BACKGROUND

• Crush tubes are structures that dissipate kinetic Energy during impact by various damage mechanisms. They are primarily used in automatic and military applications.
• Non-circular cross sections like cone, ellipse and square have been investigated by researchers.
• Shell elements are commonly used in the finite element modeling of crush tubes.

OBJECTIVES

• To understand crush behavior of composite tubes and structures using 3D solid elements and compare with 2D shell elements.
• To study the mechanisms and damage modes that arise due to axial crush: compression-shear, progressive crushing and buckling. Progressive crushing behavior is preferred as it results in maximum energy dissipation. Buckling causes minimum dissipation.
• To analyze the factors that affect the energy dissipation capabilities of the composite tube.
• To investigate the failure mechanisms and energy dissipating capabilities of composite tubes of varying trigger geometries.

APPROACH

• Use LS-DYNA progressive rate dependent MAT162 material model to conduct axial crush modeling of composite tubes.
• Conduct axial crush on composite tube with/without arbitrary crush initiator to understand the crush mechanisms and damage mechanisms.
• Perform a parametric study varying the trigger geometries and impact velocity.
• Trigger system is a device that causes localized failure area as a result of stress concentration. This area gradually extends resulting in progressive crush.

FEM OF CRUSH TUBE
• Length = 120 mm
• Inner Radius = 37.10 mm
• Thickness = 8 mm
• Arbitrary rectangular trigger system used for the tubes.
• 8 elements through the thickness.
• Impact velocity = 13.4112 m/s (30 m/h) or 1 m/s.
• Fiber orientations = 0°, 15°, 30°and 45°.
• Bottom plate constrained: U_x=U_y=U_z=0 andR_x=R_y=R_z=0.

VARIATION OF TRIGGER GEOMETRY

• Dimensions of top plate = 110 mm x 110 mm x 10 mm
• Length of cylinder = 10 mm
• Radius of cylinder = 50 mm
• Thickness of cylinder = 2.5 mm
• Impact velocity = 10, 20, 50 m/s.

RESULTS AND DISCUSSIONS

• 15° orientation has the highest initial peak load in all cases.
• Arbitrary trigger geometry used does not aid in progressive crushing. 45° orientation composite tube undergoes least catastrophic failure.

• Minimal surface area contact at impact. Fiber-crush damage centering around trigger settling in base of trigger. Dominant damage mode is transverse matrix crack. Increasing load indicates minimal crack propagation.
• 45° trigger geometry displays best energy dissipating capabilities.

• The behavior of the elements at different locations during crush was studied.
• Compressive failure at tip of trigger i.e. X = 0 mm.
• Tension-compression failure experienced by central element i.e. X = 5 mm.
• Es = Ps / (ρ.A)
• Es = Specific energy dissipation,
• Ps = Sustained load,
• A = area of the section of the profile,
• ρ = Density of material.
• Crush Force Efficiency =P ̅ ⁄ Pmax ; where Pmax and P ̅ are the maximum and average initial crushing load respectively.

CONCLUDING REMARKS

Conducted a computational study on the effect of trigger geometry on the energy dissipation capabilities of circular composite tubes.
Trigger system was found to aid in higher dissipated energy during the failure process.
45° trigger geometry exhibited greatest energy dissipating capabilities.

FUTURE WORK

• Apply the trigger geometries used to longer tubes and conduct experiments to validate the simulation results.
• Investigate and optimize the length to diameter ratio required to maximize energy dissipation.

ACKNOWLEDGEMENTS

This work is supported by the CORVID Technologies and ONR under Contract No. M67854-13-C-6582.