Torsional Fatigue Tests Research Articles

Influence of pre-torsion strengthening on the fatigue properties of 45 CrNiMoVA ultra-high strength steel

Aiming at the demand for fatigue life improvement of 45CrNiMoVA ultra-high strength steel shaft parts, a pre-torsion strengthening process using a combination of two different angles before and after was proposed based on the mechanisms of strain hardening, fine grain strengthening and dislocation strengthening. Through surface integrity measurement, torsional fatigue test and fatigue fracture characterization of the specimens, the main surface integrity factors affecting the fatigue life of the pre-torsion strengthening were analyzed. The results indicated that the grain size was refined and the plastic deformation was increased by the two-angle pre-torsion strengthening compared with the single-angle pre-torsion strengthening. The obtained surface residual compressive stress σx was the main factor affecting the fatigue life of pre-torsion strengthening, and the change in pre-torsion angle results in a significant secondary strengthening layer in the subsurface layer of the material. In summary, the study of the pre-torsion strengthening process with different combinations of angles has a guiding significance for improving the fatigue life of ultra-high strength steel.

Fatigue Life Data Fusion Method of Different Stress Ratios Based on Strain Energy Density.

To accurately evaluate the probabilistic characteristics of the fatigue properties of materials with small sample data under different stress ratios, a data fusion method for torsional fatigue life under different stress ratios is proposed based on the energy method. A finite element numerical modeling method is used to calculate the fatigue strain energy density during fatigue damage. Torsional fatigue tests under different stresses and stress ratios are carried out to obtain a database for research. Based on the test data, the Wt-Nf curves under a single stress ratio and different stress ratios are calculated. The reliability of the models is illustrated by the scatter band diagram. More than 85% of points are within ±2 scatter bands, indicating that the fatigue life under different stress ratios can be represented by the same Wt-Nf curve. Furthermore, P-Wt-Nf prediction models are established to consider the probability characteristics. According to the homogeneity of the Wt-Nf model under different stress ratios, we can fuse the fatigue life data under different stress ratios and different strain energy densities. This data fusion method can expand the small sample test data and reduce the dispersion of the test data between different stress ratios. Compared with the pre-fusion data, the standard deviations of the post-fusion data are reduced by a maximum of 21.5% for the smooth specimens and 38.5% for the notched specimens. And more accurate P-Wt-Nf curves can be obtained to respond to the probabilistic properties of the data.

Temperature effects on the lifetime reinforcement due to strain-induced crystallization in carbon black filled natural rubber under non-relaxing torsion: comparison with non-relaxing tensile loadings

This paper investigates the lifetime reinforcement at different temperatures of carbon black filled natural rubber under non-relaxing torsion. Torsion fatigue tests with different angle ratios have been carried out with axisymmetric-shaped specimens. Finite element analysis was used to predict the local mechanical state by taking into account temperature effects and stress softening heterogeneities. It has been shown that when the temperature of the test is increased, the mechanical response does no longer stabilize, contrary to what is observed when the test is carried out at ambient temperature. A strategy has been proposed to evaluate an equivalent loading ratio, in order to compute the minimum loading as the Cauchy stress normal to the crack plane for the minimum torsion angle applied and to represent the results in the Haigh diagram. It was found that the lifetime reinforcement under torsion loading decreases when the temperature is increased to 90 °C, but is still observed. The lifetime reinforcement under torsion at 90 °C was of the same amplitude as the one obtained with the same material under uniaxial tension at the same temperature. The study has been completed by post-mortem analysis at the macro- and the microscale in order to characterize damage mechanisms at 90 °C.

Torsional fatigue behaviour and damage mechanisms of martensitic spring steel in the HCF regime

This study investigates the fatigue behaviour and damage mechanisms of the martensitic spring steel SAE 9254, with emphasis on short crack propagation. Shear stress-controlled torsional fatigue tests are performed at stress ratio R = -1 and test frequency f = 79 Hz using structure-sensitive measurement techniques to detect the relevant damage mechanisms. An in-situ torsional fatigue testing device is applied for shear-controlled fatigue tests at two amplitudes of the torsion angle using R = –1 and f = 5 Hz, and the microstructural-dependent short crack propagation path is documented by means of confocal laser microscopy and electron backscattering analysis. It is found that the early development of fatigue damage is characterised by the formation of slip bands, which serve as crack initiation sites. In this process, crack initiation occurs preferentially at or near prior austenite grain boundaries. Furthermore, prior austenite grain boundaries were identified as obstacles for the propagation of short cracks, because of the change of the crystallographic orientation leading to an oscillating propagation rate of short cracks. The short crack density ρCD, which is an important parameter for fatigue damage, is rapidly growing with increasing shear stress amplitude ΔτT/2 and reveals a stress type specific behaviour.

