In the field of long-life fatigue, predicting fatigue lives and limits for mechanical components is crucial for ensuring reliability and safety. Fracture mechanics tools have enabled the estimation ...of fatigue lives for components with small cracks or defects. However, when dealing with defects larger than the microstructural characteristic size, estimating the fatigue resistance of a material requires determining the cyclic resistance curve for the defect-free matrix, which depends on knowledge of the material’s intrinsic fatigue limit. This study focuses on the experimental evidence regarding the intrinsic fatigue limit and its correlation with naturally nucleated non-propagating cracks. Fracture mechanics models for small crack propagation are introduced, and their disparities and limitations are analyzed. The concept of intrinsic fatigue limit is then introduced and applied to reanalyze a recent publication. Methods for estimating the intrinsic fatigue limit are explored and applied to experimental results reported in the literature. The need to clarify and accurately predict the intrinsic fatigue limit is highlighted in alloys where the processing generates defects larger than the microstructural size of the matrix, as often observed in materials and components produced using additive manufacturing.
The heterogeneity of welded joints’ microstructure affects their mechanical properties, which can vary significantly in relation to specific weld zones. Given the dimensional limitations of the ...available test volumes of such material zones, the determination of mechanical properties presents a certain challenge. The paper investigates X welded joint of S690QL1 grade high strength steel (HSS), welded with slightly overmatching filler metal. The experimental work is focused on tensile testing to obtain stress-strain properties, as well as fracture mechanics testing. Considering the aforementioned limitations of the material test volume, tensile testing is carried out with mini tensile specimens (MTS), determining stress-strain curves for each characteristic weld zone. Fracture mechanical testing is carried out to determine the fracture toughness using the characteristic parameters. The experimental investigation is carried out using the single edge notch bend (SENB) specimens located in several characteristic welded joint zones: base metal (BM), heat affected zone (HAZ), and weld metal (WM). Fractographic analysis provides deeper insight into crack behavior in relation to specific weld zones. The numerical simulations are carried out in order to describe the fracture behavior of SENB specimens. Damage initiation and evolution is simulated using the ductile damage material behavior. This paper demonstrates the possibility of experimental and numerical determination of fracture mechanics behavior of characteristic heterogeneous welded joint zones and their influence on crack path growth.
3D printing technique is currently one of the promising emerging technologies. It is used in many areas of human activity, including acoustic applications. This paper focuses on studying the sound ...reflection behavior of four different types of 3D-printed open-porous polylactic acid (PLA) material structures, namely cartesian, octagonal, rhomboid and starlit structures. Sound reflection properties were evaluated by means of the normal incidence sound reflection coefficient based on the transfer function method using an acoustic impedance tube. In this study, various factors affecting the sound reflection performance of the investigated PLA samples were evaluated. It can be concluded that the sound reflection behavior of the tested PLA specimens was strongly affected by different factors. It was influenced, not only by the type of 3D-printed open-porous material structure, but also by the excitation frequency, the total volume porosity, the specimen thickness, and the air gap size behind the tested specimen inside the acoustic impedance tube.
The microstructure of Ti6Al4V alloy, manufactured using laser powder bed fusion (L-PBF), is affected by process parameters and heat treatment. However, their influence on the nano-mechanical behavior ...of this widely applicable alloy is still unknown and scarcely reported. This study aims to investigate the influence of the frequently used annealing heat treatment on mechanical properties, strain-rate sensitivity, and creep behavior of L-PBF Ti6Al4V alloy. Furthermore, the influence of different utilized L-PBF laser power-scanning speed combinations on mechanical properties of annealed specimens has been studied as well. It has been found that the effect of high laser power remains present in the microstructure even after annealing, resulting in increase in nano-hardness. Moreover, the linear relation between the Young's modulus and the nano-hardness after annealing has been established. Thorough creep analysis revealed dislocation motion as a dominant deformation mechanism, both for as-built and annealed conditions of the specimens. Although annealing heat treatment is beneficial and widely recommended, it reduces the creep resistance of Ti6Al4V alloy manufactured using L-PBF. The results presented within this research article contribute to the L-PBF process parameter selection, as well as to understanding the creep behavior of these novel and widely applicable materials.
The objective of this work was to investigate the fracture behavior of a heterogeneous I-shaped welded joint in the context of yield load solutions. The weld was divided into two equal parts, using ...the metal with the higher yield strength and the metal with the lower yield strength compared to base metal. For both configurations of the I-shaped weld, one with a crack in strength in the over-matched part of the weld and one for a crack in the under-matched part of the weld, a systematic study of fracture toughness SE(B) specimen was carried out in which the crack length, the width of the weld and the strength mismatch factor for both weld metals were varied, and the yield loads were determined. As a result of the study, two mathematical models for determination of yield loads are proposed. Both models were experimentally tested with one strength mismatch configuration, and the results showed good agreement and sufficiently conservative results compared to the experimental results.
The
integral for the compact tension (CT) specimen is calculated using the estimation equation in ASTM E1457-15. This equation was developed based on the assumption of material homogeneity and is not ...applicable to a welded CT specimen. In this paper, a modified equation for estimating the
integral for a welded compact tension (CT) specimen under creep conditions is proposed. The proposed equation is defined on the basis of systematically conducted extensive finite element (FE) analyses using the ABAQUS program. A crack in the welded CT specimen is located in the center of the heat-affected zone (HAZ), because the most severe type IV cracks are located in the HAZ. The results obtained by the analysis show that the equation for estimating the
integral in ASTM E1457-15 can underestimate the value of the
integral for creep-soft HAZ and overestimate for creep-hard HAZ. Therefore, the proposed modified equation is suitable for describing the creep crack growth (CCG) of welded specimens.
