The behavior of composites under uni-axial fatigue loading has been extensively studied but still poorly understood. Another aspect that composite materials under multi-axial fatigue loading has been far less investigated. The lack of experimental data and models is even more evident in the case of multi-axial fatigue of notched composite structure. The goal of this project is to investigate the damage mechanisms and model the fatigue failure behavior of notched GFRP laminate under multi-axial fatigue loading.
Recently, discontinuous fiber composites (DFC) made of chopped and randomly distributed fibers brought a significant attention to the composite community. It has appealing properties such as ease of manufacturing, resistance to delamination, and near quasi-isotropic mechanical properties. However, due to its complex morphology, the mechanics of DFC is largely unknown. This project aims to characterize the main failure mechanisms occurring in DFC using X-ray CT scanner, and develop a physically-based computational modeling of DFCs. The findings of this study will contribute to the safety and certification of DFC structures.
Application of composite materials have been exponentially growing
attributed to their outstanding and tailorable mechanical properties.
However, their orthotropic nature, complicated damage mechanism, and
nonlinear softening makes them hard to predict a failure. Our aim on
this project is to capture the softening behavior of this complicated
material in order to simulate the fracture mechanism. Key features in this
model are spectral decomposition and a projection of strain and stress to
the microplane which is a plane forming a sphere around a point of interest
in a meso-structure. Spectral decomposition allows to isolate the modes in
the constitutive behavior of the material. These modes can actually be
interpreted with physical meaning. By doing so, the orthotropic behavior
of the material can be simplified in each mode. The stress-strain analysis
on each microplane helps to capture the complexity of the meso-structure.
VISUAL: Evolution of the normal strain in mode 1 under the tensile loading along the fiber direction
The goal of this project is to prove the possibility of design and manufacture novel
types of foldable – yet stiff – structures based on the Tachi-Miura-Polyhedron (TMP)
origami. This type of origami has received significant attention from the scientific
and engineering community due to its unique features such as rigid foldability, high
compactness, and tunable stiffness. The kinematics of this TMP structure is being
investigated in order to achieve extraordinary mechanical properties in terms of
robust deployability. Several prototypes have been tested with hinges made out of
both dry and wet fibers, using glass, carbon, and kevlar materials.
The aim of this project is to develop a better understanding of how the variation
in the extent of development of the FPZ for different sizes affects their fracture
behavior. The size effect studies are being conducted through Finite Element based
Cohesive Zone Modeling. A better insight into their size effects will lead to
improved utilization of these materials in structural applications. The plot
alongside shows the variation of the failure nominal stress with the specimen width,
obtained from cohesive zone simulations for SENT samples for two different cohesive
law (traction separation law) shapes - inward and outward, for the same value of
Fracture Energy. It is interesting to observe that for very small sized specimens
they have similar failure stress values. Subsequently, they diverge for intermediate
specimen sizes before converging again to similar values for large sample dimensions.
The final convergence towards the end can be attributed to the FPZ being fully
developed for specimens beyond a particular size.
An extensive parametric study sweeping both material and geometrical parameters is
being performed using size effect law (SEL) and Cohesive Zone Modeling (CZM). These
simulations and recent experimental results suggest that the cohesive law of
composites is bi-linear and that the Fracture Process Zone (FPZ) may be almost fully
developed already for laboratory-sized specimens. If this is true, the work done by
Bazant (1984) and Cusatis (2009) needs to be extended for both smaller and larger
scale asymptotes and for various ratios of Gf / GF. The goal of this project is to
formulate a master curve that can be used for quick, yet accurate, estimation of
energy and strength parameters.
In this project, an experimental and numerical approach is devised to investigate delamination resistance in discontinuous fiber composites (DFC). Owing to the high degree of randomization in material properties in DFC’s, conventional testing methodologies such as the double cantilever beam (DCB) and end-notched flexure (ENF) that are used to characterize Mode-I and Mode-II interlaminar failure may fail to address the non-linear behavior during delamination resistance. This project aims to use an energy-based approach, to model the delamination resistance in geometrically scaled DFC specimens, though experimental and finite element models to derive the interlaminar fracture toughness values, GI and GII, during Mode-I and Mode-II failures.
While traditional Carbon fiber composites have very high stiffness and strength, they show brittle behavior and are prone to catastrophic failure. Hybrid composites featuring both Glass and Carbon fibers retain some of the stiffness and strength of Carbon fibers and the ductile behavior of Glass fibers. Such composites can be useful in a wide range of applications which require lightweight stiff materials and at the same time cannot accommodate catastrophic failure. Uniaxial tension and some basic damage behavior of such composites have been studied by researchers hitherto, but the mechanics the failure of notched or cracked Carbon Glass hybrids have not been explored substantially. The current project is an effort towards bridging this gap and throwing some light on the damage and failure mechanics of pre-cracked or notched Carbon-Glass hybrids and studying their size effects and how the presence of a major defect affects the failure profile of the material.
While this project is still in its early stages, microstructures printed using the Nanoscribe “Photonic Professional GT” machine will be tested using nanoindentation in order to study the size effect behavior at much smaller scale.
The development of deployable structures is an active area of research. The flexibility requirements along with the need for lightweight structures drive the choice of the material which has to provide compliance without damaging and failing at large deformations. Thin composites are generally adopted because of their high flexibility and strength. However, the low thickness and the presence of holes and notches to promote compliance may lead to failure during the deployment. The objective of this project is to study the damage initiation and failure of thin composites. Capturing the effects of curvature from bending is a major focus. A multi-scale framework with a periodic boundary condition explicitly models the microstructural features of composites and damage.
Fatigue or environmental degradation needs to be taken into serious consideration since more than 60% of aircraft components fail because of that. In order to overcome this issue, graphene sheets can be an excellent candidate which is expected to reduce the coefficient of water diffusion of the polymer and minimize the water absorption. As a result, this may provide better resistance to environment aggression hence improved durability.