Cyclic Tensile Loading Research Articles

Fatigue failure characterization of multi-axial warp-knitted composites under cyclic uniaxial tensile loading

In this work, the fatigue failure mechanisms of quadri- and tri-axial warp-knitted fabric composites (QWFC and TWFC) were investigated. A series of quasi-static state tensile tests and cyclic loading experiments were carried out, and the morphology of fatigue failure was visualized by optical and scanning electron microscopes (SEM). The results showed that while the failures of QWFC and TWFC were similar with X-shape morphology, QWFC had longer fatigue life than TWFC. The SEM images illustrated that the matrix of QWFC was not uniform, and contained cracks with scaly appearance, in addition to small amount of fiber curls. Moreover, the fracture surface was flat and layered. In contrast, the matrix of TWFC had propagation of cracks along ±45° direction; its fracture surface had sphere ‘budding yeast-like’ appearance. The wrap-knitted fabric contained increased numbers of layers in the composites, thereby had longer fatigue life.

Suction caissons in dense sand, part II: vertical cyclic loading into tension

Suction caissons have been used for numerous oil and gas installations and are increasingly considered as a foundation solution for offshore wind turbines (OWTs). There can be significant differences between the two offshore energy applications in the load paths and magnitudes, soil type and caisson aspect ratio (skirt length to diameter). This paper investigates the response of suction caissons in dense sand to a range of cyclic vertical loading histories relevant to a jacket-supported OWT, with an emphasis on cyclic tensile loading. The findings are based on a series of experiments performed in a centrifuge, such that soil stresses reflect those in the prototype. The caisson was installed using suction at enhanced gravity, followed by cyclic loading and then caisson extraction. The installation and extraction results are discussed in a companion paper. This paper focuses on the caisson load–displacement response under vertical cyclic loading. The centrifuge experimental results reinforce findings from previous work, add insights into the load transfer mechanisms and provide confidence in their applicability to the prototype, both qualitatively and quantitatively. The results highlight the complexity of the caisson response, particularly under tensile loading, with the influences of average load, cyclic load amplitude and drainage discussed in detail.

Creep-fatigue and cyclically enhanced creep mechanisms in aluminium based metal matrix composites

An aluminium (Al, 2024T3) matrix composite reinforced with continuous alumina (Al2O3) fibres is investigated under tensile off-axis constant macro stress and thermal cyclic loading. The micromechanical approach to modelling and three different fibre cross-section geometries have been employed. The effect of creep is included by considering three dwell times at the peak temperature of the thermal loading history. The presence of the hold time gives rise to different sources of failure such as cyclic enhanced creep and creep ratchetting. These failure mechanisms are carefully discussed and assessed. The linear matching method framework has been used for the direct evaluation of the crucial parameters for creep-fatigue crack initiation assessment at the steady cycle. A detailed representation of the steady-state hysteresis loops is provided by using the strain range partitioning and a method for dealing with multiaxiality is reported with regard to the algebraic sign of the Mises-Hencky equivalent stress and strain. All the results obtained have been benchmarked by fully inelastic step-by-step (SBS) analyses. The design of a long fibre metal matrix composite should consider not only the detrimental effect of their dissimilar coefficient of thermal expansion, but also the state of stress at the interface between the matrix and fibre.

Effects of cyclic tensile loading on the rupture behavior of orthogonal 3-D woven SiC fiber/SiC matrix composites at elevated temperatures in air

This study examined the rupture mechanisms of an orthogonal 3D woven SiC fiber/BN interface/SiC matrix composite under combination of constant and cyclic tensile loading at elevated temperature in air. Monotonic tensile testing, constant tensile load testing, and tension–tension fatigue testing were conducted at 1100 °C. A rectangular waveform was used for fatigue testing to assess effects of unloading on the damage and failure behavior. Microscopic observation and single-fiber push-out tests were conducted to reveal the rupture mechanisms. Results show that both oxidative matrix crack propagation attributable to oxidation of the fiber–matrix interface and the decrease in the interfacial shear stress (IFSS) at the fiber–matrix interface significantly affect the lifetime of the SiC/SiC composites. A rupture strength degradation model was proposed using the combination of the oxidative matrix crack growth model and the IFSS degradation model. The prediction roughly agreed with the experimentally obtained results.

