Introduction to Ansys LS-DYNA in Crash Simulations of Vulnerable Road User
Ansys LS-DYNA uses nonlinear impact finite element code to simulate crashes involving vulnerable road users (VRUs) like pedestrians and cyclists by which uses nonlinear impact finite element code to simulate vehicle crashes creating detailed finite element models of the vehicle and utilize the impactors available in Ansys library to run simulations for different load cases allowing engineers to analyze the forces involved in a collision and design vehicle features to minimize potential injuries in real-world accidents; essentially, it helps optimize vehicle design to better protect VRUs by simulating various impact scenarios and identifying areas for improvement.
Introduction to Vulnerable Road Users (VRU)
A "vulnerable road user" refers to any person traveling on a road without the protective barrier of a vehicle, such as pedestrians, cyclists, or motorcyclists, placing them at significantly higher risk of severe injury in a collision compared to occupants inside a car due to their lack of physical protection and greater exposure to impact forces during a crash.
Over the years automotive companies have developed technologies to protect the occupants inside the vehicle through structural design, safety features (airbags, auto braking, etc.), crash avoidance (rollover resistance, electronic stability control, etc.) to meet regulatory standards and excellent consumer metric ratings.
With increasing urbanization and the rise in the number of large vehicles (SUV & Trucks) on the road, the need for enhanced protection of vulnerable road users (VRUs) has never been more critical. Addressing this issue requires a multifaceted approach that includes innovative vehicle design, implementation of advanced technology solutions, better infrastructure and increased awareness.
Governments and organizations worldwide are recognizing the importance of protecting VRUs and are investing in various initiatives to enhance their safety and create safer road environments for everyone.
In industrialized countries, road infrastructure and its environment have gradually developed in the last 50 years to meet the needs of growing traffic and mobility. The automotive manufacturers have implemented design changes as well as several safety features (safety cage design, airbag, seatbelt, crash avoidance, etc.) to reduce the injury and fatality of the occupant inside the vehicle as the regulatory requirements have been made compulsory. Figure-1 shows the fatality per million inhabitants for the last 30+ years. The addition of all the safety features has significantly reduced the fatality rates across the world. But the U.S vehicle occupant fatality rate of sedans have decreased by 36% whereas SUV fatality rate have increased by 100% respectively (Figure-2) as SUV production have increased by 150% and sedan production reduced by 50% in the last 20 years (Figure-3).
Source : https://www.iihs.org/topics/fatality-statistics/detail/passenger-vehicle-occupants
In order to reveal the casualty/injury mechanisms of VRU, researchers have carried out in-depth investigations into various traffic accidents to identify the main factors contributing are
1. Hard impact of the head against the stiff hood or windshield causing traumatic brain injury.
2. Insufficient clearance between the hood and the stiff underlying engine components
3. Injuries to the lower limb (usually to the knee joint and long bones).
4. Larger vehicle structure results in less rotation and higher energy transfer resulting in increased injury in pelvis and femur areas as compared to Sedans (Figure-4).
5. Vulnerable road users sharing the same roads as the vehicles & narrow lanes due to the addition of extra lanes to reduce vehicular traffic congestion.
To reduce vulnerable road user fatalities, vehicle design changes can focus on improving visibility, lowering impact forces through softer materials and hood design, incorporating advanced driver assistance systems (ADAS) like pedestrian detection, and implementing features that alert drivers to the presence of pedestrians and cyclists. Some of the areas in the vehicle that have been worked on are divided into 3 categories.
Exterior Design Changes (Figure-5 and Figure-6):
1. Head Impact: Lower hood design creates a better view of pedestrians (SUV and Trucks). Hoods that can actively lift upon impact to absorb energy and lessen the severity of pedestrian injuries.
2. Lower and Upper leg: A longer bumper overhang provides more space for pedestrian leg protection in a collision. Smooth, rounded corners on the front of the vehicle reduce severe leg injuries in a collision.
3. Material Selection and Visibility Features: Incorporating softer materials on the front bumper and hood to absorb impact energy in a pedestrian collision. Adding reflective strips on the front of the vehicle to enhance visibility, especially at night.
Figure-5. Hood design and Active hood Figure-6. Lower and Upper leg design change
Source: https://mikstoreph.com/products/honda-pedestrian-hood-pop-up-disabler-kit Source: https://cdn.euroncap.com/media/32288/euro-ncap-pedestrian-testing-protocolv84.pdf
Ansys LS-DYNA is used extensively across the automotive industry demonstrating its effectiveness in enhancing vehicle and occupant safety including VRU loadcase. The various protocols, impactors and methods relating VRU mean that CAE process can be complex and time consuming. Ansys LS dyna impactor models along with extensive library of material models and contact definitions will enable engineers to optimize the vehicle's front-end design to reduce VRU fatalities and testing costs.
Ansys will help the Engineers in creating process flow through workbench to set up multiple runs as the Pedestrian Protection Impact points cover the entire hood, Fascia and Windshield.
Pedestrian Impact set up for Simulation.
The setting up the Pedestrian Impact model properly is very critical in evaluating 100+ points through simulation. Here are the steps to be followed
Pre-processing (Geometry Clean up and Model Build) through Ansys Lsdyna and Ls-Prepost:
This setup includes geometry clean up that is imported from external source and have mid surface created using Ansys Discovery. Then generate finite element mesh to ensure mesh accuracy and assign accurate material assignments for these assemblies are very important in order get accurate results:
1. Hood assembly
2. Front Fascia Assembly (including Foam, Bumper stiffener, etc.)
3. Headlamp assembly
4. Wiper assembly
5. Windshield
6. Fenders and Front structure until front door
Mark-up of impact points:
Mark-up of impact points is very critical to identify test zones relating to various protocols (Legal and Consumer Metric requirements). It involves the markings on the hood, fascia, fender and windshield as shown in Figure-7 & Figure-8. This can be achieved with enhancement tool in Ansys Mechanical will calculate various reference lines that form the bounds of the test zones. Then Ansys tool can create the impact points for simulation.
