Assignment 6 - Frontal Crash Test using Hypermesh and Radioss for Neon Frontal Model
Origin
In the years 1970 attempts were made to simulate car crash events with non-linear spring-mass systems after calibration, which require as an input the results of physical destructive laboratory tests, needed to determine the mechanical crushing behavior of each spring component of the modeled system. \"First principle\" simulations like more elaborate finite element models, however, need only the definition of the structural geometry and the basic material properties (rheology of car body steel, glass, plastic parts, etc.) as an input to generate the numerical model.
The origins of industrial first principle computerized car crash simulation lie in military defense, outer space and civil nuclear power plant applications. Upon presentation of a simulation of the accidental crash of a military fighter plane into a nuclear power plant on May 30, 1978 by ESI Group in a meeting organized by the Verein Deutscher Ingenieure (VDI) in Stuttgart, car makers became alerted to the possibility of using this technology for the simulation of destructive car crash tests (Haug 1981).
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In the following years, German car makers produced more complex crash simulation studies, simulating the crash behavior of individual car body components, component assemblies, and quarter and half car bodies in white (BIW). These experiments culminated in a joint project by the Forschungsgemeinschaft Automobil-Technik (FAT), a conglomeration of all seven German car makers (Audi, BMW, Ford, Mercedes-Benz, Opel, Porsche, and Volkswagen), which tested the applicability of two emerging commercial crash simulation codes. These simulation codes recreated a frontal impact of a full passenger car structure (Haug 1986) and they ran to completion on a computer overnight. Now that turn-around time between two consecutive job-submissions (computer runs) did not exceed one day, engineers were able to make efficient and progressive improvements of the crash behavior of the analyzed car body structure.
Application
Crash simulations are used to investigate the safety of the car occupants during impacts on the front end structure of the car in a \"head-on collision\" or \"frontal impact\", the lateral structure of the car in a “side collision” or “side impact”, the rear end structure of a car in a \"rear-end collision\" or “rear impact”, and the roof structure of the car when it overturns during a \"rollover\". Crash simulations can also be used to assess injury to pedestrians hit by a car.
Benefits
A crash simulation produces results without the actual destructive testing of a new car model. This way, tests can be performed quickly and inexpensively in a computer, which permits optimization of the design before a real prototype of the car has been manufactured. Using a simulation, problems can be solved before spending time and money on an actual crash test. The great flexibility of printed output and graphical display enables designers to solve some problems that would have been nearly impossible without the help of a computer.
Analysis
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A large number of crash simulations use a method of analysis called the Finite Element Method. The complex problems are solved by dividing a surface into a large but still finite number of elements and determining the motion of these elements over very small periods of time. Another approach to crash simulations is performed by application of Macro Element Method. The difference between the two mentioned above methodologies is that the structure in case of Macro Element Method consists of a smaller number of elements. The calculation algorithm of structure deformation is based on experimental data rather than calculated from partial differential equations.
Pam-Crash started crash simulation and together with LS-DYNA is a software package which is widely used for the application of Finite Element Method. This method allows detailed modelling of a structure, but the disadvantage lies in high processing unit requirements and calculation time. The Visual Crash Studio uses Macro Element Methodology. In comparison with FEM it has some modeling and boundary condition limitations but its application does not require advanced computers and the calculation time is incomparably smaller. Two presented methods complement each other. Macro Element Method is useful at the early stage of the structure design process while Finite Element Method performs well at its final stages.
Structural analysis
In a typical crash simulation, the car body structure is analyzed using spatial discretization, that is, breaking up the continuous movement of the body in real time into smaller changes in position over small, discrete time steps. The discretization involves subdividing the surface of the constituent, thin, sheet metal parts into a large number (approaching one million in 2006) of a quadrilateral or triangular regions, each of which spans the area between \"nodes\" to which its corners are fixed. Each element has mass, which is distributed as concentrated masses and as mass moments of inertia to its connecting nodes. Each node has 6 kinematic degrees of freedom, that is, one node can move in three linear directions under translation and can rotate about three independent axes. The spatial coordinates (x), displacement (u), velocity (v), and acceleration (a) of each node is mostly expressed in a three-dimensional rectangular Cartesian coordinate system with axes X,Y, and Z.
