Cyclic Fatigue Loading Research Articles

Impact of block-loading on the high cycle fatigue strength of superalloy 617M at 973 K

The study demonstrates the positive influence of coaxing on the fatigue strength of alloy 617M under high cycle fatigue (HCF) loading at stress ratio (R) -1, frequency ≈ 85 Hz, and temperature of 973 K. Tests were performed employing resonance excitation in two patterns viz., constant amplitude loading and block loading using a ‘low-to-high’ pattern. The fatigue limit for the constant amplitude loading pattern was determined to be 320 MPa for a runout criterion of 108 cycles. Upon under-stressing the alloy by application of a block loading pattern, the limit increased to 335 MPa. The increase of stiffness and associated resonance frequency during cycling indicated an initial hardening regime brought about by work hardening followed by a period of secondary hardening owing to precipitations occurring in the matrix. In the later stage, the frequency response also indicated the influence of dynamic strain ageing (DSA) on the alloy behaviour. Comparative microstructural characterisation of specimens is conducted to extend the understanding of coaxing and the resulting changes occurring in the work-hardening behaviour of the alloy.

Superelastic behavior of novel Ni-Ti-Co shape memory alloys for seismic applications

Abstract Ni-Ti-Co is a new shape memory alloy (SMA) that has higher strength and lower superelastic temperature range than traditional binary Ni-Ti SMA. In this paper, the superelastic properties of Ni-Ti-Co bars that are important for seismic applications were studied and compared with those of Ni-Ti bars. The superelastic behavior, strength, strain recovery, low-cycle fatigue characteristics, and fracture strains of Ni-Ti-Co and Ni-Ti SMA with different heat treatment strategies and testing temperatures from -40℃ to 50℃ were investigated with seismic applications in mind. The effect of low-cycle fatigue loading and temperature variation on the superelasticity degradation of Ni-Ti-Co SMA were evaluated. The results showed that the yield strength of Ni-Ti-Co SMA at room temperature was 1.41 ~ 1.74 times that of the Ni-Ti SMA. Ni-Ti-Co SMA exhibited 100% strain recovery when unloaded from 5% strain in a temperature range from -40℃ to room temperature; while at 50℃, a residual strain of 0.6% was observed when unloaded from 5% strain. For comparison, the Ni-Ti SMA lost superelasticity when the temperature was reduced to 0℃. When subjected to low-cycle fatigue loading, the stability of the superelasticity behavior (maintaining yield stress, energy dissipation and strain recovery) of Ni-Ti-Co SMA at -40℃ was better than that at room temperature and 50℃. In particular, the superelasticity of Ni-Ti-Co SMA at -40℃ was even superior to that of the Ni-Ti SMA at 0℃. At -40℃, the Ni-Ti-Co SMA showed no loss in yield stress, damping ratio and recovery strain during the first 100 cycles of fatigue loading. Moreover, from 100th to 471st cycle, at which fracture occurred, the strain recovery of Ni-Ti-Co SMA showed almost no degradation.

Stiffness Optimization of Squirrel Cage in Aircraft Engines

squirrel cage is a component used in engine shafts to make the shaft stiffer and able to withstand high cycle fatigue loads continuously for a long time. A squirrel cage can be of many shapes and sizes. The critical speed and the stresses acting on the shaft play a key role in determining the performance of shafts in engines. The squirrel cage in the shaft absorbs stress and extends shaft life. This paper deals with the structural design and analysis of the squirrel cage of an aircraft engine. The design of the squirrel cage was done on SolidWorks, and analysis was carried out on the Ansys workbench. To meet the strength and stiffness, the squirrel cage was optimized by introducing slots in it. Stress analysis of the bearings has been carried out for axial and radial loads from 1 to 20 kg. The radial load is applied to individual bearings, and then the axial load is applied to only one bearing. Then results are plotted into curves, and a slope is obtained based on deformations obtained. The stiffness value is noted, and the same procedure is performed for an optimized slotted squirrel cage. These values are compared, and then the squirrel is proved to be weight optimized and has improved stiffness.

