The aim of this work is the development and experimental implementation of the laser-ultrasonic method for CFRPs porosity assessment with one-side access to an investigated object. The scheme with the laser thermooptical generation and backward-mode piezoelectric detection of longitudinal acoustic waves is used. The proposed method is based on the measurement of the acoustic impedance of a composite specimen by using the value of the integral of the ultrasonic pulse reflected from an immersion layer-specimen interface. The existence of the relationship between the porosity content, ultrasound velocity, and material’s density allows one to assess the porosity of CFRPs by the measured acoustic impedance. The proposed method does not require plane parallelism of the input and output surfaces of the studied objects, as well as measuring the thickness of the object. This allows studying CFRP structures with complex shapes. This method may be of use for various types of composite materials, regardless of acoustical waves propagating features related to a periodical material’s structure. CFRP specimens with three reinforcement schemes and different average volume porosity contents were investigated. It is shown that the local porosity distribution in the studied CFRPs is nonuniform along the fiber stacking plane. The porosity content obtained by the laser-ultrasonic method was checked using the X-ray computer tomography. The porosity values, averaged from the laser-ultrasonic measurement results, coincide with the X-ray tomography data within the error limits. The method can be useful for quality control of obtained composite structures with the aim of modernization of technology processes and selecting optimal production conditions and also for the detection of changes in composite structures during their operation or during fatigue testing processes.

中文翻译：

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逆向模式法声阻抗测量的激光超声技术在CFRP层压板孔隙度评估中的应用

这项工作的目的是开发和实验实施用于 CFRP 孔隙度评估的激光超声方法，可从一侧访问研究对象。使用激光热光产生和纵向声波的反向模式压电检测的方案。所提出的方法基于通过使用从浸没层-试样界面反射的超声脉冲的积分值来测量复合试样的声阻抗。孔隙度含量、超声速度和材料密度之间存在的关系使得人们可以通过测量的声阻抗来评估 CFRP 的孔隙率。所提出的方法不需要研究对象的输入和输出表面的平面平行度，以及测量物体的厚度。这允许研究具有复杂形状的 CFRP 结构。该方法可用于各种类型的复合材料，而不管与周期性材料结构相关的声波传播特征如何。研究了具有三种增强方案和不同平均体积孔隙率的 CFRP 试样。结果表明，所研究的 CFRP 中的局部孔隙率分布沿纤维堆叠平面是不均匀的。使用 X 射线计算机断层扫描检查通过激光超声方法获得的孔隙度含量。从激光超声测量结果中取平均值的孔隙率值与误差范围内的 X 射线断层扫描数据一致。