Flexural Fatigue Failure Research Articles

Flexural fatigue behavior and damage evolution analysis of aeolian sand concrete under freeze–thaw cycle

Aeolian sand is abundant and is widely distributed in western China and thus can reduce the project cost, alleviate the consumption of river sand resources, as well as protect the ecological environment in the application of concrete. Concrete structures, such as roads, bridges, and airport runways in northern cold regions, are exposed to the combined action of freeze–thaw cycle and fatigue load, and the damage mechanism is more serious and complex than that under a single factor. In the present study, the flexural fatigue behavior and damage evolution of aeolian sand concrete under freeze–thaw cycles are studied. Results show that freeze–thaw damage aggravates the flexural fatigue failure of concrete with aeolian sand, and the addition of aeolian sand enhances the average fatigue life of concrete after freeze–thaw cycle. The fatigue life of concrete that has aeolian sand content of 100% is improved more significantly than that of concrete with aeolian sand content of 50%. A double logarithmic fatigue equation is established for varying numbers of freeze–thaw cycles and different failure probabilities. Additionally, the course of flexural fatigue strain and fatigue modulus of aeolian sand concrete under freeze–thaw cycle action follows a typical three-stage development course. The effects of freezing and thawing action on fatigue behavior, such as fatigue strain growth, stiffness degradation, stress–strain relationship, and dissipated energy of 50% aeolian sand concrete (ASC50), are more significant, whereas the effects of 50 and 100 freeze–thaw cycles on the fatigue properties of 100% aeolian sand concrete (ASC100) are not evident. The research results are of great significance for the popularization and use of aeolian sand concrete in cold regions.

Flexural fatigue behavior of novel co-curing damping composite structures

In order to investigate the flexural fatigue performance of novel carbon fiber reinforced bismaleimide resin matrix damping sandwich composite structure prepared by co-curing process, three-point flexural fatigue tests were carried out on composite structure embedded with damping films of different thickness. The flexural fatigue failure mode, fatigue life and damage evolution law of sandwich beam were obtained, and different damage mechanisms of sandwich beam under flexural fatigue load were analyzed. The S-N curves of undamped composite were fitted by three-point flexural fatigue test. The fatigue characteristics of the undamped composite and damping composite were compared and analyzed. The results showed that the fatigue life of damping composite decreased with the increase of damping layer thickness under the same cyclic load. A numerical simulation model of three-point flexural fatigue performance of damping composite was established. The fatigue life variation trend of damping composite under different damping layer parameters (number of damping layers, location of damping layers, distribution of damping layers) was further simulated, which provided theoretical basis for the fatigue design of damping composite.

Prediction of flexural fatigue life and failure probability of normal weight concrete

Fatigue life has to be considered in the design of many concrete structures at various stress levels and stress ratios. Many flexural fatigue test results of plain normal-weight concrete are available in the literature and almost every set of test results provides different fatigue equations. It is necessary, though, to have a common fatigue equation to predict the design fatigue life of concrete structures under flexural load with reasonable accuracy. Therefore, a database of flexural fatigue test results was created for concrete with strengths ranging from 25 to 65 MPa; this database was used to derive new fatigue equations (Wöhler fatigue equation and S-N power relationship) for predicting the flexural fatigue life of normal-weight concrete. The concept of equivalent fatigue life was introduced to obtain a fatigue equation using the same stress ratio. A probabilistic analysis was also carried out to develop flexural fatigue equations that incorporate failure probabilities.

Effects of graphene nanoplatelets and montmorillonite nanoclay reinforcement on dental polymethyl methacrylate

AbstractThe study aimed at developing a cytocompatible dental polymer composite with high mechanical and surface characteristics by reinforcing polymethyl methacrylate resin with graphene nanoplatelets and montmorillonite nanoclay. After ball milling the particles into the polymer samples were prepared via conventional compression molding method. Samples were tested for flexural strength, polishability, fatigue failure, wear, hardness, and cytotoxicity. The fracture mode and surface polish were tested using a field emission scanning electron microscope. Most properties displayed enhancement at the tested fraction weight in relation to the control, while some properties were downgraded. However, the tested polymer composite was cytocompatible at all concentrations. Within study limits, graphene nanoplatelets and montmorillonite nanoclay reinforced polymer brought about a positive effect to a particular degree. Establishing optimal concentration of reinforcement and alternate blending methods may boost the resultant characteristics of the resin composite.

