Flexural Fatigue Strength Research Articles

Flexural fatigue strength of demolition aggregates stabilized with alkali-activated calcium carbide residue

Replacing the base and subbase layers in flexible pavements and sub-ballast layer in railway tracks with recycled materials has attracted significant attention from industry and researchers alike. The semi-rigid stabilized layers in the flexible pavement structure and railway tracks sustain high flexural and tension stresses which reduces the serviceability life of the superstructure. The flexural and compressional modulus of the stabilized materials under repeated loads and at at-rest k0 condition is highly dependent on the specifications of the binder. Industrial by-products including calcium carbide residue (CCR), Fly Ash (FA) and Slag (S) were utilized to improve the compressional resilient modulus, flexural strength, durability and fatigue life of recycled aggregates. The variation of ductility in the blend with different precursors indicated that the service life of the stabilized layers not only depended on the strength but was also controlled by the deterioration rate of the flexural modulus and stiffness of the sample. The ductility of binder with aluminosilicate gels fabric can potentially absorb the stresses of the cyclic loading and postpones development of tension cracks.

Mechanical and rheometric properties of natural rubber composites filled with untreated and chemically treated leather wastes

In this work, the effect of the incorporation of leather wastes from chrome tanning on the mechanical and rheometric properties of natural rubber-based composite was evaluated. Chrome content in leather wastes diminished as a result of a chemical treatment with a sodium bicarbonate solution. The treatment also improved the tensile properties of composites with 0, 20 and 40 parts per hundred rubber of leather wastes with particle size between 590 and 840 µm. In a second stage of the experimental work, 0, 20, 40, 60, 80 and 100 parts per hundred rubber of chemically treated leather wastes with particle size between 297 and 590 µm were incorporated to the same natural rubber compound studied in the first part of this study. All the composites were prepared in a torque rheometer, seeking to determine their rheological behaviour during mixing. Rheometric properties of all the composites during curing were monitored in a moving die rheometer. Tensile properties, tear strength, hardness, compressive modulus, compression-set, abrasive wear resistance and De Mattia flexing fatigue resistance for composites obtained in the second experimental set of this work were evaluated. It was found that leather wastes improved the hardness, stiffness, tear strength and abrasive wear resistance on the composites. However, elongation at break, flexural fatigue and tensile strength diminished when a large quantity of wastes was incorporated.

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.

Mechanical and hydrolytic degradation of an Ormocer®-based Bis-GMA-free resin composite

The aim of the study was to evaluate the mechanical stability of bisphenol A-glycidyl methacrylate (Bis-GMA) and Ormocer-based resin composites before and after water absorption and to examine water saturation. Disc-shaped specimens of the Bis-GMA (Grandio SO, Voco) and the Ormocer-based (Admira Fusion, Voco) dental resin composites were produced, stored in water, and weighed after pre-determinedtimes to measure the absorbed water. Bend bars were produced and stored for 24h in dry conditions as well as in distilled water for 14days or 60days at 37°C. The initial flexural strength (FS) under quasi-static loading and flexural fatigue strength (FFS) under cyclic loading were determined under 4-point bending.Fracture toughness (KIc) of both composites was measured using the single-edge-V-notch-beam (SEVNB) technique after the same storage conditions under 3-point bending. Within the first 14days, storage conditions did not affect the initial FS of Grandio SO, while a significant drop in initial FS was observed for Admira Fusion after 2weeks in water and most of the water was absorbed within this time. FFS for the Bis-GMA composite was not reduced before 2months in water, whereas for the Ormocer®-based composite, there has been a significant decrease in strength after cyclic fatigue already at 2weeks of water storage. KIcof Admira Fusion decreased significantly after both storage periods, while KIcof Grandio SO decreased only significantly after 2weeks of water storage. All mechanical properties of the Bis-GMA composite were superior to those of the Ormocer®-based material, except water sorption. Water storage seems to have a much more pronounced effect on the mechanical properties of Ormocer®-based dental composites in comparison to Bis-GMA-based composites.

