Marvin A. Hamstad

A review: Acoustic emission, a tool for composite-materials studies

The technique of acoustic emission has two broad applications areas. The first is nondestructive evaluation. The second is as a tool in studies or research which are not fundamentally directed towards acoustic emission. It is this second application with which we are concerned here. Acoustic emission is a very useful tool in this role because of its high sensitivity, real-time capability, volume-monitoring approach, and sensitivity to any process or mechanism which generates sound waves. This paper presents a comprehensive review of areas where acoustic emission (AE) has been used for materials studies on composite materials. The following fields, among others, will be covered: (1) time-dependent composite properties, (2) impact studies, (3) correlation of AE with stress level, (4) application to matrix cure studies, (5) relationship of AE-detected damage to other measures of damage, (6) studies of the effects of matrix material, (7) application to differences in second phase, (8) interface studies, (9) AE and dimensional stability, (10) AE applied to orientation studies, and (11) environmental effects. This review will emphasize the roles that AE can play as a tool for the materials scientist: (1) discovery of damage mechanisms, (2) characterization of damage progression with increasing time or stress, (3) optimization of fabrication variables, and (4) reduction in the numbers of test specimens required in various studies.

Finite Element and Plate Theory Modeling of Acoustic Emission Waveforms

A comparison was made between two approaches to predict acoustic emission waveforms in thin plates. A normal mode solution method for Mindlin plate theory was used to predict the response of the flexural plate mode to a point source, step-function load, applied on the plate surface. The second approach used a dynamic finite element method to model the problem using equations of motion based on exact linear elasticity. Calculations were made using properties for both isotropic (aluminum) and anisotropic (unidirectional graphite/epoxy composite) materials. For simulations of anisotropic plates, propagation along multiple directions was evaluated. In general, agreement between the two theoretical approaches was good. Discrepancies in the waveforms at longer times were caused by differences in reflections from the lateral plate boundaries. These differences resulted from the fact that the two methods used different boundary conditions. At shorter times in the signals, before reflections, the slight discrepancies in the waveforms were attributed to limitations of Mindlin plate theory, which is an approximate plate theory. The advantages of the finite element method are that it used the exact linear elasticity solutions, and that it can be used to model real source conditions and complicated, finite specimen geometries as well as thick plates. These advantages come at a cost of increased computational difficulty, requiring lengthy calculations on workstations or supercomputers. The Mindlin plate theory solutions, meanwhile, can be quickly generated on personal computers. Specimens with finite geometry can also be modeled. However, only limited simple geometries such as circular or rectangular plates can easily be accommodated with the normal mode solution technique. Likewise, very limited source configurations can be modeled and plate theory is applicable only to thin plates.

A correlation between acoustic emission and the fracture toughness of 2124-T851 aluminum☆

Abstract The paper reports on an experimental study of the possible application of acoustic emission as an inspection technique, to determine whether 2124-T851 aluminum plate meets certain, minimum, fracture toughness specifications. Unflawed specimens, taken in the three principal directions, from three separate plates are tested in compression and tension. The-true-mean-square or the root-mean-square voltage of the continuous acoustic emission generated during the tests is recorded as a function of specimen strain. The acoustic emission, chemical analysis, fracture surfaces, and large (1–20 μm) second-phase particles are studied in relation to fracture toughness. Results indicate that the generated acoustic emission from a longitudinal compression test, long-transverse tension test and short-transverse tension test all correlate with changes in plate fracture toughness. Based on the experimental results, the monitoring of acoustic emission during relatively simple tension or compression tests of aluminum may be useful in checking whether the material meets a fracture toughness specification. Also, the results indicate that acoustic emission is useful in studies to develop fructure toughness models.

Some fundamental aspects of the theory of acoustic emission

Abstract A dislocation process that seems likely to be of importance in the generation of acoustic emissions is described. A simple mathematical theory, which describes this process, is developed so that order of magnitude estimates can be made. These estimates indicate that acoustic emission events may be associated with only a small fraction of the total plastic strain. Hence, it may be difficult to correlate acoustic emission with parameters related to total strain.

