Damage monitoring of carbon fibre reinforced composites using acoustic emission

This work is aimed at practical damage assessment in large composite panels with a view to long-term monitoring of such components. Three sets of experiments have been carried out on a carbon fibre epoxy composite to assess the possibilities for source location and also to characterise the AE associated with impacts and with various damage modes. AE wave propagation was studied using a Hsu-Nielsen source on a one-metre square CFRC plate and a new approach using features of the Gabor Wavelet Transform was developed to determine the resulting wave speeds and modes. Next, a series of tests were carried out with low speed impacts and the time and frequency features identified as a function of the incident energy, size and material of the impacting particle. In particular, it was found that the time difference between two peaks in the Gabor Transform Contour Plot could be used as a measure of the impact contact time. Finally, a set of destructive tests (tension, tearing and bending) were carried out while measuring AE to identify fibre breakage, matrix cracking and fibre/matrix de-bonding. Using a classification scheme based on the modal analysis developed in the propagation studies, the proportions of the various damage modes could be assessed. The research concludes overall that modal AE analysis, aided by the novel signal processing schemes developed here is an efficient way of identifying and locating damage in CFRC panels in a way that reduces the reliance on energy methods and the consequent problems that this raises with calibration

Acoustic emission monitoring of wind turbine blades

Damage to wind turbine blades can, if left uncorrected, evolve into catastrophic failures resulting in high costs and significant losses for the operator. Detection of damage, especially in real time, has the potential to mitigate the losses associated with such catastrophic failure. To address this need various forms of online monitoring are being investigated, including acoustic emission detection. In this paper, pencil lead breaks are used as a standard reference source and tests are performed on unidirectional glass-fiber-reinforced-polymer plates. The mechanical pencil break is used to simulate an acoustic emission (AE) that generates elastic waves in the plate. Piezoelectric sensors and a data acquisition system are used to detect and record the signals. The expected dispersion curves generated for Lamb waves in plates are calculated, and the Gabor wavelet transform is used to provide dispersion curves based on experimental data. AE sources using an aluminum plate are used as a reference case for the experimental system and data processing validation. The analysis of the composite material provides information concerning the wave speed, modes, and attenuation of the waveform, which can be used to estimate maximum AE event – receiver separation, in a particular geometry and materials combination. The foundational data provided in this paper help to guide improvements in online structural health monitoring of wind turbine blades using acoustic emission

Durability and Smart Condition Assessment of Ultra-High Performance Concrete in Cold Climates

The goals of this study were to develop ecological ultra-high performance concrete (UHPC) with local materials and supplementary cementitious materials and to evaluate the long-term performance of UHPC in cold climates using effective mechanical test methods, such as “smart aggregate” technology and microstructure imaging analysis. The optimal UHPC mixture approximately exhibited compressive strength of 15 ksi, elastic modulus of 5,000 ksi, direct tensile strength of 1.27 ksi, and shrinkage of 630 at 28 days, which are characteristics comparable to those of commercial products and other studies. The tensile strength and modulus of elasticity in tension, dynamic modulus, and wave modulus show slight increases from the original values after 300 freeze-thaw (F-T) cycles, indicating that UHPC has excellent frost resistance in cold climates. Although porosity deterioration was observed in the F-T cyclic conditioning process, no internal damage (cracks or fractures) was found during imaging analysis up to 300 cycles. Since structures for which UHPC would be used are expected to have a longer service life, more F-T cycles are recommended to condition UHPC and investigate its mechanical performance over time. Moreover, continuum damage mechanic-based models have the potential to evaluate damage accumulation in UHPC and its failure mechanism under frost attack and to predict long-term material deterioration and service life

Classical wave experiments on chaotic scattering

We review recent research on the transport properties of classical waves through chaotic systems with special emphasis on microwaves and sound waves. Inasmuch as these experiments use antennas or transducers to couple waves into or out of the systems, scattering theory has to be applied for a quantitative interpretation of the measurements. Most experiments concentrate on tests of predictions from random matrix theory and the random plane wave approximation. In all studied examples a quantitative agreement between experiment and theory is achieved. To this end it is necessary, however, to take absorption and imperfect coupling into account, concepts that were ignored in most previous theoretical investigations. Classical phase space signatures of scattering are being examined in a small number of experiments.Comment: 33 pages, 13 figures; invited review for the Special Issue of J. Phys. A: Math. Gen. on "Trends in Quantum Chaotic Scattering

Low-Frequency Measurements of Seismic Moduli and Attenuation in Antigorite Serpentinite