Effect of Constant Cyclic Stress Coupling on the Fatigue Behavior of 304LN Stainless Steel.

The stress state of the primary circuit main pipeline is very complex mixed constant stress and periodic cyclic stress during the operation of nuclear power plants, which often affects the failure behavior of the stainless steel (SS) pipe. The fatigue behavior of 304LN SS under mixed constant stress and cyclic stress was investigated, and it was found that the coupling effect of constant stress and cyclic stress has a significant collaborative acceleration on the fatigue behavior. The research results showed that the cracks in the 304LN SS did not propagate even after 78 days under both constant load conditions of 5 KN and 7.5 KN, while the crack growth rate (CGR) of the specimen increased by about an order of magnitude when coupled with a cyclic fatigue load. The fatigue life of 304LN SS was the longest under 200 °C high-temperature water, and its life was roughly the same under 250 °C and 300 °C water. In a 300 °C high-temperature water environment, the fatigue life under the coupling of constant stress and cyclic fatigue stress is significantly lower than that under symmetric cyclic stress alone. There is a synergistic acceleration effect on the fatigue life of 304LN SS when constant stress is coupled with periodic cyclic stress, which is attributed to the combined effect of a corrosion environment and mechanical factors.

Modified cure cycles for increased fatigue performance of fiber metal laminates

Modified (MOD) cure cycles that deviate from the manufacturer’s recommended cure profile can reduce the inherent thermally induced residual stresses (TRS) in fiber metal laminates (FML). Literature shows that MOD cycles do not adversely affect the material properties but enhance the quasi-static material strength. However, the literature does not comprehensively discuss the impact of MOD cycles on material performance during cyclic fatigue loading. This paper, therefore, investigates how MOD cycles influence the fatigue characteristics of different FML layups made of carbon fiber-reinforced polymer (CFRP) and steel. The experimental results confirm that the quasi-static material strength and the modulus of the FML are increased when using MOD cure cycles. The evaluation of fatigue tests with digital image correlation, thermography, and electrical resistance measurements of notched specimens shows that a reduction of TRS delays damage initiation in the metal facesheets and decreases the crack growth rate until facesheet failure. The results further show that layup-dependent parameters, the maximum stress in the metal sheets, and metal sheet thickness significantly govern the evolution of fatigue damage. In summary, modified cure cycles are an efficient tool to enhance the quasi-static and fatigue performance of CFRP-steel hybrid laminates.

An experimental-cohesive zone model approach to predict fatigue life of adhesive joints with varying modes of loading and joint configurations for automotive applications

ABSTRACT Predictive fatigue life models of adhesive joints are necessary to enable the assessment of automotive-bonded structures while reducing costly experimental testing. However, contemporary models have typically been calibrated for specific joint configurations and modes of loading, limiting their applicability to large-scale structures. Additionally, available models are based on simulation of cumulative fatigue cycling, making them computationally prohibitive. In the current study, cross-tension (CT) (load angles of 0°, 45°, and 90°) and single-lap shear joint (SLJ) configurations were bonded using an epoxy adhesive (BetaGuard CI6125R; PPG, France) used in automotive production (one part) and tested under fatigue cyclic loading. A total of nine joint configurations, having symmetrical (same material and thickness) and asymmetrical (dissimilar material or unequal thickness) joints, were tested. Fatigue tests at load levels between 25% and 75% of the static peak load were performed until joint failure or to runout (two million load cycles). The static tests of the joints were simulated to failure using finite element (FE) models with the cohesive zone method (CZM). The maximum strain energy release rates ( G max ) were calculated within the adhesive bond line at static loads corresponding to the peak loads of the fatigue tests. The G max values, computed from single cycle, specimen-specific FE simulations, were correlated with the measured fatigue life ( N f ) of the adhesive joints with varying modes of loading and joint configurations. The fatigue life prediction model, based on G max − N f correlation and following a crack propagation approach, predicted the cycles to failure for 85% of the fatigue tests, and 81% of the independent validation tests. The proposed fatigue life prediction approach provides computational efficiency and large-scale compatibility.

