Several commercial CD-R and CD-RW optical recording disks are exposed to intense white light sources (Halogen Tungsten and High-Intensity Discharge) at different exposure times under three minutes. Main analog playback parameters for CD-R and CD-RW optical recording disks are identified and characterized. The contents of a test disk is recorded onto each CD-R and CD-RW disk using a commercial rewritable/recordable disk drive. For each disk, written marks are imaged under a Nomarsky/Bright field optical microscope before and after exposure. Using a dynamic tester carrier-to- noise ratio and timing jitter are determined before and after exposure. AZO-dye CD-R disks, cyanine-dye CD-R disks, and CD- RW disks show considerable reduction in written mark contrast after exposure. Pthalocyanine-dye CD-R disks do not show significant changes in the mark contrast after exposure. Dynamic tester results confirm the mark contrast results and give insight into playback behavior of exposed disks.
Extrinsic Fabry-Perot interferometric (EFPI) fiber optic sensors were used to characterize delamination size and location in laminated composite bemas. Six eight-ply glass/epoxy composite beams, each 26.04 cm long and 2.33 cm wide, were fabricated with five midplane delamination sizes ranging from 1.27 cm to 6.35 cm long. The five delaminated beams as well as undamaged beams were tested for their first five modal frequencies. The modal frequencies shifted with changes of delamination size and location. The EFPI fiber optic sensors measured identical model frequencies as piezoelectric ceramic sensors. However, EFPI fiber optic sensors showed more sensitivity and better signal-to-noise ratios. Analytical classical beam theory and finite element methods validated the EFPI modal frequency measurements. A feedforward backpropagation neural network predicted the size and location of a prescribed mid-plane delamination in the composite beam using the EFPI fiber optic sensor modal frequency measurements. Modal frequency data sets from classical beam theory were used for training and testing the network. The delamination size and location predictions from the neural network had an average error of 5.9% and 4.7% respectively.
Real-time determination of contact forces due to impact on composite plates is necessary for on-line impact damage detection and identification. We demonstrate the use of fiber optic strain sensor data as inputs to a neural network to obtain contact force history. An experimental study is conducted to determine the in-plane strains of a clamped graphite/epoxy composite plate upon low-velocity impacts using surface mounted extrinsic Fabry-Perot interferometric strain sensors. The plate is impacted with a semi-spherical impactor with various impact energies using the drop-weight technique. The impacts did not produce apparent damage in the composite plates. The significant features of the strain and contact force response are contact duration, peak strain, strain rise-time and full-width at half maximum. We have designed and built an instrumented drop-weight impact tower to facilitate the measurement of contact force during an impact event. The impact head assembly incorporates a load cell to measure the contact forces experimentally. The load cell data is used to train a three-layer feedforward neural network which utilizes the back-propagation algorithm. The output of the neural network simulation is the impact contact force history and the inputs are fiber optic sensor data in two different locations and time in 10 microsecond intervals. The efficiency and accuracy of the neural network method is discussed. The neural network scheme recovers the impact contact forces without using any complex signal processing techniques.
An experimental study is conducted to evaluate the performance of surface-mounted fiber optic strain sensors for delamination damage detection in eight-layer symmetric cross-ply Graphite/Epoxy composite plates. Teflon film and waxed paper inserts were embedded to model delamination experimentally. Extrinsic Fabry-Perot fiber optic sensors were used to detect such pre-existing delaminations. A fringe-order identification computer program for interferometric optical fiber strain sensors is used for automated strain detection. The responses of surface mounted Fabry-Perot optical fiber sensors due to real-time low velocity impact of rigid spheres with the delaminated and healthy composites were analyzed with respect to the first impact strain peak. The data was validated by measurements of out-of-plane acceleration using conventional piezo accelerometers.
An experimental study if conducted to evaluate the performance of different types of strain gauges for impact detection in eight layer symmetric cross-ply Graphite/Epoxy composite plates. The plates were impacted with rigid spheres using the drop-weight technique. The impacts did not produce apparent damage in the composite plate. The responses of calibrated surface-mounted extrinsic Fabry-Perot fiber optical sensors, a metalized polyvinylidene fluoride foil sensor, and a conventional electrical resistance rosette are compared. Characteristics of signal signatures from each sensor are demonstrated. Advantages and disadvantages of utilizing each type of strain sensor in the composite structure are discussed. The experimental strain responses are presented for various impact energies. The significant features of the strain response are contact duration, peak strain, strain rise time, and full width at half maximum. All of the sensors exhibited reproducible and similar results. The extrinsic Fabry-Perot fiber optical sensor performed superior with respect to sensitivity and accuracy of point measurement.