Costin D. Untaroiu

Rib fractures under anterior–posterior dynamic loads: Experimental and finite-element study

The purpose of this study was to investigate whether using a finite-element (FE) mesh composed entirely of hexahedral elements to model cortical and trabecular bone (all-hex model) would provide more accurate simulations than those with variable thickness shell elements for cortical bone and hexahedral elements for trabecular bone (hex-shell model) in the modeling human ribs. First, quasi-static non-injurious and dynamic injurious experiments were performed using the second, fourth, and tenth human thoracic ribs to record the structural behavior and fracture tolerance of individual ribs under anterior-posterior bending loads. Then, all-hex and hex-shell FE models for the three ribs were developed using an octree-based and multi-block hex meshing approach, respectively. Material properties of cortical bone were optimized using dynamic experimental data and the hex-shell model of the fourth rib and trabecular bone properties were taken from the literature. Overall, the reaction force-displacement relationship predicted by both all-hex and hex-shell models with nodes in the offset middle-cortical surfaces compared well with those measured experimentally for all the three ribs. With the exception of fracture locations, the predictions from all-hex and offset hex-shell models of the second and fourth ribs agreed better with experimental data than those from the tenth rib models in terms of reaction force at fracture (difference <15.4%), ultimate failure displacement and time (difference <7.3%), and cortical bone strains. The hex-shell models with shell nodes in outer cortical surfaces increased static reaction forces up to 16.6%, compared to offset hex-shell models. These results indicated that both all-hex and hex-shell modeling strategies were applicable for simulating rib responses and bone fractures for the loading conditions considered, but coarse hex-shell models with constant or variable shell thickness were more computationally efficient and therefore preferred.

A Finite Element Model of the Foot and Ankle for Automotive Impact Applications

A finite element (FE) model of the foot and leg was developed to improve understanding of injury mechanisms of the ankle and subtalar joints during vehicle collisions and to aid in the design of injury countermeasures. The FE model was developed based on the reconstructed geometry of a male volunteer close to the anthropometry of a 50th percentile male and a commercial anatomical database. While the forefoot bones were defined as rigid bodies connected by ligament models, the surrounding bones of the ankle and subtalar joints and the leg bones were modeled as deformable structures. The material and structural properties were selected based on a synthesis of current knowledge of the constitutive models for each tissue. The whole foot and leg model was validated in different loading conditions including forefoot impact, axial rotation, dorsiflexion, and combined loadings. Overall results obtained in the model validation indicated improved biofidelity relative to previous FE models. The developed model was used to investigate the injury tolerance of the ankle joint under brake pedal loading for internally and externally rotated feet. Ligament failures were predicted as the main source of injury in this loading condition. A 12% variation of failure moment was observed in the range of axial foot rotations (±15°). The most vulnerable position was the internally rotated (15°) posture among three different foot positions. Furthermore, the present foot and ankle model will be coupled together with other body region FE models into the state-of-art human FE model to be used in the field of automotive safety.

Human surrogates for injury biomechanics research

This article reviews the attributes of the human surrogates most commonly used in injury biomechanics research. In particular, the merits of human cadavers, human volunteers, animals, dummies, and computational models are assessed relative to their ability to characterize the living human response and injury in an impact environment. Although data obtained from these surrogates have enabled biomechanical engineers and designers to develop effective injury countermeasures for occupants and pedestrians involved in crashes, the magnitude of the traffic safety problem necessitates expanded efforts in research and development. This article makes the case that while there are limitations and challenges associated with any particular surrogate, each provides a critical and necessary component in the continued quest to reduce crash‐related injuries and fatalities. Clin. Anat. 24:362–371, 2011.

Influence of pre-collision occupant parameters on injury outcome in a frontal collision

Optimal performance of adaptive restraint systems in the vehicle requires an accurate assessment of occupant characteristics including physical properties and pre-collision response of the occupant. To provide a feasible framework for incorporating occupant characteristics into adaptive restraint schemes, this study evaluates the sensitivity of injury risk in frontal collisions to four occupant parameters: mass, stature, posture and bracing level. The numerical approach includes using commercial multi-body software to develop occupant models that span a range of occupant parameters representative of the real-world driver population. Coupled with a multi-body model of the vehicle interior and standard restraint system, risk of occupant injuries within specific body regions are predicted through numerical simulations in conjunction with established injury risk functions. The results show occupant posture to be the most significant parameter affecting the overall risk of injury in frontal collisions. The causal relationship as predicted using the numerical model has been compared to the traffic injury epidemiology findings, and the feasibility of an analytical methodology to provide real-time estimates of injury severity has been discussed. Preliminary estimates from the study indicate that the proposed methodology will provide a framework to optimize restraint performance and potentially reduce the risk of injuries up to 35% (based on parameter-specific optimization), using accurate information regarding the pre-collision occupant characteristics.