Healing efficiency characterization of self-repairing polymer composites based on damage continuum mechanics

Self-healing polymer composites are part of a class of materials which have the capability to repair themselves when damaged. They are often tested by inflicting damage onto them and then performing a mechanical test to initiate and measure self-healing. Self-healing efficiency is commonly defined as the ratio of a recovered property value to its original value. Applying this in practice can be challenging as most self-healing experiments are idealized scenarios. In this work, microcapsules containing epoxy and mercaptan self-healing agents were incorporated into ±45° composite weaves that were subjected to cyclic tensile loading and unloading. A damage mechanics theory was applied to quantify the change in stiffness and determine self-healing effectiveness. As this method did not enable a direct comparison between the damaged and healed states of the material, a new methodology was developed to take into account the energy levels of the material in order to quantify self-healing performance.

Mechanical behavior of sandstone under uniaxial constant cyclical compressive and tensile loading

In order to investigate mechanical behavior of red sandstone under cyclical compressive and tensile loads, a series of short-term uniaxial constant cyclical compressive and tensile loading tests and uniaxial constant cyclical compressive and tensile loading creep tests were conducted. First, based on the results of short-term uniaxial constant cyclical compressive and tensile loading experimental, the permanent residual strain, strains at peak stress were analyzed. Results show that the specimen shows permanent residual strain after suffering short-term cyclic loads; the residual strains and strains at peak stress show the “decay increase” and “steady-state increase” stages with the cyclic number N; the relationship between the strain at peak stress (tensile or compressive stress) and cyclic number can be well described by a modified Burger model. And then, in accordance with the creep experimental results, the creep behavior of the red sandstone was analyzed. Results show that there is obvious instantaneous strain, decay creep, and steady creep under each stress level of tensile or compressive stress stages; the specimen appears accelerated creep stage under the 5th tensile stress of 1.19 MPa. It was also found that power function can better express the relationship between steady strains and cycle number under tensile or compressive stress levels. In the end, a viscoelastic-plastic creep model was proposed; comparison of the test results with the proposed viscoelastic-plastic creep model predictions indicates that the proposed model is capable of describing the creep behavior of red sandstone subjected to cyclic tensile and compressive stress loading.

A viscoelastoplastic stiffening model for plant fibre unidirectional reinforced composite behaviour under monotonic and cyclic tensile loading

At room conditions and standard strain rate (ε˙∼10−4s−1), unidirectional (UD) plant-based reinforced organic polymers often exhibit nonlinear mechanical behaviour in tension. A viscoelastoplastic model (VEP model) for the simulation of UD plant fibre composite mechanical behaviour in tension, previously validated from twisted flax yarn epoxy composite under room conditions and standard strain rate, is calibrated with new data obtained from flax fibre epoxy composite under repeated progressive loading and a wide range of strain rates (ε˙∼10−3 to 10−7s−1). The VEP model does not reproduce well the experimental observations. There seems to be a lack of stiffening in this phenomenological model.We propose an improved VEP model, developed within the frameworks of thermodynamics and limited to uniaxial tension and infinitesimal strains. An internal variable s representing the stiffening is added to create a VEP-stiffening model. This internal variable represents the coupled effects of reorienting cellulose microfibrils in kink band areas, spiral spring-like extension of cellulose microfibrils, and shear-stress-induced crystallization of the amorphous cellulose of flax fibres. The stiffening phenomenon was considered viscous, without a threshold, and was related to the tension energy in the direction of the fibres. Three viscosity coefficients drive the three phenomena: η (elastic), K (plastic), and Ks (stiffening). In the chosen formalism, this leads to two thermodynamic potentials φVEPs and ΩVEPs in which the stiffening phenomenon is strongly coupled with all the others.This VEP-stiffening model of the UD flax fibre epoxy composite correlates well with experimental observations. The paper also explores the evolution of the three viscous phenomena (elastic, plastic, and stiffening) by simulation of different loading conditions: monotonic, cyclic, and creep.This VEP-stiffening model can easily enrich existing multiaxial models of UD behaviour in the fibre direction. Implemented in a finite element model, it could be used at different length scales to numerically explore the origin of the mechanical behaviour of plant-based reinforced polymers.

Piezoresistive characteristics of CNT fiber-incorporated GFRP composites prepared with diversified fabrication schemes

The present study utilized piezoresistive carbon nanotube (CNT) fibers for the structural health monitoring of composites. In particular, diversified schemes including i) Pre-casted CNT fiber patches embedded into GFRP composites, ii) CNT fibers stitched to one glass fabric of the composites during the fabrication procedure, and iii) CNT fibers attached to the top of the composites, were proposed for incorporation of CNT fibers to the composites. The fabricated samples were subjected to cyclic tensile loadings with constant and increasing amplitudes to assess their piezoresistive characteristics. All types of samples showed sharp increases and decreases in the electrical resistance values, reflecting the applied stress. They also showed good sensing integrity under extended cyclic loadings as well as against loading up to the point of failure. Moreover, the fabricated composites exhibited tensile strengths comparable to those of the control samples, demonstrating feasibility to be used as a sensor of composite structures.

