Subsequent Fatigue Tests Research Articles

Monitoring of a fatigue test on a full-size railway steel bridge with acoustic emission

Within the frame of the Rail4Future project (https://www.rail4future.com) co-funded by the Austrian Research Promotion Agency, a railway steel bridge was removed from the track and transported to a workshop in one piece. Artificial defects were introduced at different sections of the bridge structure as per advice of the scientific project partners in order to initiate fatigue cracks in the course of the subsequent fatigue test. One objective of the fatigue test was to identify suitable measurement and testing techniques for monitoring of fatigue crack growth. Among other techniques utilised by different project partners, acoustic emission monitoring was applied by TÜV AUSTRIA. This contribution describes the preparation as well as the performance of the acoustic emission monitoring and the results obtained. It is shown that this non-destructive testing technique is suitable for fatigue crack monitoring.

Criticality of volumetric defect structure on fatigue properties of stainless steel 316L fabricated by laser powder direct energy deposition

As one of the significant metal additive manufacturing (AM) processes, laser powder direct energy deposition (LP-DED) has established its role in specific applications, including component repair, coatings, and multi-material structure. However, one of the limiting factors for the wide adoption of LP-DED is the formation of volumetric defects during the printing process. These defects are one of the crucial causes of poor and scattered mechanical properties, especially fatigue performance, by accelerating fatigue crack initiation, growth, and final failure. Unraveling defect structure-fatigue correlations can help understand how defects are affecting fatigue properties, how critical they are, and, finally, what solutions can be utilized to diminish their harmful effects. In this research study, novel defect features, including size features, shape descriptors, and location features, were proposed and investigated regarding fatigue properties in LP-DED. Accordingly, stainless steel 316L (SS 316L) blocks were fabricated by LP-DED. After cutting out the dog-bone samples by wire electric discharge machining (EDM), X-ray Computed Tomography (XCT) and subsequent fatigue tests were conducted. The defect features and their statistical descriptors were extracted from XCT data. By statistical analysis of the features, valuable insights regarding their correlations with fatigue life were realized.

Comprehensive investigation and modeling of pitting corrosion effects on mechanical properties and fatigue performance of steel wires

Steel wires are widely employed in structural engineering due to their robust strength. However, the emergence of pitting corrosion significantly impacts their capacity, fatigue performance, and mechanical traits, with no specific codes addressing this issue. To assess the load-bearing capacity of corroded steel wires, predict their remaining fatigue life, and analyze failure modes, a comprehensive physical test was conducted. Mechanical evaluations of corroded wires were followed by 3D scanning to analyze corrosion attributes and fracture surface morphology. Using fuzzy reliability theory, a model estimated wire capacity under varying corrosion degrees. Subsequent fatigue testing explored performance and developed a predictive residual life model. Investigation results revealed a rapid and gradual decrease in capacity's reliability coefficient. Corroded wire fracture morphologies were classified as typical and multi-source cracks, including subtypes like cracks expanding in the same plane and step-like fractures. All wire types exhibited a cracking threshold exceeding 5.6 MPa·m1/2, with fracture intensity declining as corrosion severity increased. Notably, the model better suited slightly corroded wires, showing up to 30.93 % relative error for severely corroded wires. Enhancing accuracy for the latter entails considering additional factors to refine the model.

Which of the 37 Plates Is the Most Mechanically Appropriate for a Low-Neck Fracture of the Mandibular Condyle? A Strength Testing.

(1) Background: The mandible is the most frequently injured component of the facial skeleton, with 25-45% of mandibular fractures involving the condylar process. This study aims to mechanically compare which plates are most suitable for use in low-neck fractures of the condyle. (2) Methods: Polyurethane mandibular models with simulated low-neck fractures were tested using 37 distinct plate designs. These plates were fabricated from 1 mm thick, grade 23 titanium sheets. The models were then subjected to force tests on a strength machine, and the correlation between applied force and fracture displacement was recorded. (3) Results: For low-neck fractures, XCP side-dedicated 3+5 and ACP-T plates demonstrated strength comparable to that of two straight plates, the current gold standard in osteosynthesis. (4) Conclusions: The Mechanical Excellence Factor (MEF) introduced by the authors provides a more accurate metric for theoretically predicting a plate's mechanical strength compared to the Plate Design Factor (PDF). Eight plate characteristics were utilized to calculate the MEF. Employing the MEF allows for rapid, preliminary validation before undertaking strength tests. Furthermore, the findings of this study can guide the selection of the most durable plate designs for subsequent fatigue testing.

