Kenny Ng

Development of Micromechanics Models and Innovative Sensor Technologies to Evaluate Internal Frost Damage of Concrete

Internal frost damage is a major problem affecting the durability of concrete in cold regions. Micromechanics models and innovative sensor technologies were used to study the mechanisms of frost damage in concrete. Crystallization pressure resulting from ice nucleation within capillary pores is the primary cause of internal frost damage of concrete. Crystallization pressure of a cylinder pore was formulated with interface energy balance with thermodynamics analysis. Crystallization pressure on the pore wall was input for fracture simulation with the extended finite element model on a homogeneous beam sample with a vertical cylinder pore; this simulation led to a straight line. An image sample was obtained with imaging process and ellipse fitting techniques to capture microstructure of a tested specimen. Crack simulation of this image sample with the same cylinder pore under crystallization pressure matched fracture patterns of the single-edge-notched bending specimen. Extended finite element model simulation results were verified by open-mode fracture behavior in middle-notched single-edge-notched bending and freezing tests. An innovative time-domain reflectometry sensor was developed to nondestructively monitor the freezing process. Data show that sensor signals from time-domain reflectometry can detect the freezing degree, an important input parameter. Studies indicate that the micromechanics models and time-domain reflectometry sensor techniques can help practitioners evaluate internal frost damage of concrete. Future work will incorporate sensor measurements into micromechanics models to predict, in real time, internal frost damage process in concrete specimens. The goal of this study was to provide practical nondestructive testing and computational tools for designing concrete that is resistant to freezing damage.

Transmission X-Ray Microscope Nanoscale Characterization and 3D Micromechanical Modeling of Internal Frost Damage in Cement Paste

AbstractThis study employed the transmission X-ray microscope (TXM) characterization and three-dimensional (3D) cohesive zone modeling (CZM) techniques to investigate the internal-frost damage in cement paste samples. The microscale cement samples were tested under controlled freeze-thaw cycles. The TXM technique was applied to perform fast-image acquisition of capillary pores and micro-damage evolution at 30 nm resolutions. The constructed 3D nanostructures of tested specimens were used in two ways: digital sample generation for model simulation and model prediction on crack propagation. The thermodynamics principles were applied to calculate the ice crystallization pressure within saturated pores under subcooling temperatures. The 3D bilinear CZM techniques were applied to predict the internal-frost damage evolution under the calculated crystallization pressure exerted on pore walls. The CZM predicted crack propagation was favorably compared with the TXM captured micro-damage. The micromechanical modeli...

Novel High Pressure Sealing System for Tube Hydroforming Operations

The tube hydroforming (THF) process is a metal forming process that uses a pressurized fluid as the forming mechanism. Recently, this process has increased in popularity in the automotive industry as a method to reduce the number of required components and consolidate parts which can substantially reduce the overall automobile weight. This reduction in weight is a currently pursued method for improving the vehicles fuel economy.At the micro scale, hydroformed tubes have the potential to offer additional benefits with possible uses in medical and MEMS (Microelectromechanical systems) applications. However, this can be a challenge when the forming materials have small mating features. In many macro scale tube hydroforming processes the forming fluid is supplied to the tubes by a tapered filling nozzle inserted inside the inner diameter of the tubes. When considering forming tubes with sub-millimeter features, this poses a significant challenge.This paper explores the design of a new method for creating the required high pressure seal. Specifically, this seal is made on the outside surface of the tube by using a flexible encompassing rubber gasket and two proprietary designed seal cavities. In this study, stainless steel 304 micro tubes of varying outer diameters (1.0 mm and 2.1 mm) and thickness were tested.Copyright

Micromechanics models and innovative sensor technologies to evaluate internal-frost damage of concrete

