Tensile Fatigue Loading Research Articles

Tension Fatigue Behaviour of Woven Bamboo and Glass Fiber Reinforced Epoxy Hybrid Composites

The aim of this paper is to investigate the influence of stacking sequence of multi layered woven bamboo and glass fibers reinforced with epoxy matrix composites under static tensile and tension-tension fatigue loading. Six layers of bamboo fiber and seven layers of glass fiber has been used to prepare the samples by hand lay-up technique with [0°/90°] and [±45°]. Extensive fatigue tests (frequency 3 Hz and stress ratio 0.1) were performed in accordance with ASTM D3479. In constant amplitude and load-controlled tests, a percentage of the ultimate tensile strength (UTS) was applied to the composite specimens. Five stress levels were applied from 90% of UTS, and decreased in steps of 10% of UTS. The obtained results were used to draw S/N Curve and [0°/90°] lay-ups show better fatigue strength than [±45] laminate.

Pin hole tensile and fatigue properties of self-reinforced PET composites

This study investigates the effects of the width-to-hole diameter ratio (W/D) and the edge distance-to-hole diameter ratio (E/D) of a specimen on the bearing properties of self-reinforced recycled poly (ethylene terephthalate) (srrPET) composites with (srrPET-FR) and without flame retardant under a tensile and tensile-tensile fatigue load. The bearing strength of srrPET composites is largely dependent on the W/D ratios, whereas E/D ratios did not show any significant influence. The srrPET composites exhibited higher fatigue cycles compared with the srrPET-FR composites at the same (stress amplitude) σa. Higher W/D ratios resulted in a higher fatigue strength for srrPETs. The compact constitutive equation used to predict the curves between σa and number of cycles to failure (Nf), well agree with the experimental results.

Experimental Study on Damage Evolution Behavior of Self-Compacting Rubberized Concrete under Direct Tensile Fatigue Loading

Fatigue properties of self-compacting rubberized concrete (SCRC) under axial tension are rare. To explore the fatigue damage evolution of SCRC, the direct tensile fatigue test and residual mechanical test were conducted. According to the experimental results, the linear relationship between the fatigue life and secondary strain rate also exists in tensile fatigue test of SCRC, and the slope value is similar to the compressive and direct tensile fatigue test of ordinary concrete. Further, the downtrend of the residual secant modulus is more obvious than the residual strength. In addition, the direct tensile fatigue damage evolution of SCRC under different stress levels can be predicted by the Isojeh’s model.

Residual stress relaxation and re-shot-peening process of wet shot-peened titanium alloy during tensile fatigue load

Residual stress relaxation and re-shot-peening process of wet shot-peened titanium alloy during tensile fatigue load

Fatigue Properties for Bi-2212 High Temperature Superconducting Round Wires Under Axial Tensile Loading

Bi-2212 wires, which have high critical current density and critical magnetic field without anisotropy, offer significant potential applications for ultrahigh-field superconducting magnets. The Bi-2212 wire is very sensitive to strain. The strands are inevitably subjected to cyclic thermal or electromagnetic stress during application. There is a lack of understanding regarding the fatigue properties. In this paper, the degradation behavior of critical current for Bi-2212 wires under axial tensile fatigue loading was investigated. It was found that the fatigue life decreases with the increase in maximum fatigue stress.

Study of the Fatigue Behavior of the Unidirectional Zylon/epoxy Composite Used in Pulsed Magnets

The fatigue behavior of the Zylon/epoxy composite has been tested under quasi-static tensile and tensile fatigue loads. The distribution of the fatigue life at different stress was determined. The results show that the statistical characteristic of the static strength and the fatigue life can be satisfactorily described by the normal, lognormal or Weibull distribution model. The measured S-N curve exhibits a pronounced nonlinear behavior. Strains of the unidirectional specimens were monitored with a mechanical extensometer during the test. Three distinct stages of the strain evolutionary process were observed. The evolution curves show a sharp increase in the first stage, an approximately linear slow increasing in the middle stage and an unstable acceleration in the third stage before failure. Similar to the strain evolution, the three-stage stiffness of degradation was also observed. The maximal cycle strain is chosen as the indicator of damage evaluation. The experimental data of the damage parameters are fitted with the existing analytical model. The presented models have a good agreement with the tested results.

