• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.42 no.11
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• pp.927-936
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• 2014

This paper presents development and verification of the progressive failure analysis upon the composite laminates. Strength and stiffness of the fiber-reinforced composite are analyzed by property degradation approach with emphasis on the material nonlinearity and continuum damage mechanics (CDM). Longitudinal and transverse tensile modes derived from Hashin's failure criterion are used to predict the thresholds for damage initiation and growth. The modified Newton-Raphson iterative procedure is implemented for determining nonlinear elastic and viscoelastic constitutive relations. Laminar properties of the composite are obtained by experiments. Prediction on the un-notched tensile (UNT) specimen is performed under the laminate level. Stress-strain curves and strength results are compared with the experimental measurement. It is concluded that the present nonlinear CDM approach is capable of predicting the strength and stiffness more accurately than the corresponding linear CDM one does.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.5
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• pp.746-755
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• 1997

This paper addresses the systematic procedure using sequential approach for the analysis of the coupled thermo-mechanical behavior of a steady rolling tire. Not only the knowledge of mechanical stresses but also of the temperature loading in a rolling tire are very important because material damage and material properties are significantly affected by the temperature. In general, the thermo-mechanical behavior of a pneumatic tire is highly complex transient phenomenon that requires the solution of a dynamic nonlinear coupled themoviscoelasticity problem with heat source resulting from internal dissipation and friction. In this paper, a sequential approach, with effective calculation schemes, to modeling this system is presented in order to predict the temperature distribution with reasonable sccuracies in a steady state rolling tire. This approach has the three major analysis modules-deformation, dissipation, and thermal modules. In the dissipation module, an analytic method for the calculation of the heat source in a rolling tire is established using viscoelastic theory. For the verification of the calculated temperature profiles and rolling resistance at different velocities, they were compared with the measured ones.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.12
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• pp.1417-1426
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• 2009

Various types of damper are usually applied to reduce noise and vibration for mechanical systems. Especially, for washing machines, the free-friction stroke damper is installed. The behavior of the free-friction stroke damper has nonlinear characteristics such as hysteresis and viscoelastic properties because of its foam material. First of all, the dynamic experiments were carried out by using a MTS machine to find characteristics of the free-friction stroke damper. And the simulation model of the free-friction stroke damper and characteristics of a foam material were evaluated by using optimization technique. To make a good simulation model which can show the dynamic characteristics, it is important to understand the working mechanism of the damper. The Finite Element Method (FEM) technique can help us instinctively understand the damping phenomenon under operating conditions, because we can observe the condition of damper at every step in the simulation by using it. Also, by changing factors, we can comprehend the variation of characteristics of damper. So, in this paper, a study on the dynamic characteristics of free-friction stroke damper by FEM is focused on. Finally, the possibility which physical experiments can be replaced into simulations is shown.

• Applied Microscopy
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• v.50
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• pp.25.1-25.11
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• 2020

Scanning acoustic microscopy (SAM) or Acoustic Micro Imaging (AMI) is a powerful, non-destructive technique that can detect hidden defects in elastic and biological samples as well as non-transparent hard materials. By monitoring the internal features of a sample in three-dimensional integration, this technique can efficiently find physical defects such as cracks, voids, and delamination with high sensitivity. In recent years, advanced techniques such as ultrasound impedance microscopy, ultrasound speed microscopy, and scanning acoustic gigahertz microscopy have been developed for applications in industries and in the medical field to provide additional information on the internal stress, viscoelastic, and anisotropic, or nonlinear properties. X-ray, magnetic resonance, and infrared techniques are the other competitive and widely used methods. However, they have their own advantages and limitations owing to their inherent properties such as different light sources and sensors. This paper provides an overview of the principle of SAM and presents a few results to demonstrate the applications of modern acoustic imaging technology. A variety of inspection modes, such as vertical, horizontal, and diagonal cross-sections have been presented by employing the focus pathway and image reconstruction algorithm. Images have been reconstructed from the reflected echoes resulting from the change in the acoustic impedance at the interface of the material layers or defects. The results described in this paper indicate that the novel acoustic technology can expand the scope of SAM as a versatile diagnostic tool requiring less time and having a high efficiency.

• Journal of the Korean Institute of Telematics and Electronics
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• v.26 no.6
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• pp.71-78
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• 1989

The imput-output relations for nonlinear systems can be explicitly represented by the Volterra series and they can be characterized by the Volterra kernels. This study is concerned with modeling an elastomer force-displacement relation due to step inputs by utilizing the truncated Volterra series. Since it is practically impossible to apply step inputs that have infinite slope at zero time, the loads due to constant penetration(displacement) rate followed by constant penetration inputs are measured as an alternative approach and estimated for step inputs and then utilized for the truncated Volterra series models. One second order and one third order truncated Volterra series models have been employed to model the force-displacement relation which is one of the prominent properties to characterize the viscoelastic material. The third order truncated Volterra series model has better results, compared with those of the second order truncated Volterra series model.