Internal-frost damage is one of the major problems affecting the durability of concrete in cold regions. This paper presents micromechanics models and innovative sensor technologies to study the fundamental mechanisms of frost damage in concrete. The crystallization pressure due to ice nucleation with capillary pores is the primary cause of internal-frost damage of concrete. The crystallization pressure of a cylinder pore was formulated using interface energy balance with thermodynamics equations. The obtained crystallization pressure on the pore wall was input for the fracture simulation with the developed Extended Finite Element Model (XFEM). The XFEM fracture simulation on a homogeneous beam sample with a vertical cylinder pore leads to a straight line. The XFEM simulation was also conducted on the generated digital sample. The simulation results were favorable compared with the middle-notched single edge beam bending specimen due to the open-mode fracture behavior in both cases. An innovative Time-Domain Reflectometry (TDR) sensor was developed to nondestructively monitor the freezing process. The experimental data shows that the TDR sensor signals can detect the freezing degree, an important input parameter to micromechanics models. These studies indicate that the developed micromechanics models and TDR sensor techniques can be used by the practitioners to evaluate internal-frost damage of concrete. Future work will incorporate the TDR sensor measurements into micromechanics models to real-time predict the internal-frost damage process in concrete specimens. The predicted freeze-thaw damage process will be verified with acoustic emission detection.

Micromechanical analysis of damping performance of piezoelectric structural fiber composites

Recent studies showed that the active piezoelectric structural fiber (PSF) composites may achieve significant and simultaneous improvements in sensing/actuating, stiffness, fracture toughness and vibration damping. These characteristics can be of particular importance in various civil, mechanical and aerospace structures. This study firstly conducted the micromechanical finite element analysis to predict the elastic properties and piezoelectrical coupling parameters of a special type of an active PSF composite laminate. The PSF composite laminates are made of longitudinally poled PSFs that are unidirectionally deployed in the polymer binding matrix. The passive damping performance of these active composites was studied under the cyclic force loadings with different frequencies. It was found that the passive electric-mechanical coupling behavior can absorb limited dynamic energy and delay the structure responses with minimum viscoelastic damping. The actuating function of piezoelectric materials was then applied to reduce the dynamic mechanical deformation. The step voltage inputs were imposed to the interdigital electrodes of PSF laminate transducer along the poled direction. The cyclic pressure loading was applied transversely to the composite laminate. The electromechnical interaction with the 1-3 coupling parameter generated the transverse expansion, which can reduce the cyclic deformation evenly by shifting the response waves. This study shows the promise in using this type of active composites as actuators to improve stability of the structure dynamic.

Integration of computational model and SEM imaging technology to investigate internal frost damage in cementitious materials

This study investigates the internal-frost damage due to ice crystallization pressure in capillary pores of concrete. The pore structures have significant impact on freeze-thaw durability of cement/concrete samples. The scanning electron microscope (SEM) techniques were applied to characterize freeze-thaw damage within pore structure. The digital sample was generated from SEM imaging processing. In the microscale pore system, the crystallization pressures at subcooling temperatures were calculated using interface energy balance with thermodynamic analysis. The largest crystallization pressure on the pore wall was used for the fracture simulation with the developed Extended Finite Element Model (XFEM). The largest crystallization pressure on the pore wall was used for the fracture simulation with the developed Extended Finite Element Model (XFEM). One comparison study between model simulation and test results indicates that internalfrost damage model can reasonably predict the crack nucleation and propagation within multiphase cement microstructure.

Damage investigation of single-edge notched beam tests with concrete specimens using acoustic emission techniques

This study applied the AE techniques with statistical analysis to investigate the damage process of singleedge notched beam (SEB) tests with normal strength concrete specimens. The SEB tests with the labprepared NSC specimens were conducted by employing a clip-gauge controlled servo-hydraulic testing system and an AE damage detection system. It was found that the cumulative AE events with respect to the crack mouth opening displacement (CMOD) or the crack tip opening displacement (CTOD) correlate to the mechanical loading of the specimens. A Weibull rupture probability distribution was proposed to quantitatively describe the mechanical damage behavior under the SEB test. A bi-logarithmic regression analysis was conducted to calibrate the Weibull damage distribution with detected AE signals and to predict the damage process as a function of the crack opening displacements. The calibrated Weibull damage functions were compared among concrete specimens. The comparison results indicate the Weibull function calibrated with AE signals can describe the damage behavior of concrete beam specimens.

