Osamu Majima

Multiple delaminations and their severity in circular axisymmetric plates subjected to transverse loading

Instability problem of multiple penny-shape interlaminar delaminations in circular axisymmetric plates subjected to a transverse concentrated loading is theoretically studied. The authors consider a case that all the delaminations located at the same intervals have a same size and propagate simultaneously. Then, they consider two cases that one of the delaminations is larger or smaller than the rest. The energy release rates are obtained in closed forms and the results are compared with those obtained numerically via a finite element analysis. The present solutions agree excellently with the finite element results. When the load is constant, the energy release rate is constant regardless of the size of the delaminations and increases linearly with the number of the delaminations. When one of the delamination is larger or smaller than the others, the energy release rate of the shorter delamination or delaminations become much larger than that of the longer delamination(s), that is, only the smaller delamination(s) tends to propagate until it grows up to the size of the larger delamination(s). Multiple delaminations, if they exist, have a tendency to grow self-similar way.

Multiple Delaminations and their Severity in Nonlinear Circular Plates Subjected to Concentrated Loading

Instability problem of multiple penny-shape interlaminar delaminations in circular nonlinear axisymmetric plates subjected to a transverse loading is theoretically studied. It is an idealized problem of damage accumulation in composite laminates due to low velocity impact loading. All the delaminations having a same size are located at a same interval. Nonlinear behaviors of the plates are approximately solved through Rayleigh-Ritz method considering only two mode functions, that is, global and local mode functions which are based upon a linear exact solution. Energy release rate due to a simultaneous propagation of all the delaminations is obtained approximately and the results are compared with those obtained via a finite element analysis. The present solutions agree excellently with the finite element results except when the nonlinear effect is extremely large. The energy release rate significantly decreases with the propagation of the delaminations due to the nonlinear effects, particularly when the number of the delaminations is large. The load must be increased to keep the delaminations to propagate. The conclusions from the results obtained based on the linear assumption that the more delaminations exist, the easier delaminations propagate, may not be true when the nonlinear effect is considered.

Compressive property degradation of composite stiffened panel due to debonding and delaminations

The effect of localized damage due to impact on compressive buckling as well as postbuckling behaviors of blade stiffened composite plates was numerically studied. A partial debonding between a skin panel and a flange and multiple delaminations in the skin panel were chosen as the localized damage. The three-dimensional composite elements used to analyze the compressive behavior of the stiffened panel with multiple delaminations was found to be suitable for this kind of complex composite structure. The contact problem between the skin and flange panels was approximated by a spring element with no restraint of the positive relative displacement and a strong restraint of the negative displacement. At the delaminations in the skin panel, which tended to close during compression, the normal relative displacement was constrained to prevent the delaminated portions from overlapping; this constraint made the convergence of the solution quite smooth compared to the case where the contact problem was exactly considered. When a debonded area only was located at the edge of the flange, no notable reduction of compressive buckling load was found until the size of the debonding reached a half wavelength of the buckling mode. The compressive buckling load dropped significantly when multiple delaminations, which were small in comparison to the half wavelength of the buckling mode, accompanied the skin-flange debonding. The energy release rate distributions at the damage edges, calculated using the virtual crack closure technique, increased quite rapidly, particularly in the transverse direction after buckling became sufficient to increase the delamination.

Compressive Behavior of Composite Laminates with Multiple Delaminations of different sizes

Effects of multiple delaminations of different sizes on the compressive behavior of rectangular composite laminates are studied using a finite element analysis, because it is reported that the size of delaminations which occur in composite laminates due to impact loading increases linearly from the impacted surface to the back surface. Embedded delaminations exist at a regular interval in the thickness direction, but their diameters change linearly with the depth from the surface. The loading edges are fixed and the side edges simply-supported, following the SACMA CAI test standard. The fracture due to instability of the delamination propagation is discussed using distributions of energy release rate along the delamination edges. The effect of the variation of the size of delamination on the compressive behavior is found to be significant and the postbuckling behaviors are different from those of the plates with equal diameter delaminations. Keyword : Composite, Buckling, Postbuckling, Multiple delaminations, Energy release rate, Finite element analysis, CAI.

Damage accumulation in composite laminates during quasi-static transverse loading

A mechanism of damage accumulation in composite laminates subjected to a quasi-static concentrated load is numerically studied by using the finite element method to disclose the impact damage problem. The energy release rate distributions are calculated along the delamination edges in square composite laminates with various stacking sequences. Not only circular delaminations but also more realistic impact damage models are investigated to explain the reason why the typical impact damage must be created. The effects of transverse cracks and stacking sequence on the energy release rate are discussed.

Effect of different size multiple delaminations on compressive behavior of composite laminates

Effects of multiple delaminations of different sizes on the compressive behavior of rectangular composite laminates are studied using both shell and brick elements. Embedded delaminations exist at a regular interval in the thickness direction, but their diameters change linearly with the depth from the surface. The loading edges are fixed and the side edges simply-supported, following the SACMA CAI test standard. The brick elements are used to model the delaminated portions and surrounding portion and the shell elements to model the undelaminated portion. The energy release rate can bc well estimated, because the three dimensional brick elements are used in the neighborhood of the crack front. The fracture due to instability of the delamination is discussed using distributions of energy release rate along the delamination edges. Though the change of the sizes of delaminations in the thickness direction significantly influences on the compressive behavior of the delaminated plate effects just after the buckling, the basic behavior is similar to that of the plate with equal diameter multiple delaminations. The postbuckling behavior consists of three stages: initial local instability of the delaminated portion, global deformation of symmetric shape and global deformation of antisymmetric shape. Keyword : Composite, Buckling, Postbuckling, Multiple delaminations, Energy release rate, Finite element analysis, CAL

Severity of Multiple Delaminations in Nonlinear Plates Subjected to a Concentrated Loading

Instability problem of multiple penny-shape interlaminar delaminations in circular nonlinear axisymmetric plates subjected to a concentrated loading is analytically studied. It is an idealized problem of damage accumulation in composite laminates due to low velocity impact loading. All the delaminations having a same size are located at the center of the plate at a same interval. Not only nonlinearity due to large deflection but also effect of inplane stiffness reduction in the delaminated portion due to damage such as transverse cracks are considered. The reduction of inplane stiffness is introduced by uniformly reducing the inplane elastic modulus by a same rate in the damaged area. The nonlinear behavior is approximately solved by a Rayleigh-Ritz method considering only two mode functions, that is, a global and a local mode functions which are based upon a linear exact solution. Energy release rate due to a simultaneous propagation of all the delaminations is obtained approximately and the results. The introduction of the reduced stiffness makes the high hardening effect due to large deflection weaker. The reduction of the elastic properties in the damaged portion is an important factor for the impact damage accumulation process.