BS EN 843-3-2005 pdf download.Advanced technical ceramics — Mechanical properties of monolithic ceramics at room temperature — Part 3: Determination of subcritical crack growth parameters from constant stressing rate flexural strength tests.
In Equation (1). the value of n at room temperature is normally high, typically in the range fifteen to several hundred. At the lower end of this range, materials are very susceptible to subcritical crack growth. while at the upper end the phenomenon becomes insignificant. It should be recognised that Equation (1) implies a single simple relationship, but in practice there may be non4inearities. There are thought to be two pflnctpat causes of non-linearity:
a) At low stress intensity factors there may be no subcritical crack growth. This is termed the subcritical crack growth threshold, or “fatigue limit”.
b) At intermediate stress intensity factors, the crack growth rate may be limited by the rate at which the environment can penetrate along the crack to control fracture at the tip. This results in a plateau effect, which is maintained to K levels at which crack growth can occur in the absence of an environmental effect.
It should be noted that n and A0 are often functions of the environmental conditions employed. In particular, many ceramics show marked subcritical crack growth In humid air or in water, and much less marked effects in dry or inert conditions. The test environment shall be defined and controlled for reproducible results.
In this document, the parameter i’i and a parameter B, which is related to 4, are determined from the effect of stressing rate on flexural strength.
NOTE 4 The term “dynamic fatgue” Is frequently used to describe such tests, but lends to be misunderstood. Its use is discouraged.
Annex A shows how the mathematical formulation of the relationship between the subcritical crack growth parameters based on Equation (1) and the effect of stressing rate on strength is derived, yielding the basic formula:
to within 0.1 mm with respect to the outer support span. The spans and distances between load and support rollers shall be measured with a travelling microscope to the nearest 0.1 mm along the length of the test-piece using a travelling microscope or similar device (5,4.2).
The material from which the jig is constructed shall be compatible with the environment to be used for the tests, and shall not corrode in such a way as to impair its self-aligning and friction-free capability.
NOTE For tests in water, it is recommended that the test jig be constructed from stainless steel that does not suffer badly from crevice corrosion, e.g. 316 grade The rollers may conveniently be machined from hard ceramics such as alum4na, dense silicon nitride or silicon carbide.
5.2 Environmental control
If the tests are to be performed in any environment other than ambient air, an appropriate containment fachty shall be constructed to allow the test conditions to be controlled. For tests in water, a simple water tank shall suffice, For controlled humidity or gaseous environments, an environmental chamber and control system is required. For corrosive chemical solutions, appropriate safety precautions shall be adopted for handling and containment as required by local regulations.
5.3 Test machine
The test jig and environmental control facility shall be assembled In a suitable mechanical testing machine which is capable of applying a force to the loading roller (three-pomt flexure) or equally to the two loading rollers (four-point fiexure) in order to stress the test-piece. The machine shall be capable of applying the force at a range of constant rates covering at least four orders of magnitude within the range lOONs’andO,005Ns”.
NOTE 1 If the available machine does not offer pre-selected loading rates (load control), it is permissible to employ cross-head displacement rate control. The actual test piece stressing rate is determined from the tangent of the toc&time record at the time of failure.
The test machine shall be equipped for recording the peak force applied to the test-piece. The accuracy of the test machine shall be in accordance with EN ISO 7500-1. Grade 1 (accuracy better than I % of indicated load).
The test machine shall be subject to regular certified calibration in accordance with ISO 7500-1. Before undertaking tests, the validity of the calibration shall be checked. it is essential that the accuracy of the force rneasuflng device is maintained during loading to better than 1 % of the indicated load at all loading rates.
NOTE 2 Many older mechanical testing machines do not have an adequate response rate in load recording at fast loading rates, The minimum time to failure In such cases should be greater than 5 s.
5.4 LInear measuring devices
5.4.1 Micrometer, in accordance with ISO 3611. capable of recording to 0,01 mm and accurate to this level. BS EN 843-3-2005 pdf download.