• The Korean Journal of Rheology
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• v.11 no.2
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• pp.143-152
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• 1999

Using a Rheometrics Fluids Spectrometer(RFS II), the steady shear flow and the small-amplitude dynamic viscoelastic properties of three kinds of semi-solid food materials(mayonnaise, tomato ketchup, and wasabi) have been measured over a wide range of shear rates and angular frequencies. The shear rate dependence of steady flow behavior and the angular frequency dependence of dynamic viscoelastic behavior were reported from the experimentally measured data. In addition, some viscoplastic flow models with a yield stress term were employed to make a quantitative evaluation of the steady flow behavior, and the applicability of these models was also examined in detail. Furthermore, the correlations between steady shear flow(nonlinear behavior) and dynamic viscoelastic(linear behavior)properties were discussed using the modified power-law flow equations. Main results obtained from this study can be summarized as follows : (1) Semi-solid food materials are regarded as viscoplastic fluids having a finite magnitude of yield stress, and their flow behavior shows shear-thinning characteristics, exhibiting a decrease in steady flow viscosity with increasing shear rate. (2) The Herschel-Bulkley, Mizrahi-Berk, and Heinz-Casson models are all applicable to describe the steady flow behavior of semi-solid food materials. Among these models, the Heinz-Casson model has the best validity. (3) Semi-solid food materials show a stronger shear-thinning behavior at shear rate region higher than a critical shear rate where a more progressive structure breakdown takes place. (4) Both the storage and loss moduli are increased with increasing angular frequency, but they have a slight dependence on angular frequency. The elastic behavior is dominant to the viscous behavior over a wide range of angular frequencies. (5) All of the steady flow, dynamic, and complex viscosities are well satisfied with the power-law model behavior. The relationships between steady shear flow and dynamic viscoelastic properties can well be described by the modified forms of the power-law flow equations.

• Korea-Australia Rheology Journal
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• v.13 no.1
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• pp.29-35
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• 2001

The development of filament stretching extensional rheometers over the past decade has enabled the systematic measurement of the transient extensional stress growth in dilute and semi-dilute polymer solutions. The strain-hardening in the extensional viscosity of dilute solutions overwhelms the perturbative effects of capillarity, inertia & gravity and the kinematics of the extensional deformation become increasingly homogeneous at large strains. This permits the development of a robust open-loop control algorithm for rapidly realizing a deformation with constant stretch history that is desired for extensional rheometry. For entangled fluids such as concentrated solutions and melts the situation is less well defined since the material functions are governed by the molecular weight between entanglements, and the fluids therefore show much less pronounced strain-hardening in transient elongation. We use experiments with semi-dilute/entangled and concentrated/entangled monodisperse polystyrene solutions coupled with time-dependent numerical computations using nonlinear viscoelastic constitutive equations such as the Giesekus model in order to show that an open-loop control strategy is still viable for such fluids. Multiple iterations using a successive substitution may be necessary, however, in order to obtain the true transient extensional viscosity material function. At large strains and high extension rates the extension of fluid filaments in both dilute and concentrated polymer solutions is limited by the onset of purely elastic instabilities which result in necking or peeling of the elongating column. The mode of instability is demonstrated to be a sensitive function of the magnitude of the strain-hardening in the fluid sample. In entangled solutions of linear polymers the observed transition from necking instability to peeling instability observed at high strain rates (of order of the reciprocal of the Rouse time for the fluid) is directly connected to the cross-over from a reptative mechanism of tube orientation to one of chain extension.

• Journal of the Korean Society of Propulsion Engineers
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• v.17 no.2
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• pp.57-62
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• 2013

A crack in a solid propellant increases the area of burning surface, which leads to excessive burning that causes motor failure. Therefore, it is necessary to evaluate fracture toughness of solid propellants. However, it is very difficult to measure fracture toughness of solid propellants because of the nonlinear mechanical behavior. In this study, evaluation of fracture toughness on a solid propellant was carried out under the assumption that the solid propellant is a linear viscoelastic material. Actual displacements from fracture toughness tests using CCT specimens were converted into pseudo-elastic displacements by using stress relaxation characteristics and fracture toughness was evaluated using ASTM E399 standard. Also, effects of test temperature and speed on the fracture toughness were considered.

• Design & Manufacturing
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• v.16 no.1
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• pp.27-35
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• 2022

Thermoforming is a plastic manufacturing process that applies a force to stretch a film of heated thermoplastic material over an engineered mold to create a 3-dimensional shape. After forming, the shaped part can then be trimmed and finished to specification to meet an end-user's requirements. The process and thermoplastic materials are extremely versatile and can be utilized to manufacture parts for a very wide range of applications. In this study, based on K-BKZ nonlinear viscoelastic model, thermoforming process analysis was performed for an interior room-lamp. The predicted thickness was minimum at the corner of a molded film, and maximum at the center of the bottom. By using the Taguchi method of design of experiments, the effects of process conditions on residual stresses were investigated. The dominant factors were the liner thickness and the film heating time. As the thickness of the liner increased, the residual stress decreased. And it was found that the residual stress decreased significantly when the film heating temperature was higher than the glass transition temperature. A thermoforming mold and a trimming mold were manufactured, and the spring back was investigated through experiments. The dominant factors were film heating time, liner thickness, and lower mold temperature. As the film heating time and liner thickness increased, the spring back decreased. In addition, it was found that the spring back decreased as the lower mold temperature increased.