Damage sensing, mechanical and interfacial properties of resins suitable for new CFRP rope for elevator applications

The evaluation of damage sensing for flexural, interfacial and fatigue conditions was studied for carbon fiber reinforces plastic (CFRP) rope, with 4 different formulations of epoxy resin. Thermal analysis and tensile tests were used to investigate the thermal and mechanical properties for different epoxy formulations. Using these experimental results along with the empirical equation between tensile strength, glass transition temperature, Tg, and enthalpy, ΔH, optimum conditions for the epoxy formulations were found. Eddy current and static contact angle measurements were made to evaluate the wettability of epoxy into the carbon fibers with the different epoxy types. Flexural strength, fatigue strength and inter-laminar shear strength (ILSS) were evaluated for manufactured CFRP rope via pultrusion process. Changed ratio of electrical resistance (ER) of CFRP rope were measure during flexural testing, for signs of micro-cracking leading up to final fracture. The signal of CFRP rope were measured by changes in electrical resistance (ER) to predict the fracture failure. From the results of these studies it was concluded that an acid anhydride-based epoxy was the optimal epoxy formulation for manufacture of CFRP rope.

Mechanical performance of Y-TZP monolithic ceramic after grinding and aging: Survival estimates and fatigue strength

This study aims to evaluate the flexural fatigue strength and survival estimates of an Y-TZP monolithic ceramic after grinding and aging. Ceramic discs (1.2 mm thickness × 15.0 mm diameter - ISO, 6872, 2015) of zirconia (Zirlux FC2 – Ardent; Ivoclar Vivadent) were made and randomly allocated into 6 groups (n = 10), according to grinding and aging factors: Ctrl – as-sintered; Ctrl Sto – as-sintered and dry stored at room temperature for 2 years; Ctrl Aut Sto – as-sintered, submitted to autoclaved aging (134 °C, 2 bar, 20 h) and then dry stored for 2 years; and similar conditions for ground samples (Ground; Ground Sto; Ground Aut Sto). Grinding was performed with diamond burs (#3101G, KG Sorensen) coupled to a contra-angle torque multiplier attached to a low speed motor under constant irrigation. Fatigue testing followed a step-stress approach. Data from strength and number of cycles until fracture were recorded and analyzed through Kaplan-Meier and Mantel-Cox tests. Both grinding and aging increase monoclinic phase content. The topography was altered by grinding but not by aging procedures. Grinding did not alter the fatigue strength (Ctrl = Ground), while aging increase it only for ground groups (Ground Sto, Ground Aut Sto). Aged conditions (Ctrl Sto; Ground Sto; Ctrl Aut Sto; Ground Aut Sto) showed increased survival probabilities for both flexural fatigue strength and cycles for failure. Therefore, despite promoting monoclinic phase increase, aging and grinding did not deleteriously affect the fatigue behavior of Y-TZP ceramics.

The effect of extended glaze firing on the flexural fatigue strength of hard-machined ceramics

Statement of problemIt is unclear whether an extended glaze firing could improve the long-term mechanical performance of densely sintered CAD-CAM ceramics. PurposeThe purpose of this in vitro study was to analyze the effect of an extended glaze firing on the flexural fatigue strength (FFS) of densely sintered milled (hard-machined) leucite-based (LEU) and lithium disilicate-based (DIS) ceramics. Material and methodsDisks were machined from ceramic blocks and divided into 6 groups (n=20) according to the material, LEU or DIS, and to the applied glaze firing: manufacturer-recommended glaze (G group), extended glaze (EG group), and control/no firing (C group). The surface roughness of the disks was measured before and after firing by using a contact profilometer, and data were compared by paired sample tests. Specimens were submitted to fatigue by using the staircase test design in water (piston-on-3 balls; 500 000 cycles, 20 Hz, and sinusoidal loading). Mean (±SD) FFS values were then calculated and analyzed by using 1-way analysis of variance and post hoc Tukey test (α=.05). ResultsSurface roughness did not change after the firing (P>.05). The highest FFS value in both ceramics was obtained after EG firing (LEU-EG=80.52 ±6.3 MPa; DIS-EG=147.25 ±10.5 MPa), which was statistically superior to G firing (LEU-G=73 ±6.8 MPa, P=.003; DIS-G=134.34 ±15.6 MPa; P=.023) and C group (LEU-C=61.94 ±6.3 MPa; P<.001; DIS-C=134.13 ±17.3 MPa; P=.023). ConclusionsEG firing optimized the biaxial flexural fatigue strength of hard-machined leucite and lithium disilicate ceramics compared with conventional glaze firing.

