Simulation of Frontal Crash of a Car using Hypermesh with Radioss

AIM: To Analyze the Frontal crash (BIW) of a Car by simulating with recommended parameters.

KEYWORDS: A-pillar, B-pillar, Bumper, Rigid Wall, Rigid Body, Accelerometer, Section, Skew, Frame, etc.,

SOFTWARE: Hypermesh - Hypercrash

OBJECTIVE:

• Check the given Car BIW for any free parts that are presented.
• Check for the Penetration & Intersection.
• Create a Rigid wall with friction of 0.1.
• Add mass to locate proper COG.
• Make the vehicle dynamic with an initial velocity of 56 kmph.
• Use model checker to ensure good quality.
• Create a Timestep (0.5 - 1 microsecond) and run time (80 ms).
• Run the simulation and plot the required results.

INTRODUCTION:

The past few decades have seen an increasing application of CAE for simulation of crash phenomena particularly due to the development of high computing machines and parallel computing techniques. The increase in safety standards can be attributed to the improvement of structural crashworthiness performance through Finite Element Analysis.
The effect of crashes and impacts on structures is one problem and the second one which is of prime importance is the safety of occupants. We find that occupant safety simulation offers reasonably accurate results today which can save a lot of testing time and overall design cycle time. Today’s dummies have closer dynamic properties and also include the correct load-carrying capacity to allow interaction with the structure as compared to earlier dummy models which didn’t take into account loads entering into the body. The CAE development for this application was delayed due to the unavailability of high-end computing power and it can be said that such simulations are barely 20 years old. Although there has been a tremendous increase in the application of software to solve problems related to the automotive industry, aerospace industry, and drop test of components, still the best is yet to come and research has been very active in all aspects of this application ranging from basic physical phenomena understanding to development of efficient numerical algorithms and finally the development of a general-purpose software which can solve almost all kinds of problems. We now find the simulation of tests prescribed by the ECE and FMVSS standards and regulations quite a common application. Other simulations widely used are the bird strike impact analysis and jet engine blade containment analysis about FAR in the aerospace industry. The main aim of this chapter is to introduce the basics of structural crashworthiness simulations in terms of the solution methodology and discuss some of the practical simulations performed in the industry.

We can solve some of the basic equations in structural crashworthiness. These are basically the equilibrium equations of transient dynamics. These are written as:

Where,

M is the mass matrix,

C is the damping matrix

K is the stiffness matrix.

All that we do is just determine the evolution of the basic quantities such as displacement, velocity, and acceleration given initial conditions on displacement and velocity concerning time. All other quantities that can be derived from these and most important are the element stresses, plastic strains, contact forces, and the energies such as kinetic, potential energy, and overall energy absorption characteristics popularly called energy management. Most software would commonly implicitly solve the dynamic equilibrium equation but the most popular way that should be used for highly non-linear problems is to use explicit time integration schemes such as a central difference scheme. There are several advantages of such a procedure and the most important is that it leads to an algorithm that can be easily program-med, does not require any matrix inversion procedure, and further is extremely suitable for a fast parallel computing Methodology.

The automotive industry has probably the widest application of crash simulation. Simulating the crash-worthiness of the vehicle in terms of very simple models based on the spring-mass damper systems was the focus when the computers were very slow. Nowadays software such as RADIOSS and others have very wide practical aspects such as the use of special seat belt elements and the development of dummies for occupant safety. Several standards have originated in various countries and it is impossible to give a detailed account of these. In the following, we discuss some of the most essential applications related to the automotive industry. There are very detailed procedures such as five-star ratings and no attempt will be made here to take a detailed account of these.

I. Standards Maintained to conduct the test:

The most common standards in the USA are FMVSS (Federal Motor Vehicle Safety Standards) regulations and ECE ( Economic Commission of Europe ) regulations in Europe. In India, ARAI testing standards are used now.
FMVSS standards can be obtained free of cost from the website and there are three 3 series listed in the Vol 49 Code of Federal Regulations. The 100 series deals with active safety or crash avoidance and the 200 series forms the most important aspects of crashworthiness tests. Listed in series 200 are several standards and an attempt has been made to give the reader a brief survey of them. One can refer to the information available online on NCAP (New car assessment program), IIHS (Insurance Institute for
Highway Safety) and NHTSA (National Highway Traffic safety association).

