Fatigue Tests Of Specimens Research Articles

The effect of MWCNT concentration on the electrical resistance change characteristic of glass/fiber epoxy composites under low cycle fatigue loading

Abstract In this study, the effect of multi-walled carbon nanotube concentration on the electrical resistance change characteristics of multi-walled carbon nanotube filled glass/epoxy composites under low-cycle fatigue loading was experimentally investigated. For this purpose, multi-walled carbon nanotube concentrations of 0.2, 0.3, and 0.4 wt.% within composites were utilized to ensure electrical conductivity. The rectangular specimens for fatigue tests were manufactured by vacuum bagging method. The fatigue tests were conducted in a load-controlled manner with an ultimate strength ratio of 0.6 and at a stress ratio of 0.1. The results showed that the alteration in electrical resistance within the composites experiences a sharp and exponential rise when the concentrations of multi-walled carbon nanotube reach 0.2 and 0.3 wt.%, whereas the rate of this increase in electrical resistance is more gradual at 0.4 wt.%. multi-walled carbon nanotube concentration. The electrical resistance change curves of multi-walled carbon nanotube filled composites at various fatigue life levels were determined for statistical analysis using the Weibull distribution method. Finally, the average stiffness loss and the average residual fatigue life were determined at the electrical resistance changes corresponding to 95, 80, and 50 % Weibull reliabilities at various fatigue life levels and various multi-walled carbon nanotube concentrations.

An adaptive fatigue crack growth model at the welded joints of steel bridge

Under the influence of random loads such as those from vehicles, the welded details in steel bridges tend to gradually develop into macrocracks, typically originating from manufacturing defects or stress concentration points. As the service life of the steel bridge increases, the continuous generation and propagation of fatigue cracks lead to a decline in the bridge's service performance, potentially resulting in catastrophic failures such as bridge collapse. Accurately analyzing the fatigue crack growth patterns at welded joints is a prerequisite for reliably assessing the structural service performance. The analysis of fatigue crack propagation depends on both the crack growth step size and the cyclic stress–strain response at the welded joint under loading. Considering the complexity of the material properties and stress concentration at the welded joint, which leads to a lack of analytical solutions for the cyclic stress–strain characteristics at the crack tip, numerical analysis was employed to obtain the cyclic stress–strain response at the crack tip of the welded joints. Using the size of the cyclic plastic zone at the crack tip as an adaptive crack growth step size, a model for fatigue crack growth was developed based on the plastic strain energy of the cyclic plastic zone. Fatigue test specimens extracted from the welded toe of cruciform welded joints were tested, providing cyclic stress–strain (Ramberg-Osgood material parameters) and fatigue damage characteristics (Manson-Coffin material parameters) of the material at the welded toe. High-cycle fatigue tests using cruciform welded joint specimens determined a good correlation between the crack propagation characteristic dimensions obtained from the adaptive numerical analysis model and the experimental results. The experimental findings confirmed the applicability of the established adaptive numerical analysis model for studying fatigue crack growth in cruciform welded joints. The parameters introduced in this model are physically meaningful, easy to obtain, and suitable for engineering applications.

The Evolution of Surfaces on Medium-Carbon Steel for Fatigue Life Estimations

Early in fatigue life, fatigue cracks are often initiated at persistent slip bands (PSBs), which play the main role in surface evolution when the components are subjected to cyclic loading. Therefore, this paper aims to study the behavior of the surface development of medium-carbon steel, specifically 42CrMo4 (SAE 4140). Tests were conducted using tension–compression fatigue testing with stress amplitudes set at 30%, 40%, and 50% of the ultimate tensile strength (UTS); a load ratio of R = −1; and a frequency of f = 10 Hz. The ultimate number of test cycles was 2 × 105. The fatigue test specimens with as-machined surface quality (Ra < 100 nm) were tested on a servo-hydraulic push–pull testing machine, and the tests were interrupted a few times to bring the specimens out for surface measuring with a confocal microscope. The linear roughness values of the arithmetic mean deviation (Ra), maximum height (Rz), maximum profile peak height (Rp), and maximum profile valley depth (Rv) were investigated and further used to determine the roughness evolution during cyclic loading (REC) by analyzing the inclinations of the fitting curves of roughness and number-of-cycles diagrams. REC could then be used to estimate and classify the fatigue lifetime.

