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1

A Poisson-Process Model for Cumulative Delamination Damage in Composite Materials

Nechval Konstantin, Chatys Rafał

Transactions on Aerospace Research

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2025

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Nr 4 (281)

122--139

EN

This study presents a probabilistic model for cumulative damage in composite materials undergoing delamination. The model assumes the presence of numerous weak microvolumes (WMVs) in which fatigue damage can initiate and accumulate. Defect nucleation is described as a Poisson process whose rate does not depend on specimen size. The cumulative distribution function of the fatigue life of each microvolume is obtained using the Poisson formulation, and the overall delamination behavior is interpreted as a brittle-type failure mechanism governed by nodal attachment points in honeycomb structures. Numerical implementation is demonstrated through Excel-based inverse comparison and R-based simulation methods. The results show that Poisson-driven damage evolution provides a viable approach for estimating residual undamaged area and for modelling avalanche-type delamination growth. The methodology offers a foundation for future experimental verification and refinement of stochastic models for composite damage propagation.

2

Połączenia z profili CHS pod kątem mniejszym niż 30°. Wnioski na podstawie Eurokodu 3 dotyczące projektowania konstrukcji ze stali zwykłej i wysokiej wytrzymałości

Drewniak Katarzyna, Ataman Magdalena, Cwyl Maciej

Inżynieria i Budownictwo

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2025

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R. 81, nr 6

654--659

PL

W artykule przedstawiono obowiązujące wymagania normowe według Eurokodu 3 dotyczące połączeń z rur okrągłych CHS (Circular Hollow Section) ze stali oraz stali wysokiej wytrzymałości. Omówiono główne aspekty normowego podejścia do wymiarowania połączeń, w których pręty są łączone pod kątem mniejszym niż 30°. Przedstawiono podstawowe wymagania geometryczne i materiałowe, a także omówiono zagadnienia związane ze zmęczeniem. Wskazano aspekty, które nie zostały uwzględnione w obowiązujących normach, oraz zidentyfikowano obszary wymagające dalszych badań naukowych.

EN

This article presents the current normative requirements according to the Eurocode 3 for connection made of circular CHS hollow section from steel and high-strength steel. The main aspects of the normative approach to designing connections in which the members are joined at an angle of less than 30° are discusses. The basic geometric and material requirements are presented, along with fatigue considerations. Furthermore, aspects not covered by the normative approach are identified, and areas for further scientific research are indicated.

3

Zmęczenie odczuwane podczas pracy z urządzeniami rzeczywistości rozszerzonej - wyniki badań wstępnych

Wisełka Mariusz

Bezpieczeństwo Pracy : nauka i praktyka

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2025

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nr 2

24-27

PL

Urządzenia wirtualnej i rozszerzonej rzeczywistości są coraz częściej wykorzystywane do celów zawodowych. Ze względu na postęp technologiczny i szybki rozwój tych urządzeń mogą one stwarzać nowe zagrożenia dla zdrowia użytkowników. W artykule zaprezentowano metodę pomiaru zmęczenia oczu oraz sprawności poznawczej użytkowników okularów rzeczywistości rozszerzonej, obejmującą wybrane testy psychomotoryczne oraz badania okulistyczne. Zaprezentowano także wyniki wstępnych badań, przeprowadzonych przy użyciu tej metody.

EN

Virtual and augmented reality devices are increasingly used for professional purposes. Due to technological progress and the rapid development of these devices, they may pose new health risks to users. The article presents a new method of measuring the fatigue of users of augmented reality glasses, including selected psychomotor tests and ophthalmological examinations. The results of preliminary research conducted using this method are also presented.

4

Structural Parametric Study of a Piezoelectric Energy Harvester for a Specific Excitation Frequency of an Electric Motor, Considering Fatigue Life

