Experimental study of FRP-strengthened RC bridge girders subjected to fatigue loading

The practical application of composite materials for retrofitting of reinforced concrete bridge T-sectional girders was investigated. Carbon and glass fibre-reinforced polymers (CFRP and GFRP) saturated in an epoxy resin matrix were used to enhance the service load-carrying capacity of the bridge. Three 5-m-long simply supported beams were tested under monotonic and cyclic loads for comparison to a beam subjected to more than 10⁶ cycles in the service load range. The results show that an FRP-strengthened T-beam subjected to fatigue loading demonstrated excellent behaviour that can be expected from well-detailed retrofit schemes incorporating carbon and glass fibre laminates.     

Flexural fatigue analysis of a CFRP form reinforced concrete bridge deck

Concrete bridge decks reinforced with fiber reinforced polymer (FRP) composite panels have recently been used where the FRP panels also serve as the permanent formwork for concrete. Comparing to their short-term behavior, their long-term performance especially under repeated traffic loads (fatigue) has not yet been widely known. This paper presents a fatigue analysis tool developed for a new steel-free concrete bridge deck reinforced with carbon FRP stay-in-place form. The developed model takes into account the cyclic creep of concrete in compression, the reduction in flexural stiffness due to fatigue tensile cracking and the reduction in modulus of rupture under cyclic loading. Comparisons with experimental data show reasonable agreement where a full-size 2-span deck specimen was subjected to millions of fatigue cycles. The parametric study recommends reducing the amount of FRP reinforcement and concrete strength of the current design, and lower loading rate may introduce more stiffness degradation in the system.     

Static and fatigue characterisation of new basalt fibre reinforced composites

Basalt reinforced composites are recently developed materials. These mineral amorphous fibres are a valid alternative to carbon fibres for their lower cost, and to glass fibres for their strength. In order to use basalt reinforced composites for structural applications, it is necessary to perform a mechanical characterisation. With this aim in the present work experimental results of several static and fatigue tests are described. Two polymeric matrices are taken into account, vinylester and epoxy, to assess their influence on the evaluated parameters. In parallel to these mechanical tests, also the thermal answer of the specimens to mechanical loads is evaluated by means of thermography. This experimental technique allows defining the composite local heating during the application of mechanical loads and its behaviour in details. Final discussion on obtained results is proposed focussing the attention on basalt fibre composite behaviour, and comparing mechanical properties of BFRP with other composite materials in glass and carbon fibres.     

Effects of static–fatigue (tension) on the tension–tension fatigue life of glass fibre reinforced plastic composites

The effects of static–fatigue interaction on tension–tension fatigue life of glass fibre reinforced plastic (GFRP) composites were investigated. This paper proposed a new static–fatigue model, which is capable of predicting residual strength after a period of static loading. Also an algorithm is proposed to calculate fatigue lives with the inclusion of static–fatigue interaction. Predictions from the proposed static–fatigue model show a good agreement with the experimental results. Static–fatigue interaction has shown a considerable effect on fatigue lives of GFRP composites at intermediate and lower applied stress levels possibly due to a longer exposure to applied loads. At higher load levels approximately greater than 65% of ultimate stress, and higher stress ratios range like 0.5 < R < 0.9, fatigue lives shown to be closer to material’s static–fatigue limits which is shorter than the expected lifetime by cyclic fatigue.     

Effects of mean stress and fibre volume content on the fatigue-induced damage mechanisms in CFRP

The effect of the load type (tension and compression) in quasi-static and of the applied mean stress in fatigue tests on the mechanical behaviour and on the damage mechanisms in unidirectional (UD) carbon/epoxy laminates has been studied in combination with the influence of fibre volume content. Results show that the fibre volume content increases the mechanical properties in tension–tension fatigue tests for all tested angles 0°, 45° and 90°. The tensile damage mechanisms of off-axis specimens depend on the fibre volume content and change from matrix cracking and matrix–fibre debonding to fibre-pull out with an increasing amount of fibres as investigated in detail in a previous work. In tension–compression tests, higher fibre volume contents are only beneficial in fatigue tests at angles of 0° and 45°. Fatigue strengths of UD 90° specimens in tension–compression tests are not significantly improved by the fibre volume content which can be ascribed to breakage of entire fibre bundles and crushed fibres on the fracture surfaces.     

A simulation-based investigation of the staircase method for fatigue strength testing

A critical evaluation of the statistics of the fatigue strength distribution as determined by the staircase (or up-and-down) method is presented. The effects of test parameters (namely, step size and sample size) were analyzed using numerical simulation to determine the accuracy of fatigue strength standard deviation calculations using traditional staircase statistics, resulting in a quantification of standard deviation bias as a function of step size and sample size. A non-linear correction was formulated to mitigate this standard deviation bias inherent in small-sample tests. In addition, the simulation was used to investigate the effectiveness of a bootstrapping algorithm on standard deviation estimates. The bootstrapping algorithm was found to significantly reduce the potential of large standard deviation errors in small-sample tests. Together, the use of the non-linear correction factor and the bootstrapping algorithm may allow an improved method to estimate the statistics of a material’s fatigue strength distribution using a small-sample staircase test strategy.     

An experimental investigation into quasi-static and fatigue damage development in bolted-hole specimens

An extensive experimental program has been carried out to investigate and understand the sequence of damage development throughout the life of bolted-hole composite laminates under quasi-static loading and tension–tension fatigue. Quasi-isotropic carbon/epoxy laminates, with stacking sequence [45₂/90₂/-45₂/0₂]_S defined as ply scaled and [45/90/-45/0]_2S defined as sub-laminate scaled, were used. Specimens were cycled at 5 Hz with various amplitudes to 1 × 10⁶ cycles unless failure occurred prior to this limit. For all cases an R ratio of 0.1 was used. Bolt washer pressures of 23 MPa and 70 MPa were investigated. For the ply-level case, the quasi-static test showed both delamination and fibre-dominated pull-out failures for a washer pressure of 23 MPa, and pull-out failure only for 70 MPa. Delamination dominates in fatigue tests. For the sub-laminate case the tests failed by pull-out in both quasi-static and fatigue tests for all washer pressures. It is shown in this paper how the role of delamination is critical in the case of fatigue loading and how this interacts with bolt clamp-up forces. A number of tests were analysed for damage using X-ray CT scanning and comparisons of damage are made with tests from previous open-hole studies.     

Experimental investigation of bond characteristics between CFRP fabrics and steel plate joints under impact tensile loads

In recent years, the use of adhesively-bonded fibre-reinforced composite materials has attracted widespread attention as a viable alternative for the retrofitting of civil infrastructure such as buildings and bridges. This has been particularly the case for concrete structures. The retrofitting of metallic bridges and buildings with FRP materials, however, is still in its early stages. In real life, these structures are subjected to dynamic loads. Therefore, it is necessary to understand the bond behaviour between steel and the strengthening materials for both static and dynamic loads. To examine the bond between steel plates and carbon fibre-reinforced polymers (CFRP) fabrics, this paper describes the experimental procedures and results of double strap steel joints loaded at different loading rates (2 mm/min, 3.35, 4.43 and 5 m/s). In this test program, ultimate load-carrying capacity, effective bond length, failure mechanism and strain distribution were examined for all loading rates. Different numbers of CFRP layers with different bond lengths were investigated. Experimental findings reveal that the maximum improvement in joint capacity occurs at a rate of 3.35 m/s. It was observed that the effective bond length is insensitive to loading rate for both joints. The failure modes and strain distributions, however, exhibit little difference between static and dynamic conditions.     

Experimental dynamic characterization of an FRP composite bridge superstructure assembly

A system comprised of concrete filled, filament wound, circular carbon/epoxy girders and an E-glass/polyester deck, representative of a bridge section in the positive moment region was tested at large scale to assess the overall and component structural response. The system was characterized for stiffness and overall response under monotonic and cyclic fatigue loads. Forced vibration testing was conducted as part of a level I non-destructive evaluation (NDE) procedure at each of the test stages, including after failure. Experimental results from the tests were seen to correspond well with analytical results for mode shapes and frequencies obtained through an eigenvalue analysis of a plane-grillage finite element model. The test method was shown to be effective in indicating changes in response as a function of load level and damage accumulation, and is expected to have significant potential for eventual routine health monitoring and damage detection of such structural systems in the field.     

Fatigue strength of tubular carbon fibre composites under bending/torsion loading

Carbon fibre reinforced polymer composites have been increasingly used on structures frequently subjected to biaxial fatigue loadings. This paper studies the fatigue behaviour of tubular carbon fibre composites under in phase biaxial bending/torsion dynamic loadings. Particularly, it was analysed both the torsion stress and mean stress effects on the fatigue strength and failure mechanisms. Fatigue strength decreases significantly with increased torsional/bending stresses ratio, while the damage becomes faster. For the cases in which a torsion loading was applied the effect of the mean stress on the fatigue strength seems to be well fitted by using a quadratic equation.     

Lifetime assessment for GFRP-gliders using a representative substructure

In a research project fatigue tests on two identical wing sections that are used as representative substitute components were performed to estimate lifetime enhancement of light-weight aircraft such as general aviation gliders. Single step fatigue tests at limit load on one component were compared with spectrum loading on the other. A characteristic damage behaviour was observed. Local buckling due to high shear loading of the sandwich core of the wing shell causes the main damage. Even though an intentionally high load level was chosen no increase of micro-cracking could be detected. Additionally no delamination effects in laminates or bondings were observed except the skin-core delamination which causes the buckling effect in the wing shell due to fatigue loading. Finally the concept of using representative substructures and accompanying specimen tests is an effective approach for in-service loading investigations and could also be applied to research on wind turbine blades.     

Uniaxial and biaxial flexural fatigue of glass reinforced inter-penetrated network polymer composites

Conclusions Flexural fatigue of uniaxially and biaxially stressed IPN/glass mat composites was investigated using four point bend (4PB) and concentrically loaded (CL) specimen geometries. Regions of nearly constant bending moment between the inner spans of a 4PB beam and within the inner annulus of a CL circular plate yield quasi-uniform uniaxial and biaxial stress, respectively, on the tensile faces. The specimen dimensions were optimized for both loading geometries to give: (1) reduced specimen deflection through maximizing the ratio of the induced tensile stresses to the applied load, (2) minimized contact stresses by maximizing the induced stress with respect to the unit contact load, and (3) a large material volume exposed to the maximum cyclic stress (i.e., statistical fracture initiation).A power model was used to analyze the fatigue data for the 4PB and CL specimens. Both IPN composite materials studied fatigued more rapidly under the more severe loading conditions imposed by the CL specimen geometry.Fractography revealed that debond fracture was the dominant damage process for both geometries. The initial debond cracks were uniformly distributed throughout the stressed regions, confirming the presence of nearly uniform tensile stress. Damage localization followed after further cycling and was characterized by a locally high debond fracture density, fiber fracture, and always occurred where several glass strands crossed near the specimen surface. Final specimen failure resulted from the preferential growth of dominant cracks through the specimen thickness.     

Torsion fatigue behavior of unidirectional carbon/epoxy and glass/epoxy composites

This paper presents results of the feasibility of carbon/epoxy composites (CFRP) as a future helicopter flexbeam material. Torsional behaviors of unidirectional CFRP and glass/epoxy composites (GFRP) with the same resin matrix were investigated. The initial torsional rigidity of CFRP was almost identical to that of GFRP. The torsional rigidities calculated using finite element analyses (FEA) agreed with the experimental results: the torsional rigidities are governed mainly by the material’s shear stiffness. Torsion fatigue tests were also conducted by controlling the angle of twist of the sinusoidal wave under a constant tensile axial load. No catastrophic failure occurred with either GFRP or CFRP, although decreased amplitudes of torque and torsional rigidities were observed according to the number of cycles. Results of X-ray CT inspections and numerical calculation by FEA revealed that degradation of a torsional rigidity is caused mainly by splitting crack propagation along the fiber direction. The torsion fatigue life of CFRP was superior to that of GFRP. Consequently, results confirmed that CFRP exhibits excellent properties as a torsional element of a helicopter flexbeam in terms of torsional rigidity and tension–torsion fatigue behaviors.     

Multiaxial fatigue of a short glass fibre reinforced polyamide 6.6 – Fatigue and fracture behaviour

The multiaxial fatigue behaviour of a short glass fibre reinforced polyamide 6.6 (PA66-GF35) is investigated on hollow tubular specimens in the range of fatigue lives between 10² and 10⁷ cycles. Fatigue experiments included pure tension, pure torsion, combined tension–torsion at different biaxiality ratios and phase shifting angles between the stress components. Tests were carried out with load ratio R = 0 and R = −1 at room temperature as well as at 130 °C. The influence of biaxiality ratio, phase angle between load components and load ratio is discussed.An extensive analysis of the fracture behaviour is performed on the specimens to recognise the crack nucleation and propagation mechanisms; failure modes were evaluated via optical and scanning electron microscopy.     

Experimental and numerical analysis of a bus component in composite material

The aim of this work is the fatigue design of a structural component manufactured in composite materials. The use of composite materials in structural applications implies that the reliability and safety requirements are met, in particular from a point of view of the fatigue life. A bus component is considered: the composite material is obtained by means of the pultrusion technique: the longitudinal reinforcement is glass fibre, while the matrix is polyester resin. At first, a static and fatigue characterization of the pultruded composite is performed to identify the mechanical behaviour along the fibres and in the normal direction, following an extensive testing programme on specimens. During the fatigue tests, the variation of the elastic modulus and the residual strength are monitored to characterize the damage; further experimental tests are performed to fit parameters required for models of fatigue life prevision. Fatigue bending tests are performed on the component, showing crack propagation through the section: a check at the SEM is performed to identify the main internal type of damage. A final comparison with a numerical simulation is proposed: this numerical model will be useful for the optimal fatigue design of the section.     

The flexural fatigue performance of plain and fibrous concretes

If fatigue cracking is going to occur in concrete structures then it is more likely due to repeated flexural loadings rather than direct compression or tension. Typical examples are road and airfield pavements, bridges, offshore constructions and structures likely to experience earthquakes. Also many of these loadings have a dynamic character and a knowledge of material behaviour at rapid stressing rates as an essential preliminary requirement to understanding flexural fatigue performance. Therefore since flexural loadings are frequently encountered in practical situations then the flexural test is probably one of the most useful types to be used in the examination of both the fatigue behaviour of concrete and its ultimate strength developed at rapid loading rates. Although the designer regards concrete as an homogeneous material, it consists of two phases, the active hardened cement phase, which is the binding material to the inert phase, the aggregate. Concrete behaviour can therefore be complex and aspects interpreted at either the macroscopic, microscopic or molecular level. The composite nature of concrete may be further complicated by the introduction of steel reinforcement. Over the last few years there has been a general research interest with the incorporation of small quantities of fibres in concrete. Steel fibres of dimensions 10mm to 60mm in length, 10m to 60 m in diameter, with material properties ranging from 150 GN/m ₂ to 200 GN/m ₂ in elastic modulus and 700 N/mm₂ to 200 N/mm₂ in tensile strength may be introduced at the mixing stage in proportions between 0.5% and 3.5% of concrete volume. The consequent enhancement in flexural strength is substantial and the likelihood of a similar improvement in fatigue performance needed to be demonstrated. It has also been common practice when conducting modulus of rupture tests, to use 500mm × 100mm × 100mm specimens for mixes incorporating fibres. When casting such specimens there is a danger of fibres not being randomly orientated. Thus for this investigation, beams of 1500 times 200 times 200mm have been used. The selection of larger beams also makes instrumentation more manageable and the dimensions may be more comparable with those associated with most practical applications. Some fifty fatigue tests have been completed supported by a further five hundred strength and control tests. A complete series of strain, deflection and crack development histories have been observed. An appropriate form of S-N curve has been adapted for the examination of results. Models explaining the behaviour of both plain and fibrous concretes have been proposed and a method of flexural fatigue performance prediction has been formulated.     

