Simcenter Madymo Simcenter Madymo - How to visualize the IMM2 state of airbag elements

General:

 

Elements of the airbag fabric mesh that are in "IMM2 state" have zero stress. Only after all sides of a fabric element have reached their reference size (or when elements are enforced to convert triggered by reaching the threshold initial-to-reference airbag area ratio), the element will convert to "material state" and elements stresses will be calculated. When requested as output in the Madymo input file, FE element stresses (e.g. Von Mises stresses) are stored in the Madymo <file_name>_ani.h5 output file.  This element stress animation output data be used to distinguish elements in IMM2 state from those in material state at each animation time step in MADPost. 

The picture below shows an example of the visualization  of the element stresses of the fabric mesh of a driver airbag: Blue colored FE elements are in "IMM2 state" (zero stress), red colored FE elements have already converted to "material state" (non-zero stress).

 

 

Steps to be taken for element stress visualization / IMM2 state checking in MADPost:

 

In the Madymo XML input file (using XMADgic):

 

1) Include the animation output request in the Madymo XML input deck model. Specify in the airbag FE_MODEL the element OUTPUT_ANIMATION and select the von Mises stress.

2) Include RESULT_ANIMATION_FE under CONTROL_OUTPUT with HDF5 as animation output format. This is required to have the output written to the <file_name>_ani.h5 output file.

After running the simulation, follow the steps below in MADPost:

 

3) Load the output file <file_name>_ani.h5

4) Click/activate Contour Settings

 

 

5) In Type/Appearance tab: select Element as contour type, select Von Mises stress as result type, select the Resultant checkbox, unselect the  Extrapolation elements results to nodes checkbox and select Discrete for appearance.

 

 

6) Go to the Range/Scale tab and set:  No of Contours = 2,  Min Value = 0, Max Value = 1.0E-5 and select the Plot Absolute Value checkbox. Then press OK.

 

 

8) Click/activate the Contours On button to activate the contour plotting in the animation pane. 

 

 

This will show the Von Mises stress contour plot onto the airbag fabric mesh, where the elements in blue are the elements in IMM2 state (with zero stress).

 

Note that the contour view can also be (de)activated by clicking the icon in the top left corner in the Contour Settings menu.

 

 

Further remarks:

 

- The reprint file shows the begin and end time of the IMM2 imposed transition period.

- The imposed transition period only applies for triad membrane element. The imposed transition period is not enabled for quad membrane.

- An unfolded airbag with a references mesh which is not equal to the initial mesh (e.g. as of minor scaling due to a "settling" pre-simulation) may result in unintended motion of the airbag as of the IMM2 imposed transition. This can be resolved by increasing the AIRBAG_AREA_RATIO value from 0.7 to e.g. ~1.0. Note here that setting AIRBAG_AREA_RATIO too high can suppress the imposed transition (see example case below).

- The time window attribute in the control_imm.method2 is a transition rate parameter for the imposed transition period. Elements may get out of IMM2 (imposed transition period) faster if a smaller time window values is defined. Note: The maximum imposed transition period, which is not user-definable, is 20ms.

 

Example case:

Driver airbag typically have a "sine-shaped" stitch between the front and back panel in their inflated state. This shape depends on the thread directions in the fabric material of its front and back panels. Whether this shape is simulation correctly, also depends on the on the chosen material model for the fabric material. MATERIAL.FABRIC_SHEAR is the recommended material model to be used. However, depending for instance on the stiffness ratio between shear and tension, the membrane fabric elements may still get hampered to convert from IMM2 state to material state. This can typically occur when the imposed state transition period is suppressed due to a too high AIRBAG_AREA_RATIO. Another possible cause could be that the fabric airbag mesh contains quad membrane elements (for which no enforced state transition in implemented in the solver IMM2 algorithm).

Driver airbag with a 'sine-shaped' stitch with fabric shear material for front and back panel:

 

 

Please contact your local Simcenter Madymo support organization for further questions/details on using CONTROL_IMM.METHOD2.