How do laminated composites crack and lose their strength? It's an important question as commercial aerospace and other industries increasingly consider composites for critical load-bearing structures. Abaqus Inc. recently introduced software that promises to move predictions of crack behavior into the engineering mainstream.
Software simulations of crack propagation based on a method called Virtual Crack Closure Technique (VCCT) have been in use for more than 15 years now. This method calculates the energy required for a crack to grow at the structure's bond lines under a given set of loading conditions. It can, for example, help determine whether the shear forces on an aircraft skin-stringer panel would compromise the structure's load-bearing capacity.
In the past, VCCT has mostly been wielded by academics and specialists involved in composites R&D. But Abaqus has now made VCCT part of its suite of FEA software. The company began by licensing Boeing technology that helped integrate VCCT and FEA. After refining the Boeing technology, Abaqus made the technology part of its FEA software suite.
As implemented in the Abaqus environment, VCCT "requires very little additional modeling effort," says Dale Berry, manager of engineering applications for Abaqus. Users need only determine some of the conditions related to bond-line integrity—for example, by specifying areas with lower or no bond strength. All the VCCT calculations take place during the FEA run time, extending it noticeably only in the most complex crack scenarios. "The overhead associated with VCCT is trivial," Berry says.
Abaqus FEA software can now show how cracks propagate in laminated composite structures. In this example, the deflected shape of an aircraft skin-stinger panel exhibits some buckling from a shear force.
Having VCCT run within the FEA environment makes sense. As Berry explains, analysts formerly had to "manually mix and match" the two analyses. He goes on to describe a vicious iterative circle in which users repeatedly plug the stresses from a FEA into VCCT, then to take those VCCT results back into the next round of FEA, and repeat these steps over and over again before arriving at a final result. VCCT for Abaqus, by contrast, allows VCCT and FEA results to play off one another. "The FEA results drive the crack expansion without any further attention from the user," he says.
This integration not only saves time but also helps overcome some of VCCT's limitations. Berry points out that VCCT only addresses delamination, which represents only one of the failure modes for composites. VCCT doesn't, for example, address damage to the fiber-resin matrix from impacts or non-linear deformations of composite structures. Abaqus FEA software does. So running VCCT as part of the FEA analysis makes it easier to incorporate the effect of a crack in a more complete structural analysis—particularly one that involves non-linear behavior not accounted for by the linear VCCT method. "VCCT is not the end all of failure analysis for composites," says Berry. "But it's an important arrow in the engineer's quiver."
VCCT for Abaqus is an add-on to the company's standard FEA package. For more information, go here.