Masters Theses

Date of Award

8-1995

Degree Type

Thesis

Degree Name

Master of Science

Major

Engineering Science

Major Professor

Madhu S. Madhukar

Committee Members

John Landes

Abstract

Experiments were performed to study the effect of fiber pretension and fiber surface treatment on the interfacial failure process in single fiber glass/epoxy and carbon/epoxy composites. Single fiber/epoxy composite specimens were fabricated in a specialized fixture in which fiber tension could be monitored during the curing cycle. Different fiber tensions ranging from 0% to 40% of expected fiber failure stress were applied to two different A4 graphite fibers, namely untreated designated as AU4, and surface treated designated as AS4. Curing induced fiber stresses were calculated. The fiber-matrix interface properties were characterized by the single-fiber fragmentation test. The experiments on curing induced fiber stresses revealed two dominant mechanisms that occur during the curing process. These are - matrix thermal expansion and cure shrinkage. The results showed that the embedded fiber experienced significant tensile stresses prior to the cool down. In the cases when the fiber pretension was larger than 50% of the expected fiber failure load, the curing induced tensile stresses produced tensile fracture of the embedded fiber during the curing cycle. Optical observation of the failure process during the mechanical loading of the single fiber graphite/epoxy specimens revealed that there are several parameters which are sensitive to both fiber pretension and fiber surface modification. In particular the attention was directed to: first fiber fracture strain; fiber retraction after its failure; fiber-matrix debonding and its progression along the fiber; friction at debond; and fiber critical length. The mechanical loading of the specimens showed that weak interface bonds results in significant fiber-matrix debond near the broken fiber ends. The debonds propagate along the interface in discrete jumps (stick-slip manner). Specific experiments were conducted to show that once the debonds were formed, the stress transfer between the fiber and the matrix occurs mostly due to friction. The composites having strong interface do not show much debond. Instead, inelastic deformation near the broken fiber ends was noted in these specimens. The mechanisms that are believed to be responsible for such a behavior are identified.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS