Traceability for measurements carried out on incremental step loading equipment

The incremental step loading technique has been widely used for material hydrogen embrittlement characterization. However, due to numerous error sources that can affect the measurement results and to the current lack of standardization, users are facing difficulties in the evaluation of measurement uncertainty as well as in the metrological performance evaluation of incremental step loading equipment. The paper deals with evaluation of the metrological performance of equipment used to characterize the susceptibility of materials to hydrogen embrittlement through use of incremental step loading technique. Additionally, a procedure is developed to estimate the measurement uncertainty associated with the results obtained during the test. Two equipment configurations were verified. The first one, with a load cell of 20 kN, was used for test specimens with nominal dimensions of 30 mm, 30 mm and 200 mm in width, thickness and length, respectively; and the second, with a 1 kN load cell was used for test specimens with nominal dimensions of 10 mm x 10 mm x 60 mm. Three measurands were evaluated in each assemblage (force applied on the specimen, equipment arm positioning angle and stress). Several metrological parameters were estimated (bias, accuracy, expanded uncertainty, repeatability, maximum error, hysteresis and nonlinearity). The effectiveness of the proposed procedure for uncertainty assessment was tested by calculation of the uncertainty associated with lower value for fast fracture strength. From the obtained results, it was concluded that the two equipment configurations had excellent metrological properties, in both the loading and unloading phases. It was shown that the proposed procedure can properly estimate the uncertainty associated with measurement on incremental step loading testing. Considering the widely use of the incremental step loading technique, the results here presented can be particularly useful for ensuring the traceability of results to the International System of Units required by the ISO 17025 standard.