3D Printing of Highly Stretchable Strain Sensors Based on Carbon Nanotube Nanocomposites

This paper presents the additive manufacturing of a stretchable and electrically conductive polydimethylsiloxane (PDMS) nanocomposite for strain sensing. A long, thin PDMS strip in a zig‐zag pattern of five parallel lines is 3D printed on elastomer substrate using an in‐house modified 3D printer. Multi‐walled carbon nanotubes (MWCNTs) are uniformly deposited on the uncured PDMS lines and cured in an oven. An additional layer of PDMS is then applied on top of MWCNTs to form a thin protective coating. The 3D printed PDMS/MWCNT nanocomposites are characterized using a scanning electron microscope (SEM) to validate the thickness, uniformity, and microstructural features of the sensor cross‐section. The strain sensing capability of the nanocomposites is investigated under tensile cyclic loading at different strain rates and maximum strains. Long‐term performance is tested under cyclic tensile loads for 300 cycles. Sensing experiments indicate that under cyclic loading, the changes in piezo resistivity mimic the changes in the applied load and the measured material strain with high fidelity. In situ micro‐mechanical testing in SEM is carried to investigate the piezoresistive sensing mechanism. Due to the high flexibility of PDMS, the 3D printed sensors are tested to monitor the bending of a human wrist joint as a wearable sensor.

Cyclic loading effects on craniofacial strain and sutural growth in pigs.

Current craniofacial growth modification devices use static forces, but cyclic forces are believed by some to be more effective. The latter have not been evaluated in large animal models, and it is not known how such forces are transmitted to distant parts of the skull. In this study, we aimed to (1) develop a portable loading system capable of delivering reliable cyclic loads to the porcine nasofrontal suture (NFS), (2) explore strain transmission to distant sutures, and (3) characterize the sutural growth effects in a small pilot study. After we validated the device, cyclic (2.5Hz) tensile loads were applied unilaterally to the NFS of 6 abattoir pig heads, with strain gauges on multiple sutures. Similar loading was applied to 3-month-old live pigs (Sus scrofa, n=4 and 1 sham) 30minutes per day for 5days. These animals received fluorescent markers of mineralization on loading days 1 and 3. Suture strains were recorded on day 5. Histomorphometric analysis quantified suture width and mineral apposition rate. A wearable loading system was developed to produce an average of +900 microstrain at the targeted NFS. Substantial strains were seen at the contralateral NFS and midline sutures, but bone strains were low. Strain patterns were similar exvivo and invivo, with the latter generally having higher magnitudes. Preliminary evidence demonstrates wider sutures with higher mineral apposition rates in the loaded sutures. Daily spurts of cyclic load caused sutural strain throughout the skull. This regimen most likely enhances sutural growth and may be therapeutically useful.

Experimental investigation of the fatigue behavior of glass/epoxy composites evaluated by the stiffness degradation and damage accumulation

The present paper deals with the mechanical behavior of glass/epoxy composite materials under static and cyclic tensile loading. The tested specimens have been made with different stacking sequences ([02/902]s; [902/02]s; [03/90]s; [903/0]s) according to the vacuum infusion procedure. The results of this investigation show the static behavior of the tested laminated composites characterized by the strain to failure, Young's modulus and tensile strength. Wöhler curves corresponding to each stacking sequence were plotted in order to observe the evolution of the maximum stress Smax according to the number of cycles. Fatigue Tests were performed under various applied loading levels ( r), and these values were chosen to evaluate the effect of loading level on the stiffness degradation and damage growth in the tested specimens. Afterward the evolution of the global damage factor ( D) according to the number of cycles was calculated in function of the core characteristics, applied loading level ( r) and type of loading. The cyclic behavior of the composite laminates mainly depends on the progressive developments of damage mechanisms such as matrix cracking, delamination, and fiber breakage.