Nonlinear guided waves for fatigue crack evaluation in steel joints with digital image correlation validation

This paper presents a study of nonlinear guided-wave-based fatigue crack identification in a steel joint subjected to tensile fatigue loadings. The steel joint is considered as a section of a railway bogie, a vital component of the train structure. Finite element method and experiment were used to evaluate fatigue crack based on the second harmonic method. The symmetric mode (S1) of Lamb waves was selected as the input signal and excited in experiment by piezoelectric transducers installed on the upper surface of the joint. The sensing paths were aligned perpendicularly to and at various distances from the potential crack trajectory. When the waves interacted with the crack, higher order harmonics were produced owing to the phenomenon of contact acoustic nonlinearity. A nonlinear index based on the second harmonic characteristics was exploited to define the nonlinearity. To validate the outcomes based on nonlinear guided waves, a digital image correlation technique was implemented and the results were in good agreement. The findings indicated that the nonlinear guided-wave-based technique is capable of identifying crack initiation and growth in such joint type structures.

Characterization of crack propagation behavior in Al-7075 under in-plane biaxial fatigue loading with shear overloads

Effect of shear overloads in an otherwise constant amplitude, tensile biaxial fatigue loading is investigated using a cruciform specimen of Al 7075-T651, and the crack propagation behavior and governing micromechanisms are characterized. It was observed that with an increase in shear overload ratio fatigue life of the specimen is increased and the crack growth rate is significantly affected. This behavior is attributed to the combined effects of competing mechanisms including compressive residual stress, plasticity induced crack closure, and strain hardening. The fractographic study revealed features of shear fracture in the transient region and tensile fracture in the pre-overload region and post-overload region.

Assessment of fatigue damage in a fully pearlitic ductile cast iron by evaluation of Acoustic Emission Entropy

The paper presents the preliminary results of Acoustic Emission (AE) tests on a peralitic ductile cast iron (DCI) subjected to fatigue tensile loading. The focus is on the evaluation of the information Entropy of the AE data, as an innovative tool for a reliable assessment of fatigue damage in DCIs. Two damage indexes are proposed for the identification of the damage evolution and for the prediction of the fracture failure.

In-situ investigations of hydrogen influenced crack initiation and propagation under tensile and low cycle fatigue loadings in RPV steel

Present work aims to unveil the mechanism of hydrogen embrittlement (HE) in SA508 Grade 3 Class I low alloy reactor pressure vessel (RPV) steel. In-situ tensile and low cycle fatigue (LCF) tests are performed on specially designed specimens using tensile/fatigue testing stage under scanning electron microscope (SEM). Electrochemical hydrogen charging resulted in localized void formation at prior austenite grain boundaries (PAGBs) during tensile loading. Alongside the hydrogen induced weakening of PAGBs due to synergetic HELP (hydrogen enhanced localized plasticity) and HEDE (hydrogen enhanced decohesion) mechanisms of HE, fish-eyes formation around Al2O3–SiO2 type inclusions are the primary factors for hydrogen enhanced tensile properties degradation in subject RPV steel. During LCF loading, crack initiation and propagation is facilitated by long rod inter-lath cementite particles distributed along the bainitic ferrite lath boundaries in the un-charged specimen. In case of hydrogen charged specimen, the edge crack formed during LCF loading propagated through the specimen by cleavage. Predominantly plasticity (slip) driven transgranular crack propagation occurred in un-charged specimen. In contrary, hydrogen charging resulted in LCF crack to propagate in mixed intergranular and transgranular manner during early stages of propagation, whereas once the crack length exceeded 5 to 6 grains, cleavage type transgranular crack propagation was observed.

