Hota V. S. GangaRao

Fatigue Response of Fabric-reinforced Polymeric Composites

Mechanical fatigue response of fiber-reinforced polymeric (FRP) composites is essential to better understand the durability of composite materials systems and to develop design specifications. Currently, the fatigue response of multidirectional glass composite materials is not well-understood and much needs to be done to understand their behavior under fatigue loading. In this study, three glass fabric FRP composite material coupons and systems are tested at constant low-amplitude fatigue loading. Experimental results show that for a given FRP material and load configuration, the energy loss per cycle due to fatigue damage is linear from about 10-90% of the fatigue life of the FRP composite material. The energy loss per cycle is determined to be a characteristic value of the constituent materials, and is found to vary with the induced fatigue strain levels by a power law. Based on the experimental results, a fatigue life prediction model is proposed, with internal strain energy as damage metric, to predict the useful life of FRP composites. The experimental and predicted fatigue lives at various strain levels are compared (S-N curves) and the model is found to be conservative.

Detection of subsurface defects in fiber reinforced polymer composite bridge decks using digital infrared thermography

Fiber reinforced polymer (FRP) bridge decks are rapidly emerging as potential alternatives to conventional reinforced concrete (RC) bridge decks. The FRP decks offer higher strength-to-weight ratios compared to RC decks. However, presence of subsurface defects such as debonds and delaminations formed during initial construction and in service can adversely affect the structural integrity and service performance of the FRP bridge decks. Hence, a field monitoring technique such as infrared thermography is required to evaluate the in situ condition of these FRP decks. This paper investigates the use of digital infrared thermography for subsurface defect detection in FRP bridge decks. Air-filled and water-filled debonds were inserted between the wearing surface and the underlying FRP deck in the laboratory. Simulated subsurface delaminations (of various sizes and thickness) were also created at the flange-to-flange junction between two FRP deck modules. The infrared technique was used to detect these embedded subsurface defects. Surface temperature–time curves were established for different sizes of delaminations and debonds. In addition, field study was conducted on a FRP bridge deck to detect debonds between the wearing surface and the underlying deck. The laboratory and field testing results show that infrared thermography is a potentially useful tool for defect detection in FRP composite bridge decks. The technique can possibly be used for several applications such as quality control during pultrusion of new decks (in factories), during field construction, and for field inspection of in-service decks.

Damage detection using scanning laser vibrometer

A damage detection algorithm based on the principle of curvature changes has been developed at CFC-WVU. However, the algorithm requires accurate mode shapes with adequate spatial density. Existing contact sensors can not provide adequate spatial density without adding excessive mass. Hence, non-contact scanning techniques, such as scanning laser vibrometer (SLV) has adequate sensitivity and accuracy is yet to be proven. The applicability of SLV on large structures is also questionable. To assess the suitability of using SLV for damage detection, a beam specimen has been tested using an existing system. The results confirm that damage detection using vibration measurements from SLV is successful. Due to more spatial density, the SLV data is shown to be more sensitive than the contact sensor test.

Non-baseline detection of small damages from changes in strain energy mode shapes

Several methods for damage detection based on identifying changes in strain energy mode shapes (SEMS) have been recently described in the literature. Most of these methods require knowing strain energy distribution for the undamaged structure (baseline SEMS). This is especially true for detection of small damages, where changes in the SEMS cannot be observed otherwise. Usually, the mode shapes from the structure under test should be compared to the baseline mode shapes to provide sufficient data for damage detection. However, these methods do not cover damage detection on structures where baseline mode shapes cannot be readily obtained, for example, structures with preexisting damage. Conventional methods, like building a finite element (FE) model of a structure to be used as a baseline might be an expensive and time-consuming task that can be impossible for complex structures. This paper suggests a method for extraction of localized changes (damage peaks) from SEMS based on Fourier analysis of the strain energy distribution. A detailed analytical proof is given for the case of a pinned–pinned beam and a numerical proof for the free–free beam. The analytical predictions have been confirmed both by the FE model and impact testing experiments on a free–free aluminum beam, including single and multiple damage scenarios.

Durability of glass fiber-reinforced polymer composites under the combined effects of moisture and sustained loads

This paper deals with durability of glass fiber-reinforced polymer composites under sustained loads and simultaneously exposed to either saltwater or tap water. The tensile properties of the specimens before and after conditioning were evaluated along with their moisture uptaking behavior to reveal the durability of the specimens under combined effects. The mass change curves under saltwater and tap water immersion exhibited dual-stage diffusion process. The first stage followed Fickian profile, while mass loss occurred in the second stage with increasing immersion time, attributed to hydrolysis of resin. The weight gain in saltwater was greater than that in tap water. Moreover, the mass change curves under varying percentage of sustained loadings exhibited similar trends, while the maximum moisture uptake was obviously affected by the loading conditions. Based on Fick’s second law, the diffusion coefficients in the first stage under different conditions were determined through the least-square regression technique. It was observed that the tensile strength and modulus were increasing initially upon immersion, which was attributed to curing beyond the initial cure during manufacture. Then, the tensile properties decreased with continued immersion in saltwater or tap water. After 180 days of immersion, the tensile properties degraded at a smaller rate, both for specimens with and without sustained loading. For the saltwater immersion of 360 days, the tensile properties decreased significantly as the sustained loading increased. However, the tap water immersion had less detrimental effect on tensile properties of glass fiber-reinforced polymer. Furthermore, based on Arrhenius concept coupled with fracture mechanics principle, a new model was proposed to describe the combined effects of immersion and sustained load.