Micromechanical analysis and finite element modeling of electromechanical properties of active piezoelectric structural fiber (PSF) composites

This paper presents the combined micromechanics analysis and finite element modeling of the electromechanical properties of piezoelectric structural fiber (PSF) composites. The active piezoelectric materials are widely used due to their high stiffness, voltage-dependent actuation capability, and broadband electro-mechanical interactions. However, the fragile nature of piezoceramics limits their sensing and actuating applications. In this study, the active PSF composites were made by deploying the longitudinally poled PSFs into a polymer matrix. The PSF itself consists a silicon carbide (SiC) or carbon core fiber as reinforcement to the fragile piezoceramic shell. To predict the electromechanical properties of PSF composites, the micromechanics analysis was firstly conducted with the dilute approximation model and the Mori-Tanaka approach. The extended Rule of Mixtures was also applied to accurately predict the transverse properties by considering the effects of microstructure including inclusion sizes and geometries. The piezoelectric finite element (FE) modeling was developed with the ABAQUS software to predict the detailed mechanical and electrical field distribution within a representative volume element (RVE) of PSF composites. The simulated energy or deformation under imposed specific boundary conditions was used to calculate each individual property with constitutive laws. The comparison between micromechanical analysis and finite element modeling indicates the combination of the dilute approximation model, the Mori-Tanaka approach and the extended Rule of Mixtures can favorably predict the electromechanical properties of three-phase PSF composites.

Effect of Continuous Direct Current on the Yield Stress of Stainless Steel 304 Micro Tubes During Hydroforming Operations

Hydroforming at the macro scale offers the opportunity to create products that have superior mechanical properties and intricate complex geometries. Micro tube hydroforming is a process that is gaining popularity for similar reasons. At the same time, due to the physical size of the operations, there are many challenges including working with extremely high pressures and available materials that are typically difficult to form.Increasing the formability of micro tubes during the hydroforming process is desired. Being able to increase the formability is essential because as the tube diameters decrease in size, the required forming pressure increases. As a result, it is important to explore methods to decrease the yield stress during forming operations. Traditional methods for decreasing the materials yield stress typically involve heating either the sample or the process equipment. Using traditional methods typically sacrifice dimensional quality of the part, alter the mechanical properties and also raise the costs of the operations.Electrically Assisted Manufacturing (EAM) is a non-traditional method that is gaining popularity by reducing the necessary forces and pressures required in metal forming operations.© 2012 ASME

Influence of Continuous Direct Current on the Micro Tube Hydroforming Process

Research of the micro tube hydroforming (MTHF) process is being investigated for potential medical and fuel cell applications. This is largely due to the fact that at the macro scale the tube hydroforming (THF) process, like most metal forming processes has realized many advantages. Unfortunately, large forces and high pressures are required to form the parts so there is a large potential to create failed or defective parts. Electrically Assisted Manufacturing (EAM) and Electrically Assisted Forming (EAF) are processes that apply an electrical current to metal forming operations. The intent of both EAM and EAF is to use this applied electrical current to lower the metals required deformation energy and increase the metal’s formability. These tests have allowed the metals to be formed further than conventional methods without sacrificing strength or ductility. Currently, various metal forming processes have been investigated at the macro scale. These tests also used a variety of materials and have provided encouraging results. However, to date, there has not been any research conducted that documents the effects of applying Electrically Assisted Manufacturing (EAM) techniques to either the tube hydroforming process (THF) or the micro tube hydroforming process (MTHF). This study shows the effects of applying a continuous direct current to the MTHF process.© 2011 ASME