Shear Fatigue Research Articles

Experimental Crack Path Analysis of Aluminum Alloy Specimen under Dynamic Shear Loading using Digital Image Correlation Method

The present paper investigates crack growth in specially designed specimens of 6061 aluminum alloy sheets subjected to dynamic shear loads. The sheets were previously thoroughly tested using uni-axial static and dynamic tests, so the less-known shear fatigue tests are presented in this work. For this purpose, a special specimen and fixtures were designed, which are capable of fatigue shear testing at elevated temperatures. In addition, the shape of the specimen was designed for more predictable crack initiation and growth along the specimen surface. In this study, the specimens were tested with low frequency (0.2 Hz) alternating shear loading (amplitude 0.3 mm in the positive and negative directions) until complete fracture. Each time the peak amplitude was reached, a photograph of the shear area on the specimen surface was taken and used to investigate crack growth further. The main objective of this work was to observe the crack initialization and crack growth under alternating shear loading using Digital Image Correlation (DIC) method; this was achieved by analyzing the photographs taken with the DIC method by observing local in-plane fields of strains and displacements.

Mode II Behavior of High-Strength Concrete under Monotonic, Cyclic and Fatigue Loading.

An economically efficient yet safe design of concrete structures under high-cycle fatigue loading is a rather complex task. One of the main reasons is the insufficient understanding of the fatigue damage phenomenology of concrete. A promising hypothesis states that the evolution of fatigue damage in concrete at subcritical load levels is governed by a cumulative measure of shear sliding. To evaluate this hypothesis, an experimental program was developed which systematically investigates the fatigue behavior of high-strength concrete under mode II loading using newly adapted punch through shear tests (PTST). This paper presents the results of monotonic, cyclic, and fatigue shear tests and discusses the effect of shear-compression-interaction and load level with regard to displacement and damage evolution, fracture behavior, and fatigue life. Both, monotonic shear strength and fatigue life under mode II loading strongly depend on the concurrent confinement (compressive) stress in the ligament. However, it appears that the fatigue life is more sensitive to a variation of shear stress range than to a variation of compressive stress in the ligament.

Low-cycle fatigue life and duration-of-load effect for hybrid CLT fabricated from lumber and OSB

Given the nature of cross-laminated timber (CLT), with an orthogonal arrangement and rolling shear characteristics of transverse layers, it has a more complex duration-of-load (DOL) effect than other engineered wood products, such as glued-laminated timber. Developing hybrid CLT (HCLT) with structural composite lumber, which can improve the mechanical properties and material sources of CLT, is gaining increasing interest. Construction oriented strand board (COSB) and spruce-pine-fir (SPF) dimension lumber were used to fabricate HCLT specimens for this study. The regular SPF CLT and HCLT specimens having different lay-ups and numbers of layers were subjected to low-cycle fatigue bending and shear loading tests. The Foschi and Yao damage accumulation model was calibrated using the test results. The DOL factors for the CLT and HCLT specimens were calculated using the stress ratio evaluation method. Results showed that the primary failure modes for the fatigue test specimens were the same as those observed in previous short-term ramp loading tests, and a good fit was observed between the model's predictions and the test data. The DOL factor corresponding to a load duration of 50 years, for a three-layer shear CLT specimen, was determined to be 0.53, which was lower than that recommended in CSA O86-19. The fatigue life and long-term performance of CLT were improved by using COSB panels as CLT layers. The HCLT specimens that used COSB only as the transverse layer showed advantages in fatigue life, long-term performance, and practical applications.

Investigations on Interface Shear Fatigue of Semi-Precast Slabs with Lattice Girders

Due to their high cost efficiency and flexibility, semi-precast concrete slabs with lattice girders are widely used in constructions all over the world. Prefabricated concrete slabs, combined with in situ concrete topping, exhibit a quasi-monolithic structural behavior in which lattice girders serve as vertical shear reinforcement and ensure the transfer of longitudinal shear within the interface, acting in combination with concrete-to-concrete bonding mechanisms. To be applicable in industrial and bridge construction, semi-precast slabs need to have sufficient resistance against fatigue failure. To improve and expand the limits of application, theoretical and experimental investigations are conducted at the Institute of Structural Concrete (IMB), RWTH Aachen University. To investigate the fatigue behavior of lattice girders, small size tests with lattice girder diagonals were carried out. These test results have been used to derive an S–N curve (S: stress, N: number of load cycles) for lattice girders for a more refined fatigue design. Subsequently, the fatigue behavior of semi-precast slabs with lattice girders was investigated by fatigue tests on single-span slab segments. The fatigue design regulations of lattice girders according to technical approvals can generally be confirmed by this test program; however, they tend to be conservative. The use of the derived S–N curve leads to significantly improved agreement of fatigue behavior observed in tests and design expressions.

