scholarly journals INSPECTION POLICIES IN AIRCRAFT SERVICE

Aviation ◽  
2004 ◽  
Vol 8 (4) ◽  
pp. 3-9 ◽  
Author(s):  
Konstantin N. Nechval ◽  
Nicholas A. Nechval
Keyword(s):  
Crack Propagation ◽  
High Stress ◽  
Simple Approach ◽  
Structural Components ◽  
Aircraft Structure ◽  
Numerical Examples ◽  
Usual Approach ◽  
Long Period ◽  
Parameter Values ◽  
Initial Cracks

As aircraft structures begin to age (that is, as flight hours accumulate), existing subcritical cracks or new cracks can grow in some high‐stress points of the structural components. The usual approach is to inspect the structures periodically. Thus, a catastrophic accident during flight can be avoided. The problem then arises of choosing a sequence of inspection times which avoids both too many inspections, which may be costly, and too few inspections, that may also be costly due to a crack in an aircraft structure component not being detected for a long period. In this paper, a simple approach is proposed, where after each inspection (if a crack is not detected), we choose the next inspection point so that a crack may occur within an interval between successive inspection times with a given probability. It allows one to find the inspection policies for detection of initial cracks in critical structural components of aircraft under the assumption that the parameter values of the underlying distributions are unknown; this constraint is often met in practice. Furthermore, obtaining inspection schedules under crack propagation is considered. To illustrate the proposed technique based on ancillary statistics, numerical examples are given.

Evidence of tectonic release from underground nuclear explosions in long-period P waves

1983 ◽  
Vol 73 (2) ◽  
pp. 593-613
Author(s):  
Terry C. Wallace ◽  
Donald V. Helmberger ◽  
Gladys R. Engen
Keyword(s):  
Upper Mantle ◽  
High Stress ◽  
Test Site ◽  
Strike Slip ◽  
Body Waves ◽  
Release Time ◽  
P Waves ◽  
Underground Nuclear Explosions ◽  
Nuclear Explosions ◽  
Long Period

abstract In this paper, we study the long-period body waves at regional and upper mantle distances from large underground nuclear explosions at Pahute Mesa, Nevada Test Site. A comparison of the seismic records from neighboring explosions shows that the more recent events have much simpler waveforms than those of the earlier events. In fact, many of the early events produced waveforms which are very similar to those produced by shallow, moderate-size, strike-slip earthquakes; the phase sP is particularly obvious. The waveforms of these explosions can be modeled by assuming that the explosion is accompanied by tectonic release represented by a double couple. A clear example of this phenomenon is provided by a comparison of GREELEY (1966) and KASSERI (1975). These events are of similar yields and were detonated within 2 km of each other. The GREELEY records can be matched by simply adding synthetic waveforms appropriate for a shallow strike-slip earthquake to the KASSERI observations. The tectonic release for GREELEY has a moment of 5 ՠ1024 dyne-cm and is striking approximately 340°. The identification of the sP phase at upper mantle distances indicates that the source depth is 4 km or less. The tectonic release time function has a short duration (less than 1 sec). A comparison of these results with well-studied strike-slip earthquakes on the west coast and eastern Nevada indicate that, if tectonic release is triggered fault motion, then the tectonic release is relatively high stress drop, on the order of several hundred bars. It is possible to reduce these stress drops by a factor of 2 if the tectonic release is a driven fault; i.e., rupturing with the P velocity. The region in which the stress is released for a megaton event has a radius of about 4 km. Pahute Mesa events which are detonated within this radius of a previous explosion have a substantially reduced tectonic release.

