Jed Lyons

High-temperature deformation measurements using digital-image correlation

The ability of the computer-vision technique of digital-image correlation to measure full-field in-plane surface deformations at elevated temperatures was evaluated by a series of experiments. Samples were subjected to pure translation, free thermal expansion and uniform tensile loads. Results are presented which show that the digital-image-correlation technique remains fully capable of accurate measurement of the displacements and strains on the surface of a planar object at temperatures up to 650°C.

Microstructure and retention of superplasticity of friction stir welded superplastic 2095 sheet

Abstract Friction stir welding (FSW) was used to join superplastic, rolled sheets of an AA 2095. Microstructure was examined by light optical and transmission electron microscopy. The superplastic behavior of welded sections and of the base metal were compared. Superplasticity was retained after FSW.

On the evaluation of critical thrust for delamination-free drilling of composite laminates

The existing analytical models in the literature for estimating the critical thrust force of the drill to avoid delamination of composite laminates during drilling assume the drill force as a point load on the laminate. Also, they are based on small deflection assumptions even while considering the pushing out of the last one or two plies at drill exit. In this paper, the effects of these two assumptions on the critical thrust value have been examined by taking an alternate assumption of a distributed thrust over the drill diameter, and large deformation of un-cut plies during drill exit. The effects of both these assumptions are calculated and analyzed separately.

Factors Affecting the Bond Between Polymer Composites and Wood

Bond strength and durability play a vital role in the effectiveness of wood strengthening with fiber-reinforced polymer composites. This paper analyzes the effects that resin system, wood surface condition, moisture content, primer application, and environmental exposures have on bond strength. FRP composites were made by hand-layup with two types of epoxies and E-glass fabric on seven different wood surfaces. Bond strength was measured as fabricated, and after the following environmental exposures – soaked under water, dried in high temperature and cyclic tidal salty water. Results show a relationship between resin type and wood moisture content with respect to bond strength. Application of the composite on a rough surface overall improves the bond strength. Application of hydroxyl methanol resorcinol (HMR) overall improves the bond strength except in a dry environmental exposure.

Fracture mechanics testing of the bond between composite overlays and a concrete substrate

This paper presents a methodology for assessing the bond strength of composite overlays to concrete utilizing a fracture toughness test. The principles and practices of existing ASTM standards for determining the fracture toughness of adhesive bonds between double cantilever beam (DCB) metallic and composite specimens (D 3433-93 and D 5528-94a) have been extended to cover the case of an elastic composite layer bonded to a rigid concrete/masonry substrate. In the theoretical section, the dominant loading conditions, relevant ASTM standards, and the development of energy release rate concepts for analyzing a disbonding composite layer modeled as an elastic cantilever beam are presented. The experimental section covers specimen fabrication and preparation, experimental setup, test procedures, post-test evaluation of the specimens, and data processing. The discussion of test results focuses on explaining the variability in measured strain energy release rate, and identifies trends between the measured strain energy release rate and the fraction of the fracture surface retaining cement paste after disbonding. It was found that good-quality composite-to-concrete bond is associated with high fracture toughness of the adhesive and location of the crack path in the concrete substrate. Strict enforcement of surface preparation and adhesive handling procedures was found to play an important role in promoting good bond strength and high fracture toughness. The fracture toughness test developed in this paper can be used for screening various composite-repair systems, to assess the effect of different environmental attacks, and as a quality control tool.

Experimental Characterization of Crack Tip Deformation Fields in Alloy 718 at High Temperatures

A series of fracture mechanics tests were conducted at temperatures of 650 C and 704 C in air, using Inconel 719. A noncontacting measurement technique, based on computer vision and digital image correlation, was applied to directly measure surface displacements and strains prior to and during creep crack growth. For the first time, quantitative comparisons at elevated temperatures are presented between experimentally measured near-crack-tip deformation fields and theoretical linear elastic and viscoelastic fracture mechanics solutions. The results establish that linear elastic conditions dominate the near-crack-tip displacements and strains at 650 C during crack growth, and confirm that K{sub 1} is a viable continuum-based fracture parameter for creep crack growth characterization. Postmortem fractographic analyses indicate that grain boundary embrittlement leads to crack extension before a significant amount of creep occurs at this temperature. At higher temperatures, however, no crack growth was observed due to crack tip blunting and concurrent stress reduction after load application.

Time and temperature effects on the mechanical properties of glass-filled amide-based thermoplastics

Abstract Time-dependent mechanical design properties of four commercial thermoplastic resin systems were investigated at temperatures of 23, 100, and 150°C. The test materials were glass-filled polyamides and polyphthalamides. Experiments were performed to characterize creep, creep-rupture and tensile behaviors after isothermal aging. Creep-rupture data were used to create master curves using a Sherby–Dorn analysis. Although each material possessed a different property advantage, a polyphthalamide with 33% glass reinforcement exhibited a good combination of creep resistance, strength and ductility. The reported results provide information that is critical for the design and development of structural thermoplastic components.

Effect of Hygrothermal Aging on the Fracture of Composite Overlays on Concrete

This research assessed the effects of elevated temperature and humidity exposures on the durability of fiber reinforced epoxy composite overlays on concrete substrates. Modified double cantilever beam samples were used to determine the Mode I strain energy release rate of the composite-to-concrete bond after exposure to temperatures of 23°, 60° or 100°C, humidity levels of 50 or 95% RH, and times up to 40 days. The only statistically significant degradation in toughness was observed after exposure to the 100°C, 95% RH environment. However, chemical changes in the matrix occurred during this extreme exposure condition that may not represent the in-service aging of composite-repaired concrete structures.

Effect of Hygrothermal Environment on the Bending of PMC Laminates under Large Deflection

The effects of hygrothermal conditions on the large-deflection behavior of fiber-reinforced polymer matrix composite laminates have been studied. Cylindrical bending of laminates under in-plane and transverse loadings is analyzed using the von Karman plate theory. A cross-plied symmetric laminate is chosen as an example. The results presented show the effects of expansional strains due to moisture. Results of a similar nature are expected under thermally induced expansion strains also. The formulation is quite general as it can be used for the hygrothermo-mechanical loading of PMCs as well as for the thermo-mechanical loading of MMCs, and fiber and matrix properties may be considered to be transversely isotropic.

Experimental investigation of near crack tip creep deformation in alloy 800 at 650^C

Using a novel, noncontacting experimental method, the values for creep fracture parameters C(t) and C* have been experimentally quantified from full field, time-dependent surface deformation measurements in the crack-tip region for the first time. Near-tip experimental deformation results were obtained for a stationary crack in alloy IN800 at 650^C and compared to predictions based on both far-field measurement and finite element analyses. Results indicate that (a) for short times, the C(t)- integral is time and path dependent; (b) for long times, the C(t)-integral approached a constant for all paths which is agreement with C* values obtained from far-field experimental data. Comparisons of experimental results with FEM solutions were illustrated by means of radial and angular variations, full-field contour plots, as well as line integration form. Results from these comparisons indicate that (c) the experimental strain data portrays trends similar to those observed in the FEM solutions; (d) experimental, near-tip strain values were different in magnitude from the predicted values for all angles, and (e) the transition time obtained from the tests was about 10 times longer than that calculated from a formula proposed by Riedel and Rice (1980), indicating that primary creep cannot be neglected for this material.