In recent years, the damage assessment by means of Laser Doppler Vibrometry (LDV) has become very attractive as it provides non-contact, non-destructive, accurate and improved evaluation of advanced materials. This study deals with the development of advanced software based on LabVIEW in order accurate and automated measurements of acoustic activity to be achieved. Furthermore, this automated method was applied for damage detection in aluminum 1050 Η16 undergone cyclic mechanical loading. LDV was used to measure the amplitude of a Rayleigh surface wave propagating in aluminium specimens. Rayleigh waves are experimentally generated with a piezoelectric transducer and detected by LDV. The proposed measurement technique is used to assess the damage and its evolution, in terms of the increasing amplitude of Rayleigh wave, in 1050 H16 specimens under cyclic mechanical loading. In addition, the reduction in the Rayleigh wave velocity it depends on ultimate fatigue strength of material. The development of this process allows the automated, improved and detailed damage assessment of composite materials.
One of the important characteristics of metallic structures affecting structural integrity is their behavior in corrosive environment. In this respect, aircraft components made from aluminum alloys can catastrophically fail due to pitting corrosion and fatigue damage. Pitting, because of stress concentration, is responsible for fatigue crack nucleation in the material. In the current study, tensile-shape samples of aluminum alloy are immersed in NaCl solution, which simulates the natural exposure in a marine environment. This has an objective to induce accelerated electrochemical damage of the material under testing by the controlled pitting corrosion in a specific area of the surface using different electrochemical techniques, while the rest of the specimen remains completely sealed. In order to investigate the effect of pitting corrosion on the degradation of the material’s mechanical performance, the specimens were subjected to cyclic loading. The corrosion fatigue testing results were compared to data obtained from the uncorroded materials. Using a scanning white-light interferometer the pits' morphology was characterized and the effect of corrosion on the fatigue life was assessed. The results were validated using two complimentary nondestructive techniques, namely infrared thermography and acoustic emission.
Cement matrix composites with a conductive nano-reinforcement phase, lead to the development of innovative products. A matrix with carbon based nano-inclusions (graphene, carbon nanotubes, carbon nanofibers, carbon black) obtains multi-functional properties like enhanced mechanical, electrical, elastic and thermal properties and, therefore, the advantage of self-sensing in case of an inner defect. This research aims to characterize the nano-modified cement mortars with different concentrations of graphene nanophase. The results will be compared with data obtained from nanomaterials containing multi-walled carbon nanotubes. Comprehensive characteristics of these cement-based nanocomposites have been determined using destructive and nondestructive laboratory techniques. Flexural and compressive strength were measured. During four point bending tests, acoustic emission monitoring allowed for realtime identification of the damage process in the material. The electrical surface resistivity of graphene-reinforced cement mortars was measured by applying a known DC voltage, and compared to the electrical resistivity of nano-modified mortars with carbon nanotubes.
Scanning acoustic microscopy uses a focused acoustic beam to investigate local elastic properties on the surface of a material. The measurement is based on the difference in propagation time between the direct reflection and the Rayleigh wave. This work deals with the development of a fully automated acoustic microscopy method in order to determine the near-surface elastic property and map sub-surface features in metallic and composite materials. This method allows for the detection and analysis of Rayleigh waves, which are sensitive to subtle changes in the material’s local elasticity. Via this process, the periodicity of the V(Z) curve can be initially assessed and the local Rayleigh velocity of the material is determined. In this work, the automated acoustic microscopy method was applied for the assessment of aluminum and Al-SiC metal matrix composites.
This paper deals with the use of complimentary nondestructive methods for the evaluation of damage in engineering materials. The application of digital image correlation (DIC) to engineering materials is a useful tool for accurate, noncontact strain measurement. DIC is a 2D, full-field optical analysis technique based on gray-value digital images to measure deformation, vibration and strain a vast variety of materials. In addition, this technique can be applied from very small to large testing areas and can be used for various tests such as tensile, torsion and bending under static or dynamic loading. In this study, DIC results are benchmarked with other nondestructive techniques such as acoustic emission for damage localization and fracture mode evaluation, and IR thermography for stress field visualization and assessment. The combined use of these three nondestructive methods enables the characterization and classification of damage in materials and structures.
The corrosion behavior of metallic structures is an important factor of material performance. In case of aluminum matrix composites corrosion occurs via electrochemical reactions at the interface between the metallic matrix and the reinforcement. The corrosion rate is determined by equilibrium between two opposing electrochemical reactions, the anodic and the cathodic. When these two reactions are in equilibrium, the flow of electrons from each reaction type is balanced, and no net electron flow occurs. In the present study, aluminum alloy tensile-shape samples are immersed in NaCl solution with an objective to study the effect of the controlled pitting corrosion in a specific area. The rest of the material is completely sealed. In order to investigate the effect of pitting corrosion on the material performance, the specimens were subjected to cyclic loading. The effect of corrosion on the fatigue life was assessed using two complimentary nondestructive methods, infrared thermography and acoustic emission.
