MIL-STD 1530D requires that the certification of an aircraft part employ analytical tools that are capable of modeling crack growth. It is further stated that the durability and damage tolerance (DADT) analyses should be based on linear elastic fracture mechanics (LEFM) and follow a building block approach. This paper illustrates the durability analysis required to certify an additively manufactured part by using the examples of durability tests performed on two wire arc additively manufactured (WAAM) 18Ni 250 Maraging steel specimens.

Chemically activated reactions are important in describing the composition of reactive gases including flames, planetary atmospheres, and the interstellar medium (ISM). In a chemically activated reaction, two reactants combine to populate a vibrationally excited well that can undergo unimolecular transformations (isomerization, dissociation) or be thermalized through collisions with the bath gas. Once a well has been thermalized, it may still have sufficient energy to undergo further unimolecular reaction, in a purely thermal process. If the timescale for the thermally activated process is sufficiently short, such that it approaches that of the chemically activated reaction, the two concurrent processes become inseparable and the value of the phenomenological rate coefficient is no longer obvious. Here, we introduce the thermal decay (TD) procedure to determine phenomenological rate coefficients for chemically activated reactions proceeding on timescales approaching those of thermal reaction, principally for use in stochastic master equation simulations of multiple‐well multiple‐channel unimolecular reaction processes. By fitting the thermal decay of the initially activated well to a first‐order kinetic model, the would‐be thermal yield can be eliminated so as to arrive at the chemically activated component in a reliable and objective fashion. This technique is demonstrated here for the reaction of 1,3,6‐heptatriyne with H using the MultiWell code and a 16‐well 33‐channel C7H5 reaction model. A computer program implementing the TD method and for postprocessing of MultiWell output data, PPM, is provided.

The durability assessment of additively manufactured parts needs to account for both surface-breaking material discontinuities and surface-breaking porosity and how these material discontinuities interact with parts that have been left in the as-built state. Furthermore, to be consistent with the airworthiness standards associated with the certification of metallic parts on military aircraft the durability analysis must be able to predict crack growth, as distinct from using a crack growth analysis in which parameters are adjusted so as to match measured data. To partially address this, the authors recently showed how the durability of wire arc additively manufactured (WAAM) 18Ni-250 maraging steel specimens, where failure was due to the interaction of small surface-breaking cracks with surface roughness, could be predicted using the Hartman–Schijve variant of the NASGRO crack growth equation. This paper illustrates how the same equation, with the same material parameters, can be used to predict the durability of a specimen where failure is due to surface-breaking porosity.

Zirconium diboride (ZrB2) was formed into dense complex shapes using freeze casting as a near‐net‐shaping technique. Aqueous‐based formulations were compared with nonaqueous (cyclohexane) based formulations in terms of rheological behavior, particle packing in the green body, sintered density, macroscale porosity, and cracking. The influence of particle solids concentration and freezing rate was investigated. The aqueous formulations were found to be deficient in that they produced macroscale porosity that could not be eliminated during sintering resulting in low density and large pores in the final shaped objects. The nonaqueous‐based system was able to produce complex shaped objects with significantly reduced macroscale porosity. The higher concentration of solids in the nonaqueous‐based formulations was primarily responsible for the reduced macroscale porosity and enabled higher sintered densities (up to 90%‐91.5% of theoretical density for fast freezing). The microstructure of the ZrB2 formed at fast freezing rates and high solids content typically had isolated pores in the order of 5‐10 μm in size, mainly found along grain boundaries (grain sizes between 20 and 50 μm). Although this rapid freezing produced denser components, it tended to produce objects with internal cracks. When slower freezing rates were used, intricate complex shaped objects could be produced without cracks but their density was only between 65% and 80% of theoretical density.