Accounting for defect position in ultrasonic fatigue test specimen with heterogeneous stress distribution

To account for or neglect the defect position, i.e. the fatigue initiating defect location, both radially and axially, is evaluated for hourglass-shaped ultrasonic fatigue specimen. The commonly used analytical equations to calculate the stress is compared against a finite element (FE) based approach, which is able to fully considering the stress state at the defect position. Notably, the effects on several common fatigue analyses are evaluated: the fatigue strength distribution, the stress–life and the stress–defect relationships. Fracture mechanical assessment is also performed, for a comprehensive VHCF characterization of the EN-GJS-500-7 ductile cast iron used in the study. The VHCF properties are characterized up to 3⋅108 cycles, using the Step-Stress fatigue testing method under fully reversed loading. The FE-model and Weibull distribution as the choice of fatigue strength distribution, enables size effect evaluation by Weakest-link effective volume with the highly stressed volume method as benchmark. The work shows that it is imperative to use the local stress state at the defect position, as the distribution of failures can diverge largely from the center of the specimen, and that neglecting this causes systematic error and flawed potentially results.

Fatigue life prediction of friction-stir-welded aluminum bridge decks – Experimental tests and numerical framework

This paper presents the results of experimental tests and proposes a numerical framework for the prediction of the fatigue behaviour of friction stir welded (FSW) butt-lap joints of aluminium bridge deck extrusions. Fatigue test specimens made of welded real roadway bridge deck extrusions were subjected to cyclic bending under different loading conditions. The fatigue failure modes were thoroughly analyzed. A numerical framework incorporating a 2D finite element (FE) model based on the theory of critical distances (TCD) and a simplified linear elastic fracture mechanics (LEFM) model was developed to predict the fatigue failure initiation location and the fatigue life of the specimens under the test conditions. The location of the failure initiation played a key role in the choice of the numerical approach adopted in the prediction of fatigue life. For fatigue failures initiating from the base metal, the framework utilizes the TCD formulations while for failures in the weld joint, the LEFM model was applied. The proposed numerical framework showed a good agreement with the experimental data.

Characterization of surface asperities to understand its effect on fatigue life of electron beam powder bed fusion manufactured Ti-6Al-4 V

Surface asperities play a leading role in determining the fatigue life of as-built Ti-6Al-4 V components manufactured by electron beam powder bed fusion (PBF-EB). Several roughness parameters are available to characterize the surface asperities This study focuses on identifying the surface roughness parameter that correlates best with fatigue life. To this end, several fatigue test specimens were manufactured using the PBF-EB process and utilizing different contour melting strategies, thus producing as-built surfaces with varying roughness. The focus variation microscopy technique was employed to obtain surface roughness parameters for the as-built surfaces. Selected specimens were characterized using x-ray computed tomography (XCT). Tomography can detect surface-connected features obscured by other parts of the surface that are not visible through optical microscopy. The fatigue life of all specimens was determined using four-point bend testing. Through regression model analysis, maximum pit height (Sv) was identified as the statistically significant roughness parameter with the best fit affecting fatigue life. The fracture zone was closely inspected based on the data collected through XCT prior to fatigue tests. This led to another estimate of the worst-case value for the statistically significant roughness parameter Sv. The Sv parameter values obtained from optical microscopy and XCT were used as the initial crack size in a crack growth model to predict fatigue life. It is observed that life estimates based solely on optical measurements of Sv can be overly optimistic, a situation that must be avoided in predictive design calculations.

