M. Veidt

Pseudoelastic behaviour of cast magnesium AZ91 alloy under cyclic loading-unloading

Abstract Large stress–strain hysteresis loops are observed under cyclic loading after a small plastic prestrain. Loops have been observed in sand-cast material in a variety of tempers under tension or compression, and in high-pressure die-cast material with different cross-section thickness tested in tension. The loops are first observed after a nucleation strain of between 0.001 and 0.01% and grow to a maximum width after 1–2% plastic strain, becoming slightly narrower afterwards. When fully developed, the loops add a large (0.3–0.45%) pseudoelastic strain to the elastic strain, effectively decreasing the elastic modulus of the alloy by up to 70%. In sand-cast material of a given temper, the effects tend to be more pronounced in compression than in tension. Further, the effect is slightly larger in die-cast or aged sand-cast as compared to as-cast sand-cast material. The phenomenon is discussed in terms of the partial reversal of { 1 0 1 2 } twins upon unloading.

A Lamb-wave-based technique for damage detection in composite laminates

This paper presents the application of Lamb waves to inspect damage in composite laminates. The proposed methodology employs a network of transducers that are used to sequentially scan the structure before and after the presence of damage by transmitting and receiving Lamb wave pulses. A damage localization image is reconstructed by analyzing the cross-correlation of the scatter signal envelope with the excitation pulse envelope for each transducer pair. A potential damage area is then reconstructed by superimposing the image observed from each actuator and sensor signal path. Both numerical and experimental case studies are used to verify the proposed methodology for composite laminates. Three-dimensional finite element models with a transducer network consisting of four transducer elements are used in the numerical case studies. The experimental case studies employ a transducer network using four piezoelectric transducers as transmitter elements and a laser vibrometer to measure the response signals at four locations close to the transducers. The results show that the method enables the reliable detection of structural damage with locating inaccuracies of the order of a few millimeters inside as well as outside of an inspection area of 100 x 100 mm(2).

Scattering of the fundamental anti-symmetric Lamb wave at delaminations in composite laminates.

An analysis of the scattering characteristics of the fundamental anti-symmetric (A(0)) Lamb wave at a delamination in a quasi-isotropic composite laminate is presented. Analytical solutions for this problem do not exist due to the anisotropic nature and multilayer characteristics of composite laminates. This study uses a three-dimensional finite element (FE) method and experimental measurements to provide physical insight into the scattering phenomena. Good agreement is found between simulations and experimental measurements. The results show that the A(0) Lamb wave scattering at a delamination in composite laminates is much more complicated than the scattering at a defect in isotropic plates. Scatter amplitudes and scatter directivity distributions depend on the delamination size to wavelength ratio and the through-thickness location of the delamination damage. The study also investigates the feasibility of the common experimental practice of simulating delamination damage by bonding masses to the surface of composite laminates for guided wave damage detection and characterization methodologies verifications. The results suggest that care is required to use bonded masses to simulate delamination damage for verifying and optimizing damage characterization techniques. In summary, the results of the investigation help to further advance the use of the A(0) Lamb wave for damage detection and characterization.

Crack detection in hollow section structures through coupled response measurements

Detection of a circumferential crack in a hollow section beam is investigated using coupled response measurements. The crack section is represented by a local flexibility matrix connecting two undamaged beam segments. This matrix defines the relationship between the displacements and forces across the crack section and is derived by applying fundamental fracture mechanics theory. The suitability of the mode coupling methodology is first demonstrated analytically. Laboratory test results are then presented for circular hollow section beams with artificially generated cracks of varying severity. It is shown that this method has the potential as a damage detection tool for mechanical structures.

The elastic modulus of cast Mg–Al–Zn alloys

The effect of grain size, temper and solute content on elastic modulus values of pure Mg, a Mg–0.3Zr alloy, an AZ91 alloy and a number of Mg–Al and Mg–Zn binary alloys have been determined using two ultrasonic methods. The effect of porosity and cross-section thickness in diecast AZ91 was also assessed. Coarse-grained pure Mg and fine-grained Mg–0.3Zr have similar elastic moduli. The modulus increases then decreases with increasing content of Al or Zn in solid solution, with a maximum value at an electron concentration of ca. 2.01. The modulus of aged sand-cast AZ91 specimens increases with the volume fraction of β-Al_12M_g17 precipitates. The cross-section thickness seems to have no influence on the modulus of die cast material, but it decreases with the porosity content. An elastic modulus chart was constructed for AZ91 alloy accounting for the possible effects of solute in solution, volume fraction of precipitates and porosity content.

Protective Iron Carbonate Films— Part 1: Mechanical Removal in Single-Phase Aqueous Flow

The protective role of iron carbonate fi lms deposited on steel surfaces as by-products of carbon dioxide (CO2) corrosion may be hindered by mechanical fi lm removal due to hydrodynamic forces. Damage to the fi lm is frequently accompanied by se- vere corrosive attack, which may ultimately result in costly equipment failure. The present work reports on experimental investigations of mechanical fi lm removal (its kinetics and level of damage to the fi lm) using a rotating cylinder con- fi guration in the highly turbulent fl ow regime. Corrosion rate monitoring using the linear polarization resistance technique was used as a tool for implicit fi lm removal rate measuring, whereas scanning electron microscopy served for verifi cation and evaluation purposes. Two types of fi lms were studied, one less uniform and adherent and the other more compact and adherent. The results clearly show that pure mechani- cal removal in undisturbed single-phase fl ow does occur. However, fi lm removal starts after an initiation period and is only localized. The differences in the thickness, microstruc- ture, and topography of intact and residual fi lms are dis- cussed in detail for both fi lm types. In addition, the possible mechanism of mechanical fi lm removal from the fl uid mechan- ics point of view has been proposed.

