Tension-tension Fatigue Research Articles

Effects of marine environment and fatigue pre-damage on the residual tensile properties of SFCBs

Steel-fiber reinforced polymer (FRP) composite bars (SFCBs) present a promising solution to corrosion issues associated with conventional reinforcement in marine environments, offering a viable alternative. While the performance of SFCBs under marine environments or fatigue loading is well understood, their behavior under combined conditions remains unreported. This study conducted tension-tension fatigue tests on seawater-immersed SFCBs to assess the effects of fatigue loading (at a stress level of 0.36), various aging temperatures (23°C, 40°C, and 60°C), and durations (30, 180, and 360 days) on their residual properties. Microscopic analysis was used to investigate the degradation mechanisms due to environmental aging. The results indicate that environmental aging deteriorates the GFRP matrix in the outer layer of SFCBs, with fatigue loading further causing matrix cracking and reducing the quasi-static tensile strength. The residual tensile strength of SFCBs decreases with increasing aging times or temperatures, while aging shows no significant effect on their fatigue life. Additionally, a model predicting the residual tensile strength of SFCBs based on damage accumulation was proposed. According to the model, tensile strength initially rapidly decreases with aging time and eventually stabilizes. These findings offer practical insights for the application of SFCBs in concrete structures.

Effect of manufacturing-induced microscopic surface defects on crack initiation and fatigue mechanisms in hot-dip galvanized steel

When designing against fatigue, the surface condition of steel is of utmost importance. Since hot-dip galvanizing is a surface treatment primarily aimed at preventing corrosion, it is critical to assess its influence on the fatigue properties of steel. Given the challenges in producing large structural galvanized steel without imperfections, it is crucial to understand the influence of manufacturing-induced microscopic surface defects on crack initiation and fatigue mechanisms in hot-dip galvanized steel. In this study, to gain such insights, tension-tension fatigue testing was conducted on hot-dip galvanized steel, followed by a detailed analysis of the crack initiation sites to determine any correlation with manufacturing-induced microscopic surface defects. It was observed that under low-cycle fatigue, pre-existing surface defects did not appear to have influenced fatigue mechanisms. However, under high-cycle fatigue, crack initiation sites exhibited evidence of pre-existing manufacturing induced defects which were significantly smaller than the total crack initiation area. This indicates that the defects could not immediately penetrate the substrate but expanded within the layer and only penetrated the substrate after reaching a critical size. Therefore, it is discovered that while manufacturing-induced defects may be challenging to eradicate entirely, those below a threshold size may not immediately evolve into a stable fatigue crack. Hence, if manufacturing induced defects are kept below a critical size, they may not significantly influence the transition of stage I crack into a stable stage II crack.

The synergistic effect of gradient nanostructure and residual stress induced by expansion deformation on the tension-tension fatigue performance of Ti6Al4V holes

A large number of titanium alloy structural components in aircraft are joined and assembled through holes. The complex service conditions pose high requirements for the fatigue performance of the holes. The study aims to use expansion deformation to induced gradient nanostructures and residual stress enhance the fatigue performance of Ti6Al4V holes. By studying the evolution process of the microstructure in titanium alloys, the formation mechanism of gradient nanostructures has been obtained, and their impact on various stages of fatigue fracture is analyzed. The results showed that the split sleeve cold expansion (CE) process caused severe plastic deformation on the surface of the hole, resulting in a large amount of LAB production and a transformation of the main texture components. Meanwhile, the gradient nanostructures are formed on the hole surface, which include nanocrystalline layer, superfine lath grain layer, lath grain layer and severe deformation layer. During the expansion deformation process, the slip of dislocations leads to the formation of high-density dislocation structures, which evolve into subgrain boundaries and then undergo dynamic recrystallization through subgrain rotation to achieve grain refinement. What's more, the degree of grain refinement, depth of gradient nanostructure, and residual stress increase as the expansion deformation intensifies. The depth of gradient nanostructure is 300 μm, and the maximum residual stress is −419 MPa, when the deformation is 5 %. The combined effect of gradient nanostructures and high residual compressive stress induced by expansion deformation has increased the total fatigue life and crack propagation life of CE-5 specimen by 2.9 and 3.4 times compared to NCE specimen. This work provides an effective method and theoretical analysis for improving the fatigue life of titanium alloy holes.

