Damage identification is an important component for accurate lifetime predictions of any structure. In the case of a composite structure, however, damage can occur at several material scales: it can vary from micro damage, like fiber debonding or micro-cracking, to global damage such as buckling or delamination. These different material scales make damage identification difficult with a single type of sensing device. A single embedded optical fiber, causing little perturbation to the surrounding host structure, can multiplex hundreds of sensors, and furthermore, sensors measuring at different length scales. For example, short Bragg gratings can measure strain at given locations; long Bragg gratings can measure strain gradients; interferometric techniques can measure integrated strain along a given fiber length. The use of multi-scale measurements has been shown by the authors to improve the precision of damage identification. Still the treatment and fusion of these data is a non-trivial problem. This work presents a back propagation Neural Network algorithm used to fuse simulated multi-scale sensor data in order to identify damage. An analytical model of an isotropic plate subjected to a known load and specific forms of damage is used to train the network. The input data are: localized strain, localized strain gradient, and integrated strain measurement along a regularly spaced sensor network. This method is tested against a randomly generated set of damages. The combined use of multi-scale measurements and Neural Network analysis shows a great potential in damage identification for composite structures.
Fiber reinforced composites offer increased resistance to fracture as compared to isotropic materials. In addition, they have demonstrated great potential to support embedded sensor systems. However, to develop a truly reliable, embedded sensor for composites, the failure modes of such materials, including the influence of the embedded fiber sensor, must be known. Crack bridging by intact fibers is considered to be one of the most efficient mechanisms to slow down transverse crack propagation in a fiber reinforced composite. This paper presents non-invasive, direct measurements of bridging fiber stresses in a model epoxy/glass composite, using long gage length optical fiber Bragg gratings. Several central crack specimens, containing artificially bridged cracks, were fabricated and tested. The Bragg grating gage length of 12 mm permitted measurement of the force distribution in the reinforcing fiber extending from the crack surface to the far field region. A T-matrix simulation was used to model the grating response. Results from specimens involving both a strong and mixed interface are presented. The measured strain distribution in the bridging fibers compared well with previous analytical models. Discussion of the application of these results to structurally embedded sensors for damage detection is also presented.
This paper investigates the use of embedded optical fiber Bragg gratings to measure strain near a stress concentration within a solid structure. Due to the nature of a stress concentration (i.e. the strong non-uniformity of the strain field) the assumption that the grating spectrum in reflection remains a single peak with a constant bandwidth may not be valid. Compact tension specimens including a controlled notch shape are fabricated with embedded optical fiber Bragg gratings at identical locations but with different gauge lengths. The spectra in transmission varies between such specimens for given loading conditions. This variation is shown to be due to the difference in gauge length. By using the strain field measured on the specimen surface with electronic speckle pattern interferometry and a discretized model of the grating, the spectra in transmission are then verified analytically. Thus, by considering the non-uniformity of the strain field, the optical fiber Bragg gauge functions well as an embedded strain gauge near the stress concentration. Due to the distributed nature of the measurements within a specific gauge length, the optical fiber Bragg gauge has a large potential to measure debonding in fiber reinforced composites.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.