Tension-tension Research Articles

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.

An Out-of-Plane Bending Fatigue Assessment Approach for Offshore Mooring Chains Considering the Real-Time Updating of Interlink Bending Stiffness

Fatigue failure caused by frequent tension and bending loads is a crucial safety concern for mooring chains used on floating structures in the oil and gas industry. The bending effect for a chain’s fatigue is usually not considered by existing fatigue analysis methods, and even if it is considered, existing commercial software can only calculate the constant interlink bending stiffness without consideration of the stiffness variations due to tension and friction. To address this issue, this study develops an in-plane bending (IPB)/out-of-plane bending (OPB) fatigue assessment approach for offshore mooring chains considering the time-varying nonlinear interlink bending stiffness. Initially, the mooring tension and IPB/OPB angles are calculated by an in-house code MeCAP (multi-element coupled analysis program), considering the time-varying interlink stiffness between the chain links. The fatigue life of mooring chains for pure tension–tension (TT) and OPB combined fatigue at different hotspot locations (A, B, and C) is calculated by MeCAP-fatigue, a newly-developed module in MeCAP. Based on the analysis model, a comparative study is implemented to investigate the effects of interlink stiffness on fatigue damage. The differences and advantages of the combined fatigue calculation method over the pure TT fatigue assessment method are discussed. The results illustrate that combined fatigue would be underestimated significantly if zero interlink stiffness (hinge joint assumption) is applied or even constant interlink stiffness (calculated by mooring pretension), while pure TT fatigue will be largely unaffected. Moreover, compared to pure TT fatigue, the OPB combined fatigue assessment method, considering the effects of OPB/IPB and the time-varying properties of the connection between chain links, could evaluate fatigue life more comprehensively.

On the high cycle fatigue resistance of austenitic stainless steels with surface gradient microstructures: Effect of load ratio and associated residual stress modification

The effect of gradient microstructure generated by ultrasonic surface mechanical attrition treatment (SMAT) on the fatigue performance of an austenitic stainless steel has been evaluated in the context of localized and general plasticity by comparing two load ratios: fully reversed tension-compression (TC) RTC = -1 and tension-tension (TT) RTT = +0.1, respectively.After identical SMAT processing conditions, the fatigue limit was enhanced by +17% for RTC while it was reduced by -7% for RTT. Under RTC cyclic loading, self-heating of the specimens and sub-surface martensitic transformation occurred but not for RTT. The residual stress measurements have also revealed that while the stress gradient was smoothed after the RTC fatigue loading, it was completely reverted for the RTT one. In turns, the fatigue crack initiation sites and propagation were modified between the RTC and RTT loading conditions.Considering the stabilized surface residual stress after fatigue loading, the use of a Crossland criterion allowed to explain both the effects of load ratio and SMAT on the high cycle fatigue behavior of the stainless steel. It turns out that under the studied loading conditions the modifications of residual stress state can be considered as the primary factor governing the varying fatigue performances and the observed triggering at different initiation sites.

Universal Strain‐Life Curve Exponents for Thermoplastics and Elastomers under Tension‐Tension and Torsion

AbstractThe fatigue behavior of 28 amorphous and semi‐crystalline thermoplastic polymers and elastomers is tested under strain controlled sinusoidal tension‐tension (TT) and torsion (T) at room temperature and analyzed via strain‐life (total strain amplitude versus fatigue lifetime) and crack propagation rate versus total strain amplitude curves, analogous to Paris’ law. Investigating fatigue is extremely time‐ and resources consuming, so universal relationships between the exponents of the strain‐life power‐law and material properties are of high importance. For a brittle failure mechanism (I), the strain‐life curves are found to have fixed exponents of BTT,I = −0.27 and BT,I = −0.22, respectively, while the crack propagation versus strain amplitude in TT has an exponent of mda/dN,I = 4. For ductile failure (II), fixed strain‐life curve exponents in TT of BTT,II = −0.35 and in torsion of BT,II = −0.48 with mda/dN,II = 2.8 are obtained. In torsion, most semi‐crystalline polymers show brittle and ductile failure depending on the applied strain amplitude, so the strain‐life curve exponent changes accordingly. The universal exponents for strain‐life and crack growth‐strain amplitude curves offer a significant simplification to rapidly estimate, predict, and simulate the fatigue behavior of polymers.

