Sparse Blind Deconvolution with Nonconvex Optimization for Ultrasonic NDT Application

In the field of ultrasonic nondestructive testing (NDT), robust and accurate detection of defects is a challenging task because of the attenuation and noising of the ultrasonic wave from the structure. For determining the reflection characteristics representing the position and amplitude of ultrasonic detection signals, sparse blind deconvolution methods have been implemented to separate overlapping echoes when the ultrasonic transducer impulse response is unknown. This letter introduces the ℓ1/ℓ2 ratio regularization function to model the deconvolution as a nonconvex optimization problem. The initialization influences the accuracy of estimation and, for this purpose, the alternating direction method of multipliers (ADMM) combined with blind gain calibration is used to find the initial approximation to the real solution, given multiple observations in a joint sparsity case. The proximal alternating linearized minimization (PALM) algorithm is embedded in the iterate solution, in which the majorize-minimize (MM) approach accelerates convergence. Compared with conventional blind deconvolution algorithms, the proposed methods demonstrate the robustness and capability of separating overlapping echoes in the context of synthetic experiments.

Air-Coupled, Contact, and Immersion Ultrasonic Non-Destructive Testing: Comparison for Bonding Quality Evaluation

The objective of this study is to compare the performance of different ultrasonic non-destructive testing (NDT) techniques for bonding quality evaluation. Aluminium-epoxy-aluminium single lap joints containing debonding in the form of release film inclusions have been investigated using three types of ultrasonic NDT methods: contact testing, immersion testing, and air-coupled testing. Apart from the traditional bulk wave ultrasound, guided wave testing was also performed using air coupled and contact transducers for the excitation of guided waves. Guided wave propagation within adhesive bond was numerically simulated. A wide range of inspection frequencies causing different ultrasonic wavelengths has been investigated. Average errors in defect sizing per ultrasonic wavelength have been used as a feature to determine the performance of each ultrasonic NDT technique. The best performance is observed with bulk wave investigations. Particularly, the higher frequencies (10–50 MHz) in the immersion testing performed significantly better than air-coupled testing (300 kHz); however, air coupled investigations have other advantages as contactless inspection. Whereas guided wave inspections show relatively lower accuracy in defect sizing, they are good enough to detect the presence of the debonding and enable to inspect long range. Even though each technique has its advantages and limitations, guided wave techniques can be practical for the preliminary in-situ inspection of adhesively bonded specimens.

Determination of fracture toughness for carbon fiber reinforced plastics free of the crack initiator using the acoustic microscopy

The fracture toughness which reflects change in the elastic deformation energy of the structural element with an increase in the crack area per unit at the onset of straining is one of the crack resistance parameters of carbon fiber plastics (CFRPs). When studying the fracture toughness, the position of the crack front is determined: both the initial one and that obtained as a result of crack growth. Currently existing test standards (STO TsAGI, ASTM D7905) determine the viscosity by the shear mode GIIc using the samples with a crack initiator. However, the method does not reflect the real conditions of crack initiation in CFRPs structures and can lead to a decrease in the accuracy of determining the load of crack initiation. A new technique of the fracture viscosity determination free of the standard delamination initiator has been developed in TsAGI. We present the results of developing the proposed methodology. The GIIc values were determined for a shear crack under three-point bending conditions after wedging. To determine the position and shape of the crack front, as well as to assess the dynamics of its propagation under subsequent loads, we used ultrasonic methods — ultrasonic flaw detection (ultrasonic NDT) and acoustic microscopy instead of the standard visual observation of the crack boundaries from the end surface of the samples. It is shown that acoustic microscopy at a frequency of 50 MHz provides determination of the crack front position in CFRP samples at a depth of 3.0 – 3.5 mm with a high resolution about 100 μm. The features of the crack growth under shear conditions are discussed. The results of the study show that high accuracy of acoustic microscopy in comparison with traditional ultrasonic NDT diagnostics is strongly sought for determining the shape of the cracks, as well as for analyzing the dynamics of crack growth and revealing the mechanisms of interlayer crack propagation in a composite material.