Low cycle fatigue of components manufactured by rod extrusion: Experiments and modeling

The performance of formed components is significantly influenced by the initiation of ductile damage. Preceding forming operations, for instance, affect the service life determined in fatigue tests. In the current investigation, the effect of ductile damage in forming is isolated by changing the shoulder opening angle in forward rod extrusion. Forming-induced ductile damage is then related to measurements of void area fraction, density and Young’s modulus. Subsequent fatigue tests in the low cycle range indicate that the service life of the extruded components can be improved through a reduction of the forming-induced damage. A novel constitutive model considering forming-induced damage and fatigue damage is proposed to account for the observed behavior in axial fatigue tests of extruded components. The non-local ductile damage formulation is formulated in the framework of Generalized Standard Materials. Kinematic and isotropic hardening are considered. Based on earlier work of Lemaitre and Desmorat, the fatigue damage initiation criterion is extended to take the observed mechanical behavior in low cycle axial fatigue tests of formed components into account. The extended model is able to capture the effect of forming-induced damage on the service life.

Fatigue damage development in 10CrMo9-10 steel for power plant pipes in as-received state and after 280,000 h of exploitation

This paper discusses an effect of 280,000 h of exploitation under internal pressure of 2.9 MPa and high temperature of 540 °C on the mechanical properties of 10CrMo9-10 (10H2M) power engineering steel. The mechanical response of the specimens obtained from two pipes: a new in the as-received state and exploited for a long period was assessed through the uniaxial tensile tests and subsequent fatigue tests. The long-term, high-temperature exposure of 10H2M steel resulted in the deterioration of its mechanical properties and was quantitatively described as a function of the fatigue damage measure, φ, and the fatigue damage parameter D. Finally, the proposed methodology of power exponent approximation with both damage sensitive parameters (φ, D) enabled for successful determination of the 10H2M steel fatigue life.

Experimental and numerical studies on the hot spot stress calculation method for welded CHS X-joints stiffened by internal ring stiffeners

Ring stiffeners are usually used to enhance the strengths of welded hollow section joints, which may change the magnitude and distribution of hot spot stress around intersections related to the fatigue life of the joints. Up to date, there is no relevant research on the hot spot stress and its calculation method for welded CHS X-joints with internal ring stiffeners in combined load cases, causing a lack of confidence in the fatigue design process. Experimental tests including two types of load cases are firstly carried out to provide a reference for the finite element (FE) modeling. FE analysis is conducted to investigate the influence of internal ring stiffeners on hot spot stress, covering a variety of numbers, thicknesses and spacing of internal ring stiffeners. The hot spot stress calculation method is developed. A fatigue test is subsequently conducted. It is shown that the hot spot stress near the intersection region of the ring stiffeners and tubes is relatively large in addition to those at the crowns and saddles. The locations of interest include both traditional locations and intersection locations of ring stiffener contours and tube contours. Setting ring stiffeners can averagely reduce the hot spot stress ranges by 23.6% in chord and 27.8% in brace, respectively. The extended hot spot stress calculation method is verified for the fatigue design of welded CHS X-joints with ring stiffeners. In the subsequent fatigue test, fatigue failure is found at the intersection locations. A dynamic effect is also observed which is related to the change in load eccentricity. The fatigue strength curve specified in CIDECT design guide No.8 could also be applicable to predict the fatigue life of the welded CHS X-joint with ring stiffeners.

Pre-fatigue enhancing both long-term stability and sensitivity of direct-ink-writing printed sensors

The direct-ink-writing printed sensors possess high sensitivity and large stretchability. However, whether the high performance of these sensors keeps stable under fatigue loads is crucial but still undetermined. Here we develop the testing techniques and report performance evolution of the printed sensors under different strain ranges of 0.5–3.27% up to 105 cycles. It is found that the sensor sensitivity is enhanced by one order of magnitude through applying fatigue strain with moderate levels, accompanying improved stability in the subsequent fatigue testing. These insights may contribute to the ongoing search for long-term stable and sensitive sensors for engineering applications.