Crack Growth During Full Scale Reeling Simulation of Pipes With Girth Welds

2004 ◽  
Author(s):  
Oddvin O¨rjasaeter ◽  
Olav Jan Hauge ◽  
Guy Ba¨rs ◽  
Per Egil Kvaale
Keyword(s):  
Crack Propagation ◽  
Crack Growth ◽  
High Strain ◽  
High Stress ◽  
Strain Range ◽  
Full Scale ◽  
Small Scale ◽  
Final Fracture ◽  
Lack Of Fusion ◽  
Girth Welds

Installation of pipelines by reeling has proved to be an effective method. However, the pipe bending results in very high stress and strain and cannot be handled by conventional design rules, as stated in design codes, e.g. [2]: High strain crack growth must be assessed according to specific case-by-case selected criterions. In the present work the performance of 10” and 12 3/4” pipes with typical weld defects is studied — from initiation of cracks at notches to final fracture. Information was obtained from several sources: full scale cyclic bending of pipes, FE simulations, and small-scale tests. The plasticity during reeling operations results in substantial non-linear behavior due to varying cross section properties, cyclic creep, and different material response at tensile and compression side of the pipe. Hence, a full scale reeling simulation must be carefully planned and include sufficient tolerances. Critical cracks in pipe girth welds initiate mainly from the surface (undercuts, lack of penetration, or lack of fusion), but potentially also internally (lack of fusion or large pores). Various configurations of these parameters were investigated in full scale pipe tests. It was possible to verify both crack propagation during the reeling cycles, and the point of final fracture (for ECA verifications). In pipe design on must assure safe conditions for both reeling operations and for later in-service loading. Proper design tools must be available. Several methods for high strain crack growth analysis were considered and also compared to small-scale specimen data. Conventional strain-life methodology failed to predict the crack propagation accurately. A new approach including a tensile strain range parameter offered promising results.

Regulation of sperm motility at the axonemal level

1995 ◽  
Vol 7 (4) ◽  
pp. 847 ◽  
Author(s):  
C Gagnon
Keyword(s):  
Direct Action ◽  
Mammalian Species ◽  
Basic Structure ◽  
Structure And Function ◽  
Structural Components ◽  
Long Period ◽  
Different Types ◽  
Radial Spokes ◽  
Dynein Arms ◽  
And Function

With very few exceptions, the basic structure of the 9+2 axoneme has been well preserved over a very long period of evolution from protozoa to mammais. This stability indicates that the basic structural components of the axoneme visible by electron microscopy, as well as most of the other unidentified components, have withstood the passage of time. It also means that components of the 9+2 axoneme have sufficient diversity in function to accommodate the various types of motility patterns encountered in different species of flagella. Several of the 200 polypeptides that constitute the axoneme have been identified as components of the dynein arms, radial spokes etc. but many more remain to be identified and their function(s) remain to be determined. Because this review deals with the regulation of flagellar movement at the axonemal level, it does not include regulation of flagella by extracellular factors unless these factors have a direct action on axonemal components. In this context, it is very important firstly to understand the structural components of the axoneme and how they influence and regulate axonemal movement. Different primitive organisms are mentioned in this review since major breakthroughs in our understanding of how an axoneme generates different types of movement have been made through their study. Despite some variations in structure and function of axonemal components, the basic mechanisms involved in the regulation of flagella from Chlamydomonas or sea urchin spermatozoa should also apply to the more evolved mammalian species, including human spermatozoa.

scholarly journals Receptance-based frequency assignment for assembled structures

2020 ◽  
pp. 107754632094545
Author(s):  
Shike Zhang ◽  
Huajiang Ouyang
Keyword(s):  
Structural Properties ◽  
Natural Frequencies ◽  
Dynamic Performance ◽  
Frequency Assignment ◽  
Structural Components ◽  
Engineering Structures ◽  
Numerical Examples ◽  
Structural Modifications ◽  
Assignment Method

For engineering structures, there is a strong need to assign natural frequencies to achieve desired dynamic performance. This study proposes a receptance-based frequency assignment method for assembled structures. Very often, the substructures involved are not allowed or are difficult to change. This method uses the links between the substructures as targets of structural modifications and determines the structural properties of the links that assign the desired frequencies cast as an optimisation problem. These links could be either simple discrete structural components such as masses and springs or complex continuous structures. Only a few receptances of the substructures are required in this method, which can be measured accurately and easily in practice. Two numerical examples are presented to show the validity of this method and its strength in dealing with complex assembled structures.

scholarly journals THE DYNAMICS OF CHANGES IN STRUCTURE INDICES OF INFANT MORTALITY IN ST. PETERSBURG DURING 2005-2017 YEARS

