The above mentioned elastic constants describe the elastic behaviour in first approximation. A more precise description of the real elastic properties of a material is possible if third order elastic constants are used. Acousto-elastic constants finally are combinations of the elastic constants Young`s, shear moduli and the third order constants. As expected, these acousto-elastic constants are more sensitive to elastic properties than the moduli. In experimental investigations good correlations are found between the acousto-elastic constants and the percentages on solid solution phase of three work hardening and two heat treatable Al-alloys. Furthermore, linear correlations are found between the acousto-elastic constants and the yield strengths of the mentioned alloys as to be seen in Figure 1 (3). Similar investigations dealt with the characterization of the elastic properties of Al-based MMC`s with different types and volume percentages of reinforcements (4). The influence of fatigue damages on the third order elastic constants and on the acousto-elastic constants is shown in (5). Besides the elastic constants also their temperature dependencies are found to be suitable quantities to characterize microstructural states (6). A two frequency eddy current device has been developed for the in process evaluation of the tensile strenght of Al air bag components and for the simultaneous detection of surface defects (7). In Figure 2 the nondestructively evaluated strength values are shown in comparison with the tensile test results.
Figure 1: The acoustoelastic constant AEC33 of a longitudinal wave propagating perpendicular to the load for work hardening and precipitation hardening Al-Alloys with different content on solid solution phase (left) and with different yield strengths (right).
Figure 2: Nondestructively evaluated strength values in comparision with the tensile test results.
Complementary to the established techniques to evaluate stress states, ultrasonic techniques permit the evaluation of stresses in surface layers and in the bulk of components. One advantage of ultrasonic techniques is the possibility of a fast evaluation of stress states enabling a continuous analysis along traces to get information about the stress distribution and stress inhomogeneities. Different ultrasonic techniques have been developed to meet the requirements concerning the local resolution, the evaluation of one, two or three axial stress states, and the environmental conditions. Set-ups for the automated data aquisition and evaluation are available as well as sensors of different types and sizes. |
The stress states in plates and sheets cause sometimes difficulties during the sawing and cutting process. Hence, the stress state and the stress distribution is of interest in order to prevent the difficulties as well as unnecessary stress relief treatments. Ultrasonic stress analysis was applied to casted Al-plates. Using the skimming longitudinal wave, the surface stresses have been evaluated and using linearly polarized shear waves, the volume stress states have been characterized. The results, displayed in three-dimensional figures clearly show stress inhomogeneities (8,9).
It is known that the residual stress states due to welding strongly influence the dynamic and static behavior of a welded component. Hence, it is of significant importance to know the residual stress states after welding and after post-welding treatments. But in order to take advantage of the locus continuous ultrasonic techniques, some experimental investigations are needed because the quantitative evaluation of stress states using ultrasonic techniques presupposes the knowledge of the acousto-elastic constants. These constants have been evaluated for two Al-alloys frequently used by automotive industry. Using the evaluated acousto-elastic constants, the stress states in and around weldments in Al-sheets were mapped. The Figures 3 and 4 show the profiles of the two principal stresses acting parallel and perpendicular to the weld seam. Comparisons with the results of an established technique demonstrate the reliable applicability of ultrasonic techniques (10).
The interdependencies between the crystallographic texture and the propagation velocities of ultrasonic waves are known and have been widely used and published as are the interdependencies between the texture and the earing behavior. The ultrasonic SH-wave was found to be the most suitable ultrasonic mode to characterize the texture. The major advantage of using a SH-wave is the possibility to keep a distance between the ultrasonic sensor and the surface of the strip in order to prevent any surface damage. The distance between the ultrasonic transmitter and receiver is usually kept constant. Hence, the ultrasonic time-of-flight is the only measuring quantity. Using our prototype system, the reproducibility of the time-of-flight measurement is better than 0.3 . Measurements were performed at the positions 50%, 75% and 95% of the width of the strip in order to check on the homogeneity of texture. The absolute time-of-flight data are taken at stepwise changes of the ultrasonic propagation direction with respect to the rolling direction. As example, the relative change of the ultrasonic time-of-flight with the propagation direction of the SH-wave is displayed in Figure 5. The difference of the extreme values of the relative time-of-flight data is found to be in a very satisfying correlation with the earing parameters as shown in Figure 6. The earing parameters were evaluated by the manufacturer of the strips using the established technique 11. A similar ultrasonic system for on line texture analysis and characterization of deep drawability of ferritic steel strips is already in industrial use (12).
