Cyclic Load Range Research Articles

Fatigue Testing and Modeling of Bonded Composite Specimens in Mode I, II, and Mixed-Mode I/II Loadings

Abstract Fatigue debond growth of bonded composite specimens in mode I, mode II, and mixed-mode I/II loadings were studied experimentally and numerically. For the mode I loading, test data sets were analyzed under the linear elasticity condition. For the mode II and mixed-mode loadings, specific test data processing was used to ensure that nonlinear effects that act outside of the cyclic loading range are excluded from the fatigue behavior assessment. An important finding under the mode II and mixed-mode loadings is that the actual loading ratios in the tests varied and were different from the applied displacement ratio. The experimental characterization of the fatigue debond behaviors was complemented with numerically determined correlations between the compliance and debond length under each individual loading. This correlation was used to determine an effective debond length from the compliance determined experimentally. According to the actual loading ratio variation, the fatigue growth behaviors were characterized using Paris’ law. Numerical fatigue analyses using two codes, Abaqus finite element (FE) and AFGROW, were carried out and validated using the developed test database. The details on an FE model setup and analysis strategy for a mixed-mode specimen are presented. Good agreement in the fatigue lives was obtained between the test and numerical analyses for the three types of loading. Also, good agreements in the load versus debond growth length relations were obtained between the test and FE analysis in the mode II and mixed-mode loadings. Reasons for the result discrepancy are discussed. The study shows that the presented analysis procedure is effective. The thorough description of both the used test data processing and numerical analysis procedure fills a knowledge gap in the available literature.

Experimental Study of Reinforced Concrete T-Beam Retrofitted with Ultra-High-Performance Concrete under Cyclic and Ultimate Flexural Loading.

Structurally deficient bridges are commonly retrofitted using conventional methodologies, including reinforced concrete, steel jackets, and fiber-reinforced polymers. Although these retrofit methods aim to improve structural performance, exposure to aggressive environments may undermine the durability performance of the retrofit material. More recently, ultra-high-performance concrete (UHPC) has provided an alternative to conventional construction methods, with its superior material characteristics favoring its use in retrofit applications. In this study, a large-scale reinforced concrete (RC) T-beam is constructed and artificially damaged. The T-beam is then retrofitted with an external envelope of UHPC on all faces. Sandblasting is introduced to the surface, providing partially exposed reinforcement in the T-beam to simulate material deterioration. Additional reinforcement is placed in the web and flange, followed by casting the enveloping layer of UHPC around the specimen. The feasibility of this method is discussed, and the structural performance of the beam is assessed by subjecting the beam to cyclic and ultimate flexural loading. This paper presents the results of cyclic and ultimate testing on the RC-UHPC composite T-beam regarding load-displacement, failure mode, and strain responses. The retrofitted T-beam specimen is subjected to a cyclic loading range of 131 kN for 1.576 million cycles. Despite no visible cracks in the cyclic testing, the specimen experiences a 12.22% degradation in stiffness. During the ultimate flexural testing, the specimen shows no relative slip between the two concretes, and the typical flexural failure mode is observed. By increasing the longitudinal reinforcement ratio in the web, the failure mode can shift from localized cracking, predominantly observed in the UHPC shell, toward a more distributed cracking pattern along the length of the beam, which is similar to conventional reinforced concrete beams.

The effect of contaminating media on the static and dynamic mechanical resilience of dental implant abutments' screws: In vitro study.

This in vitro study aims to biomechanically evaluate the influence of medium contamination for example, saliva, blood, chlorhexidine (liquid and gel), and fluoride mouthwash on the biomechanical behavior of implant abutments' screws under static and dynamic loading. Forty five Ti6Al4V commercial dental implants and abutments were tested in this study. Two main mechanical tests were carried out in the selected media. The first, static, aimed to evaluate the torque loss after the first tightening. The second, dynamic, involved a random cyclic load range between 0 and 200 N to evaluate torque loss due to mastication. In addition, metallographic longitudinal and cross-sections of the abutment-implant apparatus were examined to evaluate the abutment screw-abutment-implant interface. The static torque test showed that irrespective of the media, no statistical difference in static torque loss was found prior to dynamic loading. For the dynamic tests, torque-angle evolution analysis during tightening to 30 Ncm and after the spectrum loading, showed the same global mechanical behavior for all media, but the statistical analysis indicated a difference between the groups in reverse torque values (RTV) and in the torque loss due to dynamic loading. The medium groups CHX, CHX-gel, and Fluoride mouthwash, showed a meaningful torque loss due to loading, but the medium groups, control (no medium), blood and saliva showed an opposite trend and required a higher torque to open the abutment screws. The microstructural analysis revealed clear signs of cold-welding/galling, post-dynamic loading in these latter groups. The presence of lubrication/contamination media (CHX mouthwash/CHX-gel/Fluoride mouthwash) reduces the preload generated due to tightening but prevents damage due to galling. The observed reduction of RTV clearly emphasizes the need for frequent abutment screw retightening for implant-supported prosthetic long-term stability.