This paper deals with experimental determination of the mechanical properties of the fiber glass-reinforced plastic. Nine test samples were made from fiber-glass plastic, where three series with ...three samples each were with different thickness. All samples were of nominal length 250 mm with fibers of length 50 mm. The width of the specimens was 25 mm, and the thickness was 0,35 mm; 0,8 mm and 1,16 mm, respectively. The samples were subjected to tensile testing on a tensile machine in order to determine the mechanical properties. The force and longitudinal displacement (actuator displacement) were measured on the tensile testing machine, while with the optical measuring system, strains in the transverse direction (perpendicular to the tensioning direction) were measured. The results of the measurements on the tensile test machine, presented in the form of stress-displacement diagrams, are shown first within the same series (equal thickness of the test sample) and then the results are compared for different thicknesses of the test sample. By tensometric measurements, the change of the principal strains over time was determined. The engineering stress-strain curve for all nine test specimens was obtained using the optical measuring system and given in form of diagrams. It was found that the best tensile properties of all considered specimens have test specimens with the largest thickness, which also had glass fibers of knitted structure.
Total structural life analysis methodology, applicable to actual engineering structures, is presented in this paper. It is demonstrated on the case of special purpose hydraulic press, specifically ...its C-shaped welded frame. The frame is subjected to high loads due to the operation of integrated hydraulic cylinders. In order to withstand such demanding working conditions, welded frame is made of high-strength steel. Two different designs of load-bearing welded joints, partially and fully penetrated, are considered and their lifetime is compared. Series of analyses are performed to determine the total structural life. A numerical analysis of the welded frame structure is performed considering the elasto-plastic range of material behavior. Fatigue analysis is performed based on the results obtained by EP analysis, in order to determine initial crack location and life cycles to crack initiation. Crack growth analysis is performed in order to determine the critical crack dimensions and total structural life. This approach allows to predict the welded joints behavior and timely prevent structural failure. It can be concluded that fully penetrated welded joint exhibits a significant structural life advantage in comparison to partially penetrated joint.
During the manufacturing of welded structures, some degree of residual stresses occurs. The classic approach to residual stress reduction is Post-Weld Heat Treatment (PWHT). In the case of structural ...grade mild steels, the thermal process is well established. In case of S690QL1 High Strength Steel (HSS), which is manufactured using the Quenching and Tempering (QT) process considered in this paper, only limited PWHT treatment is possible without deterioration of mechanical properties. Since this steel grade is susceptible to subsequent heat input, the challenge is to establish adequate PWHT parameters, achieving residual stress reduction while retaining sufficiently high mechanical properties. The paper considers X joint welded HSS steel plates with slightly overmatching filler metal. The welded coupon is prepared and subjected to PWHT treatment. The research on the influence of heat treatment was performed using the four different PWHT cycles and initial As-Welded (AW) material condition. The authors proposed those PWHT cycles based on available resources and the literature. Process holding temperature is considered the variable parameter directly related to the behaviors of material properties. The methodology of welded joint analysis includes experimental testing of mechanical properties, metallographic examination, and residual stress quantification. Testing of mechanical properties includes tensile testing, Charpy V-notch impact testing, and hardness testing in scope of complete welded joint (BM + HAZ + WM). Metallographic examination is performed in order to characterize the welded joint material in relation to applied PWHT cycles. In order to quantify residual stresses, all heat-treated samples were examined via the X-ray diffraction method. Mechanical properties testing determined that an increase in PWHT cycle holding temperature leads to degradation of tested mechanical properties. For specific zones of the welded joint, the decreasing trend from AW condition to Cycle D (max. 600 °C) can be quantified. Based on representative specimens comparison, strength values (BM ≤ 5.7%, WM ≤ 12.1%, HAZ ≤ 20%), impact testing absorbed energy (BM = 17.1%, WM = 25.8%, FL = 12.5%, HAZ = 0.6%), and hardness values (BM = 4.1%, WM = 3.2%, CGHAZ = 16.6%, HAZ = 24.2%) are all exhibiting decrease. Metallographic examination, using the light microscopy, after the exposure to PWHT thermal cycles, did not reveal significant changes in the material throughout all specific welded joint segments. Average relative reduction in residual stress in correlation with PWHT temperature can be observed (AW = 0%, Cycle A (max. 400 °C) = 72%, Cycle B (max. 530 °C) = 81%, Cycle C (max. 550 °C) = 93% and Cycle D (max. 600 °C) = 100% stress reduction). It can be concluded that S690QL1 HSS welded joints can generally be subjected to PWHT, while adhering to the limits of the material and process. In the authors’ shared opinion, it is advisable to use the PWHT Cycle C (max. 550 °C) with 93% RS reduction, while mechanical properties retain high values.
In this paper, our aim was to create a coupled numerical simulation method for rolling contact fatigue (RCF) analysis which takes into account the effects of results of the heat treatment process of ...the manufacturing of specimen/machine parts, such as hardness, mechanical properties, residual stresses in the carburized layers of the body. Thus, the finite element method (FEM) simulation process calculates the material properties and stress close to the realistic state and using them as an input data, the RCF analysis is carried out by this coupled method. The Dang Van, Matake and Findley multiaxial fatigue criteria were used for failure analysis. In parallel with the development of the method, validation experiments were carried out to decide which criterion is the most appropriate for our purpose and to determine the coupled method efficiency.
PDF
