Application Cases

Experiment Name: Experimental Study on the Nonlinear Vibration-Acoustic Modulation Testing Technology for Impact Damage in Composite Materials

Research Direction:

Composite materials are widely used in the aerospace industry due to their advantageous properties such as low density and high specific strength. In practical engineering applications and during service, accidental impacts or collisions with external objects during flight can lead to internal damage such as delamination and fiber fracture, significantly reducing the residual strength and structural integrity of the composite materials. Due to the relatively low impact velocity and the energy-absorbing characteristics of composite materials, such damage is often nearly invisible on the surface. However, the internal damage may exceed the designed damage tolerance, making it a type of barely visible impact damage. How to effectively detect such invisible impact damage has garnered widespread attention in the field of nondestructive testing in recent years.

Experimental Content:

Vibration-acoustic modulation technology is a nonlinear ultrasonic nondestructive testing technique with high sensitivity to micro-defects. It involves simultaneously introducing a low-frequency signal with high amplitude (typically generated by an impact hammer, vibration exciter, or stacked piezoelectric ceramics) and a high-frequency ultrasonic signal (usually from piezoelectric wafers) into the structure under inspection. The signals are then collected using piezoelectric wafers or laser vibrometers. If defects exist in the specimen, low-frequency vibrations cause changes in the contact conditions of the defects, modulating the high-frequency ultrasonic signal. By analyzing the ratio of modulation sidebands to the main frequency, the presence of defects in the structure can be determined. This method is not limited by the shape of the structure, offers advantages such as low equipment requirements, fast detection speed, and higher sensitivity compared to conventional ultrasonic testing. It provides a new approach for remote testing of complex or large structures. Based on these characteristics, applying nonlinear vibration-acoustic modulation technology to detect low-velocity impact damage in composite materials holds significant research importance. Since this technique requires low-frequency excitation at the modal frequencies of the specimen to generate strong modulation effects, frequency selection significantly influences the detection results.

Testing System:

In nonlinear vibration-acoustic modulation testing, both high-frequency ultrasonic signals and low-frequency vibration signals need to be excited in the specimen. An Agilent 33220A signal generator is used to generate a continuous sinusoidal signal, which is amplified by an Aigtek ATA-2041 high-voltage amplifier and applied to stacked piezoelectric ceramics to produce low-frequency vibrations. The vibration frequency is selected based on the modal frequencies of the composite laminate, and the amplitude is increased stepwise from 10V to 100V in 10V increments. Once a stable vibration field is established, a Tiepie Handyscope HS5 multifunctional tester is used to emit continuous high-frequency ultrasonic signals, and the modulated signals are received by a PZT piezoelectric wafer. The experimental system is shown in the figure below.

Conclusion:

This study applies nonlinear vibration-acoustic modulation technology to detect impact damage in composite laminates and investigates the appropriate selection of excitation signals. The main conclusions are as follows:

(1) Nonlinear vibration-acoustic modulation technology generates modulation sidebands through the interaction of high-frequency ultrasonic signals and low-frequency vibrations in the structure. The modulation index $MI$ is used to assess the degree of modulation effect, thereby determining the presence of defects in the specimen. By appropriately selecting the excitation frequency and amplitude, this technology can effectively identify the existence of impact damage in composite materials, with results consistent with trends observed in ultrasonic C-scanning.

Figure: Modulation Index of the Specimen with Arbitrary Waveform Selection

(2) The modulation index of the specimen shows an approximately linear increase with voltage. Modulation effects vary under different vibration modes, and selecting appropriate ultrasonic frequencies and vibration modes aids in distinguishing defects. In this study, a specific ultrasonic frequency, the fifth vibration mode, and a relatively low amplitude of low-frequency excitation were sufficient to clearly identify defective specimens.

Figure: Judgment Coefficients for Various Modes

(3) Nonlinearities introduced by the material itself and the testing system can affect the experimental results. How to minimize such effects and how to detect and quantitatively analyze the extent of damage among laminates warrant further investigation and research.

Figure: ATA-2041 High-Voltage Amplifier Parameters and Specifications