Experimental and theoretical developments for the acoustoelastic characterization and stress-monitoring of concrete materials and structures

Condition assessment of civil infrastructure often requires knowing the current stress acting on a given structural member. However, the development of an efficient nondestructive testing (NDT) technique for estimating current stresses in structural concrete elements remains open. To this end, previous research have studied the dependence of mechanical wave speed with applied stress, “the acoustoelastic effect”. Recent research on concrete elements under uniaxial compression has shown that the acoustoelastic effect can also be detected with techniques based on vibration phenomena, which offers several benefits. This thesis focuses on studying, documenting and improving the use of resonance vibration for acoustoelastic characterization and current stress determination of slender concrete structural elements under compression. An exhaustive theoretical development using analytical and numerical methods is provided, where the torsional vibration mode is selected over other vibration modes. The nonlinear material parameter βG is defined based on torsional vibration, which corresponds to the rate of change of the elastic shear modulus G with respect to the uniaxial strain. The expression of βG is analytically calculated with respect to the second and third-order elastic constants (l, m, and n) and numerically verified with finite element method (FEM) models. The effect of non-uniform torsion (warping), geometric nonlinearity (P-δ effect) and changing boundary conditions is studied analytically, numerically and experimentally, to assess their effect on βG. Experiments are carried out for three concrete mixture designs using prismatic specimens of dimensions 15 × 15 × 60 cm3; values of βG are calculated for these specimens submitted to several loading and unloading cycles, which proves the existence, dominance and repeatability of the acoustoelastic effect: torsional frequency of vibration increases with increasing compressive strains (and stresses) in elongated elements. A second experimental campaign is conducted using ultrasonic wave propagation and torsional vibration techniques simultaneously on the same mortar specimen. Conversely to the theoretical predictions based on acoustoelasticity, ultrasonic results yield a βG value an order of magnitude lower than the torsional vibration-based βG. To address this apparent contradiction the theory is completed heuristically by accounting for the slight material viscosity. Finally, a case of study of a real size post-tensioned-concrete nuclear-containment structure is presented, where the containment is submitted to gradual internal pressure. Frequencies of vibration are identified using an output-only sensing system and the tracked frequencies are correlated with internal pressure. Both the experiment and an FEM model show that frequencies of vibration increase with increasing internal pressure suggesting that geometric nonlinearity dominates over acoustoelastic effects in this case.Para evaluar el estado de las estructuras civiles es frecuentemente necesario conocer el nivel de tensión que soportan los distintos elementos estructurales. Sin embargo, el desarrollo de técnicas eficientes de ensayos no destructivos para la estimación de tensiones de estructuras de hormigón sigue siendo un tema abierto. Con este fin, investigaciones anteriores han estudiado la dependencia de la velocidad de propagación de ondas mecánicas con la tensión aplicada, esto es el "efecto acustoelástico". Estudios recientes en elementos de hormigón sometidos a compresión axial han mostrado que el efecto acustoelástico también puede ser detectado con técnicas basadas en fenómenos de vibración, lo que ofrece varias ventajas. Esta tesis se centra en el estudio, documentación y mejora del uso de técnicas de vibración (resonancia) para la caracterización acustoelástica y la determinación de tensiones en elementos estructurales alargados de hormigón, sometidos a compresión axial. Se incluye un desarrollo teórico exhaustivo con métodos analíticos y numéricos, donde el modo de vibración torsional se elige frente a los otros posibles modos de vibración. El parámetro nolineal material βG se define en base a la vibración torsional, que corresponde a la tasa de cambio del módulo elástico de corte G respecto a la deformación axial. La expresión de βG se calcula analíticamente respecto a las constantes elásticas de segundo y tercer orden (l, m, y n) y se verifica numéricamente usando modelos de elementos finitos. Los efectos de la torsión no uniforme (alabeo), la no-linealidad geométrica (efecto P-δ) y el del cambio de las condiciones de borde, son estudiados analítica, numérica y experimentalmente, para evaluar su efecto en βG. Se realizan experimentos para tres mezlcas de hormigón usando tres especímenes prismáticos de dimensiones 15 × 15 × 60 cm3; los valores de βG se calculan para estos tres especímenes sometiéndolos a varios ciclos de carga y descarga, lo que prueba la existencia, dominio y repetitividad del efecto acustoelástico: la frecuencia de vibración torsional aumenta al aumentar las deformaciones (y tensiones) de compresión en elementos alargados. Una segunda campaña experimental es realizada usando las técnicas de propagación de ondas ultrasónicas y vibración torsional simultáneamente en el mismo especimen de mortero. Contrariamente a las predicciones teóricas basadas en la acustoelasticidad, el ultrasonido arroja resultados de βG un orden de magnitud menor que los resultados de βG que arroja la vibración torsional. Para afrontar esta aparente contradicción, la teoría se completa heurísticamente teniendo en cuenta fenómenos menores de viscosidad. Finalmente, se presenta un caso de estudio de un estructura a escala real, un tanque de hormigón postensado para la contención de material nuclear, el cual se somete a incrementos graduales de presión interior. Se identifican las frecuencias de vibración usando un sistema de medición basado en técnicas de análisis modal operacional y se observa que las frecuencias de vibración se correlacionan con la presión interna. Tanto el experimento como el modelo de elementos finitos muestran que las frequencias de vibración aumentan al aumentar la presión interna, sugiriendo que, en este caso, la no-linealidad geométrica domina frente a los efectos acustoelásticos

