Harovel Wheat / Harris L. Marcus
Corrosion Fatigue Behavior of Offshore Structural Materials Under Combined Hydrostatic and Axial Loading
Steven Mear (Ph.D. student)
The high hydrostatic pressure and corrosive seawater coupled with the cyclic loading induced by wave motion or ocean currents present a particularly challenging environment for the TLP components. This project aims to determine the response of fiber-reinforced composite materials to this environment, and thus the feasibility of using these composites in tension-leg platforms or other deep sea structures.
To provide an improved understanding of the basic mechanisms and behavior involved in the loss of strength and failure of high performance composites subjected to combined fatigue loading and seawater environments.
Research has focused on the transverse fatigue crack growth rates in [90]12 unidirectional carbon-epoxy composites. Crack growth rates in ambient air, ambient seawater, and high pressure (up to 4000 psi) seawater have been determined for the as-received plate stock during tension load control fatigue testing. Seawater conditioned specimens were tested in a similar manner when they reached saturation. Besides pressure and degree of sample conditioning, other test variables will include load ratio , frequency, and water chemistry. In addition, damage fatigue in [±45]as specimens has been examined. The use of coupon fatigue specimens of [0/90]2s will be pursued in order to determine the effect of the pressurized seawater on the transverse cracking and delamination in a crossply layup. Crack density will be determined as a function of fatigue cycles. Optical and scanning electron microscopy will be used for post-failure analysis.
The effects of hydrostatic pressures have been investigated by Parry and Wronski, Shin and Pae, Kim et al and Sloan. Their observations imply that changes in moisture uptake and in failure mechanisms occur in presence of hydrostatic pressure.
The complexity of the impact that moisture absorption under high pressure has on the mechanical properties of composites reinforces the need for further empirical studies. The response of composite materials to moisture absorption and pressure effects must be evaluated in order to obtain the baseline data necessary for the design of composite structures for deepwater applications. In particular, the effect on the fatigue strength of composite materials subjected to these environmental factors will be assessed.
October 1993 to September 1997
A high pressure testing system has been designed, fabricated, and tested. The system includes a unique internal load sensing clevis pin which is used to monitor the low loads encountered during testing. In addition, this pin eliminates error due to seal friction and reduces the error resulting from pressure fluctuations of the system. The cell allows for fatigue testing of composite specimens in a seawater environment at pressures up to 5000 psi. In addition, the cell was designed with the capability of elevated temperature testing. Data from composite testing consists of fatigue crack growth rate information, as well as composite compliance and fracture toughness values. Post-failure scanning electron microscopy of the composite specimens will provide possible explanations for changes in the fatigue behavior of specimens subjected to various conditioning states and pressure loadings.
Fowkes, G.F., "An Apparatus for the Characterization of the Corrosion Fatigue Behavior of Offshore Structural Materials Under Combined Hydrostatic and Axial Loading," Master's Thesis, The University of Texas at Austin, May, 1994.
Mear, S.T., Fowkes, G.F. and Marcus, H.L., "Corrosion Fatigue Behavior of Offshore Structural Materials Under Combined Hydrostatic and Axial Loading," paper to be presented at the 124th Annual ASM/TMS Spring Meeting, Las Vegas, Nevada, February, 1995.
Mear, S.T., Stolk, J., Wheat, H.G. and Marcus, H.L., "Transverse Fatigue Crack Growth of Graphite/Epoxy Composites in High Pressure Seawater," paper to be published in the proceedings for the Sixth International Offshore and Polar Engineering Conference, ISOPE '96, Los Angeles, California, May, 1996.
Stolk, J., Mear, S.T., Wheat, H.G. and Marcus, H.L., "Corrosion Fatigue Behavior of Graphite/Epoxy Composites for Deep-Sea Structural Applications," paper to be published in the proceedings for the Johannes Weertman's Symposium at the 125th Annual ASM/TMS Spring Meeting, Anaheim, California, 2/96.
Mear, S.T., Stolk, J., Wheat, H.G. and Marcus, H.L., "Transverse Fatigue Crack Growth of Graphite/Epoxy Composites in High Pressure Seawater," Proceedings for the Sixth International Offshore and Polar Engineering Conference, ISOPE '96, Los Angeles, California, May, 1996.
Stolk, J., Mear, S.T., Wheat, H.G. and Marcus, H.L.,"Corrosion Fatigue Behavior of Graphite/Epoxy Composites for Deep-Sea Structural Applications," Proceedings for the Johannes Weertman's Symposium at the 125th Annual ASM/TMS Spring Meeting, Anaheim, California, 2/96.