Durability and prediction models of fiber-reinforced polymer composites under various environmental conditions: A critical review:

This article presents recent developments in long-term behavior of fiber-reinforced polymer composites subjected to moisture, pH, temperature, sustained stress and ultraviolet radiation, as well as response under combined effects of moisture, pH, etc. Although numerous experiments have been conducted to investigate the durability of fiber-reinforced polymers under different environments, at this stage, it is difficult to ascertain the degradation trends and mechanisms of failures of fiber-reinforced polymers because of non-standardization of various conditioning effects and variation in material constituent. Recent findings on the long-term performance of fiber-reinforced polymer composites are synthesized herein. This article critically examined not only the moisture uptake response, mechanical property and failure modes of fiber-reinforced polymers under varying environments, but also the degradation mechanism through physical and chemical reactions. Moreover, based on information in literature, long-term predictive models were described along with the application potential and limitations. The strength reduction factors were proposed from available data to account for the environmental effects on fiber-reinforced polymers.

Macro-element analysis

Abstract In recent years methods of analyzing plates in bending via large or macro-element have been studied. Herein, a method of studying plate behavior by a macro-flexibility approach is introduced. Deflected shapes of macro-elements of rectangular shapes were obtained by a shape function that satisfies all four boundary conditions and the bi-harmonic equation. The shape functions were a sum of sinusoidal and polynomial terms with undetermined coefficients. The elements that satisfy moment and shear conditions, were assembled by utilizing compatibility equations for deflection and slope. This resulted in equilibrium of forces and moments for all lines along the common edges of macro-elements. Three bounded domains were analyzed, and the results were compared to solutions obtained from classical and finite element methods. The convergence of the macro-approach was checked by progressively increasing the number of harmonics. The study of the numerical results indicates that excellent results can be obtained within the first three harmonics.

Performance Evaluation of FRP Bridge Deck Under Shear Loads

Shear behavior of glass fiber reinforced polymer (FRP) bridge deck components has been experimentally and theoretically studied under in-plane shear, out-of-plane shear, punching shear, shear of web—flange junction, and system racking shear. Experimental data revealed that the shear modulus of FRP bridge decks ranged from 2.66 to 4.14 GPa and the shear stress to failure ranged from 20.7 to 96.6 MPa. In-plane shear behavior is studied under V-notched and racking shear test (parallel and perpendicular to cell direction). Experimental results under in-plane shear loading are compared with the results from the classical finite element method. Out-of-plane shear strength and stiffness of an FRP composite deck are experimentally evaluated utilizing test data from the short beam shear test, and the beam bending test. Using experimental and numerical results, the reduction in bending rigidity due to shear deformation under several loading conditions is calculated. In addition, size limits (span to depth ratio) under transverse loading are established as: L/d>22 (for multi-cell specimen with and without joints). A theoretical model based on FRP deck types for predicting punching shear capacity is proposed and validated through experimental data. In addition, the failure modes of test specimens are identified and reported. To study the web—flange junction behavior, closed FRP sections were tested under shear-bending effect. It is clear that the web—flange junction shear strength is only one half of the shear strength obtained from flange specimens under V-notched beam testing. While testing, cracks and layer delaminations around web—flange junctions were initiated and extended along the thickness of the web portion with increasing applied loads, which eventually led to web shear-off failure. In addition, it is found that shear strengths of test specimens depend on modes of shear failures induced by different shear test methods. Higher shear strength is found on failure modes that have more influence of fiber shear.

Fiber-Reinforced Polymer Composite Bridges in West Virginia

Fiber-reinforced polymer (FRP) composites have been used more often over the past decade than before in new construction as well as in repair of deteriorated bridges. Many of these bridges are on low-volume roads, where they receive very little attention. It is imperative that new bridge construction or repair be long lasting, nearly maintenance free, and as economical as possible. Relative to those factors, FRP composite bridges have been found to be structurally adequate and feasible because of their reduced maintenance cost and limited environmental impact (i.e., no harmful chemicals leaching into the atmosphere with longer service life). In West Virginia, 23 FRP composite bridges have been constructed, among which 18 are built on low-volume roads that have an average daily traffic (ADT) of less than 1,000, including 7 with ADT less than 400. General FRP composite bridge geometry and preliminary field responses are presented as are some of the preliminary construction specifications and cost data of FRP composite bridges built on low-volume roads in West Virginia

Subsurface Defect Detection in FRP Composites Using Infrared Thermography

This paper demonstrates the use of digital infrared thermography to detect subsurface defects such as debonds and delaminations in Fiber Reinforced Polymer (FRP) bridge decks. Simulated sub‐surface debonds and delaminations were inserted between the wearing surface and the underlying FRP deck specimens. The infrared thermography technique was used to detect these embedded subsurface defects. The use of various cooling and heating methods, including solar radiation, was explored. Surface temperature‐time curves were established for different types and sizes of subsurface defects.