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

A finite element (FE) model of a vehicle occupant’s lower limb was developed in this study to improve understanding of injury mechanisms during traffic crashes. The reconstructed geometry of a male volunteer close to the anthropometry of a 50th percentile male was meshed using mostly hexahedral and quadrilateral elements to enhance the computational efficiency of the model. The material and structural properties were selected based on a synthesis of current knowledge of the constitutive models for each tissue. The models of the femur, tibia, and leg were validated against Post-Mortem Human Surrogate (PMHS) data in various loading conditions which generates the bone fractures observed in traffic accidents. The model was then used to investigate the tolerances of femur and tibia under axial compression and bending. It was shown that the bending moment induced by the axial force reduced the bone tolerance significantly more under posterior-anterior (PA) loading than under anterior-posterior (AP) loading. It is believed that the current lower limb models could be used in defining advanced injury criteria of the lower limb and in various applications as an alternative to physical testing, which may require complex setups and high cost.

A design optimization approach of vehicle hood for pedestrian protection

Abstract Head injuries are the most common cause of fatality in vehicle-to-pedestrian collisions. To reduce the incidence and severity of such injuries, subsystem tests are used in which headform impactors are impacted upon the vehicle hood. The development and validation of an adult headform impactor finite element (FE) model are presented in the study. The geometry was obtained from a drawing of a physical headform while the skin material model was modeled as viscoelastic material with parameters identified by FE optimization to match quasi-static and dynamic test data reported in literature. Overall, it was shown that the geometrical and inertial characteristics of the headform FE model developed in this study satisfy compliance and certification regulations. Then, the results from a hood design optimization using simulations of a headform-to-hood impact test are presented. The baseline design was a generic hood design consisting of two plates connected by buckling structures. Minimization of the underhood clearance space subjected to the admissible limit of head injury risk under impact as constraint was included in an optimization problem. The automated design process, which considered the geometry of connecting spools and the panel thicknesses as design variables converged to an optimum design after several iterations. The methodology and recommendations presented in this paper may assist in the hood design of new vehicle models to reduce pedestrian head injuries and meet new safety requirements.

A study of the pedestrian impact kinematics using finite element dummy models: the corridors and dimensional analysis scaling of upper-body trajectories

Pedestrian–vehicle impact experiments using cadavers have shown that factors such as vehicle shape and pedestrian anthropometry can influence pedestrian kinematics and injury mechanisms. Although a parametric study examining these factors could elucidate the complex relationships that govern pedestrian kinematics, it would be impractical with cadaver tests because of the relative expense involved in performing numerous experiments on subjects with varying anthropometry. On the other hand, finite element modelling represents a more feasible approach because numerous experiments can be conducted for a fraction of the expense. The current study examined the relationship between pedestrian anthropometry and front shape of a mid-size sedan using a PAM-CRASH model of the 50th-percentile male (50th M) Polar-II pedestrian dummy extensively validated against experimental data. To evaluate the influence of pedestrian anthropometry on response kinematics, scaled dummy models were developed on the basis of the weight and height of the 5th-percentile female (5th F), 50th-percentile female (50th F) and 95th-percentile male (95th M). Simulations of the 5th F, 50th F, 50th M, and 95th M Polar-II finite element models struck at 40 km/h by a mid-size sedan were used to generate trajectories of the head, upper thorax, mid-thorax and pelvis. In an effort to assess the validity of scaling techniques when interpreting trajectory data from vehicle–pedestrian crashes, the trajectories of the 5th F, 50th F and 95th M model were scaled to the 50th M and compared with those generated by the 50th M model. The results demonstrated non-linear behaviour of dummy kinematics that could not be accounted for with traditional dimensional analysis scaling techniques.