Laboratory measurements of seismic moduli and attenuation in antigorite serpentinite at a confining pressure of 200 MPa and temperatures up to 550 °C provide new results relevant to the interpretation of geophysical data in subduction zones. A polycrystalline antigorite specimen was tested via forced oscillations at small strain amplitudes and seismic frequencies (millihertz to hertz). The shear modulus has a temperature sensitivity, ∂G/∂T, averaging −0.017 GPa/K. Increasing temperature above 500 °C results in more intensive shear attenuation ( urn:x-wiley:grl:media:grl58579:grl58579-math-0001) and associated modulus dispersion, with urn:x-wiley:grl:media:grl58579:grl58579-math-0002 increasing monotonically with increasing oscillation period and temperature. This “background” relaxation is adequately captured by a Burgers model for viscoelasticity and possibly results from intergranular mechanisms. Attenuation is higher in antigorite ( urn:x-wiley:grl:media:grl58579:grl58579-math-0003 at 550 °C and 0.01 Hz) than in olivine ( urn:x-wiley:grl:media:grl58579:grl58579-math-0004 below 800 °C), but such contrast does not appear to be strong enough to allow robust identification of antigorite from seismic models of attenuation only

The high frequency flexural ultrasonic transducer for transmitting and receiving ultrasound in air

Flexural ultrasonic transducers are robust and low cost sensors that are typically used in industry for distance ranging, proximity sensing and flow measurement. The operating frequencies of currently available commercial flexural ultrasonic transducers are usually below 50 kHz. Higher operating frequencies would be particularly beneficial for measurement accuracy and detection sensitivity. In this paper, design principles of High Frequency Flexural Ultrasonic Transducers (HiFFUTs), guided by the classical plate theory and finite element analysis, are reported. The results show that the diameter of the piezoelectric disc element attached to the flexing plate of the HiFFUT has a significant influence on the transducer's resonant frequency, and that an optimal diameter for a HiFFUT transmitter alone is different from that for a pitch-catch ultrasonic system consisting of both a HiFFUT transmitter and a receiver. By adopting an optimal piezoelectric diameter, the HiFFUT pitch-catch system can produce an ultrasonic signal amplitude greater than that of a non-optimised system by an order of magnitude. The performance of a prototype HiFFUT is characterised through electrical impedance analysis, laser Doppler vibrometry, and pressure-field microphone measurement, before the performance of two new HiFFUTs in a pitch-catch configuration is compared with that of commercial transducers. The prototype HiFFUT can operate efficiently at a frequency of 102.1 kHz as either a transmitter or a receiver, with comparable output amplitude, wider bandwidth, and higher directivity than commercially available transducers of similar construction

Transformation seismology: composite soil lenses for steering surface elastic Rayleigh waves.

Metamaterials are artificially structured media that exibit properties beyond those usually encountered in nature. Typically they are developed for electromagnetic waves at millimetric down to nanometric scales, or for acoustics, at centimeter scales. By applying ideas from transformation optics we can steer Rayleigh-surface waves that are solutions of the vector Navier equations of elastodynamics. As a paradigm of the conformal geophysics that we are creating, we design a square arrangement of Luneburg lenses to reroute Rayleigh waves around a building with the dual aim of protection and minimizing the effect on the wavefront (cloaking). To show that this is practically realisable we deliberately choose to use material parameters readily available and this metalens consists of a composite soil structured with buried pillars made of softer material. The regular lattice of inclusions is homogenized to give an effective material with a radially varying velocity profile and hence varying the refractive index of the lens. We develop the theory and then use full 3D numerical simulations to conclusively demonstrate, at frequencies of seismological relevance 3–10 Hz, and for low-speed sedimentary soil (v(s): 300–500 m/s), that the vibration of a structure is reduced by up to 6 dB at its resonance frequency

Low-Velocity Impact, Vibration and Shock Response of Unstitched/Stitched E- And S- Glass/Vinyl Ester Composites

In this study, the effects of through-thickness stitching on the dynamic properties, low-velocity impact and shock responses of stitched and unstitched E- and S- Glass/Vinyl Ester composites have been investigated. The stiffness and loss factor (inherent damping) properties are obtained from impulse-frequency response experiments using the hammer excitation method. Drop weight impact testing machine was used to carry out low-velocity punch shear tests on the composite specimens following ASTM D3763 Standard, and shock tube apparatus was used to determine center point deflection under blast loading. The stitching increased the dynamic flexural modulus by 13% for E- and 8% for S- Glass/Vinyl Ester composites but had minimal change in the case of loss factor. The stitching also demonstrated approximately 20% increase in total energy absorption and approximately 40% increase in fracture energy for E- Glass/Vinyl Ester while showing approximately 12% increase in total energy absorption and approximately 30% increase in fracture energy for S- Glass/Vinyl Ester. Under shock loading, the stitched E- Glass/Vinyl Ester specimens exhibited lowest center point deflection of 3.7 mm center point displacement, indicating greater resistance while the rest had values within the range of 4 mm to 4.5 mm. The average energy absorption up to the point of maximum deflection was higher for the unstitched specimens compared to their stitched counterparts