Fatigue Life Extension Research Articles

VECD investigation and quantification of rest period healing within pulse-rest loading

ABSTRACT The viscoelastic continuum damage (VECD) model has been widely recognised as a characterisation method for asphalt mixture’s fatigue and damage behaviour. Rest periods between loading cycles can recover stiffness and extend fatigue life. However, there is still a lack of approach to quantify the effect of rest period after each load cycle (pulse-rest loading). In this paper, a series of simplified VECD tests are conducted with different length of rest period. Based on smeared continuum damage method, rest period based damage characteristic curves are produced at two different temperatures, with rest period healing effect critically evaluated. VECD results indicate that longer rest period improves healing effect, and higher temperature incurs greater maximum healing effect when rest period is long. Based on regression fitting technique, a novel rest period damage function has been developed. The new function can be used to overcome laboratory fluctuation of VECD tests with pulse-rest loading, to produce accurate smeared damage characteristic curves with any rest period length. It has benefit of saving experiment time and improving accuracy. This work also proposes laboratory procedures and data processing notices for the production of rest period damage functions for future use.

Application of design of experiments for laser shock peening process optimization

Laser shock peening—a very promising life enhancement technique—has demonstrated great success regarding the improvement of fatigue behavior via deep compressive residual stresses. However, the prediction and adaption of residual stress fields on basis of the laser peening parameters are still not comprehensively established. The aim of the current work is to investigate the effects of the laser pulse energy, the number of treatment overlaps as well as the laser spot size on the resulting residual stress distribution, characterized by following quantities: the residual stress close to the surface, the maximum compressive residual stress, and the integral compressive stress area over the specimen depth. For a systematic investigation of all main and interaction-based process parameter effects, and a subsequent parameter optimization, the general full factorial design is employed. The results show that laser shock peening with different process parameter combinations, inducing residual stresses with comparable integral stress area, can lead to a minimum fatigue life extension of approx. 100,000 cycles, representing a minimum fatigue life of 250% of the base material. The experimental scatter in the number of cycles to failure follows the Weibull distribution which qualitatively correlates with the standard deviation of the integral stress area.

Fatigue reinforcement during repainting for two motorway bridges

A large number of metallic structures have already reached or even exceeded the limit of their initial design fatigue life. All this paper reports and investigates about the various works carried out in order to extend the fatigue life of existing motorway bridges located in the north of France.The studied bridge is composed with two independent decks of a French motorway carrying heavy traffics for 40 years. The slabs of both bridge decks are very fine: steel-concrete composite slabs associating 8 mm steel plate and 100 mm concrete layer according to the original design of Charles Brignon, who was also a pioneer in France of modern fatigue design for bridges, using rounded gussets to attach transverse beams, which reduces the stress concentration factor at the crossing point of the flanges.The structures of the bridges were recently strengthened to extend their fatigue resistance by three different methods which are:Addition of a continuous welded steel plate inclined outside of the edge girders to increase the safety at the Ultimate Limit State (ULS) and the robustness in fatigue of the bottom steel flanges. The inclined additional plate supported by the webs and the free bottom flangesPost weld treatment with Tungsten Inert Gas (TIG) dressing of the welded cross-beams connection gussets were performed.Bonding of CFRP carbon fibres on the bottom steel flanges of the main beams at the extremities of the welded existing cover-plates. These fibres need a high elastic modulus of 400 GPa.The interactions between both fatigue and anticorrosion life extension works are discussed. An innovative option of mixed complete-partial sandblasting methods was proved more economic than the usual complete sandblasting method and performed. The initial conception with two independent decks carrying the highway showed itself particularly relevant for the implementation of heavy repairs. The traffic deviation on the other deck allowed to work on each deck without stopping the traffic.

