The Influence of the Specimen Shape and Loading Conditions on the Parameter Identification of a Viscoelastic Brain Model

The mechanical properties of brain under various loadings have been reported in the literature over the past 50 years. Step-and-hold tests have often been employed to characterize viscoelastic and nonlinear behavior of brain under high-rate shear deformation; however, the identification of brain material parameters is typically performed by neglecting the initial strain ramp and/or by assuming a uniform strain distribution in the brain samples. Using finite element (FE) simulations of shear tests, this study shows that these simplifications have a significant effect on the identified material properties in the case of cylindrical human brain specimens. Material models optimized using only the stress relaxation curve under predict the shear force during the strain ramp, mainly due to lower values of their instantaneous shear moduli. Similarly, material models optimized using an analytical approach, which assumes a uniform strain distribution, under predict peak shear forces in FE simulations. Reducing the specimen height showed to improve the model prediction, but no improvements were observed for cubic samples with heights similar to cylindrical samples. Models optimized using FE simulations show the closest response to the test data, so a FE-based optimization approach is recommended in future parameter identification studies of brain

Evaluation of Human and Anthropomorphic Test Device Finite Element Models under Spaceflight Loading Conditions

In an effort to develop occupant protection standards for future multipurpose crew vehicles, the National Aeronautics and Space Administration (NASA) has looked to evaluate the test device for human occupant restraint with the modification kit (THORK) anthropomorphic test device (ATD) in relevant impact test scenarios. With the allowance and support of the National Highway Traffic Safety Administration, NASA has performed a series of sled impact tests on the latest developed THORK ATD. These tests were performed to match test conditions from human volunteer data previously collected by the U.S. Air Force. The objective of this study was to evaluate the THORK finite element (FE) model and the Total HUman Model for Safety (THUMS) FE model with respect to the tests performed. These models were evaluated in spinal and frontal impacts against kinematic and kinetic data recorded in ATD and human testing. Methods: The FE simulations were developed based on recorded pretest ATD/human position and sled acceleration pulses measured during testing. Predicted responses by both human and ATD models were compared to test data recorded under the same impact conditions. The kinematic responses of the models were quantitatively evaluated using the ISOmetric curve rating system. In addition, ATD injury criteria and human stress/strain data were calculated to evaluate the risk of injury predicted by the ATD and human model, respectively. Results: Preliminary results show wellcorrelated response between both FE models and their physical counterparts. In addition, predicted ATD injury criteria and human model stress/strain values are shown to positively relate. Kinematic comparison between human and ATD models indicates promising biofidelic response, although a slightly stiffer response is observed within the ATD. Conclusion: As a compliment to ATD testing, numerical simulation provides efficient means to assess vehicle safety throughout the design process and further improve the design of physical ATDs. The assessment of the THORK and THUMS FE models in a spaceflight testing condition is an essential first step to implementing these models in the computational evaluation of spacecraft occupant safety. Promising results suggest future use of these models in the aerospace field

A Finite Element Model of the THOR-K Dummy for Aerospace and Aircraft Impact Simulations

1) Update and Improve the THOR Finite Element (FE) model to specifications of the latest mod kit (THOR-K). 2) Evaluate the kinematic and kinetic response of the FE model in frontal, spinal, and lateral impact loading conditions

A Numerical Investigation on the Variation in Hip Injury Tolerance With Occupant Posture During Frontal Collisions

<div><p> <b>Objective:</b> More than half of occupant lower extremity (LEX) injuries during automotive frontal crashes are in the knee–thigh–hip (KTH) complex. The objective of this study is to develop a detailed and biofidelic finite element (FE) occupant LEX model that may improve current understanding of mechanisms and thresholds of KTH injuries.</p> <p> <b>Methods:</b> Firstly, the pelvis, thigh–knee–hip, and foot models developed in our previous studies were connected into an occupant lower limb model. Further validations, including posterior cruciate ligament (PCL) stretching, thigh lateral loading, KT, and KTH impact loading were then performed to verify the injury predictability of the model under complex frontal and lateral loading corresponding to automotive impacts. Finally, a sensitivity study was performed with the whole lower limb model to investigate the effect of the hip joint angle to acetabulum injury tolerance in frontal impacts.</p> <p> <b>Results:</b> The whole lower limb model proved to be stable under severe impacts along the knee, foot, and lateral components. In addition, the biomechanical and injury responses predicted by the model correlated well with the corresponding test data. An increase in hip joint extension angle from −30 to +20° relative to neutral posture showed an increase of 19 to 58 percent hip injury tolerance.</p> <p> <b>Conclusions:</b> The stability and biofidelity response of the pelvis–lower limb (PLEX) model indicates its potential application in future frontal and lateral impact FE simulations.</p> </div