Experimental study on fatigue behavior of butt-welded thin-walled steel plates strengthened using CFRP sheets

Abstract Welded steel joints in structures are susceptible to fatigue failure. In this context, advanced carbon fiber reinforced polymer (CFRP) possess a demonstrated potential for the fatigue strengthening of steel structures. However, limited information is available on CFRP strengthening of butt-welded joints, which are the most common joint types in steel structures. This paper presents a study on fatigue behavior of butt-welded thin-walled steel plates strengthened using CFRP sheets. A total of 34 specimens with both single-sided and double-sided strengthening were performed via constant amplitude tensile fatigue loading, and another four specimens were employed for hot-spot stress measurement. The effect of CFRP strengthening on fatigue behavior was investigated by varying the strengthening scheme (single-sided or double-sided) and the number of CFRP sheet layers. Test results showed that the fatigue life of butt-welded steel plates with CFRP strengthening increased by approximately one to ten times when compared to the fatigue life of butt-welded steel plates without CFRP strengthening. Triple-layered double-sided specimens seemed to exhibit the best effect on fatigue life; their fatigue strength at 2 million cycles increased by 34% compared to that of specimens without CFRP strengthening. Specific fatigue design S–N curves of butt-welded thin-walled steel plates with and without CFRP strengthening were proposed directly. The fatigue test results were used to calibrate a fatigue design approach for CFRP strengthened butt-welded steel plates. In addition, the effect of CFRP strengthening on stiffness degradation was also discussed.

Effect of freeze-thaw cycling on fatigue behaviour in concrete

Freeze-thaw damage is a deterioration mechanism caused when saturated concrete is exposed to temperature variations that cycle above and below its freezing point. The expansion of water in the concrete pore space due to freezing can lead to internal pressures that induce cracking, thus expediting the deterioration process through freeze-thaw cycles. The purpose of this investigation was is to evaluate the effect of freeze-thaw deterioration on the tensile fatigue life of air-entrained concrete at early stages of deterioration. Concrete cylinders were prepared with 0.45 and 0.65 water to cement ratios (W/C), then cured for 28 days before being subjected to the freeze-thaw cycling. After 0, 25 and 50 freeze-thaw cycles, the pressure tension (PT) test was used to induce tensile fatigue loading cycles until failure. The PT was capable of determining the decrease in fatigue life, represented as the number of cycles to failure, of the concretes when subjected to pure cyclical tensile loading after having first been exposed to freezing and thawing cycles. The results indicated that even though the ultimate static tensile strength of the specimens did not vary significantly due to the freeze-thaw cycles, the residual fatigue properties were degraded. UPV monitoring was able to determine the increase in internal damage, as the UPV of the specimens decreased with continued freeze-thaw cycling. It was shown that freeze-thaw deterioration reduces the ability of concrete to withstand cyclic loading, even at early stages prior to a decrease in static tensile strength.

Prediction of fatigue life to crack initiation in unidirectional plies containing voids

In the present work, a model is proposed to predict the life to crack initiation in porous unidirectional composite plies subjected to tensile fatigue loadings. The average values of the Local Maximum Principal Stress (LMPS) and Local Hydrostatic Stress (LHS) in a control volume of the matrix are proposed to be the driving forces to the initiation of fatigue cracks in off-axis plies.The experimental fatigue curves for the life to crack initiation of laminates characterised by different void contents are found to fall in the same scatter band once plotted in terms of the proposed driving forces, thus proving the capability of the proposed model to predict the life to crack initiation in porous laminates based on the behaviour of the void-free material.Finally, a parametric analysis is carried out to study the influence of the global void content and average void size on the reduction of the life to crack initiation and the applicability of the proposed model to different void distributions and shapes is discussed.

