Advanced Materials Research Vols. 891-892

Abstract: The austenitic steel X6CrNiNb1810 (AISI 347) was investigated in isothermal total strain-controlled tests at ambient temperature and T = 300 °C in the LCF-and HCF-range. The phase transformation from paramagnetic austenite (fcc) into ferromagnetic α´-martensite ́(bcc) leads to cyclic hardening and to an increase in fatigue life. At 300 °C no α´-martensite formation was observed in the LCF-range and the cyclic deformation behavior depends basically on cyclic hardening processes due to an increase of the dislocation density, followed by cyclic saturation and softening due to changes in the dislocation structure. In the HCF-range an increase in fatigue life was observed due to ε- and α´-martensite formation. Measurements of the mechanical stress-strain-hysteresis as well as temperature and magnetic properties enable a characterization of the cyclic deformation behavior and phase transformation in detail. The changes in the physical data were interpreted via microstructural changes observed by scanning-and transmission-electron-microscopy as well as by x-ray investigations. Additionally electromagnetic acoustic transducers (EMATs) developed from the Fraunhofer Institute of Non-destructive Testing (IZFP) Saarbrücken were used for an in-situ characterization of the fatigue processes.

Abstract: The scattering of a fundamental symmetric wave mode by a notch on the blind side of weep hole is described in this paper. It will report on findings obtained from computational simulations to determine the effect and interaction of the impinging waves with the defect on the open hole located on the blind side of the incident wave. The finite element simulation results showed mode conversions of fundamental modes, leaky edge waves on the circumferential surface and source-like diffractions radiating from the tip of the notch and hole. These findings highlight the potential of applying this wave phenomenon to quantify defect located hard-to-inspect areas by positioning actuator and sensor in accessible regions of metallic structures and is relevant to the development and improvement of current techniques in non-destructive inspection of metallic structures

Abstract: In this study, three types of filament wound glass fiber reinforced epoxy composite specimens were tested according to the ASTM standards to evaluate preliminary, the acoustic emission wave velocity. The pipe specimens had 300.0 mm of length, 50.5 mm of diameter and 3.5 mm of wall thickness. Ten points were marked over the imaginary central circle on the pipe middle, from zero degree up to 90º with nine divisions of 10º each one and an additional point representing the crossing of plies. Next, there are positioned two collinear acoustic emission (AE) sensors along the length, positioned at 35 mm from middle, axially. After the specimen preparation, took place the HSU-NIELSEN or pencil lead break (PLB) tests to analyze the signal wave time for both sensors, by means of the 0.3mm – 2H graphite lead brake. The results were compared to considering the angular variation and the velocity for each winding angle. It was evident that the procedure above can determine with good precision the variations occurred on the acoustic emission velocity on filament wound composite pipes, regarding the three angular variations compared. This study was the parameter of control to proceed with the acousticemission evaluation over tensile tests in further studies to predict the fatigue and the damage propagation on this typology of laminates.

Abstract: This work presents a computational investigation into the scattering of edge guided waves travelling by a notch. To establish a good understanding of this scattering phenomenon, the analysis was conductedon a range of length scales. The finite element analysis indicate that the edge guided surface waves are scattered by the presence of a notch which resulted in a SH₀-like appearance wave radiating into the medium. This can be mistaken as a mode conversion of the fundamental lamb modes or even a source at notch tips. The phenomenon becomes harder to notice at higher frequency as increasing the frequency decreases the speed and both the bulk and surface waves travel at identical speeds. A clear understanding of this interaction furthers our knowledge in one of the most prominent interaction in the study of acoustic waves for structural health monitoring.

Abstract: Structural Health Monitoring (SHM) systems are developed to decrease the maintenance cost and increase the life of engineering structures by fundamentally changing the way structural inspections are performed. However, this important objective can only be achieved through the consistent and predictable performance of a SHM system under different service conditions. The capability of a Piezoelectric lead Zirconate Titanate (PZT)-based SHM system in detecting structural flaws strongly depends on the sensor signals as well as actuator performance. But service conditions can change the behaviour of transducers, raising questions about long term SHM system capability. Although having a clear understanding of the reliable sensor life is important for surface mounted systems, however, this is particularly critical for embedded sensors. This is due to the fact that opportunity for replacement of sensors exists for surface bonded transducers while for the embedded systems, sensor replacement is not straightforward. Therefore, knowledge of the long term behaviour of embedded-SHM systems is critical for their implementation. This paper reports a study on the degradation of embedded PZT transducers under cyclic loadings. Carbon/epoxy laminates with an embedded PZT were subjected to fatigue loading and their performance was monitored using Scanning Laser Vibrometery (SLV). The functionality of PZT transducers under sensing and actuating modes were studied. High and low cycle fatigue tests were performed to establish strain-voltage relationships which can be used to identify critical cyclic loading parameters (number of cycles and R value) under sensing and actuating modes.