Hygrothermal aging effect on static and fatigue behavior of three-dimensional five-directional hybrid braided composites under tension

This paper attempts to study the hygrothermal aging effect on static and fatigue behavior of carbon/glass fiber three-dimensional five-directional (3D5D) hybrid braided composites under tension. Firstly, moisture absorption tests and micro-CT scanning analysis were conducted on the composite specimens. The results show that the moisture absorption behavior follows the Fickian diffusion law well and the aging damage is dominated by matrix cracks/pores and interfacial debonding which is more severe in the braiding glass fiber yarn and surrounding regions than the axial carbon fiber yarn and surrounding regions. Then tensile tests and fatigue tests were performed on the unaged and aged composite specimens. It was found that hygrothermal aging has significant detrimental influence on the static tensile and tension-tension fatigue behavior of the composites. From experimental observations and failure analysis, it was also concluded that hygrothermal aging has impact on the failure mechanisms of the composites under both static tensile and fatigue loading owing to the extensively existed aging damage and deteriorated material properties in the aged composites.

Effect of TiN monolithic and Ti/TiN multilayer coating on the fatigue behavior of titanium alloy under tension-tension

This paper investigates the fatigue failure mechanism of mono- and multilayer coatings on the fatigue performance of TC11 titanium alloy under tension-tension. The morphology, phase composition, mechanical properties were measured by scanning electron microscope, X-ray diffractometer and nanoindentation. Electron back scatter diffraction was employed to investigated the failure mechanism. Fatigue limits obtained of uncoated TC11, TC11 with TiN coating, TiN/Ti multilayer (ML-6, ML-3, ML-1) and after 1 × 107 cycles are 855 MPa, 550 MPa, 525 MPa, 500 MPa and 400 MPa. Under fatigue loading, the hard-coating/TC11 substrate experiences fatigue failure through coating cracking hastens the substrate's fatigue failure. EBSD analysis results indicate that the main slip system of TC11 titanium alloy under tension-tension fatigue load is α phase (10-10)[-12-10]. After 1 × 107 cycles at fatigue limits, the average dislocation density on the surface of the TC11 with TiN coating is lower than that of TC11. Due to the surface defect induced by coating preparation and high crack propagation velocity, the hard coating significantly deteriorates fatigue property of TC11 by reducing fatigue crack initiation period. Therefore, instead of approaching from the perspective of coating structure design to increase the fatigue crack propagation cycles, it is more effective to reduce the surface roughness of the coating and enhance the fatigue crack initiation cycles.

Microstructure analysis of carbon-fiber-reinforced polymer laminates subjected to self-heating and fatigue strengthening under tension-tension fatigue loading

In this study, tensile-tensile fatigue testing of unidirectional (UD) carbon-fiber-reinforced polymer (CFRP) prepreg laminates was performed. Two types of specimens (open-hole and unopened-hole ones) were examined at different stress levels to elucidate the impact of fatigue damage on their static strength at different numbers of cycles. The effect of internal heat generation during fatigue on the life of unopened-hole CFRP laminates was analyzed by observing the changes in the microstructure of specimens. The results revealed that the difference in fatigue life between unopened specimens with and without cooling at 70% ultimate tension stress (UTS) level was about 88 times. In the open-hole CFRP laminates, the residual tension strength (RTS) after increases and then decreases with the number of fatigue cycles, and the interface debonding was a significant cause of fatigue strengthening. Open-hole specimens were shown to withstand continuous cycling under severe loads at 5% above their ultimate stress levels. Therefore, the fatigue damage extension of the open-hole laminates can be slowed down under intensive cycling at high stress levels, ensuring good fatigue performance of the material.

