that the more reliable and removable partial denture metal frameworks for dental prostheses may find its applications.

An Improved Model-Free Sampling Technique Based on Bootstrap Methods

The distribution and parameters of the random variables is an important part of conventional reliability analysis methods, such as Monte Carlo method, which should be known fist before using these methods, but it is often hard or impossible to obtain. Model-free sampling technique puts forward a method to get the distribution of the random variables, but the accuracy of the extended sample generated by it is not enough. This paper presented an improved model-free sampling technique, which is based on Bootstrap methods, to increase the accuracy of the extended sample and decrease the iteration times. In this improved model-free sampling technique, the method of the selection of initial sample points and the generation of iterative sample is improved. Meanwhile, a center distance criterion, which considers the local characteristics of the extended sample, is added to the generating criterion of dissimilarity measure. The effectiveness of this improved method is illustrated through some numerical examples.

The Reliability of a Component Under Multiple Cyclic Stress Levels With Distributed Cyclic Numbers

Fatigue damage is initiated through some “defects” on the surfaces of and/or inside the component and induced by the fatigue cyclic loadings. These “defects” are randomly scattered in components, and one of these “defects” will be randomly “activated” and finally developed to become the initial crack which causes the final fatigue failure. Therefore, the fatigue strength is inherently a random variable and should be treated by probabilistic models such as typical P-S-N curves. The fatigue cyclic loading could be presented or described in any form. But the fatigue loading spectrum can generally be grouped as and described by these five models: (1) a single constant cyclic stress (loading) with a given cyclic number, (2) a single constant cyclic stress with a distributed cyclic number, (3) a distributed cyclic stress (loading) at a given fatigue life (cyclic number), (4) multiple constant cyclic stress levels with given cyclic numbers, and (5) multiple constant cyclic stress levels with distributed cyclic numbers. The approaches for determining the reliability of components under fatigue loading spectrum of the models 1∼4 are available in literature and books. But few articles and books have addressed an approach for determining the reliability of components under the fatigue loading spectrum of the model 5. This paper will propose two approaches for addressing this unsolved issue. Two examples will be presented to implement the proposed approaches with detailed procedures.

Progress Toward Understanding Catastrophic Failure of Electric Vehicle Li-Ion Batteries: Multi-Physics Modeling

The goal of this work is to enhance understanding of critical design aspects that would prevent automotive lithium-ion battery packs from catastrophic failures. Modeling lithium-ion batteries is a complex multiscale multi-physics problem. The most dangerous energy producing component of a lithium ion cylindrical cell, jellyroll, is a layered spiral structure, which consists of thin layers of electrodes and separator only microns thick. In this study, we investigate the feasibility of using commercial explicit finite element code LS-DYNA to understand the structural integrity of lithium-ion batteries subjected to crushing condition through computer simulation. The jellyroll was treated as homogeneous material with an effective stress-strain curve obtained through characterization experiments of representative jellyroll samples and individual electrode layers. Physical and numerical impact tests have been conducted on cylindrical cells using developed drop test system. Results of material homogenization, experimental drop testing, and initial structural simulations are discussed. The investigation of structural cell deformations coupled with thermal heat generation and distribution after the crash brings us one step closer to accurate modeling of the entire battery pack that consists of hundreds of cells.

Influence of Basic Parameters for Fiber Reinforced Polymer Crushing Simulation

This article investigates the influence of finite element model features on Fiber Reinforced Polymer (FRP) crushing simulation results. The study focuses on two composite material tube models using single shell modeling approach. The chosen material model is MAT58 (*MAT_LAMINATED_COMPOSITE_FABRIC) from the commercial finite element analysis software LS-Dyna. The baseline models geometry and material parameters come from a model calibration conducted for lightweight vehicle investigation. Five parameters are investigated. The mesh size and the number of integration point (NIP) are generic and ERODS, TSIZE and SOFT are the non-physical parameters of MAT58. This analysis aims at discuss the influence of these parameters on the simulation results focusing on the initial force peak and the average crush load, regarding results realism and instabilities such as large elements deformation and abnormal peak values. Also, the impact of the number of CPUs involved in the simulation calculation is presented. Recommendations are given to set the mesh size and the NIP. TSIZE value should be selected regarding the simulation time step. On the other hand, ERODS has to be adjusted manually. Both are determinant for simulation robustness. Further studies are proposed to find out the reasons of large element deformation.

