Multiaxial crush of high-density aluminum honeycombs.

The results of in-plane biaxial experiments of general loading paths including off-axis loading and non-proportional loading are presented. The data are essential to characterize the stress coupling effect exhibited by honeycomb. The biaxial fixture has also been modified to conduct biaxial crush tests of honeycomb at elevated temperatures. The temperature effects on the crush strength of honeycomb are discussed. INTRODUCTION High-density aluminum honeycombs are commonly used in crashworthiness applications to mitigate impact load and to absorb energy. A honeycomb component may experience any one of an infinite number of impact environments including angle of impact and temperature. Honeycomb models need to make predictions of these cases accurately. Since models are typically developed from the observations of uniaxial experimental data, there are issues when generalizing from uniaxial to multiaxial cases. For example, in the orthotropic crush model no coupling between stress components is assumed and each stress component is treated independently. This assumption has not been evaluated experimentally. The other issue is that during uniaxial crushing of honeycomb, engineering stress and true stress have the same value, which one should be used in multiaxial conditions? To understand the mechanical behavior of honeycombs and to develop predictive models, multiaxial experiments are needed. Figure 1. The in-plane biaxial system. The South and East actuators are shown in the lower left and right corners, respectively. All four actuators are identical. Each has a half million pound capacity. A custom-designed system and fixture for biaxial compression of high-density aluminum honeycomb and experiments of simple biaxial loading paths were reported in an earlier paper [1]. The in-plane biaxial system and compression fixtures are shown in Figure 1 and 2, respectively. There are four hydraulic actuators (North, South, East and West, two opposing actuators per each loading axis) and four control channels allowing independent control of each actuator. A load cell is bolted to the end of each actuator. A biaxial compression fixture (or platen) with a capacity of 40 kips is attached to each load cell through two bearing assemblies, which fix the fixture to move with the actuator in the loading direction while accommodating the motion perpendicular to the loading direction in the loading plane. A sliding guide mechanism is mounted on each fixture plate to control and adjust relative position and motion between adjacent fixtures. Figure 2. The biaxial fixtures. Each one is attached to a load cell, which is connected to an actuator. The bearing allows the fixture to move transversely. The maximum friction between loading fixtures was found to be less than 50 pounds, and the maximum cross talk between fixtures was less than 10 pounds. The total uncertainty of load measurement was within 100 pounds. Considering a specimen with a cross section of 2”x2”, 100 pounds corresponds to 25 psi. The friction and cross talk was insignificant compared to the crush strengths of high-density aluminum honeycombs. BIAXIAL EXPERIMENT Three loading paths were designed for the experiments: uniaxial, equal biaxial, and nonproportional. In uniaxial compression the North and South actuators did not move, only confined the specimen. The East actuator moved toward the center and compressed the specimen. The motion of fixtures was demonstrated in figure 3(a). (It could also be accomplished by moving the East and West actuators toward each other at the same time, as shown in Figure 4(a).) During equal biaxial compression, all four actuators moved simultaneously toward the center at the same rate and reached the position shown in Figure 3(b) in a single step. Position 1 (b lue) Position 2 (green) E S W N Position 1 (b lue) Position 2 (green) E