Safe Life Design Research Articles

Hybrid-driven probabilistic damage assessment of creep-fatigue-oxidation interaction

This paper presents a hybrid-driven probabilistic damage assessment approach by considering creep-fatigue-oxidation damage interaction (CFO-DI). Based on generalized strain energy density exhaustion (GSEDE) framework, the hybrid-driven concept integrates the strengths of both physics-based models and machine learning, exploring the frontier from deterministic evaluation to probabilistic assessment. Experimental investigations involving generalized creep-fatigue loading tests are conducted to establish a comprehensive dataset in Inconel 718 at 650 °C. Deterministic models for fatigue, creep, and oxidation damages are developed, and their interactions are analyzed using the GSEDE framework. To tackle limited experimental data, a divide-and-conquer strategy employing machine learning models is implemented for data augmentation. Probabilistic assessments are performed incorporating uncertainties from material properties, loading conditions, and model parameters using Monte Carlo simulations and Latin Hypercube Sampling. The results demonstrate accurate life prediction accuracy and reliable probability distributions in the presence of oxidation damage. Finally, a novel three-dimensional probabilistic CFO-DI assessment diagram quantified by the confidence level is developed, providing a technical pathway for safe-life design in high-temperature structural applications.

Effects of Build Direction and Heat Treatment on the Defect Characterization and Fatigue Properties of Laser Powder Bed Fusion Ti6Al4V

Laser powder bed fusion (LPBF) is one of the high-precision additive manufacturing techniques for producing complex 3D components. It is well known that defects appear in additive-manufactured parts, and they deeply affect the fatigue properties; even heat treatment is performed after printing. In order to meet the safe-life design requirements of additive-manufactured aircraft structures, the effects of build direction and heat treatment on defects and fatigue properties need to be quantified. Hence, Ti6Al4V alloy samples with different build directions were designed and printed by LPBF. X-ray computed tomography was used to quantitatively analyze the defect size, the sphericity, and the defect orientation. And their effects on fatigue properties were studied. An extended effective defect size and a defect-based fatigue anisotropy evaluation process are proposed to qualify the effects of the defect size, sphericity, and defect orientation. It is shown that the build direction can affect the porosity distribution and maximum defect size, while the annealing treatment can cause the coalescence of small defects and higher porosity. The defect orientation exhibited a fluctuating trend of 0°–90°–0°–90°–0° as the volume increased. The elongated lack of fusion defects related to the build direction was the main crack source and could lead to fatigue anisotropy of LPBF Ti6Al4V.

Theoretical and simulation of central elliptical hole with rectangular plate

A study on design engineering components with slot, notches is very important, because there is a stress increases/failure region area, where the force/stress is concentrating more and more. The elastic stress concentration mainly depends on the mode of loading, materials, and geometry of the design engineering components. The design engineers, academicians, and researchers concentrated and focused on fail-safe design and safe life design. A Plate is considered with different slots, such as circular and elliptical. The main objective of this study is to find out the stress concentration factor in plates with various cutout shapes. This concept is used in design components/structures, for finding the elastic stress concentration. The methods compared are tabulated with their findings. Singularities of the circular hole and elliptical hole in rectangular plates are considered in the present study. The finite Element Method (FEM) was used for fine mesh and ANSYS WORKBENCH software was used for extracting the results and results were validated by analytical or experimental methods.

Probabilistic structural integrity assessment of a floating offshore wind turbine under variable amplitude loading

The objective of the present study is to assess the structural integrity of a semisubmersible-type floating offshore wind turbine probabilistically based on fracture mechanics. Although floating offshore wind turbines have been historically much more expensive than their fixed bottom peers, the levelised cost of energy of floating offshore wind turbines has decreased dramatically due to increasing wind turbine size, higher capacity factors, innovative designs, and optimal operational strategies. However, there is still much to be done to bring the cost down, especially from the operational and maintenance standpoint. In this regard, a damage-tolerant approach is a strong alternative to a safe-life design approach that leads to overdesigning. The present study assesses the structural integrity of a semisubmersible-type floating platform with an initial crack assumption using a probabilistic framework, resulting in the quantification of the uncertainty involved in the fatigue crack growth under variable amplitude loading. The present study outcomes can then be used to optimise the structural design, inspection, and maintenance plan. The cycle-by-cycle crack growth is simulated using the modified Paris’ law, which allows accounting for the overload-induced retardation on the crack propagation. Furthermore, the factors influencing crack growth are subjected to a great deal of uncertainty. The present study focuses on the variability caused by the load sequence and interaction effect. To this end, Monte Carlo Simulation is performed to quantify the uncertainty associated with the crack growth by first considering the load sequence effect and then the overload-induced retardation effect.

