D.G. Aggelis

Acoustic emission monitoring of degradation of cross ply laminates

The scope of this study is to relate the acoustic activity of damage in composites to the failure mechanisms associated with these materials. Cross ply fiber reinforced composites were subjected to tensile loading with recording of their acoustic activity. Acoustic emission (AE) parameters were employed to monitor the transition of the damage mechanism from transverse cracking (mode I) to delamination (mode II). Wave propagation measurements in between loading steps revealed an increase in the relative amplitude of the propagated wave, which was attributed to the development of delamination that confined the wave to the top longitudinal plies of the composite.

Monitoring of concrete structures using the ultrasonic pulse velocity method

Concrete is the material most produced by humanity. Its popularity is mainly based on its low production cost and great structural design flexibility. Its operational and ambient loadings including environmental effects have a great impact in the performance and overall cost of concrete structures. Thus, the quality control, the structural assessment, the maintenance and the reliable prolongation of the operational service life of the existing concrete structures have become a major issue. In the recent years, non-destructive testing (NDT) is becoming increasingly essential for reliable and affordable quality control and integrity assessment not only during the construction of new concrete structures, but also for the existing ones. Choosing the right inspection technique is always followed by a compromise between its performance and cost. In the present paper, the ultrasonic pulse velocity (UPV) method, which is the most well known and widely accepted ultrasonic concrete NDT method, is thoroughly reviewed and compared with other well-established NDT approaches. Their principles, inherent limitations and reliability are reviewed. In addition, while the majority of the current UPV techniques are based on the use of piezoelectric transducers held on the surface of the concrete, special attention is paid to a very promising technique using low-cost and aggregate-size piezoelectric transducers embedded in the material. That technique has been evaluated based on a series of parameters, such as the ease of use, cost, reliability and performance.

Performance monitoring of large-scale autonomously healed concrete beams under four-point bending through multiple non-destructive testing methods

Concrete is still the leading structural material due to its low production cost and great structural design flexibility. Although it is distinguished by such a high durability and compressive strength, it is vulnerable in a series of ambient and operational degradation factors which all too frequently result in crack formation that can adversely affect its mechanical performance. The autonomous healing system, using encapsulated polyurethane-based, expansive, healing agent embedded in concrete, is triggered by the crack formation and propagation and promises material repair and operational service life extension. As shown in our previous studies, the formed cracks on small-scale concrete beams are sealed and repaired by filling them with the healing agent. In the present study, the crack formation and propagation in autonomously healed, large-scale concrete beams are thoroughly monitored through a combination of non-destructive testing (NDT) methods. The ultrasonic pulse velocity (UPV), using embedded low-cost and aggregate-size piezoelectric transducers, the acoustic emission (AE) and the digital image correlation (DIC) are the NDT methods which are comprehensively used. The integrated ultrasonic, acoustic and optical monitoring system introduces an experimental configuration that detects and locates the four-point bending mode fracture on large-scale concrete beams, detects the healing activation process and evaluates the subsequent concrete repair.

Simultaneous acoustic and dielectric real time curing monitoring of epoxy systems

The attainment of structural integrity of the reinforcing matrix in composite materials is of primary importance for the final properties of the composite structure. The detailed monitoring of the curing process on the other hand is paramount (i) in defining the optimal conditions for the impregnation of the reinforcement by the matrix (ii) in limiting the effects of the exotherm produced by the polymerization reaction which create unwanted thermal stresses and (iii) in securing optimal behavior in matrix controlled properties, such as off axis or shear properties and in general the durability of the composite. Dielectric curing monitoring is a well known technique for distinguishing between the different stages of the polymerization of a typical epoxy system. The technique successfully predicts the gelation and the vitrification of the epoxy and has been extended for the monitoring of prepregs. Recent work has shown that distinct changes in the properties of the propagated sound in the epoxy which undergoes polymerization is as well directly related to the gelation and vitrification of the resin, as well as to the attainment of the final properties of the resin system. In this work, a typical epoxy is simultaneously monitored using acoustic and dielectric methods. The system is isothermally cured in an oven to avoid effects from the polymerization exotherm. Typical broadband sensors are employed for the acoustic monitoring, while flat interdigital sensors are employed for the dielectric scans. All stages of the polymerization process were successfully monitored and the validity of both methods was cross checked and verified.

