With the aim of increasing the efficiency of maintenance and fuel usage in airplanes, structural health monitoring (SHM) of critical composite structures is increasingly expected and required. The optimized usage of this concept is subject of intensive work in the framework of the EU COST Action CA18203 “Optimising Design for Inspection” (ODIN). In this context, a thorough review of a broad range of energy harvesting (EH) technologies to be potentially used as power sources for the acoustic emission and guided wave propagation sensors of the considered SHM systems, as well as for the respective data elaboration and wireless communication modules, is provided in this work. EH devices based on the usage of kinetic energy, thermal gradients, solar radiation, airflow, and other viable energy sources, proposed so far in the literature, are thus described with a critical review of the respective specific power levels, of their potential placement on airplanes, as well as the consequently necessary power management architectures. The guidelines provided for the selection of the most appropriate EH and power management technologies create the preconditions to develop a new class of autonomous sensor nodes for the in-process, non-destructive SHM of airplane components.

The objective of this paper is to analyze the effect of small critical flaws on the strength of polycrystalline ceramic materials. For this purpose, a finite fracture approach based on the coupled criterion (CC) is used to describe the initiation of a crack near a stress concentrator. The initiation criterion combines both a stress and an energy condition. The required input fracture-mechanics parameters are the tensile strength and the fracture toughness. Both a blunt and a sharp geometry are studied. The size of the starter crack developing near the stress concentrator can be easily estimated in each case when the CC is fulfilled. Based on the calculations, if the size of the defect is smaller than the characteristic material length, numerical predictions reveal that the defect (either sharp or blunt) has no effect on the strength, reaching the intrinsic tensile strength of the material. This result is in a good agreement with experimental results obtained from the strength measurements of ceramic materials with controlled flaws. It is also shown that combining two fracture tests after introducing flaws with controlled sizes enables to identify the fracture parameters of the ceramic material.

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Criteria for predicting initiation of cracks in brittle materials like ceramics are based on two parameters: the material fracture toughness and the tensile strength. Standardized experiments exist to estimate the former. However, the tensile strength is often taken from experiments (mainly uniaxial bending) on specimens with various geometries and surface finish, usually tested under ambient conditions at a given loading rate. The reported strength is commonly the Weibull characteristic strength, which scatters due to the critical defect size distribution on the tested specimen. In this work, we propose a definition of the "inherent" or "intrinsic" tensile strength to be used in numerical models, making a distinction between extrinsic defects due to manufacturing and intrinsic ones relying on the microstructure. Our approach is based on the Finite Fracture Mechanics theory and the Coupled Criterion applied to small surface flaws and its influence on the measured (extrinsic) strength. Numerical results are compared with experiments on alumina reported in the literature. In addition, a model for the Petch law (strength vs. grain size) in polycrystalline materials is proposed using the Coupled Criterion, which predicts an initial crack length of increasing numbers of grains as the grain size decreases.

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