231 research outputs found
Chebyshev descriptors for SHM with acoustic emission and acousto ultrasonics
No full textPurpose
– The purpose of this paper is to study the feasibility on the use of alternative parameters for representing acoustic emission (AE) and acousto-ultrasonic (AU) signals, using a wavelet-based approach and the computation of Chebyshev moments.
Design/methodology/approach
– Two tests were performed, one on AE artificial signals generated on a CFRP plate and one on an AU setup used for actively detecting impact damage. The waveforms were represented using a data reduction technique based on the Daubechies wavelet and an image processing technique using Chebyshev moments approximation, to get 32 descriptors for each waveform.
Findings
– The use of such descriptors allowed in the AE case to verify that the moments are similar when the waveforms are similar; in the AU setup the correlation coefficient of the descriptors with respect to a reference data set was found to be linked to the delimitation size.
Practical implications
– Such a data reduction while retaining all the useful information will be positive for wireless sensor networks, where power consumption during data transmission is key. With having to send only a reliable set of descriptors and not an entire waveform, the power consumption is believed to be reduced.
Originality/value
– This paper is a preliminary study that fulfils a need for a more reliable data reduction for ultrasonic transient signals, such as those used in AE and AU
Gear tooth root fatigue test monitoring with continuous acoustic emission: Advanced signal processing techniques for detection of incipient failure
No full textThe phenomenon of fatigue in gears at the tooth root can be a cause of catastrophic failure if not detected in time. Where traditional low-frequency vibration may help in detecting a well-developed crack or a completely failed tooth, a system for early detection of the nucleation and initial propagation of a fatigue crack can be of great use in condition monitoring. Acoustic emission is a potentially suitable technique, as it is sensitive to the higher frequencies generated by crack propagation and is not affected by low-frequency noise. In this article, a static gear pair is tested where a crack was initiated at a tooth root. Continuous acoustic emission was periodically recorded throughout the test. Data were processed in multiple ways to support the early detection of crack initiation. Initially, traditional feature–based acoustic emission was employed. This showed qualitative results indicating fracture initiation around 8000 cycles. A rolling cross-correlation was then employed to compare two given system states, showing a sensitivity to large changes towards the final phases of crack propagation. A banded fast Fourier transform approach showed that the 110- to 120-kHz band was sensitive to the observed crack initiation at 8000 cycles, and to the later larger propagation events at 22,000 cycles. Two advanced data processing techniques were then used to further support these observations. First, a technique based on Chebyshev polynomial decomposition was used to reduce each wavestream data to a vector of 25 descriptors; these were used to track the system deviation from a baseline state and confirmed the previously observed deviations with a higher sensitivity. Further confirmation came from the analysis of wavestream entropy content, providing support from multiple data analysis techniques on the feasibility of system state tracking using continuous acoustic emission
Non-destructive evaluation of additively manufactured materials: developing a coupled acoustic emission and thermoelastic stress analysis approach
No full textA method combining two non-destructive evaluations techniques, acoustic emission and thermoelastic stress analysis, is proposed to monitor cyclically loaded monolithic and additive manufactured metallic samples for crack growth. Current literature was used to investigate additive manufacturing processes and their current quality control methods. A review of acoustic emission and thermoelastic stress analysis theory and recent applications is also provided. It was found that there was currently a gap in monitoring methods for additive manufactured parts under fatigue loading for crack growth due to internal defects. Acoustic emission and thermoelastic stress analysis have never been combined to monitor crack growth in monolithic or additive manufactured metals in such a way and would be ideal methods for crack growth monitoring under cyclic loading due to their inherent benefits. Thermoelastic stress analysis is a full-field non-contact non-destructive evaluation technique which can monitor surface stress fields in materials subjected to cyclic loading. Acoustic emission is ideal for monitoring fatigue damage due to its continuous monitoring capabilities of energy released by crack growth. The proposed method consisted of acoustic emission capturing continuously and saving any waveforms exceeding a set threshold while the thermoelastic stress analysis captured data at regular intervals. Acoustic emission energy was extracted from captured waveforms to provide crack initiation detection as well as a measure of crack severity. Thermoelastic stress analysis provided a crack tip location and enabled crack tip tracking as well as crack growth rate monitoring to complement the acoustic emission energy findings. Acoustic emission could not locate the crack because it only used a single piezoelectric sensor but could detect crack growth anywhere on the sample while thermoelastic stress analysis could give the crack location only if the crack was near the surface or within