Polymers occupied nearly every facet of our daily lives, and enhancing their mechanical and fracture properties has long been an important topic in the field of polymer science. Based on the various need in applications, polymers are designed to have a range of characteristics such as tackiness, optical properties, mechanical strength, rheological properties, chemical resistance and thermal conductivities. To achieve the diverse properties, they are modified by methods such as the addition of fillers, blending and varying the crosslinking conditions. While there are a lot of strategies to physically or chemically modify the matrix structures of the polymers, not so many systematic fatigue characterization methods can be easily applied onto different polymer products designed for different industrial purposes. To address this problem, this thesis focuses on developing straightforward and cost-effective methodologies to tackle the challenges in understanding the long-term fatigue failure mechanisms of polymer networks, providing insights to both bulk and interfacial contributions during fatigue fracture process. These insights and interpretations are critical for industrial products, which are always expected to be durable and contribute to sustainability. Given the varieties in polymer matrix, three representative polymer model systems are chosen to cover the extensive scale of mechanical stiffness: lightly crosslinked pressure sensitive adhesives (PSA), filled rubber elastomers and filler-reinforced epoxy matrices. The fatigue characterization of each polymer matrix requires unique methods that consider both the bulk and interfacial contributions, which involves utilizing various testing geometries, such as indentation, flexural, and pure shear tensile geometries. This thesis initiates with an investigation into the fatigue resistance of silicone and natural rubbers under elevated temperature conditions, employing a pure shear tensile geometry. Subsequently, the morphologies of the crack surfaces are analyzed and discussed in relation to the conditions of energy dissipation during the fatigue tests. The fatigue characterization method involves subjecting the sample to cyclic oscillation for stress-strain hysteresis loops, which allows for the extraction of mechanical response information and the calculation of energy dissipation across different length scales. Part of the energy is dissipated in the bulk material and the remaining stored elastic energy, recovered from the unloading curve, serves as the driving force for crack propagation. Similar to rubber elastomers, the softer PSA were examined with pure shear cyclic tests and probe tack tests utilizing an indentation setup. In this scenario, the stored elastic energy is the driving force for the detachment between the indenter and the adhesive surface. For the investigation of the rigid epoxy matrix, a flexural fatigue crack propagation test was designed to study a triblock copolymer-filled dynamic epoxy matrix. This approach also demonstrated a good correlation with the Dugdale model, particularly in the context of a standard flexural three-point bending fracture toughness test.

Creator

• Chen, Qihua

DOI

• https://doi.org/10.21985/n2-kgm2-2q31

Subject

• Materials Science

Language

• en

Alternate Identifier

• etdadmin_upload_1004779
• http://dissertations.umi.com/northwestern:16673

Keyword

• Adhesive
• Fatigue
• Elastomer
• Fracture
• Composite
• Mechanical Properties

Date created

• 2023-01-01

Resource type

• Dissertation

Rights statement

• In Copyright