Very high cycle fatigue of a notched Ni-based single crystal superalloy at high temperature

A new notched geometry has been developed for ultrasonic fatigue testing of CMSX-4 single crystal Ni-based superalloy specimens at 1000 °C. Ultrasonic fatigue tests were carried out at 1000 °C, R = -1 and 20 kHz using this new geometry. Two competing crack initiation mechanisms were identified for the studied range of fatigue lives: internal crack initiation from a casting pore associated to a rough zone (RZ) formation and surface crack initiation assisted by oxidation. Failure modes associated to the identified fatigue crack initiation mechanisms are discussed. Comparing fresh fractographic evidence with available results from the literature, it is shown that fatigue crack nucleation and first stages of crack propagation seem to govern VHCF fatigue life.

Corrosion fatigue behavior of nanoparticle modified iron processed by electron powder bed fusion

Due to its excellent biocompatibility, pure iron is a very promising implant material, but often features corrosion rates that are too low. Using additive manufacturing and modified powders the microstructure and, thus, the material properties, e.g., the corrosion properties, can be tailored for specific applications. Within the scope of this study, pure iron powder was modified with different amounts of CeO2 or Fe2O3 nanoparticles and subsequently processed by Electron Beam Powder Bed Fusion (PBF-EB/M). The corrosion-fatigue behavior of CeO2 and Fe2O3 modified iron was investigated using rotation bending tests under the influence of simulated body fluid (m-SBF). While the modification using Fe2O3 showed reduced fatigue and corrosion-fatigue strengths, it could be demonstrated that the modification with CeO2 is characterized by improved fatigue properties. The superior fatigue properties in air are attributed to the positive impact of dispersion strengthening. Additionally, an increased degradation rate compared to pure iron could be observed, eventually promoting an earlier failure of the specimens in the corrosion fatigue tests.

Low Cycle Fatigue and Damage Evaluation on 16MnCr5 Alloy Steel

The 16MnCr5 alloy steel samples are evaluated in a normalized condition and put through low cycle fatigue (LCF) under constant amplitude strains of εa = 0.2%, 0.25%, 0.4%, and 0.6%. Mechanical properties are reported based on the LCF results. Fatigue damage is checked in specimens that have reached 80% of their nominal fatigue life by two methods that are different from traditional electronic microscopy: measuring the roughness of the surface and looking at it through full‐field optical microscopy. In addition, the electric resistivity is determined for the test specimens. It presents an increment from the blank value of 6.5 × 10−8 Ωm (free of fatigue damage) to 1.25 × 10−7 Ωm for the fatigue test specimen (εa = 0.6%). Finally, X‐ray diffraction patterns are determined for analyzing the residual equivalent strain at the microscale for the fatigued samples. The residual equivalent strain exhibits a trend toward increasing. For instance, it increases from 18.5 (blank value) to 34.0 με for the fatigue test specimen with εa = 0.6%.

Surface acoustic wave spectroscopy for non‐destructive coating and bulk characterization at temperatures up to 600°C enabled by piezoelectric aluminum nitride coated sensor

Surface acoustic wave spectroscopy has been established as non‐destructive and fast method for characterization of mechanical properties of surfaces and bulk materials in both research and industry. The present work shows that by application of a novel and robust aluminum nitride (AlN) coated piezoelectric contact sensor the advantages of the method can be extended from room temperature to at least 600°C. An overview of sensor concepts and applications of the method is discussed first, followed by theoretical and practical considerations for design and coating of a novel temperature stable contact sensor. After fabrication of such a sensor using magnetron sputtering, it was tested in a modified surface acoustic wave spectroscopy setup with an incorporated heating table concerning signal amplitude and frequency range. The AlN coated sensor was found to perform well up to 600°C, with temperature limited by the specification of the heating table. At room temperature, performance was acceptable when compared with a conventional contact sensor using a PVDF piezoelectric foil. Application of the high temperature capabilities of the setup was demonstrated by measuring temperature stability of hydrogen‐free amorphous carbon coatings (a‐C and ta‐C) depending on their sp3 carbon ratio. In another example, high precision temperature dependent measurement of Young's modulus for ultrasonic fatigue test specimen was taken, achieving an accuracy better than 1%. Use of the developed sensor opens up new possibilities in material science for in situ study of temperature depending mechanical properties for coatings and surfaces.

