Nonlinear acoustics for nondestructive testing is still an emergent technique in complex materials with localized cracks because linear effects can be cancelled using specific symmetries for the pre- and post-signal processing. The local nonlinear signature is accessible even in complex aeronautic structure by using the symmetry properties: i. e. time reversal invariance and pulse inversion associate to chirp-coded excitation. Excitation Symmetry Analysis Method [Dos Santos et al., JNLM, 2007] which is an extension of the pulse inverted method for extraction of third order nonlinearity is applied experimentally for the measurement of relative level of nonlinearity on a airplane steel bracket, on which visible crack was created by fatigue tests. This pre-processing method has been coupled to a 100-250 kHz bandwidth chirp-coded ultrasonic excitation, realized at various amplitude (A, A/2, 31/2A/2, etc.), and performed with DAKEL IDK09 200 kHz PZT sensors. Signal processing based on correlation measurements, allow an extraction of the nonlinear signature versus fatigue level. Experiments and data processing have been conducted with a 12 bit DAKEL-DTR system in the bandwidth 80-800 kHz and compared with a continuous 4 channel acoustic emission device.
Laser beam cutting and drilling using pulsed CO2- and Nd:YAG-laser systems is a very well-known and established industrial technique. Short pulsed lasers such as copper vapour lasers (CVL) have shown a potential in achieving precision machining (accurate circular drill holes, narrow kerf-widths, small heat affected zones) due to its high pulse power, high repetition rate, and good focusability.1
Copper VapourLasers in a MOPA-cham (MOPA, Master-Qsci11ator-Eower-mpIffier) configuration with low divergence can be used for the high precision machining of metals and ceramics. The fundamental interaction phenomena, ablation process and possible industrial applications are presented. The following paper relates the results and experiences in the operation of a Copper Vapour Laser MOPA Chain, consisting of an oscillator and up to three amplifiers, with the triggering points for these lasers exactly variable through a Master-Timing-System. In principle, a low-divergent laser beam is generated (5 1 1 and 578nm wavelengths) via an off-axis unstable resonator scheme, with precise synchronization of the Amplifiers producing average powers of over 140W. Due to the excellent beam focusability, peak power densities of some 10'°W/cm2 are achievable in a SOns pulse duration, which provides almost material-independent precision machining at high velocities. Beginning from the principles of beam-target reciprocation, the removing and cutting of metallic as well as non-metallic materials with copper vapour lasers will be described. Additionally, the potential of copper vapour lasers for industrial applications is illustrated through precision machining examples.
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