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ASME J Nondestructive Evaluation. 2018;1(3):031001-031001-19. doi:10.1115/1.4039359.

Composite materials find wide range of applications due to their high strength-to-weight ratio. Due to this increasing dependence on composite materials, there is a need to study their mechanical behavior in case of damage. There are several extended nondestructive testing (ENDT) and structural health-monitoring (SHM) methods for the assessment of the mechanical properties each with their set of advantages and disadvantages. This paper presents a comparative study of three distinct damage detection methods (infrared thermography (IRT), neutral axis (NA) method based on optical strain sensor measurements, and terahertz spectroscopy) for the detection of delamination and temperature-induced damage in a simple glass fiber reinforced polymer (GFRP) beamlike structure. The terahertz spectroscopy is a specialized technique suitable for detecting deterioration inside the structure but has limited application for in-service performance monitoring. Similarly, the IRT technique in the active domain may be used for in situ monitoring but not in in-service assessment. Both methods allow the visualization of the internal structure and hence allow identification of the type and the extent of damage. Fiber optic sensors (especially fiber Bragg grating (FBG)) due to their small diameter and no need of calibration can be permanently integrated within the sample and applied for continuous dynamic strain measurements. The measured strain is treated as an input for neutral axis (NA) method, which as a damage-sensitive feature may be used for in-service monitoring but gives absolutely no information about the type and extent of damage. The results for damage detection based on proposed comparative studies give a complete description of the analyzed structure.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2018;1(3):031002-031002-9. doi:10.1115/1.4039691.

X-ray computed tomography (CT) is a powerful tool for industrial inspection. However, the harsh conditions encountered in some production environments make accurate motion control difficult, leading to motion artifacts in CT applications. A technique is demonstrated that removes motion artifacts by using an iterative-solver CT reconstruction method that includes a bulk Radon transform shifting step to align radiographic data before reconstruction. The paper uses log scanning in a sawmill as an example application. We show how for a known nominal object density distribution (circular prismatic in the case of a log), the geometric center and radius of the log may be approximated from its radiographs and any motion compensated for. This may then be fed into a previously developed iterative reconstruction CT scheme based on a polar voxel geometry and useful for describing logs. The method is validated by taking the known density distribution of a physical phantom and producing synthetic radiographs in which the axis of object rotation does not coincide with the center of field of view for a hypothetical scanner geometry. Reconstructions could then be made on radiographs that had been corrected and compared to those that had not. This was done for progressively larger offsets between these two axes and the reduction in voxel density vector error studied. For CT applications in industrial settings in which precise motion control is impractical or too costly, radiographic data shifting and scaling based on predictive models for the Radon transform appears to be a simple but effective technique.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2018;1(3):031003-031003-10. doi:10.1115/1.4039692.

The increased usage of carbon fiber reinforced plastics (CFRP) for primary aerospace structures involves dealing with the susceptibility of composite laminates to impact loads as well as the occurrence of barely visible impact damages. One special case among impact sources is the so-called blunt impact, which may cause damage primarily to the internal structure. Therefore, the assessment of debonding of stiffening elements in CFRP structures poses an attractive application case for structural health monitoring by guided ultrasonic waves. Wave propagation phenomena at impact damages as well as the signal processing utilized to extract a damage related feature (i.e., damage index (DI)) contribute to the sensitivity, and thus, to the reliability of structural health monitoring (SHM) systems. This work is based on data from the EU-funded project SARISTU, where a generic CFRP door surrounding fuselage panel with an integrated sensor network has been built and tested by introducing a large number of impact damages. Wave interaction of delaminations and stringer debondings of different size and morphology in omega-stringer stiffened structures are examined to highlight the factors contributing to the sensitivity. Common damage indicator formulations for the use with imaging algorithms, such as the reconstruction algorithm for the probabilistic inspection of damage (RAPID), are applied on data from various damage cases. Furthermore, the difference in detectability of delaminations and debondings as well as the implications on imaging algorithms is examined.

Topics: Delamination , Damage
Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2018;1(3):031004-031004-12. doi:10.1115/1.4039938.

Devices mounted on printed circuit boards (PCBs) are subject to temperature variations resulting from power switching and ambient temperature changes, and may be subject to random dynamic load histories from sources such as vibration. Since solder material is mechanically the most ductile part, fatigue failure may occur in solder joints. Health monitoring for fatigue life under field conditions is a key issue for improving availability and serviceability for maintenance. We have developed a failure precursor detection technology and a fatigue life estimation method for ball grid array (BGA) solder joints, based on a canary circuit. This method estimates fatigue failure life of an actual circuit by detecting failure connections in a canary circuit (a dummy circuit of daisy-chained solder joints). The canary circuit is designed to fail before the actual circuit under the same failure mode by using accelerated reliability testing and inelastic stress simulation. A feasibility study of the failure probability estimation method is conducted by applying the method to a PCB on which a BGA component is mounted. It is confirmed that the fatigue life under a thermal cyclic load can be estimated from a canary circuit, that estimation of fatigue life under a random dynamic load is feasible, and that the estimation results are consistent with results from actual random vibration tests. The proposed method is found to be useful for prognostic health monitoring of solder joint fatigue failure.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2018;1(3):031005-031005-7. doi:10.1115/1.4039939.

Condition monitoring and fault diagnosis techniques are increasingly used for assurance of machine condition and safe operation of the systems and structures. For this purpose, preliminary data about the machine performance are obtained from the system and signal processing techniques are used to extract features and final information regarding the machine performance. One type of the primary data that can be obtained from a system is the vibration responses of the system. Vibration responses of dynamical systems are usually nonstationary signals. In boilers and heat exchangers, generation and collapse of the vapor bubbles due to overheating can generate vibration in the tubes. These vibrations could be detected and identified in terms of location and time of occurrence for future maintenance and modification of the system and process. In this paper, a nonstationary vibration analysis method based on Morlet wavelet transform is presented for detection of onset of nucleate boiling (ONB) in a boiler tube. Comparison of the results by wavelet transform and visual observation of the ONB shows the capability and accuracy (less than 3% difference) of the proposed technique for detection of ONB in a boiler tube.

Commentary by Dr. Valentin Fuster

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