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Aim Create the mesh for bumper assembly,mesh size should be 6mm. Run the crash tube model as it is. Change the Inacti=6 and run. Create the type 11 contact and run. Remove both notches and remove boundary condition on rigid body node then run. Create a new notch in the middle ,select the whole section and run.…
abhijeet dhillon
updated on 04 Jun 2020
Aim
Solution :
Meshing of Bumper Part
We will be doing the geometric clean up and the meshing of the geometric part as shown below :
Geometric Clean up :
We will remove the unwanted line using the delete command as shown below :
Using 6 mm as element size and quad only we get the following mesh :
Quality Check :
The following parameters are checked in order to determine the quality of the check :
1.Warpage : It is defined as angle between normals to two planes formed by splitting the quad element along diagonals. Max. angle out of the two possibilities is reported as warp angle. Warpage should be lesser then 7degreee and for complex it should be lesser than 15 degree.
2.Skew : Skewness is defined as the difference between the shape of the cell and the shape of an equilateral cell of equivalent volume. Highly skewed cells can decrease accuracy and destabilize the solution. For example, optimal quadrilateral meshes will have vertex angles close to 90 degrees, while triangular meshes should preferably have angles of close to 60 degrees and have all angles less than 90 degrees. A general rule is that the maximum skewness for a triangular/tetrahedral mesh in most flows should be kept below 0.95, with an average value that is less than 0.33.
3.Aspect ratio : It is the measure of a mesh element's deviation from having all sides of equal length. A high aspect ratio occurs with long, thin elements. Entering an overly large value for the Minimum Element Size mesh control may cause the mesh generator to create solid elements with high aspect ratios.he ideal value of aspect ratio is 1. That means all the sides of an element are of equal length. However an aspect ratio of less than 5 is generally accepted. If aspect ratio is greater than 5, it means that max length is 5 times greater than min. length of the element and the element is said to be failed in aspect ratio criteria.
4.Jacobian : Now more specifically the jacobian , which is short for the jacobian matrix is really the best measure of finite element mesh quality. It is one number which defines how good or bad an element is. IT is a measure of the normals of the element faces relative to each other. Unfortunately, Hypermesh does not show the element face normals on solid elements, but it is basically an arrow on each face pointing out perpendicular to the face. The range of it is from 1, a perfect cube, to something lower, -1 or even lower. The smallest seen was -1.45. When the element face normals start to cross, that is they are not perpendicular to each other, your element quality gets worse.
Considering these quality parameters we get the following :
Case 2 : Crash Analysis of Tube Model
Interfaces:
It is defined as how the different models are to interact with each other when they come in contact with each other .
Interface can interact with node to surface ,surface to surface and self contact node to surface.
Now we will be discussing about the various parameters associated with interfaces:
1.Igap = It determines how the size of the gap is calculated .
a.Igap = 3 :Variable gap with gap scale factor with mesh size correction
This is used when there is coarse meshing of the slave element ,the nodes are far away from each other ,instead of the node coming in contact with each other ,the edges come into contact.The gap scale factor reduces the gap by 0.8 ,hence the nodes are allowed into the gap .Because of the constant of vibrations ,this factor helps to reduce the calculation the efforts because of the fluctuation of gaps between the two.
b.Igap=1
This just uses the variable gap which is maximum of gapmin and gs+gm.
2.Gapmin= Gap minimum required for the activation of the interface.
3.Inactvi = Action to take if intial penetrations exist.
There are some cases where there is an intial penetration exisisting between the master segement and the slave segment due to the geometry. This can be removed by pushing it out but a lot of energy dissiaption occurs and hence more unstable the system becomes .
Iacti=0: Ignores the penetration which is not recommended as it reduces the time step
Inacti=6:Gap is reduced and scaled and hence the penetration is reduced .
4.Istff = It affects how the stiffness of the interface is calculated .
5.Iform = Friction formulation
6.Stmin = Minimum stiffness required to use in the interface .
Types of Interface :
1.Type 7 : General Node to Surface .
2.Type 11 : edge to edge
3.Type 24 : General node and surface to surface .
4.Type 19 : Mixture of type 7 and type 11 .
5.Type 2 : Used for welding connections
We will be focussing our interface analysis on these types .
1.Type 7 :
It used for the contact interface between a node and a surafce .The surface upon which the crash analysis is to be done is usually made of the slave nodes while the surface on which the crash will be taking place will be made of the master nodes .A gap is defined around the master surface ,the slave nodes are placed in such a manner they do not penetrate the gap , if they do reaction forces are generated and are pushed away from the gap.If the slave nodes penetrate more into the gap , the stiffness generated in the penalty method increases non lineraly and reach very high values ,as time step is inversly proportional to stiffness hence the time step becomes very small and hence it gives very small time to calculate all the parameters and less force dissaiaption takes place and hence the system becomes unstable .Hence in type 7 the stiffness is increased in such a way that the slave segements are kept out of the master segment.
In type 7 auto impact is possible and it can be widely used for many applications .One major advantage of type 7 is that it produces a cylindrical gap around the master segment hence nodes approaching from all directions for analysis .