Theoretical and experimental study on the fatigue property of material extrusion products and improvement by using vibration machining during manufacturing process

Nowadays, material extrusion (ME) is one of the most popular additive manufacturing technology, which offers a new way of manufacturing parts with specific complex geometry and mechanical properties. However, with the expansion of the application range of ME products, it is urgent to investigate their fatigue property so as to ensure the reliability and safety. In this paper, the damage mechanical fatigue life prediction model of ME products considering void defects was established. Fatigue test specimens built with different processing parameters were then fabricated, and tensile fatigue tests were carried out under different stress levels. In addition, a novel method, utilizing vibration during ME process, was proposed to improve the built products’ fatigue property, and corresponding theoretical and experimental investigations were performed to reveal the influencing mechanism of the vibration utilized. The results showed that the proposed model was validated by comparing the theoretical predictions with the experimental measurements. As layer height and printing speed were increased, the fatigue life of ME specimens dropped, while it rised with the increasing extrusion temperature. The built products’ fatigue life could be improved obviously by using vibration.

Impact of Corrosion in Simulated Body Fluid on Fatigue Characteristics of 3D-Printed Polylactic Acid-Coated AM60 Magnesium Alloys

In this research, the pure fatigue behaviors of AM60 magnesium alloy with polylactic acid (PLA) coating (PF-AM60-PLA) and the corrosion fatigue behaviors of magnesium alloy with PLA coating (CF-AM60-PLA) were evaluated. Polymer coating was made by fused deposition modeling (FDM) with a 3D printer and attached to standard fatigue test specimens with glue. Then, after 27 days of immersion in the simulated body fluid (SBF), the high-cycle bending fatigue test was performed on samples. Due to corrosion, the weight of the specimens was reduced by an average of 35%. The corrosion rate decreased in the first 7 days and then increased. PF samples with a coating had an average 49% increase in fatigue lifetime. Regarding the CF samples, despite the use of a 10-times stronger solution, the fatigue lifetime of these samples decreased by only 35%. The field-emission scanning electron microscopy (FESEM) results also showed cleavage plates and striations. In addition, the separation of the glue from the coating and Mg was observed. Corrosion products, in addition to microcracks and holes, were seen on the fracture surface of CF specimens, which caused the stress concentration and the crack initiation. Holes caused by the release of gases were also observed in polymer coatings, which were fabricated by 3D printing.

Kaji Eksperimental Karakteristik Fatik Komposit Berpenguat 2 Lapis Serat Nenas dan Orientasi Serat 45 Derajat atau 90 Derajat

The aim of this study is to experimentally investigate the effects of fiber orientations on the fatigue strength, fatigue life and surface fracture of 2 ply Pineapple Leaf Fiber Composite. Fatigue test specimens with 45o and 90o fiber orientation are loaded using rotary bending fatigue test machine at several load levels. Fatigue strength and fatigue life are measured during fatigue test and fracture surface of fatigue specimens are observed using microscope. The results show that 2 ply Pineapple Leaf Fiber Composite with 45o fiber orientation have highest fatigue strength than 2 ply Pineapple Leaf Fiber Composite with 90o fiber orientation. The shape of the surface fracture in the test specimen varies widely which is generally caused by Multiple Origins or several fault starting points at once which can erode or tear the surface of the specimen before the specimen finally fails or breaks.

Improved maximum likelihood method for P-S-N curve fitting method with small number specimens and application in T-welded joint

In fatigue data analysis, fitting accurate P-S-N curve is problematic if only a small number of specimen is available, especially to evaluate the relationship between the stress level and the standard deviation. This paper proposes a sample information reconstruction method that can effectively solve this problem. Based on this method and the life equivalent principle, a new maximum likelihood method (which is abbreviated to improved maximum likelihood method) is proposed for P-S-N curve fitting. T-joint specimens of Q450NQR1 steel were fabricated and tested, then the P-S-N curves was fitted by the improved maximum likelihood method, least square method, maximum likelihood method, standard BS7608 and standard IIW. Finally, P-S-N curves by three methods and two standards are compared and analyzed. The results show that the relevant parameters of the P-S-N curve with 99.9% survival probability fitted by the improved maximum likelihood method are similar to those in the two standards, and it is indicated that the improved maximum likelihood method is a better way for P-S-N curve fitting with the small number of fatigue test specimens.