Effect of initial damage level and patch configuration on the fatigue behaviour of reinforced steel plates

The application of carbon fibre reinforced polymer composites externally bonded on cracked steel plates is an effective system in extending the fatigue life of these structural elements. In particular, composite patches bonded on the crack tip region reduce the stress concentration and the crack opening displacement, leading to an extension of the fatigue life. In order to additionally show the effectiveness of this kind of reinforcing technique, experimental tests were performed at the laboratories of the Politecnico di Milano. Fatigue tests were executed on single edge notched tension specimens reinforced by pultruded strips bonded to a single side (non‐symmetric reinforcement). Different patch configurations (reinforcement stiffness and patch location) and initial damage levels were considered as parameters influencing the repair effectiveness in extending the fatigue life. The results showed that the use of carbon fibre reinforced polymer materials bonded around the tip region allows extending the fatigue life for different amount of initial damage level. Finally, this work provides some useful information for the more efficient repair configuration.     

Investigation of laser pulse fatigue effect on unpainted and painted CFRP structures

Laser ultrasonic based nondestructive evaluation (NDE) techniques are being increasingly used in aerospace industries. Generally, the service lifetime for an aircraft could be more than 25 years. Thus, the composite structures of the aircraft could be susceptible to laser pulse fatigue damage caused by the laser pulse energy in the long-term repetitive maintenance inspection. In this paper, the effect of repeat laser pulse scanning on the mechanical characteristics of unpainted and painted CFRP specimens (USN175BX Carbon UD prepreg, Bisphenol A epoxy resin) is investigated to verify the reliability regarding the use of laser ultrasonic scanning based NDE techniques on the inspection of the CFRP structure. A high-speed laser ultrasonic scanning system is setup to perform repeat scanning of 1300 times on both CFRP specimens at the five laser pulse energy levels using the 532-nm and 1064-nm Q-switched continuous wave lasers. Elastic modulus assessment based on the ultrasonic Lamb wave pitch–catch method is used and the surface condition of the scanned area is investigated by a microscope. As a result, the laser pulse fluences that is shown in this paper are suitable for the long-term repetitive maintenance inspection in unpainted and painted CFRP structure even if it demonstrates an embrittlement phenomenon similar to the modulus measurement resolution in the unpainted CFRP specimen. In addition, the laser pulse fluence for maximum signal-to-noise ratio without any damage is investigated in both unpainted and painted CFRP specimens. As a result, both 102.45 mJ/cm² in unpainted CFRP specimen and 51 mJ/cm² in painted CFRP specimen can be the laser pulse energy for the maximum signal-to-noise ratio without any damage.     

Modelling experimental bond fatigue failures of concrete beams strengthened with NSM CFRP rods

Near surface mounting (NSM) is a promising strengthening technique provided that the full bond between the strengthening material and the existing structure develops. Wahab et al. (2011) reported fatigue bond failures by de-bonding between the CFRP rod and the epoxy that started at mid span. As the load was cycled, the de-bonding propagated towards the support. The model presented here describes the failure of those beams. The de-bonding was modeled as a crack growing at the interface between the CFRP rod and the epoxy where the driving force was the interfacial shear stresses between the CFRP rod and the epoxy. Once the stresses at the crack tip exceeded the allowable fatigue stresses between the CFRP rod and the epoxy, de-bonding occurred and the crack length increased. Ahead of the crack tip, the CFRP rod was fully bonded to the epoxy and the forces decayed exponentially. Behind the crack tip, the rod was partially de-bonded. The experimental and calculated number of cycles until excessive slipping between the CFRP rod and the epoxy occurred and the forces in the CFRP rod at all locations in the shear span at the onset of failure were in good agreement.     

Effect of fatigue loading on the bond behaviour between UHM CFRP plates and steel plates

Extensive research has been conducted on static bond behaviour between CFRP and steel. However, very limited research is available on the effect of fatigue loading on the bond behaviour between CFRP and steel. This paper attempts to fill the knowledge gap in this area. A series of static and fatigue tests on UHM (ultra high modulus) CFRP plate and steel plate double strap joints were conducted. Five specimens were tensioned to failure under static loading as control specimens. The other 12 specimens were tested under fatigue loading with load ratios ranging from 0.2 to 0.6 (defined as the ratio of the maximum fatigue load to the average static bond strength of control specimens). After going through pre-set number of fatigue cycles, the specimens were tensioned to failure under static loading. The failure modes, residual bond strength and residual bond stiffness of such specimens were compared with those of control specimens, to facilitate the investigation of the effect of fatigue loading on the bond behaviour. Microscopic investigation was also performed to reveal the underlying failure mechanism.