Fatigue Loading Conditions Research Articles

Cohesive analysis of a 3D benchmark for delamination growth under quasi‐static and fatigue loading conditions

AbstractThis paper evaluates the capabilities of the recently developed CF20 cohesive fatigue model, which can predict crack initiation as well as the rates of crack propagation by relying on intrinsic relationships between a stress‐life diagram and its corresponding Paris law. The model is validated here using a partially reinforced double cantilever beam (R‐DCB) benchmark proposed in literature. The two parameters needed for the CF20 cohesive fatigue model were obtained by performing preliminary analyses of a conventional DCB. The analysis results indicate that the CF20 cohesive fatigue model can accurately reproduce the complex evolution of the delamination observed in the R‐DCB.

Verification of urban light rail transit (LRT) bogie frame structure design lifetime under variable fatigue loads

The bogie frame is the main structure of the train that supports the train's loads during its operation. These structures are subject to fatigue testing to ensure their design life is up to the required standards. The urban light rail transit (LRT) bogie frame used in the Greater Jakarta area is newly designed and manufactured by a commercial railway rolling stock manufacturer. The design lifetime of this newly designed bogie frame structure under various fatigue load conditions is verified experimentally by fatigue testing. Testing and evaluations were conducted following the EN 13749 standard and VDV recommendation. The fatigue test of the urban LRT bogie frame structure was carried out in the test hall of the BPPT Structural Strength Technology Center (B2TKS) using a combination of seven loadings. The bogie frame was subjected to two variable types of fatigue loads, namely driving in curves and passing points (switches), with 2,000,000 cycles, 4,000,000 cycles, and 6,000,000 cycles of fatigue loadings. The parameter measured on the bogie frame structure is the strain value during the test using a dynamic data logger. The stress values analyzed are the average stress and the stress amplitude and then plotted on the maximum and minimum stress curve. The bogie structure is inspected by the non-destructive test method in all areas of its welded joints. This inspection is carried out before and after the test to ensure whether there are cracks caused by fatigue loads. The results of the fatigue test on the bogie structure under the variable fatigue load conditions show that the maximum stress value of 91.71 MPa at 1,500,000 cycles, that occurs during the test, does not exceed the fatigue limit of the material, and there are no cracks in the structure after the test is carried out for up to 6,000,000 cycles.

A novel fatigue crack angle quantitative monitoring method based on rotating alternating current field measurement

Under complicated fatigue loading conditions, cracks initiate and grow in the arbitrary direction from corrosion pits in the aerospace equipment. The monitoring of crack propagation angle is very important for the safety assessment of the aerospace equipment, which is still a challenge by the conventional structural health monitoring (SHM) method. In this paper, a novel crack angle quantitative monitoring method is presented based on the rotating alternating current field measurement (RACFM). A theoretical model of the crack angle measuring method is established to analyze the perturbation principle of the induced electromagnetic field. The relationships between the angle, length and depth of the crack and the Bz signal are analyzed. The probe and testing system are established, and experiments are carried out. The results show that the phase of the Bz signal has a linear relationship with the crack angle for the same crack, and the amplitude of the Bz signal can correct the crack angle for the different cracks. The angle of fatigue cracks can be quantitatively measured by the Bz phase difference method based on the RACFM.

Biomechanical Comparison of 2 Double Plating Methods in a Coronal Fracture Model of Bicondylar Tibial Plateau Fractures.

Because management of bicondylar tibial plateau fractures are complicated even for expert surgeons, with using a coronal fracture model, we aimed to compare 2 kinds of double locked plating techniques that consisted of the lateral locking plate and the medial locking plate inserted medial anteriorly (MA-ly) or medial posteriorly (MP-ly). Fourteen fresh-frozen tibias stabilized with the MA or MP methods were allocated into 2 groups with similar bone mineral density values. Implanted samples were tested under incremental fatigue loading conditions using a customized load applicator. An optical motion tracking system was used to assess relative displacements and rotations of fracture fragments during loading. Static and dynamic global stiffness, failure load, failure cycles, as well as movements of fracture fragments were measured. There were no significant differences between the 2 fixation methods regarding global stiffness, failure load, or failure cycles (P = 0.67-0.98, depending on the parameter). The kinematic evaluations, however, revealed that different positions of the medial locking plates altered the directions of movements for the medial-anterior or medial-posterior fracture segments. The mechanical stability of tibia-implant constructs fixed with the double plating methods was not remarkably affected by the location of the medial locking plate. Depending on clinical conditions and surgeons' preferences, bicondylar tibial plateau fractures can be managed with either MA or MP methods.