Damage evolution of 3D woven carbon/epoxy composites under tension-tension fatigue loading based on synchrotron radiation computed tomography (SRCT)

A method combining mechanical testing, infrared thermography (IRT) and synchrotron radiation computed tomography (SRCT) is proposed for the damage evolution of 3D woven carbon/epoxy composites under tension-tension (T-T) fatigue loading. To study the relationship between the damage evolution and the stiffness degradation, fatigue tests of six specimens are carried out to different stages before detection by SRCT and optical microscope. The detected results have revealed how cracks initiate, distribute and develop with increasing fatigue cycles. Particularly, the effects of fiber yarn undulation and twist caused during the manufacturing process are discussed.

A new constant life diagram model for the longitudinal fatigue of unidirectional composites

A new constant life diagram (CLD) model featuring asymmetric bilinear constant-life curves was proposed to better describe the longitudinal fatigue behavior of unidirectional laminae (UD) under a wide range of stress ratios. This model is able to predict S-N curves with satisfactory accuracy not only in tension-tension (T-T) fatigue mode but also in tensioncompression (T-C) and compression-compression (C-C) modes, whereas the conventional Goodman CLD model shows inferior performance especially in T-C and C-C modes. Apart from static tension and compression tests, high- and low-cycle fatigue tests at two stress ratios corresponding to T-T and C-C modes should be performed to determine the parameters in the proposed model. Fatigue test data of several different GFRP UDs at various stress ratios were utilized to validate the proposed model, and the S-N curves predicted by the proposed model agreed well with the experimental results. Compared with the Goodman CLD model, the proposed CLD model demonstrates an enhanced predictive capability without losing its simplicity.

Experimental verification of anisotropic constitutive models under tension-tension and tension-compression stress states

Constitutive model is vitally important to finite element simulation which determines the accuracy and reliability of predicting material yield and plastic flow behaviors. Due to lack of accurate and sufficient experimental data for the tension-compression (T-C) stress state, the anisotropic constitutive model can only be calibrated with tension-tension (T-T) data and its accuracy cannot be thoroughly verified under the complete stress states, including T-T and T-C stress states. To address the limitation, a novel controllable biaxial loading test device was developed to conduct systematic experiment and determine sufficient data. Using such a device, arbitrary stress paths can be applied to thin-walled tubes under T-T and T-C stress states. Subsequently, the yield and plastic flow behavior of anisotropic AA6061-O tubes were analyzed through the measured stress and strain data. These were then used to verify the accuracy of common anisotropic constitutive models including Hill48, Barlat-Lian, and Yld2000-2d, those are extensively implemented in commercial finite element software, under the complete stress states. It is indicated that the yield behavior exhibited anisotropy in the complete stress states, but the yield characteristics differ for the T-T and T-C stress states. The plastic flow behavior exhibits isotropic characteristics in the T-C stress state, while anisotropic in the T-T stress state. These common anisotropic constitutive models can accurately describe yield and plastic flow behavior in either T-T or T-C stress state only. None of the models are able to provide accurate predictions of the plastic deformation behavior both in the T-T and T-C stress states simultaneously.

3D mesoscale fracture analysis of concrete under complex loading

Analyzing mechanical behavior at the mesoscale level should be a powerful tool for higher heterogeneous materials like concrete. In this study, the mechanical and damage analysis of concrete were investigated using a 3D mesoscale model under complex loading. A less time-consuming method was proposed for constructing the realistic mesostructure of concrete. The finite element simulation showed good agreements with the uniaxial and biaxial experimental results. Then complex loading conditions such as compression-compression (C-C), tension-tension (T-T), and compression-tension (C-T) were simulated. The computation showed that damage failure under biaxial loading was completely different from that under uniaxial compression. Under T-T conditions, the tensile strength decreased and an oblique crack formed perpendicular to the free plane. Finally, the obtained numerical failure envelope agreed relatively well with the corresponding experiment. The mesoscale model provided an effective way to understand the damage mechanism of concrete especially under muti-axial stress condition.

