John Montesano

Development of a synergistic damage mechanics model to predict evolution of ply cracking and stiffness changes in multidirectional composite laminates under creep

A physics-based multi-scale model that couples viscoelasticity and time-dependent damage evolution for general multidirectional laminates subjected to long-term creep loading is developed. The viscoelastic ply behavior is evaluated using a nonlinear Schapery-type viscoelastic model, while a methodology employed within the framework of classical laminate theory is used to predict the corresponding laminate time-dependent response. The evolution of microscopic ply cracks in multiple plies with different orientations during creep loading is predicted using an energy-based approach, and the corresponding laminate stiffness degradation is evaluated using a synergistic damage mechanics-based model that relies on computational micromechanics in lieu of costly experimental data. The developed model is used to predict the evolution of ply cracks and the viscoelastic stress–strain response of various cross-ply and multidirectional laminates under quasi-static and creep loading. Predicted strains, compliance changes and crack density evolutions show excellent agreement with available experimental creep data for different laminate stacking sequences, providing validation for the model. Predictions are also made for two additional multidirectional laminates in order to investigate the effect of off-axis ply angle on the creep response, which demonstrates the versatility of the model and its usefulness in assessing the long-term durability of general multidirectional composite laminates.

Progressive Failure Analysis of Polymer Composites Using a Synergistic Damage Mechanics Methodology

A synergistic damage mechanics approach was implemented into a commercial finite element package to predict progressive failure of multidirectional polymer composite laminate components. The methodology retains the framework of continuum damage mechanics, while incorporating micromechanics to define the internal damage variables of the material system computationally. A user-defined subroutine was developed and damage evolution of quasi-static tensile loaded components was simulated using nonlinear analysis. The model predictions were in good agreement with experimental observations reported in the literature. Future considerations for model development should include incorporating the effects of multiaxial stresses on damage evolution, and adding additional damage modes to the model.

Fatigue Damage in On-Axis and Off-Axis Woven-Fiber/Resin Composite

In this study the tensile static and fatigue behaviour of a woven-fabric laminate is investigated in both the on-axis and off-axis material directions. Emphasis is placed on the development of damage and its influence on the stress-strain behaviour of the laminate. The test results illustrate that there is a high degree of anisotropic behaviour due to anisotropic damage development, which is evident by the variation of the material behaviour between the on-axis and off-axis test specimens. The fatigue tests also suggest that the on-axis specimens exhibit noticeable stiffness degradation, while the off-axis specimens do not. The qualitative results provide significant insight into the type of damage mechanism responsible for the observed behaviour.

Application of fiber optic sensors for elevated temperature testing of polymer matrix composite materials

Abstract Advanced polymer matrix composite (PMC) materials have been more frequently employed for aerospace applications due to their light weight and high strength. Fiber-reinforced PMC materials are also being considered as potential candidates for elevated temperature applications such as supersonic vehicle airframes and propulsion system components. A new generation of high glass-transition temperature polymers has enabled this development to materialize. Clearly, there is a requirement to better understand the mechanical behaviour of this class of composite materials. In this study, polyimide-coated fiber optic sensors are employed to continuously monitor strain in a woven carbon fiber bismaleimide (BMI) matrix laminate subjected to tensile static and fatigue loading at elevated temperatures. A unique experimental test protocol is utilized to investigate the capability of the optical sensors to monitor strain and track stiffness degradation of the composite material. An advanced interrogation system and an optical spectrum analyzer are utilized to track the variation in the optical fiber wavelength and the wavelength spectrum for correlation with strain gage measurements. Isothermal tensile static and fatigue tests at room temperature, 105°C, 160°C and 205°C suggest that these optical sensors are capable of continuously monitoring strain and tracking the stiffness loss of a highly compliant PMC specimen during cyclic loading. The results illustrate that employing optical sensors for elevated temperature applications has significant advantages when compared to conventional strain gages.

Considerations for progressive damage in fiber-reinforced composite materials subject to fatigue

Due to the increased use of composite materials in the aerospace industry, numerous attempts have been made to develop fatigue models in order to predict the fatigue behaviour and consequently the fatigue life of these materials. Existing fatigue models have significant deficiencies, thus are not widely acceptable in the industry. A better understanding of the exhibited fatigue behaviour of composite materials is consequently required. The complex nature of fatigue behaviour in fiber-reinforced composite materials is presently investigated. An explicit progressive damage model, that is mechanistic in nature, is currently being developed using the concept of a representative volume element. A micromechanical finite element model that is capable of explicit damage initiation and propagation modeling is utilized for simulation of damage development. The predicted numerical results illustrate the capabilities of the current model. Future work is also outlined in the paper as the development of the fatigue model is continued.