Figure 1: FMVSS Standard

IIHS evaluates a vehicle's crashworthiness with the help of six tests: moderate overlap front, driver-side small overlap front, passenger-side small overlap front, side, roof strength and head restraints & seats. For front crash prevention ratings, the Institute conducts low- and moderate-speed track tests of vehicles with automatic braking systems. IIHS also conducts evaluations of headlight systems and of the child seat attachment hardware known as LATCH. The descriptions below explain how each test is conducted and how the results translate into ratings.

For the crash tests we conduct, we purchase the vehicles from dealers just like an ordinary consumer. If the test is being conducted at the manufacturer's request and isn't part of our regular testing schedule, then the manufacturer reimburses us for the vehicle.

A frontal crash is the most common type of crash resulting in fatalities. Major strides have been made in frontal protection, thanks in large part to the crash test program that the National Highway Traffic Safety Administration (NHTSA) began in the late 1970s and the crashworthiness evaluations that IIHS began in 1995.

IIHS conducts three different frontal crash tests: a moderate overlap test (formerly known as the frontal offset test), a driver-side small overlap test, and a passenger-side small overlap test.

II. Body in white (BIW) is the stage in automobile manufacturing in which a car body's frame has been joined together, that is before painting and before the motor, chassis sub-assemblies, or trim (glass, door locks/handles, seats, upholstery, electronics, etc.) have been integrated into the structure. Assembly involves different techniques such as welding (spot, MIG/MAG), riveting, clinching, bonding, and laser brazing.

• Body in white is an assembly of a frame and panels
• Homogeneous Materials - steel or aluminum sheets or composites
• The name derives from manufacturing practices before steel unibody monocoques — when automobile bodies were made by outside firms on a separate chassis with an engine, suspension, and fenders attached.

III. Frontal Impact Test Conditions:

• The impactor shall be made of steel and its mass shall be evenly distributed
• Its mass shall not be less than 1,500 kg.
• Its striking surface, rectangular and flat, shall be 2,500 mm wide and 800 mm high.
• Its edges shall be rounded to a radius of curvature of 10 mm± 5 mm.
• The impactor assembly shall be of rigid construction.

In the New Car Assessment Program (NCAP), passenger vehicles are crashed at 35 mph into a rigid barrier that covers the full width of the vehicle. The Institute runs offset frontal tests instead of full-width frontal tests. In an offset crash only one side of a vehicle's front end, not the full width, hits the barrier. As a result, a smaller part of the structure has to manage the crash energy, and intrusion into the occupant compartment is more likely. An offset test is more demanding of a vehicle's structure than a full-width test, while a full-width test is more demanding of safety belts and airbags. In a full-width test, there is less crushing of the vehicle structure so the decelerations that these restraints must handle are greater. Together, the tests provide a more complete picture of frontal crashworthiness than either test by itself. NCAP has been extremely successful. Automakers responded by improving their vehicles to earn good ratings, giving them a higher level of frontal crashworthiness than is required by law. By the 1990s, most vehicles were performing well in the full-width frontal test. In 2010, NHTSA announced an enhanced 5-star rating system that toughened criteria and combined ratings from its different tests into a single rating to provide consumers an overall view of vehicle safety. The NCAP changes apply to 2011 and later models.