Early Crack Propagation in Single Tooth Bending Fatigue: Combination of Finite Element Analysis and Critical-Planes Fatigue Criteria

Mechanical components, such as gears, are usually subjected to variable loads that induce multiaxial non-proportional stress states, which in turn can lead to failure due to fatigue. However, the material properties are usually available in the forms of bending or shear fatigue limits. Multiaxial fatigue criteria can be used to bridge the gap between the available data and the actual loading conditions. However, different criteria could lead to different results. The main goal of this paper is to evaluate the accuracy of different criteria applied to real mechanical components. With respect to this, five different criteria based on the critical plane concept (i.e., Findley, Matake, McDiarmid, Papadopoulos, and Susmel) have been investigated. These criteria were selected because they not only assess the level of damage, but also predict the direction of crack propagation just after nucleation. Therefore, measurements (crack position and direction) on different fractured gear samples tested via Single Tooth Bending Fatigue (STBF) tests on two gear geometries were used as reference. The STBF configuration was numerically simulated via Finite Elements (FE) analyses. The results of FE were elaborated based on the above-mentioned criteria. The numerical results were compared with the experimental ones. The result of the comparison showed that all the fatigue criteria agree in identifying the most critical point. The Findley and Papadopulus criteria proved to be the most accurate in estimating the level of damage. The Susmel criterion turns out to be the most conservative one. With respect to the identification of the direction of early propagation of the crack, the Findley criterion revealed the most appropriate.

Modeling Delamination in NSTX-U Inner Leg Using “EKILL”

The possibility of thermal strain–induced delamination was anticipated in the original design of the NSTX-Upgrade (NSTX-U). Current “bunching” at the toroidal field (TF) flag causes nonuniform Joule heating of the TF conductor during a shot. This produced through-thickness tensions beyond the measured capacity of the insulation bond. This however occurred where the torsional shear was at a minimum, and the Upgrade design progressed with the understanding that delamination at the core of the TF flag might occur. During the Recovery project design reviews, concern over the extent of delamination was elevated. In various early simulations, the tensile stresses reached 50 MPa. With more accurate through-thickness insulation modulus and thickness, and at end-of-flattop versus end-of-pulse temperature, the tensile stress goes down to 25 MPa, and possibly lower based on higher-fidelity modeling. Insulation delamination has been predicted analytically in coils in projects other than NSTX-U, and indications of delamination have been observed in some machines. Out-of-plane loads on the NSTX TF coil produce local and global twisting of the tokamak. The inner leg supports part of the torsion as a torque shaft or tube. Peak torsion is at the outside radius of the TF central column, away from the regions that don’t carry current, experience less Joule heat, and develop tension. Testing of the shear and tensile fatigue properties of the CTD-425 system was repeated and was a part of this requalification effort. This paper addresses simulations done in ANSYS based on a simple assumption that when the tension and shear stresses exceed an allowable, the elements are “killed” or considered much less capable of carrying tension and shear loads established by fatigue S-N tests. A competing and complementary method, the Virtual Crack Closure Technique (VCCT) was used to augment and validate the EKILL procedure. Determination of Paris constants to support the VCCT analysis is described Fig. 8.

Multiaxial low cycle fatigue behavior and life prediction method of 316LN stainless steel at 550 °C

This work studied the uniaxial, torsional and multiaxial low cycle fatigue behavior of 316LN stainless steel at 550 °C. Results show that the normalized kernel average misorientation value can be used to describe the loading path non-proportionality quantitatively. The formation of equiaxed and small-sized dislocation cells leads to the occurrence of continuously cyclic hardening under circular loading path. The crack initiation and early crack growth is dominated by intergranular mode in uniaxial test, whereas the transgranular cracking is observed in multiaxial test. Moreover, a weighting factor, evaluating the contribution of shear fatigue damage to the total multiaxial damage is proposed to modify the damage parameter of Chen-Xu-Huang (CXH) life prediction model, which enables all life data falling into the scatter band of 1.5.