Fretting Fatigue Life Extension of A Ti-Alloy by Selecting Surface Morphologies of Counterpart Contact Pad

Fretting Fatigue Life Extension of A Ti-Alloy by Selecting Surface Morphologies of Counterpart Contact Pad

Determining of the fatigue resistance of a grid-reinforced asphalt concrete by using four point bending beam test

Flexible pavement structures are prone to cracking due to the fatigue and thermal influence, while a significant problem presents the appearance of reflective cracking in semi-rigid pavement structures or in rigid pavements overlaid with asphalt concrete. Reinforcement of asphalt concrete by using geogrids substantially slow down, if not stop crack propagation. The paper presents the results of an experimental study aimed at determining the influence of geogrid application on the fatigue resistance of asphalt concrete. Two double layered sets of specimens, with and without a grid, were tested (reinforced and unreinforced). Testing beams were cut from the slabs prepared by a non-standard method, using a one-ton roller. The thickness of the bottom layer of asphalt beams was 20 mm, the thickness of the upper layer was 30 mm, the width was 60 mm and the length 400 mm. Reinforced samples contained a high-strength adhesive geosynthetic grid between layers. The testing of fatigue resistance of asphalt mixtures was carried out by application of the four-point bending beam test, while two failure criteria were used to analyze the testing results. Presented research helped to quantify the extension of fatigue life that can be obtained by reinforcement, depending on the fatigue criteria.

Biomechanical properties of plate constructs for feline ilial fracture gap stabilization.

To determine the biomechanical properties of plating techniques for comminuted feline ilial fractures. Ex vivo study on 40 paired feline hemipelves. Forty paired fresh-frozen hemipelves that had been collected from 20 cats aged 2-6 years and weighing 4.0-5.5 kg. A transverse 3-mm gap was created in each ilium. Hemipelves were fixed with one of the following methods (n = 10 per group): (1) a dorsal plate and nonlocking screws, (2) a lateral plate and nonlocking screws, (3) a lateral plate and locking screws, or (4) a lateral and dorsal locking compression plate using nonlocking screws. Each specimen was subjected to incremental, sinusoidal cyclic loading until failure, defined as 10-mm displacement. The initial stiffness and number of cycles required to reach 1-, 2-, 5-, and 10-mm axial displacement were statistically analyzed. The initial stiffness and number of cycles to failure were higher in specimens fixed with double nonlocking plates than in all other fixations (P < .05) except specimens fixed with lateral locking plate at 10-mm displacement (P = .44). Locking implants withstood more cycles to 5- (P < .05) and 10-mm (P < .05) displacement compared with other single-plate nonlocking groups. Screw loosening occurred only in the 3 nonlocking fixations. Double plating improved stiffness and resistance to failure of comminuted feline ilial fracture constructs compared with all other fixations. Single locking plates produced superior constructs compared with single nonlocking constructs. Locking implants are recommended to repair comminuted feline ilial fractures for their extended fatigue life and resistance to screw loosening. Orthogonal plating offers a strong nonlocking alternative.

Analysis of the single overload effect on fatigue crack growth in AA 2024-T3 plates repaired with composite patch

In this study, the effect of a single overload on the fatigue life of cracked aluminum 2024-T3 alloy repaired with a bonded composite patch were studied. Special consideration was given to the load history, sequence of the bonded patch repair and the overload application (repair-overload and overload-repair). The two cases were compared to a reference case involving a single overload on unrepaired specimen. Fatigue tests were performed on V-notched repaired and unrepaired specimens naturally pre-cracked under constant amplitude loading. Fractographic analysis on the failed specimens was carried out to analyze the obtained fatigue results. It was found that when the crack is repaired before the overload application, the retardation effect due to the overload is attenuated by the patch. Nonetheless, when the patch precedes the overload, very high fatigue life extension of about 38.9 times is observed due to a cumulative retardation effect of the two events (patching and overload).

Effect of self-healing on fatigue of engineered cementitious composites (ECCs)

This paper presents a pioneer study on the effects of self-healing on the flexural fatigue performance of engineered cementitious composites (ECCs), a unique class of high-performance fiber-reinforced cementitious composites (HPFRCC) exhibiting tensile strain-hardening behavior with tensile strain capacity in excess of 3% with only 2% or less fiber content by volume. Results show that self-healing greatly extends the fatigue life of ECC because water/dry conditioning not only heals the matrix cracks but also recovers the fiber/matrix interfacial bonds which leads to increased fiber-bridging strength. ECC fatigue life increases with increasing fatigue pre-damage level because higher fatigue pre-loading cycles together with water/dry conditioning troubleshoots more potential failure planes. Repeated healing at lower pre-damage level is feasible and significantly extends fatigue life of ECC.