Effect of fiber orientation distribution on constant fatigue life diagram of chopped carbon fiber chip-reinforced Sheet Molding Compound (SMC) composite

The material behavior of chopped carbon fiber chip-reinforced Sheet Molding Compound (SMC) composite is investigated under both quasi-static and cyclic fatigue loading conditions in the present study. Quasi-static tensile and compressive strength data is collected, followed by a series of fatigue tests under three different cyclic loading conditions, i.e., Tension-Tension (T-T), Compression-Compression (C-C), and Tension-Compression (T-C). The experimental results show a considerable amount of variation for each loading condition, which is found to be induced by the spatial distribution of fiber orientation within the tested specimens. An analytical approach is developed to correlate the fatigue performance of the SMC material with the local modulus, which is determined by the fiber orientation distribution. Additionally, the similar failure modes are observed under different loading conditions. Based on these findings, a new form of Constant Fatigue Life (CFL) diagram is established in which the impact of fiber orientation tensor to the fatigue behavior of SMC is considered. This developed CFL diagram is examined by comparing the measured fatigue performance with prediction where local fiber orientation distribution is quantified from microscopic images.

Influence of SC-HAZ microstructure on the mechanical behavior of Si-TRIP steel welds

Transformation induced plasticity (TRIP) steel provides enormous potential for auto-body construction in the automotive sector, owing to its enhanced mechanical behavior. In this work, Si-alloyed TRIP steel is joined by employing laser beam welding (LBW) and by utilizing two arc welding processes: gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) in order to assess the effect of the net heat input on the microstructure, the uniaxial tensile properties and the forming response. Results indicate that in spite of the Si content in TRIP steel; precipitation and growth of carbides (tempering) are observed in both: the martensite islands and the retained austenite phase, thus leading to the measurable softening at the sub-critical heat affected zone (SC-HAZ) of the arc welded samples. Although the failure location was predominantly found at the sub-critical heat affected zone of the GMAW samples, the maximum stress and elongation was basically controlled by the total extension of the weldment including fusion zone and heat affected zone. While the limiting dome height upon tension-tension (T-T) and tension-compression (T-C) depended primarily on the fusion zone hardness, weld width and geometry of the sample; the fracture location was outside the weld for T-C, whereas the fracture initiated at the weld in T-T samples. LBW specimens showed optimum forming performance.

Anisomorphic constant fatigue life diagrams of constant probability of failure and prediction of P–S–N curves for unidirectional carbon/epoxy laminates

The effect of stress ratio on statistical distribution of fatigue life of a unidirectional carbon/epoxy composite has been examined. Furthermore, a method for efficiently predicting a family of S–N curves with probability of failure as the parameter for any stress ratio of constant amplitude loading has been developed. For this purpose, fatigue life data are first collected at each of different stress levels for tension–tension (TT), tension–compression (TC) and compression–compression (CC) fatigue loadings, respectively. The experimental results show that the scatter in fatigue life tends to become larger in order of TT, TC and CC loading. Lognormal distribution is used to quantify the statistics of the static tensile and compressive strength data and the fatigue life data for each of different stress levels and stress ratios. It is found that lognormal distribution is applicable approximately not only to the static strength data in tension and compression, but also to the fatigue life data for each of different stress levels and stress ratios. The parameters of the distribution of fatigue life are shown to be dependent on the stress ratio of fatigue loading. The experimental P–S–N curves are established for each of different stress ratios using the distributions fitted to the static strength and fatigue life data. Then, a method for constructing the anisomorphic constant fatigue life (CFL) diagrams of constant probability of failure is developed. The proposed probabilistic anisomorphic CFL diagram approach allows not only accurately predicting the P–S–N curves for different stress ratios, but also adequately describing the stress-ratio dependent scatter in fatigue life of the unidirectional composite.

A New S-N Curve Model of Fiber Reinforced Plastic Composite

Based on an amount of fatigue experimental data of fiber reinforced plastic composite, a new three-parameter S-N curve model is proposed to describe the relationships between the loads and fatigue life under constant amplitude cyclic loading. As the logistic curve behaves as sigmoidal which is the similar with previous S-N models, and from this comparability, an S-N equation with logistic’ form has been established. The model can assess the fatigue behaviors of FRP under various loading conditions, such as, tension-tension (T-T), tension-compression (T-C) or compression-compression (C-C) loading under different stress ratios of the whole region of fatigue life. Several examples are employed to illustrate that the model has ability to fit several different sets of experimental data accurately.