Prediction of strength and fatigue life for 2D plain-woven high-alumina fiber reinforced alumina matrix composites under a complex in-plane stress state

This paper presents a strength criterion and fatigue life prediction method for 2D braided alumina matrix composites under a complex in-plane stress state. The static strength of the material was obtained by in-plane tensile, compression, and pure shear tests. Considering the difference between tensile and compressive properties of materials and the influence mechanism of in-plane tensile and shear coupling on material strength, a revised Hoffman strength theory was proposed. The predicted off-axis tensile strength is consistent with the test results, and the deviation is not more than 10%. Tensile fatigue tests were carried out with the off-axis angle θ=0°, 15°, 30°, 45°, the stress ratio R=0.1, and frequency f=10 Hz. The test results show that the fatigue life decreases with the increase of off-axis angle. Due to the in-plane shear stress component, the fatigue failure is gradually changed from fiber-dominated to fiber-matrix dominated mode. Based on a combination of the uniaxial tensile fatigue life curve, the Broutman-Sahu residual strength model, which is used to characterize the variation of the residual strength with the fatigue cycles, and the modified Hoffman strength theory, the paper proposes a fatigue life prediction model under complex in-plane loading conditions. The fatigue shear damage factor is defined to characterize the effect of the normal and shear stress interaction on fatigue life. The fatigue life prediction model is used to predict the fatigue life of specimens in the off-axis tensile fatigue tests. The predicted result agrees with the test result, and the deviation is within the 1-time life span. The results indicate that the proposed fatigue life prediction model can be used to predict the fatigue life of 2D braided alumina matrix composites under the complex in-plane stress condition with the given stress ratio, temperature, and fatigue load frequency.

Low cycle fatigue performance of DP900 steel under cyclic shear paths

AbstractIn this study, a sheet fatigue shear test device is designed and applied to the low‐cycle fatigue testing of DP900 with varying strain amplitudes within the range of 0.5%–6.0%. The microstructure is analyzed by using electron backscatter diffraction, and fracture surfaces are examined via scanning electron microscopy. Results indicate that the material exhibits cyclic softening behavior after the first two cycles of hardening, with a stable softening rate and a high damage evolution rate related to loading amplitude. The variation of the hysteresis curve in the cyclic process is shown. Total plastic strain energy absorbed increases as loading amplitude decreases and it reaches the peak at approximately 1%. The life prediction model based on plastic strain energy density and strain amplitude is verified to be suitable for the cyclic shear path. The influence of microinhomogeneity on the distribution of stress and strain, especially the deformation of martensite, is closely related to the bi‐linear region of fatigue life curve.

Effect of using different component combinations for orthodontic bracket bonding with self-etch primers.

To evaluate bonding quality for orthodontic bracket bonding with different component combinations of self-etch primers in vitro. Metallic brackets were bonded to bovine lower incisors and assigned to groups. Group1: comparison of self-etch (Transbond™ Plus, 3M™ Unitek, Neuss, Germany, n = 30; BrackFix® primer SE, VOCO®, Cuxhaven, Germany, n = 20) and etch-and-rinse bonding systems (Transbond™ XT, n = 20; BrackFix®, n = 20); group2: comparison of different self-etch primer (Transbond™ Plus; BrackFix® primer SE) and adhesive (Transbond™ XT, n = 20; BrackFix®, n = 20) product combinations; group3: testing cyclic fatigue bond strength of self-etch bonding systems (Transbond™ Plus, n = 20; BrackFix® primer SE, n = 20). All teeth were tested for shear bond strength according to the DIN-13990 standard, the adhesive remnant index (ARI) and enamel fractures were determined microscopically (10 ×magnification). The mean shear bond strength of the self-etch (Transbond™ Plus: 16.38 ± 3.68 MPa; BrackFix® primer SE: 16.24 ± 1.73 MPa) and etch-and-rinse bonding systems (Transbond™ XT: 18.45 ± 2.56 MPa; BrackFix®: 17 ± 5.2 MPa) were of aclinically adequate order of magnitude (≥ 6-10 MPa) and were not statistically different. The component combination BrackFix® primer SE/Transbond™ XT adhesive led to asignificantly lower shear bond strength (11.99 ± 3.68 MPa). There were no significant differences between static and fatigue shear bond strengths of self-etch bonding systems. Mean ARI scores mostly ranged between4 and5. The combination of the self-etch primer Transbond™ Plus with the BrackFix® adhesive led to asignificantly increased enamel fracture rate. Based on the present findings bond strength of self-etch primers was equal to etch-and-rinse primers for bracket bonding. Combining different self-etch bonding systems might alter the clinical performance.

