Omar Faruque

Compression Response of 2D Braided Textile Composites: Single Cell and Multiple Cell Micromechanics Based Strength Predictions

This article is concerned with the development of a finite element (FE) based micromechanics model for the prediction of compressive strength and post-peak compression response of 2D triaxial braided carbon fiber polymer matrix composites (2DTBC). This micromechanics based study was carried out on a series of single and multiple representative unit cell (RUC) 3-D FE models. The uniaxial compressive response, including unstable equilibrium paths, was studied using an arc-length method in conjunction with the ABAQUS commercial FE code. In the reported study, explicit account of the braid microstructure (geometry and packing) and the measured inelastic properties of the matrix (the in-situ properties) are accounted for via the use of the FE method. This enables accounting for the different length scales that are present in a 2DTBC. The computational model provides a means to assess the compressive response of 2DTBC and its dependence on various microstructural parameters. The model provides a means to compute the compression strength allowable for a 2DTBC structure. In particular, the dependence of compressive strength on the axial fiber tow properties and axial tow geometrical imperfections is discussed and shown to be significant in capturing the mechanism of damage development. Results are presented for 1, 4, 9, and 16 RUC representations of the 2DTBC, enabling to examine the dependence of compressive strength (or lack thereof) on the size of the region that is modeled. The predicted results are found to compare favorably against experiment.

Effects of Matrix Microcracking on the Response of 2D Braided Textile Composites Subjected to Compression Loads

This article is concerned with the implementation of a coupled damage-elastic-plastic constitutive model for the matrix material of a 2D triaxial braided carbon fiber composite (2DTBC) subjected to compression loads. Damage in the matrix of 2DTBC is in the form of matrix microcracking which is observed in laboratory experiments of 2DTBC coupons subjected to cyclic loading. In the model, the matrix is treated as a continuously evolving solid governed by a coupled elastic-plastic damage theory which is modified from the classical elasto-plastic theory. With this description of the matrix, the response of 2DTBC to compression loading is studied through the adoption of a representative unit cell that consists of a progressively damaging matrix and elastic-plastic progressively damaging fiber tows. Results from the analysis are compared against a model without evolving damage and also against available experimental data to understand the significance of matrix damage and its influence on compression load bearing capability.

Extruded Aluminum Crash Can Topology for Maximizing Specific Energy Absorption

This paper describes how specific energy absorption (SEA) is a quantitative measure of the efficiency of a structural member in absorbing impact energy. For an extruded aluminum crash can, SEA generally depends upon the topology of its cross-section. An investigation is carried out to determine the optimal cross-sectional topologies for maximizing SEA while considering manufacturing constrains such as, permissible die radii, gauges, etc. A comprehensive Department of Energy (DOE) type matrix of cross-sectional topologies has been developed by considering a wide variety of practical shapes and configurations. Since it is critical to include all feasible topologies, much thought and care has been given in developing this matrix. Detailed finite element crash analyses are carried out to simulate axial crushing of the selected crash cans topologies and the resulting SEA is estimated for each case. Evidently, topologies with an outer and an inner ring joined together by a number of straight panels yields the highest SEA. Among single cell sections, hexagon and octagon are the most efficient topologies for crash energy management. Note that all conclusions drawn in this paper are based on CAE analysis results. The authors are currently pursuing physical testing to verify the CAE analysis results.

