EN 843-8-2010 pdf download.Advanced technical ceramics – Mechanical properties of monolithic ceramics at room temperature – Part 8: Guidelines for conducting proof tests.
5 Main considerations
The short-term fracture stress of an advanced technical ceramic component is determined by the most highly stressed microstructural inhomogeneity or discontinuity, and is therefore determined by the method of manufacture and surface finishing. In general, it is not possible to predict with any certainty the forces that can be applied to a component without risking failure. For some applications where premature failure carries with it considerable costs, it can be beneficial to take steps to minimise the risks by removing from the population of items those individuals which are most at risk from failure.
Additionally, many types of advanced technical ceramic suffer from the slow growth of small cracks under maintained stress, with a consequent loss of the remaining strength. This thermally activated process may be accelerated by the presence of water, or by a corroding environment, which can react with the crystalline or amorphous bonding at the tip of crack. Thus if a component is held under stress for a prolonged period, it can weaken with time and lead to delayed failure. The tendency of a material to behave in this way can be detected, for example, by undertaking strength tests at different stressing rates (see EN 843-3) or by statically stressing the material until failure occurs. Generally, the effect is most marked in silicate glasses, and in glass- phase containing oxide ceramic materials. It is less marked in purely crystalline oxide ceramics, and least marked in non-oxide ceramics.
The principle of the proof-test (see Annex A) is to stress the item to such a level as will probe the item to determine the presence of features that would result in low strength The stress distribution should ideally match that seen in the application of the item, and should be applied smoothly and quickly, and then removed in a similar manner such that the strength of the surviving items is not reduced by non-catastrophic crack growth. There are several philosophies that can be adopted:
a) Select a stress level which pragmatically removes a certain fraction of the population, by a few percent. providing a guaranteed minimum strength for the remainder.
b) Select a stress level which is a factor of typically two or three times the expected stress level in service. providing a greater assurance that it will survive in service.
c) Numerically determine the over-stress level factor from the fracture mechanical behaviour of the material, specifically the critical stress intensity factor (see CENIIS 14425-1) and the sub-critical crack growth characteristics (see EN 843-3), combined with Weibull parameters (see EN 843-5) to provide stress- volume or stress-area predictions of the risk of failure. This method, while scientifically rigorous, is time- consuming and effective only if the fracture mechanical data that can be acquired are applicable to the item in every respect.
NOTE Components may be produced and finished In ways which are not equivalent to the oonditions employed for manufacturing, and testing test pieces of dosely defined geometry, and thus may vary in density. microstructural homogeneity, surface finishing and residual stress levels. Predictions may be poor unless the equivalence is good.
Of these three philosophies, a) and b) are pragmatic and can be set by simple judgement. They are typically used to ensure that each item, as supplied, has adequate strength at the point of delivery, but the procedures take no account of the potential of the material to age in service and to fad as a consequence of progressive loss of remaining strength with time. The third philosophy, C), additionally takes the slow loss of strength into account, and has been used successfully on safety-critical components under long-term stress.
The effectiveness of a proposed proof-testing method can be determined by evaluating the short-term strength distribution of proof-tested items compared with the strength distribution before proof testing. In the prior proof-tested batch, there should be an absence of items failing at less than the set proof-test level. The continued presence of items failing at less than the proof-test level is an indication that there is some weakening of items during the proof-test, which either has to be taken into account in selecting the proof-test loading level, or the proof-test schedule itself has to be examined to reduce or eliminate the effect.
Overload proof-testing will not be successful in guaranteeing a component in service in the following circumstances:
where the item becomes damaged in service, particularly where such damage is in regions of high stress;
— where the stresses in service are poorly defined or undefined, such as shock loading, or localised hard contact;
where temperature changes are significant:
— where the item has features that would suffer unduly in overload proof-testing, such as sharp edges, joints to other materials or surface coatings, or marking of items by the testing system:
— where the stress distribution under the service conditions cannot be conveniently modelled in a proof- testing situation;
where proof-testing cannot be performed quickly and smoothly, particularly the unloading part of the cycle;
where it may not be possible adequately to design an overload thermo-mechanical proof-test because of temperature limitations, oxidation, or unknown or undefinable heat transfer conditions.
The principal considerations are therefore the design of the system for undertaking the proof-test, and ensuring that it adequately matches the service stress condition during item testing.EN 843-8-2010 pdf download.