Andreas J. Brunner

Performance of integrated active fiber composites in fiber reinforced epoxy laminates

Active fiber composite (AFC) composed of lead zirconate titanate (PZT) fibers with interdigitated electrodes (IDEs) has been integrated into orthotropic glass fiber reinforced plastic (GFRP) laminates to characterize the performance of AFC as a smart material component in laminated materials. Monotonic cyclic tensile loading was performed on integrated specimens at different strain levels. The AFC output was monitored to determine the effect of applied strain level on the AFC performance. It was found that the AFC sensitivity degraded beyond strains of 0.20% and approached a minimum at 0.50% strain. The degradation in the AFC performance appears to be attributed to the dominating effect of PZT fiber fragmentation during testing, as opposed to depolarization. Acoustic emission (AE) monitoring was used to detect damage in laminates during testing and was correlated with crack evidence from microscopy observations during testing to characterize damage evolution in response to strain levels.

Piezoelectric Fiber Composites as Sensor Elements for Structural Health Monitoring and Adaptive Material Systems

Piezoelectric Active Fiber Composites (AFC) and Macro Fiber Composites (MFC) have the potential to provide various sensor functions for nondestructive test methods. AFC have been integrated into fiber-reinforced laminates as a first step towards structures with sensing capability. These developments constitute initial stages for developing adaptive composite structures or structures with integrated health monitoring system. So far, the use of AFC and MFC has been explored in selected nondestructive tests for defect detection in model composite systems on laboratory scale with e.g., Acoustic Emission, Acousto-Ultrasonics, and Electromechanical Impedance testing. The present article will focus on limitations and current prospects for structural health monitoring with AFC or MFC and discuss selected concepts and approaches.

Active fiber composites for the generation of Lamb waves.

Active fiber composites (AFC) are thin and conformable transducer elements with orthotropic material properties, since they are made of one layer of piezoelectric ceramic fibers. They are suitable for applications in structural health monitoring systems (SHM) with acoustic non-destructive testing methods (NDT). In the presented work the transfer behavior of an AFC as an emitter of transient elastic waves in plate-like structures is investigated. The wave field emitted by an AFC surface bonded on an isotropic plate was simulated with the finite-difference method. The model includes the piezoelectric element and the plate and allows the simulation of the elastic wave propagation. For comparison with the model experiments using a laser interferometer for non-contact measurements of particle velocities at different points around the AFC on the surface of the plate were performed. Transfer functions defined as the ratio of the electric voltage excitation signal and the resulting surface velocity at a specific point are separately determined for the two fundamental Lamb wave modes. In order to take the orthotropic behavior of the AFC into account the transfer functions are determined for several points around the AFC. Results show that the AFC is capable to excite the fundamental symmetric and antisymmetric Lamb wave mode. The antisymmetric mode is mainly radiated in the direction of the piezoelectric fibers, while the symmetric mode is spread over a larger angle. The amplitudes of the emitted waves depend on the frequency of the excitation as well as on the geometric dimensions of the transducer.

Integrated active fiber composite elements : Characterization for acoustic emission and acousto-ultrasonics

Active fiber composites (AFC) made from piezoelectric fibers show promising potential, not only as actuators integrated into smart composites but also as sensors. Selected sensor properties of AFC elements are investigated with methods from acoustic emission sensor characterization and verification and from acousto-ultrasonic testing. Model experiments with AFC elements integrated in simple composite structures show that the various approaches for analysis of acousto-ultrasonic data yield indications of damage accumulation.

Acoustic Emission Monitoring of Leaks in Pipes for Transport of Liquid and Gaseous Media: A Model Experiment

In order to explore potential applications for Active Fiber Composite (AFC) elements made from piezoelectric fibers for structural integrity monitoring, a model experiment for leak testing on pipe segments has been designed. A pipe segment made of aluminum with a diameter of 60 mm has been operated with gaseous (compressed air) and liquid media (water) for a range of operating pressures (between about 5 and 8 bar). Artificial leaks of various sizes (diameter) have been introduced. In the preliminary experiments presented here, commercial Acoustic Emission (AE) sensors have been used instead of the AFC elements. AE sensors mounted on waveguides in three different locations have monitored the flow of the media with and without leaks. AE signals and AE waveforms have been recorded and analysed for media flow with pressures ranging from about 5 to about 8 bar. The experiments to date show distinct differences in the FFT spectra depending on whether a leak is present or not.