7110 Efficacy and safety of AMG 102 in patients with advanced renal cell carcinoma (RCC)

Transformation induced plasticity (TRIP) steel provides enormous potential for auto-body construction in the automotive sector, owing to its enhanced mechanical behavior. In this work, Si-alloyed TRIP steel is joined by employing laser beam welding (LBW) and by utilizing two arc welding processes: gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) in order to assess the effect of the net heat input on the microstructure, the uniaxial tensile properties and the forming response. Results indicate that in spite of the Si content in TRIP steel; precipitation and growth of carbides (tempering) are observed in both: the martensite islands and the retained austenite phase, thus leading to the measurable softening at the sub-critical heat affected zone (SC-HAZ) of the arc welded samples. Although the failure location was predominantly found at the sub-critical heat affected zone of the GMAW samples, the maximum stress and elongation was basically controlled by the total extension of the weldment including fusion zone and heat affected zone. While the limiting dome height upon tension-tension (T-T) and tension-compression (T-C) depended primarily on the fusion zone hardness, weld width and geometry of the sample; the fracture location was outside the weld for T-C, whereas the fracture initiated at the weld in T-T samples. LBW specimens showed optimum forming performance.

Mechanical Properties of a Centrally Notched APC-2 Composite Laminate Due to Fatigue Loading at Elevated Temperature (II)

AbstractThe temperature versus life (T-N) curves of both centrally notched and unnotched AS-4/PEEK (APC-2) composite laminates due to constant stress amplitude tension-tension (T-T) fatigue loading were investigated systematically. The cross-ply laminate possesses the higher mechanical properties than those of the quasi-isotropic laminate at elevated temperature, and also the mechanical properties of both lay-ups are degraded significantly as the temperature increasing. Combining both effects of notch and temperature at various normalized stresses it is found the cross-ply laminate possesses more resistance to cyclic loading than that of the quasi-isotropic laminate. Additionally, the predictive strain model is in a practical and simple form. Alternatively, the T-N curves, instead of conventionally S-N curves, at variously applied stresses are accomplished for preliminary designs and applications.

Fatigue Response of Hybrid Magnesium/Carbon-Fiber/Peek Nanocomposite Laminates at Elevated Temperature

High performance Mg based hybrid five-layered composite laminates were fabricated by means of sandwiching the AZ31 Mg foils with the Carbon/PEEK prepregs, in which the nanoparticles SiO2 were uniformly spreaded at the interfaces, through hot press. The basic mechanical properties of the cross-ply hybrid laminates were obtained by tensile tests, and they were found in good agreement with the prediction by the rule of mixture. Then, the response due to constant stress amplitude tension-tension (T-T) cyclic loading at elevated temperature up to 10 6 cycles was investigated.

Fatigue Properties of Steel Cord-Rubber Composite

The fatigue properties of [±20°] steel cord-rubber laminated composite under tension-tension (T-T) loading are studied in this paper. The results show that the variation of the cycle strain in the fatigue process under the constant cycle loading exhibits three stages which correspond to damage initiation, steady damage propagation and catastrophic failure. Two-step experiments demonstrate that the strain variation in step-2 generally follows the similar regularity, indicating that the three-stage behavior also exists in that of the defected specimens. The S-N data in the fatigue-life diagram follow a linear trend. Studies on the effect of the frequency on the fatigue properties show that the higher the frequency of loading, the faster the cycle strain increases and the shorter the fatigue life is. The hysteretic loss caused by the viscoelasticity of the constituent materials remains constant during the steady propagation stage of the damage. Both the surface temperature and its increasing rate go up as the maximum stress and the frequency increase.

