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Technical Paper

Constitutive Modeling of Polymers Subjected to High Strain Rates

A biaxial test procedure is used to assess the constitutive properties of polymers in tension. The constitutive constants are derived for high strain rate applications such as those associated with crashworthiness studies. The test procedure is used in conjunction with a time- and strain-dependent quasi-linear viscoelastic constitutive law consisting of a Mooney-Rivlin formulation combined with Maxwell elements. The procedure is demonstrated by describing the stress vs. strain relationship of a rubber specimen subjected to a step-relaxation input. The constitutive equation is transformed from a nonlinear convolution integral to a set of first order differential equations. These equations, with the appropriate boundary conditions, are solved numerically to obtain transient stresses in two principal directions. Material constants for use in the explicit LS-Dyna non-linear finite element code are provided.
Technical Paper

A Madymo Model of the Foot and Leg for Local Impacts

It has been reported that lower extremity injuries represent a measurable portion of all moderate-to-severe automobile crash- related injuries. Thus, a simple tool to assist with the design of leg and foot injury countermeasures is desirable. The objective of this study is to develop a mathematical model which can predict load propagation and kinematics of the foot and leg in frontal automotive impacts. A multi-body model developed at the University of Virginia and validated for blunt impact to the whole foot has been used as basis for the current work. This model includes representations of the tibia, fibula, talus, hindfoot, midfoot and forefoot bones. Additionally, the model provides a means for tensioning the Achilles tendon. In the current study, the simulations conducted correspond to tests performed by the Transport Research Laboratory and the University of Nottingham on knee-amputated cadaver specimens.
Technical Paper

Effects of Humidity Fluctuations on Adsorption Columns Used for Air Purification in Closed Environments

Effects of a cabin-level humidity upset on an activated carbon column used for adsorption of trace compounds from air are examined through a series of experiments and computer simulations. Breakthrough curves measured for dichloromethane in the presence of water indicate that a rapid increase in relative humidity can displace large quantities of dichloromethane from the adsorbed phase resulting in effluent streams containing more than 20 times the feed concentration. Additionally, the breakthrough time for organic compounds is reduced significantly at high relative humidity. Numerical simulation results show favorable qualitative agreement with measured breakthrough curves, yet do not consistently predict accurate water or dichloromethane loadings at all experimental conditions.
Technical Paper

Open-Loop Chestbands for Dynamic Deformation Measurements

Originally designed for measuring closed-loop contours such as those around a human thorax, the External Peripheral Instrument for Deformation Measurement (EPIDM), or chestband, was developed to improve the measurement of dummy and cadaver thoracic response during impact. In the closed-loop configuration, the chestband wraps around on itself forming a closed contour. This study investigates the use of the chestband for dynamic deformation measurements in an open-loop configuration. In the open-loop configuration, the chestband does not generally form a closed contour. This work includes enhanced procedures and algorithms for the calculation of chestband deformation contours including the determination of static and dynamic chestband contours under several boundary conditions.
Technical Paper

A Simulation-Based Calibration and Sensitivity Analysis of a Finite Element Model of THOR Head-Neck Complex

The THOR-NT dummy has been developed and continuously improved by NHTSA to provide automotive manufacturers an advanced tool that can be used to assess the injury risk of vehicle occupants in crash tests. With the recent improvements of finite element (FE) technology and the increase of computational power, a validated FE model of THOR may provide an efficient tool for the design optimization of vehicles and their restraint systems. The main goal of this study was to improve biofidelity of a head-neck FE model of THOR-NT dummy. A three-dimensional FE model of the head and neck was developed in LS-Dyna based on the drawings of the THOR dummy. The material properties of deformable parts and the joints properties between rigid parts were assigned initially based on data found in the literature, and then calibrated using optimization techniques.
Technical Paper

Patterns of Acetabular Femoral Head Coverage

The size and shape of the acetabulum and of the femoral head influence the injury tolerance of the hip joint. The aim of this study is to quantify changes in acetabular cup geometry that occur with age, gender, height, and weight. Anonymized computed tomography (CT) scans of 1,150 individuals 16+ years of age, both with and without hip trauma, were used to describe the acetabular rim with 100 equally spaced points. Bilateral measurements were taken on uninjured patients, while only the uninjured side was valuated in those with hip trauma. Multinomial logistic regression found that after controlling for age, height, weight, and gender, each 1 degree decrease in acetabular anteversion angle (AAA) corresponded to an 8 percent increase in fracture likelihood (p≺0.001).
Technical Paper

Detached Eddy Simulation on a Swept Hybrid Model in the IRT

In recent years, there has been a growing desire to incorporate computational methods into aircraft icing certification practices. To improve understanding of ice shapes, a new experimental program in the NASA Icing Research Tunnel (IRT) will investigate swept hybrid models which are very large relative to the test section and are intended to operate at high lift coefficients. The present computations were conducted to help plan the experiments and to ascertain any effects of flow separation and unsteady forces. As they can be useful in robustly and accurately predicting large separation regions and capturing flow unsteadiness, a Detached Eddy Simulation (DES) approach has been adopted for simulating the flow over these large high-lift wing sections. The DES methodology was first validated using experimental data from an unswept NACA 0012 airfoil with leading-edge ice accretion, showing reasonable performance.
Technical Paper

