Saeed David Barbat

Comparing Time Histories for Validation of Simulation Models: Error Measures and Metrics

Computer modeling and simulation are the cornerstones of product design and development in the automotive industry. Computer-aided engineering tools have improved to the extent that virtual testing may lead to significant reduction in prototype building and testing of vehicle designs. In order to make this a reality, we need to assess our confidence in the predictive capabilities of simulation models. As a first step in this direction, this paper deals with developing measures and a metric to compare time histories obtained from simulation model outputs and experimental tests. The focus of the work is on vehicle safety applications. We restrict attention to quantifying discrepancy between time histories as the latter constitute the predominant form of responses of interest in vehicle safety considerations. First, we evaluate popular measures used to quantify discrepancy between time histories in fields such as statistics, computational mechanics, signal processing, and data mining. Three independent error measures are proposed for vehicle safety applications, associated with three physically meaningful characteristics (phase, magnitude, and slope), which utilize norms, cross-correlation measures, and algorithms such as dynamic time warping to quantify discrepancies. A combined use of these three measures can serve as a metric that encapsulates the important aspects of time history comparison. It is also shown how these measures can be used in conjunction with ratings from subject matter experts to build regression-based validation metrics.

A comprehensive metric for comparing time histories in validation of simulation models with emphasis on vehicle safety applications

Computer modeling and simulation are the cornerstones of product design and development in the automotive industry. Computer-aided engineering tools have improved to the extent that virtual testing may lead to significant reduction in prototype building and testing of vehicle designs. In order to make this a reality, we need to assess our confidence in the predictive capabilities of simulation models. As a first step in this direction, this paper deals with developing a metric to compare time histories that are outputs of simulation models to time histories from experimental tests with emphasis on vehicle safety applications. We focus on quantifying discrepancy between time histories as the latter constitute the predominant form of responses of interest in vehicle safety considerations. First we evaluate popular measures used to quantify discrepancy between time histories in fields such as statistics, computational mechanics, signal processing, and data mining. Then we propose a structured combination of some of these measures and define a comprehensive metric that encapsulates the important aspects of time history comparison. The new metric classifies error components associated with three physically meaningful characteristics (phase, magnitude and topology), and utilizes norms, cross-correlation measures and algorithms such as dynamic time warping to quantify discrepancies. Two case studies demonstrate that the proposed metric seems to be more consistent than existing metrics. It is also shown how the metric can be used in conjunction with ratings from subject matter experts to build regression-based validation models.Copyright

Dynamic response of laminated automotive glazing impacted by spherical featureless headform

Abstract During vehicle accidents, the occupants head impacting on windshield or side window is commonly observed. Head-impact safety is a significant consideration in the design of passenger vehicles, so it is necessary to investigate the mechanical behavior of automotive glazing subjected to occupant head impact. An analytical solution based on the large-deflection plate theory is presented for the dynamic response of a laminated automotive glazing subjected to simulated head impact. The results of the analytical solution are compared with those obtained using a 3-D nonlinear finite element model. In order to understand the failure behavior of laminated automotive glazing, various geometric parameters are investigated to determine their effect on the maximum principal stress and hence failure initiation.

Crack initiation in laminated automotive glazing subjected to simulated head impact

Abstract A very limited work has been done to understand the fracture in laminated automotive glazing when the head impacts on the windshields or side windows. A good understanding of the fracture plays a critical role in designing the laminated glass for utmost safety in automotive glazing. A finite element method based on the energy release rate criterion (J-integral criterion) is employed to determine the crack initiation time and location. A parametric study is done to determine the effect of various geometric parameters on the crack initiation time and location. Also, the cracking behavior of a monolithic glass is evaluated and compared with that of the laminated glass of comparable thickness.

