Channel - #Structures Loads and Mechanical Systems (SLAMS)

12/4/2024 1:36:50 AM

Channel Videos

A Unified Approach to Modal Reduction Methods

Discipline: Loads & Dynamics Original Webcast Air Date: April 22, 2015 A survey of the papers on modal reduction methods exhibits a variety of mathematical techniques and procedures utilized by the developers of each method. For example, Hurty and Craig/Bampton utilized a Rayleigh-Ritz procedure to derive their fixed-boundary methods. MacNeal used electrical circuits analogies for his free-boundary method. Rubin improved MacNeal's method with a non-Rayleigh-Ritz procedure involving a power series expansion. Craig/Chang and Benfield/Hruda offered modal reduction methods which require special modal synthesis procedures. Hintz utilized an adhoc technique with some features of other techniques to derive a mixed-boundary method. The end result of this has been confusion and uncertainty on the part of engineers with the consequence of often opting for the simplest of the methods, Craig/Bampton for all applications. In addition, some methods, such as Hintz mixed-boundary, have gone largely unrecognized due to the complex language/procedures of the paper. Another unfortunate consequence has been some methods being utilized beyond their mathematical limitations, and when not performing, unfairly eliminated as faulty. Part 1 of this two part series on modal synthesis methods presents a unified mathematical approach to the derivation of the modal reduction methods. The presenter provides a single mathematical tool capable of deriving all methods of modal reduction including the more involved mixed-boundary methods. With this, the user gains an immediate level of comfort with all approaches including the more exotic methods. In addition, with the ability to now derive the method, the assumptions, approximations, application and pitfalls of each method automatically become more clear. Another objective of this presentation is to enable the engineers to develop their own NASTRAN/DMAP or other computer programs for implementing any of the modal reduction methods. A quick preview of Part 2: The Part 2 presentation is concerned with the modal synthesis methods themselves. Again, an attempt is made to unify this important subject as much as possible. In addition to modal synthesis methods, methods for recovery of dynamic response including modal acceleration, residual flexibility, and residual vectors are considered. Part 2 also includes special topics including generalized Guyan, Improved Reduced System (IRS), and System Equivalent Reduction Expansion Process (SEREP) utilized in the development of test analytical models (TAMs).

Dr. Arya Majed

6/4/2015 5:00:00 PM

An Overview of Fastener Requirements in the new NASA-STD-5020

Discipline: Mechanical Systems NASA-STD-5020 “Requirements for Threaded Fastening Systems in Spaceflight Hardware” was created to provide an Agency-wide consensus standard to provide uniform requirements for design and analysis of threaded fastening systems in spaceflight hardware across all NASA programs and projects. The development of the standard resulted from a NESC assessment initiated in 2006 following several costly fastener issues on NASA projects and many recent debates over fastener design and analysis criteria. The new standard was also intended replace and improve upon NSTS 08307 “Criteria for Preloaded Joints,” which had become the de facto NASA standard for fastener analysis across multiple programs and projects even though it was a Space Shuttle Program standard and would be withdrawn at the conclusion of the program. This webcast will provide an overview of the contents of the new standard, including rationale for differences from NSTS 08307, and case studies to illustrate application of some of the concepts.

Robert Wingate

4/26/2013 5:00:00 PM

Ball Bearings 101

Discipline: Mechanical Systems (SLaMS Webcast Series) Presented via Webcast on April 23, 2014 The subject of ball bearings is presented. The goal is to provide a very basic explanation of ball bearings to those who may need to use bearings but are not familiar with the subject matter.

Alan Leveille

7/3/2014 5:00:00 PM

Buckling, Shells, Knockdown Factors, and Validation Testing

Discipline: Structures NASA’s Shell Buckling Knockdown Factor Project (SBKF), was established in the spring of 2007 by the NASA Engineering and Safety Center (NESC) in collaboration with NASA’s Constellation Program and Exploration Systems Mission Directorate. The SBKF project has the goal of developing improved (i.e., less-conservative, robust), shell buckling design factors (a.k.a. knockdown factors) and design and analysis technologies for launch vehicle (LV) structures. Preliminary design studies indicate that implementation of these new knockdown factors can enable significant reductions in mass and control mass-growth in these vehicles and can help mitigate some of the typical LV development and performance risks. In particular, the new design technologies are expected to reduce the reliance on testing, provide high-fidelity estimates of structural performance, reliability, robustness, and enable increased payload capability. The lecture will provide a brief summary of SBKF objectives and approach towards developing and validating these new technologies and provide a look towards the future of design, analysis and testing of the next generation of buckling-critical launch vehicle structures. In particular, a historical review of the current design recommendations for buckling-critical thin-walled cylindrical shell structures will be presented, and their limitations relative to the design of modern launch vehicle structures will be discussed. Next, the lecture will identify some key technologies that are enabling the development of updated design factors including advancements in computational tools for structural analysis, testing and measurement technologies, and manufacturing and materials, and suggest other areas of R&D investment. Finally, results from a recent (and exciting!) full-scale structural test of a 27.5-ft-diameter orthogrid-stiffened Space Shuttle External Tank barrel section, ETTA1, will be presented.