Fatigue behaviour of carbon fibre/PEEK laminate composites

The fatigue behaviour of continuous carbon fibre-reinforced poly(ether ether ketone) (PEEK) composites under tension-tension (T-T) and tension-compression (T-C) loadings was investigated. A quasi-isotropic lay-up was used, namely [0/45/90/-45] 2s AS-4/PEEK. The experimental results for fatigue life and residual tensile strength under different stress amplitudes are analysed by the median rank method. The influences of stress amplitude on the fatigue life and the residual tensile strength were investigated. The S-N curve is presented by the pooled Weibull distribution function. Two segments of the S-N curves show a different failure mode in the fatigue failure of the thermoplastic composites. Moreover, a residual strength degradation model is used to predict the residual strength for the composites subjected to a number of fatigue cycles and to simulate the effects of the stress amplitude on the fatigue life. According to the obtained results, the correlation between the experimental data and the theoretical prediction is in good agreement.

Initiation and Propagation of Delamination in a Centrally Notched Composite Laminate

Interlaminar stresses occur at the free edge and notch to cause composite laminates to delaminate. Actually, delamination is an important failure mode and affected, for example, by fiber orientation and stacking sequence as discussed. In analysis, our de veloped mathematical model yields the interlaminar edge stresses and singularities around the hole to predict the onset of delamination. Furthermore, our work mainly aims to inves tigate experimentally the initiation and propagation of delamination subjected to tension and cyclic tension-tension (T-T) tests in centrally notched T300/5208 graphite/epoxy com posite laminates, including four layups: (a) orthotropic [=±45]4s, [90/0] 4s and (b) quasi- isotropic [0/90/±45]2s, [90/0/±45]2s. Experimental result and observation associated with the obtained maximum stresses and the adopted failure criterion are in good agree ment with our predicted delamination onset. Other important findings are highlighted as follows. The failure mode of each layup recorded by X-ray radiograph can be closely cor related to the prediction of our model. The delamination growth rate has also been mea sured. An approximately linear relationship between delamination area and stiffness re duction for the centrally notched quasi-isotropic lammates is established. The critical load for delamination can be estimated by the Hashin-Rotem failure criterion satisfactorily. As a benefit of nondestructive testing, we finally find that the residual strength of the laminate first increases as applied cycles increase and then decreases during fatigue tests. However, residual stiffness always decreases monotonically with the increase of applied cycles.

Residual strength in notched composite materials

The main objective was to understand the stress concentration factor of fatigue damage for centrally notched composite laminates of various lay-ups in the proportion of 45, −45, 90 and 0 deg plies. The application of fatigue stress concentration factor was used to predict the residual strength. The Whitney-Nuismer (WN) extended point stress fracture criterion and the extended average stress fracture criterion were adopted. The characteristic length is not necessarily a constant; it depends on the shape of the damage zone, the number of applied cycles, the hole radius, etc. The tests were performed at low-frequency tension-tension (T-T) fatigue loading with constant stress amplitude at room temperature. In simple terms, after thousands of cycles, because of the geometric change of a circular hole to the deformed laminate, the stress concentration factor decreases with the increasing number of applied cycles, which is the reason for the increase of residual strength of the notched laminate. The correlation of the residual strength as predicted by the numerical methods and the fatigue stress concentration analysis is found to be satisfactory, in comparison with the experimental data.

The effect of tensile pre-straining on nucleation of martensite in an FeNiC alloy

The effect of gradient microstructure generated by ultrasonic surface mechanical attrition treatment (SMAT) on the fatigue performance of an austenitic stainless steel has been evaluated in the context of localized and general plasticity by comparing two load ratios: fully reversed tension-compression (TC) RTC = -1 and tension-tension (TT) RTT = +0.1, respectively.After identical SMAT processing conditions, the fatigue limit was enhanced by +17% for RTC while it was reduced by -7% for RTT. Under RTC cyclic loading, self-heating of the specimens and sub-surface martensitic transformation occurred but not for RTT. The residual stress measurements have also revealed that while the stress gradient was smoothed after the RTC fatigue loading, it was completely reverted for the RTT one. In turns, the fatigue crack initiation sites and propagation were modified between the RTC and RTT loading conditions.Considering the stabilized surface residual stress after fatigue loading, the use of a Crossland criterion allowed to explain both the effects of load ratio and SMAT on the high cycle fatigue behavior of the stainless steel. It turns out that under the studied loading conditions the modifications of residual stress state can be considered as the primary factor governing the varying fatigue performances and the observed triggering at different initiation sites.