Shinya Motogi

Damage mechanics approach to nonlinear behavior of FRP laminates with cracking layers

A damage mechanics model to predict the nonlinear behavior of laminated composites due to crack evolution is developed. We propose a new concept that a cracking layer can be replaced with an equivalent uniform work-softening layer. With this new concept, the constitutive equations for a cracking layer are constructed according to modem plasticity theory. A lamina damage surface, i.e. a threshold of crack evolution, is defined in the stress space of each lamina, and the constitutive equations for a cracking layer are constructed by applying the defined damage surface to the associated flow rule. Then, the derived constitutive equations for a cracking layer are introduced with classical lamination theory to predict laminate constitutive behavior in the damage process. Comparisons are made between the predictions and experimental results for some laminate configurations, and reasonable agreements are shown for all laminates.

Evaluation of Matrix Cracks in FRP Laminates Using Angular Dependence of Lamb Wave Propagation

A temperature-insensitive damage evaluation parameter for composite laminates is proposed. Ultrasonic wave velocity is directly related to material stiffness, so that it is a quantitative parameter that gives information about the mechanical state of the material. However, ultrasonic velocities vary with temperature therefore environmental factors might cause non-negligible error in inspected results. In this work, the angular dependence of Lamb wave propagation, i.e., the acoustic anisotropy, is exclusively focused on and the change in the acoustic anisotropy is used as a damage evaluation parameter. It is experimentally confirmed that the acoustic anisotropy is significantly altered by matrix cracks formation although it is insensitive to temperature variation. It is also shown that the combination of different Lamb modes is a more promising method for predicting the effective stiffness of damaged laminates.

Constructing of cure monitoring system with piezoelectric ceramics for composite laminate

The cure monitoring system with piezoelectric ceramics is constructed. An embedded type piezoelectric ceramics sensor with flat lead wires is developed. And the piezoelectric ceramics is embedded into composite laminate. A dummy piezoelectric ceramics is set in the autoclave oven. The impedance of the piezoelectric ceramics which is embedded in the composite laminate and that of the dummy piezoelectric ceramics are measured by a LCR meter. The piezoelectric ceramics have strong temperature dependency. The temperature dependency of the impedance of piezoelectric ceramics is corrected by the information from the dummy piezoelectric ceramics. A dielectric sensor is also embedded in the composite laminate as a reference sensor for the degree of cure. The change in calculated cure index shows good correspondence with change in the log ion viscosity which is measured by the dielectric cure monitoring sensor.

Composite Materials. Prediction of Stiffness Reduction in Composite Laminates Using Damage Mechanics Approach.

An analytical model for predicting the matrix crack induced stiffness reduction of FRP laminates with off-axis cracked plies is developed. The constitutive equations for a cracking ply are first proposed with the assumption of the equivalence between a cracking ply and a work-softening material. Then they are incorporated into the classical lamination theory to describe the damage evolution in FRP laminates with off-axis cracked plies. The energy release rate in a cracking ply is adopted as the criterion for crack formation, and the number of cracks is predicted by dividing the cumulative released energy in a cracked ply by the released energy due to single crack formation. With the predicted number of cracks, the variation of the laminate stiffness with the crack density is evaluated. The uniaxial tension test is performed on GFRP [0/θn/0] laminates, and the elastic modulus in the loading direction is measured at five different crack density. The experimental results are compared to the corresponding predictions of the elastic modulus for three different center ply angles.

Mechanism of Matrix Cracking Evolution in Crossply Composite Laminates.

This paper discusses the mechanism of matrix cracking evolution in crossply composite laminates. The evolution of matrix cracking cannot be understood by the notion of ‘strength’ of cracking ply, since matrix cracks are sequentially formed above the stress level of the strength of cracking ply. By assuming that the ply has inherently a distribution of defects which act as the nucleation sites of matrix cracks, the cracking mechanism is discussed. This mechanism can explain the behavior of the averaged maximum stress in the whole stage of evolution. A simple numerical simulation is performed in order to visualize the cracking behavior and the change in stress due to cracking.

Criterion for Matrix Cracking in Glass Fiber Reinforced.

The criterion for the progress of matrix cracks is investigated for glass/epoxy cross-ply laminates. A series of experiments is conducted to obtain the relation of the laminate stress and the number of transverse cracks in [0/90n]s (n=2, 3, 4) configurations. Using Hashins theory for stress in an element of a laminate, the maximum normal stress in 90° layer in the load direction is found to be the criterion for the matrix cracking. Independently of the stacking configurations, the matrix cracking progresses when the above stress exceeds a threshold value.