Jongbeom Kim

Strength evaluation and failure analysis of unidirectional composites using Monte-Carlo simulation

Abstract Tensile strength and failure process of composite materials depend on the variation in fiber strength, matrix properties and fiber–matrix interfacial shear strength. A Monte-Carlo simulation considering variation in these factors has been widely used to analyze such a complicated phenomenon as a strength and simulate the failure process of unidirectional composites. In this study, a Monte-Carlo simulation using 2-D and 3-D (square and hexagonal array) model was performed on unidirectional graphite/epoxy and glass/polyester composites. The results simulated by using 3-D hexagonal array model have a good agreement with the experimental data which were tensile strength and failure process of unidirectional composites.

Generalization of Integration Methods for Complex Inelastic Constitutive Equations with State Variables

The prediction of the inelastic behavior of the structure is an essential part of reliability assessment procedure, because most of the failures are induced by the inelastic deformation, such as creep and plastic deformation. During decades, there has been much progress in understanding of the inelastic behavior of the materials and a lot of inelastic constitutive equations have been developed. These equations consist of the definition of inelastic strain and the evolution of the state variables introduced to quantify the irreversible processes occurred in the material. With respect to the definition of the inelastic strain, the inelastic constitutive models can be categorized into elastoplastic model, unified viscoplastic model and separated viscoplastic model and the different integration methods have been applied to each category. In the present investigation, the generalized integration method applicable for various types of constitutive equations is developed and implemented into ABAQUS by means of UMAT subroutine. The solution of the non-linear system of algebraic equations arising from time discretization with the generalized midpoint rule is determined using line-search technique in combination with Newton method. The strategy to control the time increment for the improvement of the accuracy of the numerical integration is proposed. Several numerical examples are considered to demonstrate the efficiency and applicability of the present method. The prediction of the inelastic behavior of the structure is an essential part of reliability assessment procedure, because most of the failures are induced by the inelastic deformation, such as creep and plastic deformation. During decades, there has been much progress in understanding of the inelastic behavior of the materials and a lot of inelastic constitutive equations have been developed. These equations consist of the definition of inelastic strain and the evolution of the state variables introduced to quantify the irreversible processes occurred in the material. With respect to the definition of the inelastic strain, the inelastic constitutive models can be categorized into elastoplastic model, unified viscoplastic model and separated viscoplastic model and the different integration methods have been applied to each category. In the present investigation, the generalized integration method applicable for various types of constitutive equations is developed and implemented into ABAQUS by means of UMAT subroutine. The solution of the non-linear system of algebraic equations arising from time discretization with the generalized midpoint rule is determined using line-search technique in combination with Newton method. The strategy to control the time increment for the improvement of the accuracy of the numerical integration is proposed. Several numerical examples are considered to demonstrate the efficiency and applicability of the present method.

Evaluation of Ultrasonic Nonlinear Characteristics in Artificially Aged Al6061-T6

Abstract Generally, the nonlinearity of ultrasonic waves is measured using a nonlinear parameter  , which is defined as the ratio of the second harmonic’s magnitude to the power of the fundamental frequency component after the ultrasonic wave propagates through a material. Nonlinear parameter  is recognized as an effective parameter for evaluating material degradation. In this paper, we evaluated the nonlinear parameter of Al6061-T6 which had been subjected to an artificial aging heat treatment. The measurement was using the transmitted signal obtained from contact-type transducers. After the ultrasonic test, a micro Vickers hardness test was conducted. From the result of the ultrasonic nonlinear parameter, the microstructural changes resulting from the heat treatment were estimated and the hardness test proved that these estimates were reasonable. Experimental results showed a correlation between the ultrasonic nonlinear parameter and microstructural changes produced by precipitation behavior in the material. These results suggest that the evaluation of mechanical properties using ultrasonic nonlinear parameter

Absolute Measurement and Relative Measurement of Ultrasonic Nonlinear Parameters

ABSTRACT The ultrasonic nonlinear parameter is measured from the amplitudes of the harmonic frequency components generated during the propagation of ultrasonic waves in a material. There are two definitions for this parameter: absolute and relative. The absolute parameter is defined by the displacement amplitude; however, it is difficult to measure because of the very small displacement amplitude of the harmonic components. Conversely, the relative parameter is defined by the amplitude of the detected signal, regardless of displacement. Many researchers use the relative parameter because it is easier to measure, although it is only available for a relative comparison of different materials. However, it has not yet been verified that the ratio of the relative parameters between two materials is identical to that of the absolute parameters. In this study, we make it clear that the ratio of the relative parameters is inherently not identical to that of the absolute parameters, but that they can be identical to each other by compensating for material-dependent differences, such as detection-sensitivity and wavenumber. For verification, the absolute and relative parameters were measured for two different materials. The results showed that the ratios of absolute and relative parameters were in good agreement after compensation.

Evaluation of Ultrasonic Nonlinear Characteristics in Heat-Treated Aluminum Alloy (Al-Mg-Si-Cu)

The nonlinear ultrasonic technique has been known to be more sensitive to minute variation of elastic properties in material than the conventional linear ultrasonic method. In this study, the ultrasonic nonlinear characteristics in the heat-treated aluminum alloy (Al-Mg-Si-Cu) have been evaluated. For this, the specimens were heat treated for various heating period up to 50 hours at three different heating temperatures: 250°C, 300°C, and 350°C. The ultrasonic nonlinear characteristics of each specimen were evaluated by measuring the ultrasonic nonlinear parameter β from the amplitudes of fundamental and second harmonic frequency components in the transmitted ultrasonic wave. After the ultrasonic test, tensile strengths and elongations were obtained by the tensile test to compare with the parameter β. The heating time showing a peak in the parameter β was identical to that showing critical change in the tensile strength and elongation, and such peak appeared at the earlier heating time in the higher heating temperature. These results suggest that the ultrasonic nonlinear parameter β can be used for monitoring the variations in elastic properties of aluminum alloys according to the heat treatment.