If the nodes move during a crash simulation, the connected elements move, stretch, and bend with their nodes, which causes them to impart forces and moments to their nodal connections. The forces and moments at the nodes correspond to the inertia forces and moments, caused by their translational (linear) and angular accelerations and to the forces and moments transmitted by the resistance of the structural material of the connected elements as they deform. Sometimes, additional external structural loads are applied, like gravity loads from the self weight of the parts, or added loads from external masses.
The forces and moments of all nodes are collected into a column vector (or column matrix), and the time-dependent equations of motion (in dynamic equilibrium) can be written as follows.
- M a = F e x t − F i n t {\\displaystyle \\mathbf {Ma} =\\mathbf {F} _{ext}-\\mathbf {F} _{int}}
where vector M a {\\displaystyle \\mathbf {Ma} } (mass times acceleration vector) collects the inertia forces at the nodes, F e x t {\\displaystyle \\mathbf {F} _{ext}} collects the external nodal loads, and F i n t {\\displaystyle \\mathbf {F} _{int}} collects the internal resisting forces from the deformation of the material. M is a diagonal matrix of the nodal masses. Each vector (u, v, a, F, etc.) has dimension 6 times the total number of nodes in the crash model (about 6 million “degrees of freedom” for every 1 million \"nodes\" in 3-D thin shell finite element models).
Time analysis
A crash simulation uses time discretization as well to separate the continuous changes in time into very small, usable segments. The dynamic equations of motion hold at all times during a crash simulation and must be integrated in time, t, starting from an initial condition at time zero, which is just prior to the crash. According to the explicit finite difference time integration method used by most crash codes, the accelerations, velocities, and displacements of the body are related by the following equations.
- a n = M − 1 ( F e x t − F i n t ) n {\\displaystyle \\mathbf {a} _{n}=\\mathbf {M} ^{-1}(\\mathbf {F} _{ext}-\\mathbf {F} _{int})_{n}}
- v n + 1 / 2 = v n − 1 / 2 + a n Δ t n {\\displaystyle \\mathbf {v} _{n+1/2}=\\mathbf {v} _{n-1/2}+\\mathbf {a} _{n}\\Delta t_{n}}
- u n + 1 = u n + v n + 1 / 2 Δ t n + 1 / 2 {\\displaystyle \\mathbf {u} _{n+1}=\\mathbf {u} _{n}+\\mathbf {v} _{n+1/2}\\Delta t_{n+1/2}}
In these equations, the subscripts n±1/2, n, n+1 denote past, present, and future times, t, at the half and full-time intervals with time steps Δ t n {\\displaystyle \\Delta t_{n}} and Δ t n + 1 / 2 {\\displaystyle \\Delta t_{n+1/2}} , respectively.
Solution
The above system of linear equations is solved for the accelerations, a n {\\displaystyle \\mathbf {a} _{n}} , the velocities, v n + 1 / 2 {\\displaystyle \\mathbf {v} _{n+1/2}} , and the displacements, u n + 1 {\\displaystyle \\mathbf {u} _{n+1}} , at each discrete point in time, t, during the crash duration. This solution is trivial, since the mass matrix is diagonal. The computer time is proportional to the number of finite elements and the number of solution time steps. The stable solution time step, Δ t {\\displaystyle \\Delta t} , is limited for numerical stability, as expressed by the Courant–Friedrichs–Lewy condition (CFL), which states that “in any time-marching computer simulation, the time step must be less than the time for some significant action to occur, and preferably considerably less.\" In a crash simulation, the fastest significant actions are the acoustic signals that travel inside the structural material.
The solid elastic stress wave speed amounts to
- c = E 0 / ρ {\\displaystyle c={\\sqrt {E_{0}/\\rho }}}
where E 0 {\\displaystyle E_{0}} is the initial elastic modulus (before plastic deformation) of the material and ρ {\\displaystyle \\rho } is the mass density. The largest stable time step for a given material is therefore
- Δ t = d m i n ρ / E 0 {\\displaystyle \\Delta t=d_{min}{\\sqrt {\\rho /E_{0}}}} ,
where d m i n {\\displaystyle d_{min}} is the smallest distance between any two nodes of the numerical crash simulation model
Since this distance can change during a simulation, the stable time step changes and must be updated continually as the solution proceeds in time. When using steel, the typical value of the stable time step is about one microsecond when the smallest discrete node distance in the mesh of the finite element model is about 5 millimetres. It needs then more than 100,000-time intervals to solve a crash event that lasts for one-tenth of a second. This figure is exceeded in many industrial crash models demanding optimized crash solvers with High-Performance Computing (HPC) features, such as vectorization and parallel computing.
