Hiroshi Suemasu

Effects of Multiple Delaminations on Compressive Buckling Behaviors of Composite Panels

Compressive buckling stability of composite panels with through-width, equally spaced multiple delaminations are investigated analytically and experimentally. An analytical method is formulated on the basis of Rayleigh-Ritz approximation tech nique. Timoshenko type shear effects are included. An experiment and a finite element analysis are also conducted on the present model. The analytical results agree very well with the experimental and finite element results. The buckling load, which is the com pressive strength of the panel in the case of the present model, reduces significantly due to the existence of multiple delaminations. The mechanism causing the significant loss of the compressive buckling load due to the delaminations is well explained.

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.

Damage propagation in CFRP laminates subjected to low velocity impact and static indentation

This paper describes a damage accumulation mechanism in cross-ply CFRP laminates [02/902]2S subjected to out-of-plane loading. Drop-weight impact and static indentation tests were carried out, and induced damage was observed by ultrasonic C-scan and an optical microscope. Both tests gave essentially the same results for damage modes, sizes, and load-deformation history. First, a crack occurred in the bottom 0° layer accompanying some delamination along the crack caused by bending stress. Then, transverse cracks occurred in the middle 90° layer with decreasing contact force between the specimen and the indenter. Measured local strains near the impact point showed that the stress state changed from a bending dominant state to an in-plane tensile dominant state. A cohesive interface element was used to simulate the propagation of multiple delaminations and transverse cracks under static indentation. Two types of analytical models are considered, one with multiple delaminations and the other with both multiple delaminations and transverse cracks. The damage obtained for the model with only multiple delaminations was quite different from that obtained from the experiment. However, the results obtained from the model with both delaminations and transverse cracks well explain the characteristics of the damage obtained in the experiment. The existence of the transverse cracks is essential to form the characteristic impact damage.

An experimental method to measure the mode-III interlaminar fracture toughness of composite laminates

Abstract An experimental method to measure the Mode-III interlaminar fracture toughness of composite laminates is proposed. The experiment is based on an idea that the stress field at the crack tip area becomes a mode-III fracture field when a prismatic bar specimen with uniform embedded delamination is twisted (edge crack torsion (ECT) specimen). The mechanism is simply proved through a superposition technique. An explicit approximate expression to estimate energy-release rate is derived. The possible energy release rate attained without shear failure or severe non-linearity is also given and found to be increased roughly in proportion to the thickness of the specimen. Since the attainable G III value without severe damage in the other portions may not be large enough to measure the fracture toughness of tough fiber-reinforced-plastic composites of the usual coupon size, it may be necessary for the specimens to be modified in order to obtain the mode-III interlaminar toughness, G IIIc . The present experimental method is still attractive as a test method to measure mode-III fracture toughness of the composite laminates owing to its simplicity which compares well with the DCB and ENF testing methods.

Compressive Behavior of Multiply Delaminated Composite Laminates Part 1: Experiment and Analytical Development

The basic mechanics and mechanism concerning compressive stability of composite laminates with multiple circular delaminations is studied analytically and experimentally. An experimental program, employing two types of quasi-isotropic laminates with a conventional and toughened epoxy resin, is used to evaluate the validity of the mechanistic model and further demonstrate the accuracy of finite element analysis conducted in the associated paper. Embedded delaminations are introduced at regular intervals in the thickness direction. The loading edges are fixed, and the side edges are simply supported. Although the buckling load does not depend on the matrix resin toughness, the strength is affected by the toughness. In the analysis, a buckling equation is derived using the Rayleigh-Ritz method, based on classical plate theory and solved as an eigenvalue problem. This method is chosen due to its efficiency. As the buckling mode of the lowest buckling load becomes physically admissible due to the assumptions of equally spaced delaminations and the classical plate theory, the contact problem does not need to be considered, that is, all of the delaminated portions deform by the same amount and do not overlap one another even without any constraints. The buckling loads analytically obtained agree well with experimental and finite element results described in the associated paper. The effects of size and number of circular delaminations on the buckling and failure load are also discussed in detail.