Experimental and computational assessment of disbond growth and fatigue life of bonded joints and patch repairs for primary airframe structures

The applicability of a damage slow growth management strategy to bonded joints/patch repairs of primary aircraft structures was evaluated through an experimental and computational study. Fatigue tests were conducted to investigate the entire process of disbond growth from initiation up to joint ultimate failure. The residual static strength of the joint as a function of disbond length was established using finite element modelling, in which the mesh size was calibrated using the static strength of the specimens measured in room temperature and dry (RD) and hot-wet (HW) conditions, based on the characteristic distance approach. A virtual crack close technique (VCCT) approach was utilised to assess the strain energy release rates (SERRs) as a function of disbond crack length. The measured disbond growth rates were correlated with the SERRs using a modified Paris law that enabled prediction of joint fatigue life. The fatigue test results indicated that for a joint having a sufficient static strength safety margin under a typical fatigue loading that would propagate disbond, the disbond growth would be stable in a particular length range. Thus, the slow growth approach would be feasible for a bonded joint/patch repairs if the patch is designed to be sufficiently large to allow extended damage propagation (whilst in the case when patch size must be limited, safe-life design for the patch termination region in critical repairs must be considered. Should disbond growth occur in this case, the joint must be repaired or replaced). The work presented in this paper validated the framework/procedure proposed previously by the authors (Tanulia et al., 2020) for managing damage slow growth in bonded joints/patch repairs. In the last part of this paper the planned follow-on research is briefly described.

A methodology for detection of crack initiation in adhesively bonded joints under constant and variable amplitude fatigue loading

ABSTRACT The use of adhesively bonded joints is increasing in the lightweight design; however, their fatigue performance in safe-life design is challenging. Therefore, a combination of damage-tolerant design and Structural Health Monitoring (SHM) is often considered. The present work deals with a new approach for detection of crack initiation in adhesively bonded single lap joints (SLJ) subjected to constant and variable amplitude fatigue loading. For this purpose, the so-called backface strain method is used, in which a strain gauge for damage monitoring is placed at a highly sensitive point in the overlap area of SLJs. A striking converging-diverging trend in strain amplitude curves was attained in the tests subjected to constant amplitude loading, which was in a good agreement with the onset of crack initiation observed with video-microscopy. Additionally, this methodology was demonstrated on the SLJs subjected to variable amplitude loading using an additional farfield (outside of the overlap area) strain gauge, which showed a high potential for detection of crack initiation.

Fatigue Evaluation of Offshore Steel Structures Considering Stress Concentration Factor

The installation of offshore structures and facilities in the marine environment, usually for the production and transmission of oil, gas exploration, electricity, and other natural resources is referred to as offshore construction. Since offshore structures are subjected to changing threats to the environment year-round. Fatigue behavior prediction noticed on these structures should be considered during the design stage. Fatigue is one of the failure mechanisms of offshore steel structures, and it must be investigated properly during system design. The fatigue analysis of offshore structures under drag wave force, total wave force, total moment about the sea bed, and other variables are reviewed thoroughly. The structure's dynamic response becomes a critical aspect in the whole design process. The fatigue analysis was carried out using MATLAB software, material properties of the offshore structure, and wave spectrum characteristics in this study. This study shows the JONSWAP spectrum and stress concentration analysis prediction. The offshore support structure that is predicted during the design phase will help to keep the stress concentration factor below the fatigue threshold and anticipate safe life design, according to the results of the fatigue study. The fatigue performances of tripod and jacket steel support structures in intermediate waters depth are compared in this project (20-50 m). The North Atlantic Ocean is used as a reference site, with a sea depth of 45 meters. The tripod and jacket support structures will be designed by using current industry standards. Keywords: [Fatigue evaluation, North Atlantic Ocean and Failure].

Mechanism analysis of a main landing gear of transporting aircraft: A design learning perspective

The landing gear lifetime is estimated based on the safe-life design concept and its design requires thorough calculations. The individual parts and sub-assemblies must be sensibly designed to function safely and efficiently for the specified design lifetime without failure. In a design process of a main landing gear system, the kinematics/mechanisms analysis shall be first carried out as a primary task for feasibility of the gear retraction/extension with the satisfactory link paths. This paper presents the analytical kinematics analysis of a complex mechanism involving multiple four-bar linkages with multiple coupler points of the main landing gear (MLG) of CN-235 aircraft. The kinematics analyses were performed in order to figure out the overall link positions of the main landing gear mechanism and make scenarios of a deformed component from its designed geometry for possibilities of a lock failed position.