FINITE ELEMENT MODELING AND DEVELOPMENT OF THE DEFORMABLE FEATURELESS HEADFORM AND ITS APPLICATION TO VEHICLE INTERIOR HEAD IMPACT TESTING

This paper describes the steps and procedures involved in the development, calibration, and validation of a finite element model (FEM) of a deformable featureless headform (Hybrid Ill head without nose). Development efforts included: a headform scan to verify geometric accuracy, quantification of general-purpose construction of the FEM from the scanned data, viscoelastic parameters for the constitutive model definition of the headform skin, and models of drop tests with impact speeds of 9.775, 14.484, 19.312, and 24.140 km/h (6.074, 9, 12, and 15 mph). The predictions of all pertinent headform responses during the calibration were in excellent agreement with related experiments. The validity of the headform model and the headform impact methodology were verified in both component and full vehicle environments. This was accomplished through comparisons of FE simulations with tests of the headform responses at 24.140 km/h (15 mph) impact. The 24.140 km/h (15 mph) impact responses obtained with the deformable headform model were also compared, in some cases, with those obtained by impacting the same locations with a rigid featureless headform model with a reduced speed of 22.692 km/h (14.1 mph). The headform models and methodology have been proven to be valid and easy to implement, and can now be used to simplify the tasks of designing for compliance with head impact regulations. (A) For the covering abstract of the conference see IRRD E201376.

FINITE ELEMENT MODELING OF STRUCTURAL FOAM AND HEAD IMPACT INTERACTION WITH VEHICLE UPPER INTERIOR

Finite element modelling (FEM) and analysis of 15 mph spherical headform impact with component sections of upper interior structures of a pasenger compartment is presented. The finite element model validations are carried out through very good correlations of the predicted headform responses to those obtained from laboratory tests of spherical headform impact with vehicle components in unpadded and padded configurations. Such techniques can demonstrate the capacity of a well-defined model to help predict headform responses and the effect of adding foam padding and structural design changes on so called Head Injury Criteria (HIC). It is concluded that the application of finite element technique to head impact design issues has great potential for understanding structural design attributes and their effect on HIC measurements. (A) For the covering abstract of the conference see IRRD 875168.

Modeling Fracture in Laminated Automotive Glazing Impacted by Spherical Featureless Headform

Laminated glass consisting of two soda lime glass plies adhered by a polyvinyl butyral interlayer (PVB) is used for automotive glazing. This paper describes the application of a dynamic, nonlinear finite element method toinvestigate the failure modes of a laminated glass subjected to low-velocity impact with a spherical headform. Crack type, crack location and crack initiation time are evaluated using the maximum principal stress and J-integral criterion. Failure occurred due to lexural stresses and not bearing stresses. The first crack always initiated at the center of the outer impacted ply and PVB interface, and later on the exterior surface of the inner ply. The PVB thickness and velocity of impact had little or no effect on the first crack initiation.

Bumper and Grille Airbags Concept for Enhanced Vehicle Compatibility in Side Impact: Phase II

Objectives: Fundamental physics and numerous field studies have shown a higher injury and fatality risk for occupants in smaller and lighter vehicles when struck by heavier, taller and higher vehicles. The consensus is that the significant parameters influencing compatibility in front-to-side crashes are geometric interaction, vehicle stiffness, and vehicle mass. The objective of this research is to develop a concept of deployable bumper and grille airbags for improved vehicle compatibility in side impact. The external airbags, deployed upon signals from sensors, may help mitigate the effect of weight, geometry and stiffness differences and reduce side intrusions. However, a highly reliable pre-crash sensing system is required to enable the reliable deployment, which is currently not technologically feasible. Methods: Analytical and numerical methods and hardware testing were used to help develop the deployable external airbags concept. Various Finite Element (FE) models at different stages were developed and an extensive number of iterations were conducted to help optimize airbag and inflator parameters to achieve desired targets. The concept development was executed and validated in two phases. This paper covers Phase II ONLY, which includes: (1) Re-design of the airbag geometry, pressure, and deployment strategies; (2) Further validation using a Via sled test of a 48 kph perpendicular side impact of an SUV-type impactor against a stationary car with US-SID-H3 crash dummy in the struck side; (3) Design of the reaction surface necessary for the bumper airbag functionality. The concept was demonstrated through live deployment of external airbags with a reaction surface in a full-scale perpendicular side impact of an SUV against a stationary passenger car at 48 kph. This research investigated only the concept of the inflatable devices since pre-crash sensing development was beyond the scope of this research. Results: The concept design parameters of the bumper and grille airbags are presented in this paper. Full vehicle-to-vehicle crash test results, Via sled test, and simulation results are also presented. Head peak acceleration, Head Injury Criteria (HIC), Thoracic Trauma Index (TTI), and Pelvic acceleration for the SID-H3 dummy and structural intrusion profiles were used as performance metrics for the bumper and grille airbags. Results obtained from the Via sled tests and the full vehicle-to-vehicle tests with bumper and grille airbags were compared to those of baseline test results with no external airbags.

