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At least 217 records · Page 12

All sources NASA NTRS DOE OSTI

NASA NTRS1991

Dynamic characteristics of joint dominated space trusses

The rotational stiffness of hinge joints, and the gap of the joints applied in large deployable trusses, have been experimentally shown to have a significant role in determining such structures' dynamic behavior; an analytical validation of these results is presented for the case where linear rotation springs are used to model the hinge joints employed in a simple beam in trusses. The results obtained indicate that the natural frequencies of these structures depend not only on joint stiffness but also on joint location. Such gap parameters as gap size, stiffness, position, and excitation-force levels, are discussed with a view to a deeper understanding of their effects on a space interferometry system's simulated dynamic responses.

Shih, Choon-Foo↗

NASA NTRS1992

Structural characterization of an articulated-truss joint

A first generation space crane articulated-truss joint was statically and dynamically characterized in a configuration which approximated an operational environment. The articulated-truss joint was integrated into a test bed for structural characterization. Static characterization was performed by applying known loads and measuring the corresponding deflections to obtain load-deflection curves. Dynamic characterization was performed using modal testing to experimentally determine the first six mode shapes, frequencies, and modal damping values. The static and dynamic characteristics were also determined for a reference truss, which served as a characterization baseline. Load-deflection curves are presented for the reference truss, and the articulated-truss joint mounted in the test bed. The static and dynamic experimental results are compared to analytical predictions obtained from finite element analyses. The load-deflection response is also presented for one of the linear actuators used in the articulated-truss joint. Finally, an assessment is presented for the predictability of the truss hardware used in the reference truss and articulated-truss joint based upon hardware stiffness properties which were previously obtained during the Precision Segmented Reflector (PSR) Technology Development Program.

Sutter, Thomas R.↗

NASA NTRS1940

Mechanical Properties of Flush-Riveted Joints

The strength of representative types of flush-riveted joints has been determined by testing 865 single-shearing, double-shearing, and tensile specimens representing 7 types of rivet and 18 types of joint. The results, presented in graphic form, show the stress at failure, type of failure, and d/t ratio. In general, dimpled joints were appreciably stronger than countersunk or protruding-head joints, but their strength was greatly influenced by constructional details. The optimum d/t ratios have been determined for the several kinds of joints. Photomicrographs of each type show constructional details and, in several instances, cracks in the sheet.

Brueggeman, W C↗

NASA NTRS1993

Joint pain and Doppler-detectable bubbles in altitude (Hypobaric) decompression

The observation that altitude decompression sickness (DCS) is associated with pain in the lower extremities is not new, although it is not a consistent finding. DCS in divers is generally in the upper body, an effect often attributed to non-loading of the body while immersed. In caisson workers, DCS is reported more in the lower extremities. Surprisingly, many researchers do not mention the location of DCS joint pain, apparently considering it to be random. This is not the case for the tissue ratios encountered in studying decompression associated with simulated EVA. In NASA/JSC tests, altitude DCS generally presented first in either the ankle, knee, or hip (83 percent = 73/88). There was a definite statistical relation between the maximum Spencer precordial Doppler Grade and the incidence of DCS in the extremity, although this is not meant to imply a casual relation between circulating gas bubbles and joint pain. The risk of DCS with Grade 4 was considerably higher than that of Grades 0 to 3. The DCS risk was independent of the 'tissue ratio.' There was a predominance of lower extremity DCS even when exercise was performed with the upper body. The reason for these locations we hypothesize to be attributed to the formation of tissue gas micronuclei from kinetic and tensile forces (stress-assisted nucleation) and are the result of the individuals ambulating in a 1g environment. Additionally, since these showers of Doppler bubbles can persist for hours, it is difficult to imagine that they are emanating solely from tendons and ligaments, the supposed site of joint pain. This follows from Henry's law linking the volume of joint tissue (the solvent) and the solubility coefficient of inert gas; there is volumetrically insufficient connective tissue to produce the prolonged release of gas bubbles. If gas bubbles are spawned and released from connective tissue, their volume is increased by those from muscle tissue. Therefore, the nexus between Doppler-detectable gas bubbles and joint-pain decompression sickness is essentially a statistical, rather than a direct, one.

