Jaeung Chung

Compressive Response and Failure of Circular Cell Polycarbonate Honeycombs Under Inplane Uniaxial Stresses

The crushing response of polycarbonate circular cell honeycomb to inplane uniaxial loading under displacement control is analyzed through a combination of experiment and numerical simulation. The experiments, which correspond to two different uniaxial loading conditions, are performed using honeycomb material which has a nearly periodic microstructure. In the initial part of the response, the specimens deform in a uniform fashion. Next, a nonlinear phase characterized by progressive localization of deformation is observed. The progressive localization causes the walls of each cell to contact. These experimental results are simulated through numerical analysis using the finite element method. The reasons for the orthotropic response of the honeycombs are discussed.

Compressive Response of Honeycombs Under In-Plane Uniaxial Static and Dynamic Loading, Part 1: Experiments

The crushing response of polycarbonate circular cell honeycombs under in-plane uniaxial loading is studied through a combination of static and dynamic experiments. The circular cell honeycomb material has a nearly periodic microstructure. The static experiments correspond to a uniaxial loading condition under displacement control in two different principal in-plane directions. The corresponding dynamic experiments are carried out using a low-velocity impact drop test facility. Three different initial conditions (corresponding to different drop heights) are used in the dynamic tests. In the initial part of the response, the specimens deform in an almost uniform fashion. Next, a nonlinear phase characterized by progressive localization of deformation is observed. The progressive localization causes the walls of each cell to contact. The reasons for the observed orthotropic response of the honeycombs are discussed. A comparison of the collapse mechanisms between static and dynamic experiments is included. The experimental work is presented first (Part 1), followed by the simulation studies (Part 2).

Strain-Rate Effects on Unidirectional Carbon-Fiber Composites

Strain-rate effects on mode I fracture of unidirectional carbon-e ber tow composites corresponding to crack propagation parallel to the e ber tow direction was investigated. Precracked unidirectional stitched carbon-e ber specimensweresubjected to a staticandlow-velocity-impactthree-pointbend test.Thecrack position asa function oftime and hencethecrack-propagation velocity were measured with thehelp ofspecial crack-propagation gauges and a high-resolution digital camera. Load vs load point displacement was measured for every test. The effect of strain rate on fracture energy was characterized. The Iosipescu shear test under static and low-velocity-impact loading conditions wasused to characterizetherate-dependent shearresponse ofthematerial. In addition, tension and compression responses were characterized using American Society for Testing and Materials standard test cone gurations. It is found that the mode I fracture energy decreases with an increase in the rate of loading. with the failure event. The resin used for the tubes is slightly rate sensitive; however,thisindicatesthat theresin isstiffer and stronger under dynamic conditions. Yet, the dynamically crushed tubes con- sistently absorblessenergythan thestatically crushedtubes,always atalowermeanplateauload.Thus,itappearsthattheratesensitivity of the fracture events warrant a careful examination. The basic building block of braided composite plaques are e ber tows that are braided into different microstructural architectures prior to being infused with resin. Different types of braided archi- tectures are summarized in the text. 1 Prior to studying the fracture propertiesofthebraidedplaques (thebraidedplaquescontainacom- plex internal microstructure ), it is prudent to understand the various fracturemechanisms andfractureproperties of thetows themselves. Fundamental issues related to mode I, mode II, and mixed mode fracture of stitched tow-reinforced composites need investigation. The mode I, mode II, and mixed mode fracture energies are fun- damental properties of a e ber-reinforced composite. Consequently, measurement of these fracture energies is necessary for properly characterizing the response and failure of structures made of these composites. In this paper we present the results of an experimental study that examined the mode I fracture of unidirectional carbon- e ber tow composites with crack propagation along the e ber tow direction. The present investigation of crack growth is limited to low ve- locity impact (LVI) conditions. Under these conditions the dy- namic stress e eld produced by the impact loading subsides, and this transient e eld occurs at the very early stages of loading. In the present experiments the maximum impactor velocity is 4.6 m/s, re- sulting in maximum crack-propagation velocities on the order of 350 m/s. These velocities are a small fraction of the shear wave speed (1540 m/s) and Rayleigh wave speed of the material. Con- sequently, dynamic effects can be neglected. Researchers 7i11 have conducted an extensive experimental and numerical investigation of dynamic crack propagation in unidirectional continuous e ber (prepreg)-laminated composites. The present study examines con- tinuous e ber (e ber tows) unidirectional composites under LVI con- ditions.

