A new progressive failure analysis capability for stiffened composite panels was developed at Collier Research Corporation and the NASA Glenn Research Center. It combines Collier’s HyperSizer (ref. 1) stiffened panel design software with Glenn’s Micromechanics Analysis Code with Generalized Method of Cells (MAC/GMC) (ref. 2). MAC/GMC discretizes a composite material’s microstructure into a number of subvolumes and solves for the stress and strain state in each while providing the homogenized composite properties. As a result, local failure criteria can be employed to predict local subvolume failure and the effects of these local failures on the overall composite response. When combined with HyperSizer, MAC/GMC represents the ply-level composite material response within the laminates that constitute a composite stiffened panel. The effects of local subvolume failures can then be tracked as loading on the stiffened panel progresses.
T-stiffened panel-to-ply-to-micromechanics simulation now available within HyperSizer in the context of time-dependence and progressive failure. On the basis of the panel-level loads, HyperSizer determines the laminate- and ply-level stresses and strains, which are passed to MAC/GMC. MAC/GMC then determines fiber- and matrix-level stresses and strains and can predict local damage and failure, the effects of which are then passed back to the higher scales. Thus, the effects of local damage progression on the panel-level response can be simulated.
Long description of figure 1.
Progressive failure occurs when a structure experiences a significant amount of damage prior to final failure. The life of the structure is, therefore, not accurately represented by the point at which failure initiates, rather, failure progresses from initiation to final failure in some way. Clearly, in order to predict the life of such a structure, a methodology that tracks the failure progression is needed. This type of methodology has been enabled for arbitrary composite laminates and stiffened panels via the combination of HyperSizer with MAC/GMC. MAC/GMC can provide the ply-level composite material properties to HyperSizer, from which HyperSizer can determine laminate- and panel-level properties (see the illustration). Furthermore, because MAC/GMC localizes to the level of the fiber and matrix constituents, microscale failures can be predicted given ply-level stresses and strains from HyperSizer. The new ability of HyperSizer to apply loading to a stiffened panel incrementally (i.e., time-dependent loading) has now enabled progressive failure analysis based on local failures predicted within MAC/GMC while eliminating the stiffness contribution of the failed regions within the fiber and matrix constituents.
These two plots compare predictions of the described analysis technology with experimental results from the literature. Left: Biaxial stress-versus-strain response of the laminate; curves representing both midplane strains, εxx and εyy are plotted against the stress σyy. The curves are nonlinear because of progressive damage, and the model and experiment match very well. The ultimate strength of the laminate in the experiment was 847 MPa, whereas the model predicted 820 MPa. Right: Several failure envelopes plotted on σyy versus σxx axes. The predicted ultimate failure envelope matches well with the experimental data, whereas the predicted failure initiation is somewhat inside the associated experimental data and well within the experimental and predicted ultimate failure envelopes. The envelopes that represent common first-ply failure criteria (maximum stress, maximum strain, Tsai-Hill, Tsai-Wu, Hashin, and LaRC03) are well inside the experimental ultimate failure envelope in the tensile regime and closely match the HyperSizer-predicted initiation curve. In compression, all the failure envelopes match reasonably closely.
Example results are shown in the preceding graphs for a quasi-isotropic [0/±45/90]s AS4 graphite/3501-6 epoxy composite whose response was tested as part of the World Wide Failure Exercise (ref. 3). In the graph on the left, the HyperSizer--MAC/GMC progressive-failure prediction matches well with the nonlinearity in the experimental laminate stress-strain curves, while matching the ultimate failure stress within approximately 3 percent. The graph on the right shows that the predictions agree well with the full normal stress experimental failure envelope. Furthermore, in contrast to common ply-level failure criteria (which significantly underpredict ultimate failure), the current progressive failure methodology enables the prediction of both damage initiation and ultimate failure envelopes. The following graphs show similar results for a T-stiffened [0/±45/90]s AS4 graphite/3501-6 epoxy composite panel (see the illustration shown earlier). Although no experimental results are available in this case, the progressive failure capabilities of the new analysis tool are illustrated.
Two plots for a T-stiffened panel--one representing deformation and the other representing failure envelopes. No experimental data are plotted in this case. Left: Midplane strains εxx and εyy plotted versus the panel-force resultant Nyy. Nonlinearity due to damage is evident. Right: HyperSizer damage initiation and ultimate failure envelopes are plotted with the initiation envelope falling well within the ultimate failure envelope, even in the compressive regime. The common first-ply failure criteria (maximum stress, maximum strain, Tsai-Hill, Tsai-Wu, Hashin, and LaRC03) are between the predicted initiation and ultimate failure envelopes in the compressive regime and close to the predicted initiation envelope in the tensile regime.
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Last updated: October 11, 2006
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