High Acoustic Emission Activity Research Articles

An evaluation of acoustic emission from fibre-reinforced composites

An analysis of acoustic emission(AE) from epoxy matrices of different amounts of hardener and model composites containing a glass bead, carbon and glass fibres has been carried out to identify the sources of emission. A few AE events generated by microcracking were observed for epoxy matrix near the final fracture strain. From microscopic and emission observations it was found that the emission was generated by interfacial debonding at the pole for the model composite containing a single particle of the glass bead, and that the source of AE bursts for a continuous single carbon fibre/epoxy composite was succeeding fibre fractures along fibre length. The high AE activity due to fibre fracture was observed for a composite consisting of a bundle of glass fibres. The total of AE events was in agreement with the number of fibre fracture counted with the aid of a microscope in a carbon/epoxy composite. The shear strength at the carbon/epoxy interface was evaluated by a critical length of the fractured fibres using the AE results.

Effect of environment, mechanical conditions, and materials characteristics on AE behavior during corrosion fatigue processes of an austenitic stainless steel

Acoustic emission (AE) behavior during corrosion fatigue (CF) processes has been studied in a commercial grade 304 austenitic stainless steel with special attention to the effect of environment, mechanical conditions, and materials characteristics. Precracked compact tension specimens were tested under cyclic tension-tension load, polarized potentiostatically in 1N H 2SO 4 + 0.5M HaCl or 1N H 2SO 4 solutions at room temperature. Valuable AE signals discriminated from environmental and mechanical noises by source location were monitored at the loading phase near the maximum load. The experimental results showed that the AE activity observed in corrosive environments increased significantly with the acceleration of crack growth rates, compared with that observed in air (inert environment). Detailed SEM observations found cleavage-like cracking, intergranular-like cracking, separations, etc. on the crack surfaces. The AE sources which provided the high AE activity during CF crack growth were shown to be such microcracking processes as cleavage-like cracking, intergranular-like cracking, and separations caused by the cooperation of environments (intergranular corrosion or hydrogen), mechanical conditions (triaxial stress), and materials characteristics (non-metallic inclusions or carbides precipitated along grain boundaries by sensitization). The effect of these three factors on the AE behavior and cracking mechanisms is discussed in detail.

Hypocenter distribution and focal mechanism of AE events during two stress stage creep in Yugawara andesite

More than a thousand acoustic-emission (AE) hypocenters were determined in a cylindrical andesite specimen under two-stage uniaxial creep at stresses of 204 and 214 MPa. Strains were monitored for 6 peripheral points at the middle part of the cylindrical specimen's wall. The strain data indicate gradual increase of nonuniform deformation during steady creep and strong intensification of the nonuniformity during acceleration creep and, therefore, biased stress distribution within the specimen. The correlation between dilatant strain and AE hypocenters was investigated for whether or not tensile cracks emit AE. The region with high AE activity shows only a small dilatant strain. This negative correlation between AE and the dilatant region may eliminate tensile cracks as possible AE sources. A composite focal-mechanism solution of local AE events, covering a wide solid angle of the focal hemisphere, indicates that shear fractures emit AE waves. The direction of the compressional axis in this solution shows a significant deviation from that inferred from the applied external force, suggesting that the local stress field is governed by preexisting weak zones that are, presumably, produced by tensile cracks within the specimen. AE hypocenters tended to form clusters during steady creep under the constant compressional stress. During acceleration creep caused by a small step increase of the external stress, the preceding clusters disappeared while a new cluster appeared in an incipient fault plane. This suggests that changes in seismicity pattern such as migrations or quiescences of swarm—important clues for earthquake predictions—may be caused by an instantaneous change in the tectonic-stress levels.