Retrofit against fatigue cracking at web penetration details with a slot in steel girder

Fatigue cracking in steel girder web penetration details is so dangerous that it can break steel girders. The purpose of this study is to clarify fatigue behavior of web penetration details with cross beam flange welded to upper or lower surface of a slot by fillet welding, and verify the effect of retrofitting methods against fatigue cracking in web penetration details. As a result, fatigue cracks were initiated at the web-side toe of the fillet weld along cross beam lower flange edges both in upper and lower welded specimen. In the upper welded specimen, fatigue cracks were propagated into the web plate diagonally upward almost perpendicularly to the maximum principal stress direction. Whereas in the lower welded specimen, fatigue cracks were propagated into the web plate vertically downward faster than in the case of the upper welded specimen, and broke the lower flange. The fatigue life of the lower welded specimen was shorter than that of the upper welded specimen. As a preventive measure, weld toe grinding can extend fatigue life a little, but cannot prevent crack initiation perfectly. As a post cracking measure, a stop hole can stop crack propagation of long cracks, whereas crack removal and shaping up method by grinding can prevent crack propagation of short cracks.

Durability of self-healing dental composites: A comparison of performance under monotonic and cyclic loading

Durability is an important quality of dental restorative materials, and the ability to autonomously heal damage incurred during their oral function is highly desirable. ObjectiveThe objective was to evaluate the improvement in durability of self-healing dental composites (SHDCs) in terms of their resistance to fracture and capacity for healing of damage under monotonic and cyclic loading. MethodsSHDCs were prepared by incorporating dental resin composites with microcapsules containing healing liquid. Control specimens with the same mass fraction (5% and 25%) of microcapsules filled with water were also evaluated. Two sets of SHDCs were distinguished by the silane coupling agents that functionalized and bonded the microcapsules to resin network. One set used a methacrylate silane (MA-silane) that connected resin network through covalent bonds, and the other used a H-bonding forming hydroxyl silane (OH-silane). The fatigue crack growth resistance was assessed in terms of the threshold stress intensity range and the conventional Paris Law parameters. Cyclic loading was conducted at 5 Hz with maximum cyclic load ranged between approximately 1 N and 5 N. The efficiency of the autonomous healing was determined per the recovering of the fracture toughness and the extension of fatigue life. ResultsThe SHDCs with 5 wt% of healing microcapsules exhibited a larger fracture toughness than those with 25 wt% microcapsules. MA-silane SHDCs had approximately five times more responsive microcapsules triggered by fracturing of the composites. Consequently, the MA-silane SHDCs with 5 wt% of microcapsules achieved the best performance in terms of fracture toughness and healing efficiency. In regards to the fatigue crack growth behavior, there was a significant increase in the resistance to fatigue crack growth and 580 ± 15% improvement in the fatigue life. SignificanceStrong silanization is vital in SHDCs to simultaneously achieve clinically applicable mechanical performance and substantial healing capability. Moreover, the evaluation of self-healing under cyclic loading is a promising tool in quantifying the degree of fracture-induced healing.

Site-specific controller design for monopile offshore wind turbines

The fatigue life of offshore wind turbine (OWT) support structures is sensitive to variations in site-specific conditions such as the water depth and soil properties. Site condtions may vary significantly within a wind farm, and they may change throughout the lifetime of the OWT. This paper analyses how control strategies for fatigue life extension can compensate for differing fatigue loads due to varying site conditions. Control strategies applicable for both power production and idling situations are analysed, and methodology to reduce undesirable side-effects is proposed. The design case is a 10 MW monopile OWT located in 30 m water depth at the Dogger Bank in the North Sea, and results are based on time-domain simulations performed using an aero-hydro-servo-elastic simulation tool. The results show that, when all the investigated control strategies are utilized, a fatigue damage reduction over the 20-year lifetime of approximately 50% is possible. Furthermore, it is shown that adverse side-effects such as wear of pitch actuators and fluctuations in the power output can be significantly reduced by limiting the use of control strategies to some predefined situations. With only moderate cost to other system components, the control system is able to compensate for 20% variation in soil stiffness, and 5% (1.5 m) variation in water depth.

