Piyush K. Dutta

Low-temperature and freeze-thaw durability of thick composites

Low temperature produces internal stresses in composites of polymeric materials. The polymeric matrix phase becomes stiffer, and may suffer from damage-inducing stresses resulting from thermal coefficient mismatch of fibers and resins. These influences have been studied by subjecting two types of FRP composites to flexural tests. A commercially procured fiber reinforced plastic (FRP) composite indeed produced cracks on prolonged thermal cycling between 50°C (122°F) and -60°C (-76°F) temperature. But a specially manufactured woven glass reinforced FRP did not produce any visual cracks for two and half times more thermal cycling in the same temperature range. It is suspected that the resin type and the curing process control the thermal cycle response and ultimate durability of such FRP composites in extreme temperature environments.

Creep rupture of a GFRP composite at elevated temperatures

Abstract A study of the tension and compression behavior of glass-fiber reinforced polyester composite material under sustained loads and elevated temperature is presented. Time-dependent thermal deformation and failure stresses were measured at three different temperatures ( T=25 , 50, and 80°C). Using curve-fitting equations a simple empirical model was developed to predict the time-to-failure. The model takes into account superposition of time–temperature effects. Creep curves predicted by this model are similar to those reported in literature.

On the Design of Polymeric Composite Structures for Cold Regions Applications

This study focuses attention on low-temperature hygrothermal effects which influence the short- and long-term behavior and characterization of polymeric composite materials. A review of the literature reveals a scarcity of low-temperature material performance data needed for design of composite materials for cold regions applications. Four problem areas are identified: (1) hygrothermal residual stresses, (2) material degradation due to low-temperature environmental cycling, (3) moisture effects on freeze-thaw cycling, and (4) long-term synergistic effects of combined loading history and environmental exposure on material durability. A brief review of past work is presented and areas identified where more research is needed to develop the data base required for design of composite materials for cold environments.

Dynamic mode II delamination fracture of unidirectional graphite/epoxy composites

Abstract Dynamic fracture and delamination of unidirectional graphite/epoxy composites are investigated for end-notched flexure (ENF) and center-notched flexure (CNF) pure mode II loading configurations using a modified split Hopkinson pressure bar. Results show that delamination and energy absorbed in fracture increase with impact energy with CNF>ENF. A power law analytical model reasonably describes the variation of energy release rate with delamination and energy absorbed. A crack embedded deeper in a specimen (as in CNF) contributes more to dynamic fragmentation than cracks at the surface or near the edge (as in ENF). Hackle features on mode II fracture surfaces decrease with impact energy.

Comparison of the Fatigue Behaviors of FRP Bridge Decks and Reinforced Concrete Conventional Decks Under Extreme Environmental Conditions

This paper summarizes the results of the fatigue test of four composite bridge decks in extreme temperatures (-30°C and 50°C). The work was performed as part of a research program to evaluate and install multiple FRP bridge deck systems in Dayton, Ohio. A two-span continuous concrete deck was also built on three steel girders for the benchmark tests. Simulated wheel loads were applied simultaneously at two points by two servo-controlled hydraulic actuators specially designed and fabricated to perform under extreme temperatures. Each deck was initially subjected to one million wheel load cycles at low temperature and another one million cycles at high temperature. The results presented in this paper correspond to the fatigue response of each deck for four million load cycles at low temperature and another four million cycles at high temperature. Thus, the deck was subjected to a total of ten million cycles. Quasi-static load-deflection and load-strain responses were determined at predetermined fatigue cycle levels. Except for the progressive reduction in stiffness, no significant distress was observed in any of the composite deck prototypes during ten million load cycles. The effects of extreme temperatures and accumulated load cycles on the load-deflection and load-strain response of FRP composite and FRP-concrete hybrid bridge decks are discussed based on the experimental results.

Experimental studies on the high strain rate compression response of woven graphite/epoxy composites at room and elevated temperatures

In this study, experimental investigations on affordable woven graphite/epoxy laminates under high strain rate compression loading at room and elevated temperatures are discussed. 17-layered woven graphite/epoxy laminates are fabricated with plain and satin weave fabrics with room temperature curing SC-15 epoxy resin using affordable vacuum assisted resin infusion molding (VARIM) process. Samples were tested at strain rates ranging from 200 to 1100/s at four different temperatures: room, 125, 175, and 225 F. Upper limit on the temperature was selected based on the supplier’s data sheet for SC-15 epoxy resin system, which has a dry glass transition temperature of 220 F. Failure mechanisms were characterized through optical microscopy. Failure modes were influenced by the temperature and fabric architecture. Results of the study indicate the softening of fiber–matrix interface with increasing temperature, which affects the dynamic compression strength. Satin weave samples exhibit higher compressive strength as compared to plain weave samples due to straighter fabric architecture.