Yang Ping, Mohamad Usamah Nabil, Saad Abdullah Aziz, Ahmad Mazlan Ahmad Zhafran

Archives of Acoustics

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2025

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Vol. 50, nr 2

265--275

EN

Over the past decade, extensive research has been conducted in the field of piezoelectric energy harvesting, which drives advancements in novel designs and techniques. In this study, the vibration of an electric motor is characterized, and a piezoelectric energy harvester (PEH) beam with a natural frequency of 50 Hz is designed and fabricated. Then, the electromechanical characteristics of the PEH are simulated and tested through both finite element simulation and experiment. The validated simulation model can accurately predict the vibration characteristics of the PEH, which can be utilized for design improvements. Based on this beam structure, three sets of PEHs with different sizes are designed. A study of the output voltage and fatigue life of these different PEH sizes is conducted, and the relationship between the electromechanical coupling effect and its varying values is discussed. Based on the results, design schemes 1∼6 demonstrate advantages in terms of output voltage efficiency and fatigue strength, making them suitable for various environments and application purposes. This study establishes an efficient method for analyzing the structural parametric performance of piezoelectric cantilever beams, which paves the way for future research on fatigue-based structural design guidelines for PEH in electric motors.

5

Mechanical performance and fatigue analysis of transfemoral prosthetic socket

Ridha Hind Dhia’a, Abbas Saif M., Sadiq Ghanim Sh., Ibraheem Marwa Qasim

Advances in Science and Technology. Research Journal

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2025

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Vol. 19, no. 4

377--385

EN

This work is studying the simulation and testing prosthetic socket made from new arrangement of composite materials. two composite arrangements. The first arrangement is consisting of ten layers of carbon fiber while the second group is made of ten Perlon layers. This research prosthetic is for a 20-year-old female patient with a height of 155 cm and a reported weight of 75 kg. The results of the mechanical properties explained (Ϭult) and (Ϭy) for group 2 was 145 MPa and 137 MPa, while for group 1 was 285 MPa and 280 MPa. The limit of group (2) fatigue was 145 MPa and group (1) was 78 MPa. The interface pressure applied on the stump was measured using F-Socket for four regions of the stump; this data is (Anterior 495kPa, Lateral 427kPa, Posterior 384kPa and Medial 351kPa). The fatigue safety factor was found using the Ansys14.5 software, it was found that the fatigue safety factor of the prosthetic limb socket which made of carbon fiber is (1.2) which could be considered sufficient in terms of design requirements. On the other hand, the fatigue safety factor of the prosthetic limb which is made of Perlon fibers is (0.096) where it considered as a failure.

6

Calf-corset patella tendon weightbearing orthosis modeling and manufacturing

Humadi Rasha Qasim, Abbas Saif M., Ibraheem Marwa Qasim

Advances in Science and Technology. Research Journal

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2025

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Vol. 19, no. 2

418--428

EN

It has proven to be a difficult task to design and provide an ankle-foot orthosis (AFO) that enables the client to walk securely and comfortably without bearing weight through the lower leg and foot skeletal parts. Although it is widely acknowledged that the patella tendon weightbearing (PTB) ankle-loot orthosis only partially unweights the lower tibia, ankle, and foot, it is nevertheless frequently prescribed for this purpose. In this work, two ankle foot orthoses of the AFO PTB type of Calf-Corset were manufactured using a vacuum molding technique based on two kinds of materials as composite material reinforcement. The first AFO material was based on 8 layers of Perlon, while the second was based on 8 layers of fiberglass. A tensile and a fatigue test had been used to investigate the mechanical properties of the AFOs' material. The findings revealed that the yield strength (Ϭy) is 42.897 MPa, the tensile strength (Ϭult) for Perlon is 42.993 MPa, and the elongation at break is 1.138 mm, whereas fiber glass has a tensile strength (Ϭult), yield strength (Ϭy), and elongation at break of 224 MPa, 170 MPa, and 2.17 mm, respectively. Additionally, the gait cycle and the collected data on distributed pressure are measured using force plates and F-socket devices. The patellar tendon-bearing model was constructed using the SolidWorks software tool. In addition, for the fiber glass and Perlon PTB orthosis models, the total deformation, safety factor of fatigue, and Von-Mises stress were calculated using the FEM (ANSYS). The safety factor of fatigue values for the material PTBO with 8 layers of fiber glass was 2.2895, and for 8 layers of Perlon, it was 0.083515.