Flexural fatigue performance of concrete made with recycled concrete aggregates and ternary blended cements

Results of an investigation to evaluate the flexural fatigue performance of concretes made with fly ash, silica fume (SF) as partial replacement of Portland cement and 100% coarse recycled concrete aggregates (RCA) are reported. Flexural fatigue test data were obtained based on 168 static flexural tests and flexural fatigue tests conducted using Servo-controlled Actuator. The test data thus obtained were analysed and equations to estimate the flexural fatigue strength of concrete mixes were proposed. The probability distributions for the fatigue life of various concretes were also established. The flexural fatigue performance of various concrete mixes was assessed in terms of mean and design fatigue lives. The results obtained from the study show that flexural fatigue performance of concrete containing 100% RCA with 10% SF performed better than concrete containing 100% coarse natural aggregate.

Flexural fatigue behavior of ultra-lightweight cement composite and high strength lightweight aggregate concrete

This paper investigated the fatigue performance of ultra-lightweight cement composite (ULCC) and lightweight aggregate concrete (LWAC) subjected to flexural load. The ULCC having mean density of 1450 kg/m3 contained cenosphere as micro aggregates and 0.9% volume of polyvinyl alcohol (PVA) fibers. The average 28-days cylinder compressive strengths of the ULCC and LWAC were 62 MPa and 63 MPa, respectively. 108 specimens were tested to measure the flexural fatigue strength under third-point loading. All the specimens were sized as 100 × 76 × 406 mm with an effective span of 300 mm. Using the experimental results, S-N curves were plotted and regression analysis was conducted to propose the equations (called Wöhler equations) for predicting the flexural fatigue strength of ULCC and LWAC. Also, the probabilistic distributions of fatigue life of ULCC and LWAC at a given stress level were modeled using the two-parameter Weibull distribution. The distribution parameters were obtained using three different methods. Design fatigue lives were obtained at different stress levels for ULCC and LWAC corresponding to different failure probabilities. The S-N relationship incorporating the failure probability is found more conservative than that found by Wöhler fatigue equation. The flexural fatigue performance of ULCC is better than that of LWAC, both of having similar strength.

Effect of Grinding and Multi-Stimuli Aging on the Fatigue Strength of a Y-TZP Ceramic.

This study aimed to investigate the effect of grinding and multi-stimuli aging on the fatigue strength, surface topography and the phase transformation of Y-TZP ceramic. Discs were manufactured according to ISO-6872:2008 for biaxial flexure testing (diameter: 15 mm; thickness: 1.2 mm) and randomly assigned considering two factors "grinding" and "aging": C- control (as-sintered); CA- control + aging; G- ground; GA- ground + aging. Grinding was carried out with coarse diamond burs under water-cooling. Aging protocols consisted of: autoclave (134°C, 2 bars pressure, 20 hours), followed by storage for 365 days (samples were kept untouched at room temperature), and by mechanical cycling (106 cycles by 20 Hz under a load of 50% from the biaxial flexure monotonic tests). Flexural fatigue strengths (20,000 cycles; 6 Hz) were determined under sinusoidal cyclic loading using staircase approach. Additionally, surface topography analysis by FE-SEM and phase transformation analysis by X-ray Diffractometry were performed. Dixon and Mood methodology was used to analyze the fatigue strength data. Grinding promotes alterations of topographical pattern, while aging apparently did not alter it. Grinding triggered t-m phase transformation without impacting the fatigue strength of the Y-TZP ceramic; and aging promoted an intense t-m transformation that resulted in a toughening mechanism leading to higher fatigue strength for as-sintered condition, and a tendency of increase for ground condition (C < CA; G = GA). It concludes that grinding and aging procedures did not affect deleteriously the fatigue strength of the evaluated Y-TZP ceramic, although, it promotes surface topography alterations, except to aging, and t-m phase transformation.