Powell, Michael R.↗

NASA NTRS1994

Strength evaluation of socket joints

This report documents the development of a set of equations that can be used to provide a relatively simple solution for identifying the strength of socket joints and for most cases avoid the need of more lengthy analyses. The analytical approach was verified by comparison of the contact load distributions to results obtained from a finite element analysis. The contacting surfaces for the specific joint in this analysis are in the shape of frustrums of a cone and are representative of the tapered surfaces in the socket-type joints used to join segments of model support systems for wind tunnels. The results are in the form of equations that can be used to determine the contact loads and stresses in the joint from the given geometry and externally applied loads. Equations were determined to define the bending moments and stresses along the length of the joints based on strength and materials principles. The results have also been programmed for a personal computer and a copy of the program is included.

Rash, Larry C.↗

NASA NTRS1994

Adjustable bias column end joint assembly

An adjustable mechanical end joint system for connecting structural column elements and eliminating the possibility of free movement between joint halves during loading or vibration has a node joint body having a cylindrical engaging end and a column end body having a cylindrical engaging end. The column end joint body has a compressible preload mechanism and plunger means housed therein. The compressible preload mechanism may be adjusted from the exterior of the column end joint body through a port.

Wallsom, Richard E.↗

NASA NTRS1995

On the joint spectral density of bivariate random sequences

For univariate random sequences, the power spectral density acts like a probability density function of the frequencies present in the sequence. This dissertation extends that concept to bivariate random sequences. For this purpose, a function called the joint spectral density is defined that represents a joint probability weighing of the frequency content of pairs of random sequences. Given a pair of random sequences, the joint spectral density is not uniquely determined in the absence of any constraints. Two approaches to constraining the sequences are suggested: (1) assume the sequences are the margins of some stationary random field, (2) assume the sequences conform to a particular model that is linked to the joint spectral density. For both approaches, the properties of the resulting sequences are investigated in some detail, and simulation is used to corroborate theoretical results. It is concluded that under either of these two constraints, the joint spectral density can be computed from the non-stationary cross-correlation.

Aalfs, David D.↗

NASA NTRS1995

Improved Automatically Locking/Unlocking Orthotic Knee Joint

Proposed orthotic knee joint improved version of one described in "Automatically Locking/Unlocking Orthotic Knee Joint" (MFS-28633). Locks automatically upon initial application of radial force (wearer's weight) and unlocks automatically, but only when all loads (radial force and bending) relieved. Joints lock whenever wearer applies weight to knee at any joint angle between full extension and 45 degree bend. Both devices offer increased safety and convenience relative to conventional orthotic knee joints.

Weddendorf, Bruce↗

NASA NTRS1998

Structural Stiffness Characteristics of the Solid Rocket Booster Field Joint

The Space Shuttle Solid Rocket Booster (SRB) is a complex assembly of segments of which the field and factory joints contain an unpredictable amount of freeplay. Freeplay is inherent due to tolerances in being able to assemble the segments and the gaps that occur at each joint. The purpose of this work is to quantify the amount of freeplay for the field joint as well as identify the effects of introducing multiple joints that contain freeplay into a system. To provide a baseline for the above study an additional model was made to quantify the behavior of a field joint that does not have any freeplay in the system.

Bodeker, Dan A., III↗

NASA NTRS1999

Fabrication and Assembly of High-Precision Hinge and Latch Joints for Deployable Optical Instruments

Descriptions are presented of high-precision hinge and latch joints that have been co-developed, for application to deployable optical instruments, by NASA Langley Research Center and Nyma/ADF. Page-sized versions of engineering drawings are included in two appendices to describe all mechanical components of both joints. Procedures for assembling the mechanical components of both joints are also presented. The information herein is intended to facilitate the fabrication and assembly of the high-precision hinge and latch joints, and enable the incorporation of these joints into the design of deployable optical instrument systems.