7

A concept for application of B-spline algorithm to density estimation of fatigue failures in 18Ni300 steel

Blacha Łukasz, Deptuła Adam, Urbanowicz Kamil

Advances in Science and Technology. Research Journal

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2025

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Vol. 19, no. 5

113--127

EN

P-s-n curves detailing fatigue are basic data required to ensure high reliability design. Since fatigue testing requires significant time and resource costs, the development of reliable small-sample methods is crucial to ensure the reduction in time and costs of design process. This paper investigates the possibility for applying B-spline interpolation basis functions to approximate the probability density functions on various levels of high-cycle uniaxial fatigue using the sample data of only selected stress levels. It was found that using the proposed model an iterative approach can be used to ensure acceptable fitting accuracy and to predict fatigue life on the desired probability level. As a result, the findings show the possibility for rapid modeling of the probability density function and the resulting reliability in high-cycle fatigue by using the well-known interpolation techniques based only on the basic dispersion parameters. The proposed approach provides a robust framework for fatigue life prediction, paving the way for broader applications in engineering analysis. The proposed data augmentation method seems to demonstrate the potential to reduce fatigue testing costs and simplify testing procedures.

8

Lifetime determination of ultralight helicopter blades

Dudnik Vitaly

Transport Problems

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2024

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T. 19, z. 3

81--92

EN

Determining the guaranteed operational time of helicopter blades is one of the important tasks necessary for successful vehicle use. Flight tests on different modes and special fatigue benches are used to solve such problems. The loads recorded during the flight are applied to ground tests on a bench. The ground fatigue tests of the ultralight helicopter blades have features associated with the influence of the scale factor comparison with the big helicopters. These features are presented in this article as the example of a ground test for a VA115 helicopter blade and a method of recalculating data. The purpose of these tests was to confirm the guaranteed service life of blades and define a reliable method for it.

9

Development of the modified clothoidal (MCL) shape of composite dowels against the background of fatigue and technological issues

Lorenc Wojciech, Kożuch Maciej

Studia Geotechnica et Mechanica

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2024

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Vol. 46, nr 2

91--110

EN

Composite dowels opened new possibilities for engineers designing composite structures. The fundamental and most important characteristic of composite dowels is the shape of the cutting pattern (called line). It is important to understand why only one particular shape of the cutting line, the modified clothoidal (MCL) shape, is being used in bridge engineering, while so many different shapes have been investigated by many researchers. The essential part of the process of developing composite dowels – the development of shape of the cutting line – is presented in this paper. The investigation, development, and evolution of the MCL shape, which is the final form of composite dowels for bridges and has been widely introduced in bridge engineering, are presented. The results of comparative tests of different shapes under cyclic loads are discussed. The background for the design formulas for the steel part and the fabrication aspects are highlighted.

10

The impact of corrosion on the fatigue strength of steel used in bridge structures

Wysokowski Adam

Ochrona przed Korozją

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2024

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nr 1

2--8

EN

The phenomenon of corrosion, its processes and dependence on the physicochemical properties of the corroded material and the environment in which it occurs, as well as issues related to corrosion protection, are the subject of numerous research papers. Despite the preliminary recognition of the impact of corrosion pitting on the fatigue of structural steel elements, in particular, the impact of notches on the reduction of fatigue strength, appropriate coefficients have not yet been introduced in most of the standards for the design of steel bridges in the world, including Poland. The paper will present the results of the author’s research work on the estimation of the effect of corrosion on the fatigue capacity of rail and road steel bridge structures in service. The study was performed on two types of bridge steel commonly used in existing, in-service bridge structures. In order to determine the magnitude of the title phenomenon, an atmospheric corrosion simulation facility was set up for the tests. The experimental results unequivocally showed a significant decrease in the fatigue limit for both bridge steels and up to 50% of the initial condition.

PL

Zjawisko korozji, jej procesy oraz zależności od właściwości fizykochemicznych korodowanego materiału i środowiska, w jakim zachodzi, a także zagadnienia związane z ochroną antykorozyjną są tematem licznych prac badawczych. Pomimo wstępnego rozpoznania w zakresie wpływu wżerów korozyjnych na zmęczenie stali konstrukcyjnych, w szczególności wpływu karbów na obniżenie wytrzymałości zmęczeniowej, nie wprowadzono dotychczas odpowiednich współczynników w większości światowych, także polskich normatywów dotyczących projektowania mostów stalowych. Artykuł przedstawia wyniki autorskich prac badawczych nad oszacowaniem wpływu korozji na nośność zmęczeniową eksploatowanych stalowych konstrukcji mostowych – kolejowych i drogowych. Badania przeprowadzono na dwóch rodzajach stali mostowej powszechnie stosowanej w konstrukcjach eksploatowanych obiektów mostowych. W celu określenia skali wpływu korozji na wytrzymałość zmęczeniową stali stosowanej w konstrukcjach mostowych na potrzeby badań wykonano stanowisko do symulacji korozji w warunkach atmosferycznych. Wyniki badań jednoznacznie wykazały znaczny spadek granicy wytrzymałości zmęczeniowej w wypadku obu stali mostowych, i to nawet na poziomie 50% stanu wyjściowego.