Modeling and prediction of fatigue life of brass and EN24 steel using soft computing tool

This paper presents an approach for predicting the fatigue life of Brass and EN24 steel using Artificial Neural Network (ANN). The input required for the ANN model such as surface roughness, materials specifications etc have been obtained by conducting experiments on eighteen standard fatigue test specimens in the laboratory. The effect of cyclic loading range and surface roughness (Ra index) on to the fatigue life of brass and EN24 steel were discussed. Using multilayer feedback network (Levenberg–Marquardt), the ANN model was developed to predict the parameters like maximum stress and number of cycles. The ‘nntool’ in MATLAB toolbox has been used for training and testing the data in ANN model. The ANN model with five input neurons, one hidden layer with three neurons and two output neurons is used to get accurate results. The coefficient of regression (R2) for the ANN model is 0.999. Thus, the comparison between the experimental results and predicted values using ANN was agreeable and good correlation between the results were achieved. From the experimental study, it is concluded that the change in the Ra index from 2 to 0.8 μm, increases the fatigue life by 20% which shows that surface roughness also plays important role in finding the fatigue behaviour of the materials.

Fatigue limit assessment of a 6061 aluminum alloy based on infrared thermography and steady ratcheting effect

To quickly predict the fatigue limit of 6061 aluminum alloy, two assessment methods based on the temperature evolution and the steady ratcheting strain difference under cyclic loading, respectively, were proposed. The temperature evolutions during static and cyclic loadings were both measured by infrared thermography. Fatigue tests show that the temperature evolution was closely related to the cyclic loading, and the cyclic loading range can be divided into three sections according to the regular of temperature evolution in different section. The mechanism of temperature evolution under different cyclic loadings was also analyzed due to the thermoelastic, viscous, and thermoplastic effects. Additionally, ratcheting strain under cyclic loading was also measured, and the results show that the evolution of the ratcheting strain under cyclic loading above the fatigue limit undergone three stages: the first increasing stage, the second steady state, and the final abrupt increase stage. The fatigue limit of the 6061 aluminum alloy was quickly estimated based on transition point of linear fitting of temperature increase and the steady value of ratcheting strain difference. Besides, it is feasible and quick of the two methods by the proof of the traditional S-N curve.

Biomechanics of prophylactic tethering for proximal junctional kyphosis: effects of cyclic loading on tether strength and failure properties.

Biomechanical evaluation of woven polyester tethers. To quantify changes in tether elongation, stiffness, and failure characteristics after cyclic loading. Ligamentous augmentation is gaining interest as a technique to prevent proximal junctional kyphosis (PJK) in adult spinal fusions. There are a lack of data regarding the effects of cyclic loading on polyester tether mechanical properties. Tether stretch may lead to loss of stabilization and increased risk of tether failure. Biomechanical data are needed to determine the effects of cyclic loading on tether integrity. Testing was done in two materials: (1) a synthetic cortical bone composite to determine baseline mechanical properties, and (2) nine cadaveric L1 spinous processes. 5mm woven polyester tethers were looped through 2.5mm holes drilled in each material. First, five tethers were tested directly to failure in the synthetic bone to establish baseline failure properties. Next, tethers were tested at one of the three cyclic load ranges [5%, 25%, and 50% (n = 5 each) of baseline failure] for 1000 cycles and then loaded to failure. Cadaveric tests were done at the 25% range and compared to synthetic bone tests at the same range. Cadaveric failure tests were classified as either tether failure or spinous process bone failure. Greater cyclic loading range had a significant effect on tether loop elongation, increased stiffness, and decreased ultimate tensile force. Among the cadaveric failure tests, 56% resulted in tether failure and the remaining 44% resulted in bone failure. Polyester tethers stretch significantly when loaded to physiological ranges. Anticipation of tether stretch may be an important consideration for a tethering strategy to prevent PJK. Improved understanding of tether material properties can provide guidance for the evaluation of clinical outcomes associated with techniques to reduce the risk of PJK caused by ligamentous laxity. Biomechanical study.

