RD-E: 4900 Bird Strike on Windshield

A bird weighing about 4 pounds (1.93kg) with an initial speed of 80m/s impacts a windshield of a plane. The bird model is extremely simplified with one cylinder (diameter: 106mm; height: 195mm) but allows getting familiar with the major options used in a bird strike analysis. Fix the edge of the plane model.

Figure 2. Problem Description

Units: mm, ms, kg, KN, GPa

For the windshield in this example, use composite material with matrix glass and plastic fabric. Both use /MAT/PLAS_BRIT (composite with isotropic layers).

Glass Matrix

Material Properties

Initial density

1.74e-6 $\left[\frac{kg}{m{m}^3}\right]$

E-Modules

61 [GPa]

Poisson ratio

0.3

Yield stress, a

0.045 [GPa]

Plasticity hardening, b

0.05

$${\epsilon }_{t1}$$

0.01

$${\epsilon }_{m1}$$

0.03

element deleted at $${\epsilon }_{f1}$$

0.06

Figure 3.

Fabric Plastic

Material Properties

Initial density

8e-7 $\left[\frac{kg}{m{m}^3}\right]$

E-Modules

3 [GPa]

Poisson ratio

0.3

Yield stress, a

1e30 [GPa], with /PROP/SH_SANDW

Thickness of windshield

9 [mm]

Number of layers, N

9

I_thick

1

Figure 4.

/PROP/BEAM

Area

50.265 [mm²]

Iyy = Izz

201.061 [mm⁴]

Ixx

403.12 [mm⁴]

Model Method

• Bird modeling
  A 4 pound bird model with 11190 particles, the distributions follow a hexagonal compact net. The cell distribution must be cubic centered. The diameter for each particle is about 5.0 mm.

  
  
  Figure 5. Perspective View of the Cubic Centered Faces Net

• SPH simulation setup
  In order to define the SPH bird, the following keywords are needed:

  • Use /PROP/SPH to define the bird SPH cells property.
  • Use /SPHCEL to define SPH cells for bird. Use bird part ID in this keyword.
  • Use /SPHGLO to defined global parameter for this simulation.
  • Set $${\alpha }_{sort}$$ = 0.25 (default value). $${\alpha }_{sort}$$ is a security coefficient which is used when searching for neighbors, so that for each particle more than the actual neighbors are found. The computational time can be reduced.
• Contact between SPH bird and structure and impact setup
  • Use interface TYPE7 to simulate the Contact between SPH bird and structure. Set the structure as master and the bird as slave.
  • Set I_gap=1 (use variable gap). The gap is 1/2(particle diameter)+1/2(thickness of the target). This is the physical value for contact.
  • The stiffness between bird and structure are quite different. In order to get results more close to reality. Normally set I_stf=0 and Stfac=0.1. This means interface stiffness equals to 0.1 times stiffness of master side (structure).
  • Use /INIVEL/TRA to set bird part with initial velocity 80m/s strike on the structure.
• Connection between the windshield and fuselage
  • Use spring beam to simulate the weld connection between windshield and fuselage, and use tied contact to connect them.
• Other remarks

In order to decrease the size of animation files and get the best display of SPH particles, use /ANIM/VERS/44 in Engine file.

Conclusion

• The size of the mesh and the number of particle seems to be a good compromise between the accuracy of the problem and the CPU considerations (especially for larger models such as 4 and 8 pound birds).
• For interface parameters default values could be used.
• The simplified viscous law is good.
• Impact velocity remains small compared to the sound speed in the material (that is less than 300 m/s).
• The h value should be set to a value larger than the inter-cell distance (to avoid excessive de-cohesion), but not too large to avoid excessive energy absorption by the bird.
• Using this model, the discrepancies between the real tests and the simulations are usually due to parameters independent from bird modeling: boundary conditions, target material laws and test sensitivity and so on.
• The final shape of the bird is more "physical", so more experimental data would be required, for instance multiple penetration cases.