Design Theory and Experimental Research of Ultrasonic Fatigue Test

Ultrasonic fatigue testing is a key technology that is more efficient and energy saving compared to conventional fatigue testing. In order to investigate the behavior of metallic materials at ultra-high frequencies and to verify the validity of ultrasonic fatigue test results, this paper builds a test apparatus that can be used to conduct ultrasonic fatigue tests, and this paper proposes a complete procedure from theoretical analysis to the investigation of test laws for 20 kHz tensile and the compression test. Firstly, the initial sizes of the sample are calculated with an analytical method, then the three-dimensional model is simulated and optimized with finite element software, and the optimum result for the sample size is then obtained according to the sensitivity of the sample size to the effect of frequency. The next step is to analyze the influenced trend of the sample size, including L1, L2, L3, R1 and R2, on the resonant frequency and maximum stress of the sample. According to the optimized results, the sample was processed, and an ultrasonic fatigue test was carried out to ensure the sample fatigue fracture finally occurred. Finally, the S-N curve of the material was plotted based on the data recorded in the test and compared with the conventional fatigue life curve to verify the feasibility of the ultrasonic fatigue test device and test method. The fracture of the sample was observed using an optical microscope, and its macroscopic fracture morphology was analyzed. The fracture morphology of the sample can be divided into three typical zones: the fatigue crack source zone, the extension zone and the transient zone, where the fatigue cracks all originate from on the surface of the sample. The results demonstrate the validity of the ultrasonic fatigue test results and provide new ideas for the design and optimization of ultrasonic fatigue samples and shorter processing times, providing a reference for subsequent ultrasonic fatigue tests on typical materials.

Employing sequential experiments to gain insights on the interaction between creep and fatigue damage in Ni‐base superalloys

AbstractThe interaction between creep and fatigue damage in Ni‐base superalloy CMSX‐8 was explored using a sequential workflow that decouples creep and fatigue steps to quantify how accumulated creep damage interacts with subsequent isothermal low‐cycle fatigue (LCF) and vice versa. Two workflows were considered: creep followed by LCF and LCF followed by creep. For the first workflow, the subsequent LCF was evaluated at temperatures of 20°C, 750°C, and 1100°C. It was found that the extent of deleterious interaction of prior creep on LCF depended on increasing subsequent fatigue test temperature. For the LCF followed by creep, fatigue tests were first conducted at 20°C and 1100°C to 60% of life, and then creep was conducted at 800°C, where primary creep is promoted, or 900°C, where primary creep is absent. LCF damage accumulated at 1100°C accelerated creep and decreased ductility, while fatigue damage at room temperature had minimal effect on the creep behavior.

Predicting fatigue life of metal LPBF components by combining a large fatigue database for different sample conditions with novel simulation strategies

This study presents a unique methodology to systematically predict and evaluate the fatigue behaviour of additively manufactured components. A multitude of factors influence the fatigue performance of end-use application components. To analyse this effect, fatigue behaviour of samples manufactured by laser powder bed fusion (LPBF) is evaluated for different process conditions, namely, build orientation, heat treatment and surface treatment. An extensive experimental test campaign is performed to develop a database of fatigue results capturing all these influencing factors. A Gaussian process is then trained with the observed experimental results to predict the fatigue behaviour for unknown process conditions. This machine learning method is subsequently validated to confirm its capability to represent the experimental data on one hand and to generalize well to unseen data on the other hand. In addition, the method is integrated in a fatigue durability prediction solver. The predictive capability of large-scale simulations was demonstrated for low pressure steam turbine blades manufactured by LPBF and subjected to different combinations of post processes. Subsequent experimental fatigue testing of the blades indicated that the durability solver accurately predicted the crack initiation location. For a combination of stress relieved microstructure and as-fabricated surface, the experimental fatigue lives lie within 95% confidence interval of the lifetimes predicted by the durability solver. For the components with full annealed microstructure and machined surface, the durability solver predicted conservative lifetimes compared to the experimental fatigue lives, demonstrating the capability to predict fatigue behaviour of end-use components in the field. • A new methodology is developed to predict fatigue of additively manufactured metals. • Fatigue behaviour of samples is investigated for different process conditions. • Machine learning and finite element analysis are used to predict fatigue lifetimes of components. • Predicted crack initiation sites coincide with experimental tests.

Investigation of the effect of nanosecond laser pulses processing on the microstructure and fatigue resistance of commercially pure titanium

The effect has been studied of treatment with nanosecond laser pulses on the fatigue resistance of plate samples of recrystallized (grain size of the order of 2-3 μm) commercially pure titanium (grade VT1-0) under cyclic tensile loading. The results of investigations by scanning and transmission electron microscopy of the subsurface layer microstructure of the alloy under study after exposure to nanosecond laser irradiation and subsequent fatigue tests are presented. Keywords: titanium, surface, pulsed nanosecond laser irradiation, shock-wave action, fatigue properties.