Robust and non-fragile fuzzy H ∞ controller design for discrete-time systems with parameter uncertainties and time delay

In this paper, we show a controller design method for a class of discrete-time nonlinear systems described in T-S(Takagi-Sugeno) fuzzy model, which has uncertainties in state and input. The robust and non-fragile fuzzy H∞ controller is designed based on Lyapunov function theory, PDC(Parallel Distributed Compensation) and PLMI (Parameterized Linear Matrix Inequality) method. Those ensure the asymptotically stability and the performance of the resulting closed-loop systems for uncertainties. The numerical example and simulation are presented to show the feasibility and effectiveness of the proposed method.

Implementation of wireless industrial networks for industrial smart grids

Though industry is one of the major consumers of electricity but Demand Response (DR) research in the industrial sector is relatively immature. The use of wireless technologies as a part of demand response infrastructure in industry has benefits, including increased flexibility of operation and reduced installation and maintenance costs. In this study, a demonstration system of the infrastructure for demand response in industries is developed using two independent (and competing) standards for industrial wireless networks: WirelessHART and ISA100.11a. The demonstration system is tested by controlling the electrical loads using industrial wireless networks and obtaining the energy consumption data of the electrical loads on a host application for demand response.

Imaging of contact acoustic nonlinearity using synthetic aperture technique.

The angle beam incidence and reflection technique for the evaluation of contact acoustic nonlinearity (CAN) at solid-solid contact interfaces (e.g., closed cracks) has recently been developed to overcome the disadvantage of accessing both the inner and outer surfaces of structures for attaching pulsing and receiving transducers in the through-transmission of normal incidence technique. This paper proposes a technique for B-mode imaging of CAN based on the above reflection technique, which uses the synthetic aperture focusing technique (SAFT) and short-time Fourier transform (STFT) to visualize the distribution of the CAN-induced second harmonic magnitude as well as the nonlinear parameter. In order to verify the usefulness of the proposed method, a solid-solid contact interface was tested and the change of the contact acoustic nonlinearity according to the increasing contact pressure was visualized in images of the second harmonic magnitude and the relative nonlinear parameter. The experimental results showed good agreement with the previously developed theory identifying the dependence of the scattered second harmonics on the contact pressure. This technique can be used for the detection and improvement of the sizing accuracy of closed cracks that are difficult to detect using the conventional linear ultrasonic technique.

Evaluation of Ultrasonic Nonlinear Characteristics in Heat-Treated Aluminum Alloy

In this study, ultrasonic nonlinear characteristics in the heat-treated aluminum alloy have been evaluated. The nonlinearity of ultrasonic wave has been measured as the acoustic nonlinear parameter , depending upon the amplitude ratio of the second-order harmonic and the fundamental frequency component of ultrasonic wave propagating through the materials. The parameter measurement has been carried out with the reflected signals from the back-wall of specimens at the same plane using the contact-type transducers. The heat-treatment, aging, has been achieved at for various durations in the range of 1 to 50 hours. The tensile strength and elongation are obtained by the tensile test and then compared with the parameter . There is a peak of the acoustic nonlinear parameter on 5 hours aging and the decreases thereafter, exhibiting closed relations with tensile strength and elongation. Also, the heat-treatment time showing peak in the parameter was identical to that showing severe change in the curve. These results suggest that the acoustic nonlinear parameter can be used for monitoring the strength variations with aging of aluminum alloys.

Dependence of nonlinear ultrasonic characteristic on second-phase precipitation in heat-treated Al 6061-T6 alloy

HIGHLIGHTSThe nonlinear ultrasonic characteristic in heat‐treated Al 6061‐T6 was investigated.The nonlinearity parameter and the yield strength were measured for aged specimens.The nonlinearity parameter and yield strength showed an interesting correlation.The variation of nonlinearity parameter was explained by the precipitation sequence.The nonlinearity parameter can be used to obtain the maximum strength of Al 6061‐T6. ABSTRACT It is well known that nonlinear ultrasound is sensitive to certain microstructural features in materials such as dislocations and precipitates. This paper investigates the dependence of the nonlinear ultrasonic characteristic on Symbol precipitation in heat‐treated Al 6061‐T6 alloy specimens. The specimens were heat‐treated at a constant temperature of 220 °C for different exposure times up to 6000 min. The accuracy of the ultrasonic measurement setup that determines the absolute ultrasonic nonlinearity parameter Symbol was first validated. The nonlinearity parameter and the yield strength were measured for each of the artificially aged specimens. The experimental results show fluctuations in the nonlinearity parameter and yield strength over the aging time, but with an interesting correlation between the nonlinearity parameter and the yield strength over the aging time. Microstructural observations confirmed that those fluctuations are due to the formation and evolution of precipitates that occurs in a unique precipitation sequence in this alloy. These results suggest that the nonlinear ultrasonic measurement can be useful for monitoring second phase precipitation and related mechanical properties in the Al 6061‐T6 alloy. Symbol. No caption available. Symbol. No caption available.