Mathematical Modelling for Failure Analysis of Composite Drive Shaft Using Modal Flexibility and Curvature Method

Early damage identification in composite structures has attracted a lot of attention in post-failure examinations in recent years. In the research, torsional fatigue test of composite drive shafts has been done and numerical methodologies have been employed for the validation for premature failure using modal flexibility and modal curvature methods. Initially, the eigenvalue and eigenvectors of healthy and fractured drive shafts are evaluated using a numerical model and verified by fatigue experimental approach. Flexibility variance and the absolute curvature difference between the drive shafts that are fractured and those that are healthy are evaluated after mass normalization. Variations in absolute modal curvatures and variations in local flexibilities that are utilized to identify the existence, location, and severity of a drive shaft fracture. The position and severity of the fracture are predicted more precisely by the modal curvature approach than by the modal flexibility approach.

The effects of submerged laser peening, cavitation peening, and shot peening on the improvement of the torsional fatigue strength of powder bed fused Ti6Al4V produced through laser sintering

This study demonstrates the improvement in the fatigue strength of additive manufacturing (AM) metals such as laser-based powder bed fusion of metals by post-processing. Titanium alloy samples manufactured by powder bed fused (PBF) Ti6Al4V produced through laser sintering (LS), treated by submerged laser peening (SLP), cavitation peening (CP), and shot peening accelerated via a water jet (SPwj), were subjected to torsional fatigue testing and compared with the as-built specimen. At SLP, the samples were treated by laser ablation (LA) and laser cavitation (LC) which was developed following LA. A cavitating jet was used for CP. For comparison, conventional post-processing using SPwj was also performed. To characterize the microstructural modification caused by the three post-processing methods, the cross-section of the treated surface was observed by electron backscatter diffraction. The fatigue strengths at 107 cycles were found to be 217, 361, 313, and 285 MPa for the as-built, SLP, CP, and SPwj specimens, respectively. The primary factors contributing to fatigue strength improvement by post-processing were surface smoothing and the introduction of compressive residual stress. The experimental observations were used to derive correlation formulas to estimate the fatigue life improvement due to post-processing as the function of the surface roughness and surface residual stress.

Comprehensive Characterization of Blue Wire NiTi File Failure: A Comparative Analysis of Cyclic Fatigue and Torsional Resistance Properties

This study compared the fatigue resistance and elemental composition of two blue heat-treated nickel–titanium (NiTi) files used in root canal preparation as follows: Tia Tornado Blue (TTB) and Race Evo (RE) file systems. For cyclic fatigue testing, the two systems were tested where each file was rotated inside an artificial metal canal submerged in either sodium hypochlorite or saline solution until fracture. Time to fracture was recorded. For torsional fatigue testing, the file tip was secured while the file was allowed to rotate at a fixed rate until fracture. Torque at failure was recorded. The two experiments were performed at simulated body temperature and the length of fractured segments was measured. Statistical analysis was carried out with a significance level (p-value) set at 5%. The mean cycles to fracture for RE were superior to that of TTB irrespective of the solution used (p < 0.05). TTB’s cyclic fatigue resistance decreased in NaOCl (p < 0.0001). RE demonstrated lower torque at failure (p = 0.002). All files were fractured at comparable lengths (p = 0.218). Although RE is considered more resistant to cyclic fatigue, it showed inferior torsional resistance compared with TTB. The NaOCl negatively affected the TTB’s cyclic fatigue resistance.

Fatigue life reinforcement of carbon black filled natural rubber under non‐relaxing torsion loadings and comparison with non‐relaxing tension loadings

AbstractIn this paper, the effect of non‐relaxing torsion loadings on the fatigue lifetime reinforcement of carbon black filled natural rubber is fully addressed. Torsion fatigue tests with different loading ratios have been performed with highly notched axisymmetric‐shaped specimens. Finite element analysis is used to predict the mechanical state at any point in the specimens. The prediction is significantly improved by taking into account the heterogeneous accommodation. The first Haigh diagram under fatigue torsion is built and analyzed by calculating the minimum loading in terms of the Cauchy stress as the normal stress to the crack for the minimum angle prescribed during the cycle. A strong lifetime reinforcement has been observed, which is found to be of the same amplitude to the one obtained under uniaxial tension with the same material. This has been confirmed by specific tests that combined non‐relaxing torsion and non‐relaxing tension loadings. Post‐mortem analysis carried out at the macroscopic and the microscopic scales enabled us to identify damage mechanisms, especially in the cases where the lifetime reinforcement was observed.