The effects of laser shock peening scanning patterns on residual stress distribution and fatigue life of AA2024 aluminium alloy

Laser shock peening (LSP) utilizes the action of laser induced plasma to cause plastic deformation in metallic components. As a result, compressive residual stress is induced on the surface to improve its fatigue life. Our contribution in this paper is to investigate the effects of LSP scanning patterns on residual stress distribution and fatigue performance in Al 2024-T351 specimens. We use a 3D finite element analysis to simulate the residual stress distribution and experiment to establish correlation between the residual stress distribution and fatigue performance. We employ X-Y scanning pattern, Y-X scanning pattern and L-Spiral scanning pattern as the advancing scanning methods in LSP dog-bone specimens. Our results show that, tensile residual stresses are sandwiched between two compressive residual stresses at the front and back surfaces of the specimen. We therefore, observe some variations in residual stress distribution at the laser affected zone. In which case, the L-Spiral scanning pattern produces favorable residual stress distribution and considers as relatively more efficient to extend fatigue performance. We further observe that the extended fatigue life of 287% at 103 MPa shows that the LSP advancing scanning pattern has a great impact on residual stress and fatigue life respectively.

Fatigue performance test on inclined central cracked steel plates repaired with CFRP strand sheets

An experimental study was conducted on central-cracked steel plates with different crack inclination angles that had been repaired with carbon fiber reinforced polymer (CFRP) strand sheets. Initial cracks with five different inclined angles (0–60°) were artificially induced from a central hole of steel plates. All the cracks had the same projection length, running perpendicular to the loading axis. Crack lengths, crack trajectories, failure modes and the effects of different repair configurations were investigated, and fatigue crack propagation lives were compared. A typical adhesive failure mode, oblique compression failure in the adhesive layer, was observed. A conclusion was drawn that fatigue life was directly determined by the projection length, while the projection length was calculated from the initial crack length and inclination angle. The average fatigue life extension ratio of specimens with single-sided repairs was 1.44; that of specimens with double-sided repairs was 3.03.

Shape Optimization of Passenger Vehicle Wheel on Fatigue Failure

Based on the basic passenger vehicle steel wheel shape design, revised design was proposed to solve the common fatigue failure problems. Both basic and optimized design were modeled using Abaqus 3D software to calculate the stress and strain contours in the case of bending load and radial load. Consequently, fatigue life contours of models for each load case were calculated with the help of Brown-Miller biaxial fatigue theory. It was shown that, at the condition of unchanged wheel material and number of disc vents, optimized rim profile can extend fatigue life index of disc by 73.8% and 58.5% under bending load and radial load respectively. Meanwhile, the fatigue life index of rim could be extended by 17.5% and 134.4% under bending load and radial load respectively.

Prediction of Bending Fatigue Life of Cracked Out-of-Plane Gusset Joint Repaired by CFRP Plates

Carbon fiber reinforced polymer (CFRP), plates bonding repair method is one of the simple repair methods for cracked steel structures. In this study, the influence of width of CFRP plates on bending fatigue life of out-of-plane gusset joint strengthened with CFRP plates was investigated from the experimental and numerical point of view. In the bending fatigue test of cracked out-of-plane gusset joint strengthened with CFRP plates, the effect of width of CFRP plates on crack growth life was clarified experimentally. Namely, it was revealed that the crack growth life becomes larger with increasing the width of CFRP plates. In the numerical approach, the stress intensity factor (SIF) at the surface point of a semi-elliptical surface crack was estimated based on the linear fracture mechanics. Furthermore, the extended fatigue life of cracked out-of-plane gusset joint strengthened with CFRP plates was evaluated by using the estimated SIF at the surface point and the empirical formula of the aspect ratio of semi-elliptical crack. As the results of numerical analysis, the estimated fatigue life of the specimen strengthened with CFRP plates showed the good agreement with the test results.

A Finite Element and Experimental Analysis of Composite T-Joints Used in Wind Turbine Blades

In this study, a finite element failure model was created using ABAQUS to determine the location where delamination is initiated and its subsequent propagation. The effect of fibre-reinforced structures on delamination behaviour was studied. The composite T-joints were made of glass fabric infused with epoxy resin using a vacuum assisted resin transfer moulding technique. The veil layer and 3D weave techniques were employed to improve the properties in the through-thickness direction that can delay or prevent delamination when in service. All the pull-out tensile tests were conducted in an Instron testing machine using a specially designed test fixture. The 3D weave T-joints were found to have improved performance under both static and fatigue loading. Increasing the static properties increases fatigue life performance. The location for the through-thickness reinforcement plays an important role in extending fatigue life of the T-joints.