the field of observation. The method successfully detected cracks and tracked crack growth in monolithic aluminium samples, additive manufactured aluminium and additive manufactured titanium alloy samples under fatigue loading conditions. This method involves little modification to the part being observed, the surface must be painted with a thin coat of matt black paint for thermoelastic stress analysis, and a sensor must be attached to the part for acoustic emission. Acoustic emission energy detected crack growth before thermoelastic stress analysis in most tests however this was due to the experimental method, acoustic emission captures continuously while thermoelastic stress analysis is at regular time intervals. Issues with experimental method came to light in early tests but were addressed in the final tests using additive manufactured titanium samples. Future work includes testing additive manufactured samples with built-in subsurface defects and developing a chip-based system in order to make a smaller and cheaper monitoring system
Piezoelectric-silicone structure for vibration energy harvesting: testing and modelling
No full textMechanical vibrations from heavy machines, building structures or the human body can be harvested and directly converted into electrical energy. In this paper the potential to effectively harvest mechanical vibrations and locally generate electrical energy using a novel piezoelectric-rubber composite structure is explored. Piezoelectric lead zirconate titanate (PZT) is bonded to silicone rubber to form a cylindrical composite-like energy harvesting device which has the potential to structurally dampen high acceleration forces and generate electrical power. The device was experimentally load tested and an advanced model was verified against experimental data. While an experimental output power of 57 μW/cm3 was obtained, the advanced model further optimises the device geometry via a relative optimisation approach. The proposed energy harvesting device generates sufficient electrical power for structural health monitoring and remote sensing applications, while also providing structural damping properties for low frequency mechanical vibrations
Whither vortex tubes?
No full textWe review research aimed at the development of an analytical and numerical framework for tracking the evolution, in an incompressible viscous fluid, of scalar fields, called 'vortex surface fields' (VSFs), whose instantaneous isosurfaces always contain continuous vortex lines. A set of equations describing the evolution of VSFs starting from a known initial condition is proposed and discussed. Non-uniqueness in the initial-value problem is resolved with the introduction of evolution in a pseudo-time variable where the vorticity, frozen in real time, plays the role of an advecting field. A numerical method for following both the real and pseudo-time evolution is described and its regularization properties are discussed. Examples are given of following VSFs in a viscous Taylor-Green flow (Taylor and Green 1937 Proc. R. Soc. A 158 499-521). The prospects for extending these ideas to fully turbulent flows are discussed.MechanicsPhysics, Fluids & PlasmasSCI(E)EICPCI-S(ISTP)
A New (DSMC) Collision Model for Polyatomic Molecules
No full textWe present a collision model for polyatomic gases which is intermediate between those that use a simple classical representation of the molecule (Melville 1972, Pullin 1974, Pullin and Harvey 1976) and the statistical echange schemes of Borgnakke and Larsen (1975) and Pullin (1978). The energy exchange in collision is calculated from a simplified representation of a collision in a multi-dimensional space. Any number of degrees of freedom may be included, but we present it here for three translational and two internal degrees of freedom. Unlike in the statistical exchange schemes of Borgnakke and Larsen (1975) and Pullin (1978), the post-collision state is not selected directly from an equilibrium distribution, but arises from a pseudo-collision calculation with a random choice of pre-collision impact parameters. The scheme can be used as part of a two-class exchange scheme, as in the standard Borgnakke-Larsen model. In order to satisfy detail balance, the model requires that collision pairs be chosen with a Maxwell collision probability - equal propability for all collision pairs. This appeared to be a serious limitation of the new model, if it meant that only a linear viscosity law could be used in the DSMC calculations. However, since the development of 'collision-rate DSMC' (Macrossan, M. N. J. Comput. Physics, v173, p600, 2001) this restriction no longer applies. Maxwell collision probability can be used, while any viscosity can be any specified function of temperature
New research findings on non-proportional low cycle fatigue
No full textOne of the challenges regarding multiaxial fatigue damage predictions is non-proportional loading. Relevant studies have shown that these multiaxial loadings cause significant additional hardening and reduction in durability due to non-proportionality. Fatigue life predictions due to non-proportional loadings are based on an equivalent non-proportional strain range that considers a material constant related to additional hardening and a non-proportionality factor. In this paper an analysis of the non-proportional factor for three multiaxial loadings forming a square in γ/√3 – ε coordinates is carried out. One of the observations revealed by this analysis is the sensitivity of the non-proportional factor to variable shear strain rate
The early embryogeny of the primitive streak and the vitelline rampart in the chick blastoderm, 1955
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