Hwai Chung Wu

On stress concentrations for isotropic/orthotropic plates and cylinders with a circular hole

Scale factors (SFs) are widely used in engineering applications to describe the stress concentration factor (SCF) of a finite width isotropic plate with a circular hole and under uniaxial loading. In this paper, these SFs were also found to be valid in an isotropic plate with biaxial loading and an isotropic cylinder with uniaxial loading or internal pressure, if a suitable hole to structure dimension ratio was chosen. The study was further expanded to consider orthotropic plates and cylinders with a center hole and under uniaxial loading. The applicable range of the SFs was given based on the orthotropic material parameters. The influence of the structural dimension on the SCF was also studied. An empirical calculation method for the stress concentrations for isotropic/orthotropic plates and cylinders with a circular hole was proposed and the results agreed well with the FEM simulations. This research work may provide structure engineers a simple and efficient way to estimate the hole effect on plate structures or pressure vessels made of isotropic or orthotropic materials.

Failure analysis of FRP sandwich bus panels by finite element method

The weakest part of fiber reinforced polymer composites (FRP) sandwich structures is commonly the interfacial zone between layers. Under service loading, delamination failure will be most likely to initiate from these interfaces. Typical failure analysis for such sandwich structures usually depends on the modeling details of the interface. In this paper, the sandwich panel was modeled as separated layers with appropriate constrains imposed between them. These constrains guaranteed a smooth stress transfer from the skins to the core. The proposed FEM model was applied to simulate the failure behavior of a FRP sandwich panel that is used in bus body. The simulation results were compared with other numerical predictions and the experiment. It was concluded that this model is very efficient computationally for analyzing the failure issues of FRP sandwich structures.

Nonlinear acoustic nondestructive testing for concrete durability

Several nondestructive testing methods can be used to determine the damage in a concrete structure. Linear ultrasonic techniques, e.g. pulse-velocity and amplitude attenuation, are very common in nondestructive evaluation. Velocity of propagation is not very sensitive to the degrees of damage unless a great deal of micro-damage having evolving into localized macro-damage. This transition typically takes place around 80% of the ultimate compressive strength. Amplitude attenuation is potentially more sensitive than pulse-velocity. However, this method depends strongly on the coupling conditions between transducers and concrete, hence unreliable. A baseline test of the linear acoustics of several mortar samples was conducted. These mortar samples have been previously damaged to different levels. Several other testing methods were also performed on the same samples to form a comparison. The focus is in comparing the sensitivity of a new testing method (Non-linear Acoustic NDE) with several more traditional testing methods. Non-linearity of the material stiffness is expressed in non-linear acoustics as the effect that damage and flaws have on the modulation of a signal as it propagates through the material. Spectral (non-linear) analysis is much more sensitive to lower damage states and less dependent on the repeatability of the coupling of the transducers.

Application of Hilbert-Huang transformation for detection of damage in concrete

Several nondestructive testing methods can be used to estimate the extent of damage in a concrete structure. Pulse-velocity and amplitude attenuation methods are very common in nondestructive ultrasonic evaluation. Velocity of propagation is not very sensitive to the degree of damage unless a great deal of micro-damages have evolved into localized macro-damages. The amplitude attenuation method is potentially more sensitive to damage than the pulse-velocity method. However, this method depends strongly on the coupling conditions between the transducers and the concrete and hence is unreliable. In a previous study, a new active modulation approach, Nonlinear Active Wave Modulation Spectroscopy, was developed and found promising for early detection of damage in concrete. In this procedure, a probe wave is passed through the system in a fashion similar to regular acoustic methods for inspection. Simultaneously, a second, low-frequency modulating wave is applied to the system to effectively change the size and stiffness of flaws microscopically and cyclically, thereby causing the frequency modulation to change cyclically as well. The resulting amplified modulations can be correlated to the extent of damage and quantification of small damage becomes possible. In this paper, we present the use of Hilbert-Huang transform to significantly enhance the damage detection sensitivity of this modulation method by performing time-frequency decomposition of nonlinear non-stationary time-domain responses.

Effect of geometric nonlinearity on acoustic modulation

Non-linear nondestructive testing is different from linear acoustic in that it correlates the presence and characteristics of a defect with acoustical signals whose frequencies differ from the frequencies of the emitted probe signal. The difference in frequencies between the probe signal and the resulting frequencies is due to a nonlinear transformation of the probe signal as it passes through a defect. Under acoustic interrogation due to longitudinal waves, as the compression phase passes the defect the two sides of the interface are in direct contact and the contact area increases. Similarly, the tensile phase passes through the defect, the two sides separate and the contact area decreases, thereby modulating the signal amplitude. The contact area depends on the roughness of the surface and on the magnitude of the cohesive forces that arise from the small crack openings. Such cohesive forces may be attributed to aggregate interlock (in plain concrete), fiber bridging (in fiber reinforced concrete) or both. In this paper, the frequency shifts of the probe elastic wave will be analytically related to the roughness and varying cohesive forces of the crack-like defect.

Strategic sensor locations of FPR bridge decks

Advanced fiber-reinforced polymer composite (FRP) has been increasingly used in bridge deck to replace concrete or steel. A FRP bridge deck can be designed to meet AASHTO HS-25 load requirements. FRP decks have many advantages over the conventional reinforced concrete or steel decks owing to their lightweight, high strength and corrosion resistance. However, such new deck system requires extensive monitoring to ensure its designed performance before its widespread acceptance by the bridge community. For inspection and evaluation purpose, a proper monitoring system consisting of various kinds of sensors installed in the FRP deck is critical. This paper provides a framework for designing an efficient monitoring system. The strategic sensor locations are identified based on the stress analysis of the FRP deck.

Nonlinear active wave modulation approach for evaluating bond conditions between adhesively bonded FRP sheet/concrete substrate

Many recent studies have indicated that externally bonded Fiber Reinforced Plastic (FRP) composite sheets can be used effectively to strengthen concrete structures. Composite action suggests that the individual parts of a composite work together as one. Stresses must be transferred from one constituent to the other through some interface. If there is little or no bond at the interface then there is little or no transfer of stress and therefore poor composite action. In this study, we intend to develop a NDE tool for evaluating such interfacial bond conditions. An innovative active modulation approach, Nonlinear Active Wave Modulation Spectroscopy (NAWMS), is adopted in our study. In this procedure, a probe wave will be passed through the system. Simultaneously, a second, modulating wave will be applied to the system. Using three-inch thick concrete slabs we have prepared several FRP laminated samples with artificial flaws (one-inch square) designed into the interface. The sender is on one side of the flaw and the receiver on the other. Maintaining the separation of this pair of transducers, we progressively move them away from the flaw location. The resulting frequency modulations will be analyzed, and will be correlated to the presence of the flaws.