Search Results (10)

With the increasing service life of asphalt pavements, fatigue cracking has become a typical distress that significantly affects the structural lifespan of in-service asphalt pavements. To address the issues of high variability in laboratory fatigue tests and the poor correlation between test results and actual pavement cracking conditions, this study collected field core samples from in-service highways with varying degrees of cracking. The study employed push-pull fatigue tests on asphalt mixtures and linear amplitude sweep tests on extracted and recovered asphalt to evaluate fatigue performance. Quantitative relationships between transverse crack spacing and fatigue indices were established. With the inclusion of laboratory-compacted asphalt mixtures, the relationship between transverse crack spacing and the average pseudo-stiffness degradation rates was well described by a logarithmic function (R² = 0.928), while the apparent damage capacity achieved a stronger correlation (R² = 0.945). The findings demonstrate that the push-pull fatigue test, based on a viscoelastic continuum damage model, shows high stability and repeatability, with the coefficients of variation for the indices consistently below 11%. Compared to the average pseudo-stiffness degradation rates, the apparent damage capacity demonstrates better discriminative capability as an evaluation index for the intermediate-temperature fatigue performance of asphalt mixtures. The fatigue life from linear amplitude sweep tests on extracted and recovered asphalt from core samples shows a high degree of consistency with the patterns observed in various parameters of push-pull fatigue tests. The findings confirm the reliability of the push-pull fatigue test for evaluating intermediate-temperature fatigue performance of asphalt mixtures and provide the necessary theoretical support for establishing a fatigue performance evaluation system for in-service asphalt pavements. Full article

This study explores the fatigue properties of EN AW-6082-T6 aluminum alloy in the gigacycle range (10⁶–10⁹ cycles), using ultrasonic resonance fatigue testing at 20 kHz in a push–pull mode with a symmetric load cycle (R = −1). A custom-built ultrasonic fatigue machine, developed at TU-Varna, comprising a generator, ultrasonic train (including a high-power transducer, booster, custom-made sonotrode, and specimen), monitoring, data logging systems, and an air-cooling capability, was used for the experiments conducted. A Bezier curve sonotrode, with an amplification ratio of 1:6, was designed and produced for the test. Hourglass-shaped specimens were designed on the base of the dynamic Young’s modulus E = 71.3 GPa, determined through the impulse resonance method (ASTM E1876-01), and validated with FEM analysis for resonance length and stress amplitude. The fatigue testing revealed a fatigue strength reduction of approximately 60 MPa between 10⁶ and 10⁹ cycles. The percentile of failure curves based on a Cactillo–Canteli model fits well with the experimental data and gives a fatigue limit at 10⁹ cycles σl = 104 MPa and “endurance strength” σw = 84 MPa. Surface crack initiation was consistently observed with predominately cleavage transgranular fractures in the fatigue zone. The present research highlights the utility of ultrasonic testing for examining fatigue behavior in the gigacycle regime. Full article

The industry of off-highway vehicles is one of the fields of major application of nodular cast irons, which guarantee the manufacture of complex geometries and ensure good mechanical properties. The present investigation deals with the fatigue design of off-highway axles made of EN-GJS-500-7. Typically, off-highway axles are weakened by stress risers which must be assessed against fatigue. In this investigation, laboratory specimens have been extracted from an off-highway axle to take into account the manufacturing process effects. Different specimens’ geometries have been prepared, including plain, bluntly notched and sharply V-notched specimens, and constant amplitude, load-controlled axial fatigue tests were conducted using two nominal load ratios, namely push–pull and pulsating tension loading. As a result, both the notch and the mean stress effects on the fatigue behaviour of EN-GJS-500-7 have been experimentally investigated for the first time. A well-known local approach, which takes the strain energy density (SED) averaged over a properly defined structural volume as a fatigue damage parameter, has been applied both in the linear elastic and elastic plastic formulations. Since the SED correlated the geometrical notch effects of the specimens as well as the mean stress effects, a master curve based on the averaged SED has been defined for the first time, to the best of the authors’ knowledge, for the fatigue design of off-highway axles made of EN-GJS-500-7. Full article