Development of an ultrasonic NDE&T tool for yield detection in steel structures

Nondestructive Evaluation and Testing (NDE&T) is a commonly used and rapidly growing field that offers successful solutions for health assessment of structures. NDE&T methods have gained increasing attention in the last few decades especially with the contribution of the advancements in computer and instrumentation technologies. The applications of numerous NDE&T methods in civil engineering mostly focus on material characterization and defect detection. Techniques for nondestructively identifying the stress state in materials, on the other hand, mostly rely on the Theory of Acoustoelasticity. However, the sensitivity and the accuracy of acoustoelasticity are affected by several factors such as the microstructure of the material, temperature conditions, and the type, propagation and polarization directions of the signals used. This dissertation presents the results of an experimental study that investigates the changes in the characteristics of ultrasonic signals due to the applied stresses. Using a specially built testing system, ultrasonic signals were acquired from four different groups of steel specimens subjected to uniaxial tension below and above the yield stress of the material. The experimental database was first analyzed in terms of the acoustoelastic theory. Then, well known Digital Signal Processing (DSP) methods were used to calculate a total of seven time and frequency domain characteristics of the first three echoes of the acquired signals. The investigated time domain parameters were the peak positive amplitudes and the signal energies of the echoes, while the peak amplitude of the Fast Fourier and Chirp-Z Transforms, peak and peak-to-peak amplitudes and the root mean square of the Wavelet coefficients were used for the spectral analyses. Even though the acoustoelastic effects can be very small for certain measurement cases and they can be influenced by several other factors, clear distinctions between prior to and post yielding were observed for all investigated time and frequency domain parameters. The results were further analyzed with statistical methods and Receiver Operating Characteristics (ROC) curves in order to investigate the potential of the presented study for being used as a nondestructive testing tool for yield detection in steel structures

Novel applications of pulse pre-pump Brillouin Optical Time Domain Analysis for behavior evaluation of structures under thermal and mechanical loading

This study aims to: (1) develop an analytical model for the strain transfer effect of distributed fiber optic sensors in a uniform or non-uniform stress field; (2) develop a measurement approach to monitor strains in concrete and detect damage (e.g. crack and delamination) in bonded and unbonded concrete overlays; (3) characterize the strain and temperature sensitivities of distributed fiber optic sensors at elevated temperatures; (4) develop a thermal annealing approach to enhance the thermal stability and temperature sensitivity of the distributed sensors; and (5) apply the distributed sensors to assess structural behaviors of concrete and steel structures exposed to fire. The pulse pre-pump Brillouin Optical Time Domain Analysis (PPP-BOTDA) was employed to measure strain and temperature distributions along a fused silica single-mode optical fiber. Strain distributions in concrete were measured from the distributed fiber optic sensors embedded in bonded and unbonded concrete overlays. Peaks of the strain distributions represent the effect of concrete cracks and delamination. The strain sensitivity coefficient of distributed sensors was reduced from 0.054 MHz/µε to 0.042 MHz/µε when temperature increased from 22 ⁰C to 750 ⁰C. The temperature sensitivity coefficient of distributed sensors was reduced from 1.349x10-3 GHz/⁰C to 0.419x10-3 GHz/⁰C when temperature increased from 22 ⁰C to 1000 ⁰C. The distributed sensors embedded in concrete beams measured non-uniform temperature distributions with local peaks representing a sudden increase of temperature through concrete cracks. Temperature distributions measured from the distributed sensors attached on steel beams enabled an enhanced thermo-mechanical analysis to understand the structural behaviors of steel beams subjected to fire --Abstract, page iii