On [formula omitted] characterization of elastic–plastic crack-tip fields under fatigue loading conditions

In the present paper, the applicability of the ΔJ-integral is computationally investigated under the Masing postulate. It confirms that the path-dependency of ΔJ is more severe than the J-integral under incremental plasticity. ΔJ from loading is up to 20% larger than ΔJ from unloading, which is independent of loading amplitudes but due to non-proportional stress variations. The stress range field can be characterized by ΔJ and shows an analogous mathematical structure as the known HRR solution, but not for the strain field under fatigue loading. The ΔJ dominant zone is much smaller than the HRR zone and further decreases with applied loading.

Static and Fatigue Load Bearing Investigation on Porous Structure Titanium Additively Manufactured Anterior Cervical Cages.

This study investigates the static and fatigue behavior of porous and conventional anterior cervical cages. Porous structure titanium anterior cervical cages were manufactured using direct selective laser sintering technique. Four different types of cervical cages were designed and manufactured, among which three designs consist of porous structure (type 1, type 2, and type 3) and manufactured using metal 3D printing. Remaining one design (type 4) was manufactured using conventional machining and did not consist any porous structure. All types of manufactured cages were tested in compression under static and fatigue loading conditions as per ASTM F2077 standard. Static and fatigue subsidence testing was performed using ASTM F2267 standard. Static compression testing results of type 1 and type 4 cages reported higher yield load when compared to the type 2 and type 3 cages. Static subsidence testing results reported almost 11% less subsidence rate for additively manufactured cages than the conventional cages. Fatigue subsidence testing results showed that type 2 and type 3 cages can withstood approximately 21% higher number of cycles before subsidence as compare to the type 1 and type 4 cages. During fatigue testing, all the cages design survived 5 million cycles at the 3000 N loading. For 6000 N and 8000 N, loading rate type 2 and type 3 cages showed lower fatigue life when compared to other cages design. Since fatigue life of type 2 and type 3 cage designs were reported lower than other cages design, it is concluded that the performance of the additively manufactured porous cages can be significantly varied based upon the cage design features.

Metal/Carbon-Fiber Hybrid Composites—Damage Evolution and Monitoring of Isothermal Fatigue at Low and Elevated Temperatures

Carbon-fiber-reinforced polymers (CFRPs) are the standard lightweight composite material for structural applications in aviation. The addition of metallic fibers to CFRPs to form metal/carbon-fiber hybrid composites (MCFRPs) has been shown to improve the elastic and plastic properties and to enable a non-destructive method for structural health monitoring over the material’s service life. In this paper, the results from the fatigue experiments on these hybrid composites at −55, 25 and 120 °C are discussed. Multidirectional CFRP and MCFRP laminates, fabricated using the autoclave method, were tested and compared under different fatigue loading conditions, while being simultaneously monitored for temperature and electrical resistance. Magnetic phase measurements were additionally carried out for the chosen metastable austenitic steel fibers in the MCFRPs. The results show that the improved ductility of the hybrid composite due to the presence of the steel fibers leads to better performance under fatigue loads and a less-brittle failure behavior. Based on the chemical composition of the metastable austenitic steel fibers, a temperature and plastic deformation-dependent phase transformation was observed, which could potentially lead to a method for non-destructive structural health monitoring of the hybrid composite over its service life.