Experimental and numerical investigation of mixed mode fatigue crack growth models in aluminum 6061-T6

Damage tolerance philosophy and safe life design principles are widely used for critical structural components, and in such cases, fatigue life prediction using the numerical techniques is essential. In the present work, mixed mode fatigue crack growth experiments are performed using the compact tension shear specimens made of Al 6061-T6 alloy for mode mixity angles of 30°, 45°, and 60°. These experimental studies are supported by numerical and fractographic studies on the selected specimens. A three-parameter double exponential model for fitting the fatigue crack growth curves (crack length vs. the number of fatigue life cycles) is also proposed for both the mode I and mixed mode (I/II) loading conditions. Numerical prediction of mixed mode (I/II) fatigue life is carried out using the Paris’ law in combination with various ΔKeq models. The results of the present investigation show that highly satisfactory best fits to the scattered experimental data can be obtained with the help of the proposed double exponential model. The predicted life is compared with the experimental fatigue life using various measures of error. Based on the overall performance of the models during the entire crack growth regime, the results of the present investigation clearly show that Irwin’s model and one of the Tanaka’s model are consistently found to predict the mixed mode fatigue life close to the experimental data. Whereas, Richard’s and Yan’s models, again based on the overall performance, are found to be conservative models consistently for the prediction of the mixed mode fatigue life. These conclusions are verified with the published results.

Critical Crack Size Investigation Method for a Land Launcher

Structural integrity evaluation of military systems is vital in such applications as rocket launchers. In safe life design, for a certain operational life the system is ensured to function well and no failure would occur even in the presence of some imperfections or flaws. This paper considers the fracture analysis as part of the safe life design approach that is used in the design of a rocket launcher. A methodology based on sub modelling technique is introduced. It was made sure that unstable crack growth would not occur upto certain crack sizes. Subsequently, after actual manufacturing of the launcher, critical locations of welds in the system were checked for presence of any cracks after repeated firing loads. After these controls no cracks were detected due to operational conditions until the time when this document was prepared.

Fatigue crack growth under rotating bending loading on aluminium alloy 7075-T6 and the effect of a steady torsion

Axles and shafts are of prime importance concerning safety in energy equipment, transportation industry and general machinery. Case studies relating failure of axles and shafts are widely reported in the literature. Design rules for axles and shafts are mainly based on endurance curves for the material used according to long time established standards and procedures. Safe life design, based on endurance curves is being complemented by damage tolerance, where crack growth is taken into account. Recently, the knowledge of fatigue crack growth under typical loading conditions of axles and shafts with rotating bending and steady torsion has been object of research in order to apply damage tolerance concepts, mainly for maintenance and failure analysis purposes. The effect of a steady torsion on fatigue crack growth under rotating bending is focused in this paper. While axles and shafts in the transportation industry are traditionally designed on steels, the need for weight reduction due to fuel economy and eco-design constraints, lightweight materials must be considered for these applications. In this study, fatigue crack growth on rotating bending axles and shafts with or without an applied steady torsion is presented. Fracture mechanics approaches are used to analyze the results based on stress intensity factors developed for bending and torsion in shafts. Results are compared for the same stress ratio and relevant differences on fatigue crack growth are commented based on crack closure concepts. Discussion of the effect of crack growth retardation due to steady torsion is also discussed. Fractographic observations are presented and helped to explain fatigue crack growth under this mixed mode loading when steady torsion is applied to rotating bending.

A review on fatigue design of heavy section EN-GJS-400-18-LT ductile iron wind turbine castings

A comprehensive review of fatigue behavior and fatigue design of heavy section wind turbine castings made of ductile iron EN-GJS-400-18-LT is performed. Common metallurgical defects that are affecting fatigue behavior of this material were studied. Review of different researches on quantitative effect of chunky graphite and micro structure on fatigue behavior of EN-GJS-400-18-LT was performed. Finally, safe life design and damage tolerant design of heavy section wind turbine cast iron components were reviewed and compared with each other. It is tried to cover all the required points that should be considered in fatigue design of large EN-GJS-400-18-LT wind turbine castings.

궤간가변 윤축시스템의 내구성 평가

이종궤간(표준줴/광궤)이 존재하는 유라시아 철도네트워크(TKR, TCR 및 TSR)의 화물운송시간 및 비용을 절 감하기 위해서는 이 구간을 자유롭게 운행할 수 있는 궤간가변 윤축시스템의 개발이 중요하다. 국내에서는 기존 해외 윤축시스템을 보완하고자 새로운 한국형 궤간가변 윤축시스템을 개발 및 제작중이다. 본 연구에서는 안전-수명설계관 점에서 강도와 내구성해석을 수행하였다. 본 시스템의 내구성해석결과로서, 주요부품은 1x10 7 사이클까지 안전하였다. 또한, 주행중 안전성을 확인하기위하여 국제철도규정 UIC에 따라 리그피로시험을 수행하였다. 이 시험을 통하여 본 궤간가변 윤축시스템은 관련 안전규정에 만족함을 확인하였다.