IV. Procedure:

Initially, the model was to be imported to Hypermesh or Hypercrash. Here hypercrash is used to modify the parameters and to check the model for Free parts, penetration, and Intersection. The model is given as the Front half body of a car to study the frontal impact with a certain velocity of 56 Kmph. After checked the model for penetration, a Rigid wall is created with friction 0.1and the distance is maintained as 150mm. The rigid wall should be perpendicular to the model in order to get the proper impact of BIW of a car with a search distance of 2000. There are some mass is added to locate the proper Center of Gravity and also to reach the target weight of a Car by comparing the full car model. The Lumped mass is 581.87 kg which is assigned for front seats and back seats. Before running the simulation check for quality if any issue means need to resolve so that proper result can be obtained if not the simulation will fail. The engine run time is set to 80ms and time_step is followed as 0.5 microseconds. The model consists of 70 parts which are connected with a rigid body and assigned with some mass value. Expect Bumper part rest all the parts are shell elements. The contact interface used is Type7 with recommended properties. To get the impact on a particular part, Sections are created along with the frame and added to the output block. Sections are created in Support rail, A-pillars, Shotgun (Fender). To get the compression distance Intrusion (TH/Node) is created. An accelerometer is created on Both sides of the B-Pillar rocker to get the acceleration curve. To get the Axial force received on the rails from the bumper, a new contact interface has been created for the Bumper and Rail.

Figure 2: Front Mid-portion of Car BIW

Figure 3: Rigid Wall

Figure 4: Free parts & Penetration check

Figure 5: Mass & COG details

Figure 6: Mass distribution

Figure 7: Accelerometer on B-pillar Rocker

Figure 8: Section, Frame & Initial velocity

Figure 9: Property of Shell

Figure 10: Property of Solid

Figure 11: Type_7 Contact Interface of all parts

Figure 12: Type_7 Contact Interface for Bumper & Rail (Axial Force)

RESULTS:

Figure 13: Simulation Animation

The car is moving at a velocity of 56Kmph which hits the Rigid wall, due to this impact some of the parts are largely deformed and shown the animation. The maximum stress is developed as 0.3585 N/mm2 in 80 millisecond.

Figure 14: Internal & Kinetic Energy

The Kinetic & Internal Energy is constant for 15 ms because the object does not get in contact with a rigid wall. After 15 ms, the Kinetic energy is decreasing from 85000 to 45000 due to frontal impact parallelly the internal energy is increasing 0 to 36000.

Figure 15: Contact Energy

When objects collide, contact forces transfer energy so as to change the objects' motions. When two objects interact, each one exerts a force on the other, and these forces can transfer energy between them. Fields contain energy that depends on the arrangement of the objects in the field. The max energy formed as 1100 J.

Figure 16: Total Energy

Figure 17: Time step variation

Figure 18: Rigid Wall force

Figure 19: Dashboard Intrusion at Node 66244

Figure 20: Dashboard Intrusion at Node 66695

Figure 21: Section Support Rail

The graph value is negative because the normal axis indicated towards the rigid wall, so as per the study we can consider assuming a positive value. There is a sudden peak at 28 ms due to the front impact on the support rail.

Figure 22: Section A-Pillar-L

The Above figure shows the impact on the Driver side A-pillar where there is fluctuation due to force acting on the pillar and also mass distribution. The max Normal force in the X direction is 2.1 kN.

Figure 23: Section A-Pillar-R

The Above figure shows the impact on the Right side A-pillar where there is fluctuation due to force acting on the pillar and also mass distribution. The max Normal force in the X direction is 1.9 kN.

Figure 24: Section in Shotgun-L

The graph value is negative because the normal axis indicated towards the rigid wall, so as per the study we can consider assuming a positive value. The max Normal Force in the X direction is 6.4 kN.

Figure 25: Section in Shotgun-R

The Above figure shows the impact on the Shotgun (fender) right side where there is fluctuation due to force acting and mass distribution. The max Normal force in the X direction is 8.5 kN.

CONCLUSION:

The model has been imported to the Hypercrash / Hypermesh.

The model has checked for Free parts, Penetration & Intersection.

Added mass to locate proper COG.

Sections, Accelerometer & Intrusion are created, and obtained the results accordingly.

The energy error obtained as -1.6%, No. of the cycle is 160000, Time elapsed is 7963 secs, and the simulation completed with Normal termination.