EN 1006-2009 pdf download.Advanced technical ceramics – Monolithic ceramics – Guidance on the selection of test pieces for the evaluation of properties.
3.3 Test method accuracy
3.3.1 Most test methods specifically developed for advanced technical ceramics have associated with them a possible uncertainty of result determined by the accuracy of individual contributions from each parameter involved in the measurement, The potential uncertainties arising shall be taken into account when examining the consistency of a parameter within a batch of units or between batches, or examining whether it meets a given specification level.
3.3.2 It should be noted that in cases where the scatter of results of a test is similar to or less than that attributable to the accuracy of test method, the test is clearly unable to distinguish between individual test-pieces or samples. An improvement 01 the accuracy level of the chosen test or an alternative test method should be sought.
3.3.3 Certain tests for advanced technical ceramics produce a wide scatter of results as a consequence, for example, of the influence of occasional flaws or other defects. e.g. a strength test or dielectric breakdown test. The results from such tests shall be treated statistically (see e.g. EN 843-5 for strength tests), and the confidence level of the mean result or other parameters should be calculated such that the degree of discrimination between results from different batches, or between a set of results and a specification value, is clearly understood.
3.4 Sampling schemes for individual manufactured items
3.4.1 Sampling schemes are conventionally divided Into those for inspection by attributes. e.g. ISO
2859-1, or by variables. e.g. ISO 3951 (all parts).
NOTE See the Bibliography for these and other ISO standards on statistics.
The selection of an appropriate scheme should be subject to agreement between parties.
3.4.2 Inspection by attributes consists of examining a sampled unit and deciding whether or not it achieves an appropriate criterion. A decision on the fitness for purpose of a batch is by counting the number of non-achieving units in the batch sample. An example might be the presence of cracks (see EN 623-1) or surface blisters in an as-fired ceramic component. Sampling plans for inspection by attributes are given in ISO 2859-1.
3.4.3 Inspection by variables involves the measurement of a property or properties using a recognised test method producing numerical values for each unit in the sample. These values are used in conjunction with the sampling plan to decide on the fitness for purpose of a batch compared with a pre-set criterion. Typical properties measured in this type of inspection are density (see EN 623-2) and flexural strength (see EN 843-1). Schemes for sampling by variables are given in ISO
3951 (all parts).
NOTE ISO 5022 contains methods of sampling for shaped refractory products which can have some relevance to some types of advanced technical ceramic components.
3.4.4 Some tests involving determination of properties may be used to inspect by attributes by placing an upper or lower acceptance limit on the design value, e.g. a lower limit on density in any unit. The permitted deviation from the design value, based on known uncertainties In test result from manufacturing, selection and testin should also be defined. e.g. if the known uncertainty in measurement of density is ± 20 kg m at the 90 % confidence level, a unit would be deemed to have a satisfactory attribute if the measured density is not more than 20 kg m below the selected design value.
3.4.5 In either type of inspection, it should be emphasised that the samples chosen have a random chance of selection, i.e. not selecting the most conveniently accessible units, and that the data provided also indude elements of test method uncertainty (see ISO 5725 (all parts)). The latter is assumed to be reduced to a minimum by the standardisation of methods, but nevertheless is still inevitably present to varying degrees in most tests for advanced technical ceramics. Equally, it should be noted that sampling schemes do not guarantee that the test data are truly indicative of the quality of the sampled batch. They indicate only a probability, and thus contain an element of risk that a sample apparently meeting a given criterion may have been selected from a batch which overall does not. or that a sample apparently not meeting a given criterion may have been selected from a batch which overall does so. The important point is that the level of risk is calculated and known. The degree of risk may be different for supplier and customer but decreases with increasing severity of inspection, either by increased sample size or by testing more than one attribute and/or property.
3.4.6 Sampling for production consistency should be made at a time during production when it is known Iron, process indications that the product is likely to have stable attributes. This deariy does not apply to small batch supplies of ceramic components.
3.5 SamplIng attributes of physically large units or blocks of material
3.5.1 In some circumstances. test-pieces will need to be cut from large units or from supplied blanks or blocks of material, Consideration should be given not only to the position from which the test-pieces can be cut, but also the means for doing this, and the relationship the Cut test-pieces have to the unit or block as a whole.
3.5.2 Many ceramic materials when supplied in an as-fired and unmachined condition possess a surface skin, which may be of different composition or have other different attributes from the bulk material exposed by cutting. Recognition shall be given to this factor, since It can influence the result of a test in various ways. Some examples of factors to consider include:
a) the skin may be impermeable, but the bulk not;
b) the process of cutting may change the mechanical condition of the test material, and relieve undetectable internal stresses;
c) the material may be structurally an isotropic, but test-pieces may be prepared only with orientations which are not relevant to the important attributes of the unit as a whole.EN 1006-2009 pdf download.