Summary When an earthquake strikes an area, unless the local infrastructure has a structural health monitoring system, it is difficult to obtain its health condition due to a lack of data. A smartphone could be competently used for many monitoring tasks to gather large amounts of data at a low cost. In this study, a smartphone is used as a sensor to obtain the interstory drift of buildings by recording a video of the ceiling with the front camera using a feature point matching algorithm. In addition, static and dynamic tests were carried out. In the static tests, the precision and error were investigated and compared with the reference sensor. In the dynamic tests, the displacement under a seismic wave was acquired by a smartphone and compared with the reference sensor, and the illumination condition was varied to test the robustness of the algorithm. The results indicated that feature point matching was suitable for calculating the displacement, and the smartphone was competent in monitoring the interstory drift during an earthquake.

Smartphones which are built into the suite of sensors, network transmission, data storage, and embedded processing capabilities provide a wide range of response measurement opportunities for structural health monitoring (SHM). The objective of this work was to evaluate and validate the use of smartphones for monitoring damage states in a three-dimensional (3D) steel frame structure subjected to shaking table earthquake excitation. The steel frame is a single-layer structure with four viscous dampers mounted at the beam-column joints to simulate different damage states at their respective locations. The structural acceleration and displacement responses of undamaged and damaged frames were obtained simultaneously by using smartphones and conventional sensors, while the collected response data were compared. Since smartphones can be used to monitor 3D acceleration in a given space and biaxial displacement in a given plane, the acceleration and displacement responses of the Y-axis of the model structure were obtained. Wavelet packet decomposition and relative wavelet entropy (RWE) were employed to analyze the acceleration data to detect damage. The results show that the acceleration responses that were monitored by the smartphones are well matched with the traditional sensors and the errors are generally within 5%. The comparison of the displacement acquired by smartphones and laser displacement sensors is basically good, and error analysis shows that smartphones with a displacement response sampling rate of 30 Hz are more suitable for monitoring structures with low natural frequencies. The damage detection using two kinds of sensors are relatively good. However, the asymmetry of the structure’s spatial stiffness will lead to greater RWE value errors being obtained from the smartphones monitoring data.

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The objective of this study is to evaluate and improve the accuracy and stability of a strain measurement method that uses the speeded-up robust feature (SURF) method to trace the displacement of feature points in microimages and obtain the strain in objects. The microimages were acquired using a smartphone with a portable microscope, which has a broad prospect of application. An experiment was performed using an unpacked optical fiber as the experimental carrier. The matching effect of the SURF method was analyzed in the microimage, and the M-estimator sample consensus (MSAC) algorithm was used to reject outliers generated by SURF. The results indicated that the accuracy of strain measurement using the proposed method is improved by modifying the feature point tracking method and measurement method. When compared with the fiber Bragg grating (FBG) data, the maximum standard error corresponded to 2.5 με, which satisfies the requirement of structural health monitoring (SHM) in practical engineering.

Composite lattice cores sandwich structures have shown obvious advantages in specific mechanical property and potential multifunctional integration. 3D-Kagome lattice core is regarded as a classic core configuration with relative optimal theoretical performance. However, it is still unavailable due to preparation problem of the overlap joint-node of composite cores. In this paper, a self-locking mortise-tenon joint method is presented to sucessfully fabricate the integrated 3D-Kagome cores with the expected mechanical properties. The out-of-plane compressive behaviour and energy absorption characteristics are experimentally studied. For the composite structures with various relative densities, three kinds of compressive curves are observed, following by different failure modes. An obvious bearing reinforcement emerges for the lattice cores with high relative density. As the length-to-thickness ratio of the core-rods increases, the initial peak strength and modulus of structures both decrease with a slowing rate. Though the mortises weaken the load-bearing capacity of core-rods, the active constraint of mortise-tenon joint suppresses an obvious degradation. The dominated failure more depends on the core-height than rod-thickness. The composite 3D-Kagome cores show more excellent mechanical properties than other similar structures, especially for the elastic strain before the initial peak stress. The semi-rigid power-wasting mortise-tenon joint and load-bearing characteristics of Kagome core together provide a large deformation tolerance at a relatively high stress level. All suggest that the presented 3D-Kagome lattice cores could be considered as a potential energy absorbing material.