Influence of Driver Input on the Touchdown Conditions and Risk of Rollover in Case of Steering Induced Soil-Trip Rollover Crashes

Some rollover testing methodologies require specification of vehicle kinematic parameters including travel speed, vertical velocity, roll rate, and pitch angle, etc. at the initiation of vehicle to ground contact, which have been referred to as touchdown conditions. The complexity of the vehicle, as well as environmental and driving input characteristics make prediction of realistic touchdown conditions for rollover crashes, and moreover, identification of parameter sensitivities of these characteristics, is difficult and expensive without simulation tools. The goal of this study was to study the sensitivity of driver input on touchdown parameters and the risk of rollover in cases of steering-induced soil-tripped rollovers, which are the most prevalent type of rollover crashes. Knowing the range and variation of touchdown parameters and their sensitivities would help in picking realistic parameters for simulating controlled rollover tests.
Technical Paper

Intelligent Selection of Materials for Brake Linings

Friction materials used in the brake linings of automobiles, trucks, buses and other vehicles are required to satisfy a number of performance demands: they must provide a dependable, consistent level of friction, excellent resistance to wear, adequate heat dissipation, structural integrity, low cost and, if possible, light weight. No single material can meet all of these often conflicting performance criteria, and as a consequence, multiphase composites have been developed, consisting typically of a dozen or more different materials. The choice of materials is crucial in determining the performance attained, yet to date, braking material compositions have been developed largely on the basis of empirical observations.
Technical Paper

A Finite Element Model of the Lower Limb for Simulating Pedestrian Impacts

A finite element (FE) model of the lower limb was developed to improve the understanding of injury mechanisms of thigh, knee, and leg during car-to-pedestrian impacts and to aid in the design of injury countermeasures for vehicle front-ends. The geometry of the model was reconstructed from CT scans of the Visible Human Project Database and commercial anatomical databases. The geometry and mass were scaled to those of a 50th percentile male and the entire lower limb was positioned in a standing position according to the published anthropometric references. A "structural approach" was utilized to generate the FE mesh using mostly hexahedral and quadrilateral elements to enhance the computational efficiency of the model. The material properties were selected based on a synthesis on current knowledge of the constitutive models for each tissue.
Technical Paper

Viscous Fan Drive Model for Robust Cooling Air Flow Simulation

One Dimensional models for front end air flows through the cooling system package are very useful for evaluating the effects of component and front end geometry changes. To solve such models for the air flow requires a robust iterative process that involves a number of non-linear sub-models. The cooling fan (s) constitute a major part of the difficulty, especially when they employ a viscous or “thermal” fan drive. This drive varies the torque coupling between the input and output shafts based on the radiator outlet air temperature. The coupling is achieved by viscous shear between two grooved disks and is regulated by a bimetal strip valve that varies the amount of fluid between the disks. This paper presents a mathematical model by which the input/output speed ratio may be determined as a function of the air temperature and input speed. Coefficients in the model are estimated from standard supplier performance information.
Technical Paper

Robust Compressor Model for AC System Simulation

Simple component models are advantageous when simulating vehicle AC systems so that overall model complexity and computation time can be minimized. These models must be robust enough to avoid instability in the iteration method used for determining the AC system operating or “balance” point. Simplicity and stability are especially important when the AC system model is coupled with a vehicle interior model for studies of transient performance because these are more computationally intensive. This paper presents a semi-empirical modeling method for compressors based on dimensionless parameters. Application to some sample compressor data is illustrated. The model equations are simple to employ and will not introduce significant stability problems when used as part of a system simulation.
Technical Paper

A Multi-Body Computational Study of the Kinematic and Injury Response of a Pedestrian with Variable Stance upon Impact with a Vehicle

This research investigates the variation of pedestrian stance in pedestrian-automobile impact using a validated multi-body vehicle and human model. Detailed vehicle models of a small family car and a sport utility vehicle (SUV) are developed and validated for impact with a 50th percentile human male anthropometric ellipsoid model, and different pedestrian stances (struck limb forward, feet together, and struck limb backward) are investigated. The models calculate the physical trajectory of the multi-body models including head and torso accelerations, as well as pelvic force loads. This study shows that lower limb orientation during a pedestrian-automobile impact plays a dominant role in upper body kinematics of the pedestrian. Specifically, stance has a substantial effect on the subsequent impacts of the head and thorax with the vehicle. The variation in stance can change the severity of an injury incurred during an impact by changing the impact region.
Technical Paper

Cooling Fan Modeling to Support Robust AC/Cooling System Simulation

Advanced design of modern engine cooling and vehicle HVAC components involves sophisticated simulation. In particular, front end air flow models must be able to cover the complete range of conditions from idle to high road speeds involving multiple fans of varying types both powered and unpowered. This paper presents a model for electric radiator cooling fans which covers the complete range of powered and unpowered (freewheel) operation. The model applies equally well to mechanical drive fans.
Journal Article