Critical damage events of 3D printed AlSi10Mg alloy via in situ synchrotron X-ray tomography

Fish-scale-like melt pool structures and internal defects are unique characteristic in additively manufactured (AM) metals. These features play a critical role in the damage and fracture processes under different service conditions, governing the material's performance. However, the relationship between these damage features and loading conditions, as well as the spatial interactions between melt pool structures and internal defects remains incompletely understood. By in situ time-lapse synchrotron X-ray tomography and diffraction, we identify the initiation and growth events of life-limiting damage under tensile, low cycle fatigue (LCF), and high cycle fatigue (HCF) loading. A novel transition from meso-structure insensitive, defect-dominated short fatigue crack propagation to a meso-structure sensitive mechanism occurs as the plastic zone expands of a growing crack. Under tension and LCF, the damage accumulation gradually increases and micro-voids nucleate at the melt pool boundaries (MPBs) after which the crack path follows the MPBs. In contrast, under HCF, surface defects initiate fatigue cracking and the MPBs have a very limited effect on the crack propagation path. Finally, physics-informed machine learning method is introduced to develop a novel methodology for predicting fatigue life by including three-dimensional features of defects in AM parts.

Elevated-temperature performances of Al-Si-Cu casting alloys for cylinder head applications

In this study, high-temperature properties of two newly developed Al-Si-Cu alloys (2Cu and 3.5Cu alloys) were investigated and compared to the commercial-grade A356 + 0.5Cu alloy (R alloy). 3.5Cu alloy exhibited the highest strength, outperforming R alloy by over 50 MPa at room temperature and by more than 20 MPa at elevated temperatures in ultimate tensile strength. However, R alloy demonstrated three to four times higher elongation than 3.5Cu alloy at room temperature, though this difference diminished at high temperatures. The minimum creep rate of 3.5Cu alloy was 2.4 times lower than that of 2Cu alloy and 14.5 times lower than that of R alloy, showing the superior creep resistance. Under low cycle fatigue loadings, the fatigue lifetimes of R and 2Cu alloys were similar, and slightly longer than that of 3.5Cu alloy. Conversely, in the high cycle fatigue regime, 3.5Cu alloy exhibited the highest fatigue resistance, followed by 2Cu and R alloys. The superior high-temperature performances of 3.5Cu alloy were attributed to the enhanced thermal stability of θ' precipitates compared to β' precipitates in R alloy, as confirmed during long-term thermal exposures at 250 and 300 °C. These findings suggest that 3.5Cu alloy is a promising candidate to replace the traditional A356 + 0.5Cu alloy for cylinder head applications.

Study on hysteresis behavior of two kinds of reinforced CHS X-joints

Ultra-low cycle fatigue loading tests were conducted on two reinforced CHS X-joints (external stiffening rings and plates) and an unreinforced joint to analyze their post-yield failure modes, fracture mechanisms, and hysteretic properties. The findings indicate that these three joints exhibit varying failure modes post-yield: the unreinforced joints fail primarily due to chord fracture at the weld; the ring-reinforced joints initially see ring fracture, followed by chord crack at the weld; while the plate-reinforced joints initially fail from chord fracture and subsequently develop cracks at the plate-chord interface. The reinforced joints display plumper hysteretic loops and higher ultimate bearing capacities than the unreinforced joint. Compared to X-1, the tensile and compressive ultimate bearing capacities of RX-1 have increased significantly by 56.9 % and 74.1 %, respectively. In comparison, those of TX-1 have increased moderately by 2.2 % and 67.6 %, respectively, albeit with a slight compromise in ductility. The cumulative energy dissipation of RX-1 and TX-1 has risen by 22.62 % and 131.99 %, respectively, compared to the unreinforced joint. Notably, the dissipated energy before cracking accounts for 16.50 %, 18.16 %, and 22.29 % of the cumulative energy dissipationfor the three joints, respectively, highlighting the substantial contribution of crack propagation to energy dissipation under large deformations. The VUSDFLD subroutine based on Cyclic Void Growth Model (CVGM) is embedded into ABAQUS, this subroutine considers the initiation and propagation of joint cracks. Three finite element models with the same dimensions of the specimens were built to simulate the damage process of the joint. The results show that this method can accurately simulate such joints’ cracking and crack growth behavior under cyclic axial load. The simulated hysteresis curve is in good agreement with the test curve.

