Keith A. Berube

Interlaminar fracture toughness of woven E-glass fabric composites:

Composite materials fabricated using woven fabrics are characterized for fracture toughness. Crack propagation behavior in woven fabric composites is investigated with respect to the periodic pattern produced as a result of the weave structure. Additionally, experimental methods for determining fracture toughness are investigated for woven fabric composites and a numerical technique to detect crack onset is proposed. Fracture characterization methods for mode I, mode II, and mixed-mode are investigated. A case study encompassing DCB, ENF, and three MMB test configurations is presented for a typical marine-grade E-glass fiber reinforced composite with a toughened vinyl ester resin matrix. The aim of this article is to investigate the fracture behavior of a heavy woven fabric composite. The most important outcomes of this study are a numerical technique to detect crack initiation and a discussion of the fracture behavior of woven fabric composites.

Variability in the Material Properties of Polymer Matrix Composites for Marine Structures

The test results from a composite material round robin manufacturing study are presented. The objective of the study was to investigate material property variability when different manufacturers were used to fabricate identical composite parts. The study was part of an ongoing Office of Naval Research project to determine the causes of material property variability of E-glass/vinyl-ester structural composites fabricated with a vacuum-assisted resin transfer molding process. The manufacturers that participated in the study consisted of five industrial composite fabricators that either had experience with U.S. naval fabrication projects or possessed commercial marine fabrication experience. The materials specified for the study were a 24 oz woven roving E-glass fabric and a rubber-toughened vinyl-ester resin system. The tests performed included constituent volume, tension, compression, in-plane shear, and flexure. The material coupon tests were performed on 5.1 mm (0.20 in.) thick cross-ply laminates in a warps parallel lay-up. A three-dimensional digital image correlation system was used to measure strain during testing to reduce the variability often experienced when using conventional foil strain gauges on heavy woven fabrics. The results of the testing indicated that compression had the most variability in both strength and modulus, which was attributed to the waviness of the fabric, while the flexural strength and tensile modulus had the least amount of variability.

Cavitation erosion resistance of various material systems

Abstract Advancement in both the design and construction of high-speed ships necessitates the evaluation of cavitation erosion resistant materials. Given their weight advantages, aluminum and laminated composite materials are often chosen as construction materials for high-speed designs. Historically, neither of these materials performs well in a cavitating environment. The objective of this effort is to evaluate potential cavitation erosion protection alternatives. Screening of the various material alternatives was performed using a modified ASTM G32 ultrasonically induced cavitation test method. A relative ranking is provided for materials including metals, composites, elastomers, polymers, and hard ceramic coatings using the maximum erosion rate as a parameter. A potential solution identified during this study involves the use of a durable elastomer material as a protective barrier. Results also show that a sandwich core composite system can be used to increase the cavitation erosion resistance of laminated composite materials.

Analysis of a hybrid composite/metal ship hull structural system with removable panels

Abstract This article presents the finite element analysis of a bolted composite/metal hybrid panel assembly subjected to uniform pressure loading. The structure consists of four EG/VE stiffened panels joined together by a hybrid connection configuration. A simplified modeling approach is presented, where effective section properties of the hybrid joint regions are computed according to the behavior of each section. The modeling approach is verified with available experimental data, and the global panel deflections as well as the strains within the bulk of the panels are in good agreement with the test results. Parametric studies are presented to quantify the influence of the geometry of the joint constituents on the global response of the hybrid assembly. The model response is most sensitive to changes in the steel component geometries, while the composite laminate geometry has a modest localized effect at the joint region. This approach is useful for global modeling of hybrid joints in large-scale structures, where extensive detailing of the joint region is unfeasible.

Determining the Flexural and Shear Moduli of Fiber-Reinforced Polymer Composites Using Three-Dimensional Digital Image Correlation

A three-dimensional digital image correlation systemwas implemented into the flexural tests of fiber-reinforced polymer composite beams to characterize shear deformation. An optimization routine that minimized the error between the analytical and experimental data was implemented with first-order shear deformation beam theory used to compute the flexural and shear moduli using the deflection and slope of the mid-plane of the beam. A relatively coarse 814 g/m2 woven roving E-glass fabric and a rubber-toughened vinyl ester resin system were used to fabricate the 10.0mmthick laminates in a quasi-isotropic laminate configuration. Span-to-thickness ratios of 8, 12, 16, and 24-to-1 were adopted for the laminate beams at a width-to-thickness ratio of 1.5-to-1. The full-field displacement and slope-optimization fitting methods were compared with conventional discrete point methods to determine flexural and shear moduli. Slope optimization produced consistent and reasonable values for the flexural modulus at all span-to-thickness ratios but produced higher than expected values for the shear modulus at shorter spans. Deflection optimization produced more variability in the flexural and shear moduli than slope optimization and lower than expected values of shear modulus at larger span-to-thickness ratios. Tests that used higher resolution images produced slightly larger values for shear modulus. Overall, the slope-optimization method produced the least amount of variability in the results for the flexural and shear moduli.

Effect of Resin Cure Recipe and Ambient Processing Temperature on the Material Properties of Marine Grade Polymer Matrix Composite Materials

The effects of the resin curing recipe and ambient processing temperature on the mechanical properties of composite laminates were investigated. Woven roving E-glass/vinyl-ester composite plates were fabricated with a pliable-bag vacuum assisted resin transfer molding process over a range of ambient temperatures and resin gel times commonly encountered when fabricating polymer composite parts for the marine industry. Standardized tests for Mode-I interlaminar fracture toughness, compression, constituent volume fraction, and Barcol hardness were conducted. Interlaminar fracture toughness exhibited the most variability among the measured properties. Although there were few direct correlations between the measured properties and the parameters in the study, there were several statistically significant differences that could not be discarded as random, as they were consistent among the replicate panels fabricated for each combination of parameters in the study.

Integrated Monitoring System for Carbon Composite Strands in Cable-Stayed Bridge, Penobscot Narrows, Maine

An integrated monitoring system for carbon fiber composite cable (CFCC) strands recently implemented in the Penobscot Narrows Bridge in Maine is presented. This cable-stayed bridge features a cradle-stay system in the pylons that provides a continuous cable stay between the main span bridge deck anchorage and the back span bridge deck anchorage. The cradle-stay system allowed for replacing epoxy-coated steel strands with CFCC strands. The monitoring system installed in the bridge is composed of a set of sensors and devices that provide complementary structural information: (a) load cells supported on steel anchorage chairs, (b) linear voltage differential transformers (LVDTs) mounted on the steel anchorage chairs, (c) fiber-optic strain (FOS) sensors embedded in composite sleeves, and (d) temperature sensors. The monitoring system was designed to detect variations over time in individual carbon fiber strand force and strains at the anchorage locations and to correlate the measurements with ambient temperature. Laboratory tests were conducted to proof-load the steel anchorage chairs and to calibrate the LVDTs and embedded FOS strain measurement systems. The baseline response of the CFCC strands that was based on load cell, FOS sensors, and LVDTs measurements after the strand stressing operation is presented. The CFCC strands experienced changes in tensile force between +3% and −5% over the first 4 months in service, which were consistent with ambient temperature fluctuations. The FOS measured strand strains and the LVDT deflections of the anchored chair exhibited trends consistent with the variations in CFCC strand force.