A numerical investigation of mid-femoral injury tolerance in axial compression and bending loading

Bone fractures occur frequently at mid-shaft femoral site of the front seat vehicle occupants during frontal and offset automotive crashes. A numerical investigation of femoral shaft tolerance under axial and bending loading corresponding to traffic accidents is presented in the current study. A subject specific finite element (FE) model of a femur is developed and the parameters of two material models of cortical bone (isotropic elastic–plastic and elastic transversally isotropic) are identified based on three-point bending test data using optimisation techniques. A Monte Carlo analysis is performed on a surface approximation of the optimised models over a domain of +/− 20% of the optimised parameter values and showed that the elastic moduli of femur are the most influential parameters on the bone stiffness curve prior to bone fracture. The mid-shaft femoral tolerance curves demonstrate sensitivity with respect to the impact direction of transversal load due to the initial curvature of the femur, but insignificant dependence on the material model, or the failure criteria used for femoral cortical bone. The results highlight the predominant role of bending in a combined axial-bending loading of the femur, which may be used in redefining the current injury criteria of femur used in anthropometric test devices.

Injury tolerance and moment response of the knee joint to combined valgus bending and shear loading.

Valgus bending and shearing of the knee have been identified as primary mechanisms of injuries in a lateral loading environment applicable to pedestrian-car collisions. Previous studies have reported on the structural response of the knee joint to pure valgus bending and lateral shearing, as well as the estimated injury thresholds for the knee bending angle and shear displacement based on experimental tests. However, epidemiological studies indicate that most knee injuries are due to the combined effects of bending and shear loading. Therefore, characterization of knee stiffness for combined loading and the associated injury tolerances is necessary for developing vehicle countermeasures to mitigate pedestrian injuries. Isolated knee joint specimens (n=40) from postmortem human subjects were tested in valgus bending at a loading rate representative of a pedestrian-car impact. The effect of lateral shear force combined with the bending moment on the stiffness response and the injury tolerances of the knee was concurrently evaluated. In addition to the knee moment-angle response, the bending angle and shear displacement corresponding to the first instance of primary ligament failure were determined in each test. The failure displacements were subsequently used to estimate an injury threshold function based on a simplified analytical model of the knee. The validity of the determined injury threshold function was subsequently verified using a finite element model. Post-test necropsy of the knees indicated medial collateral ligament injury consistent with the clinical injuries observed in pedestrian victims. The moment-angle response in valgus bending was determined at quasistatic and dynamic loading rates and compared to previously published test data. The peak bending moment values scaled to an average adult male showed no significant change with variation in the superimposed shear load. An injury threshold function for the knee in terms of bending angle and shear displacement was determined by performing regression analysis on the experimental data. The threshold values of the bending angle (16.2 deg) and shear displacement (25.2 mm) estimated from the injury threshold function were in agreement with previously published knee injury threshold data. The continuous knee injury function expressed in terms of bending angle and shear displacement enabled injury prediction for combined loading conditions such as those observed in pedestrian-car collisions.

Material characterization of liver parenchyma using specimen-specific finite element models

The liver is one of the most frequently injured abdominal organs during motor vehicle crashes. Realistic car crash simulations require incorporating strain-rate dependent mechanical properties of soft tissue in finite element (FE) material models. This study presents a total of 30 tension tests performed on fresh bovine liver parenchyma at various loading rates in order to characterize the biomechanical and failure properties of liver parenchyma. Each specimen, cut in a standard dog-bone shape, was tested until failure at one of three loading rates (0.01 s(-1), 0.1s(-1), 1 s(-1)) using a tensile testing setup. Load and acceleration recorded from each specimen grip were employed to calculate the time history of force at specimen ends. The shapes of all specimens were reconstructed from laser scans recorded prior to each test and then used to develop specimen-specific FE models. A first-order Ogden material model and the time histories of specimen end displacement were assigned to each specimen FE model. The failure Green-Lagrangian strain showed averages around 50% and no significant dependence on loading rates, but the failure 2nd Piola-Kirchhoff stress showed rate-dependence with average values ranging from 33 kPa to 94 kPa. The FE models with material model parameters identified using a simulation-based optimization replicated well the time history of load recorded during the test. The FE simulations with model parameters identified using an analytical approach or based on the displacement of optical markers showed a significantly stiffer response and lower failure stress/strain than the FE specimen-specific models. This study provides novel biomechanical and failure data which can be easily implemented in FE models and used to assess injury risk in automobile collisions.