Consistent Structural Integrity in Preliminary Design Using Experimentally Validated Analysis

Abstract

Coupling analytical methods to experimental results forms the basis of consistent structural integrity by analysis. By establishing repeatable statistical variance from building block test data for unique failure modes, it is possible to identify correlation factors (CFs) that account not only for analysis inaccuracy, but also observed scatter in test results. Industry accepted failure analysis predictions then can be used to design more robustly and to avoid unanticipated design flaws discovered in final design, or worse yet lead to part failure. The CFs can be used to adjust the individual margins-of-safety to produce more consistent structural integrity in the design and dependability in weight predictions of an aerospace vehicle. Such a capability is most useful during preliminary design where 80% of the design decisions carry forward, including the uninformed ones that bring with them undesirable difficulties of meeting weight goals, passing structural testing on the first try, and costly certification. The presented approach has been implemented in the HyperSizer® automated design tool that results in significant design cycle time reduction with the ability to analyze orders of magnitude more design configurations. Substantial risk reduction in final design is achieved from the integration and use of correlated, higher fidelity tools earlier in the design process. Presented are summary results from a recent Long Range Strike Aircraft preliminary design that compares the traditional, zero-margin for all failure modes approach, vs. the presented approach that achieves the same % reliability for all potential failure modes. Included are identified areas of the vehicle sized the traditional zero-margin method that results in an unexpected and unacceptable low reliability even though it is 9% heavier then reliability based sizing.

Spherical Cryogenic Hydrogen Tank Preliminary Design Trade Studies

“…This is due to the fact that curved panel buckling is highly dependent on slight variations in thickness and flaws that occur randomly in structures. The necessary buckling knockdown factor is also a function of thickness, as the small variations and flaws become of greater importance as the structure becomes thinner 8,9 . The buckling lengths characterize the controlling buckling mode shapes for the curved panel and are a function of the thickness and radius of curvature of the panel.…”

“…Furthermore, HyperSizer adds to the methods of NASA SP8007 by including the effects of the D11, D22, and D33 bending stiffness terms and reliability analysis, see ref. 9. These factors are different for the stiffened and unstiffened configurations because they have different effective thicknesses.…”