Application Cases

Experiment Name: Detection of Impact Fatigue Specimens in Composite Materials Using Nonlinear Acoustic Resonance Method

Experiment Purpose: To study impact fatigue damage in composite materials, research on detection using the nonlinear acoustic resonance method was conducted and compared with vibration acoustic modulation detection. Similar to the vibration acoustic modulation detection method, the nonlinear acoustic resonance method also falls under the category of non-classical nonlinearity. Its detection principle is based on the gradual accumulation of damage in the material, which leads to a decrease in the material's strength or stiffness, causing elastic wave distortion. As the amplitude of the excitation voltage increases, the resonance frequency of the material structure shifts to the left.

Testing Equipment: Voltage amplifier, charge amplifier, dynamic data acquisition device, function signal generator, computer, etc.

Experiment Process:

Figure 1: (a) Nonlinear Vibration Acoustic Modulation Experimental Setup; (b) Composite Material Specimen

Figure 1(a) shows the experimental setup for the nonlinear acoustic resonance method, and Figure 1(b) shows the composite material specimen. A sine wave signal is output from the function signal generator, amplified by a wideband voltage amplifier, and used to drive a 3W, 4Ω loudspeaker. The loudspeaker is attached to the center of the composite material specimen surface with paraffin wax to excite the specimen and generate vibrations. The response signal is captured by a miniature accelerometer with a mass of approximately 1.6g, which is also attached to the right edge of the composite material specimen at a distance of 28mm using paraffin wax. The received signal is amplified by a charge amplifier and then collected by a dynamic data acquisition device with a sampling frequency set at 10kHz. Finally, the collected signal is processed using a fast Fourier transform on a computer. The entire composite material specimen is placed on a sponge to simulate free boundary conditions.

After frequency response testing of the composite material specimen, the function signal generator's sweep range is reset to 50Hz on either side of the resonance frequency (a scan length of 100Hz) for nonlinear acoustic resonance detection. The excitation voltage amplitude of the loudspeaker is adjusted from 52.5V to 65.0V, with a point taken every 2.5V for repeated experiments on the specimen. After the detection experiment, the composite material specimen is placed on a heating platform to melt the paraffin wax and remove the loudspeaker and accelerometer. Subsequently, impact fatigue damage is introduced into the specimen, and cyclic testing is performed until the composite material specimen is damaged.

Figure 2: Resonance Frequency Response Spectrum of an Undamaged Specimen

Figure 2 shows the resonance frequency response spectrum of an undamaged composite material specimen. The frequencies corresponding to each vibration mode are 720Hz, 1360Hz, and 2350Hz, respectively. The vibration response amplitude is relatively high at 1360Hz. Therefore, to obtain a strong vibration response, 1360Hz is chosen as the resonance frequency f0 for this nonlinear acoustic resonance detection experiment.

Figure 3: Nonlinear Acoustic Resonance Spectrum of an Undamaged Specimen

Figure 3 shows the nonlinear acoustic resonance spectrum of an undamaged specimen. It can be seen that the amplitude-frequency curves at each excitation voltage can be clearly distinguished. The amplitude increases with the increase of excitation voltage, and the resonance frequency remains almost unchanged.

Figure 4: Nonlinear Acoustic Resonance Spectra of Damaged Specimens: (a) 2800 impacts at 3J; (b) 2400 impacts at 4J; (c) 1600 impacts at 5J; (d) 25 impacts at 7J

Figure 4 shows the nonlinear acoustic resonance spectra of damaged specimens corresponding to (a) 2800 impacts at 3J; (b) 2400 impacts at 4J; (c) 1600 impacts at 5J; (d) 25 impacts at 7J. As the excitation voltage increases, the amplitude of the amplitude-frequency curves also increases significantly. However, unlike the undamaged specimen, the stiffness of the composite material specimen changes due to impact fatigue damage. Under increasing excitation voltage, the resonance frequency exhibits a noticeable leftward shift, and the change in resonance frequency  increases with the increase of excitation voltage. Therefore, in acoustic resonance detection, the presence of damage in the specimen can be determined by observing whether the resonance frequency in the frequency domain spectrum of the received signal shifts.

Experimental Results:

Both nonlinear vibration acoustic modulation and nonlinear acoustic resonance detection techniques are based on non-classical nonlinear effects and can quickly and effectively identify impact fatigue damage in carbon fiber composite materials. As the degree of impact fatigue damage increases, the strength or stiffness properties of the material structure gradually degrade, leading to an increase in additional nonlinearity caused by the damage. In acoustic resonance detection, as the driving amplitude increases, the resonance frequency in the frequency domain spectrum of the received signal shifts to the left. In vibration acoustic modulation, both low-frequency vibration signals and high-frequency ultrasonic signals are excited simultaneously. Under the interaction of low-frequency vibration, high-frequency ultrasonic waves, and damage defects, modulation sideband nonlinear signals are generated in the frequency domain spectrum of the received signal.

Due to the different mechanisms of acoustic resonance detection and vibration acoustic modulation detection, different requirements are placed on sensors. For acoustic resonance detection, magnetostrictive sensors and loudspeakers are commonly used to excite structural vibrations. Significant structural vibrations are a necessary condition for analyzing changes in resonance frequency, and high excitation voltages are not required. For vibration acoustic modulation, piezoelectric transducers are the most commonly used excitation and reception sensors. During detection experiments, both low-frequency vibration and high-frequency ultrasonic waves need to be excited simultaneously. The normal excitation voltage in the high-frequency ultrasonic signal is sufficient for vibration acoustic modulation. However, for the low-frequency vibration signal, a higher low-frequency vibration excitation voltage is required to obtain a strong vibration response. Only under such experimental conditions can the contact surfaces in the damage area close more frequently, enhancing the modulation effect of high-frequency ultrasonic waves passing through the damage defect, thereby improving the detection effect of nonlinear vibration acoustic modulation.

Voltage Amplifier Recommendation: ATA-2088

Figure: Specification Parameters of the ATA-2088 High-Voltage Amplifier

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