Week-7 Head Impact

Import the model in the form of .k file.

The rigid wall is created normally in the direction of z-axis & placed away from the head geometry.

The section card is defined for the simple head foam.

By using Material_Elastic card for head foam the material & section are assigned for the respective part ID'S

Initial velocity is defined as the simple head foam is created to impact with the rigid wall.

Contact_Automatic_Single_Surface is defined with slave as simple head foam where the rigid wall is going to act as very high stiffness with a 0 deformation mode.

Control termination is defined where it has been set at which time the simulation should end.

Database Creation:

Binary_D3plot: It defines the frequency at which time the animation file should be created.

Database_History_Node: A nodes on tyhe bottom face of the head were defined to measure the acceleration & HIC value.

ASCll_Option_Keyword

The results has been requested in ASCll format. Thus the following keyword will be activate.

• GLSTAT - Global data
• MATSUM - Material energies
• NODOUT - Nodal point data
• RWFORC - Rigid wall force

ANIMATION:

EFFECTIVE STRESS:

EFFECTIVE STRAIN:

PLOTS FOR ENERGIES:

The above energy plot for a simple head impact on a rigid wall simulation. We can absorb that kinetic energy decreases & internal energy increases at the instant of the impact. It occurs due to the reduction of the velocity during impact. The simple head geometry is a elastic material bounces back without major loss of energy& attains velocity in other direction. Thus sliding, hour-glass & total energy remains constant. 

HEAD INJURY CRITERIA:

Thus the HIC for a simple head model which impacted with a rigid wall is measured from a node on the simple head model.From the plot we can absorb that resultant acceleration is increased & decreased gradually after the impact, beacuse the body comes again & bounces back from the bottom of the rigid surface.

CASE-2: CHILD HEAD FOAM & RIGID WALL

The *INCLUDE_TRANSFORM card is necessary for the file to be transformed & set up for the analysis. In this card the original geometry is known as filename & defined transformation is carried out on the geometry. Transformation ID (TRANID) is set for this card.The original input file is not changed but only known as transformation.

Since the transformation of original geometry is necessary to replicate the real impact situation. So define TRANSFORMATION_CARD is used to rotate the original body about Y-axis.

*CONSTRAINED_EXTRA_NODE_SET was used to assign the contact between the outer skin & inner sphere.

*CONSTRAINED_RIGID_BODIES was used between the backplate & sphere & accelerometer block.

CONTACT_AUTOMATIC_SURFACE_TO_SURFACE was used to assign the contact between outer skin & inner sphere.

 Each component of physical head foam & complete assembly were weighed to obtain their masses. Density of the each component were adjusted based on the geometry & total mass weighed.

The rigid wall is generated normal to z-axis & the plate is placed 20mm away from the child head foam model. Material & section are already predefined for the child head foam assembly.

The initial velocity is defined along x & y direction to the head foam to impact the rigid wall. An initial velocity is given which the head impacts the rigid wall, but here it will be in a form of relative motion.So the component motion in the x & z direction should be calculated.

It calculates the time step automatically for all the elements & uses the minimum value for the analysis.

It defines the frequency at which the animation file is to be created. 

*DATABASE_HISTORY_NODE where nodes on the acceleration block of the head were defined to measure the acceleration & calculated HIC value.

Then we raise the output request in ASCll format. The following keywords are;

• GLSTAT - Global data
• MATSUM - Material energies
• NODOUT - Nodal point data
• RWFORC - Rigid wall force

ANIMATION:

EFFECTIVE STRESS

EFFECTIVE STRAIN:

PLOTS FOR ENERGIES:

In this energy plot for head foam impact on a rigid wall, the kinetic energy decreases as internal energy, it increases the impact & since the head foam is modeled with hyperelestic material there is a gradual change in the energised after impact is compared to the simple head model case, where some energy is absorbed in deformation causes due to impact. Thus hourglass & sliding energy remains constant. 

HEAD INJURY CRITERIA:

Hic-15 value is placed & measured form the acceleromter where its been placed in the component. We can absorb from the plot whereas accelerometer is increased & decreased after the impact because of the body comes to halt & it bounces back again from the rigidd surface at the bottom.

CASE-3: CHILD HEAD FOAM & BONNET HEAD

In this case the rigid wall is replaced with a bonnet to study the real life situation of child pedestrian head impact simulation.

*DEFINE_TRANSFORMATION & *INCLUDE_TRANSFORMATION to define the position of the child's head. Since the material & section properties are predefined only for the child head assembly & have to be defined for the car hood.

Section creation define dection properties for different elements. Since the car hood model are meshed by using 2d element, we will define them with SECTION_SHELL_CARD

We will use aluminium material properties for the car hood. Many material models are used for metals. One of the the most important model is *MAT_PIECEWISE_LINEAR_PLASTICITY. This material model is recommended for the starting point of elastic plastic simulation of metals & general plastic simulation of metals & general plastics since it also can handle the viscoelastic behaviour. For defining the material MAT_024(MAT-PIESEWISE_LINEAR_PLASTICITY) this type of material is TYPE 24 which is an elastoplastic material.

AUTOMATIC_SURFACE_TO_SURFACE contact is defined between the hood & headfoam, where hood as master & headfoam as slave. As the headfoam contains 5 components in it, slave node set-type is given as part set ID & hood as master segment type which is also given as part set ID.

 The initial velocity is defined along x & y direction to the head foam to impact on the car hood. Head impacts the rigid wall but here it will be a resultant direction. So the component motion in the negative x & z direction has to be calculated.

 The boundary spc is defined for the hood to constraint all the degree of freedom for the set of nodes on the edges of the hood.

Control termination is defined at what time the simulation ends.

Control time step calculates the time automatically  for all the elements uses the value for the analysis.

BINARY_D3PLOT defines the frequency at which the animation file is to be created.

A nodes on the acceleration block of the head wave defined to measure the acceleration & calculated the HIC values.

DATABASE_EXTEND_BINARY writes the strain sensor to d3 plot when STRFLG is set to 1.

Then we raise the output request in ASCll format. The following keywords are;

• GLSTAT - Global data
• MATSUM - Material energies
• NODOUT - Nodal point data
• RWFORC - Rigid wall force

 ANIMATION:

EFFECTIVE STRESS:

EFFECTIVE STRAIN:

PLOTS FOR ENERGIES:

Since the hood deforms kinetic energy decreases & internal energy increases gradually but in previous study we can see that some abrupt change in these energy values during impact.

HEAD INJURY CRITERIA:

HIC-15 value is placed & measured form the acceleromter where its been placed in the component. We can absorb from the plot whereas resultant acceleration is increased & during the initial impact stage decreased because the hood deforms gradully & slow down the head velocity. But not in a quick time but in a margin of time which will save our head from several injuries.

RESULTS:

• The simple head model was defined with MAT_ELASTIC material card which implies the material is pure elastic that's why the maximum stress is only there in the time of the impact since there is no deformation in the simple head geometry. So HIC values sre high.
• The standard model of the child head foam with proper material was used. Since the rigid wall is undeformable the total stress is transfered  to the head foam geometry. Thus HIC values is lower than the case-1.
• With the standard head model but in this case, it impacts on the car hood to study the real life situation the hood was defined with a piecewise linear plasticity material model(MAT_024) which is deformable model so during the impact  the hood deforms significantly & reduces stress, strain on the head foam & thus HIC values are lower than the case-2.