The Quest for a Lighter, Stronger Aircraft

The aluminum age in aircraft design is all but over. Aircraft manufacturers in every market are increasingly looking at composite materials to create vehicles that are lighter, stronger and easier to maintain. Lighter aircrafts mean increased range, which in turn mean lower fuel costs – a critical factor in a petroleum-dependent world.

The Boeing 787 Dreamliner will be the first commercial jet to be more than 50% composite by weight. The Airbus A380, with deliveries that started in 2008, is also increasing its reliance on composites. With the materials paradigm shifting in aerospace, it was predicted at the CompositesWorld 2008 conference that demand for composites would grow 30% in the general aviation market during the next three years.

Carbon fiber reinforced polymer (CFRP) is the most common composite used in the aerospace industry. Carbon fibers have a micro-graphite crystalline structure and a pattern similar to chicken wire; they derive their strength from layering, or sandwiching, multiple sheets in a polymer matrix. CFRP composites, with their attractive weight-to-strength ratio and other beneficial material properties – high tensile strength, high elastic modulus, heat resistance, low thermal expansion and chemical stability – are highly desirable in high-performance aerospace applications. Like any material, composites have their own set of manufacturing, assembly and lifespan challenges that must be fully understood to make their use in critical applications, such as commercial flying, acceptable and safe.

Aircraft manufacturing is evolving into a process where a variety of specialized manufacturers are contracted to produce structures or sub-assemblies that are then assembled into a finished aircraft by the parent company. One such specialty firm is Grupo TAM, headquartered near Madrid, Spain. Grupo TAM manufactures auxiliary components with state-of-the-art CNC tools. Approximately 40% of its business is in aeronautics, including the design and manufacture of composite structures.

To fully understand the performance of these composite components, as well as assembly and maintenance challenges, a Grupo TAM structural analysis engineering team, headed by Abel Pardo and Jose Carlos Fernandez, conducted a series of in-depth 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.

Composite panel with shear loads applied (top) and with axial, or aerodynamic, loads applied (bottom)

Composite Analysis
For the intact panel analysis, the Grupo TAM engineers chose Abaqus FEA from Simulia, the Dassault Systèmes brand for realistic simulation, in large part for its ability to handle both implicit and explicit non-linear analysis. “We needed more than our in-house tools to conduct the analysis,” Fernandez says. “We chose Abaqus for its extensive composite capabilities and to meet the high quality standards required by our customers.”

They also chose Isight from Simulia for its Monte Carlo and Stochastic Design Improvement components, its sampling capability, and the ease with which it can interface with in-house software. Isight allowed the team to conduct trade-off studies with their Abaqus models and achieve rapid design optimization.

To carry out their FEA analysis of the intact panel, the team 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 towards 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, the other with two disbondings under the panel stiffeners.

The team constructed their geometry model in CATIA v5 from Dassault Systèmes, using the following: S4R planar elements for the skin and stiffeners; the C3D8R element for the adhesive; shell composite with a single ply for the delamination analysis; and for the disbonding analysis, a homogeneous solid in which mechanical properties were reduced six orders of magnitude. The model had approximately 49,500 elements, 45,400 nodes and 272,600 variables.

The analysis was run on a Windows server with four Intel Xeon processors, each with 64 bits and 8GB of RAM. The team conducted multiple Monte Carlo simulations, with each full cycle analysis – including nominal, delamination and disbonding analysis – taking 30 minutes.

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

Composite Performance
As with many materials and structures, the number of variables to consider when designing a composite panel for an airplane is large, and it is difficult to sort out which variables might be the key to improving structural strength and performance. In such instances, a stochastic approach is useful for managing the enormous amount of data inherent in composite analysis. Isight streamlines this iterative solution process by using an interactive graphical interface and automation features built into the software to enable tools like Monte Carlo, Design of Experiments and/or Six Sigma for optimization.

In this case, the Grupo TAM team chose the Monte Carlo method, which is particularly useful when there is significant uncertainty in the variables and inputs.

To begin the stochastic analysis in Isight, the Grupo TAM engineering team looked at the manufacturing variables and tolerances, 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 described above or standard industry values. The team then built a calculation flow chart that was accomplished using Isight’s intuitive graphical tools and icons. Isight then automatically and repeatedly ran this analysis string without the need for individual manual FEA analyses, with each Monte Carlo simulation including between 100 and 800 samples. According to Fernandez, “Descriptive sampling was chosen because it has better convergence to the statistical distributions and requires less iteration. In the end, this powerful computational process identified the most critical tolerances and variables for us.”

With the results of the study in, Pardo says, “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.”

Composite panel with loads applied shows shear buckling (left), pressure buckling (center), and composite strain (right).

Cost Reduction
While the goal of optimizing the composite panel, with Abaqus FEA and Isight, was to increase panel strength and ultimately performance, the analysis process also provided insight into associated manufacturing, assembly and maintenance costs. The engineering team reached a number of interesting conclusions. They found that buckling pressure was the most critical factor, and that a tightening of material tolerance would lead to improved performance along with lower costs for quality control and maintenance. They 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.

Looking to the future, the Grupo TAM structural analysis department identified a number of developments that will further improve the overall cost evaluation process. For instance, parallel computing in Isight will cut computing time in half. Additionally, a design-to-cost strategy will be employed in which cost functionality will be added to the analysis using software that is currently under development in the industry and can be incorporated into Isight.

“This analysis process would lead to what we at Grupo TAM call robust design,” Fernandez concludes. “Robust, because it takes into consideration the entire product life cycle as well as all associated costs.” Incorporating such cost considerations within stochastic analyses will undoubtedly provide tremendous value to manufacturers in any industry.

Dassault Systèmes, Simulia
Providence, RI
simulia.com