Acoustic Emission Results Research Articles

Microcracking in Ceramics and Acoustic Emission

One of the limitations in the use of ceramics in critial applications is due to the presence of microcracks, which may arise from differential thermal expansion and phase changes, among others. Acoustic emission signals occur when there are abrupt microdeformations in a material and thus offer a convenient means of non-destructive detection of microcracking. Examples of a study of acoustic emission from microcracking due to anisotropic thermal expansion in low thermal expansion single phase ceramics such as niobia and sodium zirconium phosphate ceramics and due to phase changes in zirconia and superconducting YBa2Cu3O7-x ceramics are piesented, together with the case of lead titanate ceramics, which exhibits both a phase change (paraelectric to ferroelectric) and an anisotropic thermal expansion. The role of grain size on the extent of microcracking is illustrated in the case of niobia ceramics. Some indirect evidence of healing of microcracks on heating niobia and lead titanate ceramics is presented from the acoustic emission results.

The kinetics and micromechanics of hydrogen assisted cracking in Fe-3 pct Si single crystals

The kinetic and micromechanical behavior of hydrogen-assisted cracking (HAC) was evaluated on Fe-3 wt pct Si single crystals with {001} and {00l} orientations. This was accomplished by sustained load tests in gaseous hydrogen under various temperatures and in humid air at ambient temperature. Severe plastic deformation was observed to accompany the hydrogenassisted sustained load slow crack growth. The crack growth proceeded by a tunneling behavior in both orientations, indicating that plane strain conditions promoted the kinetic process. Microscopically, the crack propagated discontinuously with a 1-μm size instability along (1l0) directions, as revealed by fractographic and acoustic emission (AE) results. The crack growth rate was found to exhibit a plateau region with regard to the applied stress intensity. In this stage II regime, the growth rate increased with temperature in the lower temperature Arrhenius rate regime, with an apparent activation energy of 25 kJ/mol. After reaching its maximum at about 100 °C, the growth rate dropped down rapidly, and no slow crack growth was observed above 160 °C. The experimental observations were analyzed to show that the kinetics of HAC were controlled by hydrogen availability rather than by plasticity. Based on this, a transient kinetic model was applied to fit the data. The present interpretation of the crack tip stress field and the HAC nucleation site strongly implies that long-range diffusion of hydrogen over distances ranging from one to several microns is required. The kinetic aspects of this study are presented here with the underlying micromechanics being considered in terms of a decohesion mechanism

Fracture Behavior of Extren at Room Temperature and 77 K

Unlike most fiber-reinforced polymer composites, the measured inter laminar fracture energy of the glass-polyester Extren is significantly greater at 77 K than at room temperature (298 K). Composite microfailure is adhesive at room temperature but cohesive at 77 K. SEM and acoustic emission results suggest the enhanced interlaminar fracture toughness at 77 K is associated with increased matrix cracking which occurs at cryogenic conditions. The additional fracture surfaces increase the amount of released sur face energy.

Results of acoustic emission during mechanical and thermal loadings of vessel components and their fracture mechanical interpretation : Gries, H.; Waschkies, E. Nuclear engineering and design, vol. 106, no. 3, pp. 399–403 (Mar. 1988)

Results of acoustic emission during mechanical and thermal loadings of vessel components and their fracture mechanical interpretation : Gries, H.; Waschkies, E. Nuclear engineering and design, vol. 106, no. 3, pp. 399–403 (Mar. 1988)

Effect of γ precipitates on the martensitic transformation of β CuZnAl studied by calorimetry

1. i) The temperatures for forward transformation are always lower than standard ones in the domain of t ★ studied. This cannot be explained by a change in the composition of the β phase and therefore implies a relative stabilisation of β phase versus martensite. 2. ii) The temperatures for reverse transformation are lower than the standard ones for growing times t ★ ⩽ 20 s and higher for t ★ > / 40 s. A change of behaviour in this time interval is also present in acoustic emission results. It could be atributed to a change in the coherence of the precipitates. 3. iii) The cycle of hysteresis is always wider than the standard one. This is associated with an increase of friction. All the aforementioned effects are permanent. Therefore, it is possible to modify the martensitic transformation of shape memory alloys in a reproducible and controllable manner.

