Residual stress measurement of titanium - 6Al-4V alloy pressure vessel material
Diffractometer and X ray stress measurements of directional elastic residual stresses on surfaces of 6Al-4V titanium alloy weldments and parent metal
Kol, P. K.↗
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Stress Measurement System
Under the Aircraft Structural Integrity program, Langley Research Center and Stress Photonics developed an infrared-based stress measurement system for use in nondestructive evaluation of materials and structures. Stress Photonics commercialized the technology in the DeltaTherm 1000 system, used to compare designs and detect cracks in structures, especially for aging aircraft and bridges. The system combines digital signal processing technology with a special infrared camera to provide instantaneous thermal images and live differential images.
Source record↗
Magnetoacoustic stress measurements in steel
Uniaxial stress effects on the low-field magnetoacoustic interaction have been studied using bulk compressional waves and Rayleigh surface waves in numerous steel samples having various impurity concentrations (Namkung et al., 1984). The results invariably showed that the initial slope of acoustic natural velocity variations, with respect to net induced magnetization parallel to the stress axis, is positive under tension and negative under compression. The results of current measurements in railroad rail steel having about 0.68 wt percent carbon content are typical for medium range carbon steels. The low-field natural velocity slope in this particular type of steel, which is almost zero when unstressed, becomes steeper with increased magnitude of stress in both directions. Hence, the nondestructive determination of the sign of residual stress in railroad wheels and rails is possible using this technique. This paper discusses the basic physical mechanism underlying the experimental observations and presents the results obtained in railroad rail steel.
Namkung, M.↗
Determination of calibration constants for the hole-drilling residual stress measurement technique applied to orthotropic composites. I - Theoretical considerations
The hole-drilling technique for the measurement of residual stresses using electrical resistance strain gages has been widely used for isotropic materials and has been adopted by the ASTM as a standard method. For thin isotropic plates, with a hole drilled through the thickness, the idealized hole-drilling calibration constants are obtained by making use of the well-known Kirsch's solution. In this paper, an analogous attempt is made to theoretically determine the three idealized hole-drilling calibration constants for thin orthotropic materials by employing Savin's (1961) complex stress function approach.
Prasad, C. B.↗
Demonstration of acoustic spectrometer for measuring stress and locating flaws in materials Final formal progress report
Testing and evaluation of acoustic spectrometer for nondestructive measurement of stress and location of flaws in materials
FLAW DETECTION↗
Determination of calibration constants for the hole-drilling residual stress measurement technique applied to orthotropic composites. II - Experimental evaluations
The first step in the extension of the semidestructive hole-drilling technique for residual stress measurement to orthotropic composite materials is the determination of the three calibration constants. Attention is presently given to an experimental determination of these calibration constants for a highly orthotropic, unidirectionally-reinforced graphite fiber-reinforced polyimide composite. A comparison of the measured values with theoretically obtained ones shows agreement to be good, in view of the many possible sources of experimental variation.
Prasad, C. B.↗
Residual Stress Measurements of Ni-base Superalloys through Neutron Diffraction
Linear friction welding (LFW) is being considered for joining single crystal to polycrystalline nickel-base alloys for advanced high temperature gas turbine disks. Unfortunately, the transient thermal cycle during the welding generates undesirable residual stresses. Additionally, upon thermal treatment to relieve these residual stresses the weld joints often crack. To understand this issue, state-of-the-art neutron diffraction was performed near weld joints to quantify the residual stresses created by varying linear friction weld conditions. The residual stresses were measured along three orthogonal axes in both the polycrystalline LSHR and single crystal SC-180 alloys. Residual stress measurements using a contour method were used to compare the calculations obtained by neutron diffraction. The results suggest high residual stresses can be successfully reduced using both pre- and post-weld processing steps. Moreover, the effect of grain size and pre-heat temperatures on the LFW process were explored.
Superalloys↗
Microwave Photoelasticity: Exploiting Multiple Resonances to Measure Stress Changes within Yttria-Partially-Stabilized-Zirconia
The NASA Glenn Research Center is developing non-destructive-testing (NDT) methods to enable the measurement of stresses embedded in optically opaque materials using microwave radiation in a free-space quasi-optical system. This methodology tracks microwave resonances observed in reflected scattering parameters extracted from materials under load. In this paper, we report the successful measurement of the stress-optic-coefficient of bulk yttria-partially stabilized zirconia (YTZP) ceramic of C = 1.42 x 10-4 ± 6.65 x 10-6 (1/GPa), across W-Band (80-100 GHz), and determined that this result is independent of sample thickness. The primary goal of this research is to establish a methodology to quantify and assess the life expectancy of ceramic thermal and environmental barrier coating (TBCs/EBCs). Bulk YTZP samples can undergo multiple resonances within a contiguous measurement bandwidth, each corresponding to an integer multiple wave number ∝. This allows for the acquisition and analysis of multiple stress measurement points within a single sample. As an additional benefit, one can approximate the refractive index of YTZP across a wide bandwidth by observing multiple resonances produced by a set of samples with varying thicknesses. Using this approach, the refractive index of bulk YTZP was found to be n = 5.80 ± 0.043 across the 85-115 GHz frequency band.
Seth W. Waldstein↗
Enhancement to Non-Contacting Stress Measurement of Blade Vibration Frequency
A system for turbo machinery blade vibration has been developed that combines time-of-arrival sensors for blade vibration amplitude measurement and radar sensors for vibration frequency and mode identification. The enabling technology for this continuous blade monitoring system is the radar sensor, which provides a continuous time series of blade displacement over a portion of a revolution. This allows the data reduction algorithms to directly calculate the blade vibration frequency and to correctly identify the active modes of vibration. The work in this project represents a significant enhancement in the mode identification and stress calculation accuracy in non-contacting stress measurement system (NSMS) technology when compared to time-of-arrival measurements alone.
