Thermal ageing test design

Experience has also shown that in cases such as stress rupture and thermal ageing the test parameters may have to be designed progressively. Shorter tests at higher loads (or temperatures) are set up first and the times to failure measured. The test conditions for longer lifetimes are then set on the basis of these results. This is particularly important where the validity of the final result depends on obtaining a failure within a particular time interval, e.g., over 5,000 h as in IEC 60216 [2], or where the measurements must be completed within a set time. 

A systematic investigation of intrinsic chemical stability was carried out on devices specially designed for high power applications [7]. For this purpose, accelerated ageing test procedures were developed for nc-DSCs and it turned out that, to first order, a separation can be made between the effects of visible light soaking, UV illumination and thermal treatment on long term stability. 

Note that the unprotected circuits of the control had substantial drift of the three that did not fail during the test run. Failures were evident upon inspection of the part as occurring from disintegrated resistors. Dip coated PSS and HMDS had fewer failures and less drift. The best protection from moisture was offered by the plasma-polymerized HMDS. The indicated 1.7% drift is close to the acceptable limits of the design for power-on thermal aging effects. 

Aging at elevated temperatures typically involves exposing test specimens or products at different temperatures for different extended times (see chapters 3-6). Tests are performed at room or die respective testing temperatures for whatever mechanical, physical, or electrical property is of interest. These tests of aging can be used as a measure of thermal stability in design as is done with other materials. 

Early material testing to characterize structural property degradation due to environmental effects such as irradiation, thermal aging, gas corrosion, fission product release, and material sublimation in vacuum should be pursued to reduce uncertainty in concept designs. 

Oilfields in the North Sea provide some of the harshest environments for polymers, coupled with a requirement for reliability. Many environmental tests have therefore been performed to demonstrate the fitness-for-purpose of the materials and the products before they are put into service. Of recent examples [33-35], a complete test rig has been set up to test 250-300 mm diameter pipes, made of steel with a polypropylene jacket for thermal insulation and corrosion protection, with a design temperature of 140 °C, internal pressures of up to 50 MPa (500 bar) and a water depth of 350 m (external pressure 3.5 MPa or 35 bar). In the test rig the oil filled pipes are maintained at 140 °C in constantly renewed sea water at a pressure of 30 bar. Tests last for 3 years and after 2 years there have been no significant changes in melt flow index or mechanical properties. A separate programme was established for the selection of materials for the internal sheath of pipelines, whose purpose is to contain the oil and protect the main steel armour windings. Environmental ageing was performed first (immersion in oil, sea water and acid) and followed by mechanical tests as well as specialised tests (rapid gas decompression, methane permeability) related to the application. Creep was measured separately. 

The influence of simultaneous thermal and chemical cycling on commercial three-way catalysts has been examined after ageing in a specifically designed automated laboratory bench. For all cycles tested, reproducing repeated fiiel cutoff procedures between two temperatures (850°C-850°C, cycle 1 850°C-950°C, cycle 2 850°C-1050°C, cycle 3), X-ray diffraction evidences the formation of platinum/rhodium alloys only when the atmosphere cycle comprises a reducing step. Evaluation of the rhodium concentration in alloyed phases suggests that some rhodium remains unalloyed in catalysts. 

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