An investigation into specimen property to part performance relationships for laser beam powder bed fusion additive manufacturing

The influence of part size and geometry on the melt pool size, microstructural features, and resulting mechanical properties of additive manufactured 17-4 precipitation hardening (PH) stainless steel (SS), fabricated using a laser beam powder bed fusion (LB-PBF) process is investigated. Three different types of geometries including a dog-bone part (i.e. representing the specimen or the witness coupon) and two square rods with different cross-sectional sizes (i.e. representing the parts) were designed and fabricated using the same set of process parameters. Mechanical properties were determined under quasi-static tensile and uniaxial strain-controlled fully-reversed fatigue loading conditions. Experimental results confirmed a minimal effect of part geometry on the tensile behavior of the LB-PBF 17-4 PH SS as the yield strength ranged from 1145 to 1198 MPa and ultimate tensile strength varied from 1171 to 1231 MPa. On the other hand, part geometry had a significant impact on the fatigue behavior in the high cycle regime. Specimens fabricated from the large square blocks contained the lowest amount of porosity, and consequently, exhibited the highest fatigue resistance. On the contrary, the highest amount of porosity was observed in the specimens fabricated from the dog-bone parts resulting in an inferior fatigue resistance as compared to other geometries. Microstructural analysis was performed to estimate the effect of part geometry on the thermal history experienced during manufacturing by measuring the melt pool dimensions. Some effects of geometry on the melt pool dimensions were observed as the melt pool depth was found to be shorter for dog-bone specimens and longer for large block specimens. The less elongated melt pools in dog-bone specimens can be attributed to higher cooling rate experienced by the specimens resulting in more entrapped gas pores.

Fatigue studies on patch repaired carbon/epoxy woven composites

This work investigates the behaviour of carbon/epoxy composite laminates repaired by external patches under load controlled constant amplitude tensile fatigue loading. The specimens with laminate configurations [0°/45°/45°/0°] and [45°/0°/0°/45°] are repaired by bonding the pre-cured patches of two configurations viz. [0°]2 and [45°]2 over a hole made in the specimens, which mimic the damage on laminates. The strain field is captured by using Digital Image Correlation (DIC). The strain contours enable to develop understanding of the difference in compliance of both ([0°]2 and [45°]2) patches under fatigue loading. The examination of repaired specimens during tests reveal that the damage initiates in the form of delamination and failure of patch-parent interface which grows further as the number of cycles increase and lead to ultimate failure. Moreover from life aspect, the external patch repair shows significant potential to restore life under fatigue loading. The damaged specimen (with hole) of [45°/0°/0°/45°] configuration has life of about 1k cycles, while the specimens repaired with [45°]2 and [0°]2 patches have a life of around 5k cycles and 2k cycles respectively. Furthermore, it is observed that for both parent laminate configurations, the specimens repaired with [45°]2 patch have an overall higher life than the specimens repaired with [0°]2 patch.

Adaptive cycle jump and limits of degradation in micromechanical fatigue simulations of fibre-reinforced plastics

This research investigates the influence of numerical parameters of micromechanical fatigue damage models on the obtained progressive damage behaviour of fibre-reinforced plastics at transverse tensile fatigue loads. The simulated damage behaviour is evaluated using experimentally observed crack patters published in the literature. The investigated numerical model parameters are (1) whether or not the model considers static failure within a simulated load cycle, (2) the degree of material property degradation after sudden failure and (3) the size of the cycle jump. The results reveal a significant influence of the degree of material degradation and of the cycle jump on the simulated matrix crack formation at both higher and lower fatigue loads. Static failure within a simulated load cycle primarily affects the damage behaviour at higher fatigue loads. The paper gives recommendations of the parameter choice for plausible progressive fatigue damage simulation results. Regarding the cycle jump, an adaptive algorithm is proposed and implemented. This approach leads to plausible fatigue damage results paired with a significant reduction of computation time comparing to a cycle-by-cycle analysis.