Experimental investigation of the behavior of concrete‐filled high‐strength glass fiber‐reinforced polymer tubes under static and cyclic axial compression

AbstractThis experimental study investigated the load–displacement behavior, ultimate strength, and failure modes of hollow and concrete‐filled high‐strength glass fiber‐reinforced polymer (GFRP) tubes under static and cyclic axial compression. For this purpose, pultruded GFRP tubes were used which contained fibers oriented at 0°, +45°, and −45° to the longitudinal axes of the tubes. Self‐compacting concrete (SCC) was used for in‐filling the hollow GFRP tubes. The main parameters considered in the test program were the tube cross‐section, column length, and cyclic loading range. A total of 28 specimens of lengths equal to either 0.5 or 1.5 m were tested under static and cyclic axial compressive loading. The experimentally obtained load–displacement and load‐axial/hoop strain behaviors are discussed in this paper. It was verified that in‐filling the GFRP tubes with concrete can enhance their axial strength and stiffness, though it is not able to prevent the brittle nature of the failure. Hollow GFRP tubes displayed linear‐elastic behavior with a constant axial stiffness up to failure under both static and cyclic loading, while the concrete‐filled specimens exhibited a bilinear load–displacement behavior with the change in stiffness occurring at the point at which the GFRP tube starts to act as a confining jacket. Under cyclic loading, the concrete‐filled specimens suffered degradation in axial stiffness with the rate of deterioration decreasing with increasing number of cycles. However, a consistent relationship between the cyclic load range magnitude and number of cycles to failure could not be found using the experimentally obtained data.

Flexible and Stretchable Antennas for Biointegrated Electronics.

Combined advances in material science, mechanical engineering, and electrical engineering form the foundations of thin, soft electronic/optoelectronic platforms that have unique capabilities in wireless monitoring and control of various biological processes in cells, tissues, and organs. Miniaturized, stretchable antennas represent an essential link between such devices and external systems for control, power delivery, data processing, and/or communication. Applications typically involve a demanding set of considerations in performance, size, and stretchability. Some of the most effective strategies rely on unusual materials such as liquid metals, nanowires, and woven textiles or on optimally configured 2D/3D structures such as serpentines and helical coils of conventional materials. In the best cases, the performance metrics of small, stretchable, radio frequency (RF) antennas realized using these strategies compare favorably to those of traditional devices. Examples range from dipole, monopole, and patch antennas for far-field RF operation, to magnetic loop antennas for near-field communication (NFC), where the key parameters include operating frequency, Q factor, radiation pattern, and reflection coefficient S11 across a range of mechanical deformations and cyclic loads. Despite significant progress over the last several years, many challenges and associated research opportunities remain in the development of high-efficiency antennas for biointegrated electronic/optoelectronic systems.

Fatigue Stress-Life Model of RC Beams Based on an Accelerated Fatigue Method

Several standard fatigue testing methods are used to determine the fatigue stress-life prediction model (S-N curve) and the endurance limit of Reinforced Concrete (RC) beams, including the application of constant cyclic tension-tension loads at different stress or strain ranges. The standard fatigue testing methods are time-consuming and expensive to perform, as a large number of specimens is needed to obtain valid results. The purpose of this paper is to examine a fatigue stress-life predication model of RC beams that are developed with an accelerated fatigue approach. This approach is based on the hypothesis of linear accumulative damage of the Palmgren–Miner rule, whereby the applied cyclic load range is linearly increased with respect to the number of cycles until the specimen fails. A three-dimensional RC beam was modeled and validated using ANSYS software. Numerical simulations were performed for the RC beam under linearly increased cyclic loading with different initial loading conditions. A fatigue stress-life model was developed that was based on the analyzed data of three specimens. The accelerated fatigue approach has a higher rate of damage accumulations than the standard testing approach. All of the analyzed specimens failed due to an unstable cracking of concrete. The developed fatigue stress-life model fits the upper 95% prediction band of RC beams that were tested under constant amplitude cyclic loading.