Flexural fatigue properties and failure propagation of 3D stitched composites under 3-point bending loading

This work investigated the static and fatigue properties of 3D stitched composite and 2D laminated composite subjected to three-point bending loads. Four stress levels were carried out to investigate fatigue characteristics of the composites. The failure propagation of the composites and their fracture morphology were captured in detail using the high-speed camera. The results demonstrated that the static flexural strength of 3D stitched composite was lower than 2D laminated composite but the modulus of 3D stitched composite was higher than that of the latter. Besides, the fatigue failure mechanism of 3D stitching composite was dependent on the stress levels. Though the fatigue life of the 3D stitching composite was not as good as 2D laminated composite, the failure of stitching thread proved to be able to judge the catastrophic damage of the 3D stitching composite, which is important for indicating the safe use of the composite structure.

English

BACKGROUND Polymethyl-methacrylate (PMMA) resin is the most commonly used denture base material which satisfies colour stability, aesthetic demands, accurate fit, affordable cost, and ease in manipulation. The common disadvantages are incomplete flow, impact strength, transverse strength and fatigue resistance. The fracture of denture base is mainly due to flexural fatigue failure which can be minimised by reinforcing with various artificial and natural fibres. There are very few studies which have incorporated natural fibres to enhance the strength of PMMA resin. So, in this study both natural and artificial fibres were used to enhance the property and find the best fibres to strengthen the denture base. METHODS A total of 80 samples were prepared using a standardized stainless steel metal die and these samples were grouped into Group A as control group, Group B PMMA samples reinforced with raw teak wood fibres, Group C reinforced with alkali treated teak wood fibres, Group D reinforced with heat treated teak wood fibres and Group E samples were reinforced with Ultra-High molecular weight (UHMW) polyethylene fibres and processing was carried out by conventional method. The flexural strength of each sample was calculated. RESULTS The mean flexural strength of Group A, Group B, Group C, Group D and Group E were 82.338, 58.680, 62.259, 105.878, 90.2263 respectively with a P value of 0.001. Group D samples showed a P value < 0.001 which was statistically significant when compared to other groups. The obtained values were statistically analysed by oneway ANOVA and Tukey’s HSD test using statistical software SPSS 17. CONCLUSIONS This in-vitro study infers that the samples of Group D incorporated with heat treated teak fibres had shown increased flexural strength, which could be a good replacement for various synthetic and natural fibres to enhance the strength of denture base resins. KEY WORDS Teakwood Fibres, Heat Treatment, Alkali Treatment, Sandwich Technique, Polyethylene Fibres

Reliability assessment of flexural fatigue failure of asphalt mixture: A new perspective

This study aims to evaluate the evolution of reliability with number of cycles for asphalt mixture under repeated fatigue loading, i.e. degradation reliability. A novel methodological framework was developed to analyze the reliability, which integrates four theories (reliability theory, probability distribution function transformation, fatigue damage and Weibull distribution). Based on the developed framework, a novel and simple method was established to calculate the degradation reliability through a probabilistic simulation technique. Flexural fatigue performance of asphalt mixture was then analyzed statistically by Weibull distribution function. The five numerical methods were adopted and compared to estimate Weibull parameters. Kolmogorov-Smirnov test was conducted for further confirming the validity of this distribution. The reliability of asphalt mixture under fatigue loading was assessed using the established method. Results show that the randomness evolution behaviors of asphalt mixture under fatigue loading can be described by the developed framework. The degradation reliability under non-fatigue damage behaviors may be able to be calculated by the established method. The two-parameters Weibull distribution allows to describe fatigue life data of asphalt mixture. The evolutions of reliability with number of cycles show a decrease trend with three-stage. The degradation reliability of asphalt mixture in practical project should be larger than the value corresponding to mean fatigue life as far as possible. Method of moments should be adopted to estimate distribution parameters for the evaluation of degradation reliability of asphalt mixture, the health state of asphalt mixture is thus early warned timely.