Phelps, James E.↗

NASA NTRS1999

Microgravity Vibration Output Testing of Space Station Rotary Joints

The mission of the International Space Station is to provide a working laboratory in orbit for research in engineering, life sciences, and microgravity. Ensuring that the mechanical equipment on Space Station does not unduly disturb the microgravity environment is of paramount importance in meeting the Station's mission. The large inertia being moved by the Space Station's solar array and thermal radiator rotary joints make them one of the largest potential disturbance sources. The present paper describes the mechanical and control system design of these joints, their disturbance producing characteristics, and analytical predictions of some key performance indicators. The component and system functional tests performed to measure the actual vibration output of the joints are detailed and the test results discussed. Results of the rotary joint test program presented here show that the joints do meet the Space Station microgravity requirements, ensuring that this unique laboratory for microgravity research will be unaffected by the operation of the largest moving machinery on board.

Boucher, Robert L.↗

NASA NTRS2001

Work and Fatigue Characteristics of Unsuited and Suited Humans During Isolated, Isokinetic Joint Motions

The effects of a pressurized suit on human performance were investigated. The suit is known as an Extra-vehicular Mobility Unit (EMU) and is worn by astronauts while working outside of their space craft in low earth orbit. Isolated isokinetic joint torques of three female and three male subjects (all experienced users of the suit) were measured while working at 100% and 80% of their maximum voluntary torque (MVT). It was found that the average decrease in the total amount of work done when the subjects were wearing the EMU was 48% and 41% while working at 100% and 80% MVT, respectively. There is a clear relationship between the MVT and the time and amount of work done until fatigue. In general the stronger joints took longer to fatigue and did more work than the weaker joints. However, it is not clear which joints are most affected by the EMU suit in terms of the amount of work done. The average amount of total work done increased by 5.2% and 20.4% for the unsuited and suited cases, respectively, when the subject went from working at 100% to 80% MVT. Also, the average time to fatigue increased by 9.2% and 25.6% for the unsuited and suited cases, respectively, when the subjects went from working at 100% to 80% MVT. The EMU also decreased the joint range of motion. It was also found that the experimentally measured torque decay could be predicted by a logarithmic equation. The absolute average error in the predictions was found to be 18.3% and 18.9% for the unsuited and suited subject, respectively, working at 100% MVT, and 22.5% and 18.8% for the unsuited and suited subject, respectively, working at 80% MVT. These results could be very useful in the design of future EMU suits, and planning of Extra-Vehicular Activit). (EVA) for the upcoming International Space Station assembly operations.

Gonzalez, L. Javier↗

NASA NTRS2004

Elasto-Plastic Analysis of Tee Joints Using HOT-SMAC

The Higher Order Theory - Structural/Micro Analysis Code (HOT-SMAC) software package is applied to analyze the linearly elastic and elasto-plastic response of adhesively bonded tee joints. Joints of this type are finding an increasing number of applications with the increased use of composite materials within advanced aerospace vehicles, and improved tools for the design and analysis of these joints are needed. The linearly elastic results of the code are validated vs. finite element analysis results from the literature under different loading and boundary conditions, and new results are generated to investigate the inelastic behavior of the tee joint. The comparison with the finite element results indicates that HOT-SMAC is an efficient and accurate alternative to the finite element method and has a great deal of potential as an analysis tool for a wide range of bonded joints.