A novel normalized fatigue progressive damage model for complete stress levels based on artificial neural network

The fatigue life model and fatigue stiffness degradation model are most crucial components in establishment of fatigue progressive damage model (FPDM). Based on numerous static and fatigue experiments of glass fiber reinforced polymer-matrix composites (GFRP) 3232A/Glass, a novel normalized FPDM for complete stress level based on artificial neural network (ANN) has been built: (1) By analyzing fatigue experimental data, the construction method of fatigue life model is deeply investigated. Furthermore, a normalized fatigue life model (NFLM) predicting fatigue life considering simultaneously tension–tension (T-T) fatigue and compression-compression (C–C) fatigue is developed, which has been validated by experimental data. It is expected to reduce experimental time and costs by nearly half; (2) To fully consider the influence of stress level and remove the constraint of the shape of stiffness degradation function, the traditional models are no longer suitable. Thus, a fatigue stiffness degradation model based on ANN (FSDM-ANN) has been constructed, which overcomes drawbacks of the traditional models. The results predicted by FSDM-ANN are consistent with fatigue data in different stress levels; (3) Due to distortion of the fatigue life prediction of extreme high-cycle and low-cycle, an empirical data extrapolation method is proposed here to construct a modified FSDM-ANN, which greatly improves prediction accuracy of stiffness degradation for complete stress levels.

Notch effect in tension-tension fatigue of short glass fibre reinforced polyphenylene sulfide composites

An experimental investigation of the fatigue behaviour of plain and notched specimens made of 40 % wt. short glass fibre-reinforced PolyPhenylene sulfide is presented. To this end, plain and V-notched specimens (with notch root radius ranging from 0.2 mm to 10 mm) were produced by injection moulding, and the final geometries were obtained by means of two different technological processes (i.e., directly after the injection moulding process and by machining them from injected plates). To investigate the effect of notch root radius and fibre orientation, tension–tension fatigue tests were carried out in load control mode, and the experimental results were reanalysed in terms of net stress amplitude defining the traditional stress-life curves. During the fatigue tests, the damage evolution was monitored using a travelling microscope to define the number of cycles spent for fatigue crack nucleation. Furthermore, the fracture surfaces were observed at the microscopic level to investigate the damage mechanisms and the actual fibre orientation was evaluated by computed tomography analysis. Then, by employing coupled finite element analysis of the injection moulding process and the structural behaviour, four different approaches available in the open literature were used to correlate the geometrical and/or manufacturing process effects in the fatigue strength of the considered material.

Tensile fatigue behavior and damage evolution of z-pinned carbon/phenolic woven composite using in-situ monitoring technology

Carbon/phenolic composite, as structural materials for aerospace, undergoes static and dynamic loading which can affect the structural integrity of components. This paper aims to comprehensively investigate the mechanical properties and damage evolution mechanisms of z-pinned carbon/phenolic woven composite subjected to static tensile and different levels of tension-tension fatigue loading. The essential correlation between damage modes and acoustic emission (AE) signals of carbon/phenolic composite was first established. The AE signals were processed using the Hilbert-Huang transform method to obtain the percentage of damage modes at various load levels. The results showed that the peak frequency ranges of the three damage modes, matrix cracking, debonding, and fiber breakage, were 100–200 kHz, 200–300 kHz, and over 300 kHz, respectively. Moreover, the damage evolution mechanisms can be revealed using the OM-AE monitoring method. It was found that the dominant damage modes were all matrix cracking. The fatigue damage occurs at the edge of the specimen, and the three damage modes interact with each other on a microscopic scale. Cracks expand from the edge to the center at low stress. The developed OM-AE online damage monitoring system allows real-time damage monitoring and accurate recognition of damage modes.

Fatigue behaviour of 10% wt. short glass fibre reinforced recycled Polypropylene with mineral filler in presence of notches

An experimental investigation of the fatigue behaviour of plain and notched specimens made of 10% wt. short glass fibre reinforced partially recycled Polypropylene filled with mineral filler is presented. To this end, plain and V-notched specimens (with notch radius ranging from 0.07 mm to 10 mm) were produced by injection moulding and tested under tension-tension fatigue. During the fatigue test, the damage evolution was monitored using a traveling microscope to define the number of cycles spent for the fatigue crack initiation. The fracture surfaces were analysed at microscopic level in order to investigate the fatigue damage mechanisms of plain and notched specimens. The fatigue tests were first analysed in terms of net stress amplitude to define the stress-life curves. Then, an energy-based approach was proposed to reduce the scatter of the fatigue data of plain and notched specimens. Starting from the damage evolution observed during the fatigue tests, the model relies on the strain energy density numerically evaluated in the matrix and averaged on a structural volume embracing the notch tip.

