"Structural Health Monitoring for Life Management of Aircraft"
J. D. Achenbach, S. Krishnaswamy, and I. M. Daniel, Northwestern University, Center for Quality Engineering and Failure Prevention
This project brings together the participants’ previous experience in two areas: structural health monitoring of aircraft structures and the mechanics of degradation and failure in composites. An intelligent health monitoring system includes a network of sensors which continuously monitors several critical parameters, as damage accumulates. Measurements of the chosen damage parameters are translated into a probabilistic assessment of structural integrity. The project has three primary components: (1) selection and implementation of suitable sensor technology for damage monitoring in composites; (2) investigation of damage evolution, and selection of the damage parameters; and (3) utilization of the measured damage parameters in probabilistic failure analysis.
Sensor Technology
For sensors intended to be permanently installed as part of a health-monitoring network, a simple technological solution that can generate unobtrusive, robust and inexpensive elements, has to be used. It is also desirable that the sensors be integrated with wireless telemetry for data uplink to a central processing unit. Lamb waves have been proven to be a powerful tool to investigate composite parts over extended areas. Their effectiveness lies in that they probe the entire thickness of the plate, over large distances. Here, the chosen technique is that of an array of piezoelectric sources, which can excite and detect single Lamb modes selectively. The figure shows a typical configuration of Lamb sensors for damage detection. Each sensor can serve either as generator or receiver, and its response can be tailored for increased sensitivity to a particular flaw size and type.
Figure 1: SAW sensors for flaw sizing
A related issue is the determination of the optimal number and location of the sensors. The sensors can be attached to the exterior of the monitored structure, or they can be embedded. Sensors should be distributed to probe structurally critical locations and their sensitivity should be adequate for detecting incipient damage. Critical areas of flaw initiation need to be identified and an appropriate distribution of sensors needs to be installed in such locations. At the same time, it is essential that sensors do not adversely affect structural integrity.
Damage evolution monitoring and selection of damage parameters
The objective of this part of the project is to characterize and monitor damage mechanisms and damage evolution in composite laminates and to select damage parameters relevant to the fatigue life of the part. Under quasi-static loading the failure sequence in composite laminates consists of matrix cracking in the transversely loaded plies, matrix cracking in the axial (longitudinal) plies, local delaminations at the intersections of these sets of cracks, and finally fiber breakage accompanied by ultimate failure. This failure sequence is abruptly accelerated if impact is the cause of damage. The damage parameters considered for the chosen sensing technique in conjunction with the investigated composite specimens must be derived from received signal amplitudes and wave group velocities.
Use of damage parameters in probabilistic failue analysis
Probabilistic failure methods are applied in conjunction with structural health monitoring so that damage can be assessed and predicted and components can be repaired or replaced. Quantified measures of reliability (provided by probabilistic methods) allow maximization of inspection benefits through optimization of the health monitoring technique. A systematic approach to reliability assessment for a structural component containing damage is illustrated below. As can be seen in the diagram, the underlying concept in developing an accept/reject criteria for a component is based on detecting and characterizing damage and evaluating it in terms of failure mechanics and a damage growth law. The aim is to determine whether damage in a structure will be sufficiently small that failure can be precluded with a high degree of certainty within a preset time interval.
The understanding gained from this work will set the stage for health condition monitoring combining fatigue reliability assessment and inspection methodologies for composite aircraft components, and will provide important guidelines on computational efficiency requirements for the reliability assessment in relation to sensor efficacy and the processing of sensor data. 
"Damage Tolerance Testing and Analysis Protocols for Full-Scale Composite Airframe Structures"
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