Strain and damage self-sensing of basalt fiber reinforced polymer laminates fabricated with carbon nanofibers/epoxy composites under tension

This study investigated the strain and damage self-sensing capabilities of basalt fiber reinforced polymer (BFRP) laminates fabricated with carbon nanofibers (CNFs)/epoxy composites subjected to tensile loadings. The conduction mechanisms based on the tunnel conduction and percolation conduction theories as well as the damage evolution were also explored. A compensation circuit with a half-bridge configuration was proposed. The results indicated the resistivity of the CNFs/BFRP laminates and CNFs/epoxy composites exhibited similar change rule, indicating that the conductive networks of CNFs/BFRP laminates were governed by CNFs/epoxy composites. With the increase of strain under monotonic tensile loading, the electrical resistance response could be classified into three stages corresponding to different damage modes. This confirmed CNFs/BFRP laminates have excellent self-sensing abilities to monitor their internal damages. Moreover, stable and repeatable strain self-sensing capacity of the CNFs/BFRP laminates was verified under cyclic tensile loading because the electrical resistance varied synchronously with the applied strain.

In Situ Strain and Damage Monitoring of GFRP Laminates Incorporating Carbon Nanofibers under Tension.

In this study, conductive carbon nanofibers (CNFs) were dispersed into epoxy resin and then infused into glass fiber fabric to fabricate CNF/glass fiber-reinforced polymer (GFRP) laminates. The electrical resistance and strain of CNF/GFRP laminates were measured simultaneously during tensile loadings to investigate the in situ strain and damage monitoring capability of CNF/GFRP laminates. The damage evolution and conduction mechanisms of the laminates were also presented. The results indicated that the percolation threshold of CNFs content for CNF/GFRP laminates was 0.86 wt % based on a typical power law. The resistance response during monotonic tensile loading could be classified into three stages corresponding to different damage mechanisms, which demonstrated a good ability of in situ damage monitoring of the CNF/GFRP laminates. In addition, the capacity of in situ strain monitoring of the laminates during small strain stages was also confirmed according to the synchronous and reversible resistance responses to strain under constant cyclic tensile loading. Moreover, the analysis of the resistance responses during incremental amplitude cyclic tensile loading with the maximum strain of 1.5% suggested that in situ strain and damage monitoring of the CNF/GFRP laminates were feasible and stable.

Specimen for cyclic delamination crack growth rates from ply-drops

The cyclic growth rate of delaminations initiated from ply-drops under tensile cyclic load was experimentally measured using a test specimen geometry, with internal ply-drops, that allows stable crack growth. It is found that the fatigue delaminations grow at a constant rate.

Pullout behavior of polymeric strip in compacted dry granular soil under cyclic tensile load conditions

Assessment of the reinforcement behavior of soil under cyclic and monotonic loads is of great importance in the safe design of mechanically stabilized earth walls. In this article, the method of conducting a multistage pullout (MSP) test on the polymeric strip (PS) is presented. The post-cyclic behavior of the reinforcement can be evaluated using a large-scale pullout apparatus adopting MSP test and one-stage pullout (OSP) test procedures. This research investigates the effects of various factors including load amplitude, load frequency, number of load cycles and vertical effective stress on the peak apparent coefficient of friction mobilized at the soil-PS interface and the pullout resistance of the PS buried in dry sandy soil. The results illustrate that changing the cyclic tensile load frequency from 0.1 Hz to 0.5 Hz does not affect the pullout resistance. Moreover, the influence of increasing the number of load cycles from 30 to 250 on the peak pullout resistance is negligible. Finally, the effect of increasing the cyclic tensile load amplitude from 20% to 40% on the monotonic pullout resistance can be ignored. The peak apparent coefficient of friction mobilized at the soil-PS interface under monotonic and cyclic load conditions decreases with the increase in vertical effective stress.

Constitutive modeling of randomly oriented electrospun nanofibrous membranes

In this paper, a simple phenomenological model describing the macroscopic mechanical response of electrospun nanofibrous structures is proposed. Motivated by the experimental observation, the model development starts from the description of membrane response at fiber scale in order to capture individual fiber response and irreversible inter-fiber interactions using hyperelastic and large strain elasto-plastic frameworks, respectively. The macroscopic response is subsequently obtained by integrating the fiber responses in all possible fiber orientations. The efficiency of the proposed model is assessed using experimental data of PVDF electrospun nanofibrous membranes. It is found that the model is qualitatively in good agreement with uniaxial monotonic and cyclic tensile loading tests. Two other deformation modes, i.e., equibiaxial extension and pure shear (planar extension), are simulated to further evaluate the model responses. Finally, the deformation-induced fiber re-orientation is investigated for different modes of deformations.

