Course Sampler Design and Analysis of Bolted Joints DABJ—Design and Analysis of Bolted Joints for Aerospace Engineers 3-day Course—Developed and Taught by Tom Sarafin Objectives: • Help you understand how to design bolted joints that – can withstand mission environments and function as required – are relatively inexpensive and easy to assemble – are trouble-free • Help you understand the mechanics of a preloaded joint and how they relate to failure. • Share methods of analysis and help you understand their applications and limitations. • Help you understand and learn to use NASA-STD-5020A for threaded fastening systems, and provide insight into its development. • Provide a valuable reference and a trail to data sources. Target audience: Structural and mechanical engineers (design and analysis), responsible/cognizant engineers, and others interested in the topic June 2019 Copyright Instar Engineering and Consulting, Inc.• instarengineering.com Materials may be reproduced in complete form only, with header and footer Course Sampler Design and Analysis of Bolted Joints DABJ History First version developed in 1998 at the request of NASA Goddard – Titled “Design and Analysis of Fastened Joints” (DAFJ); 8-hour course, taught twice in 1998 Expanded into a 2-day course in 1999—Taught 21 times in this format; course materials revised (improved) after nearly each class Revised into a 3-day course in 2005 at NASA JSC request to include a section on compliance with NSTS 08307—Renamed “Design and Analysis of Bolted Joints” (DABJ) – This exercise included several meetings with JSC experts; all concluded that NSTS 08307 should be revised or replaced Helped fuel the NASA-STD-5020 project (new standard for threaded fastening systems), which started in 2007. Tom Sarafin served as one of the core team members for this project. – Taught 19 times in this format, with periodic revision to capture additional information Major revision in June 2010 to include a section on analysis criteria per the draft NASA-STD- 5020, which was being developed at that time—Taught 10 times between June 2010 and March 2012 Revised in April 2012 to be consistent with the final version of NASA-STD-5020, which was released in March 2012—Taught 40 times, with gradual improvement to the charts over time Revised in March 2019 to address NASA-STD-5020A (Rev A, released in September 2018)— Taught 3 times through June 2019 Through May 2019, Tom Sarafin has taught this course, in its evolving versions, a total of 95 times to more than 1800 engineers June 2019 Copyright Instar Engineering and Consulting, Inc.• instarengineering.com Materials may be reproduced in complete form only, with header and footer Course Sampler Design and Analysis of Bolted Joints Topics by Section Number Introduction 1. Overview 2. Screw Threads: Evolution and Important Characteristics 3. Developing a Concept for the Joint 4. Calculating Bolt Loads when Ignoring Preload 5. Failure Modes and Assessment Methods 6. Thread Stripping and Pull-out Strength 7. Selecting Hardware and Detailing the Design 8. Mechanics of a Preloaded Joint Under Applied Tension 9. Fastening System Analysis per NASA-STD-5020A 10. Special Topics Summary June 2019 Copyright Instar Engineering and Consulting, Inc.• instarengineering.com Materials may be reproduced in complete form only, with header and footer Course Sampler Design and Analysis of Bolted Joints Representative Fastener Issues on the Space Shuttle and International Space Station Programs 1996 Space Shuttle mission (STS-80): A small screw with no locking feature backed out and jammed a gear in external airlock, preventing the astronauts from opening the hatch and performing the Extra-Vehicular Activity (EVA) part of the mission. In a 2006 EVA, while astronauts tried to activate the Solar Alpha Rotary Joint on the International Space Station, a bolt in the launch restraint seized and required extremely difficult removal, injuring a crew member. Multi-Purpose Logistics Module (MPLM): Hundreds of fasteners too short to fully engage threads and engage the locking feature, not detected during installation. Running torque not verified for any of the fasteners used to assemble the MPLM. Space Shuttle Ku Band Antenna: After several missions, 2 of 4 main attachment bolts were discovered in 2006 to be too short to engage locking features or provide adequate strength; required risky repair on launch pad. During a 2006 Shuttle mission, an EVA camera came loose and was lost because the mounting screws backed out. Clearly the space industry needs to improve how threaded fasteners are used, controlled, and assessed! June 2019 Copyright Instar Engineering and Consulting, Inc.• instarengineering.com Materials may be reproduced in complete form only, with header and footer 1-4 Course Sampler Design and Analysis of Bolted Joints Designing a Bolted Joint Where discussed Sec. 1 1. Identify functional requirements and constraints for the structure being designed. Sec. 3 2. Develop a concept. – Structural configuration and form of construction – Method of attachment: welding, bonding, or fastening – Concept for the joint: configuration, types of fasteners, access for assembly Sec. 1 3. Quantify requirements and identify design considerations for the joint. – Life-cycle environments, design loads, loading cycles, temperatures – Stiffness, allowable permanent deformation, design criteria – Cost, lead time, ease of assembly, schedule Secs. 4. Size the joint. 