Hybrid BOFDA/BOCDA system for distributed static and dynamic strain measurements.

A robust ceramic strain gage has been developed for propulsion health monitoring which is capable of measuring both static and dynamic strains at temperatures up to 1400/spl deg/C. These thin film sensors are ideally suited for in-situ strain measurement in harsh environments such as those encountered in the hot sections of gas turbine engines. The sensors are non-intrusive and robust enough to withstand the high g loading associated with rotating engine components. Since gage thickness is considerably less than the boundary layer thickness, these sensors do not adversely affect the gas flow path through the engine. Results of recent laboratory tests have shown that these strain gages can meet many of the current demands of high performance propulsion engines. Specifically, static strain tests have been performed at temperatures as high as 1400/spl deg/C, which have resulted in a relatively large and reproducible piezoresistive response. In addition, high frequency dynamic strain tests were performed at temperatures up to 500/spl deg/C and at frequencies up to 2000 Hz to simulate conditions that would be encountered during high cycle fatigue. The results indicated that not only did these sensors survive severe testing conditions, including an applied strain of /spl plusmn/500 /spl mu//spl epsiv/, but that they were sensitive enough to isolate possible causes of high cycle fatigue. The results of both the static and dynamic strain measurements at elevated temperature using these ceramic strain gages are presented and the prospects of using these strain gages for structural health monitoring are discussed.

Fiber Bragg grating (FBG) sensors offer a significant advantage for structural health monitoring due to their ability to simultaneously monitor both static and dynamic strain while being durable, lightweight, capable of multiplexing, and immune to electro-magnetic interference. Drawing upon the benefits of FBG sensors, this research explores the use of a series of long-gage fiber optic sensors for damage detection of a structure through dynamic strain measurements and curvature analysis. Typically structural monitoring relies upon detecting structural changes through frequency and acceleration based analysis. However, curvature and strain based analysis may be a more reliable means for structural monitoring as they show more sensitivity to damage compared to modal parameters such as displacement mode shapes and natural frequency. Additionally, long gage FBG strain sensors offer a promising alternative to traditional dynamic measurement methods as the curvature can be computed directly from the FBG strain measurements without the need for numerical differentiation. Small scale experimental testing was performed using an aluminum beam instrumented with a series of FBG optical fiber sensors. Dynamic strain measurements were obtained as the aluminum beam was subjected to various loading and support conditions. From this, a novel normalized parameter based on the curvature from the dynamic strain measurements has been identified as a potential damage sensitive feature. Theoretical predictions and experimental data were compared and conclusions carried out. The results demonstrated the potential of the novel normalized parameter to facilitate dynamic monitoring at both the local and global scale, thus allowing assessment of the structures health.

Combined measurements of shear stress relaxation and differential dynamic storage and loss shear moduli G′ and G″ have been made on styrene-butadiene rubber (type 1502) containing 50 phr N299 carbon black and 10 phr Sundex 790 oil, both cured and uncured, and compared with similar measurements on the cured and uncured gum rubber. The range of temperature was −22.5° to 63°C, and of static shear strain 0.01 to 0.40; the maximum oscillatory shear strain was about 0.005 and the frequency was usually 0.64 Hz. Dynamic measurements of G′ and G″ with no superposed static strain and stress relaxation measurements at small strains on the filled samples could be reduced by frequency-temperature superposition with αT shift factors that were somewhat larger than those that applied to the raw or crosslinked gum. At about 0.6 Hz, the filler increased the storage modulus by about a factor of 5 for the cured sample and about 10 for the uncured; the moduli for the cured and uncured filled samples were almost identical. The order of rates of relaxation was unfilled uncured > filled uncured > filled cured > unfilled cured. At small static strains, the stress relaxation of the filled samples (both uncured and cured) was substantial, but the differential dynamic moduli G′ and G″ remained nearly equal to their values with zero static strain throughout the relaxation process. With increasing static strain γ, the strain-dependent relaxation modulus G(γ;t) decreased by as much as a factor of 3 for the filled, cured sample and 4 for the filled, uncured sample. The modulus G(γ;t) could not be factored into a strain-dependent and a time-dependent function. During stress relaxation of the filled samples (both cured and uncured) at large strains, the differential storage modulus G′(ω,γ;t) experienced a drop followed by slow recovery toward its initial value at 25°C; the loss modulus G″(ω,γ;t) was unchanged. For the filled, cured sample, complete recovery to the initial value G′(ω,0) (where 0 refers to zero static strain) was accomplished by removing the shearing stress and heating for one day at 63° followed by return of the temperature to 25°C. The results are interpreted in terms of a structure which combines a crosslinked (or, in the case of uncured, an entangled) polymer network and a network of carbon black particles. At small strains, stress relaxation is thought to be accomplished primarily by rearrangements of carbon particles and/or polymer molecules bridging them, without structural damage in the sense that the rearranged structure has the same properties as the original. These rearrangements are impeded by crosslinking in the cured vis-a-vis the uncured filled rubber. The kinetics of rearrangement may be governed by configurational changes of the polymer molecules since the temperature shift factors do not differ greatly from those for the gum rubber. At large strains, the particle network can be damaged but can regain its structure by a healing process which is accelerated at higher temperatures. The conclusions apply to the particular compound studied here and not necessarily to other rubber-black compounds, which according to the literature show great diversity of properties.