Concerns in tension-tension fatigue testing of unidirectional composites: Specimen design and test setup

Tension-tension fatigue of unidirectional (UD) composites is often used to represent the fatigue behavior of composites. The standard proposes to use end tabs for UD composites. However, obtaining a reliable S–N curve requires avoiding premature failure, which in turn requires minimizing stress concentrations near the grips and end tabs. This work examines the fatigue life of different specimen and end tab designs. Conventional end tabs and other well-known designs, along with novel arrow end tabs, are tested to identify the method that is least influenced by the stress concentrations and yields the highest and the most valid fatigue life. Rectangular specimens with rectangular and tapered end tabs, which is the standard configuration, yielded the highest fatigue life. This differs from the preferences in the quasi-static test, where arrow end tabs performed the best. Moreover, the effect of parameters in the manufacturing process and test setup on the fatigue life of two different carbon fiber/epoxy prepregs material systems are discussed. The results reveal the significant effect of the test setup on tension-tension fatigue of UD composites and will inform the community on how to perform more reliable fatigue tests.

Tension-tension fatigue properties of multiaxial laminated carbon/epoxy composites molded by high-pressure resin transfer molding (HP-RTM) process

Engineering structures are often subjected to cyclic-loading conditions, which detrimentally affect the component’s service life and damage tolerance. The fatigue behavior of multiaxial laminated composites manufactured with unidirectional plies has been analyzed. Carbon/epoxy composite laminate was molded by a high-pressure resin transfer molding (HP-RTM) process. Specimens were subjected to tension-tension (T-T) fatigue stress of 65 %–75 % of static tensile strength until failure or up to one million cycles. An analysis of the maximum stress-cycle number (S-N) curve is conducted to determine the fatigue limits of the material. The predetermined fatigue limit has been obtained at 717.5 MPa stress, which is about 67 % of static tensile strength under tension-tension stress conditions. Fatigue failures under tension-tension loading conditions are associated with fiber breakage, fiber pull-out, and plies delamination. This research aimed to understand the fatigue behavior and fracture mechanism of multiaxial laminated composites under tension-tension cyclic loading conditions and collect more data for future research.

Polymer Fatigue Evaluation Methodology for Unbonded Flexible Pipes

Polymer layers have been used in the offshore Oil & Gas industry for many years in unbonded flexible pipes for static and dynamic applications. The evaluation of fatigue performance for polymer grades in such applications is complex and no qualification method is well defined nor standardized at the time this article is written.Though historically no reported polymer fatigue field failures the complexity of unbonded flexible pipes, the conservatism of this industry makes the introduction of new polymer grades in risers for dynamic applications challenging. Development of a polymer fatigue methodology will allow the industry to quantitatively evaluate risks of fatigue failure of liners and thereby be important in introducing new material grades.In this paper, a design fatigue methodology is built based on inputs from a specific project and results from small- and full-scale tests. The pipe curvature ranges, extracted from the project location, are used to obtain an accumulated tensile strain which is then converted into an accumulated tensile stress. The fatigue damage is then determined, using a design curve established from bending or tension-tension fatigue testing performed on dumbbells from pipe samples. Parameters are included to reflect geometrical effects from manufacturing process but also service life conditions with several temperature profiles. This method can be applied for pressure sheaths as well as sealed outer sheaths with relevant inputs related to their operating conditions. The obtained fatigue damage can then be used to validate the use of polymer grades for specific unbonded pipe designs under their service conditions.

Effects of thermal conditions on fatigue behaviour of laminated glass/epoxy plates under tension-tension cycle

The fatigue behavior study on laminated glass/epoxy composite plates at elevated temperature is attempted in the present research work. INSTRON 8862 servo-electric universal testing machine with hydraulic suspension is utilized to perform low cyclic tension-tension fatigue tests at 0.5Hz and 0.7Hz frequencies. Bluehill universal software compatible with INSTRON 8862 is employed to obtain the relationship of fatigue stress upon cycles to failure (S – N) for each specimen. The parametric investigation is done for the loading frequency, lamination sequence and number of layers to understand their effects on fatigue behavior of composite laminated plates under ambient and thermal environment. The elicited results lead to the conclusion that above parameters greatly influenced the fatigue behavior of composite laminated plates under thermal loading. The rising temperature has significant adverse effects on fatigue life. The present research is beneficial for the analysis and design of laminated composite plate or plate-like structures in the domain of fatigue analysis.