Investigations on Flexural Fatigue Strength of Conductor Paths Fabricated by LPKF-LDS® Technology

Reliability aspects are crucial for the success of every technology in industrial application. Regarding interconnect devices, several methods are applied to evaluate reliability of conductor paths like accelerated environmental tests. Especially, molded interconnect devices (MID), which enable numerous applications with three-dimensional (3D) circuitry on 3D shaped injection-molded thermoplastic parts are often under particular stress, e.g., as component of a housing. In this study, a new test method for evaluating the flexural fatigue strength of conductor paths produced by the laser-based LPKF-LDS® technology is presented. For characterization of test samples, a test bench for flexural fatigue test was built up. A result of the flexural fatigue test is a characteristic Woehler curve of the metal layer system. Applying this new test method, essential influencing parameters on the reliability of MID under mechanical load can be identified. So, the metal layer system as well as the geometric parameters of the metal layer is crucial for the performance. Furthermore, test specimens are tested under different types of mechanical load, i.e., tensile stress and compressive stress. For a holistic view on reliability of MID, experimental results are discussed and supported by simulations. An important finding of the study is the advantage of nickel-free layer systems in contrast to the Cu/Ni/Au layer system, which is often used in MID technology.

Flexural fatigue behavior of thin laminated cementitious composites incorporating cenosphere fillers

Flexural fatigue behavior of thin laminated cementitious composites (LCCs) is evaluated in this paper. LCCs were developed by incorporating fly ash cenosphere (FAC), as lightweight filler material, in various weight fractions of 40%, 50%, and 60%. The composites were reinforced with continuous reinforcement (welded wire mesh and woven glass fiber mesh), and discontinuous reinforcement (PVA fibers). Both single and double layer reinforced composites were cast. The developed composites were tested for static flexural strength as well as flexural fatigue strength under three – point flexural loading. The fatigue lives of LCC were determined at various stress levels and stress ratios. The results thus obtained are used to develop S-N relationships, and equations have been proposed to predict the flexural fatigue strength of LCC. The results indicate that the probabilistic distribution of fatigue life of LCC can be modeled by two-parameter Weibull distribution. The Weibull distribution has been employed to incorporate the failure probabilities into the fatigue life of LCC. The fatigue lives corresponding to different failure probabilities have been calculated and the data so obtained have been used to generate Pf-S-N diagrams. It is found that FAC is useful in producing structural lightweight composites with enhanced ductility and improved flexural fatigue characteristics.

Fatigue behavior of pure polypropylene and recycled polypropylene reinforced with short glass fiber

In this study, the fatigue behavior of recycled polypropylene reinforced with short glass fiber with different weight fractions (5 wt%, 10 wt%, 20 wt%, and 30 wt%) was compared to pure polypropylene under dry and wet conditions. The specimens were manufactured using injection molding process. The results show that addition of short glass fiber to recycled polypropylene resulted in increasing water uptake compared to pure polypropylene. However, flexural fatigue strength of recycled polypropylene reinforced with short glass fiber with 10 wt%, 20 wt%, and 30 wt% exhibits higher relative fatigue strength than pure polypropylene under dry and wet conditions. Fatigue strength increases with the increase in fiber contents increased in recycled polypropylene matrix.