Torsional Fatigue and Static Torsion Strength and Test Validations of Composite Tube Hybrid Drive Shafts

Drive shafts are the transmission components which transfer power from power source to required location. The connection may be between steering wheel and rack & pinion mechanism or gearbox to differential. As in gearbox to differential connection, operational conditions are harsh and high loads are generated, steel materials are mainly preferred to be used. Though steels have such advantages in cost, performance and durability, it is heavier compared to other engineering materials. Since international regulations dictate to decrease the carbon emissions rates due to global concerns, weight decrement of automobile components plays an important role in solving such problem. Considering the operational requirements and boundary conditions, weight reduction in steel parts might be tough. For this reason, alternative materials must be deeply investigated and implemented into the traditional technologies. In the scope of this study, fatigue and torsional strength of propeller shaft with composite tube and aluminum subcomponents were investigated.

Static axial tension or compression stress effects on shear cracks initiating under cyclic torsion loads in multiaxial fatigue tests

This work proposes models that can better fit and explain apparently contradictory mean stress effects observed when superimposing axial tension or compression loads on cyclic torsion fatigue tests. It introduces new shear-based multiaxial fatigue damage models, modifying the classic Fatemi-Socie equation to consider the peak deviatoric stress normal to the initiating shear crack as the actual driving force for the peak or mean effects of the normal stresses. This deviatoric hypothesis can quantify the odd detrimental effects of static axial compression observed on axial shear cracks tested under cyclic torsion. Additionally, applying the peak deviatoric correction term only to the elastic component of the shear strain range allows an even better description of the increasing mean normal stress effects in high-cycle fatigue, eliminating the need for a variable normal stress sensitivity coefficient in the multiaxial fatigue damage parameter. To evaluate the proposed shear-based models, their life and crack initiation direction predictions are compared for cyclic torsion experiments both with and without superimposed static axial stresses for steels and aluminum alloys, including both literature and original data. The experiments show that axial compression can be detrimental to fatigue crack initiation lives, depending on the cracking mode and on the presence of bifurcations, which effects could be explained from the proposed deviatoric damage parameters.

Fundamental relationship between rolling contact fatigue driven surface damage and torsional fatigue

This paper presents fundamental findings on equivalence of rolling contact fatigue (RCF) driven surface damage and torsional fatigue modes of failure. Fatigue experiments were conducted for both modes of failure using AISI 4130 case carburized steel specimens. A Thrust Bearing Surface Pitting Rig (TBSPR) was designed and developed to perform RCF tests on flat test specimens using thrust ball bearings. The experiments were conducted in boundary lubrication regime, simulating the contact conditions in tribological machine components. The current investigation presents a novel approach where the effect of contact pressure and number of fatigue cycles on the initiation of surface damage was characterized by surface profilometry of the wear track after each test to generate a stress-life (S-N) map. The S-N map provides unique insights to early detection of RCF driven fatigue damage. The S-N map was utilized to generate the surface damage S-N curve. Torsional fatigue experiments were conducted on the same material, and the S-N curves of both failure modes were compared. The comparison shows key similarities between the two S-N curves, as they differ by a simple scaling factor. This investigation demonstrates that RCF driven surface damage characteristics can be predicted by conducting much simpler, and faster, torsion fatigue tests. This paper also presents a novel methodology to determine contact pressures to avoid incipient micropitting/microdenting damage.

Cyclic and torsional fatigue resistance of a new rotary file on a rotary and reciprocating motion.

This study evaluated the cyclic and torsional fatigue resistance of a new nickel-titanium (Flat File 25.04) instrument on the continuous and reciprocating motion. Sixty instruments of the ProDesign Logic2 25.03 and 25.05 (Easy Equipamentos Odontológicos, Belo Horizonte, Brazil), and MK Flat File 25.04 (n = 20) (MK Life, Porto Alegre, Brazil) were used. For the cyclic fatigue test, an artificial stainless steel simulated canal with an angle of 60° and a radius of curvature of 5 mm located 5 mm from its tip was used. The torque and rotation angle at the instruments' failure on the torsional fatigue test was based on the ISO 3630-1 protocol, in which the 3 mm tip of each instrument was fixed and connected to an electric motor and a load cell. The fractured surface of each fragment was examined by scanning electron microscopy. Data were analyzed using a 1-way analysis of variance and Tukey's test with a significance level of 5%. Flat File 25.04 had lower cyclic fatigue in both kinematics than the Logic instruments (p < .05). Reciprocating motion improved the cyclic fatigue of the tested instruments (p < .05). Flat File 25.04 had similar torque to Logic2 25.05 (p > .05), and both were superior to Logic2 25.03 (p < .05). The angular deflection values were different for the three tested instruments (p < .05), in the decreasing order: Logic2 25.03, 25.05, and Flat File 25.04. Flat File presented good resistance to cyclic and torsional fatigue resistance. Reciprocating motion improved the cyclic fatigue resistance of the instruments and can be considered when using programmable motors. RESEARCH HIGHLIGHTS: Scanning electron microscopy evaluation of different endodontic rotary file and fatigue resistance tests.