Abstract: The Australian Defence Science and Technology Organisation (DSTO) is developing a variety of in-situ structural health monitoring (SHM) approaches for potential use in high value platforms across the Australian Defence Force (ADF). The implementation of SHM systems would allow the ADF to move from expensive interval based inspection and maintenance regimes for ageing platforms to more cost-effective condition-based approaches, and therefore reduce aircraft through - life support costs. One critical issue is determining the optimal means of supplying power to these in-situ SHM systems. To address this issue DSTO has developed a bi-axial vibration energy harvesting approach based on a vibrating spherical-mass, magnet and wire-coil transducer arrangement. It is important that the vibration energy harvesting devices themselves are resistant to fatigue and wear related damage as they may need to operate in service for many years. This paper examines work done on mitigating wear effects in vibration energy harvesting devices, with the goal of ensuring device longevity.

Abstract: The performance and reliability of Structural Health Monitoring (SHM) techniques remain largely unquantified. This is in contrast to the probability of detection (POD) and sensitivity of manual non destructive inspection methods which are well characterised. In this study factors influencing the rates of emission of Acoustic Emission (AE) signals from propagating fatigue cracks were investigated. Fatigue crack growth experiments were performed in 2014 T6 aluminium sheet to observe the effects of changes in crack length, loading spectrum and sample geometry on rates of emission and the probability of detecting and locating the fatigue crack. Significant variation was found in the rates of AE signal generation during crack progression from initiation to final failure. AE signals at any point in the failure process were found to result from different failure mechanisms operating at particular stages in the failure process.

Abstract: Under cyclic thermomechanical loading conditions, various damage mechanisms such as strain accumulation, creep cavitation, ageing, fatigue surface cracking etc. may take place in the material of a gas turbine blade. Depending on the loading conditions, all these effects can contribute to reduce the lifetime of the component. Subject of the present work is the development of a material model to describe the mechanical effects mentioned above, as well as the development of a lifetime model able to discriminate the different damage mechanisms.

Abstract: In this paper the TMF crack initiation behaviour of the single-crystal nickel-base superalloyMD2 is investigated and modelled. TMF tests were performed in both IP and OP for varying mechanicalstrain ranges in the [001] crystallographic direction until TMF crack initiation was obtained. Acrystal plasticity-creep model was used in conjunction with a critical-plane approach, to evaluate thenumber of cycles to TMF crack initiation. The critical-plane model was evaluated and calibrated ata stable TMF cycle, where the effect of the stress relaxation had attenuated. This calibrated criticalplanemodel is able to describe the TMF crack initiation, taking tension/compression asymmetry aswell as stress relaxation anisotropy into account, with good correlation to the real fatigue behaviour.

Abstract: Turbine discs in the gas turbine engine experience a wide range of stress and temperature during service. These result in distinct deformation mechanisms characteristic of different temperatures and stresses, and the cumulative effect of these on fatigue life is of much interest as turbines run hotter and thermal stresses rise. Thermo-mechanical fatigue tests are used to investigate performance at specific disc locations and have been performed on the alloy RR1000, a nickel based superalloy. This work describes a series of examinations to explore the interaction between temperature-loading conditions, deformation and failure mechanisms of an out-of-phase, largely compressive, thermo-mechanical cycle. Fatigue cracks initiated in the corners of the rectangular-sectioned test pieces and propagated to form approximately quarter-circle cracks. The interaction of the microstructure and deformation mechanism with the crack growth path has been investigated using Electron Backscatter Diffraction (EBSD) analysis complemented by Transmission Electron Microscopy (TEM) of specimens sampled from specific locations in the vicinity of a secondary crack.