Static and Fatigue Debond Resistance between the Composite Facesheet and Al Cores under Mode-1 in Sandwich Beams

The debonding toughness between unidirectional glass fiber reinforced polymer face sheets and cellularic cores of sandwich structures is experimentally measured under static and fatigue loading conditions. The effect of various core geometries, such as regular honeycomb and closed-cell foams of two relative densities on the adhesive interfacial toughness is explored using the single cantilever beam (SCB) testing method. The steady-state crack growth measurements are used to plot the Paris curves. The uniformity of adhesive filleting and the crack path was found to affect the interfacial toughness. The static Mode-1 interfacial toughness of high-density foam cores was witnessed to be maximal, followed by low-density honeycomb, high-density honeycomb, and low-density foam core. Similarly, the fatigue behavior of the low-density honeycomb core has the lowest crack growth rates compared to the other samples, primarily due to uniform adhesive filleting.

Bolt insertion damage and mechanical behaviors investigation of CFRP/CFRP interference fit bolted joints

Interference fit has advantages in improving fatigue behaviors of composite bolted joints; however, interference fit bolt insertion tends to cause damages in laminates weakening joint mechanical properties. Therefore, an experimental study was conducted to investigate bolt insertion damages of Carbon Fiber Reinforced Polymer (CFRP)/CFRP interference fit bolted joints. Mechanical behaviors of joints were also evaluated experimentally under both quasi-static loads and cyclic loads. Scanning Electron Microscope (SEM) and high-resolution X-ray micro-CT scan were used to examine micro damages in laminates. Damage and failure behaviors of joints were characterized. The results demonstrated that the hole entrance in upper laminate and the laminate boundary near the hole wall were the most critical regions for damages during bolt insertions. However, the influence of those damages on quasi-static failure loads and fatigue failure modes of joints was minimal. Delamination and matrix cracking occurred first in laminates following fiber and matrix fracture in quasi-static tensile tests. Interference fit could improve the fatigue resistance of the laminate hole; however, the bolt seemed to suffer a more critical local fatigue loading condition. This paper can contribute to composite structure designs, especially in understanding damage and failure behaviors of composite bolted joints.

On the static and fatigue life of nano‐reinforced Al‐GFRP bonded joints under different dispersion treatments

AbstractAs bonded components of engineering structures inevitably experience fatigue loading conditions during their service life, it is significant to find ways to enhance their endurance. Also, adding nanoparticles to the adhesive can be introduced as an approach to design more durable bonded joints. In this way, it is important to determine a nanoparticle dispersion method which can lead to the longest fatigue life. Accordingly, this paper aims to investigate the influence of various nanoparticle dispersion treatments on static and fatigue responses of aluminum‐to‐composite bonded joints under four‐point bending. For this purpose, four dispersion techniques of mechanical stirring, bath‐sonication, low‐power and high‐power probe‐sonication were used to disperse nanoparticles into the adhesive. Also, four specimen groups of neat (with no additive particles), graphene nano platelet (GNP)‐reinforced, nano silica‐reinforced, and hybrid‐reinforced (including GNP + nano silica) were prepared. Results indicated that, for the joints including GNPs, the longest fatigue life was achieved for the specimens prepared by low‐power sonication method, while for hybrid‐type reinforcements, using high‐power method led to the highest endurance. Moreover, for 0.5 and 1.0 wt.% of nano silica, the endurance of bath‐sonicated specimens was 12.77% and 9.28% longer than low and high‐power probe‐sonicated joints, respectively. Using the backface strain (BFS) measurement technique, the influence of dispersion methods was studied on extending the crack initiation life (CIL) of each joint group. It was revealed that, for 0.5 wt.% and 1.0 wt.% of nano silica, the CIL is independent of the dispersion method, while, for 1.5 wt.%, probe sonicated specimens had the longest CIL. The microstructural characterization of each dispersion method and major reinforcing mechanisms were interpreted by scanning electron microscopy (SEM). For future researches, using X‐ray tomography and health monitoring methods is recommended to detect size and location of damage for nanoparticle reinforced composite structures.