The micropolar elasticity constants of circular cell honeycombs

In this paper, a set of expressions for the characterization of circular cell honeycombs as micropolar elastic solids is derived using a combination of non-dimensional analysis and numerical analysis. Closed-form expressions for the four in-plane (i.e. the plane normal to the generators of the cells) micropolar compliances are derived in terms of the cell size, cell thickness and the linear elastic properties of the cell wall material. Independent analyses are conducted to verify the accuracy of the derived constants.

Elastic imperfection sensitivity of hexagonally packed circular–cell honeycombs

The sensitivities of the in–plane macroscopic linear stiffnesses of perfectly elliptical–cell honeycombs to geometric imperfections are derived through an analytical method and a finite–element–based numerical solution. The sensitivities of the honeycomb stiffness to three types of initial imperfections are studied. These imperfections are the deviation from circularity of a cell, a uniform change in thickness of honeycomb cell walls and thickness variations along the honeycomb cell wall. The sensitivities of the in–plane macroscopic stiffnesses to the deviation from circularity of the cell and to the thickness change of the honeycomb cells are derived analytically. The sensitivities of the in–plane properties to thickness variations along the cell wall are obtained numerically.

Compressive Response of Honeycombs Under In-Plane Uniaxial Static and Dynamic Loading, Part 2: Simulations

The static and dynamic experimental results of polycarbonate circular cell honeycombs subjected to in-plane uniaxial loading are simulated through numerical analysis using the finite element method. The experimental results were presented in Part 1 (Chung, J., and Waas, A. M., Compressive Response of Honeycombs Under In-Plane Uniaxial Static and Dynamic loading, Part 1: Experiments. AIAA Journal, Vol. 40, No. 5, 2002, pp. 966-973). Through a comparison between the experimental results and numerical analysis, the crushing mechanism of the circular cell honeycomb material is studied. The influence of friction between the loading plate and the honeycomb material is studied numerically in the simulation of the dynamic experimental results.

Collapse, Crushing and Energy Absorption of Circular-Celled Honeycombs

The crushing response of polycarbonate circular cell honeycomb to inplane uniaxial loading under displacement control is analyzed through a combination of experiment and numerical simula- tion. The experiments corresonding to two differ- ent uniaxial loading conditions are performed using honeycomb material which has a nearly periodic mi- crostructure. In the initial part of the response, the material deforms in a uniform fashion. Next, a non- linear phase characterized by progressive localiza- tion of deformation is observed. The progressive lo- calization causes the walls of each cell to contact. These experimental results are simulated through numerical analysis using the finite element method.

Compressive Response of Circular Cell Polycarbonate Honeycomb Under Inplane Static and Dynamic Loads

The crushing response of polycarbonate circular cell honeycombs to inplane uniaxial load- ing is studied through a combination of static and dynamic experiments. The circular cell hexagonally packed honeycomb material has a nearly periodic microstructure. The static experiments correspond to a uniaxial loading condition under displacement control in two different principal inplane directions. The corresponding dynamic experiments are carried out using a low velocity impact drop test facility. Three different initial conditions (corresponding to different drop heights) are used in the dynamic tests. In the initial part of the response, the specimens de- form in a almost uniform fashion. Next, a nonlin- ear phase characterized by progressive localization of deformation is observed. The progressive local- ization causes the walls of each cell to contact. The reasons for the observed orthotropic response of the honeycombs are discussed. A comparison of the col- lapse mechanisms between static and dynamic ex- periments is included.