Combined NDT methods for characterization of subsurface cracks in concrete

One of the most frequent problems in concrete structures is corrosion of metal reinforcement. It occurs when the steel reinforcement is exposed to environmental agents. The corrosion products occupy greater volume than the steel consumed, leading to internal expansion stresses. When the stresses exceed concrete strength, eventually lead to corrosion-induced cracking beneath the surface. These cracks do not show any visual sign until they break the surface, exposing the structure to more accelerated deterioration. In order to develop a methodology for sub-surface damage characterization, a combination of non destructive testing (NDT) techniques was applied. Thermography is specialized in subsurface damage identification due to anomalies that inhomogeneities impose on the temperature field. Additionally, ultrasonic surface waves are constrained near the surface and therefore, are ideal for characterization of near-surface damage. In this study, an infrared camera scans the specimen in order to indicate the position of potential damage. For cases of small cracks, the specimens are allowed to cool and the cooling-off curve is monitored for more precise results. Consequently, ultrasonic sensors are placed on the specified part of the surface in order to make a more detailed assessment for the depth of the crack. Although there is no visual sign of damage, surface waves are influenced in terms of velocity and attenuation. The combination of the NDT techniques seems promising for real structures assessment.

Thermography and Ultrasound for Characterizing Subsurface Defects in Concrete

A combination of non destructive testing (NDT) techniques is applied for subsurface damage inspection. Thermography and ultrasound are used complimentarily to detect and characterize near surface cracking. In this paper, specimens with subsurface cracks, are scanned by an infrared camera in order to indicate the position of the cracked area. For cases of small and thin cracks the cooling off curves over a specified time span are examined in order to identify the damage areas more reliably. At the specific position indicated by thermography, ultrasonic sensors are placed in order to make a more detailed assessment for the depth of the crack. Although there is no visual sign of damage, ultrasonic waves are influenced in terms of velocity and attenuation.

Real-time characterization of damage in ceramic matrix composites using IR thermography and acoustic emission

Infrared thermography is one of several non-destructive testing techniques which can be used for detection of damage in materials such as ceramic matrix composites. The purpose of this study is to apply a non-destructive methodology for analyzing the thermal effects in ceramic matrix composites caused by cyclic loading. Mechanical stresses induced by cyclic loading cause heat release in the composite due to failure of the interface, which results in increasing the materials temperature. The heat wave, generated by the thermo-mechanical coupling, and the intrinsic energy dissipated during mechanical cyclic loading of the sample were detected by an infrared camera. The results were correlated with acoustic emission events.

Monitoring of fatigue damage in metal plates by acoustic emission and thermography

Acoustic Emission (AE) supplies information on the fracturing behavior of different materials. In this study, AE activity was recorded during fatigue experiments in metal CT specimens with a V-shape notch which were loaded in fatigue until final failure. AE parameters exhibit a sharp increase approximately 1000 cycles before than final failure. Therefore, the use of acoustic emission parameters is discussed both in terms of characterization of the damage mechanisms, as well as a tool for the prediction of ultimate life of the material under fatigue. Additionally, an innovative nondestructive methodology based on lock-in thermography is developed to determine the crack growth rate using thermographic mapping of the material undergoing fatigue. The thermographic results on the crack growth rate of aluminium alloys were then correlated with measurements obtained by the conventional compliance method, and found to be in agreement.

Measurement of elastic wave dispersion on human femur tissue

Cortical bone is one of the most complex heterogeneous media exhibiting strong wave dispersion. In such media when a burst of energy goes into the formation of elastic waves the different modes tend to separate according to the velocities of the frequency components as usually occurs in waveguides. In this study human femur specimens were subjected to elastic wave measurements. The main objective of the study is using broadband acoustic emission sensors to measure parameters like wave velocity dispersion and attenuation. Additionally, waveform parameters like the duration, rise time and average frequency, are also examined relatively to the propagation distance as a preparation for acoustic emission monitoring during fracture. To do so, four sensors were placed at adjacent positions on the surface of the cortical bone in order to record the transient response after pencil lead break excitation. The results are compared to similar measurements on a bulk metal piece which does not exhibit heterogeneity at the scale of the propagating wave lengths. It is shown that the microstructure of the tissue imposes a dispersive behavior for frequencies below 1 MHz and care should be taken for interpretation of the signals.

New Trends in Materials Nondestructive Characterization Using Surface Acoustic Wave Methodologies

The surface of the materials is usually the most sensitive part due to exposure to environmental influence, as well as higher bending and torsional loads than the interior. Therefore, degradation is bound to initiate from the surface in most engineering components. Surface wave propagation in heterogeneous media is a topic concentrating many efforts in the engineering community. The main aim is quality characterization via nondestructive evaluation (NDE) methodologies by correlation of propagation characteristics with material properties. In the present chapter surface waves are examined in structural materials of outmost significance such as aerospace composites and concrete.