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. Full article

A non-destructive thermographic methodology based on the temperature field is utilized to determine the crack tip position during the very high cycle fatigue (VHCF) test of pure iron and deduce the corresponding fatigue crack growth rate (FCGR). To this end, a piezoelectric fatigue machine is employed to test 1 mm thick pure iron samples at 20 kHz in push–pull fatigue loading. Two cameras are placed on each side of the plate sample, an infrared one for measuring the temperature fields on the specimen surface and an optical one for visualizing the crack tip verification. The centre section of the specimen is notched to initiate the crack. The temperature field is converted into intrinsic dissipation to quantify the inelastic strain energy according to energy conservation. The maximum value of intrinsic dissipation in each thermal image is related to the position of the crack tip and thus allows monitoring of the crack evolution during the fatigue test. The obtained results show that one specific specimen broke at 7.25 × 10⁷ cycles in the presence of a very low-stress amplitude (122 MPa). It is observed that the intrinsic dissipation has a low-constant level during the initiation and the short cracking, then sharply grows during the long cracking. This transition is visible on the polished surface of the sample, where the plasticity appears during the long cracking and slightly before. The material parameters in the Paris equation obtained from the intrinsic dissipation in the short crack growth are close to the results available in the literature as well as those obtained by the optical camera. Full article

Manual demolition tasks are heavy, physically demanding tasks that could cause muscle fatigue accumulation and lead to work-related musculoskeletal disorders (WMSDs). Fatigue and recovery models of muscles are essential in understanding the accumulation and the reduction in muscle fatigue for forceful exertion tasks. This study aims to explore the onset of muscle fatigue under different work/rest arrangements during manual demolition tasks and the offset of fatigue over time after the tasks were performed. An experiment, including a muscle fatigue test and a muscle fatigue recovery test, was performed. Seventeen male adults without experience in demolition hammer operation were recruited as human participants. Two demolition hammers (large and small) were adopted. The push force was either 20 or 40 N. The posture mimicked that of a demolition task on a wall. In the muscle fatigue test, the muscle strength (MS) before and after the demolition task, maximum endurance time (MET), and the Borg category-ratio-10 (CR-10) ratings of perceived exertion after the demolition task were measured. In the muscle fatigue recovery test, MS and CR-10 at times 1, 2, 3, 4, 5, and 6 min were recorded. Statistical analyses were performed to explore the influence of push force and the weight of the tool on MS, MET, and CR-10. Both muscle fatigue models and muscle fatigue recovery models were established and validated. The results showed that push force affected MET significantly (p < 0.05). The weight of the tool was significant (p < 0.05) only on the CR-10 rating after the first pull. During the muscle fatigue recovery test, the MS increase and the CR-10 decrease were both significant (p < 0.05) after one or more breaks. Models of MET and MS prediction were established to assess muscle fatigue recovery, respectively. The absolute (AD) and relative (RD) deviations of the MET model were 1.83 (±1.94) min and 34.80 (±31.48)%, respectively. The AD and RD of the MS model were 1.39 (±0.81) N and 1.9 (±1.2)%, respectively. These models are capable of predicting the progress and recovery of muscle fatigue, respectively, and may be adopted in work/rest arrangements for novice workers performing demolition tasks. Full article