Probability of failure of RCA concrete with ternary blended cements

The paper presents the results of an investigation carried out to examine the flexural fatigue strength and failure probability of concrete made with 50% Coarse Recycled Concrete Aggregates (RCA) and ternary blended cements using Silica Fume (SF) and Metakaolin (MK). A control mix containing 100% Coarse Natural Aggregates (NA) was also tested for comparing the flexural fatigue results. A total of 128 concrete beam specimens were tested under flexural fatigue using 100 kN MTS Servo-controlled actuator under four-point flexural fatigue loads applied at frequency of 10 Hz. The material coefficients of various fatigue test models have been calculated to predict the flexural fatigue strength of different concrete mixes. The S-N-Pf curves have been generated for all the concrete mixes using the experimental fatigue data at various probabilities of failure. Two million cycles endurance limit has also been calculated for all the concrete mixes to assess the flexural fatigue strength of concrete made with 50% RCA. A minute decrease occurs in the endurance limit of concrete made with 50% RCA (i.e. 56% of average static flexural strength) with respect to the control concrete (i.e. 58%). Whereas, the endurance limits of 66% and 68% have been observed for the concretes made with 50% RCA with SF and 50% RCA with MK respectively. Therefore, the use of SF and MK has been found to enhance the flexural fatigue performance of concrete made with RCA.

Flexural fatigue failure of concrete reinforced with smooth and mixing hooked-end steel fibers

This paper presents an experimental study on flexural fatigue dynamic behavior of reinforced concrete. Steel fiber is used for the retrofitting and strengthening flexural members, by mixing short s...

Probability of Flexural Fatigue Failure of Concrete made with Recycled Concrete Aggregates

The paper presents results of an investigation conducted to quantify the flexural fatigue performance of concrete in which the Coarse Natural Aggregates (NA) were systematically replaced with Coarse Recycled Concrete Aggregates (RCA). Concrete mix containing 100% NA was also tested for comparison purpose. The compressive strength results of 100% RCA showed maximum reduction followed by the concrete made with 50% RCA. The fatigue test data of concrete made with 50% and 100% RCA and 100% NA was obtained by conducting flexural fatigue tests on beam specimens of 100×100×500 mm size. The specimens were tested using 100 kN MTS Servo-controlled actuator under four-point flexural fatigue loads applied at frequency of 10 Hz. Static flexural tests were also conducted to facilitate fatigue testing. The maximum flexural strength of 5.10 MPa was observed by the concrete mix containing 0% RCA (100% NA) followed by 4.89 MPa and 4.53 MPa for 50% RCA and 100% RCA respectively. To predict the flexural fatigue strength of concrete made with RCA, the materials coefficients of fatigue strength prediction models have been obtained. The S-N-Pf models for three concrete mixes have also been generated from the fatigue test data analytically. Two-Million cycles fatigue strength has also been estimated which was coming out to be 50% and 56% of the actual flexural strength for concrete mixes containing 50% and 100% RCA respectively.

Fatigue behavior of steel reinforced concrete (SRC) beams with different shear span-to-depth ratios

There is an increase in the use of steel reinforced concrete (SRC) beams in high-speed railway platforms and bridges that are subjected to fatigue loading. It was found by the authors in their previous research on SRC beams under fatigue loading that flexural fatigue failure was the dominating failure mode when a large shear span-to-beam depth ratio (λ) of 2.63 was used. Research on reinforced concrete (RC) beams under fatigue loading and research on SRC beams under static loading revealed that shear failure may dominate when the shear span-to-beam depth ratio gets smaller. It is unknown if shear fatigue failure will happen in SRC beams with smaller shear span-to-beam depth ratio. This paper presents an experimental investigation of fatigue behavior of SRC beams with shear span-to-beam depth ratio of 1.8, 1.5, 1.1 and 1.0. It was found that shear fatigue failure did not occur in all twenty-seven SRC beams tested even for λ ratio as low as 1.0. Because the failure mode is found to be the same (i.e. dominated by flexural fatigue failure) for all the SRC beams with λ ranging from 1.0 to 2.63, the test results of all 39 SRC beams under fatigue loading were grouped together to derive fatigue strength curves for SRC beam design.

