Observations of Failure Phenomena in Periodic Media

New manufacturing techniques, such as 3D printing, allow for greater control over material properties and can be used to create custom heterogeneous materials. Heterogeneities can be leveraged to increase fracture toughness by redistributing the stresses, such as due to an elastic heterogeneity, or by impeding crack propagation, such as the renucleation at a material interface or edge of a void. The goal of this research is to study the mechanisms by which heterogeneities work to make composite materials more resistant to fracture than either of the individual base materials. The influence of heterogeneities on the deformation and fracture of 3D printed fracture specimens is investigated. Brick-like heterogeneities are studied in compact tension and plate specimens with soft, stiff, and void heterogeneities. Horizontally layered heterogeneities are studied in compact tension specimens. The specimens are manufactured using a printer capable of printing multiple materials. The specimens are loaded until failure, and full-field displacement and strain data are collected using digital image correlation. The evolution of resistance to fracture is quantified by the energy release rate and fracture toughness values calculated using load, load-point displacement measurements, and crack extension data determined from images of the specimen. Both in soft specimens with stiff heterogeneities and in stiff specimens with soft heterogeneities, stresses are observed to be higher in the stiffer material. Fracture toughness is observed to increase in the presence of stiff inclusions and voids, although in the case of voids this is due to the crack terminating at the edge of the void and renucleating at the other edge. The effects of interfaces on crack propagation in periodic media are experimentally studied. Comparative experiments on two proposed heterogeneity architectures aim to separate the effects of elastic deformation caused by heterogeneous inclusions in a composite from the effects of passing through an interface during crack propagation. The first, 'stripe' specimens, alternate equal width stripes perpendicular to the plane of the crack. The second, 'cross' specimens, have the same stripe pattern but with a narrow strip of one of the constituent materials in the plane of crack propagation. The 'cross' is wide enough to contain the crack to an area without material interfaces but thin enough that its overall effect on elastic deformation is minimal. Specimens are manufactured from two polymers using polyjet 3D printing. Energy release rate for fracture is calculated from load and displacement measurements. Digital image correlation is used to study strain and stress fields during crack propagation. While the stress fields during crack propagation appear similar, the fracture toughness in the 'stripe' specimens was found to be higher than that of the 'cross' specimens, indicating that fracture toughness is enhanced by renucleation at the interfaces. Additionally, the amount of enhancement was observed to depend on the width of the heterogeneous layers. The interaction between the cohesive zone and elastic stiffness heterogeneity in the peeling of an adhesive tape from a rigid substrate is examined experimentally and with finite element simulations. It is understood that elastic stiffness heterogeneities can greatly enhance the adhesion of a tape without changing the properties of the interface. However, in peeling experiments performed on pressure sensitive adhesive tapes with both an elastic stiffness heterogeneity and a substantial cohesive zone, muted adhesion enhancement was observed. It is proposed that the cohesive zone acts to smooth out the effect of the discontinuity at the edge of the elastic stiffness heterogeneities, suppressing their effect on peel force enhancement. The results of numerical simulations show that the peel force enhancement depends on the strength of the adhesive and the size of the cohesive zone.