Effect of magnetic field on crystallographic orientation for stainless steel 316L laser-MIG hybrid welds and its strengthening mechanism on fatigue resistance

In order to extend fatigue life of structures, traditional methods involving controlled creation of boundaries and local microstructure discontinuities are used to obstruct dislocation motion and change the crack propagation path during fatigue service. However, such strategies inevitably sacrifice ductility due to the reduced ability to accommodate dislocations. Herein, an external magnetic field is used to assist laser-MIG hybrid welding for 316L austenitic stainless steel. Fatigue crack propagation rate is decreased by 33% at f = 0.1 Hz and 12% at f = 1 Hz in air. The external magnetic field optimizes fatigue resistance of 316L welds from two aspects: diverse orientation distributions of ferrite and non-K-S orientation relationship (OR) between ferrite (δ) and austenite (γ). Diverse orientation distributions of δ induced by magnetic field activate different slip systems, promoting crack deflection in γ grain interior during cyclic deformation. Semi-coherent interface between δ-γ with non- Kurdjumov-Sachs (K-S) OR not only impedes transmission of the dislocation slip but also accommodates a considerable amount of plastic strain by continual loss of coherency. Thus, external magnetic field assisted welding of 316L extends fatigue life by increasing strength and ductility, and promoting crack deflection in grain interior.

Fatigue retrofitting of cracked steel beams with CFRP laminates

In recent years, carbon fiber-reinforced polymer (CFRP) externally bonded to steel members has been considered an effective technique for fatigue crack repair of steel components. This paper investigated the fatigue behavior of six cracked steel beams strengthened using CFRP laminates via two approaches, i.e., sole adhesive bonding and adhesive bonding together with mechanical anchorage. Two types of epoxies were selected to compare the effectiveness. The tensile flange of the steel beam was artificially cut to simulate an initial crack. A digital image correlation (DIC) system was adopted to detect strain distribution along the retrofitting patches. Different failure modes were observed for specimens with different retrofitting schemes. Test results showed that, in comparison with control specimens with sole adhesive bonding, application of mechanical anchorage significantly retarded crack propagation and extended fatigue life of the steel beams attributed to the pronounced frictional bond. The crack mouth opening displacement (CMOD) was also considerably reduced. The displacement fields obtained from the DIC recorded at specific crack lengths were analyzed to provide information on strain distribution along the CFRP laminates and corresponding bond-slip relationship. This study extends the understanding of fatigue repair for steel beams and provides useful suggestions for the strengthening method.

Influence of ring deformation on the dynamic characteristics of a roller bearing in clearance fit with housing

Modern bearings have thin-walled rings that can easily produce elastic structural deformations. Such deformation will induce significant changes on the load distribution of the bearing, thereby significantly affecting the dynamic characteristics and fatigue life. In this study, an improved quasi-dynamic model was established to investigate the influence of the ring deformation on a thin-walled roller bearing in clearance fit with housing. The dynamic performance of the bearing was studied. Results indicated that a small fit clearance between the outer ring and the housing could optimize load distribution, improve lubrication, and extend fatigue life. However, the clearance could not exceed a certain critical value; otherwise, it would have negative influence on load distribution and fatigue life.

Design of Wing Spar Cross Section for Optimum Fatigue Life

Aircraft structure is the most obvious example where functional requirements demand light weight and strong structures. Shape and sizing optimization are being increasingly used nowadays for designing lightweight structural components. The aim of this paper is to present optimization of I-section integral wing spar made of aluminum 2024-T3. The efficient design, based on optimum fatigue life, was achieved using Extended Finite Element Method (XFEM) and its ability to simulate crack growth in complex geometry. The computations were carried out in Morfeo/Crack for Abaqus software which relies on the implementation of XFEM. Shape optimization of the aircraft wing spar beam was conducted by comparing the fatigue crack growth lives for different cross section shapes, but constant cross section area of the spar. The analysis revealed that XFEM is efficient tool for complex three-dimensional configurations optimization where extended fatigue life is one of the most important objectives.