Casey Holycross

Forced-Response Verification of the Inherent Damping in Additive Manufactured Specimens

The laser powder bed fusion AM process has been used to manufacture beams with unique internal geometries that are capable of increasing inherent damping in a part. The concept of the internal design is to have densely packed, unfused powder pockets that dissipate energy via particle interaction. Four Inconel (IN) 718 beams have been tested and all demonstrated the capability to suppress vibration 10X more effectively than a fully fused beam. The mechanism presumed to dissipate energy and thus suppress vibration is the sliding of unfused particles. This mechanism has been associated with a crack opening under Mode II fracture. Based on this assumption, a proportional expression has been developed as a criterion for optimizing unfused powder locations for vibration suppression effectiveness and was validated with 3.175 mm thick beams. This study investigates five uniquely designed IN-718 beams created via the optimizing criterion to assess accuracy of the expression. The intent of this study is to investigate the predictability of the unfused pocket optimization criterion. The results of this study will lead to a more robust design criterion for more complex 3D structures with improved damping capability.

Improved Hybrid Specimen for Vibration Bending Fatigue

A hybrid specimen was developed to minimize material used when assessing bending fatigue behavior with a vibration-based experimental procedure. Motivation for this work comes from the eagerness to quantify critical three-dimensionally printed gas turbine engine components at low cost under representative operating and stress conditions. The original vibration-based fatigue specimen (a whole, square plate) is capable of providing airfoil representative data at a lower cost than failing a fully manufactured airfoil. The reduced cost, though significant, was further reduced with the hybrid specimen. The most recently published iteration of the hybrid specimen produced an insert-plate system that required 95 % less material than the original specimen in order to gather fatigue data, and the hybrid specimen data compared favorably within a 95 % prediction interval of the original for Aluminum 6061-T6. Despite the successful comparison, improvements to the hybrid plate specimen were still necessary for accurately assessing fatigue behavior of more commonly used aerospace alloys. Specifically, improving repeatability in the experimental response, increasing the hybrid specimen excitability (i.e. reduce specimen system damping), and minimizing damage accumulation on the carrier plate of the hybrid specimen were critical to the efficiency of the characterized bending fatigue behavior. The proposed investigation addressed these issues for Titanium 6Al-4V, resulting in a more repeatable experimental procedure and results that agree with whole plate data.

Fatigue and Strength Studies of Titanium 6Al-4V Fabricated by Direct Metal Laser Sintering

Vibratory bending fatigue life behavior of Titanium (Ti) 6Al-4V plate specimens has been assessed. The plates were manufactured via direct metal laser sintering (DMLS), which is a powder bed, laser deposition additive manufacturing process. Motivation for this work is based on unprecedented performance demands for sixth generation gas turbine engine technology. For example, the inclusion of a third stream flow for improving engine performance may add complexity and weight that could offset anticipated thrust and fuel efficiency gains. Therefore, complex, lightweight components with improved functionalities are desired. Novel component design concepts have been cost, schedule, and feasibility limited when using conventional manufacturing methods. Additive manufacturing, however, can extend the thresholds of component concepts. Though additive manufacturing can be a promising addition to the turbine engine community, the manufacturing process controls required to achieve consistency in material properties have not been fully identified. The work presented in this manuscript investigates variability in vibration-based bending fatigue life of DMLS Ti 6Al-4V compared to cold-rolled Ti 6Al-4V. Results show discrepancies between the fatigue life variation of DMLS and cold-rolled data. Along with the support of fusion and post-fusion process parameters, the fatigue results are also supported by tensile property characterization, fractography, and microscopy.Copyright