In Situ Crack Propagation Monitoring In Aerospace Structures By Digital Sensor
Mario Marchetti, G. Pisciolini · WIT transactions on modelling and simulation · 1970
The aim of this paper was to develop a passive system sensor in order to analyse the rise and behaviour of crack propagation coming into the surface in a plane structural elements. The peculiarity of this system is to monitor the condition of the residual reliability of the structural element following a progressive damage, in situ and in the normal load condition which it has to provide. The sensor is made by a film which support a grid, row-column, of electric conductor which will be subsequently attached on the surface of the structural object. The propagation of the defect is noticed observing the progressive interruption of the electric contact on the surface row-column of the grid. To collect information of on-off type from the sensor it was realised a hardware interface device which collect it to a personal computer. The database is used by the managing software. The software display in a monitor the defect shape and the database values which come progressively during the process, as time, crack length, propagation speed and the propagation direction. The main goal of the sensor and of the entire system is toward of structural objects which have surface crack. INTRODUCTION The Fail-Safe (FS) design methodology take into account the initial defects of a structure. Their dimension must be equal to the sensitivity threshold of the Non Destructive Controls adopted in the inspection and the operative life must be equal to the range of the inspection [1]. This means that in FS methodology it is necessary to know the time-loading and the crack grow law. The materials must be choised to support long cracks without break them. Also they must Transactions on Modelling and Simulation vol 16, © 1997 WIT Press, www.witpress.com, ISSN 1743-355X 770 Computer Methods and Experimental Measurements have low crack grow speed. For a reliable FS design it is essential to have very precise data about the crack grow speed in relation with load, and also of its variation in time and of the most significant physic and environmental parameters. The necessity of precise data, related to structural object for which it is difficult to forecast mathematical traditional models, has been the inspiration of the present work. The in situ monitoring system, based on a digital sensor, can constitute valuable instrument for design because it provide interesting data in real time and with high accuracy. If it is employed by aeronautic and astronautic fields, it can be integrated in the on board avionic system which will on real time the post-process of the data. SYSTEM DESCRIPTION The system has been realised as a prototype and utilised, in this version, for the fatigue test. The general plan of the set-up of acquisition and visualisation system is shown in Fig 1. MOMTOR COLUMN DETECTOR SPECIMEN SOF DA/ARE