Load Estimation of Offshore Wind Turbines

The influence of 3 MW Hywind-II wind turbine wakes from an upstream offshore floating wind turbine on a downstream turbine with a separation distance of seven rotor diameters was studied for a site in the Gulf of Maine. The turbines and the platforms were subjected to atmospheric boundary layer flows. Various sensitivity studies on fatigue loads with respect to the positions of the downstream turbine were performed and validated with a large-eddy simulation tool. In particular, the effect of various lateral positions of the downstream turbine relative to the upstream turbine were considered using time-series turbine wake data generated from the large-eddy simulation tool which served as an input to an aero-elastic wind turbine model to assess the loads. The load response from the rotor, tower, and the floating platform for the downstream turbine were sensitive to the lateral offset positions where turbines that were partially exposed to upstream turbine wakes yielded significant increases in the cyclic load range. For the given set of lateral positions for the downstream turbine, the largest damage equivalent load occurred when the turbine was one rotor diameter to the left of the centerline, when looking upstream, which is the position of the turbine fully exposed to upstream turbine wake. On the other hand, the fatigue load on the downstream turbine placed on the right side of the position fully exposed to the upstream turbine wake, yielded lower stress due to the non-symmetric shape of the turbine wake. The configuration associated with the largest damage equivalent loads was further investigated in a large-eddy simulation, modeling both the upstream and downstream turbines. It was found that the energy spectra at the blade rotational frequency were a magnitude order higher for the downstream turbine, especially for surge, heave, pitch, and yaw motion of the platform. The increase of the damage equivalent load for the flapwise blade root moment was 45% compared to the upstream turbine, which can potentially reduce the turbine service life time.

Open Screw Placement in a 1.5 mm LCP Over a Fracture Gap Decreases Fatigue Life.

ObjectiveTo investigate the influence of plate and screw hole position on the stability of simulated radial fractures stabilized with a 1.5 mm condylar locking compression plate (LCP).Study DesignIn vitro mechanical testing of paired cadaveric limbs.Sample PopulationPaired radii (n = 7) stabilized with a 1.5 mm condylar LCP with an open screw hole positioned either proximal to (PG), or over (OG), a simulated small fracture gap.MethodsConstructs were cycled in axial compression at a simulated trot load until failure or a maximum of 200,000 cycles. Specimens that sustained 200,000 cycles without failure were then loaded in axial compression in a single cycle to failure. Construct cyclic axial stiffness and gap strain, fatigue life, and residual strength were evaluated and compared between constructs using analysis of variance.ResultsOf pairs that had a failure during cyclic loading, OG constructs survived fewer cycles (54,700 ± 60,600) than PG (116,800 ± 49,300). OG constructs had significantly lower initial stiffness throughout cyclic loading and higher gap strain range within the first 1,000 cycles than PG constructs. Residual strength variables were not significantly different between constructs, however yield loads occurred at loads only marginally higher than approximated trot loads. Fatigue life decreased with increasing body weight.ConclusionFracture fixation stability is compromised by an open screw hole directly over a fracture gap compared to the open screw hole being buttressed by bone in the model studied. The 1.5 mm condylar LCP may be insufficient stabilization in dogs with appropriate radial geometry but high body weights.

Characteristics of soft marine clay under cyclic loading: a review

Soft clay forms a big proportion of the seabed ground. In a marine environment, waves, wind, and earthquakes, often known as cyclic loads, are a main contributor to the loading of the seabed. The safety of offshore structures relies on the response of soft marine clay to these cyclic loadings. Its very fine-grained composition, and hence low permeability, is the defining factor for soft marine clay showing a very different response to cyclic loading than sand. The responses of soft marine clay subjected to cyclic loading range from degradation in soil strength and stiffness to principal stress rotation, excess pore pressure generation, and deformation accumulation. These dynamic characteristics of soft marine clay are strongly influenced by several factors, including loading level (amplitude, frequency, and number of cycles), loading history, and path. The purpose of the present paper is to present an overview of the research results on the characteristics of soft marine clay under cyclic loading in the past decades. Some remarks on future research for improving the understanding of dynamic characteristics of soft marine clay are also provided based on the results of up-to-date research.