Criteria for determining the fatigue limit of Ni-based superalloy 718, considering the competition between opening- and shear-mode fatigue crack-growth

The mechanism that determines the fatigue limit associated with small-crack arrest in Ni-based superalloy 718 remains under debate. In order to comprehensively assess the alloy’s fatigue limit in relation to the growth threshold of a small fatigue crack, (i) a ΔKI-decreasing fatigue crack-growth test, (ii) torsional fatigue tests and (iii) tension-compression fatigue tests were conducted under fully-reversed loading conditions. In addition, elastic, crack-tip stress fields were analyzed for cracks under various loading conditions. Based on the results, a novel strategy for evaluating the alloy’s fatigue limit is proposed after consideration of two crack-arresting modes (i.e., opening and shear modes).

Evaluation of cyclic and torsional fatigue resistance of several heat-treated reciprocating nickel-titanium instruments.

This study investigated the cyclic fatigue and torsional resistance of Unicone Plus (UCP 25.06), Unicone (UC 25.06), Reciproc Blue (RB 25.08) and Wave One Gold (WOG 25.07) performed at body temperature (35° ± 1°C). Time and number of cycles to fracture (NCF), as well as torque and angular deflection were recorded. Fractured surfaces were evaluated by scanning electron microscopy (SEM). Data were analysed using one-way ANOVA and Holm-Sidak's tests for multiple comparison. The RB had a significantly higher time to fracture, followed by the WOG and UCP (p < 0.05). Regarding the NCF, there was no significantly difference between RB and WOG (p > 0.05). UC presented highest torque values and the lowest angular deflection (p < 0.05). SEM analysis demonstrated typical failures features in both cyclic and torsional fatigue tests. Overall, UC had the lowest time, NCF and angular deflection at fracture. RB presented the highest time to fracture and angular deflection values.

Role of grain boundaries and precipitates in small, fatigue crack-growth resistance of additively-manufactured, Ni-based superalloy 718 under torsional cyclic loading

In essence, additively-manufactured, Ni-based superalloy 718 (AM Alloy 718) possesses a strong crystallographic texture, δ-phases and Laves phases, whose effect on fatigue resistance under torsional cyclic loading is yet to be investigated. In order to elucidate the role of microstructural features in small-crack behavior, torsional fatigue testing was conducted on an AM Alloy 718. Test results revealed that crack-growth was controlled by the twist angle, but also by the tilt angle which had been reported to play a minor role in fatigue resistance under uniaxial cyclic loading. Furthermore, the impact of precipitates appeared to be insignificant, with no strong correlation made between their dispersion and the crack-growth rate.

Inferior fatigue resistance of additively-manufactured Ni-based superalloy 718 and its dominating factor

Push-pull and torsional fatigue tests were conducted using additively-manufactured, Ni-based superalloy 718 samples with various defects and different microstructures, aiming to comprehensively examine fatigue limit determining factors such as detrimental defects and microstructural features. Test results revealed that the fatigue limit in each loading condition was governed by a shear-mode, crack-growth threshold, exhibiting a crack-size dependence similar to that of the wrought alloy. The additively-manufactured materials displayed a substantially reduced crack threshold as compared to wrought ones, potentially attributed to a high volume of low twist angles between neighboring crack-planes.

Effects of non‐proportionality and tension–compression asymmetry on the fatigue life prediction of equivalent stress criteria

AbstractBiaxial tension/compression–torsion fatigue tests with varying levels of non‐proportionality were performed employing a structural adhesive designed for wind turbine rotor blades. The cycles to failure were found to be independent of the level of non‐proportionality. It is demonstrated that numerical fatigue life predictions via rainflow‐counted equivalent stress histories are not able to replicate these experimental observations and overestimate the fatigue life up to a hundredfold. The tension–compression asymmetry of the adhesive resulted in significant damage prediction differences depending on the stress space representation of the Haigh diagram. If not properly taken care of, the asymmetry will also lead to non‐conservative results. While demonstrated with a short fiber‐reinforced adhesive, the results can be transferred to other materials.

