Structural health monitoring (SHM) is being widely evaluated by the aerospace industry as a method to improve the safety and reliability of aircraft structures and also reduce operational cost. Built-in sensor networks on an aircraft structure can provide crucial information regarding the condition, damage state and/or service environment of the structure. Among the various types of transducers used for SHM, piezoelectric materials are widely used because they can be employed as either actuators or sensors due to their piezoelectric effect and vice versa. This paper provides a brief overview of piezoelectric transducer-based SHM system technology developed for aircraft applications in the past two decades. The requirements for practical implementation and use of structural health monitoring systems in aircraft application are then introduced. State-of-the-art techniques for solving some practical issues, such as sensor network integration, scalability to large structures, reliability and effect of environmental conditions, robust damage detection and quantification are discussed. Development trend of SHM technology is also discussed.

In applications that demand single-input dual-output (SIDO) or dual-input single-output (DISO) dc-dc converters, employing two separate single-input single-output (SISO) converters is a solution. However, the number of components is doubled, resulting in high overall cost. In order to reduce costs, this paper proposes a novel topology synthesis methodology, with which a variety of SIDO and DISO dc-dc converters with reduced components can be derived. The principle of topology synthesis states that integrated SIDO and DISO dc-dc converters can be easily developed from conventional SISO converters by replacing a diode with a basic cell inclusive of additional input/output port. The principle is effective for many SISO dc-dc converters, and as an example, topology synthesis based on buck, boost, buck-boost, Cuk, sepic, and zeta SISO converters is performed in this paper. In order to achieve better understanding of the proposed converters, the integrated SIDO Cuk converter is specifically analyzed and experimentally verified. In comparison with the conventional scheme of two separate SISO Cuk converters, good cross regulation is retained while the number of diodes, inductors, and capacitors is reduced in the proposed SIDO Cuk converter. In addition, zero-voltage-switching operation of one switch is achieved, contributing to lower switching losses. Finally, a prototype circuit with 48-V input and 156 V/1 A, 24 V/4 A outputs is built to validate the theoretical analysis.

Single-inductor multi-input multi-output (SI-MIMO) dc-dc converters are attractive in the engineering applications due to the advantage of high power density and low cost. In order to explore as many as possible SI-MIMO topologies, this article proposes a simple and effective topology derivation principle that only requires three steps. First, three basic cells consisting of a single inductor and multiple sets of unidirectional switches as well as inputs/outputs are proposed. Second, integrate the mentioned three basic cells with the inductor branch of the typical single-input single-output converters. Finally, implement the topology simplification by removing unnecessary switches/diodes. Based on the proposed principle, a large number of SI-MIMO topologies are derived from buck, boost, buck-boost, and noninverting buck-boost converters in this article. With more topology choices having different performance characteristics, it is very beneficial for engineers to gain an optimized design that a preferred one can be selected out after comprehensive comparison. As an example, topology comparison and selection among a family of single-inductor single-input dual-output converters is also conducted in this article. Besides, performance analysis, design considerations, and simulation/experiment results of the selected optimum topology are demonstrated in detail to verify its advantages.

Modern structures on aircraft make increasing use of large-scale composite structures. Quantitative damage monitoring for composites, including damage occurrence, number, localization, and size estimation, will help reduce maintenance costs, improve fleet management efficiency through condition-based maintenance, and potentially more rapidly enable new material systems and structural concepts by integrating health monitoring into the design itself. With the advantage of easily interpretable, intuitive, and accurate imaging result, the delay-and-sum imaging algorithm is frequently researched and applied to damage monitoring. However, when it is applied to multidamage monitoring in large-scale composites, the consumed time and computation resource are too much for pixel value calculation. Besides, due to the material anisotropy and existence of bolt holes and stiffeners in the researched complex carbon fiber composite laminate, propagation mechanism of Lamb wave is quite complicated, which makes the conventional localization result not accurate by delay-and-sum algorithm according to the point with pixel peak value. What is more, the imaging quality is deteriorated with concurrence of multiple damages, and thus, quantitative damage information cannot be extracted. Hence, the damage index merging algorithm is introduced for quick damage identification and damage merging in every subarea divided by piezoelectric sensor array. The delay-and-sum imaging algorithm is performed afterward only in subareas identified with damages, which significantly improves the efficiency of damage imaging for large-scale composites. The nonlinear normalization of pixel values compensates the deterioration of multidamage imaging quality. Then, a weighted average algorithm is introduced for more accurate localization. A further damage size level estimation is realized with probabilities by extracting the image pixel peak value. Experiments with six damages simultaneously on the complex carbon fiber laminate verify the effectiveness of the proposed quantitative multidamage monitoring algorithm with localization error below 2 cm and correct damage number and size level estimation.