The Effects of Mechanical Loading on Resonant Response of a Conformal Load-Bearing Antenna System.

Glass fibre-reinforced polymer (GFRP) is a suitable substrate material for constructing a Conformal Load-Bearing Antenna Structure (CLAS). The relative permittivity of the CLAS substrate, which determines its resonant frequency, is affected by damage sustained by GFRP. This paper investigates the effects of damage (induced by mechanically loading the substrate) on the resonant response of the CLAS. Decoupling the antenna from the substrate was essential to evaluate the CLAS's true response to the induced damage. This paper details a systematic investigation examining how the frequency response of a "pristine" antenna and a surface-mounted antenna respond to a substrate subjected to quasi-statically induced mechanical damage and cyclic fatigue loading. The results demonstrate that the resonant frequency of the CLAS varies as a function of the substrate's mechanical damage. The prepared CLAS is tolerant to a certain degree of mechanical loading and related damage with its resonant frequency remaining within an acceptable bandwidth.

Experimental investigations on combined high and low cycle fatigue: Material-level specimen design and strain response characteristics

The paper designs a novel material-level specimen and its dedicated fixture suitable for applying Combined high- and low- Cycle Fatigue (CCF) loads. Unlike full-scale or simulation specimens, the CCF specimen eliminates geometrically induced stress gradients in the test region. Experimental data on CCF life and strain responses of ZSGH4169 alloy are acquired under different CCF loads. The Maximum Strain within Each(MSE) CCF cycle is demonstrated to be independent of the Low-Cycle Fatigue (LCF) loads and High-Cycle Fatigue (HCF) stress amplitudes, but exhibits a correlation with the Cycle Ratio of HCF/LCF (Rf). The growth law of MSE changes from linear to logarithmic as Rf decreases. Strain amplitudes in the dwell stage, observed unaffected by Rf, are quantified as a function of LCF nominal stresses and HCF stress amplitudes. However, under a defined CCF load, strain amplitudes in the dwell stage remain constant. Strain peaks in the dwell stage in a single CCF cycle decrease in a power function with increasing HCF cycles.

Quantification of delamination resistance data of FRP composites and its limits

Quantitative delamination resistance data of fibre-reinforced polymer-matrix (FRP) composites for quasi-static or cyclic fatigue loads are determined under different loading modes and load rates, respectively. Such data find use, e.g., in FRP materials’ development, materials’ selection, assessment of durability, or structural design. Round robins during test development yielded repeatability and reproducibility (coefficients of variation) of roughly 10 to 25%. This scatter has several different sources. Intrinsic material variability amounts to a few percent at best, at least for advanced manufacturing processes. This intrinsic scatter is essential for material comparisons and structural design. Measurement resolution specified in standardised test procedures yields a maximum of 4–5% variability. Most of the remaining scatter comes from other, extrinsic sources. Test operator actions, e.g., choice of test set-up, manual data acquisition or data analysis can yield extrinsic scatter. Damage mechanisms during delamination initiation and propagation act on the micro- and meso-scale, typically a few micrometer to a few hundred micrometer in size, with corresponding time-scales estimated to between a few tens of nanoseconds and a few microseconds. Defect size-scales are hence several orders of magnitudes lower than test specimen and structural scales, respectively. Predictive capability of models using such test data for structures are, therefore, limited. Major issues are up-scaling from straight beam-like specimens to larger shell-like structures, possibly with complex shape and varying thickness as well as from unidirectional fibre orientation to multidirectional lay-ups.