11

Development of a Robust Multiaxial Fatigue Model for A/C Metallic Assemblies in an Industrial Context

Delpuech Benjamin, Nutte Maxime, Jacques Vincent, Morlet Benoît

Fatigue of Aircraft Structures

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2024

|

Iss.16

102--118

EN

In a context of growing importance of mass reduction and reliability of structures towards greener aircrafts, fatigue of metallic materials is a key issue in the structural optimization. The process used by aeronautic industrials to compute the fatigue life is often based on a large empirical experience and meets a need for efficiency in their application, requiring a compromise between accuracy and ease of use. According to legacy crack initiation methodologies, lifetime computation is based on the analysis of elastic stress fields, calculated analytically or by Finite Element Method. Evaluation of lifetime is calibrated on elementary tests, mainly uniaxial, with geometric specificity (bone, hole, notch…). One of the limits of this approach appears when parts are subjected to multi-axial loads. Nowadays, these particular stress states are justified by conservative approaches to ensure flight safety and by tests on full-scale aircrafts. Whether for the operational maintenance or the structural optimization of new aircrafts, it is intended to enhance crack initiation methodologies, taking into account multiaxiality of loads, stress gradient effects, and complex material behaviours. Dassault-Aviation implements a crack initiation lifetime computation based on a local approach. These developments go hand in hand with a PhD (Nutte, 2023) on a multiaxial fatigue criterion in order to predict crack initiation in metallic assemblies. This work was supported by an innovative dedicated test campaign. The identification of material’s parameters is based on uniaxial and multi-axial mechanical tests, specifically designed to calibrate these models. Then, novel geometry of specimen for bolted assemblies, facilitating various biaxial non-proportional loadings, is used to evaluate the methodology. Also, multiaxial fatigue models require a precise assessment of the local mechanical fields to which the structure is subjected. For this, a finite element analysis must be conducted with a level of complexity associated with the level of accuracy targeted. The material constitutive equations used in the finite element analysis are therefore at the heart of these fatigue substantiation approaches. Applications to complex aeronautical structures such as massive 3D parts or assembly by fastener will highlight the benefits and perspectives for this local fatigue approach. It will require the use of multi-scale data science.

12

Strength analysis of welded joints in support structures of heavy machinery subjected to changeable loading conditions

Smolnicki Tadeusz, Sokolski Piotr

Eksploatacja i Niezawodność

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2024

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Vol. 26, no. 1

art. no. 175889

EN

The durability of bearing units of large machines depends mainly on the condition of their welded joints. With this in mind, we developed numerical models of the analyzed bearing units, for which we performed FEM simulations of the stresses in welded joints inseveral basic load cases. In each of the respective variants the technical condition of the bearing nodes was different and it corresponded to the severity of the degradation processes. Different positions of the superstructure in relation to the undercarriage were also taken into account. The simulations used the hot-spot method dedicated to FEM analyses of complex welded structures. We discovered that the loads have a significant influence on the values and distribution of von Mises principal stresses and their axial components. Based on the carried out analyses, we identified the most unfavorable load cases that generate the highest stresses in the welded joints of the assessed nodes. We also demonstrated that the applied method effectively assesses the stresses of welded joints subjected to variable working loads.

13

Fatigue fracture cross-sections after cyclic tests with a combination of cyclic bending and torsion of samples made of aluminum alloy 6060

Łagoda Tadeusz, Małecka Joanna, Małys Slawomir, Derda Szymon, Kuś Juliusz, Krysiński Paweł

Eksploatacja i Niezawodność

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2024

|

Vol. 26, no. 1

art. no. 176203

EN

The paper presents the results of fatigue tests of the 6060 aluminum alloy. The test material was taken from the profiles used for the production of side windows and external doors of the passenger trains by the RAWAG company. The tests were carried out for cyclic loads with pure bending, pure torsion, and two combinations of bending and torsion. Fatigue tests were performed at zero mean values. Using scanning electron microscopy, a fractographic analysis was made, which is a supplementary basic for considerations about the mechanism of initiation and development of fatigue cracks. Based on the appearance of individual zones and the characteristics of cracks, a picture of the behavior of the material under specific conditions was obtained. Finally, the plastic property of fatigue cracks was indicated.