Relative change of the ultrasonic time-of-flight versus the angle between the sound propagation and the rolling direction of hot rolled aluminium sheets.
Correlation between the ultrasonic quantity and the earing parameter of hot rolled aluminium sheets.
In order to visualize the distribution of the density-Al2O3 short fibres in a AlSi12CuMgNi-matrix, a longitudinal ultrasonic sensor with a centre frequency of 50 or 80 MHz is manipulated across the surface of the part under test. Ultrasonic scattering occurs at the matrix / fiber interfaces and after significant amplifications, the peaks of the backscattered ultrasonic pulse amplitudes are detected. The amplitudes are digitized and colour coded, corresponding to their values. High values, corresponding with lighter colours in the so called C-scans indicate areas where the reinforcement in the matrix increases the ultrasonic backreflection and backscattering. In Figure 7 the inhomogeneous distribution of the reinforcement of a piston crown is shown. Using advanced computational possibilities, the visualization of the distribution of the reinforcement along the thickness direction of the component is also possible.
|Figure 7 :|
Distribution of Al2O3 short fibers in an automotiv piston crown with combustion bowl.
The density of MMC components can be calculated from the densities of the two constituents and their volume or weight percentage using the law of mixture. The Young's- and shear moduli do not follow the law of mixture. Calculations using the Voigt or Reuss model result in the extreme values; the average of both (approximately the Hill model) usually fits with experimental results. The change of the elastic constants with the volume percentage of short fibers are calculated for the above mentioned MMC. Using the calculated dependencies of the Young's and shear moduli on the volume percentage of short fiber reinforcement and the corresponding dependency of the density, the ultrasonic longitudinal and shear wave velocities in MMC`s are calculated as function of the volume percentage of Al2O3-fibers. The relative changes of the longitudinal and shear wave velocities with the volume percentages are found to be different: 1% change of volume percentage causes about 4.3 change of the longitudinal velocity and a change of about 7.8 of the shear wave velocity. Applying linearly polarized shear waves, the change of the volume percentage on Al2O3 short fibers in a piston crown is evaluated along a diametral trace. The vibration of the wave was parallel to the trace and perpendicular to it. As to be seen in Figure 8, there is no difference of the elastic properties of the material found along the directions of wave vibration. Hence, the piston is planar isotropic. Using the earlier mentioned calculations, the variation on the volume percentage of Al2O3 short fibers across the diameter of the piston is evaluated. 13.
|Figure 8: Variation of shear wave velocity and evaluated volume percentage of Al2O3 short fibers along the diameter of an automotive piston crown.|
|Aluminium pressure castings are frequently used semifinished products of high functional versatility. The burrs caused by the joining process of different streams of the molten metal passing through the matrix can be areas of poor mechanical properties. Reduction in strength and toughness and a bad fatigue behavior are the major drawbacks of the material in and around bad burrs. Up to now the quality of the burr is checked by common destructive tests using samples cut from the product. The nondestructive ultrasonic approach to characterize the burr and the material properties takes advantage of the microstructural changes in the burr and in its vicinity. Changes of the grain size and shape as well as the presence of second phases like aluminiumoxide in the matrix material influence the ultrasonic scattering. Hence, ultrasonic scattering technique is applied to visualize the burr. The recrystallization process changes not only the sizes and shapes of the grains in the region of the burr but also their orientations. Hence, the direction dependencies of ultrasonic waves are applied to determine the texture in the area. Using samples cut at different length positions from a pressure casted hollow girder, the position of the burr and its shape is visualized as shown in Figure 9. Together with the results of the texture analysis, displayed in Figure 10, the part of the length of the girder is determined in which the burr is not perfectly developed. The comparison of the mechanical properties of samples cut from that area with the data of samples cut from the remainding part of the girder with a perfectly developed burr confirms the non-destructive result 14.|
The paper was presented on the DGZfP regional working Group meeting in Hamburg in April '97
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