Side Crash Pressure Sensor Prediction for Body-on-Frame Vehicles: An ALE Approach

In an attempt to assist pressure sensor algorithm and calibration development using computer simulations, an Arbitrary Lagrangian Eulerian (ALE) approach was adopted in this study to predict the responses of side crash pressure sensors for body-on-frame vehicles. Acceleration based, also called G-based, crash sensors have been used extensively to deploy restraint devices, such as airbags, curtain airbags, seatbelt pre-tensioners, and inflatable seatbelts, in vehicle crashes. With advancements in crash sensor technologies, pressure sensors that measure pressure changes in vehicle side doors have been developed recently and their applications in vehicle crash safety are increasing. The pressure sensors are able to detect and record the dynamic pressure change when the volume of a vehicle door changes as a result of a crash. Due to the nature of pressure change, data obtained from the pressure sensors exhibits lower frequency and less noise in the responses which are significantly different from those of the acceleration-based crash sensors. This technology is very suitable for side crash applications due to its ability to discriminate crash severities and deploy restraint devices earlier in the event. The lower frequency and less noise in the responses are also more suitable for non-linear finite element codes to simulate.To help understand the responses of pressure sensors and the capabilities of the ALE method in the prediction of pressure sensor responses, fifteen different benchmark tests were designed and performed in previous research. The fifteen benchmark tests were divided into three groups so that the capabilities of the ALE method could be examined in detail. The first group of benchmark tests included a rectangular steel container with one side being compressed while all other sides were fixed to simulate a piston compression condition. Two different gases were tested in the first group of benchmark tests. Solutions for the first group of benchmark tests can be derived theoretically. The second group of benchmark tests, a series of eight, involved a rigid impactor or a deformable barrier hitting a rectangular steel box with and without a hole. In addition, different speeds were chosen in the second group of component tests to obtain their corresponding responses. The third group of benchmark tests, a series of five, involved a rigid impactor or a deformable barrier hitting a vehicle side door with different openings. Similar to the second group of benchmark tests, different speeds were chosen to create different crash severities. Computer simulations conducted employing the ALE method for all fifteen benchmark tests were compared to their corresponding theoretical solutions or test data. Reasonable correlations had been found between the benchmark tests and the computer simulations as presented and discussed in a previous paper.The success of the benchmark study allowed the advancement of the research into its final stage, full vehicle tests. The full vehicle tests contained both body-on-frame and unitized vehicles which are the two main vehicle architectures used in the automotive industry. This paper focused on the body-on-frame vehicles with fifteen tests, including a combination of different body styles, powertrains, drive-trains, wheel bases, test modes, and impact speeds, being investigated. In this study, an approach was developed to correlate the structural responses and to predict the pressure sensor responses for body-on-frame vehicles. The results obtained from the developed method are compared to those obtained from tests. Contrary to common thoughts, it was found that the pressure responses of the low speed test conditions are more challenging to predict than those of the high speed test conditions. This is because the pressure responses for the low speed test conditions are usually very weak. The errors obtained from the numerical simulations become predominant when the magnitudes of the pressure responses are small. The numerical fluctuations induced by the coupling of Lagrangian and Eulerian calculation need to be distinguished and ignored (or filtered) when processing the pressure information. Overall, the slopes, peak values, and shapes of the predicted pressure responses correlate reasonably well with those of the fifteen full vehicle tests selected. The pre-peak responses seem to correlate better to those of the tests than the post-peak responses which involve air leakage. The door pressure changes due to the impacts of oblique pole, IIHS MDB, and FMVSS 214 MDB, can be captured reasonably by the computer simulations.