: Static and dynamic strain measurements which were taken during test stand operations of the gas dynamic laser (GDL) for the AF Airborne Laser Laboratory indicated that higher than expected vibrational stress levels may possibly limit the fatigue life of the laser structure. Particularly the diffuser sidewall structure exhibited large amplitude random vibrations which were excited by the internal gas flow. The diffuser structure consists of two layers of brazed stainless steel, AISI-347, panels. Cooling ducts were milled into the outer face sheet. These in turn are backed by the inner face sheet. So called T-rail stiffeners silverbrazed to the outer face sheets add the required stiffness and divide the sidewall into smaller rectangular plate sections.

A ceramic thin film strain gage has been developed for static strain measurements at temperatures up to 1400°C. These thin film sensors are ideally suited for in situ strain measurement in harsh environments, such as those encountered in the hot sections of gas turbine engines. However, the wide bandgap semiconductor used as the active strain elements in these devices also exhibited a large temperature coefficient of resistance (TCR). Thus, to reduce the apparent strain or thermally induced strain in these static strain sensors to an acceptable level, a novel self-compensation scheme was demonstrated using thin film platinum resistors placed in series with the active strain element. A mathematical model was developed and design rules were established for the self-compensated circuitry using this approach. Close agreement between the model and actual static strain results have been achieved using this approach, and reliable static strain measurements have been made over an extended temperature range.

Fiber optic sensor systems for static and dynamic strain measurements of geotechnical structures

We demonstrated a MHz speed wavelength-swept fiber laser based on the active mode locking (AML) technique and applied to interrogation system of an array of fiber Bragg grating (FBG) sensors. MHz speed wavelength sweeping of wavelength-swept fiber laser can be obtained by programmable frequency modulation of the semiconductor optical amplifier (SOA) without any wavelength tunable filter. Both static and dynamic strain measurement of FBG sensors were successfully characterized with high linearity of an R-square value of 0.9999 at sweeping speed of 50 kHz.

Experimental data was collected for structural identification (St-Id) of an old bridge: (1) static displacement and strain measurements taken under proof-load level, and; (2) multi-reference impact test (MRIT) data from one of the spans of a three span, cast-in-place reinforced concrete (RC) T-beam Bridge. MRIT was used to generate the modal data for computation of modal flexibility and displacement profiles. The St-Id procedure used during this application was designed to mitigate epistemic uncertainty in the data interpretation process. Successful results from MRIT demonstrated the reliability of applications for bridge condition assessment based on impact testing.

A mercury target vessel, designed to simulate some aspects of the eventual target design for the proposed Spallation Neutron Source (SNS) to be built in Oak Ridge by the Department of Energy, was used in a test at the Los Alamos Neutron Science Center (LANSCE) to study the strain induced from thermal shock of bombarding protons. In the SNS, intense thermal shock loads are expected to cause an enormous rate of temperature rise ({approximately}10{sup 7} K/s), with resulting pressure waves in the mercury that may lead to large stresses on the thin walls of the mercury target. To guide the mercury target design and to benchmark the computer design codes, transient strain was measured using fiber optic Fabry-Perot sensors. Twenty strain sensors were attached in various axial and transverse orientations to a cylindrical stainless steel target vessel containing mercury. The vessel was 10 cm in diameter, about 15 cm long, and with a 5-cm radius hemispherical shell welded to the forward end. The test was done at the LANSCE Weapons Neutron Research (WNR) beam facility on 30-31 January 1999. The sensors were attached with gauge lengths of about two centimeters, and were located in pairs in most areas, for redundancy and facilitation of data analysis. The 800-MeV proton deposition of 0.5--2.3 x 10{sup 13} over a full-width at half maximum beam size of {approximately}25 mm, produced axial strains peaking at a few microstrains, with transverse (hoop) strains more than an order of magnitude higher. We describe the experiments, including the sensors and measurement configuration, and discuss the strain data analysis.

We have proposed and constructed a system to multiplex fiber Bragg grating (FBG) strain sensors with same reflection wavelength, adopting the synthesis of optical coherence function technique (SOCF), which has realized dynamic strain measurement. The ability to control the coherence peak position of SOCF allows random access in the spatial domain. In this paper, the function of random access strain measurement has been realized by switching the coherence peak position at every FBG, synchronizing with FBG spectrum measurement, to use the sampling rate to the full. In addition, we have proposed and confirmed the novel method for improving FBG interval by simply modulating the LD intensity.