Postbuckling Behaviors of Composite Panels with Multiple Delaminations

Compressive behaviors of composite panels with through-width, equal- size, equally-spaced multiple delaminations are investigated analytically. An analytical method is formulated on the basis of the Rayleigh-Ritz approximation technique. Contact problem on the delaminated surfaces is considered by the introduction of a constraint, which restricts a relative out-of-plane displacement and a relative slope between the upper and lower portions at a contact point. The postbuckling paths of delaminated panels are solved numerically and the results are compared with the experimental results. If appro priate initial imperfections are assumed, the analytical results can explain well the be haviors of the delaminated panels which are observed in the compressive experiment. The results of the postbuckling analysis indicate that the simplified buckling analysis, where the opening of the delamination is not considered, is sufficient to estimate the buckling load. A method to estimate a total energy release rate is also proposed.

Compressive behavior of multiply delaminated composite laminates. Part 2 : Finite element analysis

Postbuckling behavior of rectangular composite plates with multiple circular delaminations is studied numerically using finite element analysis. Embedded circular delaminations are placed at regular intervals in the thickness direction at the plate center. The plate is fixed along its loading edges and simply supported at its side edges. The three-dimensional block element is adopted to accurately obtain distributions of energy release rates G I , G II , and G III , along the delamination fronts based on the virtual crack closure technique. The contact problem of the delaminated surface is considered through introducing a spring element between nodes at the upper and lower surfaces of the delaminations, which resist only compression force. Buckling loads obtained in the present analysis agree well with those obtained from experiment and classical Rayleigh-Ritz analysis (see companion paper). The complex compressive behavior observed in the experiment can be explained through the results obtained in the present finite element method, which strongly depends on initial imperfections. The distribution of energy release rates in the postbuckling region is also consistent with the failure of the delaminated laminates observed in the experiment

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.

Mechanical and thermal properties of silicon-carbide composites fabricated with short Tyranno® Si-Zr-C-O fibre

Silicon carbide (SiC) composites reinforced with 10–50 mass% (10.5–51.2 vol%) of short Tyranno® Si-Zr-C-O fibre (average length ∼0.5 mm) and 0–10 mol% of Al4C3as a sintering aid were fabricated using the hot-pressing technique. Firstly, the effect of Si-Zr-C-O fibre addition on the relative density (bulk density/true density) of the SiC composite hot-pressed at 1800 °C for 30 min was examined by fixing the amount of Al4C3to be 5 mol%. Although the relative density was reduced to 87.4% for 10 mass% of Si-Zr-C-O addition, further increases in the amount of Si-Zr-C-O fibre increased density to a maximum of 92.8% at 40 mass% of fibre addition. Secondly, the effect of varying the amount of Al4C3addition on the relative density was examined by fixing the amount of Si-Zr-C-O fibre to be 40 mass%. The optimum amount of Al4C3addition for the fabrication of dense SiC composite was found to be 5 mol%. The fracture toughness of the hot-pressed SiC composites with 20–40 mass% of Si-Zr-C-O fibre addition (amount of Al4C3: 5 mol%) was 3.2–3.4 MPa · m1/2and approximately 1.5 times higher than that (2.39 MPa · m1/2) of the hot-pressed SiC composite with no Si-Zr-C-O fibre addition. SEM observation showed evidence of Si-Zr-C-O fibre debonding and pull-out at the fracture surfaces. The hot-pressed SiC composite with 5 mol% of Al4C3and 40 mass% of Si-Zr-C-O fibre additions showed excellent heat-resistance at 1300 °C in air due to the formation of a SiO2layer at and near exposed surfaces.

Novel Test Method for Mixed Mode II and III Interlaminar Fracture Toughness

A novel test method is proposed to evaluate mixed mode II and III interlaminar fracture toughness of composite laminates. The method is a well-improved test method of the split cantilever test method and easy to conduct, while a computational analysis is necessary to specify the critical toughness values. The aim of the test is to obtain mixed mode fracture toughness. The twisting force that exists in the case of the split cantilever test method is avoided by making the specimen symmetric by introducing two symmetrically located delaminations. By the present method, the two cracks grow stably keeping their configuration with the increase of applied displacement, that is, stable self-similar crack growth is realized. The energy release rate can be numerically evaluated from the experimentally obtained data, such as, the applied load (or applied displacement) as well as the crack configuration. The energy release rate varies along the curved crack front and the dependence of the mixed mode critical energy release rate on the ratio of the mode components can be obtained by one test trial. The method can be conveniently used to roughly estimate the failure criterion for mode II and III mixed mode Interlaminar fracture.