Acoustic emission analysis and micro-mechanical interpretation of mode i fracture toughness tests on composite materials

Abstract Mode I fracture toughness tests on Double Cantilever Beam specimens from carbon-fibre and glassfibre reinforced polymer-matrix composites were monitored with Acoustic Emission (AE) and loaddisplacement traces and delamination lengths were recorded. AE characterized the kinetics of delamination propagation. The progress of AE activity and AE intensity with load and AE source location plots are used to determine the delamination onset on the microscopic and macroscopic scale. Energy dissipating processes initiated in the damage zone near the delamination tip. Low interface adhesion results in lower debonding stresses and larger damage zones compared with composites with good adhesion. Time-dependent linear location of AE sources yields the average length of the damage zone and the average speed of delamination propagation. Parameter analysis has been used empirically for identifying AE source mechanisms. A new classification software for transient AE waveforms permits identification of the source mechanism of individual AE signals. A micro-mechanical fracture model based on the AE results describes the contributions of microscopic matrix and interface mechanisms to the interlaminar fracture energy.

Fracture Behavior of GFRP Laminates with Nanocomposite Epoxy Resin Matrix

Nano-sized, functionalized organo-silicate fillers dispersed in the epoxy matrix are one approach that is investigated for improving the delamination resistance of fiber-reinforced composites. The variety of nano-silicate fillers available on the market, of processing conditions and the related characterization effort make it desirable to have a simple, easily analyzed screening method for both, nano-modified resins and laminates. An impact test using hail simulation equipment and visual assessment on glass-fiber laminates with nano-modified epoxy matrix yields rough indications of the effect of nano-modification on the fracture behavior of the specimens that correlate with more sophisticated macroscopic and microscopic characterization.

Damage evolution in wood: synchrotron radiation micro-computed tomography (SRμCT) as a complementary tool for interpreting acoustic emission (AE) behavior

Abstract Tensile tests of miniature spruce wood specimens have been performed to investigate the damage evolution in wood at the microscopic scale. For this purpose, the samples were stepwise tensile loaded in the longitudinal (L) and radial (R) directions and the damage evolution was monitored in real-time by acoustic emission (AE) and synchrotron radiation micro-computed tomography (SRμCT). This combination is of outstanding benefit as SRμCT monitoring provides an insight on the crack evolution and the final fracture at microscopic scale, whereas AE permits the detection of the associated accumulation and interaction of single damage events on all length scales with high time resolution. A significant drawback of the AE testing of wood has been overcome by means of calibrating the AE amplitudes with the underlying crack length development. Thus, a setup-dependent and wood species-dependent calibration value was estimated, which associates 1 μm2 crack area generating of 0.0038 mV in the detected AE amplitude. Furthermore, for both L and R specimens, AE signals were classified into two clusters by using a frequency-based approach of unsupervised pattern recognition. The shares of AE signals of both clusters correlate with the ratio of the relative crack area of the interwall and transwall cracks gained from the fractographic analysis of SRμCT scans.

Effects of Mesostructure on Crack Growth Control Characteristics in Z-Pinned Laminates

ABSTRACT Standard Mode I Double Cantilever Beam specimens for delamination testing of a unidirectional (UD) IM7/977-2 composite were Z-pinned with two separate blocks of Z-Fiber® reinforcement. The reinforced beam configuration was such as to provoke an unstable delamination, propagating between the two Z-pin blocks. Crack resistance curves for these specific geometry specimens of IM7/977-2 indicate that the unstable delamination cracks are arrested by the second Z-pin block, with the crack propagation resistance being dictated primarily by the Z-pinning density within a block. Acoustic emission analysis is used to interpret visual observations and other test data.

Damage evolution in wood – pattern recognition based on acoustic emission (AE) frequency spectra

Abstract Tensile tests on miniature spruce specimens have been performed by means of acoustic emission (AE) analysis. Stress was applied perpendicular (radial direction) and parallel to the grain. Nine features were selected from the AE frequency spectra. The signals were classified by means of an unsupervised pattern recognition approach, and natural classes of AE signals were identified based on the selected features. The algorithm calculates the numerically best partition based on subset combinations of the features provided for the analysis and leads to the most significant partition including the respective feature combination and the most probable number of clusters. For both specimen types investigated, the pattern recognition technique indicates two AE signal clusters. Cluster A comprises AE signals with a relatively high share of low-frequency components, and the opposite is true for cluster B. It is hypothesized that the signature of rapid and slow crack growths might be the origin for this cluster formation.