An instrument for simultaneous measurement of dynamic strain and temperature in a thermally unstable ambience has been proposed, based on fiber Bragg grating technology. The instrument can function as a compact and stand-alone broadband thermometer and a dynamic strain gauge. It employs a source wavelength tracking procedure for linear dependence of the output on the measurand, offering high dynamic range. Two schemes have been demonstrated with their relative merits. As a thermometer, the present instrumental configuration can offer a linear response in excess of 500 °C that can be easily extended by adding a suitable grating and source without any alteration in the procedure. Temperature sensitivity is about 0.06 °C for a bandwidth of 1 Hz. For the current grating, the upper limit of strain measurement is about 150 µε with a sensitivity of about 80 nε Hz−1/2. The major source of uncertainty associated with dynamic strain measurement is the laser source intensity noise, which is of broad spectral band. A low noise source device or the use of optical power regulators can offer improved performance. The total harmonic distortion is less than 0.5% up to about 50 µε,1.2% at 100 µε and about 2.3% at 150 µε. Calibrated results of temperature and strain measurement with the instrument have been presented. Traces of ultrasound signals recorded by the system at 200 kHz, in an ambience of 100–200 °C temperature fluctuation, have been included. Also, the vibration spectrum and engine temperature of a running internal combustion engine has been recorded as a realistic application of the system.

We propose and experimentally demonstrate the feasibility of an integrated hybrid optical fiber sensing interrogation technique that efficiently combines distributed Raman-based temperature sensing with fiber Bragg grating (FBG)-based dynamic strain measurements. The proposed sensing system is highly integrated, making use of a common optical source/receiver block and exploiting the advantages of both (distributed and point) sensing technologies simultaneously. A multimode fiber is used for distributed temperature sensing, and a pair of FBGs in each discrete sensing point, partially overlapped in the spectral domain, allows for temperature-independent discrete strain measurements. Experimental results report a dynamic strain resolution of 7.8 nε/√Hz within a full range of 1700 με and a distributed temperature resolution of 1°C at 20 km distance with 2.7 m spatial resolution.

A photonic approach for generating low frequency drifting noise, arbitrary and large frequency chirping rate (FCR) optical pulses based on the Kerr effect in the nonlinear optical fiber is theoretically analyzed and experimentally demonstrated. Due to the Kerr effect-induced sinusoidal phase modulation in the nonlinear fiber, high order Kerr pulse with a large chirping rate is generated. In the concept-proof experiments, the FCR of the mth Kerr pulse has been significantly improved by a factor of 2m+1. In addition, dynamic strain measurement along with a random fiber grating array (RFGA) sensor by using different order Kerr pulse is carried out for demonstrating a large strain measurement range with lower uncertainty sensing capability. Benefiting from the use of a single laser source and large FCR Kerr pulse, the system exhibits a 3.9 µɛ static strain measurable range, 0.24 µɛ measurement uncertainty by using -4th order Kerr pulse that has an FCR up to 0.8 GHz/ns. Note that the FCR of the chirped pulse could be further enhanced by using larger FCR chirped pulse seed or choosing higher order Kerr pulses.

In this paper, the Support Vector Regression (SVR) technique is use for speed estimation of a Three Phase Induction Motor (TIM) by the dynamic strain measurement analysis running at different load condition. The dynamic strain induced by mechanical and electrical forces is measure by fiber Bragg gratings (FBGs), installed inside the motor, in between two stator teeth. A frequency inverter driver is operating at 60 Hz to run the motor at 0%, 23.15%, 36.28%, 40.03% and 41.3% of the nominal load. The speed is estimate based on SVR with a maximum absolute error of 0.2%. The dynamic strain measured by FBGs has a good signal-to-noise ratio due to the intrinsic characteristics of optical sensors.

In this paper, an ultra-precision positioning stage is introduced. The finite element method is used to establish the theoretical model, and the strain test system of experiment is established. The system includes the static strain test and the dynamic strain test. The test system contains the software and hardware system. The software system includes the static strain measurement software of DH3816, the programmable constant temperature and humidity chamber control software TEMI300, and the dynamic strain test system developed from NI Labview. The hardware system contains industrial personal computer (IPC), piezoelectric (PZT) actuator, static strain gauge of DH3816, strain amplifier of YE1817, NI USB-6009 multifunction data acquisition card, strain gauge, programmable temperature and humidity chamber, and precision positioning stage. The experimental test is carried out to examine the theoretical analysis considering thermal effect. Experimental results and analytical results indicate that whether the stresses increase or decrease linearly or nonlinearly depend on the function of the temperature to the time during the transient response same as the stresses increase or decrease linearly during the steady response under uniform temperature change. The dynamic responses of platform are affected by the temperature change, and it is essential to consider thermal effect when analyzing compliant mechanisms.