This work deals with the development of a new class of metamaterials based on phononic composite structures that can offer vibration protection in a wide range of applications. Such phononic heterostructures is a class of phononic crystals that exhibit spectral gaps with lattice constants of a few orders of magnitude smaller than the relevant acoustic wavelength. The design of a phononic composite metamaterial is based on the formation of omnidirectional frequency gaps. This is very much relevant to the dimensionality of a finite slab of the crystal. In this respect, two dimensional structures are used to cut off acoustic waves. In this study, different infrared thermography techniques were used in order to assess the phononic structure’s geometry, as well as to determine the thermal properties of the metamaterial.
The present paper describes the acoustic emission (AE) behavior and the mechanical properties of Portlant cement-based mortars due to the addition of multi wall carbon nanotubes (MWCNTs). This research aims in investigating the crack growth behavior of modified cement mortar with MWCNTs that act as nanoreinforcement during an unaxial compression test using acoustic emission technique. MWCNTs were used in various concentrations inside the matrix. Density, sound's speed, modulus, bending strength, compression strength were studied for five different concentrations. The adding and the increase of MWCNTs concentrations upper to 0.2 % by weight of cement not improving the mechanical properties of cement-based mortar but increase the acoustic emission activity.
The addition of a conductive admixture in a cement-based material could lead to innovative products with multifunctional features. These materials are designed to possess enhanced properties, such as improved mechanical properties, electrical and thermal conductivity, and piezo-electric characteristics. Carbon nanotubes (CNTs) can be used as nano-reinforcement in cement-based materials because of their huge aspect ratio as well as their extremely large specific surface area. For cement-based composites, one of the major types of environmental attack is the chloride ingress, which leads to corrosion of the material and, subsequently, to the reduction of strength and serviceability of the structure. A common method of preventing such deterioration is to avert chlorides from penetrating the structure. The penetration of the concrete by chloride ions is a slow process. It cannot be determined directly in a time frame that would be useful as a quality control measure. Therefore, in order to assess chloride penetration, a test method that accelerates the process is needed, to allow the determination of diffusion values in a reasonable time. In the present research, nanomodified mortars with various concentrations of multi-wall carbon nanotubes (0.2% wt. cement CNTs - 0.6% wt. cement CNTs) were used. The chloride penetration in these materials was monitored according to ASTM C1202 standard. This is known as the Coulomb test or Rapid Chloride Permeability Test (RCPT).
This research aims to investigate the influence of the nano-reinforcement on the thermal properties of cement mortar.
Nano-modified cement mortar with carbon nanotubes (CNTs) leading to the development of innovative materials
possessing multi-functionality and smartness. Such multifunctional properties include enhanced mechanical behavior,
electrical and thermal conductivity, and piezo-electric characteristics. The assessment of the thermal behavior was
evaluated using IR Thermography. Two different thermographic techniques are used to monitor the influence of the
nano-reinforcement. To eliminate any extrinsic effects (e.g. humidity) the specimens were dried in an oven before
testing. The electrical resistivity was measured with a contact test method using a custom made apparatus and applying a
known D.C. voltage. This study indicate that the CNTs nano-reinforcement enhance the thermal and electrical properties
and demonstrate them useful as sensors in a wide variety of applications.
Cement-based materials have in general low electrical conductivity. Electrical conductivity is the measure of the ability of the material to resist the passage of electrical current. The addition of a conductive admixture such as Multi-Walled Carbon Nanotubes (MWCNTs) in a cement-based material increases the conductivity of the structure. This research aims to characterize nano-modified cement mortars with MWCNT reinforcements. Such nano-composites would possess smartness and multi-functionality. Multifunctional properties include electrical, thermal and piezo-electric characteristics. One of these properties, the electrical conductivity, was measured using a custom made apparatus that allows application of known D.C. voltage on the nano-composite. In this study, the influence of different surfactants/plasticizers on CNT nano-modified cement mortar specimens with various concentrations of CNTs (0.2% wt. cement CNTs - 0.8% wt. cement CNTs) on the electrical conductivity is assessed.
This research aims in characterizing modified cement mortar with carbon nanotubes (CNTs) that act as nanoreinforcements
leading to the development of innovative materials possessing multi-functionality and smartness. Such
multifunctional properties include enhanced mechanical behavior, electrical and thermal conductivity, and piezo-electric
characteristics. The effective thermal properties of the modified nano-composites were evaluated using IR
Thermography. The electrical resistivity was measured with a contact test method using a custom made apparatus and
applying a known D.C. voltage. To eliminate any polarization effects the specimens were dried in an oven before testing.
In this work, the thermal and electrical properties of the nano-modified materials were studied by nondestructively
monitoring their structural integrity in real time using the intrinsic multi-functional properties of the material as damage