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Interferometric instrument response is linearly related to axial tensile stresses, and, under idealized conditions, measurement errors are within approximately plus or minus 1 percent. Ultimate accuracy of instrument depends on a number of variables, such as bolt material, dimensions, and geometry and uniformity of stresses and temperature.
The thermal contact resistance of a bolted joint is composed of two components: large scale constriction which is influenced by the extent of the interfacial pressure distribution and small scale constriction which is influenced by the magnitude of the local pressure. It is analytically shown how the roughness asperities at the contact surfaces of the joint affect these two constrictions but in opposite senses. The sum of the two effects, the total resistance, may increase or decrease with changing resistance and examples are given where an optimum roughness will minimize total joint resistance.
Closed-loop feedback circuit system used to measure bolt tension. Advantages are its simplicity, higher accuracy, and potential low cost.
The design, fabrication, static test, and fatigue test of both tension and compression graphite-epoxy candidates for a wing splice representative of a next-generation transport aircraft was the objective of the reported research program. A single-scarf bolted joint was selected as the design concept. Test specimens were designed and fabricated to represent an upper-surface and a lower-surface panel containing the splice. The load spectrum was a flight-by-flight random-load history including ground-air-ground loads. The results of the fatigue testing indicate that, for this type of joint, the inherent fatigue resistance of the laminate is reflected in the joint behavior and, consequently, the rate of damage accumulation is very slow under realistic fatigue loadings.
Graphite-epoxy bolted joint specimens were designed and fabricated. These specimens were to be representative of a side-of-body wing skin splice with a 20-year life expectancy in a commercial transport environment. Preliminary tests were performed to determine design values of bearing and net tension stresses. Based upon the information developed, a three-fastener-wide representative wing skin splice was designed for a load of 2627 KN/m (15,000 lbf/in.). One joint specimen was fabricated and tested at NASA. The wing skin splice failed at 106 percent of design ultimate load. This joint design achieved all static load objectives. Fabrication of six specimens, together with their loading fixtures, was completed, and the specimens were delivered to NASA-LRC.
Extensometer using ultrasonic pulse reflections to measure elongations in tightened belts and studs is much more accurate than conventional torque wrenches in application of specified preload to bolts and other threaded fasteners.
Simple lock, designed for hydraulic system, is used to keep any nut or bolt in place under vibration.
A CW ultrasonic device is described for measuring frequency shifts of the peak of a mechanical resonance in a body. One application of the device is measuring the strain in a bolt, and other applications such as measuring the thickness of a body, measuring the depth of a flaw in a body, measuring the elongation of a body, and measuring changes in velocity of sound in a body. The body is connected, by means of a CW transducer, to electrical circuit means including a narrow band RF amplifier to form a closed loop feedback marginal oscillator that frequency locks the device to the peak of a mechanical resonance in the body. When the frequency of this peak changes, because of a physical change in the body, the frequency of the oscillator changes. The device includes an automatic frequency resonant peak tracker that produces a voltage that is related to a change in frequency of the oscillator. This voltage is applied to the RF amplifier to change the center of its frequency band to include the frequency of the peak and is a measure of the frequency shift.
The determination of the stress distribution in the inner lap of double-lap, double-bolt joints using photoelastic models of the joint is discussed. The principal idea is to fabricate the inner lap of a photoelastic material and to use a photoelastically sensitive material for the two outer laps. With this setup, polarized light transmitted through the stressed model responds principally to the stressed inner lap. The model geometry, the procedures for making and testing the model, and test results are described.
Data are presented for the static and viscoelastic performance of Celion 6000/PMR-15 graphite/polyimide composite belted joints over the 21 to 315 C temperature range. Two laminate configurations were obtained by sectioning a panel into subpanels oriented parallel and perpendicular to the laminate principal direction. Three replicates of each specimen geometry were tested at each of the test temperatures for the two laminate configurations, and failure load was defined as the maximum load attained as indicated by the load/deflection curve. Effects of strain-rate on strength and failure mode behavior were assessed at room temperature for strain rates of 0.002, 0.01, 0.10, and 1.00/s, and creep behavior was examined for the case of bearing deformations at temperatures of 21 and 177 C. Results are presented, including: (1) two distinct mechanisms were responsible for bearing failure and related to laminate configuration, (2) the + or - 45 deg plies carried the shearing load in composite bolted joints, and (3) the material exhibited strain-rate sensitivity which varied with laminate configuration.
An equation for bolt clampup relaxation for transient temperature-moisture (T-M) conditions was derived starting with a relaxation equation for steady-state conditions, and then using an incremental time approach that exploits the superposition principle for linear viscoelasticity. The resulting equation uses the initial T-M condition (at the time of clamping), the T-M history after clamping, and elastic clampup coefficients for temperature and moisture changes. For a given material and joint configuration, the clampup coefficients are constants that can be calculated by elastic analyses. The clampup equation was used to calculate the changes in clampup occurring in a T300/5208 graphite/epoxy joint exposed to a one-year history of temperature and moisture. Two cases were considered: one was a dry joint exposed to a relatively humid environment and the other was a nearly saturated joint exposed to an arid environment.
A semi-empirical shearout strength model has been formulated for the analysis of composite bolted joints with allowance for the effects of joint geometry. The model employs a polynomial stress function in conjunction with a point stress failure criterion to predict strength as a function of fastener size, edge distance, and half spacing. The stress function is obtained by two-dimensional plane-stress finite element analysis using quadrilateral elements with orthotropic material properties. Comparison of experimentally determined shearout strength data with model predicted failures has substantiated the accuracy of the model.
An attempt is made to study the stresses in multiple-bolt connectors, focusing on the stress distribution in a two-pin connector, the two pins being in line and in parallel with the direction of the applied load. The photoelastic modeling approach with two-dimensional transmission photoelasticity is used. The joint models and model fringe patterns are discussed, with special attention given to the existence of a photoelastic isotropic point and to the separation of stresses.
Modified clamp transfers torque to bolthead. Readily-available springloaded clamp modified, transfers measured torque to bolt with knurled head.
Stainless steel bolt fasteners have adequate grounding paths for protection of structures against lightning strikes. Fasteners particularly useful in sections where insulating material required, such as thermal panels of external storage tank.
The factors that must be taken into consideration when applying NASTRAN's linear analysis to structures whose principle boundaries are formed by bolting, and for which localized stress peaking is important are discussed. An illustration is given in an application to a mounting bracket.
Four types of high-temperature joints designed for control surfaces. Design, analysis, and testing performed to develop four types of graphite/polyimide bonded and bolted composite joints for lightly loaded control surfaces on advanced transportation systems that operate at temperatures up to 550 degrees F (288 degrees C).
Corroded fasteners taken apart more easily. Lubricating holes bored to thread from three of flats. Holes facilitate application of penetrating oil to help loosen nut when rusted onto bolt. Holes make it possible to apply lubricants and rust removers directly to more of thread than otherwise reachable.