The Slope Determination by the Analysis of Residuals (SDAR) algorithm has recently been standardized in ASTM E3076-18 for the determination of the slope in the linear region of a test record. It offers an automated and objective linear-fitting method, based on the least normalized residual and (in its full form) provides metrics for evaluating the quality of the test record and of the resultant fit. In this investigation, SDAR was used to calculate values of crack size in elastic-plastic fracture toughness tests, conducted in accordance with ASTM E1820-18 by means of the single- specimen elastic compliance methodology. Our analyses showed that the use of SDAR is not advisable in case of noisy unloading/reloading cycles and/or in the presence of significant hysteresis, since the most linear portion identified by the algorithm might not necessarily represent the true elastic behavior of the specimen. Such issues could be alleviated by increasing the minimum number of data points used from 20 % of the search range (E3076) to at least 50 %. Nevertheless, SDAR remains an extremely useful tool to determine a repeatable and operator- independent slope in numerous types of mechanical tests, such as tensile tests, linear elastic fracture mechanics tests, and others.
Technical Note (NIST TN) - 2050
Analysis of residuals, ASTM E1820, ASTM E3076, elastic compliance, linear fitting, mechanical tests, SDAR, slope determination