10th National conference on NDT

Conferencereports 3rd Colloquium on acoustic emission Zittau, West Germany 16-18 April 1980 Twenty four papers were presented at the Colloquium organised by the Physics Department of the Engineering High School in Zittau. The papers were on subjects of acoustic emission in metallic materials, application of acoustic emission in vessel testing, equipment, localisation and selection of acoustic emission signals from noise, and acoustic emission in non-metallic materials. Acoustic signals arising during bending fatigue have mainly two sources, crack ,propagation were carried out on fatigue surfaces, These two kinds of signals were separated by applying 'windows' to register acoustic events during the tensile and compression part of the fatigue cycles. Comparative measurements of acoustic emission, the potential-drop method and optical visualisation of crack propagation were carried out on samples used in fracture mechanics. The samples were high strength material and austenitic vessel steel. Although there were some difficulties in interpretation of results it is clear that such complex testing is an important aid in fracture analysis. Stress corrosion crack propagation in steel H60-3 was observed using acoustic emission. Various conditions of loading in bending and electrochemical potential values were applied. When testing samples from 60 year old grey cast iron with lamellar graphite it was found that the acoustic emission response to static and dynamic loading was highly reproducible and the Kaiser effect clearly observable. The possibility of predicting the final fatigue life from acoustic emission count rate was discussed. This possibility arises from a statistical interpretation of the results. It was found that the logarithm of the mean value of count rate is proportional to the normalized fatigue life. Results were shown on the observation of acoustic emission and mechano-

luminescence during tension of specimens from steel, copper and plastic. There was a good coincidence, in time, of acoustic emission activity and mechano-tuminescence. It is claimed that this method may help to discriminate noise signals more accurately. Experiments were conducted on the registration of acoustic emission during machining of metallic materials. The aim was better recognition of phenomena appearing during machining. To detect stochastic signals in the presence of noise and the selection of optimal discrimination level is important. A simple model was considered from a statistical point and it was assumed that impulse like signals are superposed on broadband noise. It is not only the accuracy of measuring time difference for the localisation of acoustic emission sources that is of importance as signals can reach the pickup unit as longitudinal, transverse, surface or Lamb waves. The possibilities of microprocessors were discussed with the aim of using the instrumentation in-line for registration the level of

signals, signal power and density as well as amplitude distribution. The microprocessor was also applied to steering the monitor in four channel acoustic emission instrumentation. Conclusions were drawn on microprocessor applications in nonmetallic materials and the importance of respective models for the analysis of results were stressed. The temperature cycles in porcellain manufacture lead to microcracks. This fracturing process during the cooling could be detected as acoustic signals. A temperature range was found in which the greatest cracking activity appeared. The acoustic burst signals were then classified according to their duration. This damage process was also observed in various high impact strength polymers, in glass fibre reinforced polyamid and possible damage mechanisms were discussed. The acoustic emission was registered during fracture and creep tests in samples of wood and hard fibre board and gave indications on the onset of microfractures.

Reference Proceedings of the Colloquium will be printed by the Ingenieurhoehschule Zittau. Further information can be obtained from: Dr. M. Leistner, Ingenieurhochschule Zittau, Zittau, Postfach 261, West Germany.

Z. Pawlowski

10th National conference on NDT Jadwasin, Poland 24-26 April 1980 Fifty five papers were presented at the conference and about 180 delegates had the opportunity to discuss the papers at the plenary sessions and at a poster session. Seven papers were devoted to an extensive presentation of the topics of the 9th World Conference held in Melbourne last year. Almost 50% of the papers delivered were on ultrasonics. Other subjects treated were on radiography, infra-red, acoustic emission and various methods of N D T . 1

Ultrasonics Ultrasonic testing procedures and measurements are factors which contribute to the improvement in reproducibility of results obtained. For educational purposes written ultrasonic procedures were presented in relation to the cali-

NDT INTERNATIONAL. DECEMBER 1980

bration of instruments for longitudinal and shear waves, velocity measurements, attenuation measurements, transfer loses and flaw size determination using DGS diagrams. 22 The correlation between the strength of PMM-Epoxy resin joints and the reflection coefficient of ultrasonic waves at the interface was found to be dependent on frequency. It was claimed that, according to the model of joint used, the gradient of the relationship between the strength and the reflection coefficient decreases with the frequency of ultrasonic

waves.45

The lack of bonding in valves for combustion engines was detected using ultrasonic waves reflected at the interface between the stellite layer and

