Advanced Materials Research Vols. 13-14

Abstract: Dynamic behavior of two types of martensitic transformations during tensile deformation of Cu-Al-Ni shape memory alloy single crystal has been investigated using an acoustic emission waveform analysis. Two kinds of martensitic transformations consist of β1 ⇔ β1′ (structural change of DO3 to 18R) and β1 ⇒ γ1′ (structural change of DO3 to 2H), each of which is called super-elastic and thermo-elastic martensitic transformations, respectively. These two types of martensitic transformations could be obtained during tensile deformation because of different heat treatment. The rise time at the source (the source rise time) in finite elastic solid by the modified Takashima’s method was analyzed using the acoustic emission waveform detected during the martensitic transformation. The mean source rise time to the γ1′ phase was smaller than that to the β1′ phase before yielding and became the same after yielding. The former result means that the nucleation of the γ1′ phase is faster than that of the β1′ phase because of different crystallographic structure. The latter result is that the growth rate of the γ1′ phase is the same as that of the β1′ phase.

Abstract: The relationship between acoustic emission and internal friction is investigated during cyclic deformation of copper single- and poly-crystals at intermediate amplitudes. Good agreement between these two phenomena has been demonstrated in all materials tested as long as the stress relaxation occurs uniformly within the sample whereas the difference between them becomes pronounced when strain/fracture localisation takes place. The similarity between acoustic emission and internal friction can be extended to materials deforming without appreciable plastic deformation, such as metal matrix composites where the main source of stress relaxation and strain accumulation is the particle breakage. The significance of the relation between internal friction and acoustic emission for understanding of fundamentals of AE is discussed.

Abstract: Acoustic emission (AE) is an attractive technique for the structural health monitoring (SHM) of aerospace systems. To reach its full potential in this role a quantitative approach must be adopted to study damage mechanisms in composite materials. In this paper, some of the practical issues regarding acoustic emission testing in composites are addressed. A model describing Lamb wave propagation through plates is described and used to make phase velocity and attenuation measurements in both aluminium and carbon fibre reinforced plastic plates. Results are then implemented in the frequency domain to conduct an experimental study of normal incidence Lamb wave reflections. Comparisons are made with finite element analysis (FEA) models with good results.

Abstract: Large structural composite sandwich panels (approx. 2x4 m2) were found to have cracks in the honeycomb core. The core was made out of a number of Nomex honeycomb blocks having different densities. These were first machined and adhesively bonded into the final shape. The CFRP skins were then added and the sandwich panel was cured in an autoclave applying elevated temperature and pressure. Acoustic emission monitoring was done during 17 hours of processing in the autoclave using a six channel AE system. AE events from cracking of the honeycomb core were registered and could be identified in time and space. The results from AE monitoring were very useful for modifying the process parameters and overcome the cracking of the core.

Abstract: Cost-effective and reliable damage detection is critical for the utilization of composite materials due to the relatively localised nature of damage formation and the resultant reduction in structural integrity. Of the methods available, Acoustic Emission (AE) is considered as one potential technology for on-line and in situ monitoring of structural degradation of composite materials. Purpose of this work was to study the interaction between embedded PVDF (polyvinylidene fluoride) transducers and composite samples as well as detect and characterize the failure mechanisms in aramid/epoxy flexural test specimens using acoustic emission data obtained by embedded PVDF film sensors. Furthermore, it has been realized a comparison with surface mounted PVDF data. Results of our previous works (Caneva et al., 2005) dealing with monitoring tensile and flexural behaviour of glass/epoxy composites enabled to extend this methodology to aramid/epoxy composites. The use of Acoustic Emission and Scanning Electron Microscopy (SEM) observations enabled to identify and understand the failure mechanisms of the composites tested. Furthermore, satisfactory results of this work highlighted that the application of PVDF shows promise as a suitable acoustic emission transducer for fibrous composite materials.