A fracture mechanics interpretation of acoustic emission results

The examination of the crack growth processes during thermal shock experiments performed with a corrosive medium on a cladded nozzle of the HDR-RPV was carried out by an acoustic emission and analysis system, that is able to automatically identify acoustic emission events as crack growth or as other processes (such as crack friction). This classification is based on a statistical model that considers different signal parameters (rise time, endurance, energy of the signal) for each acoustic event. More often, additional signal parameters are the position and time of starting. Using an evaluation system it is possible to pick out the events with certain parameters, using many different combinations, and plot and compare them against other results obtained from measurements or calculations. In this way the acoustic emission location method is confirmed by a general comparison with a nondestructive test result. In contrast, the timing of the signal start compared with the cyclic crack loading was unexpected. Most of the acoustic signals did not occur when the thermal load was at a maximum, but during an intervening period during the relief phase which is caused by reheating. In order to find the reasons for this, the signals are separated according to fracture mechanics models. From this it was discovered that the acoustic emission model and the fracture mechanics model are partly in agreement: that is, for the friction aspects. Finally the acoustic emission results suggest the following important and new conclusion regarding the mechanism of crack propagation: in the particular case of the nozzle experiments the crack growth is not primarily controlled by the load parameter delta-K, but occurs only during a short period of adverse temperature. This conclusion should be proved by means of further investigations on the HDR-RPV. The results are conclusive especially when applying the results from samples to components. This paper demonstrates that current ideas on fracture mechanics processes are only possible: • - through the use of acoustic emission testing, • - through a powerful evaluation and presentation system for the measured results which include acoustic emission, • - and not least through the simulation of the real-life conditions found in a nuclear power station (HDR).

Results of acoustic emission during mechanical and thermal loadings of vessel components and their fracture mechanical interpretation

AE measurements are presented which were performed at pressure vessel components during different loadings: thermal shocking, hydrotest, cyclic loading. The tests were executed with the IzfP AE system. With this it is possible to locate the AE sources and to identify two source mechanisms: crack growth and crack flank friction. The AE results are discussed and compared with the results of calculations about the residual stress in a cladded vessel wall: • — AE signals originating from crack growth processes are detected very sensitively. • — During thermal shock a lot of crack border friction occurs in consequence of compressive stresses on the crack flanks. • — However, during the hydrotests no crack border friction appears, because the crack flanks are mainly under tensile stresses.

Analytical compliance method for Mode I interlaminar fracture toughness testing of composites

An analytical compliance method has been successfully applied to Mode I interlaminar fracture toughness tests for laminated composites. A new double cantilever beam (DCB) specimen was designed to measure load-line crack opening displacement with high accuracy, and an interpolation function proposed to calculate the crack length and the energy release rate from the analytical results of compliance of the DCB specimen. A computer-aided testing system was also developed based on the analytical compliance method. Unidirectionally reinforced carbon/epoxy laminates were tested by the computer-aided testing system in which the measurement of crack length and fracture toughness was carried out automatically. The validity of the analytical compliance method was confirmed theoretically and experimentally. Results of acoustic emission (AE) indicated good correlation between the AE amplitude and fracture toughness. Constant Felicity ratio was also observed during stable crack propagation.

Acoustic emission characterization of flexural loading damage in wood

The paper is concerned with damage accumulation monitored by acoustic emission (AE) techniques during monotonic four-point bending tests on Douglas fir, oak and beech beams. The purpose of the study was to discriminate between the AE responses of the different materials by elementary analysis of ring-down cumulative counting. It appears that this simple method gives results that are species-dependent; in particular, high emission rates were found, to be typical of oak samples. The acoustic emission results are related to the observed structural mechanisms of failure of Douglas fir specimens tested in the radial or tangential directions.

Formation of aluminum oxide scales in sulfur-containing high temperature environments

A model alloy consisting of Fe-18 wt pct Cr-6 wt pct Al was used to study the formation of α-aluminum oxide under various oxygen (1 to 10−20 atm) and sulfur (10−13 to 10−6 atm) partial pressures at 900 °C. Acoustic emission results indicate that at constanti,e2051-01 (10−20 atm) the oxide scale became much more resistant to isothermal cracking with increasing sulfur potentials. In addition, adherence of the oxide scale to the alloy was also enhanced and the oxidation rates increased with increasing sulfur potentials. Using inert palladium markers, the diffusion processes in the growing aluminum oxide scale were studied. In environments where oxygen was the only oxidant present, the results indicated that inward oxygen diffusion is the predominant scale growth mechanism. In contrast, oxides grown in sulfur containing environments revealed a drastic change of the marker position, indicating that the oxide growth occurred predominantly by outward aluminum ion diffusion. Oxide scale morphologies are shown using scanning and transmission electron microscopy.