Flexible riser tensile armour stress assessment in the bend stiffener region

The flexible riser within the bend stiffener in the top connection is a structurally vulnerable region for extreme and fatigue loads as it is subjected to large dynamic tensile forces and curvatures. This paper presents a nonlinear finite element model (FEM) for the flexible riser with bend stiffener taking into account interlayer friction and riser-bend stiffener contact interaction subjected to combined axisymmetric and bending loads. A case study with typical extreme and fatigue load cases is carried out to assess the curvature variation and bend stiffener contact pressure influence in the riser tensile armours stresses. For the extreme load case, detailed finite element results such as contact pressures and friction, normal and transverse bending stresses are compared with a uniform curvature FEM and available analytical models. A fatigue load case is also performed, where the stress amplitudes and cumulative fatigue damages for the internal tensile armour wires are compared with a uniform curvature FEM and analytical models. From the case study results under extreme loading, it can be observed that the model with bend stiffener interaction predicts larger stresses in comparison to the uniform curvature FEM and analytical models. Under typical fatigue loading conditions, significant differences in the cumulative damage are observed between the models, highlighting the importance of an accurate modeling for lifetime integrity assessment in a region of highly variable riser curvature.

Cohesive zone modeling of fatigue crack propagation in slab track interface under cyclic temperature load

The ballasless track has been massively applied nowadays, while the interface damage evolution of ballastless tracks is still rarely investigated. This work aims to investigate the damage evolution property of ballastless track under low-cycle fatigue load condition. The application of cyclic cohesive model as a key approach to simulated the damage evolution of ballastless track interface. Therefore, a low-cycle fatigue cohesive model (LCFC) that follows a bilinear law is established by combining and modifying existed cohesive models. The LCFC model is then implemented by a secondary development in Abaqus through implicit user-defined material subroutine (UMAT), and the validity of the model is verified though a double cantilever beam (DCB) model. To obtain the key parameters of the LCFC model for analyzing the fatigue damage of CRTS III (Chinese Railway Track System type III) ballastless track, the split tension test of specimen that is composed of the self-compacting concrete and the slab concrete is conducted, and the damage evolution process is captured through digital image correlation (DIC) technique. Further, a simulation model that involves the LCFC model is created for simulating and analyzing the interface fatigue damage of CRTS III- ballastless track under cyclic temperature gradient for the first time. The results show that the established LCFC model is suitable for studying the interface fatigue damage of CRTS III ballastless track under extreme temperature gradient cycles. The interface near the slab edge will be completely damaged within only several cycles when the cyclic temperature gradient is ± 61℃/m. The failure mode of the interface element is a kind of mixed mode failure which gradually changes from normal-dominated failure mode to shear-dominated failure mode with the increase of temperature gradient loading cycles.

Numerical simulation of damage and inelastic deformation of porous thermal barrier coatings system under high-temperature fatigue loading condition

Numerical simulation of damage and inelastic deformation of porous thermal barrier coatings system under high-temperature fatigue loading condition

Proposal of a Thermal Fatigue Damage Evaluation Method for Ceramic Thermal Barrier Coating Systems.

In recent years, the application of thermal barrier coatings to the components used in gas turbines for thermal power generation has become indispensable as higher inlet temperatures for requirement to further improve the efficiency of thermal power generation. Thermal barrier coatings, generally referred to as TBCs, consist of a top coat, which is a thermal barrier layer, and a bond coat, which improves the adhesion strength between the TC and the base material. As the development of new thermal barrier materials is progressed, it is also demanded to clarify the stress and damage behavior of TBCs under the complex thermal fatigue loading conditions in actual gas turbine operation. In this study, comprehensive finite element analysis is performed on a TBC system under cyclic thermal fatigue loading conditions with temperature and stress variations to clarify the deformation behavior and crack initiation life of the TBC, and to develop a fatigue life evaluation method for TBCs.