Investigation on the weld damage behavior of steel beam-to-column connection

A large number of steel moment frames fractured at the welds of beam-to-column connections in earthquake. As the weld damage behavior is a crucial factor for aseismic performance of steel frame connections, 20 local welded connections representing beam-to-column connections were tested under monotonic and cyclic loads to study the weld damage behavior in this paper. The failure modes and deformation curves were recorded; the effects of material strength, load types, and beam geometries on the connection damage behavior were analyzed. Three damage evolution models were calibrated, the relation between weld damage and macro connection performance was established. The results indicate weld cracking and plate buckling are the main causes of the connection damage; the range of cyclic loads is the key factor affecting the weld damage level; the plastic strain based damage model illustrate the damage process with good accuracy. This study provide technical basis for the damage evaluation and fracture prevention of the welded steel beam-to-column connections.

Analysis of a Casting Fracture and Identification of Fatigue From Tire Track Markings

Abstract Knuckle connectors on railcars are required to sustain thousands of cyclic stress events. Manufacturers continue to develop, test, and improve the life of these castings, which are now designed to exceed 400,000 cycles when subjected to tests involving cumulative cyclic sinusoidal load range settings varying between 6800 and 127,000 kgf at room temperature. This paper addressed real-world difficulties of differentiating between component failures due to overload and casting defects, and those that result from fatigue damage. Fatigue fractures in castings are rarely planar and comprise areas where solidification kinetics are different, and locations where porosity or large inclusions are present. The cumulative effect these factors have on fracture is to generate a rough topography, which may exhibit no beachmarks or other outward sign by which a fatigue fracture is commonly recognized. In this paper, an advantage was gained over a field failure because the fracture was produced in a laboratory test. Cycles to failure was recorded and details of the applied cyclic stress were also known. The difficulty regarding scanning electron microscope (SEM) analysis was that classic fatigue features normally visible to the naked eye, such as beachmarks, thumbnail initiation sites and planar areas, were not evident on the fracture face. In addition to documentation of numerous defect features on the fracture surface, SEM analysis revealed a rare but proof-positive form of evidence for propagation of the early stages of fracture by fatigue. The feature has been described in the literature as “tire tracks,” alluding to their similarity to ATV tire impressions or Lunar Rover tracks on the moon. A brief review of earlier literature examples of tire track features is provided, and the generally accepted mechanism for their formation is assessed.

Linear strain sensing performance of continuous high strength carbon fibre reinforced polymer composites

Abstract A feasible method of pre-tension was proposed to improve the sensing performances of carbon fibre reinforced polymer (CFRP) composites through reducing the misalignment and winding of carbon fibres. The CFRP composites consisted of high strength carbon fibres as active elements, epoxy resin as a matrix and sensing electrodes. Firstly, the sensing model was established based on the conductivity analysis of the CFRP composites. Then, a series of experiments were carried out to study the strain sensing linearity and stability under cyclic loading. The experiments showed that the linearity of the CFRP composites increases with increasing pre-tension amplitudes, and that a stable and linear sensing behavior can be obtained when the pre-tension amplitude is larger than 200 μe. The ΔR/R0-strain curves can be linearly fitted with a correlation coefficient larger than 0.985. It was revealed that the loading manner also has some influence on the sensing behavior of the CFRP composite sensors. The gauge factors for an intermittent cyclic loading (range: 2.85–3.37) are larger than those for a continuous cyclic loading (range: 1.89–2.16). Furthermore, the long term experimental results reveal that the electrical behavior of the as-researched composites are characterized by excellent long-term linearity and recoverability.

Fatigue in bamboo

This paper describes the results of an experimental programme to determine the fatigue behaviour of bamboo. Bamboo is subjected to cyclic loading, both in the plant itself and subsequently when the material is used in load-bearing applications in the construction industry. However, there is currently no data in the literature describing fatigue in this material. We found that sections of bamboo culm loaded parallel to the culm axis did not undergo fatigue failure: samples either failed on the first loading cycle, or not at all. By contrast, fatigue was readily apparent in samples loaded in compression across the diameter of the culm. The number of cycles to failure increased as the cyclic load range decreased in a manner similar to that found in many engineering materials: fatigue occurred at applied loads as small as 40% of the ultimate strength. Two different species of bamboo were tested and found to have different ultimate strengths but similar high-cycle fatigue strengths. Finite element analysis was used to help understand the progression of fatigue damage and the effect of stress concentration features. Some tentative design rules are proposed to define stress levels for the safe use of bamboo, taking fatigue into account.