Fatigue device driven by a three degree of freedom tripodal piezoelectric actuator.

A miniature piezoelectric-driven fatigue device with three degrees of freedom is developed. The device integrates two fatigue testing functions, including uniaxial tensile fatigue and tensile-bending combined loading modes. The synchronous tensile-bending loading principle is described, which is applicable for calculating the vector displacements along two orthogonal directions and investigating the anisotropic fatigue properties. Regarding the combined loading mode, maximum load/displacement amplitudes for tensile and bending vector components that could be achieved are 16.9 N/22.8 µm and 3.3 N/5.6 µm, respectively. Based on tensile and tensile-bending combined fatigue loading modes, the displacement responses and fatigue lives at loading frequencies ranging from 1 Hz to 100 Hz are valuated experimentally to indicate the validation.

Numerical analysis of fatigue performance of CFRP–repaired steel plates with central inclined cracks

Previous experimental and numerical studies on central inclined cracked steel plates demonstrated that when the initial crack inclination angles range from 0° to 60°, fatigue lives are similar if the initial crack projection lengths on the direction perpendicular to the loading direction are the same. This is applicable to both unrepaired and repaired specimens. This numerical study extended the inclination angle range from 60° to 80° and was conducted on the stress intensity factor (SIF) and fatigue life of CFRP–repaired steel plates with inclined central cracks subjected to tensile fatigue loading. In the case of unrepaired specimens with a crack inclination angle of 0–70° and double-sided repaired specimens with a crack inclination angle of 0–60°, it was concluded that the crack projection length on the direction perpendicular to the loading axis is the main influence of fatigue life, even though the projection length is calculated from the crack length and inclination angle. Beyond these limits, there is an obvious increase of fatigue life as the crack inclination angle rises. To minimize the effect of mode II component of SIF on the crack tip, two tail slots perpendicular to the loading axis were induced in the front of the inclined crack tips. The tail slots caused the effective SIF to increase and this led to a reduction of fatigue life. However, the limits of the initial crack angle of fatigue life calculation did not change. The results of previous research were also compared to the findings of this study; they supported the conclusions to some extent.

Microstructure-based RVE modeling of the failure behavior and LCF resistance of ductile cast iron

In this work the failure behavior of ductile cast iron microstructure subjected to tensile and low-cycle fatigue loadings is simulated by a 3-D, FE Reference Volume Element approach. A fully ferritic matrix is considered as representative of the low-hardness, high-ductility material class of nodular cast irons. Plastic flow potential rule, ductile and low cycle fatigue damage models are implemented at the micro-scale for the matrix constituent in conjunction with nonlinear cyclic hardening laws, and periodic boundary conditions are imposed over the RVE at the meso-scale. Different values of triaxiality are imposed. Numerical results confirm experimental findings of the behavior at the meso-scale and correctly predict the LCF lifetime, driving the interpretation of inner strain distribution, voids interaction and triaxiality effects on failure mechanisms.

Investigation of fretting fatigue failure mechanism of lug-bush connection members

Lug-bush connection members are widely used in aerospace industry, specifically in the connection of rotor to helicopter main body. Under small amplitude cyclic loadings, a fraction of the contact area experiences relative motion which causes fretting on the contacting surfaces. In this study, failure mechanisms of four different lug-bush members subjected to high cycle tensile fatigue loading are investigated. The contact surfaces are inspected using a digital microscope and a scanning electron microscope (SEM). Different surface regimes are observed at the mating surfaces which are shown to be the results of partial stick and slip of the contacting bodies. Energy dispersive spectroscopy (EDS) analysis in SEM showed that rubbing of lug-bush surfaces created wear debris at the sliding regions which then corroded and formed a tribolayer. A crack is observed to initiate in the vicinity of tribolayer boundary. FEA of these members are carried out using ABAQUS by defining interference and contact to mating surfaces of the lug and bush. The results showed qualitative agreement with the experimental investigations in terms of peak slip locations.