Managing manufacturing tolerances for composites

  • 12-Jul-2010 03:51 EDT
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Strain levels are identified in a Grupo TAM analysis of an A380 loaded composite panel.

Like any material, composites have their own set of manufacturing, assembly, and life-span challenges that must be understood to ensure safety in critical applications, such as commercial flying. To understand the performance of composite components, as well as assembly and maintenance challenges, a structural analysis engineering team from Grupo TAM, a Madrid-based provider of engineering, manufacture, metrology, assembly, maintenance, and logistic support, conducted a series of analyses of components, including a curved, stiffened composite panel, typical of a fuselage or fan cowls.

The panel and stiffeners are made of uniaxial and biaxial carbon fibers that are bonded with adhesive. The team focused on the composite manufacturing variables and tolerances for the panel, including material properties, geometric tolerances, thicknesses, and lay-up alignment axes, as well as the delaminations and disbonding that can occur during the manufacture, assembly, and service life of the composite structure.

“The objective of our analyses was to identify the influence of deviations, defects, and damage and to consider it during the initial design phase,” said team co-leader Abel Pardo. “In this way, nonconforming parts would be minimized, with associated cost savings.”

For intact panel analysis, Grupo TAM engineers used Abaqus FEA from Simulia for its ability to handle both implicit and explicit nonlinear analysis.

“We needed more than our in-house tools to conduct the analysis,” said co-leader Jose Carlos Fernandez. “We chose Abaqus for its extensive composite capabilities and to meet the high quality standards required by our customers.”

When performing the FEA analysis, engineers started with nominal values typical of the aeronautics industry for all the variables. They considered three load cases—two with a uniform aerodynamic pressure on the panel (one directed toward the inside of the structure, the other directed out) and a third with a shear load directed axially across the face of the panel. The team then performed two additional analyses of damaged panels—one with a delamination in the middle of the panel and the other with two disbondings under the panel stiffeners.

The results of all the FEA analyses provided baseline data that were then used to optimize the design and build of the composite panel using Isight.

For the stochastic analysis, the Grupo TAM engineering team looked at the manufacturing variables and tolerances in Isight from Simulia, as well as the range of damage during the component life cycle, determining that there were 58 important input variables. Statistical distributions for each variable were taken from either the baseline analysis data or standard industry values.

As a result of the study, Pardo said, “We now have a clear understanding of which variables are most critical to the manufacture of composite panels that will meet our stringent quality and safety criteria.”

The engineering team found via the study that buckling pressure was the most critical factor and that a tightening of material tolerance would lead to improved performance and lower costs for quality control and maintenance. It also determined that other less critical tolerances could be relaxed, resulting in both material cost savings for the carbon-fiber sheets and manufacturing cost savings where lay-up tolerances are involved. In addition, the analysis demonstrated that delaminate damage had a high impact on performance, while disbonding could be tolerated, especially with a new lay-up procedure.

“All of these results lead to resource optimization—with a quality and maintenance plan focused on the most influential inputs,” Pardo said.

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