3 – 6, – Select bolt pattern 10 Goal: a joint that ... – Calculate bolt loads; size bolts – Identify potential failure modes in the – functions as needed fittings throughout its intended (region of joined members near bolts) life cycle Sec. 7 and test-substantiated methods of assessment – uses affordable and – Size fittings available hardware Sec. 7 Iterate as needed 5. Select hardware and design details. – is easy to assemble and – Specific bolts, nuts, washers, pins Secs. – Edge distance, wrench clearance, hole disassemble 8 & 9 June 2019 Copyrightsize Instar Engineering and Consulting, Inc.• instarengineering.com Materials may be reproduced in complete form only, with header and footer 1-5 6. Specify assembly requirements. Course Sampler Design and Analysis of Bolted Joints Most Bolted Joints Don’t Work Without Preload! Tightening the nut or bolt creates a tensile load in the bolt and an equal clamp load between fittings. It’s the clamp load that’s important. A high preload ... – minimizes cyclic loading in the fastener; increases fatigue life – increases a joint’s stiffness – keeps shear joints from slipping back and forth within clearance holes; prevents fretting (corrosion resulting from breakdown of protective oxides on surfaces) – helps maintain alignment – helps lock the fastener But too high of a preload … – may cause bolts to fail during installation (combined effects of tension and torsion) – may cause excessive yielding during installation, using up much of the bolt material’s elongation and leading to greater risk of rupture under applied load – may crush a clamped brittle material Two design challenges: addressed 1. Establishing the right preload at assembly in Sec. 7 2. Maintaining preload during service June 2019 Copyright Instar Engineering and Consulting, Inc.• instarengineering.com Materials may be reproduced in complete form only, with header and footer 1-10 Course Sampler Design and Analysis of Bolted Joints Thread Cutting Vs. Thread Rolling Thread rolling Thread cutting Thread rolling Image credit: Horstengineering.com Image credit: Fastenal.com Material grain comparison Most procured fasteners have rolled threads because, although the tooling is more expensive, the recurring cost is lower. The dies used for thread rolling have a radius, and rolled UN threads typically meet Cut Rolled after heat the requirements for UNR (Ref. treatment (images from Horstengineering.com) Fastenal.com). June 2019 Copyright Instar Engineering and Consulting, Inc.• instarengineering.com Materials may be reproduced in complete form only, with header and footer 2-7 Course Sampler Design and Analysis of Bolted Joints Goal: A Joint with High, Linear Stiffness but Ductile Failure High stiffness: keeps the structure’s natural Ductile failure: signifi- frequencies higher, which usually helps avoid cant plastic deformation high dynamic loads, and makes structural prior to rupture behavior more predictable Load Linear relationship between load and displacement: makes the structure more predictable with linear- elastic analysis (the vast majority of structural Goal analyses, especially loads analysis) Linear region. Keep limit But we don’t want the joint to be linear all the way up Avoid to rupture. loads within this range. – If there’s an unanticipated high load or distribution of load (or an energy-limited load), ductility often 0 allows loads to redistribute (or allows the joint to Displacement absorb energy) before anything ruptures. Designing to ensure that failure is ductile is hugely important but often neglected, and is addressed multiple times in upcoming sections of this course. The following pages address the goal of high, linear stiffness. 0 June 2019 Copyright Instar Engineering and Consulting, Inc.• instarengineering.com Materials may be reproduced in complete form only, with header and footer 3-10 Course Sampler Design and Analysis of Bolted Joints Calculating Bolt Loads is Often Based Simply on Statics Example: Tension joint with 2 bolts Case 3: Combined loads Case 1: Tensile applied load, Case 2: Applied with resultant load vector moment centered between bolts 1600 in-lb 1600 in-lb 1000 lb 1000 lb FBD Super- 4.00” 4.00” 4.00” position applies 1600 in-lb 1000 lb 1000 lb With loads carried only 1600 in-lb Ignoring preload … by the bolts, What’s the peak What’s the peak What’s the peak bolt load? bolt load? bolt load? June 2019 Copyright Instar Engineering and Consulting, Inc.• instarengineering.com Materials may be reproduced in complete form only, with header and footer 4-7 Course Sampler Design and Analysis of Bolted Joints A Bolt that Failed in a Tension Test Full-diameter body (greater cross- sectional area than at threads) The threaded region between the nut and the full-diameter body is often quite short.