MultiMechanics Blog

High Velocity Impact on Composites - Past, Present, and Future

Posted by MultiMechanics on Jun 16, 2015 1:22:18 PM

Where it all started?

In 1969 Grumman Aerospace was the first company to successfully introduce advanced composites into a commercial airplane. The boron-epoxy laminated horizontal stabilizer, was 15% lighter and 18% less costly than its metal counterpart. 
f14-detail-horizstab
This pioneering application opened the doors to a new era of materials engineering, but also engendered the need for more robust and revealing test methods.  Up until this point, composite testing techniques were usually just adapted forms of existing metal or unreinforced plastic testing techniques. 
However, as more complex parts were getting put on faster planes, these decades-old test methods, being used on century old materials, were no longer sufficient. 

 

Whats the impact?

image006
As these carbon-based composites came into use, the most essential features (aside from their high strength -to-weight ratios) were damage tolerance and damage resistance under impact loading. This is because they were routinely exposed to unplanned impact loading of numerous kinds during the manufacturing process and in service.  
For the record, are several categories of impact loading and all carry with them unique challenges and considerations. 
  1. Low velocity (large mass) - 10 m/s (22 mph)
  2. Intermediate velocity - 10 - 50 m/s (22-111 mph)
  3. High/ballistic velocity (small mass) - 50 - 1000 m/s (111-2230 mph)
  4. Hyper velocity impact - 2000 - 5000 m/s (5000 - 11100 mph)

In this blog, we will take a look at high velocity impact.

Not so fast!

As 4th generation warplanes were reaching supersonic speeds, high velocity impact characterization became the most crucially important and complicated of the impact failure categories.  
figher-jets
There are many complexities associated with high velocity ballistic impact. There are high pressures, high temperatures, large strains and high strain rates. A wide variety of materials can be involved, all of which interact with one another and are subject to failure and fragmentation. Further, these failure events generally occur during a small fraction of a second.
For these reasons physical testing can be very expensive and time-consuming, and it is possible to obtain only limited data from physical tests. Thus, it can be extremely advantageous to develop computational methods ad CAE programs which can provide a detailed look into the complicated processes that occur during the course of an impact event.  
Computer programs can be used to examine conditions that cannot be readily tested, such as impact velocities and materials that are not yet attainable.  They also give engineers the ability to rapidly test a wide variety of designs, without the costly manufacturing and setup. Putting it all together, they can give engineers insight into a part's behavior and can help engineers design and optimize weight and cost-efficient barriers with desired ballistic resistance properties.
Ultimately, the goal of CAE tools is to provide the designer and researcher the computational tools required to design and analyze projectiles, armors and other systems, in an accurate and efficient manner

Zooming In

The challenge with using CAE tools is that the response of the structural part is governed by the ‘local’ behavior of the material in the neighborhood of the impacted zone.  
For example, for a woven composite armor, several different damage and energy absorbing mechanisms during ballistic impact have been identified. These include the following mixes of local and global failure mechanicsms. 
  • Cone formation on the back face of the target,
  • Tensile failure of primary yarns, 
  • Deformation of secondary yarns,
  • Delamination,
  • Matrix cracking,
  • Shear plugging
  • Friction during penetration.

In other words, the root cause of strength and damage is at a scale that is very difficult to represent within a standard Computer Aided Design (CAD) Model. 

MultiMechanics’ breakthrough two-way coupled multi-scale technology was developed for these very reasons. Using our tools, engineers are able to quickly and accurately relate material micro-structural details to overall structural performance and part service-life.

Upgraded_Leopard_2A4_SG_NDP_2010An example of how to capture this complex behavior under ballistic load can be seen when examining a sample of a multi-component armor plate. These armors, used on tanks and ships are made of of several dissociated layers, such as Aluminum, Rubber and Steel.  Recently, researchers have started investigating the use of composites as a reinforcing material. Because of the complicated global and local scale interactions within this part, this is   test case for two-way couple multiscale analysis.  In this situation, individual material behavior and Interaction between different layers must be captured.  
In the example below we model two layers of woven composite atop a dense rubber sub layer. As you can see, the local scale weaves are driving the behavior of the global scale composite layers. As damage accumulates locally, the stiffness response of the global scale is updated until it is eventually deleted. Further, the interaction between the rubber sublayer and the composite is accurate, as the rubber starts to yield before the stronger woven composite. 

 

 

This is a simplified example, but is nonetheless and interesting mix of local and global phenomena. As discussed, the ability to accurately predict these interactions has vast implications for transportation both on and off this planet.

As Elon Musk (founder/CEO of Tesla and SpaceX said): "I would like to die on Mars, just not on impact"

To see more examples such as these, please check out are latest series on Balistic Impact on Composites, sponsored by Altair. 

Bonus Fact: Hyper Velocity Impact

Hyper velocity impact involves projectiles moving at extremely high velocities such that the local target materials behave like fluids and the stress induced by the impact is many times the material strength.

If a spacecraft collides with an object with a relative velocity exceeding the speed of sound in solid material (this is about 4-5 kilometers per second), then this is known as a ‘hypervelocity impact’ (HVI). Impacts from man-made debris and from natural meteoroids are very similar, apart from their speeds. Typical impact velocities encountered by orbiting spacecraft are 10 kilometers per second for space debris and 20 kilometers per second for meteoroids. 

Topics: Impact Analysis