Development of a Biofidelic Rollover Dummy-Part II: Validation of the Kinematic Response of THOR Multi-Body and Finite Element Models Relative to Response of the Physical THOR Dummy under Laboratory Rollover Conditions

While over 30% of US occupant fatalities occur in rollover crashes, no dummy has been developed for such a condition. Currently, an efficient, cost-effective methodology is being implemented to develop a biofidelic rollover dummy. Instead of designing a rollover dummy from scratch, this methodology identifies a baseline dummy and modifies it to improve its response in a rollover crash. Using computational models of the baseline dummy, including both multibody (MB) and finite element (FE) models, the dummy’s structure is continually modified until its response is aligned (using BioRank/CORA metric) with biofidelity targets. A previous study (Part I) identified the THOR dummy as a suitable baseline dummy by comparing the kinematic responses of six existing dummies with PMHS response corridors through laboratory rollover testing.
Journal Article

External Biofidelity Evaluation of Pedestrian Leg-Form Impactors

Current state-of-the-art vehicles implement pedestrian protection features that rely on pedestrian detection sensors and algorithms to trigger when impacting a pedestrian. During the development phase, the vehicle must “learn” to discriminate pedestrians from the rest of potential impacting objects. Part of the training data used in this process is often obtained in physical tests utilizing legform impactors whose external biofidelity is still to be evaluated. This study uses THUMS as a reference to assess the external biofidelity of the most commonly used impactors (Flex-PLI, PDI-1 and PDI-2). This biofidelity assessment was performed by finite element simulation measuring the bumper beam forces exerted by each surrogate on a sedan and a SUV. The bumper beam was divided in 50 mm sections to capture the force distribution in both vehicles. This study, unlike most of the pedestrian-related literature, examines different impact locations and velocities.
Technical Paper

The Contribution of Pre-impact Spine Posture on Human Body Model Response in Whole-body Side Impact

The objective of the study was to analyze independently the contribution of pre-impact spine posture on impact response by subjecting a finite element human body model (HBM) to whole-body, lateral impacts. Seven postured models were created from the original HBM: one matching the standard driving posture and six matching pre-impact posture measured for each of six subjects tested in previously published experiments. The same measurements as those obtained during the experiments were calculated from the simulations, and biofidelity metrics based on signals correlation were established to compare the response of HBM to that of the cadavers. HBM responses showed good correlation with the subject response for the reaction forces, the rib strain (correlation score=0.8) and the overall kinematics. The pre-impact posture was found to greatly alter the reaction forces, deflections and the strain time histories mainly in terms of time delay.
Journal Article

A Computational Study of Rear-Facing and Forward-Facing Child Restraints

A recent study of U.S. crash data has shown that children 0-23 months of age in forward-facing child restraint systems (FFCRS) are 76% more likely to be seriously injured in comparison to children in rear-facing child restraint systems (RFCRS). Motivated by the epidemiological data, seven sled tests of dummies in child seats were performed at the University of Virginia using a crash pulse similar to FMVSS 213 test conditions. The tests showed an advantage for RFCRS; however, real-world crashes include a great deal of variability among factors that may affect the relative performance of FFCRS and RFCRS. Therefore, this research developed MADYMO computational models of these tests and varied several real-world parameters. These models used ellipsoid models of Q-series child dummies and facet surface models of American- and Swedish- style convertible child restraints (CRS).
Journal Article

Thermal Modeling of Power Steering System Performance

Power steering systems provide significant design challenges. They are detrimental to fuel economy since most require the continuous operation of a hydraulic pump. This generates heat that must be dissipated by fluid lines and heat exchangers. This paper presents a simple one-dimensional transient model for power steering components. The model accounts for the pump power, heat dissipation from fluid lines, the power steering cooler, and the influence of radiation heat from exhaust system components. The paper also shows how to use a transient thermal model of the entire system to simulate the temperatures during cyclic operation of the system. The implications to design, drive cycle simulation, and selection of components are highlighted.
Journal Article

A Quantitative Safety Assessment Methodology for Safety-Critical Programmable Electronic Systems Using Fault Injection

Given the increased use of programmable embedded electronic systems (PEES) in automotive applications and their vital importance, it is not only important for engineers to design PEES in such a way to meet or exceed safety requirements but also quantify how “safe” these systems are. At the University of Virginia's Center for Safety-Critical Systems, we have developed a safety quantification methodology for embedded real time safety-related systems. The goal of the safety quantification methodology is to provide a generic but rigorous and systematic way of characterizing the dependability behavior of embedded systems that is applicable to a broad range of applications from automotive to nuclear. This paper presents a quantitative safety assessment methodology for safety-critical embedded systems using fault injection (FI). This methodology has been developed, refined and applied to a number of commercial safety-grade systems in the railway, nuclear and avionics industries.