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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.13, #3)
Fatigue Life Recovery in Corroded Aluminium Alloys Using Bonded Composite Reinforcements by Alan A. Baker (pp. 127-146).
Bonded composite reinforcements, such as boron/epoxy or carbon/epoxy, are becoming widely used to repair fatigue or stress corrosion cracks in aluminium alloy aircraft components. They also have considerable potential as repairs for corrosion damage, although the non-discrete nature of corrosion damage is a considerable complication. This paper describes studies on bonded repairs of two types of typical corrosion damage, both of which cause a dramatic reduction in fatigue life: a) relatively severe exfoliation and b) relatively minor pitting. Both studies are based on earlier Australian experimental studies on the structural significance of corrosion. In the studies described in this paper, other than the standard surface treatment used to attach the reinforcement no attempt was made to remove the corrosion damage. It is concluded that bonded repairs can provide useful fatigue life recovery even with serious exfoliation damage; however, such repairs should be regarded only as a temporary measure. For permanent and far more effective repairs most of the damage should be removed (ground-out) prior to reinforcement. In contrast, with minor pitting, greater than full life recovery is feasible. This observation also leads to the conclusion that bonded repairs will be tolerant of minor pitting inadvertently left after grind-out. Strain reduction by the reinforcement, as expected, is the major contributor to life extension. However, in the case of minor pitting environmental isolation by the adhesive is probably highly beneficial. Residual stresses in the metal resulting from the composite reinforcement causes some loss in patching efficiency.
Tensile Stress Rupture Behavior of a Woven Ceramic Matrix Composite in Humid Environments at Intermediate Temperature — Part I by K. J. LaRochelle; G. N. Morscher (pp. 147-172).
The stress rupture strength of the SYL-iBN/BN/SiC composite was evaluated at 550 and 750 °C with moisture content levels of 0.0, 0.2, and 0.6 atm partial pressure of water vapor, pH2O. The stress rupture strengths decreased with respect to time with the rate of decrease related to the temperature and the amount of moisture content. In all cases the degradation was more severe initially and then approached a run-out threshold level. The thresholds were reached at approximately 100+, 60, 80 h for the 550 °C with 0.0, 0.2, and 0.6 pH2O, respectively. The thresholds were reached at approximately 40, 20, and 10 h for the 750 °C cases. The interpolated stress rupture strengths at 100 h for 0.0, 0.2, and 0.6 pH2O at 550 °C were 82%, 68%, and 51% of the room temperature monotonic tensile strength. At 750 °C these strengths were 67%, 51%, and 50%. Analysis of Field Emission Scanning Electron Microscopy images showed evidence of embrittlement of the fiber/matrix interphase. Little to no embrittlement was observed at both temperatures with 0.0 pH2O. At both 550 and 750 °C with 0.2 and 0.6 pH2O, evidence of embrittlement increased with temperature and test duration with the most extensive embrittlement observed at 750 °C with 0.6 pH2O.
Keywords: ceramic matrix composites; silicon carbide; boron nitride; embrittlement; stress rupture; moisture; intermediate temperature; sylramic
Influence of Compression and Shear on the Strength of Composite Laminates with Z-Pinned Reinforcement by T. Kevin O’Brien; Ronald Krueger (pp. 173-189).
The influence of compression and shear loads on the strength of composite laminates with z-pins is evaluated parametrically using a 2D Finite Element Code (FLASH) based on Cosserat couple stress theory. Meshes were generated for three unique combinations of z-pin diameter and density. A laminated plate theory analysis was performed on several layups to determine the bi-axial stresses in the zero degree plies. These stresses, in turn, were used to determine the magnitude of the relative load steps prescribed in the FLASH analyses. Results indicated that increasing pin density was more detrimental to in-plane compression strength than increasing pin diameter. Compression strengths of lamina without z-pins agreed well with a closed form expression derived by Budiansky and Fleck. FLASH results for lamina with z-pins were consistent with the closed form results, and FLASH results without z-pins, if the initial fiber waviness due to z-pin insertion was added to the fiber waviness in the material to yield a total misalignment. Addition of 10% shear to the compression loading significantly reduced the lamina strength compared to pure compression loading. Addition of 50% shear to the compression indicated shear yielding rather than kink band formation as the likely failure mode. Two different stiffener reinforced skin configurations with z-pins, one quasi-isotropic and one orthotropic, were also analyzed. Six unique loading cases ranging from pure compression to compression plus 50% shear were analyzed assuming material fiber waviness misalignment angles of 0, 1, and 2°. Compression strength decreased with increased shear loading for both configurations, with the quasi-isotropic configuration yielding lower strengths than the orthotropic configuration.
Keywords: z-pin; compression strength; shear; laminates
Processing and Mechanical Properties of Boron Carbide–Titanium Diboride Ceramic Matrix Composites by Hamid Reza Baharvandi; A. M. Hadian; A. Alizadeh (pp. 191-198).
The objective of the present investigation was to study the effect of TiB2 addition on sintering behavior and mechanical properties of pressureless-sintered B4C ceramic. Different amounts of TiB2, mainly 5 t0 30 wt.% were added to the base material. Pressureless sintering was conducted at 2,050 and at 2,150 °C. Addition of 30 wt.% TiB2 and sintering at 2,150 °C resulted in improving the density of the samples to about 99% of theoretical density. The composite samples exhibited very good mechanical properties (hardness, flexural strength and fracture toughness). As the amount of TiB2 was increased further, the mechanical properties were reduced, except for the fracture toughness, apparently due to too much TiB2 in the specimen.
Keywords: TiB2 ; B4C; ceramic; sinter; fracture toughness; composite