Mechanical Response of Out-Of-Autoclave Complex Fiber Architecture Composites for Marine Structures

Open Access
Author:
Haluza, Rudy T
Graduate Program:
Engineering Science and Mechanics
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 30, 2017
Committee Members:
  • Kevin L Koudela, Thesis Advisor
Keywords:
  • hydro
  • kinetic
  • turbine
  • hydrokinetic
  • composite
  • glass
  • fiber
  • epoxy
  • water
  • marine
  • fatigue
  • tension
  • ooa
  • out-of-autoclave
  • damage
  • fail
  • condtioning
  • elevated
  • temperature
  • seawater
  • absorption
  • model
  • quasi-laminar
  • quasi-static
  • 0.1
  • tension-tension
  • sun-li
  • modulus
  • residual
  • strength
  • reduction
  • VARTM
  • woven
  • weave
  • stitch
  • stitch-bonded
  • textile
  • wet
Abstract:
Hydrokinetic turbines have shown promise as a novel method for harvesting power from natural waterways. The customizability of these turbines allows for smaller turbine systems compared to large, geographically demanding hydroelectric plants. However, maintenance costs stemming from relatively short service lives of existing glass/epoxy turbine blades impede the growth of hydrokinetic power. In prototype blades, fatigue loading in salt water caused relatively rapid degradation and subsequent high maintenance costs. Thus, fatigue-resistant blades designed for multi-decade service life would lower the net cost of hydrokinetic turbine usage and increase the feasibility, and therefore growth, of hydrokinetic turbine usage. Furthermore, material systems chosen for hydrokinetic blade use must be studied in order to understand their behavior in long-term under-sea conditions. This study researched the tensile-mechanical response of a quasi-isotropic woven and stitched laminate under quasi-static and fatigue loading at a stress ratio (R) of 0.1 and a frequency of 10 Hz. Some fatigue samples were fatigued until failure, while others underwent residual modulus and strength measurements. Both woven and stitched laminates were found to survive ten million cycles with a maximum stress of +13.75 ksi, but fail prior to ten-million cycles with a maximum stress of +18 ksi. In room-temperature ambient conditions, the polyester-stitched composite proved to have superior fatigue life only in long-life (>105 cycles) fatigue tests. Damaged, but not failed samples showed similar trends in that stitched samples would have more damage at lower cycle counts, but less damage at higher cycle counts compared to woven samples. However, there was more statistical scatter within the stitched specimens compared to woven specimens. Samples that were conditioned and tested while submerged in water had 30% reduction in tensile strength compared to the non-conditioned samples tested in ambient conditions. The partially saturated samples also showed damage accumulation and failure occurring nearly a decade earlier than the non-conditioned samples. Through optical macroscopic and microscopic investigation, intralaminar cracks and delaminations were found to occur in damaged woven samples, while stitched samples showed higher densities of unconnected intralaminar cracks before failure. Delaminations were found in near-failure stitch-bonded samples, especially in those tested at higher maximum fatigue stresses. More intralaminar cracks were found within stitched specimens compared to woven specimens that had similar reductions in elastic modulus, although, stitched specimens showed greater strength retention compared to the woven specimens. Future research could utilize these macroscopic and microscopic crack densities to develop models to better predict turbine blade damage at given loading levels and cycles.