Research on fatigue failure of SiCf/SiC composites at high temperature based on X-ray computed tomography

The woven architecture of ceramic matrix composites and the existence of pores lead to the anisotropy and non-uniformity of the materials, which affect the mechanical behavior and failure of the materials in service environment. In this paper, tension-tension fatigue behavior of a plain-woven SiCf/SiC composite was investigated via X-ray computed tomography. The meso-scale architecture (pore, woven, etc.) characteristics of the SiCf/SiC specimen were obtained by X-ray computed tomography before and after the tests, and the conclusions can be drawn as following: (1) Changes in hysteresis loops, elastic modulus and area of hysteresis loop are correlated: as the cycle number increases, the elastic modulus decreases and the area of hysteresis loop increases; The larger the area of hysteresis loop, the greater the decrease rate of elastic modulus. (2) the porosity distribution of SiCf/SiC specimen shows the characteristics of both periodic and aperiodic. The periodic phenomenon is related to the meso-woven architecture, while the aperiodic phenomenon is caused by the distribution of stitching holes. (3) The fracture location of the specimen is significantly affected by the location of the stitching holes and the porosity distribution, but not completely determined by them. (4) When the load on the specimen is large, the stepped cracks are more likely to occur, and most cracks initiate from the surface/boundary of the fiber.

Experimental and numerical fatigue damage characterization in multidirectional thermoplastic glass/polypropylene laminates based on in-situ damage observations

Quantifying microscopic damage mechanisms in opaque composite laminates with multiple off-axis plies throughout fatigue experiments is challenging. However, if successful, important relations can be revealed between actual evolving damages and their contribution to the mechanical laminate response. This study characterizes the evolution of matrix cracking in ±45 and 90 embedded off-axis plies together with effects of delamination and relates them to the global mechanical property reductions in Glass/Polypropylene laminates. [0/45/0/-45]s and [0/45/90/-45]s laminates are tested under tension-tension fatigue over 500 000 cycles at three different stress levels. In-situ edge damage detection by optical measurements are performed as well as the validation of damages by detailed post-mortem microscopy. Moreover, through-the-width sectioning is applied to reveal the damages inside the laminate after testing. Additionally, the effects of different fabrication histories on damage progression are assessed. Influences of 45-ply thickness and different ply neighbouring orientations on the damage behaviour are discussed with their contribution to the normalized stiffness degradation and Poisson's ratio evolution. Moreover, experimental results are compared with a modelling approach by which the effects of damages in multiple plies can be considered, showing very good accordance.

Effect of brittle TiN coating on fatigue performance of TC11 titanium alloy under rotating bending and tension-tension

In this paper, the effect of brittle TiN coating on fatigue performance of TC11 titanium alloy (Ti-6.5Al-3.1Mo-1.05Zr0.35Si) under rotating bending and tension-tension were investigated. TiN coating causes a reduction in high and low cycle fatigue performance of TC11 under both conditions. The median fatigue strength at 1 × 107 cycles of TC11 decreases from 582.5 MPa and 896.0–547.5 MPa and 531.0 MPa under rotating bending and tension-tension fatigue loading, respectively. In scenarios involving rotating bending fatigue, fatigue failure initiation for both TiN coated and uncoated TC11 specimens reside on the droplet in coating surface, irrespective of high or low stress conditions. As for tension-tension fatigue loading, both TiN coated and uncoated TC11 specimens experience surface-based fatigue initiation points under high fatigue stress. However, during low-stress tension-tension loading, the incorporation of a TiN coating prompts a shift in TC11's fatigue initiation source from the interior to its surface. Furthermore, under both fatigue loading condition, fatigue cracks initiate at droplets on the coating surface and propagate to the surface of the substrate and eventually its interior. And the fatigue failure of the TC11 substrate is accelerated due to the coating causing balanced tensile stress. The inherent compressive characteristics and lack of tensile strength in TiN ceramic materials render them more susceptible to premature cracking when subjected to tensile fatigue loads. This vulnerability leads to heightened damage to the fatigue performance of TC11.

