D. Van Hemelrijck

Detecting the Activation of a Self-Healing Mechanism in Concrete by Acoustic Emission and Digital Image Correlation

Autonomous crack healing in concrete is obtained when encapsulated healing agent is embedded into the material. Cracking damage in concrete elements ruptures the capsules and activates the healing process by healing agent release. Previously, the strength and stiffness recovery as well as the sealing efficiency after autonomous crack repair was well established. However, the mechanisms that trigger capsule breakage remain unknown. In parallel, the conditions under which the crack interacts with embedded capsules stay black-box. In this research, an experimental approach implementing an advanced optical and acoustic method sets up scopes to monitor and justify the crack formation and capsule breakage of concrete samples tested under three-point bending. Digital Image Correlation was used to visualize the crack opening. The optical information was the basis for an extensive and analytical study of the damage by Acoustic Emission analysis. The influence of embedding capsules on the concrete fracture process, the location of capsule damage, and the differentiation between emissions due to capsule rupture and crack formation are presented in this research. A profound observation of the capsules performance provides a clear view of the healing activation process.

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.

A Review of Biaxial Test Methods for Composites

This paper will give an overview of the existing biaxial test methods for composite materials from the early beginning till the currently used methods. An evolution in both the biaxial testing capabilities and the applied inspection methods will be shown. The use of composite materials in aerospace, aviation and automotive industry has increased rapidly in recent years, In general, composite laminates are developing multiaxial stress states [1]. However, there is little existing capability to evaluate the full multiaxial (or even the biaxial) response of composite materials, even though large demand for such information exists [2].

Thermal characterization of anisotropic media in photothermal point, line, and grating configuration

This article is dedicated to the simulation of heat diffusion in layered anisotropic materials. The three-dimensional heat diffusion equation for layered, anisotropic materials is used to calculate a laser induced dynamic temperature distribution, the so-called thermal-wave field, in composites. In the case of isotropic materials, the thermal-wave distribution is always axisymmetric around the center of the heat source. In multilayered fiber reinforced composites, however, the distribution of the thermal-wave field depends on their characteristic stacking sequence as well as on the geometry and the frequency of the source. Together with undergoing experimental work, these theoretical simulations allow us to compare the feasibility of different spatial excitation geometries, namely, point source, line source, and grating source, to determine the thermal conductivity tensor of composite materials. It is proven that the use of a grating suppresses the ill posedness of the inverse problem and simplifies the p...

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.

Biaxial testing of fibre-reinforced composite laminates:

Advanced composite material systems are increasingly used in almost every industrial branch. The structural components manufactured from these composite material systems are usually subjected to complex loading that leads to multi-axial stress and strain fields at critical surface locations. The current practice of using solely uniaxial test data to validate proposed material models is wholly inadequate. In order to test closer to reality, a biaxial test bench using four servo-hydraulic actuators with four load cells was developed. Besides the development of the test facility, a mixed numerical/experimental method was developed to determine the in-plane stiffness parameters from testing a single cruciform test specimen. To obtain the strength data an optimized geometry for the cruciform type specimen was designed. For the optimization procedure a full three-dimensional finite element model was used. The numerical results were validated with strain gauge, digital image correlation, and electronic speckle pattern interferometry data. The material system used for the experimental validation was glass fibre-reinforced epoxy with a lay-up [(+45°−45° 0°)4(+45°−45°)] typically used for wind turbine blades.

The effects of motion on thermoelastic stress analysis

Abstract An indepth examination of the effects of specimen motion on the technique of thermoelastic stress analysis is presented. It is shown that the presence of a non-uniform temperature distribution, coupled with the material motion due to cyclic loading, can greatly corrupt the measured data. This is particularly important when analysing composite materials in which heat is generated due to viscoelastic processes and/or to internal friction at delamination sites. A series of experiments using delaminated composite specimens as well as an externally heated specimen are presented to illustrate this problem and a mathematical model is developed to account for this phenomenon. Methods to remedy this problem are suggested and in the case of symmetrical conditions, it is shown that this effect can be effectively removed by appropriate post-processing of the data.

Influence of Re-adhesion on the Wear and Friction of Glass Fibre–Reinforced Polyester Composites

Based on the well-known pin-on-disc test rig, a new test setup for online measuring of wear and friction behaviour of polymer matrix composites has been developed. In contrast to a traditional friction-and-wear test rig, a steel pin and composite disc are used for studying the influence of wear debris and fibre orientation. During sliding, a thin adhesive film is possibly formed on the wear track of a composite disc, consisting of wear debris that is squeezed under the steel pin and that finally smoothens onto the composite surface. By optical microscopy, it was observed that most of the debris particles originate from the edges of the wear track. The thin film deforms continuously, with large and dark wear particles observed at the edge of the wear track. A lower coefficient of friction is achieved when the particles are re-adhered to the mating surface. The film formation mechanism depends on the normal force, sliding velocity, and bulk composite structure: because pultruded composite profiles are presently used with a layered structure, a change in film properties is observed depending on the wear depth.

Visualization Of The Healing Process OnReinforced Concrete Beams By Application OfDigital Image Correlation (DIC)

Fabrication of concrete with self-healing capabilities has recently become a hot research topic. In general, material science is focused on the development of smart engineering concrete and cementitious composites with an extended service life. Indeed, materials that remain durable and keep their mechanical performance, damage mechanisms occurring should heal by themselves. In the case of this study, formation of damage and recovery of the mechanical properties is investigated by application of an encapsulated healing agent. On an experimental level, it is imperative to implement an optical, non-contact and online technique to visualize and compare the crack propagation at the loading and reloading (when the initial cracks are filled by the healing agent) stage. For that reason, optical measurements by application of Digital Image Correlation (DIC) are performed during the tests. Processing images captured by a 4-digital cameras system during all the loading stages of four-point bending tests give a fullfield view of the crack displacement and strain profiles. A step further, the visualization of the cracking phenomena by DIC offers a useful tool to apply fracture theories of concrete on healing systems.

Thermoelastic Stress Analysis of Fibre Reinforced Composite Systems

The thermoelastic effect first analysed by Lord Kelvin is governed by a simple relation between amplitude of temperature and amplitude of the sum of principle stress change as long as adiabatic conditions are maintained. Although valid for isotropic materials it is shown that this simple relation leads to very poor results for anisotropic materials. A non-adiabatic theory taking into account the interlaminar heat transfer shows a better agreement between theory and experiment.