Fatigue durability of FRP composite bridge decks at extreme temperatures

Decks manufactured with Fibre-Reinforced Polymer (FRP) composite materials are used in bridges. A performance evaluation of FRP composite decks subjected to simulated traffic loads that induce repetitive stress cycles under extremely high and low temperature is presented. Fatigue testing of three FRP composite bridge deck prototypes and one FRP-concrete hybrid bridge deck prototype under two extreme temperature conditions: −30°C (−22°F), and 50°C (122°F) was conducted. The fatigue response of the deck prototypes was correlated with the baseline performance of a conventional reinforced-concrete deck. Design loads were applied simultaneously at two points. Quasi-static load-deflection and load-strain characteristics were determined at predetermined fatigue cycle levels. No significant distress was observed in any of the composite deck prototypes during ten million load cycles. The effects of extreme temperatures and accumulated load cycles on the load-deflection and load-strain response of FRP composite and FRP-concrete hybrid bridge decks are discussed based on the experimental results.

Effects of low temperature on the dynamic moduli of thick composite beams with absorbed moisture

Abstract The present investigation concentrates on the influence of low temperature and moisture on the dynamic moduli of thick S2-glass composite beams. By supporting the beam sample in a free–free configuration, its natural frequencies were obtained through impact testing. Frequency dependence on the hydrothermal conditions was disclosed by testing the sample at different temperatures and with different moisture contents. Based on the frequency measurements, a method was developed to predict the longitudinal Youngs modulus and the transverse shear modulus of the sample. The process involves iterative tuning of the moduli of a Timoshenko beam model via a stable characterization scheme. Numerical sensitivity study shows that the moduli thus determined are insensitive to measurement errors rendering the method a possible supplement for conventional static tests. Both frequencies and moduli of the beam sample were found to exhibit an increasing trend with reducing temperature. Besides, freezing of the absorbed moisture enlarged the longitudinal Youngs modulus of the material in a significant manner.

Characterization and Modeling of the Effect of Environmental Degradation on Interlaminar Shear Strength of Carbon/Epoxy Composites

Accelerated ageing experiments have been conducted to address durability issues of carbon/epoxy composites to be used for emerging facilities and infrastructure, such as, bridges and buildings, in different climatic zones. The degradation of carbon/epoxy composites under UV, hygrothermal exposure, and applied tensile stress has been investigated. The tests were designed to capture the synergistic effects of field exposure and extreme temperatures, viz., hot/dry, hot/wet, cold/dry, and cold/wet conditions. Short beam shear tests (SBST) were performed for the determination of interlaminar shear strength (ILSS) of conditioned composite specimens. The hot/dry samples showed increased strength, while the hot/wet ones showed a decrease in strength. It is conjectured that conditioning at 90 °C possibly contributed to an increase in the ILSS from post curing. For the hot/wet samples (90 °C, immersed in water) the results indicate that strength degradation due to moisture-induced hydrolysis overshadowed the post-curing effect. The samples subjected to shear stress under hot conditions (90 °C) showed a higher ILSS, possibly due to improved crosslink density arising from post-cure. There is insignificant variation in the ILSS of UV treated and the UV untreated control samples. All the SBST test data reported in this work are from tests performed at room temperature and ambient humidity after environmental ageing. A two-dimensional cohesive layer constitutive model with a prescribed traction-separation law constructed from the basic principles of continuum mechanics, taking into account hygrothermal mechanisms that are likely to occur within a cohesive bi-material interface, such as between adjacent plies in a laminate, was applied to simulate interlaminar failure in the SBST specimens, using Finite Element Analysis (FEA). A phenomenological predictive model was developed using the finite element results.

A microstructural study of Gr/Ep composite material subjected to impact

Abstract Fracture morphology and texture were observed on the impact generated graphite/epoxy composite fragments. The work identifies and explains changes in the surface texture of the fragments generated at different impact velocities (122 to 610 m/s, 400 to 2000 ft/s) and over a wide range of specimen temperatures (−54 to 24°C, −64 to 75°F). The composite panels were impacted by spherical steel projectiles, and the entire spall was carefully collected after the impact. The spall was differentiated according to different sizes and shapes. A few fragments representing each shape and size were selected to analyze the surface morphology using scanning electron microscopy (SEM). Change in the surface texture was observed according to the different sizes and shapes, and the change in size and shape of the fragments was credited to the change in impact force. The results following from the close and intense observation of several SEM fractographs revealed that the surface texture of the fragments is strongly dependent on the type of forces acting at the point of impact resulting in four different modes of failure: delamination, transverse matrix cracking, fiber fracture and fiber-matrix interface debonding.