Operating Stress Levels Research Articles

Study on operator's SA reliability in digital NPPs. Part 2: Data-driven causality model of SA

This paper tries to overcome shortcomings of the traditional assessment method of situation awareness (SA) reliability and build more robust causality model of SA. Firstly, the organization-oriented analysis framework or method of SA error was used to analyze human factor events in nuclear power plants (NPPs) and the data of 132 samples were obtained. Then, the correlation analysis method is used to identify the correlation relationships between factors influencing SA and next factor analysis method is used to identify the scenes triggering SA error. It includes: operator’s mental level, operator's work attitude, stress level and system situation display level. Finally, based on the study of the results highlighted above, a data-driven SA causality model is established. These results shows that, the data-based SA causality model can identify the scenes triggering SA error, and it is very useful to improve the accuracy of quantitative assessment of SA reliability because of considering the causality relationships of performance shaping factors (PSFs).

On the deformation response and cyclic stability of Ni50Ti35Hf15 high temperature shape memory alloy wires

The processing - phase transformation - deformation response relationship of Ni50Ti35Hf15 high temperature shape memory alloy was studied. As the material was hot extruded and further underwent wire drawing, the phase transformation temperatures decreased and the strength levels increased, yet the initial brittle response was replaced by near-perfect superelastic response above the austenite finish temperature. Moreover, the Ni50Ti35Hf15 wires exhibited a remarkably stable cyclic actuation response under 300MPa, as compared to the poor cyclic stability of the hot extruded samples, which is beyond the operating stress levels for known shape memory actuators.

Design of validation experiments for life prediction models

This paper proposes a novel validation experiment design optimization (VEDO) method for the assurance of life prediction model, which is one of the key steps in guaranteeing the reliable design of products in meeting the target service life. Life testing data collected from experiments are important for the validation of time-dependent models. However, directly collecting life data for model validation at the operating stress level is usually time-consuming and expensive. In order to overcome this challenge, the accelerated life testing (ALT) method is employed in the proposed method to collect data for model validation. The connection between ALT and model validation is established first; then a VEDO model is developed using the prior information obtained from the computer simulation model. In the VEDO model, the information gain for model validation is maximized within the testing budget and available testing chamber constraints. The obtained optimal number of tests and testing stress levels are designed to maximize the confidence in the validation results. Various sources of uncertainty such as prediction uncertainty, uncertainty of prior information, and observation errors are included within the optimization process in order to improve the robustness of validation experiment design. A composite helicopter rotor hub component is used to demonstrate the effectiveness of the proposed VEDO method.

Quality control of cylinder head casting

Abstract New challenges for the Aluminium alloys used for the production of castings for automotive engine components result from an evolutionary trend of internal combustion engines towards higher specific power output. Cylinder heads, in particular, have to withstand higher operating temperature and stress levels. Present work describes quality control of microstructure (Si-morphologhy and Si-size) and mechanical properties (UTS, elongation, Brinell hardness) of cylinder head casting as effect of different T6 heat treatment (solution heat treatment time - 2, 3, 4, 5, 6, 7 hours). The data obtained from this study will be used to improve process control, and to help the selection of heat treatment of the casting for future products.

A Cooperative Automation Concept for User-oriented Support of Air Target Identification

For over 40 years science in a variety of domains has conducted an impressive amount of research in the field of assistance and automation concepts, which apply human factors and needs in the design stage of cooperative and cognitive technical systems. Cooperative automation is an approach that tries to keep the user in the control loop by means of a continuous interaction during task execution. Cooperative automation aims to support the harmonization of the actor's activities (user and automation) in a human-machine-system. To enable this, both the internal (processing and computation) and external (communications and interfaces) design of the automation has to be compatible with human competence. This contribution addresses the key features of cooperative automation. These features have been used for creating a design space, which can be used as method repository in order to develop assistance systems based on the cognitive capabilities of the human. The key elements are represented as dimensions of the design space. Corresponding theories and methods are discussed. The design process for a cognitive and cooperative assistance system based on the dimensions of the design space are described. The developed Cognitive and Cooperative Assistance System (COGAS), has been implemented in the domain of air target identification on German naval ships. An evaluation of COGAS has been conducted with naval operators in order to verify its operability and to optimize and extend the COGAS functionality. The results show that COGAS improves the operator's performance in terms of increasing the amount of correct decisions and accelerating the identification process without increasing the operator's stress level. In addition it was found that COGAS helps to enhance the situational awareness due to the workload optimization.