Analysis of flexural fatigue failure of concrete made with 100% coarse recycled and natural aggregates

In this study, the flexural fatigue performance of concrete beams made with 100% Coarse Recycled Concrete Aggregates (RCA) and 100% Coarse Natural Aggregates (NA) were statistically commanded. For this purpose, the experimental fatigue test results of earlier researcher were investigated using two parameter Weibull distribution. The shape and scale parameters of Weibull distribution function was evaluated using seven numerical methods namely, Graphical method (GM), Least-Squares (LS) regression of Y on X, Least-Squares (LS) regression of * on Y, Empherical Method of Lysen (EML), Mean Standard DeviationMethod (MSDM), Energy Pattern FactorMethod (EPFM) andMethod of Moments (MOM). The average of Weibull parameters was used to incorporate survival probability into stress (S)-fatigue life (N) relationships. Based on the Weibull theory, as single and double logarithm fatigue equations for RCA and NA under different survival probability were provided. The results revealed that, by considering 0.9 level survival probability, the theoretical stress level corresponding to a fatigue failure number equal to one million cycle, decreases by 8.77% (calculated using single-logarithm fatigue equation) and 6.62%(calculated using double logarithm fatigue equation) in RCAwhen compared to NAconcrete.

Protocol for Testing Flexural Strength, Flexural Modulus, and Fatigue Failure of Cementitiously Stabilized Materials Using Third-Point Flexural Beam Tests

Abstract In this study, a testing protocol was developed to measure the flexural strength, flexural modulus, and fatigue failure of cementitiously stabilized materials (CSMs) for both lightly and heavily stabilized soils. Four soils (sand, gravel, silt, and clay) and four binders (cement, lime, class C fly ash, and class F fly ash) were used in this study. Beam specimens (100 mm high by 100 mm wide by 400 mm long) were prepared and tested using third-point flexural beam tests. A fatigue distress model was developed, and the data from this study was validated using other existing stress-based fatigue models. Results indicated that the testing protocol could be used to determine the flexural strength, flexural modulus, and fatigue behavior of the cementitiously stabilized layers (CSL). A stress-based fatigue performance model was a good fit for the whole range of CSMs, which is needed to predict the fatigue performance of CSL in the field and to determine the fatigue life of the CSLs.

Analysis of flexural fatigue failure of concrete made with 100% Coarse Recycled Concrete Aggregates

The paper presents results of an investigation carried out to analyze the flexural fatigue performance of concrete beams made with 100% Coarse Recycled Concrete Aggregates (RCA) and its comparison with that of concrete made with 100% Coarse Natural Aggregates (NA). The fatigue performance of concrete beams made with RCA as well as NA has been assessed in terms flexural fatigue life distributions and two-million cycle endurance limit. Experiments were conducted to obtain the flexural fatigue lives of concrete beam specimens made with 100% RCA as well as 100% NA under different stress levels and further compared with literature available on NA. Specimens of size 100mm×100mm×500mm were tested under four point flexural fatigue loads applied at a frequency of 10Hz. It has been shown that the fatigue life distributions of concrete mixes made with 100% RCA and 100% NA can be modeled by the two-parameter Weibull distribution. The values of the shape parameters of the Weibull distribution obtained for concrete made with RCA have been found to be smaller than that of concrete made with NA in present investigation and previous studies on NA, thus indicating higher variability in the distribution of flexural fatigue life of concrete made with RCA viz. a viz. concrete made with NA. The two-million cycles endurance limit for concrete made with 100% RCA has been found to be 50%, which is about 8% and 7% lower than that of concrete made with NA in present and previous studies respectively.