Mechanically compliant locking plates for diaphyseal fracture fixation: A biomechanical study.

Axial micromotion between bone fragments can stimulate callus formation and fracture healing. In this study, we propose a novel mechanically compliant locking plate which achieves up to 0.6 mm of interfragmentary motion as flexures machined into the plate elastically deflect under physiological load. We investigated the biomechanical performance of three compliant plate variations in comparison to rigid control plates with small and large working lengths in a comminuted bridge plating scenario using humeral diaphysis surrogates. Under static axial loading, average interfragmentary motion was 6 times larger at 100 N (0.38 vs. 0.05 mm) and nearly three times larger at 350 N (0.58 vs. 0.2 mm) for compliant plates than rigid plates, respectively. Compliant plates delivered between 2.5 and 3.4 times more symmetric interfragmentary motion than rigid plates (p < 0.01). The bi-phasic stiffness of compliant pates provided 74%-96% lower initial axial stiffness up to approximately 100 N (p < 0.01), after which compliant plate stiffness was similar to rigid plates with increased working length (p > 0.3). The strength to failure of compliant plates under dynamic loading was on average 48%-55% lower than rigid plate groups (p < 0.01); however, all plates survived cyclic fatigue loading of 100,000 cycles at 350 N. This work characterizes the improvement in interfragmentary motion and the reduction in strength to failure of compliant plates compared to control rigid plates. Compliant plates may offer potential in comminuted fracture healing due to their ability to deliver symmetric interfragmentary motion into the range known to stimulate callus formation while surviving moderate fatigue loading with no signs of failure.

Mechanical Performance of a Hot Mix Asphalt Modified with Biochar Obtained from Oil Palm Mesocarp Fiber

A recently used material that shows environmental and technical advantages for use as an asphalt binder modifier is biochar (BC). Different biomasses can be converted into BC by pyrolysis. One agro-industrial biomass that is abundant in copious quantities is oil palm mesocarp fiber (OPMF) obtained from African palm cultivation. In the present study, the use of a BC obtained from OPMF (BC-OPMF) as a modifier of asphalt binder (AC type) to produce a hot mix asphalt (HMA) was evaluated. This type of BC has not been investigated or reported in the reference literature as a binder and/or asphalt mix modifier. Initially, AC was modified with BC in three ratios (BC/AC = 5, 10, and 15%, with respect to mass) to perform penetration, softening point, and rotational viscosity tests; rheological characterization at high and intermediate temperatures; and scanning electron microscope (SEM) visualization. Based on this experimental phase, BC/AC = 10% was chosen to manufacture the modified HMA. Resistance parameters under monotonic loading (stability—S, flow—F, S/F ratio of the Marshall test, and indirect tensile strength in dry—ITSD and wet—ITSC conditions) and cyclic loading (resilient modulus, permanent deformation, and fatigue resistance under stress-controlled conditions) were evaluated on the control HMA (AC unmodified) and the modified HMA. Additionally, the tensile strength ratio (TSR) was calculated to evaluate the resistance to moisture damage. Abrasion and raveling resistance were evaluated by performing Cantabro tests. BC-OPMF is shown to be a sustainable and promising material for modifying asphalt binders for those seeking to increase stiffness and rutting resistance in high-temperature climates, resistance to moisture damage, raveling, and fatigue without increasing the optimum asphalt binder content (OAC), changing the volumetric composition of the HMA or increasing the manufacturing and construction temperatures.

