Nonlinearities Associated with Impaired Sensors in a Typical SHM Experimental Set-Up

Structural Health Monitoring (SHM) gives a diagnosis of a structure assessing the structural integrity and predicting the residual life through appropriate data processing and interpretation. A structure must remain in the design domain, although it can be subjected to normal aging due to usage, action of the environment, and accidental events. SHM involves the integration of electronic devices in the inspected structure that sometimes are Piezoelectric Transducers (PZT). These are lightweight and small and can be produced in different geometries. They are used both in guided wave-based and electromechanical impedance-based methods. The PZT bonding requires essential steps such as preparation of the surfaces, application of the adhesive, and assembly that make the bonding process not so easy to be realised. Furthermore, adhesives are susceptible to environmental degradation. Transducer debonding or non-uniform distributed glue underneath the sensor causes the reduction of the performance and can affect the reliability of the SHM system. In this paper, a sensor diagnostic method for the monitoring of the PZT operational status is proposed in order to detect bonding defect/damage between a PZT patch and a host structure. The authors propose a method based on the nonlinear behaviour of the contact PZT/structure that allows the identification of the damaged PZT and the geometrical characterization of the debonding. The feasibility of the diagnostic procedure is demonstrated by numerical studies and experiments, where disbonds were created by inhibiting the adhesive action on a part of the interface through Teflon film. The proposed method can be used to evaluate the sensor functionality after an extreme loading event or over a long period of service time.

Design and characterization of a smog chamber for studying gas-phase chemical mechanisms and aerosol chemistry

We describe here characterization of a new state-of-the-art smog chamber facility for studying atmospheric gas phase and aerosol chemistry. The chamber consists of a 30 m3 fluorinated ethylene propylene (FEP) Teflon film reactor suspended in a temperature-controlled enclosure equipped with two banks of black lamps as the light source. Temperature can be set in the range from −10 °C to 40 °C at accuracy of ±1 °C as measured by eight temperature sensors inside the enclosure and one just inside the reactor. Matrix air can be purified with NMHCs < 0.5 ppb, NOx/O3/carbonyls < 1 ppb and particles < 1 cm

Buckling Analysis of Stitched and Unstitched Glass Epoxy Composites

In this paper, the effects of delamination on critical buckling load failure of unidirectional E-glass/epoxy composite laminated plate consisting of delamination at the middle of the mid plane were studied. Experimental work was carried out to determine the buckling load for rectangular composite plates consisting of delamination for various aspect ratios ranging from 0.5 to 3. Stitched and unstitched laminates were prepared by introducing inplane delamination at the mid of the mid plane using Teflon film. These specimens were made with unidirectional fiber of orientation of (0o/-45o/45o/90o)s with single inplane delamination at the mid of the mid plane of through width and various lengths of 130 mm and 50 mm. The Plates were made to buckle experimentally by simply supporting the top and bottom edges, and sides were kept free. Buckling load has been found from the graph. The critical buckling load is found to be increased in the stitched laminates when compared with unstitched laminates and is decreased in both stitched and unstitched laminates as the aspect ratio increases. The experimental results were validated by using commercial finite element software of ANSYS.

NUMERICAL SIMULATION AND TESTING OF A COMPOSITE-STIFFENED STRUCTURE UNDER COMBINED BUCKLING LOADS

Prediction of the buckling behavior of structures is of great interest in the aerospace industry, and extensive research is taking place worldwide in that area. The current work concerns numerical simulation of the collapse test of a closed stiffened composite box subjected to compression followed by torsion. Numerical simulation is performed and the results are correlated with experimental findings. The objective is to validate the numerical model and detect any deficiencies of the modeling procedure. For this purpose, a series of quantities numerically predicted are directly compared with experimental ones: strains, displacements, deformation plots and load–displacement curves. The physical test article also contains artificial stringer–skin debondings realized via Teflon film inserts. The energy release rates are calculated at the debonding front using the virtual crack closure technique. The FE model is slightly stiffer than the actual structure but the numerical results are at a reasonable level of agreement with the experimental data.

A NOVEL HEMISPHERICAL MICROBOND SPECIMEN FOR EVALUATING THE INTERFACIAL SHEAR STRENGTH OF A SINGLE FIBER COMPOSITE

In this paper, a novel hemispherical microbond specimen is proposed for evaluating the interfacial shear strength of fiber-reinforced composites. A hemispherical microbond specimen was developed with the insertion of a pin-holed, Teflon film into a droplet matrix surrounding a single fiber. This experimental test offered more reliable strength data in the hemispherical specimen. Thus, the hemispherical microbond specimen is recommended to be suitable for evaluation of interfacial shear strength as a convenient alternative for the cylindrical pull-out test.