Validation of a booted finite element model of the WIAMan ATD lower limb in component and whole-body vertical loading impacts with an assessment of the boot influence model on response

<p><b>Objective</b>: A novel anthropomorphic test device (ATD) representative of the 50th percentile male soldier is being developed to predict injuries to a vehicle occupant during an underbody blast (UBB). The main objective of this study was to develop and validate a finite element (FE) model of the ATD lower limb outfitted with a military combat boot and to insert the validated lower limb into a model of the full ATD and simulate vertical loading experiments.</p> <p><b>Methods</b>: A Belleville desert combat boot model was assigned contacts and material properties based on previous experiments. The boot model was fit to a previously developed model of the barefoot ATD. Validation was performed through 6 matched pair component tests conducted on the Vertically Accelerated Loads Transfer System (VALTS). The load transfer capabilities of the FE model were assessed along with the force-mitigating properties of the boot. The booted lower limb subassembly was then incorporated into a whole-body model of the ATD. Two whole-body VALTS experiments were simulated to evaluate lower limb performance in the whole body.</p> <p><b>Results</b>: The lower limb model accurately predicted axial loads measured at heel, tibia, and knee load cells during matched pair component tests. Forces in booted simulations were compared to unbooted simulations and an amount of mitigation similar to that of experiments was observed. In a whole-body loading environment, the model kinematics match those recorded in experiments. The shape and magnitude of experimental force–time curves were accurately predicted by the model. Correlation between the experiments and simulations was backed up by high objective rating scores for all experiments.</p> <p><b>Conclusion</b>: The booted lower limb model is accurate in its ability to articulate and transfer loads similar to the physical dummy in simulated underbody loading experiments. The performance of the model leads to the recommendation to use it appropriately as an alternative to costly ATD experiments.</p

Periodic Stability and Sensitivity Analysis of Rotating Machinery

This work presents the evaluation of periodic stability of rotating machinery and its sensitivity analysis. The stability criterion is based on the characteristic exponents or more generally the eigenvalues of the monodromy matrix of the periodic system following Floquet's Theory. Parametric sensitivity of the stability is formulated to provide a methodology for robustness in rotating machinery analysis and design. The problem is formulated for a generic rotating dynamical system having periodic coefficients and demonstrated on a helicopter blade in forward flight and on a cracked rotating shaft

Development, Calibration, and Validation of a Head–Neck Complex of THOR Mod Kit Finite Element Model

<div><p><b>Introduction/Objective:</b> In an effort to continually improve upon the design of the test device for human occupant restraint (THOR) dummy, a series of modifications have recently been applied. The first objective of this study was to update the THOR head–neck finite element (FE) model to the specifications of the latest dummy modifications. The second objective was to develop and apply a new optimization-based methodology to calibrate the FE head–neck model based on experimental test data. The calibrated head–neck model was validated against both frontal and lateral impact test data. Finally, the sensitivities of the model, in terms of head and neck injury criteria, to pretest positioning conditions were evaluated in a frontal crash test simulation.</p><p><b>Methods:</b> The updated parts of the head–neck THOR FE model were remeshed from CAD geometries of the modified parts. In addition, further model modifications were made to improve the effectiveness of the model (e.g., model stability). A novel calibration methodology, which incorporates the CORA (CORelation and Analysis) rating system with an optimization algorithm implemented in Isight software, was developed to improve both kinematic and kinetic responses of the model in various THOR dummy certification and biomechanical response tests. A parametric study was performed to evaluate head and neck injury criteria values in the calibrated head–neck model during a 40 km/h frontal crash test with respect to variation in the THOR model upper body and belt pretest position.</p><p><b>Results:</b> Material parameter optimization was shown to greatly improve the updated model response by increasing the average rating score from 0.794 ± 0.073 to 0.964 ± 0.019. The calibrated neck showed the biggest improvement in the pendulum flexion simulation from 0.681 in the original model up to 0.96 in the calibrated model. The fully calibrated model proved to be effective at predicting dummy response in frontal and lateral loading conditions during the validation phase (0.942 average score). Upper body position was shown to have a greater effect on head–neck response than belt position. The pretest positioning variation resulted in a 10 percent maximum change in HIC₃₆ values and 14 percent maximum change in <i>N</i>_IJ values.</p><p><b>Conclusion:</b> The optimization-based calibration methodology was effective as it markedly improved model performance. The calibrated head–neck model demonstrated application in a crash safety analysis, showing slight head–neck injury sensitivity to pretest positioning in a frontal crash impact scenario.</p></div