Accelerated tests design

This test is used most frequently it is an accelerated test designed to determine the stability of an explosive and also the compatibility of an explosive with a polymer or contact material. The assembly of the set-up and method of its determination are described in detail in the literature [14]. A dried and accurately weighed sample of a secondary explosive (5.0 0.01 g) or of a primary explosive (1.0 0.01 g) is placed in the heating tube followed by its assembly and evacuation. The heating tube is immersed in a constant temperature bath (100 °C or 120 °C) for a period of 40 h and the volume of evolved gases is recorded. Most explosives or explosive formulations yield less than 1 cm3 of gas per gram of an explosive during 40 h at 120 °C. 

Table 13.4 Standards for Accelerated Testing Designation Title...

Before developing accelerated tests, the performance of the product over time must be determined. An example of MEA performance over time is shown in Fig. 1. Once the time-dependant performance has been established, the next step is to postulate failure modes followed by propriate accelerated tests designed to mimic the failure mode. For example, the loss in cell voltage at 0.05 A cm may be due to increasing crossover, the loss in current at 0.8 V may be due to loss of catalyst activity or surface area, and the loss in current at 0.4 V may be due to increasing mass-transport resistance from gas-diffusion-layer (GDL) flooding. Once the failure mode is understood, the next step is to establish an accelerated test to accelerate that particular failure mode. 

The relative susceptibHity of several commercial aHoys is presented in Table 8. The index used is a relative rating based on integrating performance in various environments. These environments include the harsh condition of exposure to moist ammonia, Hght-to-moderate industrial atmospheres, marine atmosphere, and an accelerated test in Mattsson s solution. The latter testing is described in ASTM G30 and G37 (35,36) and is intended to simulate industrial atmospheres. The index is linear. A rating of 1000 relates to the most susceptible and zero designates immunity to stress corrosion. 

Table II shows, as an example, the combinations of low and high levels for three factors selected by a design team for an accelerated test Involving photovoltaic solar cells. In column 2 the three factors are seen to be temperature T (50 C, 95 C), relative humidity RH (60%, 85%), and ultraviolet radiation UV (five suns, 15 suns). The eight combinations of the high and low levels are shown, together with the predicted months to failure for each combination. In this example the documentation to support each prediction is symbolically referenced as shown in the last column. The documentation includes assumptions, calculations, references to the literature, laboratory data, computer simulation results, and other related material. Such a factorial table is first completed by each scientist independently. Subsequently, the team alms to generate a single consensus factorial table has the same form as that shown in Table II.

E. G. Derouane, V. Parmon, F. Lemos and F. R. Ribeiro, Principles and methods for accelerated catalyst design and testing, Series 11 Mathematics, Physics and Chemistry, vol. 69, (Kluwer Academic Publishers, Dordrecht, 2002). 

Estimates of service life are usually made either by natural or simulated trials or, most commonly, by accelerated tests with extrapolation to predict performance at longer times under less severe conditions. An alternative approach is to subject the product to environmental exposures which equate to the whole design life, and then to assess performance by real or simulated service tests (the end performance assessment). The exposures usually have to involve accelerated procedures and can be composed of several environmental agents applied simultaneously or sequentially. 

It is important that the purpose of any accelerated test programme is clearly established, because this has a large bearing on the design of the trials and the interpretation of the results. As a generality, tests where the objective is to make direct predictions from the results are a great deal more difficult to design and will be considerably more costly than tests for the other purposes. 

It is also clear from the foregoing that prediction from accelerated tests is at best a hazardous procedure. To minimise the limitations in any particular case it is essential to design the accelerated trials to simplify and ease as far as possible the prediction process. Of the limitations the most important can be summarised as ... 

Pressurised pipes for water and gas provide an example of design lives being predicted with confidence on the basis of a large assembly of data, of benchmark quality control tests for existing products, and of two-parameter accelerated testing for new ones. Some types of pipe are particularly sensitive to pressure extremes. Lifetime prediction cannot however take into account poor installation conditions. 

F. Ram6a Ribeiro, NATO Science Series, II Mathematics, Physics and Chemistry 69 (Principles and Methods for Accelerated Catalyst Design and Testing) (2002)... 

Based on factors such as cast, mechanical properties, physical properties, ease of fabrication (design) and the corrosion resistance data available in the literature the choice of materials can lead to a short list of two or three materials. At this stage it is prudent that the engineer design prototype laboratory scale model equipment from the short-list of selected metals or alloys, and determine the corrosion rates in the environment of interest. These accelerated tests will enable the engineer to select the best candidate material, making proper allowance for the corrosion of the metal or alloy over the lifetime of the equipment. 

Figures 2 and 3 show that the DRAM chip perforMance has been iMproved even though the chip functionality has increased for the accelerated tests used by the seMiconductor industry. The 85 C/85X RH results are better because of a coMbination of iMproveMents in the chip design, the Manufacturing procedures and the epoxy encapsu-lent. The teMperature cycle test results, however, were priMarily improved by converting to a "low stress" epoxy encapsulant. The im-proveMent in the pressure cooker and the 125 C operating life (Figure 3) was also due to a coMbination of iMproveMents, including those in the epoxy encapsulant. These iMproveMents in device reliability are especially reMarkable when it is realized that the chip susceptibility to contaminants and stress has increased tremendously due to the 60-fold increase to functionality.

Standard Practice for Accelerated Testing for Color Stability of Plastics Exposed to Indoor Fluorescent Lighting and Widow-Filtered Daylight, ASTM Designation D 4674-A689. 

For photostability studies, accelerated tests for liquid preparations may involve the use of some controlled, high intensity photon sources, and should take into account maintenance of a uniform temperature and adequate mixing of the preparation. If the protocol of the photostability study is properly designed, the extent of degradation should be directly proportional to the number of photons absorbed, as reported in the study by Shin et al. (97), and presented in Figure 12. 

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