14

Inspections of aircraft composite components

Zeltiņa Sintija, Kleinhofs Mārtiņš, Chatys Rafał

Composites Theory and Practice

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2024

|

R. 24, nr 3

200--204

EN

Inspections and intervals between inspections are often controlled by various parameters in the aviation industry. One of them is the method related to the p-set function, which determines the probability of crack formation and its impact on the service life of a component. It especially concerns polymer matrix composite components. Consequently, a deeper look into the basic principles and analysis of economics is needed, which will develop the theory and broaden the perspective from different angles.

15

Effect of Mn Addition on the Corrosion and Fatigue Properties of a Progressive Secondary A357 Alloy

Pastierovičová Lucia, Kuchariková Lenka, Tillová Eva, Zatkalíková Viera, Uhríčik Milan, Liptáková Tatiana, Chalupová Mária

Archives of Metallurgy and Materials

|

2024

|

Vol. 69, iss. 3

1123--1130

EN

As the modern automotive industry is looking for lightweight alternatives to minimize car emissions and fuel consumption, recycled Al-Si alloys play a key role in achieving this due to their lightweight, high specific strength, good castability, and corrosion resistance. In contrast to many other benefits, these alloys have reduced metallurgical micropurity as a result of recycling. The most significant complication of alloys is iron contamination. Higher Fe contents cause β-Fe-intermetallic phases in the form of long and brittle platelets that negatively affect corrosion resistance and fatigue. Neutralizing elements lead to the formation of less harmful α-Fe-rich phases, therefore a positive effect on properties is also expected. For this reason, the study investigates the effect of Mn addition on the corrosion properties achieved by immersion test and potentiodynamic polarization test and fatigue of secondary AlSi7Mg0.6 secondary alloy.

16

Developing a High-Performance System to Strengthen Construction Structures Against Mechanical Fatigue Using Shape Memory Materials

Riad Amine, Zohra Mouna Ben, Alhamany Abdelilah

Advances in Science and Technology. Research Journal

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2024

|

Vol. 18, no. 5

186--194

EN

To construct resilient structures, systems and sustainable buildings, capable of enduring fatigue, inclement weather, and seismic activity, researchers are actively seeking effective solutions to minimize vibrations and cyclic loading. Although these factors may not have immediate effects, they contribute to residual deformation in structures that gradually grows over time. For this reason, shape memory alloy (SMA) can be used as a perfect damper to dissipate the mechanical load in structures construction and buildings. The SMA actuators characterized by several thermo-mechanical functions, they are generally used in different applications as Mechatronics, Biomedical, Mechanical engineering and building systems. This study aims to adapt SMA actuator with structures for construction and buildings, in order to ensure a high displacement and vigilance taking into account fatigue phenomena to repulse mechanical fatigue and fretting. Accordingly, a thermomechanical analysis has been developed using finite element techniques to describe shape memory alloys' behavior and can integrate these material as a thermomechanical actuator dampers in building engineering systems. Furthermore, the suggested model elucidates the actuator's thermomechanical response, showcasing its adaptable behavior to both superelasticity and the shape memory effect within the desired structure in the building. Thus, the numerical findings affirm the efficacy of the proposed design that based on shape memory materials in addressing thermomechanical fatigue within buildings, concurrently enhancing structural resilience against mechanical fatigue. The primary outcome of this study is the successful preservation of the Ni-Ti superelastic response within the proposed system. This preservation is validated through cycling variations of up to 7.6% strain, significantly surpassing the requirements typically mandated for applications in earthquake engineering.