Investigation into rolling contact fatigue performance of aerospace bearing steels

In this investigation common aerospace-quality bearing steels was evaluated in rolling contact fatigue both experimentally and analytically. Three aerospace-quality bearing steels was procured and evaluated. First, the bearing steels were evaluated using a 3 ball-on-rod rolling contact fatigue test rig. Next, the same bearing steels were evaluated using a torsion fatigue test rig in order to quantify these materials’ performance against the damage causing stress in RCF – shear reversal. The torsion S-N data provided the foundation for the determination of material constants that were used in a continuum damage mechanics finite element model (CDM-FE model), which considered the Fatemi-Socie critical plane approach as the failure criteria. These material constants captured the material cleanliness effect between the various materials investigated. Additionally, the CDM-FE model utilized Voronoi tessellations to capture the material topological effect. RCF simulations were performed at the same operating conditions as in the 3 ball-on-rod test apparatus. Torsional fatigue results from this investigation indicated which material possessed the largest ultimate shear strength, and which material performed best in low cycle and high cycle fatigue. The three ball-on-rod results established experimentally which material performed superior in RCF. It was observed that good corroboration existed between the analytical simulation life predictions and the 3 ball-on-rod experimental results.

Mechanical Surface Treatment of Titanium Alloy Ti6Al4V Manufactured by Direct Metal Laser Sintering Using Laser Cavitation

Additive manufactured (AM) metals are attractive materials for medical implants, as their geometries are directly produced from computer-aided design (CAD)/computer-aided manufacturing (CAM) data. However, the fatigue properties of AM metals are weak compared with bulk metals, which is an obstacle to the practical applications of AM metals. To improve the fatigue properties of AM metals, we developed a mechanical surface treatment using laser cavitation. When we irradiate a pulsed laser to a metallic surface in water, laser ablation is generated, and a bubble that behaves like a cavitation is produced. The bubble is referred to as a “laser cavitation”. In the surface treatment using laser cavitation, we use the plastic deformation caused by the impact force at the bubble collapse and pulsed laser energy that produces local melting at the same time. Thus, the mechanical surface treatment using laser cavitation is a type of surface mechanical alloying. In this study, to demonstrate the improvement in the fatigue properties of AM metals, we treated titanium alloy Ti6Al4V, which was manufactured by direct metal laser sintering (DMLS), with laser cavitation, and we evaluated the surface morphology, roughness, residual stress, hardness, and finally tested it using a torsion fatigue test. Unmelted particles on the DMLS surface, which cause fatigue cracks, were melted and resolidified using laser cavitation, resulting in a reduction of the maximum heights of roughness (Rz) of about 75% and the arithmetical mean roughness (Ra) of about 84% of the non-peened one. Although tensile residual stresses of about 80–180 MPa were generated on the as-built surface, compressive residual stresses of about −80 MPa were introduced by laser cavitation. Furthermore, laser cavitation formed Ti4O5 oxide film, which increased the surface hardness by about 106%. Finally, we performed torsional fatigue tests and revealed that laser cavitation extended the fatigue life from 19,791 cycles to 36,288 cycles at an applied shear stress (τa) at 460 MPa, which is effective in suppressing crack initiation.

Effects of building direction and loading mode on the high cycle fatigue strength of the laser powder bed fusion 316L

The present study aims to investigate the high cycle fatigue (HCF) performance of steel 316L fabricated by the laser powder bed fusion (LPBF) process. Bending and torsional fatigue test specimens built horizontally (0°), inclined (45°), and vertically (90°) have been prepared and tested. Stress-relieving heat treatment was carried out to reduce the residual stresses. The 90°, 45°, and 0° built near-net-shape and polished specimens have fatigue strengths between 140 and 250 MPa under bending loading and 150 and 170 MPa under torsion loading. This difference illustrates a more pronounced defect sensitivity in bending compared to torsion. Fractographical analyses revealed the fatigue failure mechanisms. As the building direction inclines from vertical to horizontal, the effective areas of inherent defects diminish contributing to improving fatigue strength under bending loading whilst the differences from building directions in torsional fatigue strengths are minor. Microstructural features are seen to compete with inherent defects to affect fatigue performance in the condition that the effective defect sizes are close to the critical fatigue crack size.