Side Crash Pressure Sensor Prediction for Unitized Vehicles: An ALE Approach

With a goal to help develop pressure sensor calibration and deployment algorithms using computer simulations, an Arbitrary Lagrangian Eulerian (ALE) approach was adopted in this research to predict the responses of side crash pressure sensors for unitized vehicles. For occupant protection, acceleration-based crash sensors have been used in the automotive industry to deploy restraint devices when vehicle crashes occur. With improvements in the crash sensor technology, pressure sensors that detect pressure changes in door cavities have been developed recently for vehicle crash safety applications. Instead of using acceleration (or deceleration) in the acceleration-based crash sensors, the pressure sensors utilize pressure change in a door structure to determine the deployment of restraint devices. The crash pulses recorded by the acceleration-based crash sensors usually exhibit high frequency and noisy responses. Different from those of the acceleration-based crash sensors, the data obtained from the pressure sensors exhibit lower frequency and less noisy responses. Due to its ability to discriminate crash severities and allow the restraint devices to deploy earlier, the pressure sensor technology has gained its popularity for side crash applications. The lower frequency and less noisy characteristics are also more suitable for non-linear finite element codes to predict.Fifteen different benchmark problems were designed and tested in the first stage of this research to investigate the responses of pressure sensors in different impact conditions and the capabilities of the ALE method in the predictions of different pressure sensor responses. The fifteen benchmark problems were divided into three groups to examine the capabilities of the ALE method in detail. Different structures, gases, hole locations, sensor locations, hole sizes, impact speeds, and impactors, were chosen in the fifteen benchmark problems so that the sensitivity of the pressure responses to different factors could be obtained and understood. Computer simulations conducted by employing the ALE method for all fifteen benchmark problems were compared to their corresponding theoretical solutions or test data. The correlations between the tests and the computer simulations were found to be reasonable as reported in a paper published previously.The research was advanced into its final stage, full vehicle tests, after the positive results obtained from the benchmark study. The full vehicle study included two major vehicle architectures, body-on-frame and unitized, that are commonly used to design vehicles in the automotive industry. This paper focuses on the unitized vehicles. A total of thirteen tests, including different body styles, powertrains, drivetrains, test modes, and impact speeds, were investigated.A simulation methodology was developed in this study to correlate the structural responses and to predict the pressure sensor responses for unitized vehicles. The results obtained from the developed methodology using the ALE simulations are compared to those obtained from the corresponding tests. In the full vehicle study, low speed impact conditions were found to be more challenging to predict compared to those of the high speed impact conditions. This is because the pressure responses for the low speed impacts are usually much weaker than those of the high speed impacts. The numerical errors obtained from the simulations become more significant when the magnitudes of the pressure responses are low. The numerical errors induced by the coupling of Lagrangian and Eulerian calculation need to be distinguished and ignored (or filtered) when processing the pressure information. The slopes, peak values, and overall shapes of the predicted pressure responses correlate reasonably with most of the full vehicle tests selected. The correlations of the pre-peak responses are better than those of the post-peak responses which involve air leakage. The oblique pole, IIHS MDB, and FMVSS 214 MDB test modes create distinct door deformations and pressure responses which can be predicted by the computer simulations reasonably. Sensor engineers analyzed the results obtained from a FMVSS 214 simulation and confirmed that replacing the test data with the predicted results would result in the same deployment algorithm. Language: en