Impact of machining on the flexural fatigue strength of glass and polycrystalline CAD/CAM ceramics

ObjectivesTo assess the effect of machining on the flexural fatigue strength and on the surface roughness of different computer-aided design, computer-aided manufacturing (CAD/CAM) ceramics by comparing machined and polished after machining specimens. MethodsDisc-shaped specimens of yttria-stabilized polycrystalline tetragonal zirconia (Y-TZP), leucite-, and lithium disilicate-based glass ceramics were prepared by CAD/CAM machining, and divided into two groups: machining (M) and machining followed by polishing (MP). The surface roughness was measured and the flexural fatigue strength was evaluated by the step-test method (n=20). The initial load and the load increment for each ceramic material were based on a monotonic test (n=5). A maximum of 10,000 cycles was applied in each load step, at 1.4Hz. Weibull probability statistics was used for the analysis of the flexural fatigue strength, and Mann-Whitney test (α=5%) to compare roughness between the M and MP conditions. ResultsMachining resulted in lower values of characteristic flexural fatigue strength than machining followed by polishing. The greatest reduction in flexural fatigue strength from MP to M was observed for Y-TZP (40%; M=536.48MPa; MP=894.50MPa), followed by lithium disilicate (33%; M=187.71MPa; MP=278.93MPa) and leucite (29%; M=72.61MPa; MP=102.55MPa). Significantly higher values of roughness (Ra) were observed for M compared to MP (leucite: M=1.59μm and MP=0.08μm; lithium disilicate: M=1.84μm and MP=0.13μm; Y-TZP: M=1.79μm and MP=0.18μm). SignificanceMachining negatively affected the flexural fatigue strength of CAD/CAM ceramics, indicating that machining of partially or fully sintered ceramics is deleterious to fatigue strength.

Analysis of Flexural Fatigue Strength of Self Compacting Fibre Reinforced Concrete Beams

This study presents the extensive statistical investigation ofvariations in flexural fatigue life of self-compacting Fibrous Concrete (FC) beams. For this purpose, the experimental data of earlier researchers were examined by two parameter Weibull distribution.Two methods namely Graphical and moment wereused to analyse the variations in experimental data and the results have been presented in the form of probability of survival. The Weibull parameters values obtained from graphical and method of moments are precise. At 0.7 stress level, the fatigue life shows 59861 cyclesfor areliability of 90%.

CAD/CAM machining Vs pre-sintering in-lab fabrication techniques of Y-TZP ceramic specimens: Effects on their mechanical fatigue behavior

This study evaluated the effects of different pre-sintering fabrication processing techniques of Y-TZP ceramic (CAD/CAM Vs. in-lab), considering surface characteristics and mechanical performance outcomes. Pre-sintered discs of Y-TZP ceramic (IPS e.max ZirCAD, Ivoclar Vivadent) were produced using different pre-sintering fabrication processing techniques: Machined- milling with a CAD/CAM system; Polished- fabrication using a cutting device followed by polishing (600 and 1200 SiC papers); Xfine- fabrication using a cutting machine followed by grinding with extra-fine diamond bur (grit size 30 μm); Fine- fabrication using a cutting machine followed by grinding with fine diamond bur (grit size 46 μm); SiC- fabrication using a cutting machine followed by grinding with 220 SiC paper. Afterwards, the discs were sintered and submitted to roughness (n=35), surface topography (n=2), phase transformation (n=2), biaxial flexural strength (n=20), and biaxial flexural fatigue strength (fatigue limit) (n=15) analyses. No monoclinic-phase content was observed in all processing techniques. It can be observed that obtaining a surface with similar characteristics to CAD/CAM milling is essential for the observation of similar mechanical performance. On this sense, grinding with fine diamond bur before sintering (Fine group) was the best mimic protocol in comparison to the CAD/CAM milling.