Effect of interference fitting and/or bolt clamping on the fatigue behavior of Al alloy 2024-T3 double shear lap joints in different cyclic load ranges

Two applied fatigue life improvement techniques of joints, interference fitting and bolt clamping, have been investigated comparing available experimental and finite element simulation results for Al-alloy 2024-T3 double shear lap joint specimens. Fatigue test results of interference fitted, bolt clamped and combined interference fitted-bolt clamped specimens were used to determine the effect of the different techniques on the fatigue life improvement in comparison with clearance fitted specimens. Also the fracture surfaces of the specimens were analyzed post-mortem to determine the fatigue crack initiation (and growth) locations. These locations were correlated to the applied cyclic load range. Results of the finite element analysis were used to help explain the trends observed in the experimentally obtained S-N data. Results showed that the fatigue life improvement not only changes for different techniques and magnitude of interference fit or tightening torque, but also depends on the applied cyclic load range.

Resistance spot weld fatigue behavior and dislocation substructures in two different heats of AZ31 magnesium alloy

Abstract Fatigue life and dislocation substructure were evaluated in two groups of AZ31 Mg alloy resistance spot welds (heats SA and SB respectively, which had similar chemical compositions but different fusion zone microstructures). The results showed that the SA welds with a refined microstructure displayed a higher fatigue resistance than the SB welds when tested under conditions of higher cyclic load range causing interfacial failure across the fusion zone. TEM examinations revealed that typical dislocation configurations in the coarse-grained SB welds were parallel dislocation lines and parallelogram dislocation cells produced by basal slip, while elongated dislocation cells arising from basal and pyramidal multiple slips occurred in the SA welds. Twinning was observed to occur in both SA and SB welds, with more twins present in the SB welds. The strong slip incompatibilities between adjacent dendritic grains led to high local stress concentrations that activated twinning in the coarse-grained SB welds, while pyramidal slip together with twinning occurred in the fine-grained SA welds. This resulted in increased number and dispersion of slip systems which improved fatigue life in the SA welds.

Surface Roughness and Stress Responses of a Feldspar Porcelain to Fatigue Impact

Dental ceramics are attractive in restorative dentistry due to their approximations to the appearances and functions of human teeth with which we chew our food. Chewing processes generally occur at cyclic loading range of 70–800 N and up to 1400 cycles per day. Most fatigue studies on dental ceramics were conducted at the loads up to 250 N. These loads are much smaller than the maximum bite forces of 500–700 N and tooth clench or grinding forces of up to 800 N. This paper reports on an investigation of fatigue response of a dental porcelain at the higher end of the load range impacted by a tungsten carbide ball. The responses of surface roughness and contact stress to the applied loads and cyclic numbers were quantitatively studied. The results show that the cyclic numbers had significant influences on both average surface roughness Ra and maximum roughness Rmax (ANOVA, p<0.05). However, the applied loads did not significantly affect Ra and Rmax values (ANOVA, p>0.05). It is also found that the contact stress significantly reduced with the cycles (ANOVA, p<0.05) but did not show a significant change with the applied loads (ANOVA, p>0.05).

Improvement of ASTM compliance offset method for precise determination of crack opening load

There are two serious drawbacks to the ASTM offset method for determination of crack opening load, discontinuity and inconsistency problems. The former is the lack of opening load value in the lower 5% load range due to the procedure that the compliance offset is plotted against the mean load of the segment span of 10% of the cyclic load range. The latter is that the ASTM method underestimates, estimates accurately, or overestimates the opening load, depending on the maximum compliance offset value and the stress ratio. To overcome the two drawbacks, the so-called normalized-extended ASTM method is proposed and evaluated quantitatively in detail using the data of 7475-T7351 and 2024-T351 aluminum alloys. The normalized-extended ASTM method improves the standard ASTM method significantly and provides very good correlation of crack growth rate stress ratio effects. The normalized-extended ASTM method is strongly recommended for crack opening load determination.