Experimental investigation on the residual strength of concrete under the multi-factor coupling action of chloride penetration, freeze-thaw, and fatigue effect

With the ongoing process of globalization and the rapid expansion of the global economy, the proliferation of coastal constructions and infrastructures has witnessed a notable surge. These structures often encounter a confluence of environmental stressors in practical engineering, including cyclic fatigue loading, chloride salt erosion, and freeze-thaw cycles. These synergistic factors accelerate concrete’s failure, resulting in accelerated deterioration and significantly reduced durability. A comprehensive understanding of the damage characteristics of concrete materials in offshore environments, under the multi-factor coupled action of freeze-thaw phenomena, along with an assessment of their residual strength, is crucial for the development of relevant structural analysis and lifetime prediction models. To address this, twenty-seven dedicated specimens were designed and fabricated for this investigation. They were employed to scrutinize the residual strength of concrete under the combined influences of fatigue, freeze-thaw, and chloride penetration. Nine distinct multi-factor coupling actions were meticulously devised and characterized by different parameters. Following the multi-factor coupling action, tests were conducted to assess the residual flexural and compressive strengths, as well as the chloride ion content near the specimen edges. The findings revealed a substantial negative correlation between the residual flexural and compressive strengths with both the number of freeze-thaw cycles and fatigue loading iterations. In contrast, chloride ion concentration and the stress level of fatigue loading (stress ratio: 0.1–0.3) exhibited a comparatively marginal influence. Despite the relatively short test duration, it was evident that both freeze-thaw cycles and fatigue loading exerted a discernible promotional effect on chloride ion ingress in the concrete of the specimens.

Experimental Investigation into the Mechanical and Piezoresistive Sensing Properties of Recycled Carbon-Fiber-Reinforced Polymer Composites for Self-Sensing Applications.

This study investigates the mechanical and piezoresistive sensing properties of recycled carbon-fiber-reinforced polymer composites (rCFRPs) for self-sensing applications, which were prepared from recycled carbon fibers (rCFs) with fiber lengths of 6, 12, 18, and 24 mm using a vacuum infusion method. Mechanical properties of the rCFRPs were examined using uniaxial tensile tests, while sensing characteristics were examined by monitoring the in situ electrical resistance under cyclic and low fatigue loads. Longer fibers (24 mm) showed the superior tensile strength (92.6 MPa) and modulus (8.4 GPa), with improvements of 962.1% and 1061.1%, respectively. Shorter fibers (6 mm) demonstrated enhanced sensing capabilities with the highest sensitivity under low fatigue testing (1000 cycles at 10 MPa), showing an average maximum electrical resistance change rate of 0.7315% and a gauge factor of 4.5876. All the composites displayed a stable electrical response under cyclic and low fatigue loadings. These results provide insights into optimizing rCF incorporation, balancing structural integrity with self-sensing capabilities and contributing to the development of sustainable multifunctional materials.

Long-term performance of concrete pipes under fatigue traffic loads

In recent years, there has been a concerning increase in road collapses triggered by failures in urban drainage systems. Concrete pipes, commonly uesd in urban drainage pipelines, endure prolonged cyclic loading from traffic above. However, the mechanisms governing the long-term performance and fatigue damage remain unclear. Through conducting fatigue model box tests on concrete pipes, the effects of different fatigue loading cycles on the circumferential strain of concrete pipes were investigated. A fatigue life prediction equation for concrete pipes was proposed, and the crack propagation under various fatigue loading cycles was observed. Additionally, corresponding 3D FE models of concrete pipe-soil interaction with bell-and-spigot joints and gaskets were constructed. These models were used to explore the vertical displacements, circumferential bending moments, and circumferential stresses of the concrete pipes under different fatigue loading cycles, the damage and failure mechanisms of the concrete pipes under fatigue loading were revealed. The results indicate that the potential failure location of concrete pipes is within the inner crown of the bell under the fatigue traffic loads. The circumferential strains and crack propagation exhibiting a three-stage evolution pattern under fatigue loads. The proposed fatigue life prediction equation accurately predicts the remaining life of concrete pipes. Upon reaching 21.89 million loading cycles, the strain at the inner crown of the bell reaches 575.0 με, resulting in complete failure. Cracks on the inner crown of the bell extend inward and to the right from the middle of the joint, forming a channel for crack propagation. The vertical displacements at the crown and the circumferential bending moments of the bell and spigot exhibit rapid increases, stabilization, and subsequent declines with the increasing loading cycles. When concrete pipes undergo fatigue fracture, the maximum vertical displacement and circumferential bending moment at the bell are measured as 2.26 mm and 17.82 kN·m/m, respectively. Stress concentration at the bell and spigot during fatigue loading leads to crack propagation and convergence, causing redistribution of stress fields characterized by an initial increase followed by a decrease in the inner crown and invert of the bell.