Development of Nickel Based Superalloys for Advanced Turbine Engines

Superalloys have been developed for specific, dedicated properties and applications. One of the main application for this material is advanced, high-performance aircraft engines elements. Turbine engine creates harsh environments for materials due to the high operating temperature and stress level. Hence, as described in this article, many alloys used in the turbine section of these engines are very complex and highly optimized. This article provides an overview of structural changes that occur during the aging process of wrought and cast alloys and provides insight into the use of precipitated particles to achieve desired structures. Example will focus on alloy Inconel 718 and CMSX-4. Functional properties of these alloys can be achieved by choosing proper heat treatment parameters to obtain required rate between secondary phases. The paper also attempts to determine structural perfection and changes of crystallographic orientation along the axis of growth of single crystal nickel superalloys cast using X-ray topography and Laue diffraction method. Single crystal bars and turbine blades were manufactured in VIM furnace using the Bridgeman method. Withdrawing rates typical for CMSX-4 superalloy were used. It has been found that with increasing withdrawing rate the nature of distribution along the axis of growth of the angle of [001] direction deviation from the axis of single crystal blades growth had changed. The change of the withdrawing rate results also in the rotation of γ’ phase in the form of cubes against the axis of single crystal blades growth.

Investigation of Trochoidal Milling Nickel-Based Superalloy

Nickel-based superalloy is a relatively new class of materials that exhibit high mechanical strength, ductility, creep resistance at high operating temperatures and stress levels. It is widely used in aero engine parts such as blade and blisk. Due to its high hardness and high strength at high temperature, it’s difficult to machine nickel-based alloy. In order to reduce tool wear, trochoidal milling strategy is studied in this paper. With trochoidal tool path strategy, tool load can be reduced and the tool has enough time to be cooled under high cutting temperature. Tool immersion angle in trochoidal milling is smaller than slot milling, thus the tool load can be reduced. Experiment results show that tool flank wear with normal slot milling is about four times larger than that with trochoidal milling. Trochoidal tool path strategy can be further developed for rough milling of compressor blades or rough milling hub of blisks.

In situ SEM testing for crack closure investigation and virtual crack annealing model development

In this paper, an in situ scanning electron microscope (SEM) fatigue testing approach is proposed to investigate the crack closure phenomenon within one cyclic loading under plane stress conditions. One objective of this experimental study is to verify the existence and significance of crack closure by directly measuring the variation of crack tip opening displacement (CTOD) and evaluating the crack opening stress levels. During the testing, the loading cycle is divided into a certain number of levels. At each level, high resolution images are taken around the crack tip region by SEM. Following this, imaging analysis is used to process these images in order to quantify the crack tip behavior at any time instant. Four constant amplitude loading cases with different stress ratios are investigated. Crack closure phenomenon is directly observed and measured. A simple mechanical model (named virtual crack annealing model) is developed based on the in situ SEM testing observations. The calculated crack opening stress level is compared with experimental observations. A detailed discussion is given based on the current investigation to explain some well-known issues in the classical fatigue theory.

Influence of rubber dam on objective and subjective parameters of stress during dental treatment of children and adolescents – a randomized controlled clinical pilot study

Rubber dam is recommended for isolating the working field during adhesive dentistry procedures; however, dentists often omit rubber dam, particularly in paediatric dentistry, supposing that it would stress the patient. The aim of this study was to evaluate stress parameters during a standardized dental treatment procedure performed with or without rubber dam. The treatment time was measured as a secondary outcome variable. This study was designed as a randomized, controlled, clinical study with 72 patients (6-16 years; mean age, 11.1). During standardized fissure sealing procedures, objective parameters of stress (e.g., skin resistance, breath rate) were recorded. The operator's stress level was measured by pulse rate. Subjective pain (patients) and stress perception (operator) were evaluated by an interview. The breath rate was significantly (P<0.05) lower and the skin resistance level was significantly higher during treatment with rubber dam compared to the control group. Subjective pain perception was significantly lower for the test group. The treatment time needed for the fissure sealing procedure was 12.4% less in the test group. Isolation with rubber dam caused less stress in children and adolescents compared to relative isolation with cotton rolls if applied by an experienced dentist.

