Cryogenic Model Materials

An overview and status of current activities seeking alternatives to 200 grade 18Ni Steel CVM alloy for cryogenic wind tunnel models is presented. Specific improvements in material selection have been researched including availability, strength, fracture toughness and potential for use in transonic wind tunnel testing. Potential benefits from utilizing damage tolerant life-prediction methods, recently developed fatigue crack growth codes and upgraded NDE methods are also investigated. Two candidate alloys are identified and accepted for cryogenic/transonic wind tunnel models and hardware. INTRODUCTION Early in the development of the National Transonic Facility (NTF) at Langley Research Center (LaRC), much research was performed to identify and characterize materials suitable for building wind tunnel models and support hardware. These activities have not continued or pursued since the early 1980’s. A motivation for these activities has resurfaced in recent years due to the increasing difficulty in procuring 200 grade maraging steel. Due to current trends in the industry, no depots or other vendors maintain stock of this material. This has impacted the costs and development cycle times for almost all NTF research hardware projects and forced a mass buy strategy that is limiting research opportunities. Additionally, NTF models need to simulate aircraft performance under more severe aerodynamic conditions than in previous decades. Increases in dynamic pressure (Q) and unsteady loading require new materials and techniques to provide increased model performance while eliminating structural failure at the higher static and dynamic loads. Furthermore, models are tested at cryogenic (-275F) temperature provide high Reynolds number flows. Therefore, model alloys must have desirable a number of other desirable characteristics at cryogenic conditions. Under high static loads two failure modes exist; yielding and fracture. Model alloys need high tensile strength and fracture toughness to prevent yielding and fracture failures, respectively. Prevention of structural failure under cyclic or fatigue loading is more difficult. Initially, a structure may be known to contain no cracks large enough for failure to occur under anticipated loads. However, under fatigue loading small cracks may grow to a critical size where fracture occurs. To prevent fatigue crack failures a damage tolerant life-prediction method has been developed. Here, fatigue life is assumed equivalent to fatigue crack growth (FCG) from an initial defect to a critical crack size. Life prediction is a function of final and initial crack sizes, anticipated loads, and FCG behavior. Damage tolerance is superior to existing fatigue prevention schemes, in terms of safety and cost of replacement parts, and will be evaluated to ensure safe operation of NTF models. Because reliable detection of small cracks is important, nondestructive evaluation (NDE) techniques will also be studied.