Dr. Mark Hilburger

11/16/2012 4:30:00 PM

Damage Arresting Composites, Part 1: Building Blocks

Discipline: Structures Webcast Air Date: October 5, 2016 NASA conducted the Environmentally Responsible Aviation Project to explore and document the feasibility, benefits, and technical risk of advanced vehicle configurations and enabling technologies that will reduce the impact of aviation on the environment. A critical aspect of this pursuit is the development of a lighter, more robust airframe that will enable the introduction of unconventional aircraft configurations that have higher lift to drag ratios, reduced drag, and lower community noise. Although such novel configurations like the Hybrid Wing Body (HWB) offer better aerodynamic performance as compared to traditional tube-and-wing aircraft, the blended wing shape with its almost-flat sided pressure vessel poses significant design challenges. Developing an improved structural concept for a non circular pressurized cabin is the primary obstacle in implementing large lifting body designs. To address this challenge, researchers at NASA and The Boeing Company worked together to advance new structural concepts like the Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS), which is an integrally stiffened panel design that is stitched together and designed to maintain residual load carrying capabilities under a variety of damage scenarios. A building block approach was used in this technology development effort. This topic will be addressed in two parts. Part one will focus on the structural building blocks from small elements through large panels designed to demonstrate that the concept of out-of-autoclave cured, stitched composite structure with no mechanical fasteners in the acreage of flat and curved panels would efficiently support the axial, bending internal cabin pressure loads representative of the passenger compartment of a HWB vehicle. Part two, to be covered in a separate webcast, will focus on the design, analysis and testing of the complex pressurized structures of a PRSEUS “cube” and a 30-foot-long, 80%-scale, multi-bay box. All building blocks and the built-up structures were analyzed and tested and the results documented to demonstrate the feasibility of the concept for application to commercial transport aircraft.

Dawn Jegley

10/28/2016 5:00:00 PM

Damage Arresting Composites, Part 2: Large-Scale Multi-Bay Box

Webcast Air Date: 12/09/2016 Discipline: Structures NASA conducted the Environmentally Responsible Aviation Project to explore and document the feasibility, benefits, and technical risk of advanced vehicle configurations and enabling technologies that will reduce the impact of aviation on the environment. A critical aspect of this pursuit is the development of a lighter, more robust airframe that will enable the introduction of unconventional aircraft configurations that have higher lift to drag ratios, reduced drag, and lower community noise. Although such novel configurations like the Hybrid Wing Body (HWB) offer better aerodynamic performance as compared to traditional tube-and-wing aircraft, the blended wing shape with its almost-flat sided pressure vessel poses significant design challenges. Developing an improved structural concept for a non circular pressurized cabin is the primary obstacle in implementing large lifting body designs. To address this challenge, researchers at NASA and The Boeing Company worked together to advance new structural concepts like the Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS), which is an integrally stiffened panel design that is stitched together and designed to maintain residual load carrying capabilities under a variety of damage scenarios. A building block approach was used in this technology development effort. This topic is being addressed in two parts. Part one, presented by Dawn Jegley on 10/05/2016, was focused on the structural building blocks from small elements through large panels designed to demonstrate that the concept of out-of-autoclave cured, stitched composite structure with no mechanical fasteners in the acreage of flat and curved panels would efficiently support the axial, bending internal cabin pressure loads representative of the passenger compartment of a HWB vehicle. In part two, Adam Przekop will focus on the design, analysis and testing of the complex pressurized structures of a PRSEUS “cube” and a 30-foot-long, 80%-scale, multi-bay box. All building blocks and the built-up structures were analyzed and tested and the results documented to demonstrate the feasibility of the concept for application to commercial transport aircraft.

Dr. Adam Przekop

12/19/2016 6:00:00 PM

Force Limited Vibration Testing: A Review of Existing and New Methods

Discipline: Loads & Dynamics Originally webcast Live on: June 23, 2014 The force limited vibration test approaches discussed in NASA-7004C were developed to reduce overtesting associated with base shake vibration tests of aerospace hardware, where the interface responses are excited coherently. This handbook outlines several different methods of specifying the force limits. The rationale for force limiting is based on the disparity between the impedances of typical aerospace mounting structures and the large impedances of vibration test shakers. Among these approaches, the semi-empirical method is presently the most widely used method to derive the force limiting specifications. The inclusion of the incoherent excitation of the aerospace structures at mounting interfaces provides the basis for more realistic force limits to qualify the flight hardware using shaker testing. In this presentation the existing methods for defining the force limiting specifications discussed in the NASA handbook are reviewed using data recently obtained from a series of acoustic and vibration tests. New approaches based on considering the incoherent excitation of the structural mounting interfaces using acoustic test data are discussed. It is believed that the new approaches provide much more realistic force limits that may further remove conservatism inherent in shaker vibration testing not accounted for by methods discussed in the NASA handbook. A discussion on using FEM/BEM analysis to obtain force limits for flight hardware is also provided. Discipline: Loads & Dynamics