Origin of Burst-Type Acoustic Emission in Unflawed 7075-T6 Aluminum

Tensile tests were used to study the acoustic emission from several 7075-T6 aluminum plates. Specimens from one plate produced a large burst-type emission in addition to the continuous emission. Metallographic and fracture surface studies indicated that the burst-type emission resulted from the brittle fracture of large primary inclusions in the microstructure.

Correlation of Residual Strength with Acoustic Emission from Impact-Damaged Composite Structures under Constant Biaxial Load

Small, filament-wound, Kevlar/epoxy, biaxial test specimens were sub jected to various levels of impact damage at the Oak Ridge Y-12 Plant.** The specimens were pressurized in a proof test cycle to 58% of their nominal, undamaged strength and then pressurized to failure. Acoustic emission data were gathered by multiple sensors dur ing a 10 minute hold at peak proof pressure. Post-test filtering of the data was performed to study composite behavior in the damaged region and other areas. The rate and total amount of AE produced depends on the duration of the static load and degree of damage. The concept of the event rate moment is introduced as a method of quantifying a struc tures total AE behavior when under static load. Average event rate, total long duration events, and event rate moments provided various degrees of correlation between AE and residual strength.

Acoustic Emission from Single and Multiple Kevlar 49 Filament Breaks

Acoustic Emission (AE) was monitored during single filament tension tests of Kevlar 49 fiber. AE was also monitored during dry and lubricated bundle tests of the same material. These later tests were carried out in such a way that the individual filament breaks could be independently verified. Statistical studies were made of the AE event characterization parameters for the source mechanism of filament failure. Studies of effect of cumulative damage, test fixturing, first vs. second AE sensor hit, and friction between fibers were made. Characterization by AE events made up of multiple filament fractures was examined relative to events due to single filament fracture in a bundle. These results are preliminary studies prior to using AE to study fundamental variables which control damage progression in fiber composites upon repeated loading.

On the origin of the first peak of acoustic emission in 7075 aluminium alloy

The origin of acoustic emission (AE) in 7075 aluminium alloy was investigated in tension and compression tests. It is suggested that the first peak associated with the root-mean-square (rms) AE plot against time (or strain) is due to the unpinning of dislocation segments from solute atom clusters and is not due to the fracture of particles. Alteration in the value of the AE first peak and also in electrical resistivity values due to heat treatment of this alloy support the proposed origin of the AE first peak near yield.

Modeling of acoustic emission signal propagation in waveguides.

Acoustic emission (AE) testing is a widely used nondestructive testing (NDT) method to investigate material failure. When environmental conditions are harmful for the operation of the sensors, waveguides are typically mounted in between the inspected structure and the sensor. Such waveguides can be built from different materials or have different designs in accordance with the experimental needs. All these variations can cause changes in the acoustic emission signals in terms of modal conversion, additional attenuation or shift in frequency content. A finite element method (FEM) was used to model acoustic emission signal propagation in an aluminum plate with an attached waveguide and was validated against experimental data. The geometry of the waveguide is systematically changed by varying the radius and height to investigate the influence on the detected signals. Different waveguide materials were implemented and change of material properties as function of temperature were taken into account. Development of the option of modeling different waveguide options replaces the time consuming and expensive trial and error alternative of experiments. Thus, the aim of this research has important implications for those who use waveguides for AE testing.

Development of practical wideband high-fidelity acoustic emission sensors

The development of a series of wideband acoustic emission sensor/preamplifier systems is described. Key design factors are discussed along with the actual design and characterization of the sensors. These new sensors with integral amplification are out-of-plane, displacement response sensors nearly independent of frequency over a range from a slow as 30 to 50 kHz up to 1.2 MHz. The sensor design includes electromagnetic shielding and mechanical protection of the sensitive elements. More importantly, these practical sensors have signal-to-noise sensitivity that is equivalent to typical commercial resonant sensor/preamplifier systems operating from 100 kHz to 300 kHz. A mounting fixture for the sensors has also been developed.