Showing papers in "Journal of Nondestructive Evaluation in 2008"

Ten Lectures on Impact-Echo

Abstract: During the last seven years various aspects of the impact-echo method for non-destructive testing of concrete structures were studied in great detail by the authors. This work was performed within the framework of the German collaborative research initiative FOR 384, “Non-destructive evaluation of concrete structures using acoustic and electromagnetic methods”, and was supported by the German National Science Foundation (Deutsche Forschungsgemeinschaft, DFG). During this period significant new and important results were obtained and well known facts were re-discussed from a different angle of view. It is the belief of the authors that most of the new and even some of the old facts are not well-established in the impact-echo community. The main aim of the present paper therefore is to summarize these important aspects of impact-echo in terms of ten short lectures. In doing so, we will not only present results of our own work but will also include contributions from other groups with significant relevance for the practical use of impact-echo in field tests.

Ultrasonic Imaging Methods for Investigation of Post-tensioned Concrete Structures: A Study of Interfaces at Artificial Grouting Faults and Its Verification

Abstract: This paper presents the progress of successful location of grouting faults in tendon ducts with ultrasonic imaging. The examples were obtained in the research group FOR 384 funded by DFG (German Research Foundation). The co-operation of experimental research and modeling allowed imaging and identification of grouted and ungrouted areas of tendon ducts (including strands) in a large test specimen (40 m2). In addition to the criteria for indicating grouting faults in post-tensioned ducts known until now the phase evaluation of reflected ultrasonic pulses is described. Experiments and modeling of wave propagation are presented for reflections at metal plates in concrete (thickness range 0.5 mm to 40 mm) and for tendon ducts including strands. The main part of the progress was achieved by automated measurements using dry contact transducers, 3D-SAFT reconstruction including phase evaluation and modeling considering wave propagation for typical elastic parameters and exact experimental site conditions. The results for shear waves as well as for pressure waves are compared in the frequency range from 50 kHz to 120 kHz.

Imaging of the Elastic Wave Propagation in Concrete Using Scanning Techniques: Application for Impact-Echo and Ultrasonic Echo Methods

Abstract: Acoustic NDT methods like ultrasonic echo and impact-echo are successfully used for NDT of concrete structures. This paper describes useful techniques for a detailed experimental study of the elastic wave propagation, which is highly relevant for the interpretation of the results obtained from practical measurement applications.

Comparative Performance Tests and Validation of NDT Methods for Concrete Testing

Abstract: Validation of non-destructive testing methods is necessary to create a common basis where different systems can be compared and their applications and limitations be identified. This can be achieved through comparing the measurements taken by several systems used for a common diagnostic purpose under practical but controlled testing conditions. Well-designed small and large laboratory or field specimens promise such conditions. The special concrete specimen (LCS) at BAM was constructed for validation purposes, in particular, to be used for evaluating the performance of echo methods. The thickness of the specimen is varying and it contains carefully designed built-in faults, such as voids, honeycombs and tendon ducts with various degrees of grouting defects. Since the geometry and condition of the defects are known, it can be used to compare the performance of radar, ultrasonic, impact-echo. The research was conducted within the Research group FOR384, sponsored by the German Research Society DFG.

Characterization of Reflector Types by Phase-Sensitive Ultrasonic Data Processing and Imaging

Abstract: Ultrasonic and radar imaging techniques are limited in resolution by the wavelength in the material, yet information beyond those limits is hidden in complex frequency dependent reflection coefficients. The analysis of the phase of complex reflection coefficients together with the properties of imaging algorithms can help to characterize and to classify indications of defects in concrete buildings. This paper describes a method to extract phase information from measurements and SAFT reconstructed images. The influencing factors like material properties, transducer characteristics, and imaging algorithms based on Born or Physical Optics approximations are elaborated. Simulated and experimental results are briefly discussed.

Evaluation of Radar and Complementary Echo Methods for NDT of Concrete Elements

Abstract: Non-destructive testing (NDT) of concrete structures plays an increasing role in civil engineering. This paper presents the results of systematic measurements carried out in the laboratory at BAM and on-site at several bridges using reconstructed and fused radar and ultrasonic echo data sets. For investigating the influence of concrete mixture, radar and ultrasonic measurements were performed at test specimens consisting of concrete mixtures with different pore content and distribution as well as with steel fibres. Further, it is demonstrated how the fusion of data sets recorded with different methods at the same structure (here: concrete bridges) enhances the information content in the fused data set. Different approaches for data fusion algorithms are discussed. The results of these investigations show the high potential of reconstruction and data fusion for the improvement and simplification of the interpretability of large data sets measured with impulse-echo methods. The presented results are based on the research project FOR384 funded by the DFG (Deutsche Forschungsgemeinschaft).

Artificial Neural Network Prediction of Ultimate Strength of Unidirectional T-300/914 Tensile Specimens Using Acoustic Emission Response

Abstract: Acoustic Emission (AE) Monitoring was used to evaluate unidirectional carbon epoxy specimens when tensile loaded with a 100 kN Universal Testing Machine. A series of eighteen samples were loaded to failure to generate AE data for this analysis. After data acquisition, AE response from each test was filtered to include only data collected up to 50% of the actual failure load for further analysis. Amplitude, Duration and Energy are effective parameters utilized to differentiate various failure modes in composites viz., matrix crazing, fiber cut, and delamination with several sub categories such as matrix splitting, fiber/matrix debonding, fiber pull-out etc.