Fatigue strength and failure probability of concrete made with RCA

The paper presents results of an investigation into the flexural fatigue strength of concrete in which all the coarse natural aggregates (NA) were replaced with coarse recycled concrete aggregates (RCA). Concrete specimens containing 100% NA were also tested. The fatigue life data of concrete made with RCA and NA has been obtained by performing flexural fatigue tests on 64 beam specimens under four-point loading. In total, 48 static flexural tests were also conducted to facilitate fatigue testing. To predict the flexural fatigue strength of concrete made with RCA, the materials coefficients of fatigue strength prediction models have been estimated. The S–N–Pf curves have been generated from the fatigue test data for concrete made with RCA. Theoretical fatigue lives for concrete made with RCA have also been estimated using a single-log fatigue equation corresponding to 10% probability of failure and compared with those of concrete made with NA. The theoretical fatigue lives of concrete made with RCA have been found to be lower by 42·0% and 60·5% compared to concrete made with NA at stress levels S = 0·90 and S = 0·65, respectively.

Long-term life prediction of woven CFRP laminates under three point flexural fatigue

Abstract This technical contribution reports details of an investigation of the long-term flexural fatigue life behavior of plain weave carbon 3K-70P/epoxy (CFRP) laminates using accelerated testing methodology (ATM) coupled with time temperature superposition principle (TTSP). Initially, dynamic mechanical analysis (DMA) tests were performed on the CFRP laminates to determine the time-temperature shift factors followed by constant strain rate (CSR) tests at different temperatures to construct the creep strength master curve. The time-temperature shift factors were computed using an automated curve shifting programme. Horizontal shift factors were found to follow Arrhenius linear relationship for both above and below glass transition temperatures with different gradients. The scatter in the strength data was considered using Weibull analysis. Flexural fatigue tests were then conducted at different temperatures and loads. Finally, the master curves of flexural fatigue strength were constructed to predict the life of the CFRP laminates at arbitrary frequencies and load cycle ratios. Predicted fatigue behavior at non-default test parameters was found to be in good agreement with experimental data.

Experimental Evaluation of Flexural Static and Fatigue Strength of an Innovative Splice for Prestressed Precast Concrete Girders

AbstractThis paper describes flexural static and fatigue laboratory testing of a new connection design for spliced, prestressed precast I-girders. Intended for use as a field-spliced moment-carrying connection, the joint is prestressed at the jobsite using hydraulic jacks in the joint area. The new design was developed to achieve longer spans than currently permitted by transportation limits for precast segments. Behavior at the splice is similar to that of unbonded joints in segmental bridges. Whether the prestressed joint is considered bonded or unbonded for ultimate strength calculations, the spliced test specimens outperformed established calculated values when either loaded statically or after fatigue loading.

Loading frequencies up to 20 Hz as an alternative to accelerate fatigue strength tests in a Y-TZP ceramic

Considering the interest of the research community in the fatigue behavior of all-ceramic restorations and the time consumed in low-frequency cyclic fatigue tests, this study aimed to investigate the influence of the loading frequency on the zirconia fatigue strength. The biaxial flexural fatigue strength of Y-TZP discs was determined by the staircase approach after 500,000 cycles. The investigated frequencies were 2Hz (control-simulation of the chewing activity; n=20), 10Hz (n=20), 20Hz (n=20), and 40Hz (n=21). The fatigue strength data were analyzed using one-way ANOVA and post-hoc Tukey׳s test (α=0.05). Pearson coefficient (r) was calculated to assess the existence of a correlation between fatigue strength and loading frequency. X-ray diffraction analysis was used to determine the relative amount of monoclinic phase under each fatigue test condition. The fatigue strength was significantly higher for 40Hz group (630.7±62.1MPa) and did not differ among the groups 2Hz (550.3±89.7MPa), 10Hz (574.0±47MPa) and 20Hz (605.1±30.7MPa). Pearson correlation coefficient indicated a significantly moderate correlation (r=0.57) between fatigue strength and loading frequency. The percentage of monoclinic phase was similar among the groups. Therefore, the use of loading frequencies up to 20Hz seems a good alternative to expedite the cycling strength fatigue tests in polycrystalline ceramics without significantly changing the fatigue behavior showed by zirconia in tests employing the frequency of the masticatory cycle.