Conceptual design and sizing of airframe panels according to safe-life acoustic fatigue criteria

AbstractThis paper presents a methodology that permits accounting for acoustic fatigue effects when sizing safe-life structural skin-stringer panels at the conceptual design stage of aircraft product development. The approach is based upon estimation of the maximum noise radiated from an entry-into-service year 2020 turbofan. Sonic fatigue endurance is assessed for different skin-stringer panels having different values of skin thickness, rib pitch and stringer pitch. Three different materials were considered in this study: aluminium 2024-T3 alloys (Al 2024-T3); carbon fibre-reinforced plastics (CFRP); and, glass reinforced fibre metal laminate (Hybrid Glare-3). The study resulted in CFRP having the most favourable sonic fatigue performance. In order to link economic considerations into technical decision making, the sonic endurance methods were coupled with an industry grade costing analysis tool (SEER-HTM) to examine the impact of safe-life design on the panel cost and weight. The presented methodology has been shown to be sufficiently generic in nature and robust. This will not only assist in identifying acoustic fatigue as a potential critical design scenario, but will also increase throughput during conceptual design sizing and optimisation.

Some factors influencing damage tolerance under helicopter spectra

The design of helicopter structural components has been based principally on the safe life design approach, although in the last two decades there has been increasing use of damage tolerance methodologies which focus explicitly on the progressive change in structural risk with time. However, the benefits to a fleet in terms of cost, safety, and fleet management, of using damage tolerant design over a structure designed using a safe life design approach, are still subject to some debate. This paper focuses on the comparison of analytical results from damage tolerance and safe life approach on a hypothetical component under a real helicopter spectrum. The load spectrum is reconstituted from data from the Black Hawk helicopters in use with the Australian Defence Force, using two methods; namely peak-valley reconstitution and reverse rainflow reconstitution. The effect of the reconstitution method on the life analyses is also explored.

Fatigue Crack Growth Simulation in Components with Random Defects

Abstract The three dimensional distribution of defects within a fatigue specimen cut from a large spheroidal graphite cast iron component has been obtained by X-ray computed tomography. This distribution has been used as input for a fatigue assessment postprocessor for the calculation of fatigue life distribution in cast components.

A durability model incorporating safe life methodology and damage tolerance approach to assess first inspection and maintenance period for structures

This paper outlines a new durability model to assess the first inspection and maintenance period for structures. Practical scatter factor formulae are presented to determine the safe fatigue crack initiation and propagation lives from the results of a single full-scale test of a complete structure. New theoretical solutions are proposed to determine the s a − s m − N surfaces of fatigue crack initiation and propagation. Prediction techniques are then developed to establish the relationship equation between safe fatigue crack initiation and propagation lives with a specific reliability level using a two-stage fatigue damage cumulative rule. A new durability model incorporating safe life and damage tolerance design approaches is derived to assess the first inspection and maintenance period. Finally, the proposed models are applied to assess the first inspection and maintenance period of a fastening structure at the root of helicopter blade.

Monitoring of aircraft landing gear structure

AbstractLanding gear structure is developed predominantly using safe life design criteria. Health monitoring and structural prognosis techniques for landing gear cannot focus on crack detection; techniques for determining input loads and calculating damage or methods for directly measuring material damage must be employed. This paper will discuss Messier-Dowty’s research into structural monitoring over the past several years. Principally, direct damage detection systems and load monitoring systems will be discussed.

Study of Ultra-Long Life Fatigue of High Strength Steel with Duplex-Phase of Carbide-Free Bainite and Martensite

The ultra-long life fatigue properties of novel high strength steel with duplex-phase of carbide-free bainite and martensite (CFB/M) were studied in this article. The specimens were heat-treated by the following procedure: austenitizing at 900°C for 20 min followed by air cooling and then tempering at 340°C for 2 h. All the fatigue tests were carried out by ultrasonic fatigue testing equipment, at a cycling frequency of 20 kHz. Experimental results show that there is no conventional fatigue limit for the steel studied in this paper. From the S-N curve it could be suggested that it is more reliable to use the fatigue strength at 10 8 cycles than that at 10 7 cycles for the safe-life design. Based on the observations of fracture surface, it is obvious that the fatigue fracture surface can be classified into two types: initiating from surface and interior. A fish-eye mark and an area with a particular morphology surrounding the fracture origin termed ODA (optically dark area) by Japanese scholars are always found in the latter type, but no obvious inclusion is observed inside it. By calculating the values of ΔK at ODA and fish-eye, it is found that the values of Δk fish-eye and ΔK ODA tend to be constant respectively, independent of the stress amplitude, fatigue life and the distance of the initiation site to the edge of specimen, therefore, it is thought that the contribution of hydrogen to the formation of ODA should not be considered.

Mechanical Design of Machine Elements and Machines: A Failure Prevention Perspective

Mechanical Design of Machine Elements and Machines: A Failure Prevention Perspective