A crash simulation is a virtual recreation of a destructive crash test of a car or a highway guard rail system using a computer simulation in order to examine the level of safety of the car and its occupants. Crash simulations are used by automakers during computer-aided engineering (CAE) analysis for crashworthiness in the computer-aided design (CAD) process of modelling new cars. During a crash simulation, the kinetic energy, or energy of motion, that a vehicle has before the impact is transformed into deformation energy, mostly by plastic deformation (plasticity) of the car body material (Body in White), at the end of the impact.
Data obtained from a crash simulation indicate the capability of the car body or guard rail structure to protect the vehicle occupants during a collision (and also pedestrians hit by a car) against injury. Important results are the deformations (for example, steering wheel intrusions) of the occupant space (driver, passengers) and the decelerations (for example, head acceleration) felt by them, which must fall below threshold values fixed in legal car safety regulations. To model real crash tests, today\'s crash simulations include virtual models of crash test dummies and of passive safety devices (seat belts, airbags, shock-absorbing dashboards, etc.). Guide rail tests evaluate vehicle deceleration and rollover potential, as well as penetration of the barrier by vehicles.
Presentation - https://drive.google.com/open?id=14_viYccotzcT_kWcO7pNrnwxnfg3zote
Assignment 6 Contents -https://drive.google.com/open?id=1KK3Jgm3U3Ya1vbQ0P0h2JlO-fKagXQHX
Objective - To perform a frontal impact analysis of non-linear explicit nature on a partial BIW model to study how the impact forces are distributed through the structure to prevent damage to the occupants.
Analysis Setup:
- A partial BIW model is checked for necessary inputs with recommended parameters.
- Contact Interfaces of type 7 are defined with recommending friction value 0.2 to make sure zero penetration.
- Masses are added to mimic occupants and seats with four people in the vehicle.
- An initial velocity of 15.64 mm/ms is applied as per given
- sections and accelerometers are defined where time history is to be studied.
- A rigid body is defined where zero deformation is expected.
1. Unit System
By opening the file neon_front_0000.rad ( starter input deck), It was possible to check that unit system was set to [kg], [mm], [ms] as the following image shows, it was decided to stick to this system.
2. Rigid Wall Creation
In order to avoid issues related to the initial contact between surfaces, the rigid wall was created a few millimetres away from the front bumper based on the node displayed as showing in the following image.
3. Penetration Check
To Perform the penetration check, The total penetration in the tools menu was used. All the component were selected in order to check all the penetration in the model. the result was no penetration in the model. as showing in the following image.
The penetration is not showing me because i am already setting up using Inacti = 6. Which calculates a particular spring stiffness to be applied to the elements that are penetrating the surface.
3. COG and Mass Adjustment
In Hypermesh, it was determined that are the mass of the front part of the car was 188.125 kg and that it\'s COG coordinates are;
4. Balanced Mass and COG:
Mass is 121.6 kg
The distance between the two centres of gravity is 99.507 mm. But, In this particular case, the target weight is 700 kg, so that\'s the weight should be achieved in this model.
But, in this particular case. the target weight is 700 kg, so that\'s the weight that should be achieved in this model. This means that a concentrated mass of 511.875 kg should be placed in the model so that the centre of gravity of the whole model maintains it\'s original position.
This means that the recommanded mass must be placed in a node that is alligned with the nodes of both COG\'s in the model nd 136.078mm apart from the COG of the front part of the car.
The above image shows the relative position of the mentioned COG\'s
in the adams menu, I set the mass 511.875 kg and the type to 0, Since the mass is only applied to one node.
5. Rigid Body
Following image showing the rigid body is created and the master node is selected to be at COG node by using Node ID 9904894.
The Image shows the way the concentrated mass is applied to the actual model. I chose to apply the rigid to the COG\'s this is further away from the wall o avoid perturbations of the result in the crash data.
1. ICoG is the centre of gravity computation flag for the rigid body. Suppose in your model you have chosen an independent node for the/RBODY and ICoG=1 is defined. In that case once the run starts the master node is computed internally by the solver and it will be moved to the computing centre of gravity. Whereas in ICoG=3, the master node is not moved and the same node selected by the user will be maintained as the master node throughout the run.
2.It depends on the application. If you select only the edge nodes, the mass assigned will be distributed only between those nodes. But please remember that if you select all the nodes for making rigid the component will become a non-deformable object, that is rbody will make the component with infinite stiffness.
