Rollie E. Dutton

Verification and validation of ICME methods and models for aerospace applications

Integrated Computational Materials Engineering (ICME) model verification and validation (V&V) is difficult because materials processing, microstructural evolution, and property development contain a rich mix of length and time scales with an equally complex set of interacting phenomena and mechanisms. Beyond these difficulties, engineers who adapt these models rarely generate independent validation data sets to confirm model adequacy, quantify uncertainty, and identify potential error sources. Even when a validation data set is produced and applied, the range of model applicability is limited by the range on input model parameters contained within the data set. In this paper we provide a summary of a recommended approach to ICME V&V and include descriptions of V&V planning checklists, an ICME Tool Maturity Level assessment guide and examples of how such practitioner aids might be employed.

Fabrication of dense thin sheets of γ-TiAl by hot isostatic pressing of tape-cast monotapes

Abstract A method is described for the fabrication of dense thin sheets of γ titanium aluminide (γ-TiAl) by a powder metallurgy route involving hot isostatic pressing (HIP) of tape-cast monotapes. Gamma-TiAl powder (particle size

Modeling of hot isostatic pressing and hot triaxial compaction of Ti-6Al-4V powder

Abstract The accuracy of a hybrid continuum–micromechanical model for predicting the rate-dependent consolidation of metal powder was established using measurements from hot isostatic pressing (HIP) and hot triaxial compaction (HTC) experiments. The experiments were performed with PREP® Ti–6Al–4V powder encapsulated in thin-walled containers and yielded relative density vs time and height vs relative density data for direct comparisons with model predictions. The model was found to underpredict the densification rate in HIP, primarily during the early stages of consolidation, and the degree of consolidation during HTC. Adjustments to the values of the strain-rate sensitivity or a state variable representing the geometric structure of powder compacts resulted in good agreement between hybrid model predictions and measurements. The physical basis for discrepancies between predictions and measurements and justification for the postulated/required parameter adjustments were attributed to two factors: (1) differences in microstructure between the unconsolidated powder and the fully consolidated material used in the compression testing to obtain the material coefficients for the model; and (2) interparticle bonding characteristics/particle sliding effects not taken into account in determining the material properties for the model.

Effect of Solid Solution Additives on the Densification and Creep of Granular Ceramics

Abstract The effect of solid solution additives on the densification and creep of granular ceramics was investigated for a model system consisting of CeO2 solid solutions with Y2O3 as the additive. In the sintering of powder compacts at 1150°C, the densification rate of CeO2 at a given density decreased significantly with increasing Y3+ concentration. The reduction in the densification rate reached a factor of ≈ 100 for a Y3+ concentration of 6 at.%. Creep of dense specimens was investigated at constant strain rates of 10−5 and 10−4 s−1 in air at 1200°C. After compensation for differences in grain size, the creep rate was also found to decrease significantly with increasing Y3+ concentration. If the creep rate is assumed to be controlled by a mechanism of grain boundary diffusion, then the magnitude of the decrease is in good agreement with that observed in the sintering experiments. The results strongly indicate that it may be possible to predict changes in the steady-state creep behavior from observed changes in the sintering behavior provided that matter transport occurs by the same mechanism. They also indicate that the solid solution approach may have considerable merit for controlling the creep resistance of rare earth oxides that commonly have a high solubility for many cations.

Creep-sintering of polycrystalline ceramic particulate composites

The sintering of particulate composites consisting of a polycrystalline zinc oxide matrix with 10 vol % zirconia inclusions of two different sizes (3 and 14 μm) was investigated at a constant heating rate of 4 °C min−1 under an applied stress of ≈ 300 kPa. The presence of the inclusions produced a decrease in both the creep rate and the densification rate but the ratio of the densification to creep rate remained constant during the experiment. The ratio of the densification rate to creep rate for the composites was ≈ 1.5 times greater than that of the unreinforced matrix regardless of inclusion size. The creep viscosity of the composites was higher than that of the unreinforced matrix and increased slightly with decreasing inclusion size.

Probabilistic Materials Engineering and Certification

The high cost of certification as well as the extended length of time required to certify a new weapons system is limiting the Air Force’s ability to develop new systems. The extended times between major DOD procurements, the small quantities purchased, and the low production rates of these systems make it difficult to affordably achieve high reliability and performance through traditional R&D-based materials engineering. In addition, the time required for new material development and validation often extends beyond the decision point for incorporation of such materials in new weapons systems. The timeliness and cost-effectiveness of materials engineering processes for new designs and design changes can be radically improved by replacing today’s experimentally-based materials qualification process with modeling and probabilistically-based materials engineering.