Quasi-static crack tip fields in rate-sensitive FCC single crystals

In this work, the effects of loading rate, material rate sensitivity and constraint level on quasi-static crack tip fields in a FCC single crystal are studied. Finite element simulations are performed within a mode I, plane strain modified boundary layer framework by prescribing the two term (K − T) elastic crack tip field as remote boundary conditions. The material is assumed to obey a rate-dependent crystal plasticity theory. The orientation of the single crystal is chosen so that the crack surface coincides with the crystallographic (010) plane and the crack front lies along \([10\overline 1]\) direction. Solutions corresponding to different stress intensity rates \(\dot{{K}}\), T-stress values and strain rate exponents m are obtained. The results show that the stress levels ahead of the crack tip increase with \(\dot{{K}}\) which is accompanied by gradual shrinking of the plastic zone size. However, the nature of the shear band patterns around the crack tip is not affected by the loading rate. Further, it is found that while positive T-stress enhances the opening and hydrostatic stress levels ahead of crack tip, they are considerably reduced with imposition of negative T-stress. Also, negative T-stress promotes formation of shear bands in the forward sector ahead of the crack tip and suppresses them behind the tip.

A model of crack opening stresses in variable amplitude loading using smooth specimen fatigue test data for three steels

Crack growth under variable amplitude loading can be largely explained through changes in fatigue crack closure and crack opening stress. This paper presents a methodology for modeling changes in crack opening stress level and fatigue damage using data derived from periodic underload fatigue tests of smooth specimens for three steels with diverse hardness (soft, medium, and hard). The predicted crack closure stress levels are modeled under constant and variable amplitude loading (SAE Log Skidder History) and agree well with experimental measurements made with a high magnification microscope.

Computer Vision – A Versatile Technology in Automation of Agricultural Machinery

The agricultural market in developed countries is evolving from being mechanised in the 20th century, with still larger machines being automated in the 21st century. Development of automation technology for agricultural machinery includes many aspects. As the degree of automation increases, the complexity of automation would rise. The paper focuses on the sensor sub-system, and more specifically, computer vision technology. The emergence of widely available vision technologies has enabled the manufacturers to automate a wide range of tasks. Weed control and auto-guidance systems are two areas in agriculture that have driven the development of vision technology. Applying computer vision technology makes it possible to automate the navigation tasks when the vehicle is driven along crops and other field structures. The operator stress level can be generally reduced and high precision steering in sensitive crops can be maintained through out the day. The driver can concentrate on optimizing the machine settings and ensure a safe operation of the system. Machine settings can be optimised yielding a higher quality in the field operations. The working speed can be increased, thus saving time and money.

Modeling crack closure and damage in variable amplitude fatigue using smooth specimen fatigue test data

During overloads in variable amplitude fatigue, local stresses at small cracks growing from notches reach yield stress magnitude. Such high stress levels result in a large decrease in crack opening stress and an increase in the fatigue damage of subsequent smaller stress cycles. This paper presents a methodology for modeling changes in crack opening stress level and fatigue damage using data derived from periodic underload fatigue tests of smooth specimens. Predicted crack closure stress levels agree well with those obtained from crack growth observations made with a high magnification microscope.

Simultaneous simulation of closure behavior and shape development of fatigue surface cracks

The shape development and crack closure behavior of a surface crack subjected to fatigue loadings are two important features studied by researchers in the fatigue community. In the current study, both features are considered in the finite element analysis, and closure behavior and shape development of surface cracks are obtained simultaneously. In the analysis, two types of finite element models are constructed, one developed for the closure analysis and the other for the stress intensity factor computation of surface cracks. In the closure analysis, a new node release scheme is developed and the crack front nodes are unnecessarily simultaneously released during a loading cycle. Hence, the closure analysis of a surface crack with an evolving front of arbitrary shape becomes possible. The calculated results show that the proposed two-finite element model approach can obtain reasonable opening stress levels and crack shapes.

Simulation of Fatigue Crack Growth Under Random Overloads with Retardation

In order to study the behavior of fatigue crack propagation under random overloads, a Monte Carlo simulation scheme is proposed. Overloads of Poisson flow with uniform distribution on base-line constant-amplitude cyclic loads are considered. The retardation effect of overload is taken into account using crack closure model and the crack opening stress level is assumed to vary linearly in the yield zone produced by the overload. The fatigue crack growth curve from initial crack size till fatigue failure is simulated step by step. Through the large number of the simulated samples, the average fatigue crack propagation life is calculated. Finally, the influence of overload intensity and magnitude on fatigue crack propagation life is studied.

