John W. Holmes

A Technique for Tensile Fatigue and Creep Testing of Fiber-Reinforced Ceramics

An experimental technique for the elevated temperature tensile fatigue and creep testing of fiber-reinforced ceramics is discussed. The experimental approach utilizes edge-loaded specimens with rectangular gage-sections. Novel furnace and grip designs which allow testing in air to 1500°C are provided. The specimen, furnace and grip designs discussed in the paper have been successfully used to test unidirectional and cross-ply SiCf/Si3N 4, SiCf/SiC, Cf/SiC and SiCf/calcium-aluminosilicate composites.

Development of a Bamboo-Based Composite as a Sustainable Green Material for Wind Turbine Blades

Bamboo has many engineering and environmental attributes that make it an attractive material for utilization in wind turbine blades. This paper examines the mechanical properties of a novel bamboo-poplar epoxy laminate which is being developed for wind turbine blades. Information provided in this paper includes an overview of the laminate construction and initial data for the monotonic tensile and compressive stress-strain behavior and tension-tension fatigue life of panels formed by hot-pressing. In addition, a discussion of fracture resistance of the bamboo-poplar laminate, under Mode I and Mode II loading conditions is provided.

Improvement in the fatigue life of fiber-reinforced ceramics by use of interfacial lubrication

Continuous-fiber reinforced ceramics are potential materials for high temperature load carrying structures. Most applications involve cyclic loading. Recent work has shown that the interfacial shear stress in ceramic matrix composites decreases rapidly in the initial stages of cyclic loading. This brings up a key question. Namely can fatigue life be improved by altering the initial wear rate of the fiber/matrix interface. This letter reports such an attempt, where micro-cracked specimens were immersed in oil prior to cyclic loads, in an attempt to change the interface wear rate. A Nicalon SiC-fiber/calcium aluminosilicate matrix (SiC{sub t}/CAS-II, Corning Glass Works, Corning, NY) composite was used in the study. The composite was processed with 16 plies and had a nominal fiber content of 40 vol.%. Edge loaded specimens, similar to those used in earlier studies were machined from 6 in. x 6 in. panels with fibers parallel to the tensile loading direction. To facilitate the observation of matrix cracking, the specimens were polished along an edge (minor face) with diamond paste to a final finish of 1 {mu}m.

Blade Materials, Testing MethodsAnd Structural Design

A major trend in wind energy is the development of larger wind turbines for offshore wind farms. Since access to offshore wind turbines is diffi cult and costly, it is of great importance that they operate safely and reliable. The wind turbine rotor blades, which are the largest rotating component of a wind turbine, are designed for an expected lifetime of 20 years. During this period of time, the blades will be subjected to varying loads. Large wind turbine blades are made of composite materials and can develop a number of interacting failure modes. High structural reliability can be achieved by designing the blades against the development of these failure modes. This chapter provides an overview of experimental and modeling tools for the design of wind turbine blades, with particular emphasis on evolution and interaction of various failure modes. This involves knowledge of materials, testing methods and structural design.

Experimental approach for mixed-mode fatigue delamination crack growth with large-scale bridging in polymer composites:

An experimental apparatus utilizing double cantilever beam specimens loaded with uneven bending moments was developed to study the mixed-mode fatigue crack growth in composites. The approach is suitable when large-scale bridging of cracks is present. To illustrate the testing method, cyclic growth of delaminations in a typical fibre-reinforced polymer composite was investigated under a constant cyclic loading amplitude. Pure mode I, mode II and mixed-mode crack growth conditions were examined. The results, analysed using a J-integral approach, show that the double cantilever beam loaded with uneven bending moments configuration provides a robust approach to investigate the fatigue crack growth of composites for pure mode and mixed-mode cracking. A steady-state crack growth regime was observed for mode I and mixed-mode loading. For mode II loading, steady-state was absent, and a progressively decreasing crack growth rate observed. In addition to details concerning the equipment, a general discussion of the development of cyclic bridging laws for delamination growth in the presence of large-scale bridging is provided.

Microstructural stability of directionally solidified eutectic NiAl-Mo under static and thermal cycling conditions

The quasi-binary eutectic NiAl-9% Mo with faceted molybdenum fibers was subjected to both thermal annealing conditions and to annealing under thermal cycling conditions to determine the microstructural stability of this alloy. The static temperature tests were run at 0.85T{sub M}--0.97T{sub M} in an argon gas atmosphere. The thermal cycling tests were performed between temperatures of 700 C and 1,200 C by induction heating disk-shaped specimens in an argon gas atmosphere using time-temperature heating and cooling profiles to approximate potential engine applications. To quantify microstructural changes, the fiber size and size distribution and number of fibers per unit area were measured as a function of time at temperature. The overall results demonstrate that the directionally solidified eutectic NiAl-9Mo subjected to thermal fatigue conditions exhibits cell boundary coarsening and large shape changes, whereas the microstructure under static stress-free annealing is stable.

Effect of Oxidation on the Mechanical Properties of Nextel™ 312/BN/Blackglas™ Composites

Preceramic polymers are attractive precursor materials for the production of low cost continuous fiber reinforced ceramic composites (CFCC) into near net shape components. Currently, CFCC components based upon a Blackglas™ matrix reinforced with Nextel™ 312 fibers with a BN rich surface layer are being investigated for gas turbine engine applications. The effects of oxidation on tensile and bend properties were investigated after exposure to flowing air at 600°C for 20–1000h. A significant reduction in the tensile properties accompanied by increases in fiber pull-out was observed after oxidation for 96 hours. After 500 hours oxidation, strength decreased by 50% relative to as-prepared composites. These results indicate that oxidation beyond 200 hours may be embrittling the composite.

Microstructural stability of a NiAl-Mo eutectic alloy

The microstructural stability of a directionally-solidified NiAl-9 at.% Mo quasi-binary alloy was investigated under conditions of thermal cycling between the temperatures 973K and 1,473K utilizing time-temperature heating and cooling profiles which approximate potential engine applications. Two different microstructures were examined: a cellular microstructure in which the faceted second-phase Mo rods in the NiAl matrix formed misaligned cell boundaries which separated aligned cells approximately 0.4 mm in width and 5--25 mm in length, and a nearly fault-free fully columnar microstructure well aligned along the [001] direction. Both microstructures resisted coarsening under thermal cycling, but plastic deformation induced by thermal stresses introduced significant specimen shape changes. Surprisingly, the cellular microstructure, for which the cell boundary region apparently acts as a deformation buffer, exhibited better resistance to thermal fatigue than the more fault-free and better aligned columnar microstructure.

Thermal Fatigue of NiAl Single Crystals

Single crystals of [001]-oriented NiAl single crystals were subjected to thermal fatigue by a method which employs induction heating of disk-shaped specimens heated in an argon atmosphere. Several time-temperature heating and cooling profiles were used to produce different thermal strain histories in specimens cycled between 973 K and 1,473 K. After thermal cycling, pronounced shape changes in the form of diametrical elongations along {l{underscore}angle}100{r{underscore}angle} directions with accompanying increases in thickness at and near the {l{underscore}angle}100{r{underscore}angle} specimen axes were observed. The deformations were analyzed in terms of operative slip systems in tension and compression, ratchetting (cyclic strain accumulation), and the elastic properties of NiAl. The experimental results correlate best with thermal stresses associated with the large elastic anisotropy of NiAl.