17

Structural integrity of AA7075-T651 UWFSW joints

Kosturek Robert, Ślęzak Tomasz, Torzewski Janusz

Advances in Materials Science

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2024

|

Vol. 24, nr 4(82)

98--110

EN

This investigation is focused on the comparison of selected low-cycle fatigue properties of AA7075-T651 friction stir welded and underwater friction stir welded joints together with the evaluation of their lifetime prediction by the Manson-Coffin-Basquin formula. Additionally, the analysis of the fractured surface was involved to describe the character of joints decohesion. The analysis of the obtained hysteresis loops revealed that FSW joint exhibits cyclic hardening, with a stable maximum stress and a decreasing minimum stress, leading to an increased contribution of compressive stresses and a lower mean stress during stabilized fatigue. In comparison, the UWFSW joint also shows cyclic hardening but with a greater contribution of tensile stresses, a higher mean stress, and a reduced participation of plastic deformation. The Manson-Coffin-Basquin equation effectively predicts the fatigue life of AA7075-T651 alloy joints, with UWFSW joints showing significantly lower standard deviation (0.0035 vs. 0.0135) and narrower dispersion bands (1.61 vs. 1.93) compared to conventional FSW joints.

18

New prediction of cracks propagation in repaired steel plate with bonded composite patch at cyclic loading

Maachou Sofiane, Mechab Belaïd, Bouiadjra Bel Abbes Bachir, Salem Mokadem

Acta Mechanica et Automatica

|

2024

|

Vol. 18, no. 4

579--584

EN

This study presents a numerical prediction of the fatigue life of steel panels repaired by a composite patch. The effect of length cracks, the stress ratio R and properties of the patch is presented. The obtained results show that the bonded composite repair significantly reduces the stress intensity factors at the tip of repaired cracks. The results are in a good agreement with those in the literature. The Monte Carlo method is used to predict the distribution function governing crack propagation in fatigue analysis. In computing the failure probability of the structure, we consider the statistical uncertainty associated with key variables, along with the previously discussed model uncertainty. The results obtained highlight the considerable impact of variations in crack length and stress ratio on the distribution function. Notably, uncertainty in these parameters significantly amplifies the probability of structural failure in plates, thereby diminishing overall structural durability.

19

Fatigue behaviour of medium carbon steel assessed by the barkhausen noise method

Makowska Katarzyna, Szymczak Tadeusz, Kowalewski Zbigniew L.

Acta Mechanica et Automatica

|

2024

|

Vol. 18, no. 1

40--47

EN

In this paper, an attempt to estimate the stage of the fatigue process using the Barkhausen noise method is studied. First, microstructural and static tensile tests were carried out and, subsequently, fatigue tests up to failure were conducted. After determination of the material behaviour in the assumed static and dynamic conditions, the interrupted fatigue tests were performed. Each specimen was stressed up to a different number of cycles corresponding to 10%, 30%, 50%, 70% and 90% of fatigue lifetime for the loading conditions considered. In the next step of the experimental programme, the specimens were subjected to the Barkhausen magnetic noise measurements. Various magnetic parameters coming from the rms Barkhausen noise envelopes were determined. The linear relationship betweenthe full-width at half-maximum (FWHM) of the Barkhausen noise envelope and the number of loading cycles to fracture was found. Specimens loaded up to a certain number of cycles were also subjected to a tensile test to assess an influence of fatigue on the fracture features.

20

The analysis of work shift patterns and risk of fatigue in aircraft maintenance personnel: a case study

Basaria Fransisca Tiur

Zeszyty Naukowe. Transport / Politechnika Śląska

|

2023

|

z. 118

5--16

EN

In response to the COVID-19 pandemic in 2020, PT. XYZ, the biggest aircraft maintenance, repair, and overhaul (MRO) company in Indonesia, implemented a new shift pattern for line-maintenance personnel. The new shift pattern allows maintenance personnel to have fewer working hours per day (7-hour shifts) in more shift varieties for 5 consecutive days. Maintenance personnel will have 2-morning shifts, 1-noon shift, and 2-night shifts followed by 3 rest days. This study aims to explore the risk of fatigue caused by the newly implemented shift pattern. Data were collected through electronic questionnaires from a total of 303 respondents. This study found that at the time of the survey, many respondents (78 of 303, 26%) felt tired and had difficulty concentrating, regardless of the shift they were in. Based on the duty time, the highest scores of level 6 (tired, difficult to concentrate) were discovered on night shift day 5. This result shows that although the organization had provided the maintenance personnel with the opportunity to obtain sleep during rest days, tiredness and fatigue were still experienced by the aircraft maintenance personnel. This study recommends necessary actions to be taken to prevent fatigue, especially from the noon shift afterwards, where fatigue level is increasing, and alertness level is decreasing.