One major drawback of Type 7 is that it does not consider the contact of the edges of the slave segment with the surface of the master segement ,hence there is always a possibility of the edges of slave segment penetrating the the gap of the master segment and hence creating error in the system and making it unstable .
Recommended Type 7 Properties :
Igap = 2
Sets variable gap to take into account the true distance between parts .
Gapmin= 0.5 mm
Specify minimum thickness of the model in order to make sure no calculation mistakes are made .
Inacti=6
Removes all intial penetrations possible ,otherwise reduces the gap by 30% of the intial gap and adjust the gap height.
Istf=4
Set stiffness based on the softer of the master or slave
Stmin =1000 N/mm
Specify a minimum stiffness so that the there is not a soft contact .
Idel = 2
Removes the slave nodes from contact because of element deletion
Iform = 2
Sliding forces are calculated using stiffness parameters of the interface .Usually results in bigger time steps .
2.Type 11
Type 11 takes into the account of the connections made by the edges of the slave segement to the edges to master segment and hence improves the accuracy of the reactive forces calculations .
3.Type 24
It is used in the case of single surface ,surface to surface or nodes to surface contact interface using penalty method with constant stiffness .The contact gap is the sum of the master gap and slave gap .Type 24 usually does not represent the physical representation of the model as in reality the stiffness in not linear and it is widely used for press fit or where there is no physical gap .
Recommended Properties for Type 24 :
Inacti=5
Shifts the gap to the level of the penetration,
Ist = 4
Considers the stiffness minumum of the master and slave segment
Stmin = 1000 N/mm
4.Type 2 :
Type 2 is a tied contact which kinematically connects a set of slave nodes to a master surface . It can be used to connect coarse and fine mesh ,rivets etc .In this type a search distance is defined and within this search distance it will search for it slave to which it will be attached .
A penalty method is defined with the type 2 in which a spring stiffness is defined between the master surface and slave and it is always constant .It is calculated the average of the slave stiffness and the master stiffness.
Terms associated with Type 2 :
1.Spotflag:It tells us the type 2 in which method the two contacts are supposed to operate
5:This is the default condition ,meaning that
1: It is used for spot welds and rivets
20,21,22:Forumlations which are used for failure with plastics ,that is on the boundary of two materials with plastics
25:It uses penalty method with default condition.
2.Ignore condition
0:The slave are ignored without the master segment
1:The slave nodes which could not find master segment are deleted
2,3:The slave nodes are deleted in such a way that the maximum no of slave nodes are kept within the search distance .
3.Idel
It is used to ignore or delete the slave nodes that do not have a master segment.
Now we will be defining the following terms :
1.Rigid Wall
Rigid walls allow for an interface to defined between a rigid surface and nodes of a deformable body .It is usually coarsed meshed .A searched distance is defined within which the found nodes are defined as the slave nodes .They cannot have imposed velocity ,they can have intial velocity.
2.Rigid Body
A rigid body constrains a set of nodes which makes sure no relative motion takes place between them
Now we will be crash analysis of a tube as shown below :
Case 1 : Default Model
Energy Error :
As you can see the maximum energy error is -3.8% hence the there is not of lot of energy loss and hence the results can be relied upon.
Results :
Case 2 : Inactiv= 6
Results :
Case 3 : Type 11
Case 4 : Without notches and removed boundary conditions
Before simulating we will be discussing about rigid body and boundary conditions :
Rigid Body :
A rigid body is set of nodes which move together so that there is no relative motion .It consists of a master node and slave nodes .The slave nodes will follow the the motion of the master node.The rigid body is implied when there is no impact happening on the the nodes ,during this time the rigid body is turned on ,just before the impact the rigid body is turned off so that the impact analysis is done on the nodes.The motion is calculated only on the master node and the rest will follow the master node.The mass and inertia is transferred to the master node from the slave node .
Icog=2: It takes account the mass and inertia of the slave ,and computes the cog accordingly .The master node shifts towards the cog .
Icog=4:It ignores the mass and inertia of the slaves.
Boundary Condition :
A node has 6 degrees of freedom , in boundary condition we constrain a few DOF according to the analysis.
Now we see the simulation as shown below :
Case 5 : Create a new notch in the middle ,select the whole section and run
A notch has been created in the middle as shown below and has used Type 11 interface :
Case 6 : Create a new notch with nodes only from opposing 2 faces and run.
Conclusion :
As you can see the notches creates more penetration , hence increasing the total energy error and hence the internal energy.
MAXIMUM DISPLACEMENT(mm) | MAXIMUM PLASTIC STRAIN | MAXIMUM STRESS | MAXIMUM INTERNAL ENERGY(J) | MAXIMUM CONTACT ENERGY | MAXIMUM RIGID WALL FORCE | ||
CASE1 | 206 | 0.75 | 0.76 | 37000 | 0 | 11000 | |
CASE2 | 255 | 0.96 | 0.74 | 2500 | 2200 | 2500 | |
CASE3 | 263 | 0.80 | 0.63 | 41000 | 2100 | 1400 | |
CASE4 | 280 | 0.83 | 0.69 | 39000 | 1900 | 1700 | |
CASE5 | 257 | 1.43 | 0.70 | 39000 | 6900 | 950 | |
CASE6 | 280 | 1.23 | 0.63 | 45000 | 2250 | 1400 |
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