TESTING THE FLEXURAL FATIGUE BEHAVIOR OF E-GLASS EPOXY LAMINATES

With the advent of fiber reinforced composite materials the application of polymer composite materials is increasing in aerospace, marine and automobile fields, and non-conventional energy harvesting sectors in view of deriving advantages of polymer composites towards enhancing the equipment performance. Composites are replacing the conventional materials almost in every application. Due to their high specific strength, stiffness, better fatigue life, wear resistance and corrosion resistance. So polymer composites play a vital role in weight reduction with better functional characteristics of critical components like leaf spring, wheel drum, crash bumper etc. The tailor-designing capability with composite materials provides large scope of mechanical properties by adopting advanced design techniques. The tailored properties of laminated composite throws a challenge on design engineers to arrive at optimum design solutions. The flexural fatigue failure is common in rotating components such as wind turbine rotors and aero plane wing structures. The present work is focusing on flexural fatigue failure behavior with the help of flexural fatigue test rig established in the mechanical engineering department of JNTU. The explicit work publishes the experimental test results to understand the influence of frequency on the fatigue failure behavior due to cyclic flexing loads on the glass epoxy laminates

Improved flexural fatigue resistance of PVA fiber-reinforced concrete subjected to freezing and thawing cycles

This paper investigates the effect of PVA fiber-reinforcement on the flexural fatigue behavior of concrete subjected to freezing and thawing cycles. Plain and PVA fiber-reinforced concrete specimens were exposed to freezing and thawing cycles, then a flexural fatigue test using a wheel load was performed with a stress level in the range of 0.5–0.9. The effect of PVA fiber reinforcement on the flexural fatigue resistance was evaluated by the stress ratio–fatigue life (S–N) relationship. The flexural fatigue test results showed that repeated freezing and thawing actions not only reduced the static strength but also dramatically diminished the resistance to fatigue failure. In addition, a low interrelationship of the S–N relationship of concrete subjected to freezing and thawing cycles was observed. PVA fiber reinforcement enhanced the flexural fatigue strength of concrete subjected to freezing and thawing cycles due to increase in (1) the resistance for freezing and thawing action and (2) the resistance for flexural fatigue failure. Finally, empirical formulas for the S–N relationship of plain and PVA fiber-reinforced concretes subjected to freezing and thawing cycles are proposed based on the experimental investigation.

Flexural fatigue failures and lives of Eco-Core sandwich beams

Eco-Core is a class of syntactic foam made from small volume of high char yield binder and large volume of a class of flyash for fire resistance application. Very little or no flexural fatigue data of this class of core material is reported in the open literature. This paper presents a flexural fatigue response of Eco-Core in a glass/vinyl ester composite face sheet sandwich beam. A four-point loaded flexural test specimen was designed and tested in static and fatigue loadings to cause tension failure in the core. The fatigue test was conducted at maximum cyclic stress (σmax) ranged from 0.7σct to 0.9σct, where σct is the static flexural strength of the core. The sinusoidal loading frequency of 2Hz with the stress ratio of 0.1 was used. Flexural fatigue failure modes of Eco-Core sandwich beam were classified: damage onset (single tension crack), damage progression (multiple tension cracks) and ultimate failure (a combination of tension and shear). These failures were characterized by 1%, 5% and 7% changes in compliance that corresponds to N1%, N5% and N7% lives. The fatigue stress-life (S–N) relationship was found to follow the well-known power law equation, σmax/σct=AoNα. The constants Ao and α were established for all three types of failures. The endurance limit was established based on 1million cycles limit and it was found to be 0.65σct, 0.70σct and 0.71σct, respectively for the three modes of failure. Flexural fatigue and static failure modes of Eco-Core sandwich beams were similar.

