Keywords: Ultrasonics; Eddy current; Composites

Abstract: Both eddy current and ultrasonic C-scans were performed to characterize the structural damages incurred in composite samples by mechanical impact and fire. The result clearly indicate that, as expected, the eddy current flow is sensitive to any structural change in the carbon fiber bundles since the high electrical conductivity and the ultrasonic propagation is mainly determined by the properties of the matrix material. Specifically, the eddy current scan results showed the image of the individual tows up to 5th layers and the impact damages in the specimen can be visualized unambiguously, while the ultrasonic scan results provide far less indication of the same damage. For the fire-damaged samples, on the other hand, the ultrasonic scan of return echo amplitude provide a good indication of damages while the eddy current scan results are very unclear. Hence, the present study proves the necessity of a multiparameter test procedure for complete characterization. This paper presents all the details of the tests results and the possible explanation on the mechanical changes created in the matrix of the fire-damaged composite samples.

Keywords: Anelasticity; Magneto-acoustics; Ultrasonics

Abstract: The time dependence of the ultrasonic natural velocity immediately following sudden changes in the applied stress states in brass and carbon steel samples was studied using the pulsed-phase-locked-loop (P2L2) method. To isolate the effects due to anelasticity, thermoelastic effects due temperature changes have been estimated separately and subtracted. The experimental results of both ferrous and nonferrous alloys showed that the thermoelastic contribution due to temperature variation dominates the time dependence of the ultrasonic natural velocity immediately following a rapid change in the external stress from tension to compression. On the other hand, a considerable anelastic effect was evident when the stress change was reversed. In addition, the magnitude of the velocity change was comparable in both types of alloys. This suggests that anelasticity is pronounced only when the stress changes from compression to tension. Such a tendency persists over a wide range of stress change, i.e., #177#30 ksi. The fact that the anelasticity exists in the alloys that do not contain solid solutions, such as carbon, clearly indicates that the effect is not due to the stress-induced directional alignment of elastic dipoles. Rather it is most probably due to the motion of dislocations which is not necessarily opposite under opposite signs of stress. Hence, the anelastic behavior when the stress state changes from compression to tension is explainable.

Keywords: Composite Materials; Ultrasonics; Nonlinear Elasticity; Energy Flux

Abstract: Ultrasonic waves suffer energy flux deviation in graphite/epoxy because of the large anisotropy. The angle of deviation is a function of the elastic coefficients. For nonlinear solids, these coefficients and thus the angle of deviation is a function of stress. Acoustoelastic theory was used to model the effect of stress on flux deviation for unidirectional T300/5208 using previously measured elastic coefficients. Computations were made for uniaxial stress along the x3 axis (fiber axis) and the x1 axis for waves propagating in the x1x3 plane. These results predict a shift as large as three degrees for the quasi-transverse wave. The shift in energy flux offers a new nondestructive technique of evaluating stress in composites.

Keywords: Composites; Ultrasonic Velocity; Nonlinear Elasticity

Abstract: Previously, the stress acoustic constants (SAC's) of unidirectional graphite/epoxy composites were measured to determine the nonlinear moduli of this material. These measurements were made under compressive loading in order to obtain the sufficient number of values needed to calculate these moduli. However, because their strength in tension along fiber directions can be several times greater, most composites are used under tensile loading. Thus, it is important to characterize the nonlinear properties of these materials in tension as well.

The SAC's which are defined as the slope of the normalized change in ultrasonic "natural" velocity as a function of stress were measured in a unidirectional laminate of T300/5208 graphite/epoxy. Tensile load was applied along the fiber axis with the ultrasonic waves propagating perpendicular to locked loop ultrasonic interferometer with the nominal frequency of the ultrasonic waves being 2.25 MHz.

Keywords: Composites; Ultrasonic Velocity; Nonlinear Elasticity

Abstract: The normalized change in ultrasonic "natural" velocity as a function of stress and temperature was measured in a unidirectional laminate of T300/5208 graphite/epoxy composite using a pulsed phase locked loop ultrasonic interferometer. These measurements were used together with the linear (second order) elastic moduli to calculate some of the nonlinear (third order) moduli of this material.

Abstract: Plate waves provided a different approach to probe the integrity of materials of laminate structure, and showed their potential application for large area inspections. Low-order modes of plate waves (Lamb waves) were excited and propagated in analuminum plate and a plate assembly, which was composed of two plates bonded by epoxy. By changing the boundary condition of the plate surface between transducers, the resultant amplitude variation of the received signals can be examined to locate unbonded areas between the plate and the epoxy layer, and/or between the plates. In the measurements, an unbonded area of less than 1 mm width has been detected. Experimental results and a possible model which includes out of plate particle displacement and mode conversions of this technique are discussed.

Keywords: Lamb Wave Application; Guided Acoustic Wave; Disbond Detection; Plate waves Application;

Abstract: For testing the capability of disbond detection of plate wave, low order modes of Lamb waves were generated and propagated across aluminum lap splice joints fabricated with various geometry of disbond. Specimens include joints bonded with different types of adhesive and built with and with out rivet rows. Amplitude as well as time-of-flight of transmitted waves were monitored while they were scanned parallel to the long dimension of a lap joint. It was observed that both these propagation properties were affected when wavescontinued their propagation through areas with disbond, and thus provided an approach, which locates disbonds by data comparison. Mode conversion from one symmetry to another was believed to occur at disbond and contributed to the changes of propagation properties. Results of the measurement are presented and compared with those obtained with standard ultrasonic c-scan.

Source:

Rolf Diederichs 01 Aug. 1996, info@ndt.net