Parts stress technique

The handbook includes a series of empirical failure rate models developed using historical piece part failure data for a wide array of component types. There are models for virtually all electrical/ electronic parts and a number of electromechanical parts as well. All models predict reliability in terms of failures per million operating hours and assume an exponential distribution (constant failure rate), which allows the addition of failure rates to determine higher assembly reliability. The handbook contains two prediction approaches, the parts stress technique and the parts count technique, and covers 14 separate operational environments, such as ground fixed, airborne inhabited, etc. 

As the names imply, the parts stress technique requires knowledge of the stress levels on each part to determine its failure rate. 

Ceramics cannot be bolted or riveted the contact stresses would cause brittle failure. Instead, ceramic components are bonded to other ceramic or metal parts by techniques which avoid or minimise stress concentrations. 

Due in part to its ability to produce extremely low shear rates, the controlled stress technique has been formd to be highly suited to the determiiration of apparent yield stress, and in this respect the controlled-stress instrument is widely claimed to be more successful than its controlled-shear rate-cormterparts. This is usually attributed to the fact that, for suitably low stresses, the structure of the material may be preserved imder the couditions of test, ludeed, the introduction of the second generation of controlled-stress instruments can be said to have provoked considerable interest in, and debate siuroimding, the field of yield stress determination, with some early advocates of the controlled-stress technique advancing the controversial notion of the yield stress myth [Barnes and Walters, 1985]. 

External antistats. External, or topical, antistats are applied to the surface of the finished plastic part through techniques such as spraying, wiping, or dipping. Since they are not subjected to the temperatures and stresses of plastic compounding, a broad range of chemistries is possible. The most common external antistatic additives are quaternary ammonium salts, or quats, applied from a water or alcohol solution. 

Note 1 The lowest level of retention is a function of use, not technique. The higher levels may well involve considerable fabrication costs, greater accuracy and yet still leave parts stressed and prone to mechanical problems. 

Ultrasonic insertion is a fast and economical method of installing metal inserts into parts. This technique offers a high degree of mechanical reliability with excellent pull-out and torque retention combined with savings resulting from rapid production cycles. If the assembly is properly designed, ultrasonic insertion results in lower residual stress compared to molded-in or pressed-in techniques since a uniform melt occurs and a minimum amount of thermal shrinkage is involved. 

FTIR Studies of Polymers For the most part, the techniques used in the measme-ment of FTIR are the same as those applied to conventional samples. Spectral features of a component in a polymer are isolated from the solvent bands and from aU other compounds present in the system. Analysis is carried out by computer programs. Figure 17.26 shows the spectrum of polystyrene with an IR microscope through a 10-pm aperture. We see that the baseline is flat even below 1000 cm . This is very important because the frequency range of the IR spectrum below 1300 cm especially that below 1000 cm , is known as the fingerprint region. Changes in frequency and intensity may be used to determine the microscopic characterization of the polymer to see if there exists any defect due to stress or the environment. 

The parts count technique assumes average stress levels as a means of providing an early design estimate of the failure rate. 

Thermal Stresses. When the wak of a cylindrical pressure vessel is subjected to a temperature gradient, every part expands in accordance with the thermal coefficient of linear expansion of the steel. Those parts of the cylinder at a lower temperature resist the expansion of those parts at a higher temperature, so setting up thermal stresses. To estimate the transient thermal stresses which arise during start-up or shutdown of continuous processes or as a result of process intermptions, it is necessary to know the temperature across the wak thickness as a function of radius and time. Techniques for evaluating transient thermal stresses are available (59) but here only steady-state thermal stresses are considered. The steady-state thermal stresses in the radial, tangential, and axial directions at a point sufficiently far away from the ends of the cylinder for there to be no end effects are as fokows ... 

A method for measuring the uniaxial extensional viscosity of polymer soHds and melts uses a tensile tester in a Hquid oil bath to remove effects of gravity and provide temperature control cylindrical rods are used as specimens (218,219). The rod extmder may be part of the apparatus and may be combined with a device for clamping the extmded material (220). However, most of the mote recent versions use prepared rods, which are placed in the apparatus and heated to soften or melt the polymer (103,111,221—223). A constant stress or a constant strain rate is appHed, and the resultant extensional strain rate or stress, respectively, is measured. Similar techniques are used to study biaxial extension (101). 

Gurtman, G.A., Review of Techniques Used to Analyze Stress Wave Propagation in Composites, Systems, Science and Software Report No. 3SR-154, Part II, La Jolla, CA, 40 pp., December 1969. 

In experimental load studies, the measurable variables are often surface strain, acceleration, weight, pressure or temperature (Haugen, 1980). A discussion of the techniques on how to measure the different types of load parameters can be found in Figliola and Beasley (1995). The measurement of stress directly would be advantageous, you would assume, for use in subsequent calculations to predict reliability. However, no translation of the dimensional variability of the part could then be accounted for in the probabilistic model to give the stress distribution. A better test would be to output the load directly as shown and then use the appropriate probabilistic model to determine the stress distribution. 

When the part will experience higher temperatures or greater stresses than a patch can normally tolerate, the flush aerodynamic technique is often the solution. This technique consists of removing the damaged area and replacing the plies. 

To minimize the stresses induced by differential thermal expansion/contraction one must (1) employ fastening techniques that allow relative movement between the component parts of the composite structure, (2) minimize the difference in coefficient of linear thermal expansion between the materials... 

Experimentally DMTA is carried out on a small specimen of polymer held in a temperature-controlled chamber. The specimen is subjected to a sinusoidal mechanical loading (stress), which induces a corresponding extension (strain) in the material. The technique of DMTA essentially uses these measurements to evaluate a property known as the complex dynamic modulus, , which is resolved into two component parts, the storage modulus, E and the loss modulus, E . Mathematically these moduli are out of phase by an angle 5, the ratio of these moduli being defined as tan 5, Le. 

A quite different approach to the study of plant responses to stress has been explored by those ecologists who have followed the example of Harper (1977) in applying to plants techniques originally deployed in investigations of animal populations. Here the methodology has been demographic and the resulting data have allowed responses to stress to be analysed in terms of fluctuations in the rates of mortality and recruitment of either plant populations or plant parts (e.g. leaves, inflorescences). 

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