Acid rain resistance of geopolymer recycled pervious concrete featuring top-bottom interconnected pores under freeze-thaw cycles and fatigue loads

While pervious concrete with top-bottom interconnected pores enhances the performance of pervious concrete pavements, its resistance to acid rain under combined freeze-thaw cycles and fatigue loads remains unexplored. To fill this gap, this study investigated the influence of three different qualities of recycled coarse aggregates on the acid resistance, acid neutralization, and permeability of geopolymer recycled pervious concrete (GRPC) (Low-quality, middle-quality and high-quality geopolymer recycled pervious concrete (L-GRPC, M-GRPC and H-GRPC)) under the couple effects of simulated acid rain wet-dry cycle spraying, fatigue loads and freeze-thaw cycles. After experiencing the combined action of fatigue load and freeze-thaw cycle, only the compressive strength of L-GRPC increased slightly by 9.1 % after acid rain spraying, but it had a negative impact on the flexural strength. With the increase of coupling environmental factors, the neutralization depth of GRPC was less than 4 mm, and the permeability coefficient was higher than 0.5 mm/s. Furthermore, more gypsum crystals were observed in the SEM images, and new peaks corresponded to the gypsum phase (CaSO4) appeared at 2θ = 29.4° and 2θ = 21.6° in the XRD spectra. The research results provide a broader perspective for the application of GRPC in concrete pavement engineering in acid rain area.

Shear and interface shear fatigue of semi‐precast slabs with lattice girders under cyclic loading

AbstractMore and more often, semi‐precast reinforced concrete slabs with lattice girders are employed for industrial buildings or bridges, where they are exposed to high cycle fatigue loading. Despite research in recent years that has led to the formulation of an S–N curve for lattice girders, the effects of cyclic loading on semi‐precast slabs have not yet been sufficiently clarified. Moreover, research has so far been limited to single‐span slabs. The effects of continuous slab systems, which are mainly realized in buildings and bridges, cannot be considered in the calculation of fatigue resistance yet. To improve the fatigue design concept for shear and interface shear, theoretical and experimental investigations were conducted at the Institute of Structural Concrete, RWTH Aachen University. In addition to the fatigue behavior of 16 tests under cyclic loading, particular attention is paid to the increase in shear and interface shear resistance at the inner support of continuous slabs. Furthermore, the influences of the support detailing were investigated. The findings illustrate further potential for optimization of the design under cyclic loading.

Pre-stress effect of metal bars in low-stress separation technology

This paper proposes a low-stress separation technology based on pre-stress effect. It connects notching and loading to take advantage of the rotating bending fatigue to achieve the cropping of metal bars. The real-time positions of the notching tool and loading hammer are calculated based on energy equivalence theory. Compared to the conventional notching method, it achieves 25.1 % reduction of cutting force in the case of AISI1045 steel bars. A testing platform is set up and the loading curve is designed to meet the requirements of deflection law and fatigue cyclic loading law. The bending stress for the minimum cutting force, crack angle and deviation under different notch parameters, and the reasonable pre-loading of different materials are determined through cropping tests and finite element simulation. Cropping results indicate that the optimized notch angle and depth are 60° and 1.5 mm, respectively.