Real and Simulated Service Conditions

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. 

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. 

The laboratory equipment designed to test the aforementioned tire properties is based mostly on a steel flywheel with the appropriate monitoring devices such as transducers and infrared temperature monitors. Data are collected directly into computers for real-time analysis and downloading to the tire engineer s work station. Many of these flywheels are also computer controlled so as to simulate service conditions. For example, aircraft tires can undergo a complete cycle of taxi, takeoff, and landing. 

It is worth noting that in service, a component is often subject to multiaxial loads, such as axial and radial stresses, at the same time, and multiple fatigue modes, such as tension, torsion, and bending cyclic stresses, simultaneously. The most reliable life perdition of a component is based on a direct component test with a real-life simulated loading condition. 

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. 

All these tests are carried out under relatively slow conditions (50 mm/ minute) and do not represent normal service conditions. There are no standard tests to simulate these real life conditions. Equipment is available to perform these dynamic tests. 

A relatively new analytical technique, chemiluminescence (CL), is an ultrasensitive technique, and it has been reported that reaction rates as low as 10 mole/year can be measured (1-5). Thus, it could monitor the aging reactions on a real-time basis while the resins are exposed to a simulated service environment. If the method can be shown to be sufficiently sensitive and reliable, the errors inherent in extrapolating accelerated aging data to the actual conditions encountered can be eliminated (6-8). 

Few would argue against real service trials being the first choice, although in practice the conditions and timescale required frequently rule it out completely. Simulated service trials using test rigs are more widely feasible and are often considered essential for critical components. 

Real weathering loads on contemporary vehicles cannot be simulated in an accelerated test. Depending on the task, a large number of very different tests are applied. The most important test is artificial weathering and simulated solar irradiation, s. Section 2.2.4.2. Investigations for BMW, for example, showed that 240 hours of sunlight simulation are sufficient to obtain reliable predictions for component lifetime. However, it is decisive that the component test be performed under actual service conditions for the vehicle [203]. 

However, the European normalization committee is in the process of looking at the issue and is working on the EN-13648-1 Cryogenic vessels -Safety devices for protection against excessive pressure - Part 1 Safety valves for cryogenic service. The tendency is to go to a much more reliable test as described hereafter which will simulate the real process conditions to a greater degree. 

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