Abstract: Steel cables play an indispensable role in a number of rather diverse fields of applications. They are extensively used in suspension and stay cables bridges. It is important to be able to detect as precociously as possible the defects affecting their performance. They can present a healthy aspect over their observable lengths and be damaged in the hidden parts (anchorages). In this present study, Acoustic Emission (AE) is used in order to detect and to locate broken wires in the anchorage zones. The study of the behaviour of a broken wire in a cable under dynamic bending contribute to understand the origin of AE source and to discuss the necessary conditions of vibrating for detection. In this case, the phenomene source of AE is related to interwire friction caused by the redistribution of elastic energy on the recovery length of the broken wire and the repetition of the process joining-slip between wires. AE parameters are very influenced by the change of surface conditions, contact strength between wires and the recovery length of the broken wire. Several cables are evaluated in different surface conditions i.e. corroded, dry and oiled.

Abstract: In order to explore potential applications for Active Fiber Composite (AFC) elements made from piezoelectric fibers for structural integrity monitoring, a model experiment for leak testing on pipe segments has been designed. A pipe segment made of aluminum with a diameter of 60 mm has been operated with gaseous (compressed air) and liquid media (water) for a range of operating pressures (between about 5 and 8 bar). Artificial leaks of various sizes (diameter) have been introduced. In the preliminary experiments presented here, commercial Acoustic Emission (AE) sensors have been used instead of the AFC elements. AE sensors mounted on waveguides in three different locations have monitored the flow of the media with and without leaks. AE signals and AE waveforms have been recorded and analysed for media flow with pressures ranging from about 5 to about 8 bar. The experiments to date show distinct differences in the FFT spectra depending on whether a leak is present or not.

Abstract: The Kaiser Effect in acoustic emission is often used for an estimation of the stress to which rocks have been subjected. However, there are cases in which the Kaiser Effect is not clear, since the noises due to the contact and/or the stick slip between the pre-induced fracture surfaces are measured during the reloading process. In such cases, estimation of previous stress is difficult by the conventional method which is based on the acoustic emission activity observed under reloading process. In the tests for the Kaiser Effect on rocks, therefore, the noises must be eliminated from the acoustic emission generated from newly created cracks during the second loading process. Such techniques as analysis of the difference between the acoustic emission activity observed in the first and second reloading and the analysis of the change in the slope of the acoustic emission amplitude distribution have been proposed. In this paper we present a new method by which the maximum previous stress in rocks can be directly estimated without any post signal analysis. In the new method, simultaneous measurement of acoustic and electromagnetic emission during loading test of rock sample is employed. The electromagnetic emission in the deformation of rock sample generates only when the fresh surfaces due to cracking are created in the material, and the source of electromagnetic emission is the electrification between the fresh crack surfaces. This paper describes the simultaneous measurement of acoustic and electromagnetic emission useful for estimating the rock in-situ stress.

Abstract: The TÜV Austria (formerly TÜV Vienna) has been testing different structures, mainly pressure equipment, since 1977 with Acoustic Emission. I will not enumerate all the different tests, which are more than 20.000 till today, but I will point out, how the test method has changed on some typical tests over the years. I will also remind you on the different possibilities, which have been offered by the rapid development of the electronic data acquisition and treatment, especially during the last 10 – 15 years.

Abstract: In spite of the availability of very versatile analysis packages, such as VisualAE™, the large variety of test objects that can be successfully tested by AE sometimes calls for the use of an application dedicated analysis algorithm, especially if large numbers of similar or identical objects shall be tested. This can save a lot of time for data analysis and interpretation and reduce the requirements on operator qualification. Those analysis algorithms can be developed by AE-experts based on AE data from a number of test objects. Up to now, such a program had no access to real time data. The more such a dedicated analysis algorithm proves to be helpful and valuable for a specific application (e.g. testing small, buried LPG vessels), the higher the desire to use that algorithm also in real time (i.e. during data acquisition). This can simplify testing and improve test control e.g. by the application of higher sophisticated alarm criteria. With the new Embedded Code Processor (ECP) the user can develop and verify software algorithms of any complexity, that run within the environment of VisualAE™. This tremendously accelerates the implementation process. Any result within VisualAE can be input into the user-developed code, any new results can be calculated, and then used by VisualAE for filtering, data presentation and alarm raising, if desired in combination with the conventional results. This paper summarizes the structure of VisualAE in short and the intentions and possibilities of an Embedded Code Processor (ECP). ECP is already available and in use.