Static immersion tests of potential injection pipe steels 42CrMo4, X20Cr13, X46Cr13, X35CrMo4, and X5CrNiCuNb16-4 at T = 60 °C and ambient pressure, as well as p = 100 bar were performed for 700–8000 h in a CO₂-saturated synthetic aquifer environment similar to CCS sites in the Northern German Basin (NGB). Corrosion rates at 100 bar are generally lower than at ambient pressure. The main corrosion products are FeCO₃ and FeOOH with surface and local corrosion phenomena directly related to the alloy composition and microstructure. The appropriate heat treatment enhances corrosion resistance. The lifetime reduction of X46Cr13, X5CrNiCuNb16-4, and duplex stainless steel X2CrNiMoN22-5-3 in a CCS environment is demonstrated in the in situ corrosion fatigue CF experiments (axial push-pull and rotation bending load, 60 °C, brine: Stuttgart Aquifer and NGB, flowing CO₂: 30 L/h, +/− applied potential). Insulating the test setup is necessary to gain reliable data. S-N plots, micrographic-, phase-, fractographic-, and surface analysis prove that the life expectancy of X2CrNiMoN22-5-3 in the axial cyclic load to failure is clearly related to the surface finish, applied stress amplitude, and stress mode. The horizontal grain attack within corrosion pit cavities, multiple fatigue cracks, and preferable deterioration of austenitic phase mainly cause fatigue failure. The CF life range increases significantly when a protective potential is applied. Full article

Compared with the fatigue properties of the material (Inconel Alloy 690), the real fatigue lives of tubes are more meaningful in the fatigue design and assessment of steam generator (SG) tube bundles. However, it is almost impossible to get a satisfactory result by conducting fatigue tests on the tube directly. A tube with a uniform and thin wall easily fails near the clamping ends under cyclic loading due to the stress concentration. In this research, a set-up for fatigue tests of real tubes is proposed to overcome the stress concentration. With the set-up, low cycle fatigue tests were conducted in accordance with an existing fatigue design curve for Alloy 690. Strain control mode was applied with fully reversed push–pull loading under different strain amplitudes (0.15%, 0.2%, 0.3%, and 0.4%). A favourable result was obtained, and the low cycle fatigue behavior was investigated. The results showed that the fatigue life tested by the real tube was below the strain–life curve of Alloy 690 which was fitted by conventional solid specimens. A cyclic hardening behavior was found by the cyclic stress–strain curve when compared with the monotonic stress–strain curve. Full article

The use of high strength steel allows the design of lighter, more slender and simpler structures due to high strength and favorable ductility. Nevertheless, the increase of yield strength does not guarantee the corresponding improvement of fatigue resistance, which becomes a major concern for engineering structure design, especially for the welded joints. The paper presents a comparison of the low cycle fatigue behaviors between 10CrNi3MoV high strength steel and its undermatched weldments. Uniaxial tension tests, Push-pull, strain-controlled fatigue tests were conducted on base metal and weldments in the strain range of 0.2–1.2%. The monotonic and cyclic stress-strain curves, stress-life, strain-life and energy-life in terms of these materials were analyzed for fatigue assessment of materials discrepancy. The stress-life results of base metal and undermatched weld metal exhibit cyclic softening behaviors. Furthermore, the shapes of 10CrNi3MoV steel hysteresis loops show a satisfactory Masing-type behavior, while the weld metal shows a non-Masing type behavior. Strain, plastic and total strain energy density amplitudes against the number of reversals to failure results demonstrate that the undermatched weld metal presents a higher resistance to fatigue crack initiation than 10CrNi3MoV high strength steel. Finally, fatigue fracture surfaces of specimens were compared by scanning electron microscopy to identify the differences of crack initiation and the propagation between them. Full article

We have successfully developed a flat-cladding fiber Bragg grating sensor for large cyclic strain amplitude tests of up to ±8,000 με. The increased contact area between the flat-cladding fiber and substrate, together with the application of a new bonding process, has significantly increased the bonding strength. In the push-pull fatigue tests of an aluminum alloy, the plastic strain amplitudes measured by three optical fiber sensors differ only by 0.43% at a cyclic strain amplitude of ±7,000 με and 1.9% at a cyclic strain amplitude of ±8,000 με. We also applied the sensor on an extruded magnesium alloy for evaluating the peculiar asymmetric hysteresis loops. The results obtained were in good agreement with those measured from the extensometer, a further validation of the sensor. Full article