Showing papers in "Experimental Techniques in 1996"

The use of thermoelasticity to evaluate stress redistribution and notch sensitivity in ceramic matrix composites

Abstract: Summary Thermoelastic stress analysis was used to document the effect of composite damage on the stress distribution in three ceramic matrix composites. Composite damage was found to significantly alter the thermoelastic response of each material, with the greatest effect noted in SiC/CAS. Thermoelastic imaging of these materials affords a more complete picture of how the various damage mechanisms affect the stress distribution. In particular, a stress concentration factor computed from thermoelastic images, serves as an indicator of stress redistribution. The stress concentration factors were computed by comparing notch root to far field temperatures, and monitored after the introduction of various amounts of damage. In each material, the stress concentration factor diminished as the damaging load approached the ultimate stress. Reduction in the stress concentration is associated with local changes in modulus, mechanistically arising from combinations of fiber, matrix and interface fracture. Stress redistribution occurs as a consequence of modulus changes, leading to lower notch sensitivity in each of the tested composites.

A hybrid technique for compression testing at intermediate strain rates

Abstract: It is difficult to obtain accurate load information at strain rates on the order of 102 s−1 using a quartz load cell. We have developed a hybrid apparatus which combines the loading capability of a hydraulic test machine with the load-measurement technique of the Hopkinson bar. Tests comparing the output of the Hopkinson bar to the quartz cell clearly demonstrate the increase in resolution obtained with the hybrid technique.

Dynamic stress analysis of gas turbine rotor airfoils using thermoelastic techniques

Abstract: Summary The SPATE system provided a means for experimental ther-moelastic analysis of high frequency modes of vibration of a complicated turbine impeller which did not facilitate analysis with conventional methods due to low stress response and high modal density. Testing also demonstrated a unique experimental stress analysis technique which was applied to a blade and vane airfoil and exhibited excellent stress distribution correlation with an established experimental technique (brittle lacquer) and with analytical stress distributions predicted by NASTRAN. Full field dynamic stress analysis of gas turbine compressor airfoils accomplished using thermoelastic techniques provided an efficient method of determining optimum strain gage locations for monitoring rig and engine vibratory stress. Significant cost savings could be realized using this technique from: (1) reduction in the amount of instrumentation required to characterize airfoil dynamic behavior and (2) increased vibratory response mode coverage thereby preventing the re-running of engine test programs to measure vibratory stress response levels of unexpected resonances encountered during initial engine testing. Care is required when using SPATE results for vibratory modes resulting in high bi-axial stress distributions since the system output indicated stress magnitudes based on the sum of the principal stresses which could be significantly higher than either of the individual principal stresses.

Generating and recording transient bending waves in plates by pulsed lasers

Abstract: We have presented a method to generate bending waves in a plate by focusing a Q-switched Nd:YAG-laser pulse on its surface. These waves are recorded by double pulsed hologram interferometry. Evaluation of the interferograms show that the bending wave pattern is very similar to what is predicted by the Kirchhoff plate equation assuming a point impact of infinitesimally short duration. This indicates that a short Nd:YAG-laser pulse may be considered as a Dirac pulse in space and time. Future investigations will be performed with a more powerful and “cleaner” laser to get a higher energy and impulse transfer to the plate thus giving bending waves of larger amplitude. Promising preliminary experiments have also begun using a double pulsed ruby laser both for generating and recording of bending waves.

Measurement of dynamic mechanical properties of advanced ceramics and ceramic matrix composites at temperatures above 1000°c

Abstract: Summary Difficulties have been encountered in measuring dynamic mechanical properties of ceramics and ceramic composites because these materials are stiff and brittle and are to be used under extreme temperature conditions. The approaches used for testing traditional metals, polymers, and their composites may fail for testing ceramic materials if the special characteristics of ceramic materials are not accounted for. In order to avoid damaging cantilever beam vibration test samples, a precision clamping vise is required, and the clamping force must be carefully controlled. A torque wrench should be used to apply the clamping force, and find the optimum clamping force which corresponds to the smallest damping or largest modulus measurement. The extraneous energy dissipation due to the support can be reduced by either using a step beam sample with integral shoulder or using a vise with large stiffness and mass. From the economics point of view, the latter is preferred because a step geometry is costly to machine. The temperature distribution on a sample should be carefully controlled so as to minimize temperature gradients. Heat conduction from the heat chamber to the support and other possible heat losses should also be minimized if possible.