Microstructure and twin behavior of Ti-2Al-2.5Zr during cold pilgering

The microstructure and twin behavior of Ti-2Al-2.5Zr tube during cold pilgering were investigated. During the deformation, the intragranular deformation is not uniform, and the microstructure characteristics are similar to low-cycle shear fatigue. Under alternating stress, the rotation tendency of grain is mainly related to the ratio of circumferential to radial strain. As the mainly primary twin, tensile {10–12}〈10–11〉 is activated in large number, which selection of twinning variant conforms to the Schmidt factor law. However, the selection of the secondary twinning variant is related to the slip system activated by the adjacent grains to accommodate the shear strain caused by the twins. The Schmidt factor of the secondary twin is determined by the slip system opened by the adjacent grains.

Influence of NMAS and groove depths on the static and fatigue shear performance of aggregate interlocking in PQC mixes

ABSTRACT In the present study, a new test methodology is proposed to characterise the shear transfer ability of aggregate interlocking in pavement quality concrete (PQC) cylindrical specimens by conducting the direct shear test in the laboratory. The influence of the nominal maximum aggregate size (NMAS) and groove depth (GD) on shear strength (τ), joint shear stiffness (K) and fracture energy mode – II (G IIF) of aggregate interlocking in pavement quality concrete (PQC) are studied under static loading. A relationship between G IIF and K is determined. Also, shear fatigue test is conducted at higher stress levels to evaluate the effect of NMAS and GD on the performance of aggregate interlocking in PQC specimens at the grooved cross-section. From the Anderson–Darling statistic test, it is found that obtained fatigue results follow three-parameter Weibull distribution. The shape parameter (β) of the distribution is between one and two, which indicates that failure is due to wearing action. It is concluded that the proposed method in this research can be effectively used to evaluate τ, K and G IIF of aggregate interlocking in PQC specimens. Also, the use of larger NMAS in the PQC mix significantly improves the performance of aggregate interlocking under shear fatigue loading.

Analysis of Shear Characteristics of Deep, Anchored Rock Mass under Creep Fatigue Loading

To study the influence of earthquakes and engineering disturbances on the deformation of deeply buried rock masses, shear tests were carried out on anchored sandstone rock masses, anchored marble rock masses, and anchored granite rock masses under creep fatigue loading, and a new creep fatigue model was established to characterize the deformation characteristics of anchored rock masses under creep fatigue loading. The creep fatigue curves of different lithologies clearly show three stages: creep attenuation, steady-state creep, and accelerated creep. Fatigue loading can increase the creep of anchored specimens, and the lower the rock strength is, the higher the creep variable under fatigue loading is. However, for the same rock strength, with the increase in load level, the creep variable produced by creep fatigue load presents a linear downward trend. Considering the changes in the mechanical properties of the anchored rock mass under creep fatigue loading, the creep fatigue model of anchored rock masses is established by introducing a function of the fatigue shear modulus, and the accuracy and applicability of the model are verified by laboratory creep fatigue test data. The model provides a theoretical basis for the study of anchored rock mass support under low-frequency earthquakes or blasting loads.

Multiaxial fatigue criteria for prestressed strengthening of steel connections

In this study, the high-cycle fatigue behavior of bridge connections subjected to distortion-induced loadings (generating multiaxial stresses) and strengthened with prestressed elements were experimentally investigated. Thereafter, appropriate critical plane-based multiaxial fatigue thresholds based on the modified Wohler curve method (MWCM) and Fatemi-Socie's (FS) criteria were used to predict the initiation of cracks in the angles considering the estimated fully reversed uniaxial and shear fatigue limits as well as residual stresses. The prestressed strengthening system was effective in reducing the mean values as a result of the prestressing force, enabling a shift in the stress state to below the initiation threshold.

Short-beam shear fatigue behavior of round curved pultruded composite

Short-beam shear test has been used to evaluate the fatigue strength of round curved pultruded carbon fiber – epoxy matrix composite specimens in three point bending test of a convex-beam. The specimens were cyclically loaded at peak stresses from 50 to 80% of the quasi-static short-beam strength, and load ratios of 0.1 to 0.5, and damage evolution was monitored using the apparent stiffness method. Basquins’ law was used to fit the experimental S-N data. The results showed that the fatigue strength of curved pultruded composites is lower than for conventional thin flat unidirectional laminates, and less sensitive to load-ratio effects. Residual strength of pre-fatigued specimens was determined by quasi-static short-beam tests, and it sometimes exceeded the pristine material performance due to fatigue-induced strengthening phenomenon. Stress redistribution and residual manufacturing-induced stress relief due to the previous cyclic loading were found the main responsible for mitigating damage growth under quasi-static loading. Fractographic survey indicated that stress redistribution and stress relief governed the strengthening process under high-cycle and low-cycle fatigue regime, respectively.