Ultrasonic point‐source/point‐receiver measurements in thin specimensa)

This paper reports on the application of the ultrasonic point‐source/point‐receiver technique (PS/PR) to thin, anisotropic specimens. By using a scanned source and viewing a large number of signals measured at adjacent source/receiver configurations, one obtains a so‐called scan image which represents the detailed spatial and temporal characteristics of the elastic wave field in a specimen. The measurement system uses either a focused, pulsed laser beam operating as a dipole source or a small aperture, piezoceramic shear transducer serving as a monopolar source. Detection of the signals is with a sensitive piezoceramic sensor that responds to the lateral or shear motions of the specimen surface. Scan images were obtained in a Silicon wafer whose thickness was 625 μm and in one‐ or two‐ply, unidirectional, graphite/epoxy laminates whose thicknesses were approximately 145 and 275 μm, respectively. The experimental data are analyzed using a simple plane‐wave, plane‐stress model that describes the propagation of quasilongitudinal and quasitransverse membrane waves in the plane of the plate. Good agreement is obtained between the experimental and theoretical group velocity curves of the membrane waves in the laminates. It is shown that the measured group velocity data of different wave modes can be inverted to recover the elastic constants of a material with excellent reliability and accuracy. The measurement system was also used to map out the group velocities in branches comprising the cuspidal region of the quasi‐transverse group velocity curve.

Influence of stacking sequence on scattering characteristics of the fundamental anti-symmetric Lamb wave at through holes in composite laminates.

This paper investigates the scattering characteristics of the fundamental anti-symmetric (A(0)) Lamb wave at through holes in composite laminates. Three-dimensional (3D) finite element (FE) simulations and experimental measurements are used to study the physical phenomenon. Unidirectional, bidirectional, and quasi-isotropic composite laminates are considered in the study. The influence of different hole diameter to wavelength aspect ratios and different stacking sequences on wave scattering characteristics are investigated. The results show that amplitudes and directivity distribution of the scattered Lamb wave depend on these parameters. In the case of quasi-isotropic composite laminates, the scattering directivity patterns are dominated by the fiber orientation of the outer layers and are quite different for composite laminates with the same number of laminae but different stacking sequence. The study provides improved physical insight into the scattering phenomena at through holes in composite laminates, which is essential to develop, validate, and optimize guided wave damage detection and characterization techniques.

Modelling of Lamb waves in composite laminated plates excited by interdigital transducers

The technique of permanently attaching interdigital transducers (IDT) to either flat or curved structural surfaces to excite single Lamb wave mode has demonstrated great potential for quantitative non-destructive evaluation and smart materials design, In this paper, the acoustic wave field in a composite laminated plate excited by an IDT is investigated. On the basis of discrete layer theory and a multiple integral transform method, an analytical-numerical approach is developed to evaluate the surface velocity response of the plate due to the IDTs excitation. In this approach, the frequency spectrum and wave number spectrum of the output of IDT are obtained directly. The corresponding time domain results are calculated by applying a standard inverse fast Fourier transformation technique. Numerical examples are presented to validate the developed method and show the ability of mode selection and isolation. A new effective way of transfer function estimation and interpretation is presented by considering the input wave number spectrum in addition to the commonly used input frequency spectrum. The new approach enables the simple physical evaluation of the influences of IDT geometrical features such as electrode finger widths and overall dimension and excitation signal properties on the input-output characteristics of IDT. Finally, considering the convenience of Mindlin plate wave theory in numerical computations as well as theoretical analysis, the validity is examined of using this approximate theory to design IDT for the excitation of the first and second anti-symmetric Lamb modes

Flexural waves transmitted by rectangular piezoceramic transducers

Rectangular piezoceramic transducers are widely used in ultrasonic evaluation and health monitoring techniques and structural vibration control applications. In this paper the flexural waves excited by rectangular transducers adhesively attached to isotropic plates are investigated. In view of the difficulties in developing accurate analytical models describing the transfer characteristics of the transducer due to the complex electromechanical transduction processes and transducer-structure interactions involved, a combined theoretical-experimental approach is developed. A multiple integral transform method is used to describe the propagation behaviour of the waves in the plates, while a heterodyne Doppler laser vibrometer is employed as a non-contact receiver device. This combined theoretical-experimental approach enables the efficient characterization of the electromechanical transfer properties of the piezoelectric transducer which is essential for the development of optimized non-destructive evaluation systems. The results show that the assumption of a uniform contact pressure distribution between the transducer and the plate can accurately predict the frequency spectrum and time domain response signals of the propagating waves along the main axes of the rectangular transmitter element.