The damage mechanism of tension-tension fatigue interaction with creep damage of the compacted graphite cast iron alloy at high temperatures

The tension-tension fatigue test of the compacted graphite cast iron (CGI) alloy was carried out by RDL100 universal testing at 500 °C and 550 °C, respectively. A tension-tension trapezoidal load is applied to the CGI specimen. Because of the time-dependent deformation at elevated temperatures, the stress–strain curve presents hysteresis loops, and the area of the hysteresis loop increases gradually with continuous cyclic loading and sustained loading times. Intergranular and transgranular cracks in the microstructure accelerate the CGI alloy fracture failure. The fatigue life is sensitive to the short loading time and decreases with the sustained loading time exponentially under the tension-tension fatigue condition. The short holding time has a great influence on the fatigue life of CGI. The fatigue behavior of CGI alloys and the influence of holding time on the fatigue life can be characterized by y = aexp(bx) (a and b are constants, can be fitted through the test data). In addition, the fatigue life of CGI alloy can be predicted by the ductility depletion method. But the equivalent stress amplitude needs to be modified to eliminate the effects of oxidation damage.

Dynamic impact behavior and notched fatigue life of GLARE laminates

As a member of the fiber metal laminates (FMLs) family, GLARE laminates have been widely used in aircraft skin structure due to the excellent resistance to dynamic impact and fatigue loadings. The low-velocity impact of GLARE laminates are conducted to explore the mechanical response on the impact energy and punch shape. The impact indentations are characterized by the damage morphology and quantitative dimension analysis. The mechanism of impact damage resistance is illuminated by the contact force-time curve, intra-laminar and inter-laminar damage modes. The lagrangian SPH modelling is employed to numerically analyze the bird-strike behavior of GLARE laminates, revealing the variation of bird particle flow and impact energy with different impact attitudes. The tension-tension fatigue of GLARE laminates is experimentally performed subjected to different stress levels. The influence of the circular open-hole on the fatigue residual life of GLARE laminates is studied. Considering the uncertainty of S-N curves, the probabilistic fatigue life P-S-N curves are developed under different survival probabilities. These would be more positive for the fatigue life estimation and reliability design for GLARE laminates in aircraft engineering application.

About the high temperature fatigue performance of thermoplastic-based composite laminates for aeronautical applications

The use of thermoplastic-based composites for applications in high temperature aeronautical parts is often considered contradictory with respect to the softening of the matrix and the subsequent degradation of their mechanical properties, especially when service temperature T exceeds glass transition temperature. This paper examines the fatigue performance of unidirectional (UD) carbon fibers reinforced PolyArylEtherKetone (PAEK) laminates subjected to tension-tension fatigue at different testing temperatures (Room Temperature, 120°C and 170°C). Laminates consisting of different portions of plies in 0°/+45°/−45°/90° orientations have been investigated to evaluate the contribution of each ply orientation to the fatigue performance in terms of S-N diagrams. From the evolution of life span as a function of applied stress, it appears that the influence of temperature on fatigue behaviour of C/PAEK composites depends on the temperature conditions. 0° plies behaviour is known to be temperature-independent as is dominated by the mechanical behaviour of UD carbon fibers. It is expected to reflect on the fatigue behaviour at high temperature. From the obtained results, one may conclude that the 0° plies primarily bear the tensile load in fatigue (47-67-91% in SOFT, QI and HARD laminates, respectively) and therefore dominate the fatigue behaviour. The deformation mechanisms in +/−45° plies (localized plastic-viscoplastic dissipative behaviors) favoured at temperatures higher than Tg (e.g. 143°C) are limited by the low deformation of 0° plies. SOFT (12% of 0° plies) and QI (25% of 0° plies) laminates have similar relative fatigue performance. Unexpectedly, the fatigue performance of HARD laminates (about 62% of 0° plies) is improved at T>Tg at high stress levels. A simple temperature-dependent analytical model was used to successfully predict the fatigue life of C/PAEK laminates with different stacking sequences.