Investigation on fracture and fatigue performance of cold recycling emulsified asphalt mixture based on acoustic emission parameters

Cold recycling technique of asphalt reduces the demand for building materials, saves resources and costs, and meets the requirements of low-carbon and high-quality development. The purpose of this study is to improve the fracture and fatigue performance of cold recycling emulsified asphalt mixture. In this paper, cold recycling ordinary, basalt fiber (BF) emulsified asphalt mixture and cold recycling styrene butadiene rubber (SBR) modified emulsified asphalt mixture were taken as the research objects. Four-point bending strength and fatigue tests of specimens with and without prefabricated cracks were carried out respectively, and acoustic emission (AE) was used for real-time monitoring during these tests. The fracture mechanics method was used to study the fracture strength of cold recycling ordinary, BF and SBR modified emulsified asphalt mixtures. The phenomenological theory and energy analysis method were adopted to research their fatigue performance. The characterization parameter analysis method was employed to analyze the AE parameters. Furthermore, the effect of BF and SBR on the performance of the mixture was compared and analyzed. The test results show a significant improvement in the fracture energy and stress intensity factor of cold recycling BF and SBR modified emulsified asphalt mixtures, indicating that both BF and SBR can reduce the cracking risk of cold recycling emulsified asphalt mixture, of which BF is more effective compared with SBR. The dynamic modulus and cumulative dissipation energy attenuation law of cold recycling emulsified asphalt mixture during the fatigue process were revealed. Based on AE technique, a nondestructive test method for the fatigue damage of cold recycling emulsified asphalt mixture was established. This study expands the application scope of cold recycling emulsified asphalt mixture, which has good energy-saving, economic and environmentally friendly benefits.

High-cycle fatigue testing on AM60 magnesium alloy samples for as-received and pre-corroded conditions in simulated body fluid

In this present work, the pure fatigue (PF) behaviors of AM60 magnesium alloys were investigated for the as-received samples, compared to the pre-corroded specimens in simulated body fluid (SBF). The standard fatigue testing specimens were held for 28 days in the SBF, so-called corrosion fatigue (CF) samples, while their weights were recorded. Afterward, bending fatigue experiments were carried out under fully-reversed rotary conditions, within high-cycle fatigue regimes. Moreover, the fracture mechanism was also investigated using field-emission scanning electron microscopy (FE-SEM). The obtained results indicated that the weight of the samples first increased and then decreased with time. The CF lifetime was 71 % of the relative difference, shorter than PF under the lowest stress level (80 MPa) and 38 % of the relative difference, shorter than PF at the highest stress level (140 MPa). The CF lifetime was entirely behind the PF lifetime, and fracture mechanisms were mostly brittle.

Influence of Build Orientation on Surface Roughness and Fatigue Life of the Al2024-RAM2 Alloy Produced by Laser Powder Bed Fusion (L-PBF)

Additive manufacturing of high strength Al alloys brings problems with hot cracking during rapid solidification. One of the ways to solve this challenge is technology developed by the Elementum 3D company. The way consists of inoculation by ceramic nanoparticles using RAM technology. When applying the L-PBF method, a very fine equiaxed microstructure with exceptional properties and without cracks is created. This paper offers the results and discussion of the microstructure, surface roughness and fatigue life of the high-strength Al2024-RAM2 alloy made from a gas atomized powder with an additive of 2 wt.% ceramic nanoparticles on the base of Ti. The specimens for fatigue tests were produced in different orientations relative to the building platform and left in the as-built conditions with different surface quality (roughness). The specimens were T6 heat-treated. The treatment caused a coarsening of a part of the fine grains. After T6 heat treatment, the hardness increased significantly, which occurred by precipitation hardening. Fatigue tests of specimens with different build orientation were performed in plane bending and the experimentally determined fatigue life was discussed in terms of surface roughness and material microstructure.