Dr. Ali Kolaini

8/21/2014 6:30:00 PM

Metal Fatigue Part 1

Structural integrity is assured via static strength and service life (fracture control) requirements. The mitigation of catastrophic failures in metallic materials resulting from fatigue damage accumulation during the useful life of a structure is one of the primary functions of fracture control. This Webcast provides a cursory overview of metal fatigue which includes the basic elements of stress-life (S-N) fatigue, strain-life, and linear elastic fracture mechanics. Details regarding the micro and macro mechanics associated with metal fatigue crack nucleation, initiation, and propagation are also addressed.

Raymond Patin

5/22/2013 5:00:00 PM

Metal Fatigue Part 2

Structural integrity is assured via static strength and service life (fracture control) requirements. The mitigation of catastrophic failures in metallic materials resulting from fatigue damage accumulation during the useful life of a structure is one of the primary functions of fracture control. This Webcast provides a cursory overview of metal fatigue which includes the basic elements of stress-life (S-N) fatigue, strain-life, and linear elastic fracture mechanics. Details regarding the micro and macro mechanics associated with metal fatigue crack nucleation, initiation, and propagation are also addressed.

Raymond Patin

6/3/2013 5:00:00 PM

Sandwich Structures Failure Modes and Their Prevention

Discipline: Structures Webcast Air Date: September 28, 2016 Typical damage modes in light honeycomb sandwich structures include face sheet/core disbonding and core fracture, both of which can pose a threat to the structural integrity of a component. These damage modes are of particular interest to aviation certification authorities since several in-service occurrences, such as rudder structural failure and other control surface malfunctions, have been attributed to face sheet/core disbonding. Extensive studies have shown that face sheet/core disbonding and core fracture can lead to damage propagation caused by internal pressure changes in the core. In order to identify, describe and address the phenomenon associated with facesheet/core disbonding, a reliable means of characterizing facesheet/core disbonding must be developed. In addition to the characterization tests, analysis tools are required, to help assess the likelihood of a structure exhibiting critical disbonding. These analysis tools need to be verified and validated. In this webcast, sandwich structures are introduced and their failure modes are discussed. Actual in-service occurrences are presented and a road map to standardization for facesheet/core disbonding in sandwich composite components is described. An overview is given on the development of test methods that yield a critical strain energy release rate associated with disbonding, with a focus on mode-I dominated loading conditions. Further, an analysis approach is discussed to compute energy release rates along an arbitrarily shaped disbond front. Finally, a brief summary of observations is presented and recommendations for improvements are provided.

Dr. Ronald Krueger

10/27/2016 5:00:00 PM

Testing and Analysis of Advanced Composite Tow-Steered Shells

The structural performance of two advanced composite tow-steered shells, manufactured using a fiber placement system, is assessed using both experimental and analytical methods. The fiber orientation angles vary continuously around the shell circumference from 10 degrees on the shell crown and keel, to 45 degrees on the shell sides. The two shells differ in that one shell has the full 24-tow course applied during each pass of the fiber placement system, while the second shell uses the fiber placement system s tow drop/add capability to achieve a more uniform shell wall thickness. The shells are tested in axial compression, and estimates of their prebuckling axial stiffnesses and bifurcation buckling loads are generated using linear finite element analyses. Cutouts, scaled to represent commercial aircraft passenger and cargo doors, are then machined into one side of each shell. The prebuckling axial stiffnesses and bifurcation buckling loads of the shells with cutouts are then computed using linear finite element analyses. When retested, large deflections were observed around the cutouts, but the shells carried an average of over 90 percent of the axial stiffness, and 85 percent of the buckling loads, of the shells without cutouts. These relatively small reductions in performance demonstrate the potential for using tow steering to mitigate the adverse effects of typical design features on the overall structural performance. Previous studies have typically shown poor correlation between experimental buckling loads and supporting linear bifurcation buckling analyses. The good correlation noted for these tow-steered shells may result from their circumferential axial stiffness variation, which may reduce sensitivity to geometric imperfections. A numerical investigation was performed using measured geometric imperfections from both shells. Finite element models of both shells were analyzed first without, and then, with the measured imperfections, superposed in different orientations around the shell longitudinal axis. Small variations in both the axial prebuckling stiffness and global buckling load of the shells were noted for the range of orientations studied.

Dr. Chauncey Wu

1/23/2017 6:00:00 PM