Experimental Study on the Degradation of Bonding Behavior between Reinforcing Bars and Concrete after Corrosion and Fatigue Damage

In marine environments, the durability of reinforced concrete structures such as bridges, which suffer from the coupled effects of corrosion and fatigue damage, is significantly reduced. Fatigue loading can result in severe deterioration of the bonds between reinforcing steel bars and the surrounding concrete, particularly when reinforcing bars are corroded. Uniaxial tension testing was conducted under static loading and fatigue loading conditions to investigate the bonding characteristics between corroded reinforcing bars and concrete. An electrolyte corrosion technique was used to accelerate steel corrosion. The results show that the bond strength was reduced under fatigue loading, although the concrete did not crack. Therefore, fatigue loading has negative effects on the bond strength between corroded steel bars and concrete. The effects of corrosion cracking on bond strength become more pronounced after corrosion cracking appears along the main reinforcing bars. When the average width of cracking along main reinforcing bars exceeds 3 mm, the bonding properties deteriorate rapidly based on the effects of corrosion cracking, whereas fatigue loading exhibits no additional effects on bond strength.

Investigating on the influence of multi‐walled carbon nanotube and graphene nanoplatelet additives on residual strength of bonded joints subjected to partial fatigue loading

AbstractAdhesively bonded joints in engineering structures, especially automotive body structures, inevitably experience long or limited fatigue loading conditions. In this article, the shear strength of steel bonded lap joints reinforced by multi‐walled carbon nanotube (MWCNT) and graphene nanoplatelet (GNP) were evaluated after being subjected to limited fatigue loading cycles. For this purpose, three specimen groups of neat, MWCNT, and GNP‐reinforced joints (with 0.2, 0.5, 1.0, and 2.0 wt% contents) were prepared. After performing the static lap shear tests, fatigue testing of each specimen group was conducted under a maximum fatigue load of 50% of its average static failure load until complete separation of the substrates. Accordingly, limited fatigue loadings were applied to each specimen group for 30%, 50%, and 70% of its total fatigue life, and subsequently, a second‐round static testing was performed to evaluate the residual strength of the joints. The results revealed that the maximum static shear strength improvements of MWCNT and GNP‐reinforced joints were associated with incorporating 1.0 and 0.5 wt% particle contents. Moreover, for 0.2 and 0.5 wt% contents, the static strength of the joints reinforced by GNPs was 3.4% and 3.9% greater than that of MWCNTs reinforced joints. On the contrary, for 1.0 and 2.0 wt% filler contents, specimens including MWCNTs had 9.1% and 18.6% higher strength than GNP‐reinforced ones. The highest fatigue life enhancements of 34.1% and 31% were obtained by adding 1.0 and 0.5 wt.% of MWCNT and GNP particles to the adhesive, respectively. It was concluded that, for 0.2 and 0.5 wt% contents, nanoparticle type did not have a noticeable effect on the residual strength of the joints after being subjected to fatigue loading for 30% of its total life. The specimens subjected to 50% and 70% of their total fatigue life indicated a minimum loss in the shear strength for the joints reinforced by dispersing 1.0 wt% of MWCNTs.

Mechanistic model for prediction of the residual tensile strength of FRP wires

In-service cables made of fiber-reinforced polymer (FRP) wires experience repeated loading during their service life, which gradually degrades their load-carrying capacity. This paper proposes a mechanistic model to predict the residual tensile strength of the FRP wire, based on the state of damage inside the FRP wire as a function of the loading cycle. The mechanistic residual strength model developed here is an extension of the progressive fatigue damage model (PFDM) proposed recently by the authors. The main failure mode in the FRP wire in the PFDM is fiber breaks, allowing for the state of damage in the FRP wire to be determined for a specified maximum fatigue stress level and number of cycles. These results are then used as the input to calculate the residual strength of the FRP wire. Results indicate that the model can predict the residual strength of FRP wires well compared with the experimental data that has appeared in the literature. The proposed model is shown to provide a better understanding of the influence of the fatigue loading conditions on the residual tensile behavior of FRP wires, specifically, the effect on the residual stress-strain response, the kinetics of fiber element breaks, and the critical damage cluster. Moreover, the proposed model allows validation of the adequacy of safety factors typically adopted by designers to consider the degradation in strength of FRP cables, which is critical for the successful design and realizing the full potential of FRP cables.