Influence of fibre orientation on acoustic emission from filament wound pipes

The effect of the winding angle of filament wound pipes internally pressurized was investigated by means of the acoustic emission technique. The acoustic emission results were correlated with the theoretical data of the state of stress at the matrix-fibre boundary. The experimental results showed that acoustic emission strongly depended on fibre orientation. Acoustic emission signals were mainly caused by two types of failure of the composite: debonding on the matrix-fibre interface and transverse cracks in the lamina. The influence of fibre rotation on the acoustic emission behaviour was also taken into account. It is stated that the significant start of acoustic emission is due to cohesive cracking at the matrix-fibre boundary when the normal stress, σ⊥ to the fibre overreached the critical value of 11.5 MPa for all considered fibre orientations.

Acoustic emission and mechanical properties of trabecular bone

The mechanical properties of trabecular bone and a simple plug prosthetic system have been determined over a range of displacement from 0.1 to 5 mm/mm. Acoustic emission, a technique which is capable of detecting dynamic processes within a material, was used to monitor the compression tests on the prosthetic system. It was found that the stiffness and strength of trabecular bone and the prosthetic system increased with increasing strain rate of testing. The acoustic emission results demonstrated that the improvement in mechanical behaviour at tho fast strain rates was accompanied by a decrease in the extent of the failure of the trabeculae. The technique of activation analysis has been applied to the results in order to identify the rate controlling fracture mechanism. Finally, the strength of the prosthetic system was correlated with the shear and compressive strengths of trabecular bone.

ACOUSTIC EMISSION IN Nb-Ti SUPERCONDUCTORS

In this paper, we present experimental results of acoustic emission in bare Nb-Ti single core superconductor and multifilamentary superconducting composite, which have undergone different excitation procedure in Si-rubber potted and dry modes.We have found that AE signals are chiefly generated by conductor motion or flux-ju-inping, but not by flux motion. The starting driving force for motion in Si-rubber potted sample is an order of magnitude higher than that in dry sample.Acoustic emission technique is a possible means for monitoring magnet quench as conductor goes through a series of microslip (or incomplete flux jump) before macromotion.

Indentation loading studies of acoustic emission from temper and hydrogen embrittled A533B steel

Isothermal tempering at 500 °C (within the region rendering low alloy steels susceptible to reversible temper embrittlement) induced acoustic emission activity in A533B steel during indentation loading. Samples, when sectioned, were found to contain small (∼10 μm long) MnS inclusions, some of which had debonded from the matrix material when they were near the indentations. Hydrogen charging prior to testing greatly enhanced the acoustic emission activity. It also resulted in the formation of small (∼20 to 200 μm) microcracks in samples tempered at 500 °C. These microcracks, when examined by optical metallography, appear to have propagated along prior austenite grain boundaries, consistent with fractographic observations of temper embrittlement in other low alloy steels. Many were nucleated by MnS inclusion debonding and all were confined to within a few hundred micrometers of the sample surface and within two or three indenter diameters from the indent. It is proposed that trace impurities (P, As, Sb, Sn) diffuse during the 500 °C temper to both the MnS inclusion interfaces and the prior austenite grain boundaries, reducing local cohesive strength. The tensile field created by the indenter debonds inclusions to form crack nuclei. Moderate acoustic emission results. In the absence of hydrogen these void nuclei may grow but do not coalesce to form observable cracks. The prior austenite grain boundaries, which in contrast to the dispersed inclusions can provide continuous crack paths, are not sufficiently temper embrittled to fracture without the assistance of hydrogen at these stresses. Hydrogen charging induces a high hydrogen concentration in a surface layer of the sample. This reduces further the grain boundary cohesion, and cracks initiated at inclusions are able to propagate along continuous grain boundary paths, generating additional energetic acoustic emission signals. This process can continue after unloading the indenter due to hydrogen diffusion to the residual stress field.

Failure mechanisms and acoustic emission amplitude distributions in automotive finishes

A complete automotive finish consists of a steel substrate with a phosphate coat and three paint coats. The complete finish and several sub-systems of the finish have been tested in tension and the acoustic emission monitored. The results of the acoustic emission, complemented with information from other techniques, such as scanning electron microscopy, have enabled the identification of a number of failure mechanisms and the strain ranges over which they occur. Analysis of the amplitude distributions in terms of overlapping Lorentzian peaks has demonstrated that each failure mechanism is associated with emissions of a characteristic amplitude.

Inspection of nuclear power component welds by in-process acoustic-emission monitoring

The author discusses the results of using in-process acoustic-emission monitoring on nuclear power plant piping welds. The technique was applied to laboratory as well as production welds. The acoustic emission results were compared with standard ndt results. This work was sponsored by the US Nuclear Regulatory Commission.