A New Composite Allocation Method on Balance of Reliability and Maintainability Index With the Goal of Availability

This paper proposes a new composite allocation method, which is composed by improved Fuzzy-AHP allocation method, old system data correction allocation method, and optimization allocation method. The objective of the new method is to minimize the system cost and allocate the reliability index and maintenance index of unit with the goal of system availability and the balance between them. For the solution of optimization problem in this paper, in order to prevent local optimum, improve the convergence efficiency and get the satisfied optimal solution, the improved GA method is put forward in this paper. First, the reliability index allocation method is proposed by the combination of optimization allocation method with the objective of minimum cost, the improved Fuzzy-AHP method with the consideration of the experts’ expectations, and the old system data correction allocation method. Then, based on constraints of the unit reliability allocation index and system availability index, the maintain-ability allocation method is proposed considering the minimum maintenance cost. In addition, the process of this new composite allocation method is formulated in this paper. Finally, the system cost, reliability index, and maintenance index of an integrated transmission device of an armored vehicle are allocated by this new composite allocation method. The result analysis shows that the allocation result of this new composite allocation method is reasonable and has engineering applicability. All in all, this new composite allocation method not only synthetically considers experts’ expectations on the new system, system cost and system reliability baseline information, but also associate the reliability index and maintenance index with the goal of system availability. In addition, this paper provides a new approach for reliability index & maintenance index of complex repairable systems in the early stages of product design.

Simulation of Leak Noise Propagation and Detection Using COMSOL Multiphysics

Pipelines leak detection represents an essential aspect in pipeline rehabilitation to avoid any unexpected failure. Several detection techniques have been adopted and received a widespread application in pipeline inspection nowadays but still present a major challenge to field operators. This paper presents an attempt to develop correlations between leaks and their effect on the fluid characteristics inside the pipeline such as fluid velocity, variation of pressure and sound level due to the presence of leaks along the pipeline. Characterization of these parameters and how they propagate with respect to time from the leak source will allow the development of a solution to detect leaks and quantify the amount of fluid being lost. This paper aims at conducting an experimental investigation to determine the sound level for specified leak sizes. The experimental data was used in COMSOL Multiphysics to simulate various fluid flow scenarios inside a 2 in. (5.08 cm.) pipe with different leak sizes.

Phantom Battery Pack for Destructive Testing of Li-Ion Batteries

The objective of this study is to find a structural alternative to jellyroll in order to safely conduct experimental crash testing of lithium-ion battery packs in academic laboratory environment. A procedure for lateral impact experiments has been developed and conducted on cylindrical cells and phantom cells using a flat rigid drop cart in a custom-built impact test apparatus. The main component of a cylindrical cell, jellyroll, is a layered spiral structure which consists of thin layers of electrodes and separator material. We investigate various phantom materials — candidates to replace the layered jellyroll with a homogeneous anisotropic material. During our experimentation with various phantom cells, material properties and internal geometries of additively manufactured components such as in-fill pattern, density and voids were adjusted in order to develop accurate deformation response. The deformation of the phantom cell was characterized and compared after impact testing with the actual lithium-ion cells. The experimental results were also compared with explicit simulations (LS-DYNA). This work shows progress toward an accurate and safe experimental procedure for structural impact testing on the entire battery pack consisting of thousands of volatile cells. Understanding battery and battery pack structural response can influence design and improve safety of electric vehicles.

PTFE and MoS2 Additives for Mineral Oil Film Formation in EHL Point Contacts

Lubricant film-forming viscosity index improvers blended with commercial engine oil have been developed and studied by using optical interferometry. The influence of the viscosity index improvers (PTFE and MoS2) mixed with oil were experimentally studied and compared with engine oil without the index improvers as the baseline. The effect of the viscosity index improvers on lubricant film thickness, contact pressure and rolling speed for the case of a steel ball loaded on a flat glass surface in point contact condition was investigated. An optical interferometry technique which utilized a monochromatic two-beam interferometry light source, a microscope and a high-speed video recording device was used for the investigation. Hamrock and Dawson calculations for EHL film thickness were also used for comparative analysis. The lubricants used were commercial SAE #30 engine oil and PTFE and MoS2 mixed with commercial SAE #30 engine oil. The oil viscosities ranged from 0.0109 Pa.s to 0.255 Pa.s. The rolling speed and the loads were varied between 0.189 m/s to 0.641 m/s and 1 N to 2.6 N respectively. The lubricant film thickness stability at the point of contact between the steel ball and the glass disc was investigated for both steady and rolling state conditions. The viscosity index improvers were found to have a significant effect on the film thickness behavior under pure rolling point contact conditions.