Figure-8. Pedestrian Impact Points Marking
Source : https://www.safetywissen.com
Simulation and Post Processing:
Each impact point files are return into individual folders that makes it very easy to simulation for several points in parallel using HPC. Once all the impact point simulation is completed, the HIC for the head impact and LS-PrePost can be used to calculate the area of HIC for legal requirement (GTR) and points for consumer metric point score manually. Similarly, tibia acceleration, bending angle and shear displacement for lower leg impacts are calculated. Later on this can automated with an enhancement in Ansys LS-DYNA Post processing workflow.
Head Impact:
Accelerometers in the headform are used to determine Head Injury Criterion (HIC). The Head Injury Criteria value can be calculated from the following:
Head Impact : Full vehicle simulation model for Yaris in NHTSA public domain is used to set up head impact model. Child Impactor available in Ansys database is used to set up four different impact points. Head Injury Criteria (HIC) can vary significantly based on the deformation space underneath the hood, hard point location (ex. headlamp to hood interaction, hood to fascia, hood to fender & windshield). Figure-9 to Figure-12 shows the head acceleration for each impact location. HIC is calculated between time period during which the significant head acceleration occurs.
Location 1 impact shows head contacting hard components (latch and reinforcement underneath hood and fascia) resulting in HIC of 1400 > threshold limit of 1360.
Figure-9. HIC Plot for Location 1
Location 2 impact shows hood deforms gradually and slows down the head velocity resulting in a HIC of 890 < threshold limit of 1360 (Figure-10).
Figure-10. HIC Plot for Location 2
Location 3 impact shows secondary head impact on hard components headlamp and edge of the hood where deformation space is not available resulting in HIC of 1865 > threshold limit of 1360. HIC can be reduced by introducing crush initiators in the headlamp attachments so that it breaks during impact.
Location 4 impact shows secondary head impact as the hood hinge and under hood components play a significant role resulting in HIC of 2420 > threshold limit of 1360. HIC can be reduced by energy-absorbing materials around the hinge area or pop-up hood and optimizing the hood's geometry to better distribute impact forces effectively allowing the hood to deform and absorb energy upon collision with a pedestrian's head, minimizing the severity of the impact.
Lower leg impact:
The LS-DYNA impactor model library contains TRL lower legform (discontinued) and the following injury criteria are measured - Acceleration, Bending angle and Shear displacement.
NOTE: Test agency around the world use Flex-PLI and a-PLI based on the region for more accurately predicting leg injuries.
Yaris model is used for lower leg impact. Since this model is mainly developed for full vehicle crash, some of the components in fascia assembly are not modeled by NHTSA. This simulation is show how the leg-form kinematics during the impact with the front of the vehicle. Figure-13 shows tibia acceleration, bending angle and shear displacement for lower leg impact.
Below video shows the head and lower leg impact simulation for Yaris.
Head Impact Simulation
Lower leg Impact Simulation
Ansys LS-DYNA impactor library contains European Commission Regulation (EC) upper leg impactor. The following injury criteria are measured - Bending moment and Force.
Figure-14. Upper legform injury measurements
Source : European Commission Regulation
In order to reduce the vulnerable road fatalities around the world regulatory agencies across different have implemented legal requirements in order to sell the vehicles in the respective countries. The European Union (EU) has established rigorous testing and type-approval requirements for vulnerable road user protection, which manufacturers must adhere to before selling a vehicle. Many Asian countries like China, Japan and Korea have implemented similar requirements to sell the vehicle. In United States, National Highway Transportation (NHTSA) have proposed to implement some of the VRU regulations on the vehicles that have 10,000 lbs. weight or less. Apart from regulations, in some of the countries like Australia, EU region, China, Korea and Japan have included the VRU scoring as part of full vehicle star rating.
Figure-7 shows regulations for Crash in Europe, United Nations, USA, China and India.
Source: https://www.safetywissen.com/requirement/.1107349076x2q7fqycw67591b69dvq63495945991/
Data below shows some of the top-rated safest vehicles in Europe. Safety rating includes occupant safety, seatbelt, vulnerable road user safety and safety features.
Source: https://www.euroncap.com/en/ratings-rewards/latest-safety-ratings/
Table-1 shows the top-rated vehicles in USA
Table-1. Safest vehicles in USA based on the segment type
Source: https://www.youtube.com/watch?v=YBJS_tKvAeU
Ansys LS-DYNA is used extensively across the automotive industry, with numerous case studies demonstrating its effectiveness in enhancing vehicle and occupant safety including VRU loadcase. External anthropomorphic test device (ATD) dummies made by Humanetics Inc. can be used in Ansys to set up crash simulation models including vulnerable road user loadcase.
As the vehicle development phase have been reduced from 36 to 18 month by lot of automotive manufacturers and everyone is going towards simulation to reduce testing cost during early development phase, an enhancement tool is required inside Ansys Mechanical to automate the head, lower and upper leg impact positioning which will reduce the time from days to hours as there are total of 200+ test locations to be run using simulation during each design phase steps . This can be achieved by automating Pedestrian impact point marking to post processing that takes several weeks (~ 4 weeks) to complete into approximately 1 week.