Исследование влияния обработки лазерными импульсами наносекундной длительности на микроструктуру и сопротивление усталости технически чистого титана

The effect of treatment with nanosecond laser pulses on the fatigue resistance of plate samples of recrystallized (grain size of the order of 2-3 µm) commercially pure titanium (grade VT1-0) under cyclic tensile loading is studied. The results of investigations by methods of scanning and transmission electron microscopy of the microstructure of the subsurface layer of the alloy under study after exposure to nanosecond laser irradiation and subsequent fatigue tests are presented.

Manufacture of a suspension subframe through CAPROCAST technology (T-RTM)

The main objective of this work has been the manufacture , and subsequent flexural and fatigue  tests, of automotive real parts, made of thermoplastic composites. This has been achieved by incorporation continuous carbon fiber (textile) to polyamide matrix, through an specific process very similar to a thermoplastic  RTM. The fatigue behaviour of any composite is governed by the toughness of the material. Because of this and the low cost, the polyamide has been considered as a polymer matrix. The process involves an in-situ polymerization, that the low viscosity of the monomer,e-caprolactama (e-CL), allows the infiltration of preforms and textiles. The demonstration part has been completely redesigned by BATZ and TECNALIA to fullfill mechanical requirements and  manufacture ability. In particular, it has been fabricated a suspension subframe with an 52%vol. of a commercial carbon fiber and thickness variations from 3 to 8mm within the same part, obtaining a significant reduction in weight relative to the metal part (6,8 kg vs 3,48 kg) and the established mechanical requirements. As far as the fatigue/durability test are concerned, they have been performed to evaluate the strength of the composite and to increase the correlation between CAE analysis and the real behavior of the hybrid component. All the tested parts support 2.000.000 cycles (the stipulated for this part in metal) without break, only with small deformations in the bushings holes. As far as the flexural test are concerned, they evaluate the maximum quasi-static strength of the component. All the parts tested deform considerably, recovering much of this deformation once the load is removed.

Numerical and databased investigations on the fatigue of cruciform joints with gaps

The paper deals with investigations on the fatigue resistance of the load-carrying cruciform joint with gaps. The aim is to define limits of specific acceptable imperfections, since these do not always necessarily affect the fatigue strength. Different geometrical influences on the fatigue strength of the load-carrying cruciform joint with gaps are analysed numerically. Numerical simulations comprise effective notch stress calculations, which have been compared to fatigue test results from literature, documented in a fatigue test database. A larger gap in combination with a smaller weld size leads to the unfavourable failure at the weld root. For the specific geometrical configurations, borderline functions defining the failure location on the basis of local weld geometry have been defined. Finally, the most interesting configurations are presented, which will be tested experimentally in a subsequent fatigue test program.

Realitätsnahe Finite-Elemente-Simulation in der präklinischen Testung von Knie- und Hüftimplantaten

In the process of developing an implant, computer simulation involving finite element (FE) methods allows the early identification of design-related issues, thus reducing the development process to aminimum. In addition, the FE simulation is used for selecting testing combinations in order to provide the relevant authority with proof of a"worst-case" construct scenario for the subsequent experimental fatigue test. Research studies with FE simulations show that implant positioning may affect mechanical loads under certain circumstances and, therefore, influence the preclinical evaluation of the prostheses. Although the FE simulation currently contributes significantly to preclinical testing, astandardization of the calculation models allowing comparability of results is lacking. Furthermore, the development of new dynamic and realistic models is necessary in order to identify complex damage modes that currently cannot be reproduced experimentally. When considering everyday clinical life in particular, models that can reproduce intraoperative kinematic changes and the resulting incorrect loads of the implant, as well as address these problems by changing the position or design of the prosthesis, are necessary and would help in future.

Capability of martensitic low transformation temperature welding consumables for increasing the fatigue strength of high strength steel joints