Improving the fracture toughness of multi-layered commercial pure aluminum via warm accumulative roll bonding

In this study, the fracture toughness of the multi-layered commercial pure aluminum samples (AA1050) prepared by warm accumulative roll bonding (WARB) was investigated for the first time. Based on the ASTM E561 standard, the R-curve method was utilized to measure the plane stress fracture toughness. Compact tension (CT) samples were prepared from the sheets that were processed by different ARB cycles. Mechanical properties, microstructure, and fracture surfaces of the CT samples were studied by uniaxial tensile test, electron backscatter diffraction (EBSD), and scanning electron microscopy (SEM), respectively. By increasing the number of WARB cycles, fracture toughness increased; after five cycles, 78% enhancement was observed compared to the pre-processed state. A correlation was seen between the fracture toughness variations and ultimate tensile strength (UTS). WARB enhanced UTS up to 95%, while the grain size showed a reduction from 35 to 1.8 μm. Measured fracture toughness values were compared with the room temperature ARB outcomes, and the effective parameters were analyzed. Fractography results indicated that the presence of tiny cliffs and furrows and hollow under fatigue loading zones and shear ductile rupture in the Quasi-static tensile loading zone.

Microstructure and Fatigue Behaviors of Dissimilar A6061/Galvannealed Steel Joints Fabricated by Friction Stir Spot Welding.

The microstructures, tensile shear properties, and tensile shear fatigue properties of dissimilar A6061/Galvannealed steel joints fabricated by friction stir spot welding (FSSW) were investigated. Fe4Al13 phases form as the intermetallic compound (IMC) layer at the joint interface between the A6061 matrix and the galvannealed layer consisting of FeZn7, Fe, and Zn. At the edge of the joint, the stirred layer in which the A6061 matrix and the galvannealed layer are stirred also forms. Moreover, the solidified part of the residual melt discharged from the joint area forms at the outer peripheries of the joint. In this study, FSSW was conducted for two total welding durations: 9 and 10 s. Although the thickness of the remaining A6061 sheet in the welded area decreased with an increase in the welding time, the effects of the total welding time on tensile shear and tensile shear fatigue properties were negligible. A fatigue fracture occurred in the A6061 matrix and at the joint interface at the high cycle fatigue region and the low cycle fatigue region, respectively. In the case of the interfacial fracture, the crack was generated in the solidified part of the residual melt or at the interface between the solidified part and the stirred layer.

Fatigue shear performance of concrete beams reinforced with hybrid (glass-fiber-reinforced polymer + steel) rebars and stirrups

Fatigue shear performance of concrete beams reinforced with hybrid (glass-fiber-reinforced polymer + steel) rebars and stirrups

Torsional Fatigue Life Prediction of 30CrMnSiNi2A Based on Meso-Inhomogeneous Deformation.

In this paper, torsional fatigue failure of 30CrMnSiNi2A steel which exhibited non-Masing behavior was studied under different constant shear strain amplitudes, using thin-walled tubular specimens. The relationship between shear fatigue and the evolution of meso-deformation inhomogeneity and the prediction method of the torsional fatigue life curve were investigated. Shear fatigue of the material under constant amplitude was researched by numerical simulation with reference to tests, by using crystal plasticity of polycrystalline representative volume element (RVE) as the material model. Considering the non-Masing behavior of material, when determining the parameter values of the crystal plasticity model the correlation between these parameters and strain amplitude was taken into account. The meso-deformation inhomogeneity with increments in the number of cycles was characterized by using the statistical shear strain standard deviation of RVE as the basic parameter. Considering the effect of strain amplitude on fatigue damage, ratio cycle peak stress/yield stress was taken as the weight to measure the torsional fatigue damage and an improved fatigue indicator parameter (FIP) to measure the inhomogeneous deformation of the material was proposed. The torsional fatigue life curve of 30CrMnSiNi2A steel was predicted by the critical value of the FIP and then the result was confirmed.

A new proposal to assess shear fatigue resistance of asphalt pavement interfaces

The degree of adherence between two bituminous layers can be assessed from shear tests carried out at constant displacement speed, which allow the shear strength and stiffness of the interface to be obtained, being usual to specify a minimum strength. But stronger bonds will be more stressed, which could negatively affect their fatigue behaviour. In this work, the shear fatigue behaviour of two interfaces of different stiffness has been analysed, based on a cyclic compression test, performed at controlled stress, using the test device B described in the NLT-382 standard, included in the draft of the European pre-norm prEN 12697-48. From these tests, fatigue laws for each type of interface have been obtained. These laws, combined with a calculation method developed by the authors, have made it possible to obtain the number of cycles that each interface would withstand, under certain load hypotheses.