Analytical behaviour of FRP strengthened reinforced concrete beams under low velocity impact load incorporating rate dependant material constitutive models

Includes bibliographical references.Since the 1980s, the use of fibre reinforced polymer (FRP) composites in strengthening and rehabilitation of existing reinforced and pre-stressed structures has gained popularity. Versatility, high strength to weight ratio, corrosion resistance, excellent creep and fatigue behaviour, and ease of installation are amongst some of the advantages offered by externally bonded FRP systems over traditional strengthening methods. In addition to strengthening for static loading, there are many scenarios where strengthening is required to elements subjected to dynamic loads. The static behaviour of FRP strengthened RC beams has been the subject of extensive research. However, the dynamic behaviour of FRP strengthened RC beam elements remains unclear. Limited experimental studies are available that are focused on the response of FRP strengthened RC beams subjected to low velocity impact events. Furthermore, many of the Finite Element (FE) analysis models developed in these studies yielded results that were inconsistent with the test data. Key shortcomings of these models relate to a lack of definition of the FRP-concrete bond interface and considering rate dependent material behaviour

Monitoring prestress in plates by sideband peak count-index (SPC-I) and nonlinear higher harmonics techniques

Propagating guided waves in a homogeneous, isotropic, prestressed, hyperelastic plate show nonlinear characteristics that depend on the state of initial prestress. These nonlinear phenomena include higher harmonic generation, occurring when Lamb wave modes of different frequencies (ωa and ωb) are allowed to mix within the material generating secondary waves at frequencies 2ωa, 2ωb and ωa ± ωb. Further, if prescribed internal-resonance conditions are satisfied, the amplitude of secondary waves increases in space, providing a response quantity which is dependent on prestress and easy to be observed. Using the finite element method, in this paper we investigatthe time and space evolution of higher harmonics arising in one-way wave mixing. The influence of prestress on the response is elucidated, observing the nonlinear parameter β. It is further shown that the nonlinear ultrasonic technique called Sideband Peak Count-Index (SPC-I) can provide an effective monitoring tool for prestress

Evaluation of Concrete Constitutive Models for Impact Simulations

The research documented in this thesis deals with computational analysis of reinforced concrete impacted by both hollow and solid missiles as a continuing effort on the work conducted by the Committee on the Safety of Nuclear Installations (CSNI) and Nuclear Energy Agency (NEA). The analysis focuses on comparing two similar material models and their ability to capture the mechanistic response of a reinforced concrete slab subjected to impact loads. The analysis was performed using the Sandia National Laboratories computing software SIERRA Solid Mechanics to run the finite element model. The two constitutive models studied were the Holmquist-Johnson-Cook and Johnson-Holmquist 2 material models. The two material models were run with identical meshes, element types, and boundary conditions and their results were compared to the experimental test data gathered by the CSNI. Both material models proved to be successful in capturing the global flexural response of the reinforced concrete target impacted. However, the fractured damage pattern produced by both material models in the two simulations (hollow/solid) proved that some degree of uncertainty was present in the modeling approach and the material model itself

Bond of reinforcement in concrete under high loading rates

The bond between concrete and reinforcing steel is fundamental to the load bearing capacity of reinforced concrete structures. Several experimental studies indicate strength or rather resistance enhancements coming with increasingly dynamic loading. The phenomenon is known as strain or loading rate effect and its causes are still not fully clarified. The work presented herein provides a numerical view of the bond of reinforcement in concrete and investigates its loading rate dependent behaviour. Finite element analyses focusing on structural and inertia effects are carried out. Modelling is conducted at the rib scale, where bond is predominately controlled by mechanical interaction. In the first step, the model is developed and calibrated. Its quality, credibility, and limitations are assessed by a series of numerical case studies and the results are compared with available experimental data. Numerical parametric studies follow. The loading rate dependence of bond is featured, loading rate dependent characteristics are identified, and conclusions on causes of the phenomenon drawn. It is shown that structural effects are strongly involved and the same holds for hydrostatic pressure stress states and inertia effects. The thesis concludes in reviewing currently available methods for incorporating the results into large-scale simulations and highlighting further investigations and developments that are necessary in order to design dynamic loading-resistant structures in the future

Eighth DOD/NASA/FAA Conference on Fibrous Composites in Structural Design, Part 2

Papers presented at the conference are compiled. The conference provided a forum for the scientific community to exchange composite structures design information and an opportunity to observe recent progress in composite structures design and technology. Part 2 contains papers related to the following subject areas: the application in design; methodology in design; and reliability in design