It seems that the decade of 2020 is going to be focused on clean air, whether it is on the street or inside of a building. Given what we know about how viruses spread and especially so about Covid-19, the importance of clean air has never been greater. Since the start of 2020 and going into 2021, we have seen a strong uptick in requests for CFD engineering services to improve air quality inside and outside of buildings. It seems obvious, but to ensure clean air inside the building one starts from the outside to keep exhaust out and clean air in. Our CFD consulting projects have ranged from hospitals, data centers, factories, large parking garages to office towers. In every case, the requirement was to digital prototype the air quality inside these buildings. We also get involved in combustion engineering to maximize the efficiency of boilers. This is often the most challenging work to optimize the combustion process to reduce air-borne particulate. All-in-all, we do live in a cleaner world than that chronicled by Dickens in the early 1800’s, where the streets of London were often night-like at noon, and I’m sure that in the following years we will see continued improvement in the air quality that we all breath.
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Even with an academic and experimental background in fatigue analysis, it is daunting to provide a hard, no-nonsense life-cycle prediction. It becomes especially daunting when your fatigue prediction can cost or save your client millions of dollars. Plus, there are tons of computer programs that promise “instant fatigue nirvana” at the press of a button; which leads one to ask: “What is a poor engineer supposed to do?” Over the years, we have learned that there are three critical components to a quality fatigue analysis: i.) accurate FEA stress results, ii.) accurate FEA stress results and iii.) accurate FEA stress results. Okay, sad, old, real-estate joke about location, location, and location; but let us just imagine that your stress numbers are good, then what? Fatigue analysis is all about the protection of structures and systems against failure from cyclic loading. This is where the ASME Boiler & Pressure Vessel Code (BPVC) provides a tried and true standard that, if your stress numbers are good, then you can be assured that your fatigue prediction will be conservative.
NASA 5020A Requirements for Threaded Fastening Systems in Spaceflight Hardware
Over the years, we have done a number of satellite analysis projects for commercial and those other government agencies. Looking back, I’m sort of surprised how close we got with what we thought were FEA best practices for linear dynamics (i.e., normal modes and PSD analysis). The big advance over the last couple of years has been in our approach to fastener modeling. In prior work, fasteners (bolts, screws, what-ever) were idealized using beams and rigid links (Nastran RBE2) while nowadays, our preference is to use six DOF springs (Nastran C-Bush) in combination with rigid links. While a bit messy, it provides an efficient methodology to meet the NASA 5020A technical specification.
The gist of this specification is how to calculate, whether or not, the fastener will fail given: bolt preload, with and without shear pins and joint slippage. It is a tall order and the specification is a algebraic joy to the mathematically inclined simulation engineer. Fastener failure is dominated by the designer’s choice of bolt preload. Interesting enough, the NASA specification favors low bolt preload. It sounds odd, but if pushed, one can avoid NASA 5020A fastener failure by lowering the bolt preload. The reason for this is due to the relationship between bolt preload and the applied tensile load. There is no free lunch and regardless of the initial bolt preload, the applied load adds to the overall bolt tensile load. The specification favors hand calculation but with some FEA modeling, one can improved upon the hand calculations and eke out a bit more headroom. If you would like to read more, the NASA 5020A specification can be downloaded here.