Experimental Study on the Crash Performance of Aluminum and Steel Rails

Increasing government mandated CAFE standards are forcing the OEMs to aggressively reduce vehicle weight. Aluminum, with a density of about a third of that of steel, has been established as a viable alternative to steel for the construction of the automotive body structure. However, for aluminum sheet metals, there are still lingering concerns about the reliability and robustness of the available joining techniques such as spot-welding, riveting etc. The investigation reported in this paper was aimed at evaluating the relative performance of self-pierced riveted aluminum rails as compared to spot-welded mild steel and high strength steel rails. A series of straight and curved (S-shaped) rails made of aluminum, mild steel, and high strength steel have been tested. Other design parameters considered in this study include sheet metal thickness, rivet/weld location, rivet/weld spacing, adhesives, temperature, and impact speed. As were observed from the tests, axial crush mode dominated the deformation of all straight rails while bending dominated the deformation of the curved rails. Statistical analysis was performed to find the relative importance and effects of each variable on the average crush load, maximum load and energy absorption. For aluminum rails, the thickness of the sheet metal was found to be the primary controlling factor for both straight and S-rails. Other factors i.e. rivet spacing/location, adhesives, temperature and impact speed, had no significant affect on the performance of the rails. For the steel rails, the sheet metal thickness, impact speed, temperature and material properties, were all found to be significant for the crash behavior. It was also found that the aluminum rails have higher specific energy absorption than the steel rails confirming that aluminum as a material is more efficient in absorbing crush energy than steel.Copyright

Rigid Angular Impact Responses of a Generic Steel Vehicle Front Bumper and Crush Can: Correlation of Two Velocity-Measurement Techniques

This study presents experimental investigations of generic steel vehicle front bumper and crush can (FBCC) assemblies subjected to a 30° front-angular impact. There is a lack of studies regarding component level tests with FBCCs. As vehicles aim to decrease weight by applying lighter-weight material to vehicle structures, component level studies become important. Computer aided models will then be valuable tools to assess performance of these structures. Thus, a novel component level test procedure is valuable to aid in CAE correlation. A sled-on-sled testing method was used to perform all the tests reported here. Impact speed was optimized to minimize bottoming-out force for this type of test. The speed of the impactor-sled was obtained based on two measurement techniques, namely: high-speed cameras and accelerometers. Several high-speed cameras were used at different locations. The sled and bumper motions were monitored using video targets. A triaxial accelerometer system was utilized to measure off-axis accelerations of the sled-beam. The results showed that good correlation exists between the two methods for measuring the sled velocity. The accelerometer data were used to generate force-time history plots based on Newton’s second law. The force-time history and force-displacement curves from different FBCC specimens were consistent and in good agreement with respect to each other with a low coefficient of variation calculated.

Force-Time History Assessment of a Generic Steel Vehicle Front Bumper and Crush Can Subjected to a Rigid Center Pole Impact

In this study, generic steel vehicle front bumper and crush cans (FBCC) were impacted against a rigid pole. The majority of studies regarding pole impact tests are related to full vehicle testing. There is a lack of studies regarding component level tests with FBCCs. Component level studies are important as vehicles utilize lighter-weight materials to vehicle structures in order to decrease weight. Computer aided models will then be helpful to assess performance of these structures. Thus, a novel component-level test procedure is valuable to aid in CAE correlation. A sled-on-sled testing method was used to conduct the tests. Two approaches were considered to assess force-time history, namely: direct force measurement and Newton’s second law. Applied impact force was directly measured by a load wall located behind the rigid pole. The load wall consisted of two uniaxial PCB 1204-03A load cells. Each load cell had a diameter of 6.06″ (153.9 mm) and load capacity of 50 klb (222 kN). A total of six accelerometers were used in the sled system. The off-axis accelerations of the sled-beam were also measured using a triaxial accelerometer system mounted on the left side of the beam. Two types of damped accelerometers were used in this study: Endevco 7264-2000 G piezoresistive and Measurement Specialties A40.

Integration of Hydroforming Analysis of Front-End Structures Into Full Vehicle Crash Analysis

Changes in gauge and material properties are the trademarks of hydroforming process used for any parts. These changes may be large enough to affect the vehicle responses under impact loading in a significant way. An investigation was carried out to determine how full vehicle crash responses are affected by these changes and the findings are reported in this paper. Key to this study has been a methodology to map the gauge and property changes, obtained from a forming simulation, to FEA models created for crash analyses. The mapping process thus allows one to consider the property and gauge changes as an initial condition during crash simulation. A number of full vehicle crash simulations were conducted and the results are compared with the corresponding test data. The entire procedure has been verified using the CAE models developed for simulating hydroforming and frontal impact behavior of a vehicle program. The implementation of the developed methodology to other vehicle programs is straightforward.Copyright