2019 ◽  
Vol 21 (3) ◽  
pp. 157-160
Author(s):  
A. S. Simakhodsky ◽  
L. D. Sevostianova ◽  
U. V. Gorelik ◽  
N. P. Akincheva ◽  
U. V. Kolechko
Keyword(s):  
Mortality Rate ◽  
Infant Mortality ◽  
Infant Mortality Rate ◽  
High Tech ◽  
Structural Components ◽  
Long Period ◽  
The Russian Federation ◽  
Structure Indices ◽  
The Individual ◽  
The City

The review presents the literature data and, revealed by authors, the patterns of changes in the dynamics of the infant mortality rate and its structure over a long period (2005-2017) in St. Petersburg. This index is one of the lowest in the Russian Federation. There are analyzed changes in the individual components of the structure of the infant mortality rate, related both to objective and subjective factors. In the city of St. Petersburg there was shown a persistent decline in the infant mortality rate due to the implementation of federal and city programs, closely correlated with structural components. The authors believe the positive dynamics of structural components in St. Petersburg to indicate the organization of all types of specialized and high-tech medical care for newborns.

Description of Small Fatigue Crack Propagation in ODS Steel

2014 ◽  
Vol 891-892 ◽  
pp. 911-916
Author(s):  
Pavel Hutař ◽  
Ivo Kuběna ◽  
Miroslav Šmíd ◽  
Martin Ševčík ◽  
Tomáš Kruml ◽  
...  
Keyword(s):  
Fatigue Crack ◽  
Crack Propagation ◽  
Fatigue Crack Propagation ◽  
Fatigue Testing ◽  
Low Cycle Fatigue ◽  
High Stress ◽  
Oxide Dispersion Strengthened ◽  
Ods Steel ◽  
Small Fatigue Crack ◽  
Propagation Law

The oxide dispersion strengthened (ODS) steels are widely studied as possible candidate material for fission and fusion technology. Due to application of the ODS materials, mainly low cycle fatigue is important. Therefore, small crack initiation and propagation for rather high stress amplitude are in focus. The main aim of the presented work is description of the small fatigue crack propagation for ODS-EUROFER steel. Due to limited quantity of ODS steel special miniaturized cylindrical specimens for fatigue testing were designed. Crack propagation law based on plastic part of J-integral is presented and applied for mentioned material. The resulting fatigue crack propagation rates for ODS-EUROFER steel and EUROFER 97 are compared and discussed.

Slow Crack Growth in Dental Composites

1985 ◽  
Vol 55 ◽  
Author(s):  
G. M. Montes-G. ◽  
R. A. Draughn ◽  
T. H. Simpson
Keyword(s):  
Crack Propagation ◽  
Crack Growth ◽  
Slow Crack Growth ◽  
High Stress ◽  
Torsion Test ◽  
Stable Crack Growth ◽  
Test Method ◽  
Volume Percent ◽  
Stick Slip ◽  
Model Composite

ABSTRACTThe fracture properties of selected commercial composite dental restorative materials and a model composite system were studied to determine the influences of the reinforcing phase, exposure to water, and particle/polymer adhesion on crack propagation. The content of inorganic fillers ranged from 36 to 62 volume percent. In the model system the polymer phase approximated that of the commercial products, a constant size distribution of quartz fillers was used, and polymer/particle adhesion was varied. The double torsion test method was employed to measure relationships between applied stress intensity factor and velocity of crack propagation during stable crack growth. In all systems, cracks propagated through regions of high stress concentration at the low end of the velocity range studied (10−7 m/sec to 10−3 m/sec). Wet materials fractured at lower stress intensities than dry materials at all velocities. At high velocities unstable (stick-slip) growth occurred in dry materials with strong filler/matrix interfaces and in wet specimens with initially strong interfaces and less than 41 volume percent filler. In wet conditions, materials with poorly bonded fillers fractured by slow crack growth at stress intensities 10% to 30% below the levels of composites with strong interfaces.