Arnold, Steve M.↗

NASA NTRS2005

Mechanical Characterization of Reactively Brazed Metal-Composite Joints for Heat Rejection Systems

The joining of metal tubes to composite plates is required for heat-rejection components in several space applications. Currently a number of different braze compositions are being evaluated as to their effectiveness. Such tube-plate configurations cannot be represented by traditional methods of testing, e.g., lap joints. The joined region is not between two flat surfaces, but rather between a flat surface and a curved surface. Therefore, several tests have been employed to ascertain the effectiveness of the different braze approaches in tension and in shear that are both simple and representative of the actual system and relatively straightforward in analysis. The results of these "tube tests" will be discussed for the three different braze compositions, Cu-ABA, Ti-Cu-Sil, and Ti-Cu-Ni. In addition, fracture analysis of the failed joints was performed and offers insights into the cause of joint failure and the distinctions which need to be made between the "strength" of a joint versus the "load carrying ability" of a joint.

Morscher, Gregory N.↗

NASA NTRS2008

Imaging and Analysis of Void-defects in Solder Joints Formed in Reduced Gravity using High-Resolution Computed Tomography

As a part of efforts to develop an electronics repair capability for long duration space missions, techniques and materials for soldering components on a circuit board in reduced gravity must be developed. This paper presents results from testing solder joint formation in low gravity on a NASA Reduced Gravity Research Aircraft. The results presented include joints formed using eutectic tin-lead solder and one of the following fluxes: (1) a no-clean flux core, (2) a rosin flux core, and (3) a solid solder wire with external liquid no-clean flux. The solder joints are analyzed with a computed tomography (CT) technique which imaged the interior of the entire solder joint. This replaced an earlier technique that required the solder joint to be destructively ground down revealing a single plane which was subsequently analyzed. The CT analysis technique is described and results presented with implications for future testing as well as implications for the overall electronics repair effort discussed.

Easton, John W.↗

NASA NTRS2008

Analysis of Bonded Joints Between the Facesheet and Flange of Corrugated Composite Panels

This paper outlines a method for the stress analysis of bonded composite corrugated panel facesheet to flange joints. The method relies on the existing HyperSizer Joints software, which analyzes the bonded joint, along with a beam analogy model that provides the necessary boundary loading conditions to the joint analysis. The method is capable of predicting the full multiaxial stress and strain fields within the flange to facesheet joint and thus can determine ply-level margins and evaluate delamination. Results comparing the method to NASTRAN finite element model stress fields are provided illustrating the accuracy of the method.

Yarrington, Phillip W.↗

NASA NTRS1940

Mechanical Properties of Flush-Riveted Joints

The strength of representative types of flush-riveted joints has been determined by testing 865 single-shearing, double-shearing, and tensile specimens representing 7 types of rivet and 18 types of joint. The results, presented in graphic form, show the stress at failure, type of failure, and d/t ratio. In general, 'dimpled' joints were appreciably stronger than countersunk or protruding-head joints, but their strength was greatly influenced by constructional details. The optimum d/t ratios have been determined for the several kinds of joints. Photomacrographs of each type show constructional details and, in several instances, cracks in the sheet.

Bruggeman, Wm. C.↗

NASA NTRS2009

A Method for and Issues Associated with the Determination of Space Suit Joint Requirements

In the design of a new space suit it is necessary to have requirements that define what mobility space suit joints should be capable of achieving in both a system and at the component level. NASA elected to divide mobility into its constituent parts-range of motion (ROM) and torque- in an effort to develop clean design requirements that limit subject performance bias and are easily verified. Unfortunately, the measurement of mobility can be difficult to obtain. Current technologies, such as the Vicon motion capture system, allow for the relatively easy benchmarking of range of motion (ROM) for a wide array of space suit systems. The ROM evaluations require subjects in the suit to accurately evaluate the ranges humans can achieve in the suit. However, when it comes to torque, there are significant challenges for both benchmarking current performance and writing requirements for future suits. This is reflected in the fact that torque definitions have been applied to very few types of space suits and with limited success in defining all the joints accurately. This paper discussed the advantages and disadvantages to historical joint torque evaluation methods, describes more recent efforts directed at benchmarking joint torques of prototype space suits, and provides an outline for how NASA intends to address joint torque in design requirements for the Constellation Space Suit System (CSSS).

Matty, Jennifer E.↗