Real Service Conditions

All testing has to be related to environmental conditions whose characteristics must be defined. The relation of accelerated corrosion test condition to real service conditions is one aspect while another is defining classes of environment and relating them to characteristic corrosion performance in order to produce useful specification guidelines. BS5493 1977 is an attempt to do this using four categories appropriate primarily to the UK (Table 13.2). Such a classification is clearly an over-simplification, but represents an important step in this particular direction. 

This approach is expensive. It is attractive for particularly complex products which cannot be tested in real service conditions, and for those where the consequences of malfunction justify the costs of testing. It applies particularly to safety-critical products in the nuclear industry which could be subjected to abnormally harsh conditions, either for the environmental exposure or the end assessment or both. 

To develop a representative test method, adequate knowledge is required of the practical use of the test results and of the real service conditions of the object of the test. Based on such knowledge, the representative properties to be determined by the test may be identified. 

PCA fibres, in one way or another, bear mechanical load in real service conditions. That is why, strength indexes and, in the first place, rupture stress and extension are of great... 

Of particular importance is the timescale over which diffusion occurs under various conditions of relative humidity (RH) and temperature. The RH determines the equilibrium moisture concentration, whereas higher temperatures will accelerate the moisture sorption process. In order to predict the moisture profile in a particular structure, it is assumed that Fickian diffusion kinetics operate. It will be seen later that many matrix resins exhibit non-Fickian effects, and other diffusion models have been examined. However, most resin systems in current use in the aerospace industry appear to exhibit Fickian behaviour over much of their service temperatures and times. Since the rate of moisture diffusion is low, it is usually necessary to use elevated temperatures to accelerate test programmes and studies intended to characterize the phenomenon. Elevated temperatures must be used with care though, because many resins only exhibit Fickian diffusion within certain temperature limits. If these temperatures are exceeded, the steady state equilibrium position may not be achieved and the Fickian predictions can then be inaccurate. This can lead to an overestimate of the moisture absorbed under real service conditions. 

If these achievements are so important why are there still difficulties in determining the behaviour of real structures in real service conditions ... 

S-N curves of polymers have to be used with caution in designing components. The fatigue strength depends much more strongly on the load frequency than in metals because the equilibrium between heat production and dissipation plays a crucial role. To design components, experiments should be as close to real service conditions as possible. 

Phase diagrams can be used to predict the reactions between refractories and various soHd, Hquid, and gaseous reactants. These diagrams are derived from phase equiHbria of relatively simple pure compounds. Real systems, however, are highly complex and may contain a large number of minor impurities that significantly affect equiHbria. Moreover, equiHbrium between the reacting phases in real refractory systems may not be reached in actual service conditions. In fact, the successful performance of a refractory may rely on the existence of nonequilibrium conditions, eg, environment (15—19). 

The problems that have been experienced in the recirculating rig test are indicative of those often met in performance testing. Attempts to reproduce the service conditions in a laboratory test inevitably involve attempting to reproduce each of the controlling conditions that exist in the real situation. Variations, which may be relatively small, in these simulations can lead to significant differences in test results. There is therefore much to be said for keeping test conditions as simple as possible rather than attempting to reproduce accurately the conditions in practice. A balance between reproducibility and realism has to be struck. 

The bearings must be adequate to handle the shaft loads without excessive wear, provided lubrication is maintained. Usually this is not a point of real question provided the manufacturer has had experience in the type of loads imposed by the service conditions, and the responsibility for adequate design must be his. 

Few would argue against real service trials being the first choice if the conditions and time scale do not completely rule them out. Even when this approach is not feasible before the launch of a product, it is highly desirable that real life trials are started as early as possible because, at the very least, they can warn of impending disasters in the field. 

Simulating service conditions avoids all the risks of using real service, and offers the possibility of moderate acceleration by simulating the worst conditions possible. The name implies that all factors present are considered, for example mechanical stress and the environment. However, the time scales will still be long and in many cases it is difficult, if not impossible, to simulate real conditions accurately. Clearly, simulated service trials are most attractive where the expected lifetimes are relatively modest and the conditions to be simulated are not too complicated. 

All the treatments discussed above have been concerned with constant conditions, i.e., where in the accelerated tests the level of the degrading agents has been held constant throughout one exposure, and any extrapolation to service implicitly assumes that conditions there will also be constant. In real life, however, it is much more likely that service conditions will be variable or cyclic. Generally, therefore, further approximations have to be made. 

Let us cite some application examples for some thermoplastics. These examples may be commercialized, in development, potential or related to very specific uses. Tlie designer must verify the possibility to use the cited thermoplastic family for their specific problem and test the right grade under real service life conditions. 

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