Phased Arrays for Ultrasonic Investigations in Concrete Components

Abstract: An innovative application of ultrasonic testing using phased arrays for concrete components was investigated. A low frequency ultrasonic phased array consisting of ten transducers was used. This configuration allowed the variation and control of the sound field directivity during the measurement. The measurements were carried out on concrete test specimens using a scanning laservibrometer in through transmission mode. The laboratory measurements were compared to theoretical calculations to investigate the influence of shape and dimension of the transducer on the resulting sound field patterns. The technique was successfully implemented to detect an ungrouted tendon duct in a concrete block.

Utilization of Non-destructive Methods for Determining the Effect of Age-Hardening on Impact Toughness of 2024 Al–Cu–Mg Alloy

Abstract: The aim of the study is to investigate the correlation between non-destructive measurements and impact toughness of 2024 Al-alloy. Following the solutionizing at 493 °C/1 h and water quench, the samples were aged at 190 °C for various periods between 0.5 h and 18 h. Hardness, impact toughness, sound velocity, and electrical conductivity were measured for each specimen group, and fracture surfaces were investigated by SEM. To focus on only the effect of precipitation, the process variables were minimized by preparing the samples from the same batch, and applying the identical procedure for solution heat treatment and quenching. Any cold deformation prior to aging of the samples was not applied. It has been concluded that sound velocity and electrical conductivity measurements are promising for the determination of variations in the impact toughness of the precipitation hardened 2024 Al-alloy.

Micro-CT Imaging of MEMS Components

Abstract: Micro-Electromechanical Systems (MEMS) have become increasingly commonplace in varied uses such as miniature pumps, motors, and sensors. As MEMS size continues to decrease, the intricacy of their construction has increased. With this increase in complexity comes a need to evaluate the assembly and functionality of these devices in a nondestructive manner. We proposed the utilization of micro-CT imaging as a method of such evaluation for MEMS devices. Computational simulations were performed in order to determine optimal source materials and imaging parameters for micro-CT scans. Multiple MEMS components of various architecture, fabricated by Sandia National Labs, were then imaged in order to verify the simulations, as well as to prove the feasibility of micro-CT imaging of such devices. The raw data from these scans was run through computational simulations to verify the best choice of filter and interpolation method when reconstructing micro-CT images. The results of the simulations, as well as the level of detail present in the three dimensional reconstructed images of various MEMS devices proved the feasibility of micro-CT as an effective tool for the evaluation of such devices.

Low-Cost Non-Destructive Inspection by Simplified Digital Speckle Interferometry

Abstract: The paper demonstrates a low-cost approach based on speckle interferometry (SI) for the nondestructive inspection (NDI) of metallic and composite plates. Speckle interferometry, and optical methods in general, provide a clear picture of the inspected area, where anomalies are immediately detected, and can yield results in real-time. They are often expensive though, and sensitive to environmental disturbances. Relevant literature in the area of nondestructive inspection is reviewed aiming to find the most suitable configuration for a low-cost system. The chosen setup is a simplified digital speckle interferometer (SDSI), which includes only three main components: a He-Ne laser, a standard CCD camera and a light-scattering window. This simplified configuration removes the requirement of mechanical stability by using a configuration close to the common-path interferometer. The SDSI device is here tested for the detection of thinning defects in Aluminum plates, and delamination in composite plates. Among the various stressing techniques, localized heating was found to be the simplest, yet most effective approach. Results show that the presence of defects creates unique features in the live correlation fringes, from which it is possible to identify the location and size of the defects. It is in conclusion demonstrated that the SDSI device is a valuable tool for fast, low-cost, and non-contact nondestructive inspection.

Non-destructive Evaluation of Adhesive Bonding Using Reflective Fringe Pattern Technique

Abstract: Adhesive bonding is an important fabrication technique for traditional and modern engineering materials in most industrial applications. Compared to the traditional joining methods (such as mechanical fastening or welding), adhesive bonding is rather simple, low cost and time-saving. The performance of the adhesive bonded assemblies strongly depends upon both the material and adhesive properties, and also the bond quality. An excessive gap between the joined elements could result in local stress concentration, and consequently reduce the bond strength. This paper suggests a full-field and optical technique, Reflective Fringe Pattern (RFP) technique, for evaluating adhesive bonding. RFP is based on reflecting the fringe pattern from a specularly reflective surface. In the measurement, two sets of the reflected fringe patterns (i.e. one before and the other after deformation) are recorded to produce the corresponding fringe phase distributions. The difference of these phase distributions depicts the surface deformation. As an adhesive disbond affects the surface deformation, the presence of adhesive disbond can be located by identifying the flaw-induced deformation anomalies in the phase-change distribution map. The potential of using RFP for inspecting adhesive bonding is demonstrated and compared with the conventional non-destructive testing technique—thermography.