Stress and crack monitoring during plasma spraying of TBC

Two types of process monitoring techniques are compared and discussed in this presentation. The first one is in-situ curvature monitoring, by which it was possible to evaluate the stress evolution during plasma spraying and separately identify the sources of stresses, i.e., the quenching stress and thermal stress as shown in Fig.1 (a). By changing the spraying parameters, it was possible to prepare specimens at largely different deposition temperatures, which resulted in significantly different levels of residual stresses. Also, it was found that the mechanical properties of the obtained YSZ coatings such as the elastic modulus are strongly dependent on the deposition temperature as shown in Fig.1 (b). Four-point bending test was conducted to these coatings, which clearly showed that the compressive residual stress effectively offset the applied tensile stress to initiate cracking in the YSZ coatings. Another method is based on acoustic emission (AE). Non-contacting laser AE sensors as shown in Fig.2 were used to detect cracking in YSZ coatings during spraying. Due to the intensive noise from the plasma spraying environment, extensive signal processing techniques have been developed to eliminate the noise in the frequency and time domains by using digital filtering and multi-threshold techniques. The obtained results so far indicate that the through thickness temperature gradient during spraying plays a major role in the formation of deep vertically segmentation cracks

Fatigue Life Prediction of Welded Joint by Microstructure-based Simulation

This paper proposes a numerical framework to predict fatigue life on welded joints by integrating several computational techniques. The framework consists of five steps: i) materials properties estimation; ii) welding simulation using thermo-mechanical finite element method; iii) macroscopic stress field analysis under cyclic loading; iv) mesoscopic stress field analysis using crystal plasticity finite element method (CPFEM); v) analysis of fatigue crack growth. The total number of cycles to failure is eventually obtained by the sum of initiation life calculated by CPFEM and propagation life calculated by X-FEM. A fatigue life of butt joint is evaluated by the proposed method. The results demonstrated the possibility of evaluating the fatigue life and its scattering by the proposed framework

Observation of Magnetic Edge State and Dangling Bond State on Nanographene in Activated Carbon Fibers

The electronic structure of nanographene in pristine and fluorinated activated carbon fibers (ACFs) have been investigated with near-edge x-ray absorption fine structure (NEXAFS) and compared with magnetic properties we reported on previously. In pristine ACFs in which magnetic properties are governed by non-bonding edge states of the \pi-electron, a pre-peak assigned to the edge state was observed below the conduction electron {\pi}* peak close to the Fermi level in NEXAFS. Via the fluorination of the ACFs, an extra peak, which was assigned to the \sigma-dangling bond state, was observed between the pre-peak of the edge state and the {\pi}* peak in the NEXAFS profile. The intensities of the extra peak correlate closely with the spin concentration created upon fluorination. The combination of the NEXAFS and magnetic measurement results confirms the coexistence of the magnetic edge states of \pi-electrons and dangling bond states of \sigma-electrons on fluorinated nanographene sheets.Comment: 4 figures, to appear in Phys. Rev.

Deformation and Anelastic Recovery of Pure Magnesium and AZ31B Alloy Investigated by AE

Extruded pure magnesium and AZ31B alloy were compressed parallel and vertical to extrusion direction. Deformation behavior was investigated in monotonous compression by acoustic emission (AE). Anelastic recovery behavior was observed in cyclic compression-quick unloading-recovery process. AE method was applied in each recovery process to investigate detwinning behavior at different strain levels. It was found that twinned samples vertically compressed easily detwin compared to parallel samples in both pure magnesium and AZ31B alloy. Compared to pure magnesium, twinning in AZ31B alloy is more stable and shows weaker psuedoelasticity. A model of strain dependence on cumulative AE counts from detwinning was proposed and fitting results are in good agreement with experimental data

Acquisition and Analysis of Continuous Acoustic Emission Waveform for Classification of Damage Sources in Ceramic Fiber Mat

Waveforms of acoustic emission (AE) events come close and sometimes overlap each other when AE activity is very high. Conventional AE measurement systems which handle discrete AE events are not suitable for this situation because miss-detection of AE event occurs frequently. A new AE measurement system named as Continuous Wave Memory (CWM) was developed to solve this problem by recording the AE waveforms continuously to hard disks for several hours throughout the testing time. This new system enabled multiple analysis of one waveform with different filtering parameters. Short time Fourier transform (STFT) gave the time–frequency–magnitude characteristic of continuous AE waveforms and useful information for evaluation of degradation of materials. In this study, the degradation of ceramic fiber mat during cyclic compression test and the effect of binder-addition were evaluated by this new system. STFT results clearly showed the classification of degradation of the mat; breakage of fibers was the main source in the early compression cycles and sporadic friction between fibers became the main source of AE in the later compression cycles. The effect of organic binder to prevent the degradation of the mat was also estimated. It was observed that the friction signal disappeared and the breakage signal weakened in the binder-added specimens. [doi:10.2320/matertrans.I-MRA2007850

Acoustic Emission Waveform Analysis in Composites

Many ceramic matrix composites have been investigated to enhance fracture toughness of ceramics. Especially in continuous fiber reinforced ceramics remarkable increase of toughness has been reported [1,2]. In such composites, it is very important that the mechanism of stress shielding can enhance fracture toughness, in which crack bridging and sliding of interface between matrix and fiber occur and then stress is transferred. The SiC fiber reinforced glass composite was used as a model material where friction between matrix and fiber provides stress transfer. The interfacial shear stress of this composite has been measured by the indentation method [3,4]. AE waveforms during this test were recorded. In this paper we try to identify the microfracture mechanisms by using AE radiation pattern which is a far-field displacement of AE waveform [5,6]. Quantitative parameters of microcracking such as size and mode in monolithic materials have been evaluated by using the advanced AE measuring system with 6 channels and analysis system [7]. However, there are many types of microfracture in these composites and mechanical model of each microfracture for AE is not clear. Here we try to classify microfractures into several types. And then fracture process of this composite will be discussed.</p