鋭敏化した304ステンレス鋼の腐食疲労過程におけるAE発生におよぼす環境，力学，材料条件の破面解析による検討

Corrosion fatigue (CF) tests were conducted for sensitized 304 austenitic stainless steel to investigate effects of environments, mechanical conditions, and sensitization on crack growth and AE (Acoustic Emission) behavior. The same experimental method as in the previous work conducted for solution treated specimens was employed. The crack growth rates of sensitized specimens were found to be accelerated in air as compared with those in the solution treated case. Those obtained in transpassive region (+1.1 V) and at corrosion potential (Ecorr) for sensitized specimens were more accelerated, due to superposition of the acceleration by sensitization and environmental effects such as intergranular corrosion, metal dissolution, or hydrogen evolved from cathodic reaction. The AE activity during crack growth of sensitized specimens was much higher than that of solution treated specimens through all the ΔK values in this study. This indicates that AE techniques are very useful to monitor crack growth processes under sensitized condition. SEM observations of the sensitized fracture surfaces revealed that many microcracks were nucleated at grain boundaries perpendicularly to the main crack surfaces at higher ΔK than ∼34 MPa·m1⁄2. These microcracks (separations) were concluded to be the main AE source during the crack growth in air under sensitized condition, since the numbers of the separations were proportional to the detected AE event counts. In addition to separations, intergranular-like cracks were observed on the sensitized fracture surfaces tested in transpassive region. The high AE activity (event count rate) at relatively low ΔK level was ascribed to the nucleation of these intergranular-like cracks which arose from intergranular corrosion and triaxial stress in the crack tip plastic zone. Intergranular-like cracks due to hydrogen and triaxial stress as well as the separations were observed on the sensitized fracture surfaces tested at Ecorr. The AE sources which provided the high AE activity during CF crack growth under sensitized condition were shown to be such microcracking processes as separations and intergranular-like cracks caused by the co-operation of environments (intergranular corrosion or hydrogen), triaxial stress (mechanical conditions), and sensitization.

鉱山における支保鋼材のAEに関する研究

Experiments were made to estimate the loads acting on the steel arch support by means of the acoustic emission (AE) technique. By the bending test of the I-steel for arch support, the following results were obtained:1) The impulsive AE signals of the I-steel began to generate from the level of half of its yielding stress and the AE activity became at a high degree during the process of its yielding. The AE activity of the I-steel formed into an arch at an ordinary temperature was very low. But the laggings on the steel arch displayed the high AE activity under a low load by the friction between laggings and steel or their collapse.2) In the bending test, it was able to estimate the stress history of I-steel by Kaiser's effect and to locate the sources of acoustic emissions. But it was difficult to estimate the stress history of the I-steel used in mine arch supports, because its AE activity was degenerated due to the bending in the formation of an arch and the noises of the dispersal of rusts from the surface of the specimen were generated.3) The yielding stress of the I-steel formed into an arch at an ordinary temperature decreased to nearly half of that of the original straight I-steel due to Bauschinger's effect.4) By normalization the formed I-steel recovered nearly 90% of the original yielding stress and the AE activity slightly.It will be able to estimate the direction of the load acting on the steel arch by the source location of AE generated between laggings and steel arch. And the I-steel formed into an arch at an ordinary temperature should be normalized.

Acoustic Emission Monitoring of Weldments

Abstract Weld tests were performed on low-carbon and high-strength, low-alloy steel weldments. The weldments were monitored with an acoustic emission detector during both the “in-process” and the “in-cooling” phases of the welding operation. Artificial defects were induced in several of the welds to promote cracking. Some high-strength weldments were designed to produce welds subject to high restraint. Nondestructive and destructive evaluation of the completed weldments revealed that high acoustic emission activity during in-cooling could be correlated with embedded defects and cold-cracking. Copious emission activity was also detected from unflawed weldments. Acoustic emissions were detected in high-strength steel weldments for periods up to 265 h. However, there was no clear relation between the duration of acoustic emission activity and cracking. On a per-electrode-deposited basis, high-stressed welds produced more emissions than low-stressed welds.