Reduced Sample Minimum Strength Estimates for Novel Material Systems: Effects on Confidence and Structural Risk
Ollikainen, Andrew Carl
MetadataShow full item record
There are ongoing efforts to standardize the materials, methods and process controls used in additive manufacturing. To support the broader use of additive manufacturing (AM) in the aerospace industry, the statistics working group of the Composite Materials Handbook (CMH-17) is evaluating whether its current methods of calculating minimum strength values are suitable for these new materials. The work presented in this dissertation investigates methods for determining minimum strength estimates in a way that will benefit application of AM materials in engineering practice. The greatest challenge in conducting such an assessment is accounting for the many variables in material, processing, and testing that could affect the basis values but are not yet fully defined. Therefore, to show that the current methods are likely to be acceptable for AM, this study demonstrates their suitability more generally. Three studies are presented. The first study leverages Monte-Carlo simulation to exercise the CMH-17 reduced sample estimates for the lower one-sided 90% reliability at 95% confidence limit on strength. The model evaluates demonstrated confidence for datasets drawn over a broad range of within-batch and between-batch variability. From this, analytical best practices and software settings are recommended that demonstrate the required 95% confidence. Next, the efficiency of the current methods is shown to perform favorably when compared to several alternate approaches. Finally, the performance of the current methods is included in a tailorable risk-based factor of uncertainty to meet standard levels of structural reliability. With these recommendations the current reduced sample method is shown to be effective not only for the levels of variation that might be expected for additively manufactured materials but for many other material performance data that are nearly symmetric or right-skewed—behavior that is typical of weakest-link type failure modes. For many years the CMH-17 methods have been used for flight-certified plastic and composite components. Validating these methods to new materials is an important step to achieving certifiable parts.