3. Assign mass in a rigid body is the comparatively easy way, but if you don\'t want to make the component a non-deformable one then go for ADMAS.
6. Initial Velocity
The initial velocity of 35 mph (15.6464 mm/ms) was applied to all nodes as showing the following image.
7. Model Checker
When the model checker was deployed, No error\'s have occurred
8. Main Controls of the Simulation
The time step was set to 0.5 ms
The Simulation time was set to 80 ms.
Hyperview Post-Processing
1. Displacement
In geometry, displacement is a vector whose length is the shortest distance from the initial to the final position of a point P undergoing motion. It quantifies both the distance and direction of the net or total motion along a straight line from the initial position to the final position of the point trajectory. A displacement may be identified with the translation that maps the initial position to the final position.
A displacement may be also described as a relative position resulting from the motion, that is, as the final position xf of a point relative to its initial position xi. The corresponding displacement vector can be defined as the difference between the final and initial positions:
- `{\\displaystyle {s}={x_{\\textrm {f}}-x_{\\textrm {i}}}=\\Delta {x}}`
Maximum Displacement is 1.251E+03 at node 800005 and Minimum is 000E+00 at node 440703
2. Von Mises Stresses
The von Mises yield criterion (also known as the maximum distortion energy criterion[1]) suggests that yielding of a ductile material begins when the second deviatoric stress invariant {\\displaystyle J_{2}} reaches a critical value.[2] It is part of plasticity theory that applies best to ductile materials, such as some metals. Prior to yield, the material response can be assumed to be of a nonlinear elastic, viscoelastic, or linear elastic behaviour.
Maximum Von Mises Stress 3.877E+01 at node 239321 and Minimum is 000E+00 at node 440703
3. Mass Changes
First, there were reports of a growing interest in the use of light-weight materials such as aluminium and plastics in car construction. While the shell body of a car contributes less than half the total weight, nevertheless any substantial reduction in mass by the use of lighter materials could have some consequence on the safety of its occupants in multi-car crashes.
Second, the trend towards the use of finite element analysis (FEA) in car design to reduce mass while improving structural stiffness has the potential to influence the degree of safety for the vehicle\'s occupants.
Both these trends need to be closely monitored to ensure that occupant safety is optimised and to highlight the need for future regulations aimed at improving vehicle safety in this country.
Maximum Mass 2.000E-01 at node 80000 and Minimum is 8.145E-05 at node 65477
4. Plastic Strain
Strain is a measure of the amount of deformation that occurs when an object is placed under stress. Strain rate is defined as the change in strain over the change in time. All materials will undergo some change in their dimensions when exposed to stress. The deformation caused by stress can be fully reversible or permanent, depending on the amount of stress applied. An elastic strain occurs when material under stress returns to its original dimensions once the stress is removed. Plastic strain occurs when an object has been exposed to very high levels of stress and will no longer return to its original shape after the stress is removed. In many materials, the reversal of elastic strain is instantaneous, meaning it occurs without a perceptible duration of time. Deformation that is fully recoverable, but occurs over time, is described in terms of the strain rate. Strain rate varies widely for different materials, and will often change at different temperatures and applied pressures. Steel is an example of a material that returns to its original state immediately after the stress is removed.
Maximum Plastic Strain 9.686E-01 at node 239321 and Minimum is -9.463E-05 at node 441164
5. Specific Energy
Design of energy-absorbing structures is achieved in several steps, usually in a highly iterative design/calculation loop. Objectives of each design step are secured by the usage of dedicated computational tools typically different for each step. The selection of an appropriate tool for a given step depends on the complexity of the problem and availability of suitable software or other predictive techniques such as engineering formulas, empirical data or handbook - type of information. For example, in the pre-design or pre - prototyping stages factors such as the specific energy absorption per unit length or the maximal moment capacity of a cross-section are of primary importance. In the case of typical cross-sections, such information is readily available in handbooks. In the case of more complex, real - word, members the desired parameters can be easily calculated by using specialized tools, such as CRASHCAD
Using commercial FE packages at the early stages of the design process are not justified due to the excessive modelling effort. On the other extreme, only professional FE packages such as PAM-CRASH, DYNA or RADIOSS guarantee the detailed, full crash simulation. In the intermediate stages of the design process usage of both computational tools is highly desirable and can 1 CRASH CAD is a highly interactive CAD/CAE Software based on the Superfolding Element approach introduced in the preceding sections.
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