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This Week's Feature Composite Example
Delamination Initiation/Propagation Failure Analysis of Reinforced Carbon-Carbon Woven Composite Specimens
The Shuttle Reinforced Carbon-Carbon (RCC) leading edge (Figure 1) is a brittle composite system which is subject to cracking and delamination from foreign debris and other impacts. The combination of cracks and delaminations can provide pathways for hot gases to enter the interior of the Shuttle wing during reentry, leading to serious consequences. If an impact occurred during launch, delaminations would be unseen by surface inspection on orbit. An investigation was carried out to demonstrate delamination prediction in RCC when impacts are known to have occurred.
|Figure 2 - Three-Point Bending (a) Test Setup and (b) Schematic and Dimension of the Short Specimen
A detailed failure analysis of two RCC specimens in a three-point bending configuration (Figure 2) was carried out by Alpha STAR Corporation (ASC) using GENOA and compared to NASA tests. GENOA progressive failure analysis (PFA) for 1.0 inch and 1.5 inch three-point bending specimens revealed that the two specimens exhibit different failure behavior prior to final failure (different delamination initiation and damage/fracture progression).
|Figure 3 - Test (a) versus Predicted  (b): Major and Minor Delamination (Initiation & Propagation) Location. Red Indicates Damaged Locations Just Prior to Final Failure. Simulation was based on a Single Layer of Shell Elements as the Finite Element Model.|
The finite element models of each specimen was constructed with 200 Mindlin-Reissner shell elements, simulating woven Reinforced Carbon-Carbon (RCC) material properties. Shell thickness was 0.229 inch. The loading was applied using displacement control. Each shell element consisted of 19 plies, significantly reducing the computational time without loss of analysis details such as stresses, strains, and damage information for the individual plies. The material properties were initially calibrated with the 1.0 inch specimen test data. The material calibration process was presented in one of our previous issues.
The predicted simulation results for the 1 and 1.5 inch specimens compared well with the reported test data (Figures 3 to 5). Alpha Star Corporation (ASC) did not have access to the 1.5 inch long specimen test results prior to submission of the analytic predictions to NASA (Figure 5).
|Figure 4 - Calibrated: Load-Displacement Response of 1.0 Inch Three-Point Bending Specimen
Figure 4 shows the test load-displacement behaviour and the corresponding progrssive failure analysis predictions for the three-point RCC bending specimens. Points A through F indicate the onset of various types of damage.
|Figure 5 - Measured and Predicted Load-Displacement Behavior for 1.5 Inch Specimen
Progressive Failure Analysis predicted failure loads of 350 lbf and 231 lbf for the 1 inch and the 1.5 inch specimens, respectively (Figures 4 and 5). Measured and predicted failure loads were in excellent agreement, with errors of less than 1% (1 inch specimen) and 5.7% (1.5 inch specimen). Both the shorter and longer specimens were predicted to fail at the specimen mid-section.This was confirmed by the test results.The analyses indicated that the failure mode of the shorter specimen was very different from that of the longer specimen. PFA of the shorter specimen predicted considerable delamination, but longer specimen PFA revealed virtually no delamination. In the shorter specimen, two delaminations were predicted (Points C, D, and E in Figure 4); a primary one and a secondary one.
Figure 3 shows the RCC damage when the load reaches point F (348 lbf) in Figure 4. The delamination, which initiated at load point C and is tracked as excessive relative rotation at ply 3, that is, between plies 2 and 3, and 3 and 4, grows toward the Y-axis (green arrow) as the applied load is further increased (Figure 3). It grows to become a secondary delamination of the specimen.
Also at an applied load of 348 lbf additional delamination was observed near the center of the ply stack, ply 9 (Figure 3). This new delamination occurs between plies 8 and 9, and 9 and 10 and is due to excessive transverse normal shear. The delamination originates where the shear is greatest, namely under the applied load and grows to become the primary delamination in the specimen. It reduces the bending stiffness of the specimen in the vicinity of the delamination.
|Figure 6 - Photograph of damaged 1.5 inch
specimen: (a) Test (b) Prediction at 231 lbf .|
Photos of the interior of the failed test specimens confirmed the predicted failure behavior. No delamination occurred in the longer specimen (Figure 6), but primary and secondary delaminations occurred in the shorter specimen (Figure 3). Figure 6 shows the similarity between the predicted and the snaps taken at the failure. The simulation results are at 231 lbf.
GENOA PFA predictions confirmed that the 1-inch three-point bending specimen will exhibit two delaminations; a primary one and a secondary one. The primary one results in considerable loss of stiffness prior to failure, whereas the secondary one does not. Photographs of the interior damage in the tested specimens indicated that the two delaminations occur near their predicted locations in the 1-inch specimen.
Confidence in the analytical predictions was reinforced since the predicted load-displacement behavior of both specimens were in excellent agreement with the measured load-displacement behavior.
Did You Know?
GENOA Custom Failure Criteria
GENOA has a recommended 'Custom Failure Criteria' that consists of a set of well known failure criteria, such as Tsai Hill, Puck, Strain Invariant and many more that help predict the strength of composites made up of wide variety of materials and several configuration (2D, 3D braid and woven). The approach is simple to use and allows implication of user defined failure criteria subroutine. For more information on this feature and trying out GENOA through our demos, please contact email@example.com.
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