Abstract The use of low transformation temperature (LTT) filler materials represents a smart approach for increasing the fatigue strength of welded high strength steel structures apart from the usual procedures of post weld treatment. The main mechanism is based on the effect of the low start temperature of martensite formation on the stress already present during welding. Thus, compressive residual stress formed due to constrained volume expansion in connection with phase transformation become highly effective. Furthermore, the weld metal has a high hardness that can delay the formation of fatigue cracks but also leads to low toughness. Fundamental investigations on the weldability of an LTT filler material are presented in this work, including the characterization of the weld microstructure, its hardness, phase transformation temperature and mechanical properties. Special attention was applied to avoid imperfections in order to ensure a high weld quality for subsequent fatigue testing. Fatigue tests were conducted on the welded joints of the base materials S355J2 and S960QL using conventional filler materials as a comparison to the LTT filler. Butt joints were used with a variation in the weld type (DY-weld and V-weld). In addition, a component-like specimen (longitudinal stiffener) was investigated where the LTT filler material was applied as an additional layer. The joints were characterized with respect to residual stress, its stability during cyclic loading and microstructure. The results show that the application of LTT consumables leads to a significant increase in fatigue strength when basic design guidelines are followed. This enables a benefit from the lightweight design potential of high-strength steel grades.

Capability of martensitic low transformation temperature welding consumables for increasing the fatigue strength of high strength steel joints

Abstract The use of low transformation temperature (LTT) filler materials represents a smart approach for increasing the fatigue strength of welded high strength steel structures apart from the usual procedures of post weld treatment. The main mechanism is based on the effect of the low start temperature of martensite formation on the stress already present during welding. Thus, compressive residual stress formed due to constrained volume expansion in connection with phase transformation become highly effective. Furthermore, the weld metal has a high hardness that can delay the formation of fatigue cracks but also leads to low toughness. Fundamental investigations on the weldability of an LTT filler material are presented in this work, including the characterization of the weld microstructure, its hardness, phase transformation temperature and mechanical properties. Special attention was applied to avoid imperfections in order to ensure a high weld quality for subsequent fatigue testing. Fatigue tests were conducted on the welded joints of the base materials S355J2 and S960QL using conventional filler materials as a comparison to the LTT filler. Butt joints were used with a variation in the weld type (DY-weld and V-weld). In addition, a component-like specimen (longitudinal stiffener) was investigated where the LTT filler material was applied as an additional layer. The joints were characterized with respect to residual stress, its stability during cyclic loading and microstructure. The results show that the application of LTT consumables leads to a significant increase in fatigue strength when basic design guidelines are followed. This enables a benefit from the lightweight design potential of high-strength steel grades.

Investigating the birth-related caudal maternal pelvic floor muscle injury: The consequences of low cycle fatigue damage

One of the major causes of pelvic organ prolapse is pelvic muscle injury sustained during a vaginal delivery. The most common site of this injury is where the pubovisceral muscle takes origin from the pubic bone. We hypothesized that it is possible for low-cycle material fatigue to occur at the origin of the pubovisceral muscle under the large repetitive loads associated with pushing during the second stage of a difficult labor. The main goal was to test if the origin of the pubovisceral muscle accumulates material damage under sub-maximal cyclic tensile loading and identify any microscopic evidence of such damage. Twenty origins of the ishiococcygeous muscle (homologous to the pubovisceral muscle in women) were dissected from female sheep pelvises. Four specimens were stretched to failure to characterize the failure properties of the specimens. Thirteen specimens were then subjected to relaxation and subsequent fatigue tests, while three specimens remained as untested controls. Histology was performed to check for microscopic damage accumulation. The fatigue stress-time curves showed continuous stress softening, a sign of material damage accumulation. Histology confirmed the presence of accumulated microdamage in the form of kinked muscle fibers and muscle fiber disruption in the areas with higher deformation, namely in the muscle near the musculotendinous junction. The origin of ovine ishiococcygeous muscle can accumulate damage under sub-maximal repetitive loading. The damage appears in the muscle near the musculotendinous junction and was sufficient to negatively affect the macroscopic mechanical properties of the specimens.

Hot spot stress and fatigue behavior of bird-beak SHS X-joints subjected to brace in-plane bending

Abstract This paper deals with the stress concentrations and fatigue behaviour of X-shape bird-beak joints under brace in-plane bending. Static load was first implemented to record the strain distribution along the hot lines, and then stress concentration factors (SCFs) were obtained through quadratic extrapolation method. Numerical analysis was then developed to further determine key parameters affecting SCFs and to propose corresponding SCF design formulas. During the subsequent high-cycle fatigue test, the fatigue cracking process including crack initiation, crack propagation, and fatigue lives, as well as the rigidity degradation were captured. Results show that crack initiation always occurred at the crown area where the SCFs are proved to be the greatest. The through-thickness lives may be utilized to conservatively represent the fatigue design lives of SHS bird-beak X-joints. Bird-beak SHS X-joints demonstrated better fatigue behaviors than those for traditional joints if the same service load is applied.