Prediction of crack opening stress levels for 1045 quenched and tempered steel under service loading spectra

ABSTRACTThe opening stresses of a crack emanating from an edge notch in a 1045 quenched and tempered steel specimen were measured under two different Society of Automotive Engineers (SAE) standard service load histories having different average mean stress levels. The two spectra are the Grapple Skidder history (GSH), which has a positive average mean stress, and the Log Skidder history (LSH), which has a zero average mean stress. To capture the behaviour of the crack opening stress in the material, the crack opening stress levels were measured at 900X using an optical video microscope, at frequent intervals for each set of histories scaled to two different maximum stress ranges.A crack growth analysis based on a fracture mechanics approach was used to model the fatigue behaviour of the steel specimens for the given load spectra and stress ranges. Crack growth analysis was based on an effective strain‐based intensity factor, a crack growth rate curve obtained during closure‐free loading cycles and a local notch strain calculation based on Neuber's rule.The crack opening stress (Sop) was modelled and the model was implemented in a fatigue notch model, and the fatigue lives of the specimens under the two different spectra scaled to several maximum stress levels were estimated. The average measured crack opening stresses were between 6 and 12% of the average calculated crack opening stresses. In the interest of simplifying the use of Sop in design, the average Sop was correlated with the frequency of occurrence of the cycle reducing the Sop to the average crack opening stress level. The use of an Sop level corresponding to the cycle causing a reduction in Sop to a level reached once per 10 cycles gave a conservative estimate of average crack opening stress for all the histories.

Fatigue crack closure of a corner crack: A comparison of experimental results with finite element predictions

Plasticity induced corner crack closure has been examined for the case of a middle tension (MT) plate of 14 mm thick 2024-T351 aluminium alloy subjected to cyclic loading. The closure load has been found by comparing the strain just ahead of the crack tip with the far field strain as the specimen was loaded and then unloaded. Special crack detection strain gauges were placed in front of the crack tips on the bore surface and on the free surface to allow measurement of crack opening and closure stress as the crack propagates. The offset observation and slope variation methods were used to determine the crack opening and closure stress levels. A three-dimensional finite element closure model was developed for a corner crack emanating from a hole. Finite element analysis gives the displacement of each node around the crack for which the opening and closure load were evaluated. The fatigue crack closure stress levels from the finite element analysis were compared with the experimental results giving a good agreement.

Partial discharge, dissipation factor, and corona aspects for high voltage electric generator stator bars and windings

High voltage stator windings in modern electric generators are subject to high electrical, mechanical, and thermal stress. Operation of these machines in both air and high pressure hydrogen results in different considerations. Operation in the air environment does not offer the dielectric advantages of operating in high-pressure hydrogen. The two major design factors that are significant for the design of high voltage stator bars to be operated in air, are corona activity and PD activity. These of course are closely related. With the movement to increasing the power density of generators, which means operation at higher voltages and higher insulation stress levels, it has become more critical to be sure the design constraints are met in the stator bar design to minimize both partial discharge activity and corona activity involving the stator bars. The stator bar insulation system must be designed for minimum partial discharge activity and ideally is corona free at the stator winding operating stress levels, and at the operating conditions (temperature and pressure) the generator is subject to. This paper will present the findings associated with the development work in designing, testing and implementing a PD reduced and corona free design.

Three-dimensional Numerical Investigation on Thickness-through Fatigue Crack Closure

In order to establish the calculation method of the fatigue crack opening stress level for the surface cracks in ship structural members, the validity of various crack-node release schemes and opening / closure stress definitions are investigated by 2- and 3-dimensional F.E. analysis of the crack closure behavior of a thickness-through crack. As results, followings are found;1) The measured and calculated opening stresses defined by the compliance method agree well when the crack tip node is released at the maximum load and the element edge length at the crack tip is chosen so that it is about 1/10 the size of the plastic zone.2) The variation of the crack propagation rate with the depth which gives a good account of the changing nature of the crack edge shape of a real thickness-through crack can be derived when the opening stress is defined as the remote stress at the time the compressive local stress at the crack tip node changes to a tensile stress.3) It is advisable to define the opening stress by the sign change of the local stress at the crack tip whenKopis evaluated locally by 3-dimensional F.E. analysis.

Three dimensional elastic-plastic finite element modelling of small fatigue crack growth under a single tensile overload

The effects of a single tensile overload on the stress, strain and displacement fields for a three dimensional semi-circular extending crack have been studied using an elastic-plastic finite element model. The results show that the application of a single tensile overload caused an immediate drop in the crack opening stress levels, but with further crack extension the opening stress levels rapidly rose to values higher than those before the application of the overload. It is found that the introduction of a single tensile overload increases the local residual compressive stress ahead of the crack front and, with the growth of the crack, the magnitude of this local compressive stress decreases. The results also show that the introduction of a single tensile overload increases the crack opening displacement, and that with further crack extension the crack opening displacement decreases rapidly to a value less than that before the application of the overload. The predicted crack opening displacements from finite element analysis were compared with those measured from direct in-situ SEM observations, and good agreement was obtained.