Mechanical Behavior of Alumina Toughened Zirconia Nanocomposites with Different Alumina Additions

2014 ◽  
Vol 96 ◽  
pp. 61-66 ◽  
Author(s):  
Luis Antonio Díaz ◽  
Sergio Rivera ◽  
Adolfo Fernández ◽  
Anna Okunkova ◽  
Yu.G. Vladimirov ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Crack Propagation ◽  
Mechanical Behavior ◽  
Crack Growth ◽  
Disperse Phase ◽  
Subcritical Crack Growth ◽  
The Other ◽  
Structural Components ◽  
Monolithic Ceramics

ZrO2 and Al2O3 are monolithic ceramics used today in a wide variety of structural components. However, both materials present important drawbacks for some specific applications. In the case of Al2O3, its moderate strength (500 MPa) and toughness (4 MPa.√m) makes it unsuitable for high loading conditions. On the other hand, ZrO2 presents higher strength and toughness values (900 MPa and 6 MPa.√m) than Al2O3 but it is a material limited in its long-term behaviour due to its bad response to hydrothermal ageing and a pronounced tendency for subcritical crack growth. Due to this fact, ceramic nanocomposites made of Al2O3 and ZrO2 (ATZ and ZTA) have been developed in the last years in order to overcome the main drawbacks of the monolithic materials as they can combine the properties of both, strong and tough materials, simultaneously, with null ageing and even higher biocompatibility. In this work, several amounts of Al2O3 disperse phase (15, 35 and 50 vol %) were added to one ZrO2 matrix (CeO2 - 10 mol %) in order to see their effect on the mechanical properties, subcritical crack propagation and long-term reliability.

An Improved Meshfree Method for Fracture Analysis of Cracks

2006 ◽  
Author(s):  
Rajib Chowdhury ◽  
B. N. Rao ◽  
A. Meher Prasad
Keyword(s):  
Crack Propagation ◽  
Galerkin Method ◽  
Meshless Method ◽  
Mixed Mode ◽  
System Of Equations ◽  
Element Free Galerkin Method ◽  
Numerical Examples ◽  
Element Free Galerkin ◽  
Cracked Structures ◽  
Element Free

This paper presents an efficient meshless method for analyzing linear-elastic cracked structures subject to single- or mixed-mode loading conditions. The method involves an element-free Galerkin formulation in conjunction with an exact implementation of essential boundary conditions and a new weight function. The proposed method eliminates the shortcomings of Lagrange multipliers typically used in element-free Galerkin formulations. Numerical examples show that the proposed method yields accurate estimates of stress-intensity factors and near-tip stress field in two-dimensional cracked structures. Since the method is meshless and no element connectivity data are needed, the burdensome remeshing required by finite element method (FEM) is avoided. By sidestepping remeshing requirement, crack-propagation analysis can be dramatically simplified. An example problem on mixed-mode condition is presented to simulate crack propagation. The agreement between the predicted crack trajectories by the proposed meshless method and FEM is excellent. In recent years, a class of Galerkin-based meshfree or meshless methods have been developed that do not require a structured mesh to discretize the problem, such as the element-free Galerkin method, and the reproducing kernel particle method. These methods employ a moving least-squares approximation method that allows resultant shape functions to be constructed entirely in terms of arbitrarily placed nodes. Meshless discretization presents significant advantages for modeling fracture propagation. Since no element connectivity data are needed, the burdensome remeshing required by the finite element method (FEM) is avoided. A growing crack can be modeled by simply extending the free surfaces, which correspond to the crack. Although meshless methods are attractive for simulating crack propagation, because of the versatility, the computational cost of a meshless method typically exceeds the cost of a regular FEM. Also in some cases, the MLS which is the bases of the meshless method may form an ill-conditioned system of equations so that the solution cannot be correctly obtained. Hence, in this paper, we propose an improved element-free Galerkin method based on an improved moving least-square approximation (IMLS) method. In the IMLS method, the orthogonal function system with a weight function is used as the basis function. The IMLS has higher computational efficiency and precision than the MLS, and will not lead to an ill-conditioned system of equations. Numerical examples are presented to illustrate the computational efficiency and accuracy of the proposed improved element-free Galerkin method.

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