Crack Initiation in Bulk Matrix of Austenitic Stainless Steel during Very High Cycle Fatigue

Abstract In the very high cycle fatigue regime, fatigue crack initiation in high-strength steels is usually correlated to a subsurface inclusion with a fine granular area (FGA). Localized stress–strain concentration at the subsurface inclusion is a critical factor. Fatigue crack initiation with an FGA in the bulk matrix without any defect has rarely been reported. In this paper, a fundamental study on the formation of FGAs in the bulk matrix of an austenitic stainless steel has been carried out using a progressive stepwise load-increasing test with a cycle step of about 108 cycles. FGA formation in the subsurface bulk matrix has been observed. The microstructural damage in the fatigue-tested specimens has been studied using the electron channeling contrast imaging electron microscopy technique. Strain localization and grain fragmentation are the main processes for the formation of FGAs. Local plasticity exhaustion leads to crack initiation due to local stress concentrations. This method can also be used to predict the fatigue damage process, especially the damage rate in individual specimens.

Kaji Eksperimental Karakteristik Fatik Komposit Polyester Partikulat Al2O3

This research investigates the fatigue characteristics of Al2O3 particulate polyester composite, including fatigue limit, fatigue life, and fracture surface morphology in the test specimens. Fatigue test specimens with variations of Al2O3 filler of 4%, 6%, 8%, and 10% were tested using a rotary bending fatigue testing machine under varying loads to determine the number of cycles until failure. The research results indicate that the polyester composite with 4% and 6% Al2O3 filler had a fatigue limit of 1.9 MPa and could withstand 1.000.000 cycles. Meanwhile, the polyester composite with 8% and 10% Al2O3 filler had a fatigue limit of 0.9 MPa and also endured 1.000.000 cycles. The average fatigue life was 28.957 cycles, 22.725 cycles, 98.572 cycles and 66.352.5 cycles for 4% mass fraction, 6% mass fraction, 8% mass fraction, and 10% mass fraction respectively. The fracture surface morphology of the test specimens varied significantly, mainly due to multiple origins, where multiple initiation points of fracture simultaneously occurred, leading to surface erosion or tearing before the specimen ultimately failed or fractured.

Residual monotonic stress–strain property of Q690 high-strength steel: Experimental investigation and constitutive model

Residual monotonic stress–strain properties of Q690 high-strength steel (HSS) with pre-fatigue damage were investigated, which provided the basis for studying the residual resistance performance of HSS structures after an earthquake. Three monotonic tensile test specimens, nine low-cycle fatigue test specimens and twenty-seven pre-fatigue test specimens were included in this study. For the fatigue strain amplitude, the 0.75%, 1.0% and 1.5% were considered. From the low-cycle fatigue experimental results, the relationship between the fatigue life and fatigue strain amplitude of Q690 HSS was clarified through a predictive equation. The number of loading cycles was controlled to perform the pre-fatigue damage test, where the fatigue damage ratios were controlled to be 0.3, 0.6 and 0.8. Cracks were observed on the surfaces of the test specimens with pre-fatigue damage. A tensile coupon test was then conducted to clarify the residual mechanical properties of Q690 HSS with pre-fatigue damage. The effects of the fatigue damage ratios and fatigue strain amplitudes on the residual monotonic mechanical properties of Q690 HSS were determined. The stress–strain curve of the specimen with pre-fatigue damage was similar to that of the virgin specimen. The strength properties of Q690 HSS were reduced by pre-fatigue damage. The effects of the fatigue damage ratios on the key mechanical properties of Q690 HSS were quantified using predictive equations. A predictive method for the residual monotonic stress–strain properties of Q690 HSS with different pre-fatigue damages was proposed, which included predictive equations for key mechanical parameters and a nonlinear constitutive model. The proposed predictive method for the residual mechanical properties of Q690 HSS with pre-fatigue damage can be used in theoretical and numerical analyses as a material model.