Multivariate Characterization of Light Emission From ZnS:Cu-PDMS Self-Sensing Composites Under Cyclic Tensile Strains

The objective of this work is to understand multivariate elastic-mechanoluminescent (EML) light emission characteristics of copper-doped zinc sulfide (ZnS:Cu)-embedded polydimethylsiloxane (PDMS) self-sensing composites subjected to sinusoidal tensile loading and unloading cycles. To investigate the effect of tensile strain and strain rate on the EML light emission profiles (i.e., intensity and color) of the ZnS:Cu-PDMS composites, which are fabricated with different gage lengths (i.e., 150 and 4.7 mm). Each specimen is subjected to cyclic tensile loading and unloading with various maximum strains at various loading frequencies, and the EML light emission is video-recorded and subjected to image processing for quantifying intensity and color of EML light. The test specimen with 150-mm gage length is loaded and unloaded between 0 and a maximum tensile strain, which varies from 16 to 30, at 1 Hz. The shorter specimen is subjected to tensile loading and unloading cycles between 0 and 15 strain at a frequency varying from 5 to 80Hz. It is revealed that the EML light emission is triggered at a threshold tensile strain rate, and the light intensity increases as the composites undergo deformation during both loading and unloading cycles. This multistep EML light emission is attributed to the unique light emission mechanism of ZnS:Cu-embedded in soft PDMS matrix, which is a result of friction at the ZnS:Cu-PDMS interface, unlike other deformation-triggered light emitting EML materials.

Energy-based low cycle fatigue indicator prediction of non-load-carrying cruciform welded joints

A new uniform analytical formulation is proposed to predict the low cycle fatigue assessment indicator of non-loading cruciform welded joints under tensile cyclic loading considering the effects of plasticity and mechanical heterogeneity of the welded materials and geometry configurations. Particularly, weld toe of the joints is dealt with a notch rounding in order to avoid the notch tip singularity in accordance with generalized Neuber concept of Fictitious Notch Rounding (FNR). The relationship between elastic and plastic energy concentration factor of weld toe is firstly determined by the combination of notch stress and strain distribution with the increases of nominal loading, then the main parameters affecting the energy concentration factor are investigated by separating elastic and plastic stages. Stress and strain field intensities are finally expressed in terms of total energy values, which are linked to the nominal energy ones. The energy prediction function which takes into account the material and geometry factors is capable of providing a closed form analytical expression about the state of energy both under small and large-scale yielding for industrial facilities by the comparison against finite element analysis results.

A simple method to determine crack opening stress for the center cracked plate under cyclic tensile loads

A new approach is proposed to determine crack opening stress (COS) on the basis of the variable quantity of maximum crack opening displacement (VMCOD). Based on elastic–plastic finite element analysis of center cracked finite plate, and accounting for the effects of crack geometry size, stress R-ratio, Young's modulus, yield stress and strain hardening, the explicit expression of relationship between COS and VMCOD is presented. This method avoids adopting parameters around crack tip and also is applicable to large-scale yielding state. Further studies show this relationship could be applied to predict crack growth rates, which correlate well with experimental results.

The Influence of Plastic Deformation on the Low-Cycle Fatigue During the Burnishing of Holes in Flat Specimens of D16chT Steel

The paper deals with one of the most important problems of such closely related industries as mechanical engineering and aircraft manufacturing, involving the determination of the service life of components and structures operating under cyclic loading conditions. The presence of local structural and manufacturing stress concentrators greatly complicates the determination of the predicted life of components and structures at their design stage. The burnishing technique, namely, the method of plastic deformation of walls of the hole, aimed at achieving the amount of the residual plastic strain on their surface, is used to strengthen components with holes. The results of low-cycle fatigue tests conducted in repeating tension are presented for flat laboratory specimens of an aircraft aluminum alloy D16chT with a central cylindrical manufacturing hole hardened by burnishing. The burnisher geometry that ensures the required value of the residual plastic strain level (1, 2, and 3%) on the hole surface is calculated. The cyclic tensile loading of specimens at the stresses in a pulsating load cycle was performed in the range of 150 to 270 MPa at a frequency of 3 Hz using the equipment Bi-02-112. The influence of the amount of plastic strain in the burnished hole of the specimen on its cyclic life and fracture as a function of the load value is observed. It is found that due to plastic strain hardening of the surface of manufacturing holes, a local area of compressive residual stresses occur, causing the stress concentration around the manufacturing hole to decrease and the level of limit loads to increase. It is shown that the specimen, having a manufacturing hole hardened by burnishing (up to a plastic strain of 3%) and being cyclically loaded by a tensile stress to 170 MPa, failed not at the hole (the stress concentrator). Both the onset of the microcrack initiation and its further propagation take place in a solid region of the specimen. This is indicative that with a certain amount of residual stresses in the burnished hole, the specimen fracture under low-cycle fatigue becomes insensitive to the presence of this concentrator.