A robust cumulative damage approach for the simulation of delamination under cyclic loading conditions

Interlaminar damage evolution is one of the topics of main interest for composite materials research. An excellent level of knowledge has been achieved in the frame of the studies on delaminations propagation under static loading conditions (as a consequence of low velocity impacts). On the contrary, delaminations evolution under cyclic loading conditions is still an open challenge for the scientific community. Since expensive experimental campaigns are needed to assess the fatigue behaviour of Carbon Fibre Reinforced Polymers (CFRPs), the development and implementation of robust and efficient computational finite elements methodologies has become relevant. In this framework, a robust numerical finite element procedure able to simulate the fatigue driven delamination growth is proposed in this paper. A Paris-law based fatigue tool has been implemented in the commercial Finite Element Code ANSYS MECHANICAL via the Ansys Parametric Design Language (APDL) together with a static delamination procedure (named SMXB) previously developed by the authors. The proposed numerical procedure, based on the Virtual Crack Closure Technique (VCCT), is characterised by load step and element size insensitivity within non-linear incremental analyses. The proposed fatigue procedure, named FT-SMXB, has been preliminary validated at coupon level, by comparing the numerical results to literature experimental data on a unidirectional graphite/epoxy Double Cantilever Beam (DCB) specimen. Actually, the excellent agreement between the achieved numerical results and the literature experimental benchmark data proves the accuracy and the potential of the proposed methodology. Subsequently, a numerical application on a composite stiffened panel with skin stringer debonding under compressive-compressive fatigue loading conditions, has been performed. The Excellent agreement found between the obtained numerical results and available experimental literature data, for this application, in terms of number of cycles to failure and debonding propagation as a function of number of cycles, proves that the use of the proposed numerical tool can be extended, with profit, to geometrically complex and more realistic fatigue test cases.

Fatigue and creep-fatigue crack growth in aviation turbine disk simulation models under variable amplitude loading

This study analyzes the simulation model configuration and its loading conditions in fatigue and subsequent crack growth tests. Following the simulation principles, a full-size 3D finite element analysis of a representative GTE turbine disc was performed. With the aim of reproducing the simulation model, the in-service critical zone of both the stress–strain state and damage accumulation is studied. Based on the parametric computations, the block type loading conditions of the simulation models are determined and verified. Experimental study of the surface crack growth rate in the simulation model under harmonic and block loading at elevated temperature is performed for the fatigue and creep-fatigue interaction conditions. To interpret the experimental data, we calculate the nonlinear stress intensity factor distributions along the semielliptical crack front for flaws growing on the inner surface of the hole in the simulation model using numerical procedures. It is found that in terms of the nonlinear fracture resistance parameters, the sections of the fatigue fracture diagrams form one common curve representing the previous and subsequent increasing stages of block loading in pure fatigue and creep-fatigue interaction loading conditions. The possibility of using the proposed simulation model, tested under variable amplitude loading, to assess the structural integrity of GTE turbine disks is discussed.

Correlation between Fractal Dimension and Areal Surface Parameters for Fracture Analysis after Bending-Torsion Fatigue

This paper investigates the fracture surface topography of two steel and aluminum alloys subject to bending-torsion fatigue loadings, as well as their susceptibility to fatigue performance and failure mechanisms. Using fracture surface topography data analysis, elements with different geometries were elaborated. A correlation between the fractal dimension, other selected parameters of surface topography such as areal Sx, and fatigue loading conditions was found. Distinctions in particular regions of cracks were also recognized through proving the correctness and universality of the total fracture surface method. The influence of fatigue loading parameters on the surface topography of fatigue fractures was demonstrated. For the analyzed cases, results show that the fractal dimension and standard surface topography parameters represent a correlation between them and loading conditions. As a single parameter, the appropriate loading ratio cannot be outright calculated with fractal dimension, but can be estimated with some approximation, taking into account additional assumptions.