Terma Aerostructures
Aircraft structural response when stoke by hail impact
Terma Aerostructures was facing a critical design challenge to ensure structural integrity of a leading edge substructure and on the other hand achieve maximum performance in terms of minimum loss of signal strength for countermeasures equipment.
The challenge
One of the structural requirements for a leading edges of aircraft structures are survival to hail impacts which can be very harsh to a component.
Another very important but yet contradicting design measure was the performance in terms of minimum dielectric damping caused by the structure. Thus thin as paper it must survive hail impact with sufficient strength.
Problem Solving with Explicit Finite Element Analysis (FEA)
The duration of the event to be investigated is in the milliseconds range and the dynamic behavior when a a hail impact shock wave develops throughout the structure.Thus the preferred type of analysis is an explicit nonlinear analysis using Dytran/Nastran SOL700.
The results from this type of analysis should always only be taken with a one outcome of events as any possible combination of the setup in terms of speed, temperature, single hail or cluster and the total number are all statistical variations just to mention a few important conditions.
The specific analysis can show a trend of a particular event similar to the one called out for certification – a metal rod dropped from a height such resulting impact is comparable.
Torben Amtoft, Project Manager, Terma Aerostructures:
"The consultancy assistance for Explicit FE analyzes delivered by Poul Dürr Pedersen have been completed with great success and credible results. The results of the analyzes have helped us to develop structures in different materials that can withstand most types of hails."
Terma has the same requirements for hail impact on military aircraft as on a regular scheduled aircraft. Therefore, the analyzes and simulations of hail impact on structures can be applied to several different aircraft types, including the F-16."
Results from an explicit FE analyzes
When performing explicit analyzes, it is important to be aware of how far down the time step one should go, in order to achieve convergence of the analysis and a reliable result.
Basic estimates are based on the characteristic lengths of the elements and the shock wave propagation rate in the material. Thus, thousands of FE analyzes are often performed to investigate, what happens in just 1 millisecond (ms).
An explicit impact analysis uses several different assumptions that should be systematically validated during the first iteration. Then you can usually transfer analogies and lessons learned from existing products to new projects/designs within the company or industry.
In the example shown, a relatively simple bilinear material yield curve and a plastic elongation to break criterion was used. More accurate material models use true strain-stress curves derived from tests.
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