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the base material. A lack of bonding equivalent to 2mm diameter could be found. 46 Similar testing of valves for Diesel engines was carried out using a mercury path, 9.5 mm thick, between the 8 MHz probe head and the rotating valve. Cracks in the carbonized and tempered layer of the valve run parallel to the surface and had a thickness between 0.05 and 0.4 mm and the length between 2.5 and 12 ram. When the Orenburg pipeline was inspected an ultrasonic atlas of weld flaws was made which consisted of 47 punched cards. Each card contains the informarion on the weld, testing condition, system of registration and results of ultrasonic, metallographic and radiographic examinations. 23 According to the Polish standard on ultrasonic weld testing the decrease of the cross section of the weld due to the presence of flaws is the basis for weld quality classification. A statistical analysis of correlation between ultrasonic findings and those of radiographic ones shows that the correspondence between both approaches depends on the type of flaw. as A multitransducer ultrasonic probe head for testing welds can detect flaws during one parallel movement along the weld instead of making the zig-zag motion. On each of the two plastic edges there are three 4 × 4 mm transducers, 2.5 MHz, connected in parallel. The three transducers generate the waves at angles of 62 ° , 65 ° and 68 ° respectively. Three other transducers serve to receive the reflected waves. As a result, a beam width can be obtained of 25 mm with a 6 dB drop at a distance of 50 mm. 27 It was shown that in the case of a multitransducer probe head for testing welds, 16.5 m thick, the detectability of artificial holes can differ with up to 3 dB from that obtained with the single probe head. s9 The length of a flaw can be determined by moving a probe head along the flaw and observing the length at which the echo amplitude drops to a prescribed level, eg 6 dB. In general, this probe movement is larger than the flaw and depends on the chosen level decrease of the echo amplitude as well as the distance between the flaw and the probe head. In these cases some corrections are needed. Statistical analysis of the elastoacoustic effect in grey cast iron have confirmed that the decrease of longitudinal wave velocity with the

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pipes of irregular shape from thermoplastic materials and it was shown that cracks 0.3 - 0.4 mm wide and 15 - 20 mm deep could be detected. iv

applied tensile stress depends on the cast iron grade, the decrease being larger for cast iron of lower strength. 42 The velocity of longitudinal, transverse and surface waves changes with the applied stress. Therefore, when stress is determined from ultrasonic velocity at various temperatures, corrections have to be made. The temperature changes of the velocity are of the same order as the changes due to the applied stress, eg, for the steel the increase in temperature by I°C in the range - 4 0 ° - + 60°C causes the increase 0.64 m/s, 0.46 m/s and 0.30 m/s for L, T and S waves respectively.a1 An ultrasonic transducer of resonant frequency 20 MHz was tuned by capacitance and inductance methods and was applied to measure the attenuation at 5, 10 and 15 MHz in steel having various grain s i z e s . 26

Acoustic emission The detection of cracks in welds was accomplished with acoustic emission. Welded samples of low alloy steel were subjected to hear treatment and temperature changes and acoustic emission response were measured in the period of about 10 hours after welding. The burst signals connected with the cracking were observed in the temperature range between 500 and 600°C. The cracking activity was observed in about 3 minutes and in intervals of 15 to 60 minutes, n Samples of steel 12 HMF (Polish designation) was subjected to cyclic temperature changes in the range 300-600°C. A trend was observed in the decrease in acoustic emission activity with the number of thermal fatigue cycles, whereas just before the final break almost complete decay of emission was observed. Acoustic emission was also used to detect the start of slip lines in monocrystals Zn and in polycrystalline Zn and the influence of the deformation rate on the acoustic activity measured. 13

Radiography The Gammamat TK-100 unit with 6°Co source, 100 mCi (3.7 x 1 0 9 Bq), was used to detect holes and reinforcement in concrete. In the case of steel bars a sensitivity of 0.4 - 1.5% was found, which amounted to 0.6 - 2.5% for longitudinalholes. 14 Radiometric measurements with gamma sources ~ Am and ~37Cs was used for testing

Infrared The application of thermovision was demonstrated in the testing of diode radiators, 18 the testing of fire resistant bricks, 19 sandstone and limestone to follow changes in humidity and salt content, 2° and the plastic deformation of metals. 21

Various methods of N D T Dislocations and vacancies appearing on the metal surface during friction give rise to the photostimulated emission of electrons. This phenomenon has been used to observe the process of running-in of steel during f r i c t i o n . 9 New possibilities have been created with the design of a camera for optical holography. A ruby laser allows the generation of pulses of about 30 ns duration with an energy of 0.4 J and a coherent path is up to 2 m . 1° A small portable instrument for the measurement of Barkhausen noise was described. It allows the magnetization of a sample in two perpendicular directions. Thus, the magnetic anistropy as well as mean values of magnetic noise can be measured. The magnitizing is sinusoidal or triangular with a frequency of 15 and 45 Hz. ss Cold working of bimetallic sleeves can be made with a roller pressed to the surface of the antifriction material. The resulting compressive and shear stresses cause a double effect. The nonhomogeneities smaller than 60/am are welded, while larger, nonhomgeneities and the base material are enlarged and made visible due to material yielding. This combination of the strengthening procedure and visual examination has been shown in industrial practice in Poland. 8

Reference 1

Proc lOth National Conference on Non-destructive Testing, Jadwisin,

Poland (in Polish). The references in the test have the number of papers in the bounded volume of the Proceedings which is available from IPPT-PAN, Swietokrzyska 21, 00-049 Warszawa, Poland. Z. P a w l o w s k i

NDT INTERNATIONAL. DECEMBER 1980