Platt, Michael↗
A study of the effect of apparent strain on thermal stress measurement for two types of elevated temperature strain gages
A weldable type strain gage was used to measure low level thermal stress in an elevated temperature environment. Foil strain gages used in a comparative manner reveal that the apparent strain of weldable strain gages is not sufficiently known to acquire accurate low level thermal stress data. Apparent strain data acquired from coupon tests reveals a large scatter in apparent strain characteristics among the weldable strain gages. It is concluded that apparent strain data for individual weldable strain gages must be required prior to installation if valid thermal stress data is to be obtained through the temperature range of room temperature to 755 K (900 F).
Jenkins, J. M.↗
Turbulence stress measurements in a nonadiabatic hypersonic boundary layer
Turbulent shear stress and direct turbulent total heat-flux measurements have been made across a nonadiabatic, zero pressure gradient, hypersonic boundary layer by using specially designed hot-wire probes free of strain-gauging and wire oscillation. Heat-flux measurements were in reasonably good agreement with values obtained by integrating the energy equation using measured profiles of velocity and temperature. The shear-stress values deduced from the measurements, by assuming zero correlation of velocity and pressure fluctuations, were lower than the values obtained by integrating the momentum equation. Statistical properties of the cross-correlations are similar to corresponding incompressible measurements at approximately the same momentum-thickness Reynolds number.
Mikulla, V.↗
On-Orbit Glass Stress Measurement Tool Development
Exposed window surfaces on-orbit are vulnerable to a variety of damage including impacts. These impacts impart stress into the panes and reduce the remaining life of the window. Spacecraft windows are evaluated before and after flight to determine the minimum strength of the glass prior to launch. Currently, no method exists to accurately evaluate the windows on-orbit. An on-orbit optical system is under development to evaluate spacecraft windowpane material for remaining flight life by directly measuring the stress in the glass. This optical system builds on the industry-standard practice of calculating the amount of impact-induced stress through birefringence measurement. In industry, the birefringence is imaged through the pane with a detector and light source on opposite sides. On-orbit, there is only access to one side of the pane. This system has been designed to image birefringence by reflecting the light off the pane to a detector on the same side of the glass. Previous studies have shown a correlation between material stress distribution and the associated K-value. This system is designed to determine low strength areas which can be caused by defects due to impact damage. Glass samples will be imaged and broken in order to correlate impact-induced stress to the fracture strength of the material. Once this correlation is determined, a quantitative assessment of the stress areas in spacecraft windows will occur prior to and during mission operations to reliably determine the overall strength of the glass.
Max J Schneiderman↗
Triboluminescent Fiber-Optic Sensors Measure Stresses
Triboluminescence exploited in fiber-optic sensor system for measuring changes in pressures, strains, vibrations, and acoustic emissions, in structural members. Sensors embedded in members for in situ monitoring of condition of structure. System passive in sense no source of radiation required to interrogate optical fiber. Technique has potential for wide range of applications in which detection and measurement of structural stress required.
Rogowski, Robert S.↗
Instrument accurately measures stress loads in threaded bolts
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.
Rollins, F. R., Jr.↗
Turbulence and shear stress measurements in hypersonic flow
An assessment has been made of the potential for hot wire and laser anemometer measurements of turbulent fluctuations in hypersonic flow fields. The results of laser velocimeter experiments conducted in two hypersonic wind tunnels are reported and comparisons made with previous hot wire turbulence measurements. A new concept for the measurement of the compressible shear stress terms which combines the attributes of both hot wire and laser anemometry is presented.
Owen, F. K.↗
Calculated and measured stresses in simple panels subject to intense random acoustic loading including the near noise field of a turbojet engine
Flat 2024-t3 aluminum panels measuring 11 inches by 13 inches were tested in the near noise fields of a 4-inch air jet and turbojet engine. The stresses which were developed in the panels are compared with those calculated by generalized harmonic analysis. The calculated and measured stresses were found to be in good agreement. In order to make the stress calculations, supplementary data relating to the transfer characteristics, damping, and static response of flat and curved panels under periodic loading are necessary and were determined experimentally. In addition, an appendix containing detailed data on the near pressure field of the turbojet engine is included.
Lassiter, Leslie W↗
Wall shear stress measurement in blade end-wall corner region
The magnitude and the direction of wall shear stress and surface pressure in the blade end-wall corner region were investigated. The measurements were obtained on a specially designed Preston tube, the tip of which could be concentrically rotated about its axis of rotation at the measurement location. The magnitude of wall shear stress in the vicinity of the corner was observed to increase significantly (170 percent) compared to its far-upstream value; the increase was consistently higher on the blade surface compared to the value on the plate surface of the blade end-wall corner. On both surfaces in the blade end-wall corner, the variation of the wall shear stress direction was found to be more predominant in the vicinity of the blade leading-edge location. The trend of the measured wall shear stress direction showed good agreement with the limiting streamline directions obtained from the flow visualization studies.
Bhargava, R.↗
Residual stress measurements in carbon steel
External dc magnetic field-induced changes in natural velocity of Rayleigh surface waves were measured in steel specimens under various stress conditions. The low field slopes of curves representing the fractional changes of natural velocity were proved to provide correct stress information in steels with different metallurgical properties. The slopes of curves under uniaxial compression, exceeding about one third of the yield stress, fell below zero in all the specimens when magnetized along the stress axis. The slopes under tension varied among different steels but remained positive in any circumstances. The stress effect was observed for both applied and residual stress. A physical interpretation of these results is given based on the stress-induced domain structure changes and the delta epsilon effect. Most importantly, it is found that the influence of detailed metallurgical properties cause only secondary effects on the obtained stress information.
Heyman, J. S.↗