Advanced Materials Research Vols. 13-14

Abstract: Corrosion is the major cause of structural degradation in industrial plant and structures; the consequences of not identifying its presence and status can be severe, leading to a myriad of methods for its evaluation and monitoring. Amongst these there are a large number based on acoustic methods, and this paper concentrates on three variations involving passive monitoring with the aim of summarizing their usual area of applications and limitations. Passive monitoring involves listening to the process of corrosion itself, which usually causes acoustic emission as a result of the fracture and de-bonding of expansive corrosion products, localised yielding, or micro-crack formation. This method is applied to reinforced concrete structures, storage tank floors, and process plant whilst in service.

Abstract: This paper discusses the mechanism of chloride SCC of sensitized and non-sensitized Type-304 stainless steel based on the data obtained by simultaneous monitoring of AE and corrosion potential fluctuation (CPF). The steel produced transgranular SCC via corrosion pits filled with corrosion product (chromium oxy-hydroxides) in 35 mass % MgCl2 solution. Both the pitting corrosion and transgranular SCC did not produce primary AEs due to anodic dissolution, while they generated rapid drop (RD) type CPF. We, however, detected the secondary AEs from hydrogen gas evolution and fracture of corrosion products. Hydrogen gas was found to produce AE with single frequency component, while the fracture of corrosion products produced AEs with broad frequency components. The sensitized steel produced primary AEs due to the falling-off of grains as well as the secondary AEs. Microscopic progression of SCC was consistently interpreted by the timing and time-lag of AEs and CPFs.

Abstract: Eight composite plates 400x410x2.15mm were fabricated from carbon/epoxy in ((0,90)4)s lay-up. To ensure there was no damage in the plates prior to the impact investigations the plates were C-scanned. A drop test rig was used to apply a low velocity impact to the undamaged plates. A rebound mechanism was employed to prevent secondary impacts. AE sensors were selected for frequency and size due to the limited space on the test rig. Super glue was used both as a couplant and also to secure the sensors in position. During the impact wave streaming, time driven data and hit driven data were used to record the impact simultaneously. One test was conducted on each sample with two impacts completed at 5J, 6J, 8J and 10J (total of 8 samples). The impacts were designed to allow only slight visible impact damage to be formed. After completion of the impact investigation the plates were C-scanned to determine the area of debonded material. Analysis of the data showed the complete record of the impact event from the wave streaming, while the hit driven and time driven absolute energy data provided increasing correlations with the area of composite damaged.

Abstract: The presence of impact damage in a carbon fibre composite can reduce its capacity to support an in-plane load, which can lead to an unexpected or premature failure. This paper reports on an investigation into two slender carbon/fibre epoxy panels, one un-damaged and one with an artificial delamination introduced using an embedded section of PTFE. The reported tests form part of a larger series of investigations using differing sizes of artificial delamination and real impact damage. An investigation of wave velocity propagation at varying angles to the composite lay up was completed to assist in source location. The specimens were loaded under, uniaxial in-plane loading and monitored using four resonant acoustic emission sensors. A full field optical measurement system was used to measure the global displacement of the specimens. Analysis of AE waveforms and AE hit rate were used to assess the buckling of the panel. The results compared favourably with the optical measurement results.

Abstract: AE studies were performed considering pressure cylinders of design1 Type II – metallic liner with hoop composite wrapping, Type III – metallic load sharing liner with full composite wrapping and Type IV – all-composite cylinder. The AE technique has to be improved so far that for different design types of cylinders standard AE equipment can be used in an easy and practicable way under normal conditions of production and service. The paper presents our 8 years of experience in this application area using conventional AE measuring technique. Potentials, requirements and limits for detection of manufacturing faults and in-service damages of highpressure composite cylinders are discussed. 1Cylinder types classification in accordance with ECE R 110 / ISO 11439

Abstract: Some initial studies for an AE characterisation of damage in composites are presented in this paper. A PAC AE PCI-2 based system and four PAC WD AE broadband sensors are used. Pencil lead breaks (PLB) were used to introduce a source of elastic waves. An initial validation of the system based on PLB on an aluminium plate showed that there was a large difference in sensitivity of the four nominally identical sensors. It is shown that the AE parameters are so strongly related to the sensitivity of the sensors that they tend to describe the AE system more than the actual sources. To characterise damage in composite materials alternative AE parameters are required that are not dependent on the sensitivity of the sensors. In the paper, it is shown that the frequency spectra of the sources have relatively little variability with respect to the sensor. A frequency-based approach that examines the spectrum of each source is proposed in the paper. The procedure is demonstrated using PLBs on a CFRP plate and it is demonstrated that the approach can indicate the position of the source in relation to orientation of the fibres and the plate edge.

Abstract: An AE measurement system that uses a laser interferometer has been developed to evaluate microfractures at elevated temperatures in various materials. This technique has the advantage that it can estimate the temperature where microfractures were generated. The results give useful information to control process conditions. AE during sintering ceramics and thermal spying of ceramic powder on metal substrates were successfully detected by this laser AE measurement system. Effect of process conditions on damage process was discussed from the detected AE behaviour during in-process monitoring.

Abstract: This research aims to characterise and quantify the acoustic emission (AE) generated during the high velocity oxy-fuel (HVOF) thermal spraying process, recorded using piezoelectric AE sensors. The HVOF process is very complex involving high temperature turbulent flow through a nozzle with entrained particles, the projection of these particles, and their interaction with the target surface. Process parameters such as gun speed, oxy-fuel pressure and powder specification affect various characteristics of the coating, including thermal residual stresses; the lamellar microstructure and the topology and geometry of pores, all formed when the fused powder hits the surface, forming “splats”. It is widely acknowledged in the thermal spray industry that existing quality control techniques and testing techniques need to be improved. New techniques which help to understand the effects of coating process parameters on the characteristics of the coating are therefore of value, and it was anticipated that recording the AE produced when the fused particles contact the surface would aid this understanding. As a first stage, we demonstrated here that AE associated with particle impact can, in fact, be discerned in the face of the considerable airborne and structure-borne noise. In order to do this, a new test method using a masking sheet with slits of varying size was developed. Thermal spraying was carried out for a range of spray gun speeds and process parameters. The AE was measured using a broad band AE sensor positioned on the back of the sample as the spot was traversed across it. The results show that the amplitude and energy of the AE signals is related to the spray gun speed, powder used and the oxy-fuel pressure. Using a simple geometrical model for particle impact, the measured AE was found to vary with the energy and number of particles impacting on the sample in a predictable way.

Abstract: The paper summarizes the experiences acquired from on-line acoustic emission monitoring (herafter AE) of heavy castings during their manufacturing (solidification and following cooling in the mould). They are usually monitored elastic waves generated above all by stress changes in the solid state. In order to exactly determine plastic-elastic transition state the investigation was focused on raising the sensitivity of detection. The suitable experimental technique is discussed in the first part of this article. The main problem of the measurement by high temperatures was solved by using waveguides. It is very important in this case to select useful signal sources from mechanical and electromagnetical disturbances. Some laboratory experiments were done for studying the signal origin in the first state of solidification. The results from on-line monitoring of two types cast rolls during manufacturing were compared. Each type of casting has its typical AE histogram. For the quality evaluation ( in our case) is significant the time period of approximate 5 days after pouring . The time delayed stress induced cracking generates high level AE signal in this time period and the presence of such signal indicates defective product. The study of high temperature tensile tests, structural phase transformation and solidification processes using AE is very important for analysis of AE sources. The use of the laboratory results for the AE source analysis on real products will be subject to futher research.