An in vitro study to compare the transverse strength of thermopressed and conventional compression-molded polymethylmethacrylate polymers

Acrylic resins have been in the center stage of Prosthodontics for more than half a century. The flexural fatigue failure of denture base materials is the primary mode of clinical failure. Hence there is a need for superior physical and mechanical properties. This in vitro study compared the transverse strength of specimens of thermopressed injection-molded and conventional compression-molded polymethylmethacrylate polymers and examined the morphology and microstructure of fractured acrylic specimens. The following denture base resins were examined: Brecrystal (Thermopressed injection-molded, modified polymethylmethacrylate) and Pyrax (compression molded, control group). Specimens of each material were tested according to the American Society for Testing and Materials standard D790-03 for flexural strength testing of reinforced plastics and subsequently examined under SEM. The data was analyzed with Student unpaired t test. Flexural strength of Brecrystal (82.08 ± 1.27 MPa) was significantly higher than Pyrax (72.76 ± 0.97 MPa). The tested denture base materials fulfilled the requirements regarding flexural strength (>65 MPa). The scanning electron microscopy image of Brecrystal revealed a ductile fracture with crazing. The fracture pattern of control group specimens exhibited poorly defined crystallographic planes with a high degree of disorganization. Flexural strength of Brecrystal was significantly higher than the control group. Brecrystal showed a higher mean transverse strength value of 82.08 ± 1.27 MPa and a more homogenous pattern at microscopic level. Based on flexural strength properties and handling characteristics, Brecrystal may prove to be an useful alternative to conventional denture base resins.

Flexural fatigue strength and failure probability of Self Compacting Fibre Reinforced Concrete beams

The paper presents an experimental investigation on the flexural fatigue strength of Self Compacting Fibre Reinforced Concrete (SCFRC) beams. The fatigue life data of SCFRC containing 0.5%, 1.0% and 1.5% by volume of steel fibres have been obtained by conducting flexural fatigue tests on approximately 188 beam specimens of size 100×100×500mm under third point loading at different stress levels, ranging from 0.90 to 0.70. Approximately 144 complimentary static flexural tests were also carried out to facilitate fatigue testing. All the static flexural and flexural fatigue tests were conducted on a 100kN closed loop servo-controlled actuator. The results have been represented in the form of S–N diagrams and to predict the flexural fatigue strength of SCFRC, material coefficients of the fatigue equations have been estimated. Subsequently, family of S–N–Pf curves has been generated from the fatigue test data to graphically represent the relationship between stress level S, fatigue life N, and probability of failure Pf, thus incorporating probability of failure into the fatigue life data of SCFRC. The experimental coefficients of the fatigue equation have also been obtained from the fatigue test data to represent the S–N–Pf curves analytically. The two-million cycles fatigue strength of SCFRC has been found to be higher than that of Normally Vibrated Fibre Reinforced Concrete (NVFRC).

Evaluation of cyclic flexural fatigue resistance of 25/0.04 and 25/0.06 twisted file rotary nickel‐titanium endodontic instruments

Twisted files (TFs) are rotary nickel-titanium (NiTi) instruments that are produced with a newly developed manufacturing process that is supposed to improve their properties, especially flexibility and resistance to cyclic flexural fatigue failure. The aim of this study was to study the cyclic flexural fatigue failure resistance of tip size International Standards Organization 25 TFs with two tapers 0.04 and 0.06 and to compare them with the Profile (PF) rotary NiTi files of similar tip size and taper. Four groups of fifteen files were used in this study. TF (25/0.04), TF (25/0.06), PF (25/0.04) and PF (25/0.06) were tested using the cyclic flexural fatigue testing device. The time to failure during cyclic flexural fatigue testing was recorded. The mean time required for the instrument to fail under cyclic flexural fatigue testing was 235.5 ± 68 s for 25/0.04 TF, 188.5 ± 75 s for 25/0.06 TF, 180.3 ± 102 s for 25/0.04 PF and 156.3 ± 17 s for 25/0.06 PF. The difference between the time to failure of 25/0.04 TF and 25/0.06 TF was not statistically significant. The difference between the time to failure of TF and PF of similar tip size and taper was not statistically significant. The findings of this study indicate that size 25/0.04 and 25/0.06 TFs had similar resistance to cyclic flexural fatigue failure. In addition, TFs were not superior, in terms of resistance to cyclic flexural fatigue failure, to PF of similar tip size and taper.