BS EN 843-8:2010 pdf download

BS EN 843-8:2010 pdf download.Advanced technical ceramics – Mechanical properties of monolithic ceramics at room temperature Part 8: Guidelines for conducting proof tests.
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 destnbution 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 miurnum strength for the remainder.
b) Select a stress level which Es a factor of typically two or three times the expected stress level Wi 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 CEN!TS 14425-1) and the sub-critical crack growth characteristics (see EN 843-3). combined with Weibult 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 v.tiacti are not equivalent to the conditions employed for manutactunng, and testing test pieces of closely defined geometry, and thus may vary I density. microstructural homogeneity, surface finishing and residual stress levels. Predictions may be poor less the eqiavalence is good.
Of these three philosophies, a) and b) are pragmatic and can be set by simple iudgement. 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 fail 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, 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 i’i 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 m a proof- testing situation:
— where proof-testing cannot be performed ciickty 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 unknowii 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.
6 Design of proof-test equipment
The principal factors in the appropriate design of proof-testing equipment are:
— clear understanding of service conditions to be experienced by the item under test, and the lifetime to be expected:
definition of the stress distribution to be achieved in the item during testing;
definition of and agreement concerning the overload factor to be employed:
evaluation of methods of achieving the stress distribution in a non-destructive manner.
— design of a proof-testing system which provides the appropnate stress distribution without otherwise marldng or damaging the items under test.
Ideally, the Proof-testing system should incorporate the following features: