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This Week's Feature Composite Example

Certification-by-Analysis (CBA)

Objective and Benefit: Certification-by-Analysis (CBA) is a major cost and time saving approach as it minimizes physical testing of structural components. Using Alpha STAR Corporation's modular engineering software package, GENOA, a product's structural response can be rigorously tested and certified, virtually, to FAA standards, with minimal substantiation testing and reduced/accelerated certification time. The software simulates testing by integrating multi-scale physics based failure mechanisms with progressive failure analysis (PFA) to assess damage and fracture of composites. It implements the building block approach advocated by FAA, commencing at the coupon level through the detail, sub-component and component levels. Figure 1 shows the PFA test simulation of open-hole, three-point bending and facesheet delamination at coupon level followed by sub-component level oblique slot and filled hole fuselage panels as part of a comprehensive building block verification. 
Structural Level
Coupon Level

Figure 1 - GENOA Building Block Supports Composite Certification from Coupon Level to Component Levels



The power of virtual testing by means of PFA lies in its ability to simulate the behavior of structural components where very few can be tested (example: fuselage panel/wing box). The simulation follows closely the FAA recommended building block approach for structural component certification shown in Figure 2. GENOA can virtually simulate the physical test of all type of coupons to accelerate the certification process for higher order parts and components. These building block elements may be notched/un-notched, stringer-stiffened, honeycomb sandwich or other construction. To achieve a valid CBA, accurate simulation of the fundamental constituent composite properties; e.g., fiber and matrix is needed. Thus, Genoa uses Material Characterization Analysis (MCA) to perform a calibration as a first step in CBA. Simple and inexpensive coupon tests provide the data needed for the MCA.  


GENOA uses the calibrated fiber/matrix properties in a layer-by-layer simulation, including through-the-thickness effects. This simulation is used in the finite element model development and analysis as well as the subsequent PFA. For each load case, the load level is incrementally changed, enabling the PFA process to pinpoint the damage initiation location and load level, the damage propagation process, fracture initiation, fracture propagation and final failure. This process tells the engineer where, when, how and why the structural design exhibits damage, fracture and failure.



Figure 2 - FAA building block approach for structural component 
certification is implemented in GENOA (Courtesy of FAA)



Validation of GENOA-CBA: The software capability for CBA is validated by comparing predictions with actual test data at all levels of the building block process. The process started with generating calibrated material allowables from existing ATSM coupon test data; then validating those allowables against tailored, design-specific coupon test data; and comparing sealed envelop analytical results of full-scale "fuselage" panels to test results. Total of three honeycomb sandwich panels were simulated with PFA: baseline panels, circular hole panels, and slotted panels. The honeycomb sandwich panel configuration for these coupon tests represented the building block for curved fuselage panels which were also evaluated. The sandwich consisted of 3 ply (45/0/45) laminate facesheets bonded to a 3/4" Nomex honeycomb core. The base line and circular holes panels were evaluated using various load configurations: pressure only, longitudinal only and combined pressure and longitudinal loading. The slotted panels were evaluated for various loading as well: longitudinal slot, oblique slot, and circumferential slot combined loading. 


GENOA's PFA failure predictions matched the test results closely (force versus tensile strain plot in Figure 3a). The red zones represent lamina/laminate damaged areas (Figure 3b). The composite can still sustain loads but at a reduced level. When the composite material can no longer carry any load fracture initiates, elements are removed and stresses are re-distributed. GENOA Virtual Testing deviated less than 10% different from the actual test results. Predictions using the building block approach with accuracy such as these are revolutionary. With well established and well verified building block strategy structural components can now be simulated for a variety of flight loads reducing the number of physical tests for certification.



Figure 3a - Force vs. tensile strain for sandwich specimen with 1" diameter hole



Figure 3b - Damage pattern of sandwich specimen with 1" diameter hole

Figures 3a - 3b   GENOA Virtual Testing of honeycomb sandwich tensile elements with through-the-sandwich hole.





1. Scott Leemans, Peter J Rohl, Dade Huang, Frank Abdi, Jonas Surdenas, Raju Keshavanarayana, "Certification By Analysis: General Aviation Honeycomb Fuselage Panels". Sampe 2009 Conference Paper, Baltimore, MD, May 18-21, 2009. 
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