J. Michael Starbuck

Energy Absorption in Polymer Composites for Automotive Crashworthiness

The energy absorption capability of a composite material is critical to developing improved human safety in an automotive crash. Energy absorption is dependent on many parameters like fiber type, matrix type, fiber architecture, specimen geometry, processing conditions, fiber volume fraction, and testing speed. Changes in these parameters can cause subsequent changes in the specific energy absorption (ES) of composite materials up to a factor of 2. This paper is a detailed review of the energy absorption characteristics in polymer composite materials. An attempt is made to draw together and categorize the work done in the field of composite energy absorption that has been published in the literature in order to better understand the effect of a particular parameter on the energy absorption capability of composite materials. A description of the various test methodologies and crushing modes in composite tubes is also presented. Finally, this paper raises certain design issues by examining the work rate decay necessary to keep the deceleration below 20g during an impact crash.

Observations of Decreased Fracture Toughness for Mixed Mode Fracture Testing of Adhesively Bonded Joints

In contrast to fracture in monolithic materials, where cracks naturally grow in a mode I manner, fracture in laminated or adhesively bonded joints often involves cracks growing under mixed mode conditions. In many cases, the mixed mode fracture energy increases as the fracture mode changes from mode I to mode II, but exceptions have been noted for several practical engineering adhesives. In some cases, G IIc may be less than G Ic, while in other situations, failure may occur at total energy release rate (G T = G I + G II) values that are smaller than either of the pure mode fracture energies. Several examples of this behavior are reported along with possible explanations for the behavior, which often involves the propagation of the growing debond into regions where less energy is dissipated by the fracture process. These examples showing that mixed mode fracture energies of adhesive joints may be lower than pure mode fracture energies remind us of the importance of developing fracture envelopes over a wide range of mode mixities for engineering design.

On the Use of a Driven Wedge Test to Acquire Dynamic Fracture Energies of Bonded Beam Specimens

A driven wedge test is used to characterize the mode I fracture resistance of adhesively bonded composite beam specimens over a range of crosshead rates up to 1 m/s. The shorter moment arms (between wedge contact and crack tip) significantly reduce inertial effects and stored energy in the debonded adherends, when compared with conventional means of testing double cantilever beam (DCB) specimens. This permitted collecting an order of magnitude more crack initiation events per specimen than could be obtained with end-loaded DCB specimens bonded with an epoxy exhibiting significant stick-slip behavior. The localized contact of the wedge with the adherends limits the amount of both elastic and kinetic energy, significantly reduces crack advance during slip events, and facilitates higher resolution imaging of the fracture zone with high speed imaging. The method appears to work well under both quasi-static and high rate loading, consistently providing substantially more discrete fracture events for specimens exhibiting pronounced stick-slip failures. Deflections associated with beam transverse shear and root rotation for the shorter beams were not negligible, so simple beam theory was inadequate for obtaining qualitative fracture energies. Finite element analysis of the specimens, however, showed that fracture energies were in good agreement with values obtained from traditional DCB tests. The method holds promise for use in dynamic testing and for characterizing bonded or laminated materials exhibiting significant stick slip behavior, reducing the number of specimens required to characterize a sufficient number of fracture events.

A dynamic crash model for energy absorption in braided composite materials - Part II: Implementation and verification

A dynamic crash model is developed and implemented to model the failure behavior and energy absorption of braided composite structures. Part I describes the development and theoretical foundation of a viscoplastic material model that captures the rate-dependent behavior present in braided composite materials. Part II presents the implementation of the model into a finite element model program and the experimental results for tubes crushed from quasi-static to 4000 mm/s rates used to verify the model. Energy absorption decreases sharply with an increase in crush rate, which is reflected in this model. Design concepts are also introduced to increase energy absorption in braided composites.

Test Methodologies for Determining Energy Absorbing Mechanisms of Automotive Composite Material Systems

To identify and quantify the energy absorbing mechanisms in automotive composite material systems, test methodologies were developed for conducting progressive crush tests on composite specimens that have simplified test geometries. The test method development focused on isolating the damage modes associated with the frond formation that occurs in dynamic testing of composite tubes. A new test fixture was designed to progressively crush composite plate specimens under quasi-static test conditions. Preliminary results are presented under a sufficient set of test conditions to validate the operation of the test fixture. The experimental data, in conjunction with test observations, will be used in future work to identify the characteristic damage and failure modes, and determine the specific energy absorption capability of candidate automotive composite material systems.

Rate-Dependent Cohesive Zone Modeling of Unstable Crack Growth in an Epoxy Adhesive

This paper presents the development and numerical implementation of a rate dependent fracture model of an epoxy adhesive. Previous mode I fracture tests conducted under quasistatic, displacement controlled loading of an aluminum double cantilever beam (DCB) bonded with the epoxy exhibited unstable crack growth in the adhesive. Results from mode I fracture tests of compact tension specimens made from bulk adhesive at increasing cross head opening speeds are reported in this paper. The compact tension tests results showed a decreasing critical strain energy release rate with increasing cross head speed, with the critical energy release rate at 1 m/s cross head speed equal to about 20% of its quasi-static value. Two rate dependent cohesive zone models are formulated based on the compact tension test data. A cohesive de-cohesive relationship was postulated between the tractions acting across the crack faces and the opening displacement and opening velocity. These rate dependent cohesive zone models are implemented in a interface finite element to model discrete crack growth in the adhesive. The reaction force history from simulation of the DCB test is in good agreement with the test data using only the rate dependent interface element to model the adhesive.Copyright

Safety Assessment of PowerBeam Flywheel Technology

The greatest technical challenge facing the developer of vehicular flywheel systems is the issue of safety. The PowerBeam flywheel system concept, developed by HyKinesys Inc., employs a pair of high aspect ratio, counter-rotating flywheels to provide surge power for hybrid vehicle applications. The PowerBeam approach to safety is to design flywheels conservatively so as to avoid full rotor burst failure modes. A conservative point design was sized for use in a mid-size sedan such as a Chevrolet Malibu. The PowerBeam rotor rims were designed with a steel tube covered by a carbon fiber reinforced composite tube. ORNL conducted rotor design analyses using both nested ring and finite element analysis design codes. The safety factor of the composite material was 7, while that of the steel was greater than 3. The design exceeded the PNGV recommendation for a safety factor of at least 4 for composite material to prevent flywheel burst.