Fatigue Testing Of Components Research Articles

Unsupervised anomaly detection in pressurized water reactor digital twins using autoencoder neural networks

Deep learning (DL), that is becoming quite popular for prediction and analysis of complex patterns in large amounts of data is used to investigate the safety behaviour of the nuclear plant items. This is achieved by using multiple layers of artificial neural networks to process and transform input data, allowing for the creation of highly accurate predictive models. Particularly to the aim the unsupervised machine learning approach and the digital twin concept in form of pressurized water reactor 2-loop simulator are used. This innovative methodology is based on neural network algorithm that makes capable to predict failures of plant structure, system, and components earlier than the activation of safety and emergency systems. Moreover, to match the objective of the study several scenarios of loss of cooling accident (LOCA) of different break size were simulated. To make the acquisition platform realistic, Gaussian noise was added to the input signals. The neural network has been fed by synthetic dataset provide by PCTRAN simulator and the efficiency in event identification was studied. Further, due to the very limited studies on the unsupervised anomaly detection by means of autoencoder neural networks applied for plant monitoring and surveillance, the methodology has been validated with experimental data from resonant test rig designed for fatigue testing of tubular components. The obtained results demonstrate the reliability and the efficiency of the methodology in detecting anomalous events prior the activation of safety system. Particularly, if the difference between the expected readings and the collected data goes beyond the predetermined threshold, then the anomalous event is identified, e.g., the model detected anomalies up to 38 min before the reactor scram intervention.

Fatigue cracking characteristics of components stiffened with vertical ribs under eccentric loading in steel bridge deck

The components in orthotropic steel decks are generally in an eccentrically loaded state for the nonuniformly distributed wheel loads. By conducting numerical simulation and fatigue tests of components stiffened by the vertical rib (CVR) with single butt weld, the propagation law of CVR cracks under eccentric loading were analyzed. By comparing with components stiffened by U-ribs (CUR), the difference of crack-growth characteristics between components with single and multi butt welds under eccentric loading was explored. The results show that CVR cracks will initiate at the eccentrically loaded side of the weld and propagate obliquely, but still perform the bilateral cracking mode as that under symmetric loading. The CUR and CVR cracks present the same three-stage characteristics of fast-slow-rapid growth and components will accelerate to failure in the third stage, which shall be avoided. The section-damage rate of CUR will be greatly relieved for the unilateral cracking under eccentric loading compared with that of bilateral cracking under symmetric loading, which delays the crack to enter the rapid-growth stage. Since there’s little effect of eccentric loading on the demarcations of crack-growth stages in CVR, it is reasonable to take the demarcation 80 mm between the second and third crack-growth stages as the warning length for maintenance. The semi-elliptical CVR cracks will quickly penetrate base metal after extending to 75 % of thickness with the average ratio of crack depth to length being 0.09, which can be treated as the operation timing of maintenance methods for non-penetrating cracks.

Full-scale fatigue testing of a cast-iron wind turbine rotor shaft

Spheroidal graphite cast iron (EN-GJS), which provides a high degree of design flexibility and the possibility for lightweight design, has benefits as a material for use in structural parts in wind turbines. Comparing components made using the sand casting technique to those made using the chill casting process reveals significant potential to boost strength. However, at present, there is neither a proven design guideline nor reliable material input data for a lightweight component based on this material and fabrication process. This publication presents the results from the Gusswelleproject in chronological order. It starts with the explanation of the final setup and test plan for the full-scale rotor shaft fatigue experiment. The elaborated sensor and operational concept are then presented together with an adequate finite element method (FEM) model of the specimen and relevant neighboring components. The validation of this FEM model to ensure that the loading and the resulting local strains representing the real test bench situation is described. The usage of non-destructive testing to document the condition of the specimen from initial crack formation until integrity loss is explained followed by a comparison between the component fatigue test results and the material-based life-time forecast. A strength increase for chill-cast large components in the range of 50% is indicated. Simulation-based crack propagation studies are performed to qualitatively verify the loads responsible for the observed cracks of the component test and to further develop the possible method for crack predictions.

Failure analysis of aircraft tubular form-fit joints in rotating-bending fatigue tests

Tubular form-fit joints play a key role of connecting and sealing in the pipeline system of aircraft. However, they experience the complex fatigue behavior in operating conditions of compound excitation with internal fluid pulse pressure and external structural high-frequency vibration. To investigate the above fatigue behavior, the tubing component composed of tubular form-fit joints is fabricated by elastomeric swaging firstly; then the rotating-bending experimental platform for the tubing component is built to simulate the operating conditions in the above actual service period, and the rotating-bending fatigue (RBF) tests of tubing components under different deflections are carried out, so that the relationship between the fatigue strength and life is obtained; then, the bending-torsion multiaxial mechanical behavior of tubing component is analyzed when it is filled with constant internal pressure, bent at a deflection and rotated at a high speed in the RBF test. It can be found that the area near the sharp edge of sleeve groove is the most prone to crack initiation. Furthermore, the mechanism causing the fatigue fracture of tubing component is explored at the certain position of the joined tube by means of numerical simulation. Eventually, with the aid of scanning electron microscope, the mechanism of bending-torsion multiaxial fatigue crack initiation and propagation is revealed by fracture morphology observation and analysis. It can be found that the fatigue crack source on the outer wall of the joined tube is various, and the crack propagation direction is from the outer wall to the inner wall along the radial direction.

A novel accelerated fatigue testing system for pulsatile applications of cardiac devices using widely translatable cam and linkage-based mechanisms.

Fatigue testing of mechanical components is important for designing safe implantable medical prosthetics, and accelerated systems can be used to increase the speed of evaluation. We developed a platform for accelerated testing of linear force applications of cardiac devices, called the Fatigue Acceleration System Tester (FAST). FAST operates using a core translation mechanism, converting motor-driven rotary motion to linear actuation. The advantages of using this mechanism include 40x rate increases with largely 3D-printed components, versatility based on modular design paradigms, and accessible manufacturability with 3D-printable forms, enabling access for small and large research laboratories alike. FAST has been crucial in informing our designs for continuing device development. Over two fatigue cycle courses of 52 and 110 days, the motor cycled at rotational frequencies up to 1500rpm, 43 times faster than those experienced in a typical heart and equating to approximate life cycles of five and ten years, respectively. In designing FAST, our goal was to accessibly bring a strong mechanical basis to study the long-term effects of repeated loading, and we present a design that can be applied across many industries to not only evaluate fatigue performance, but also generate any cycling linear motion.

Fatigue strength analysis of an axial compressor blade using the modified Locati method

The Locati method for accelerated fatigue testing can be successfully applied in preliminary fatigue testing of components and in production quality control. The method uses only one test sample and assumes that the slope of the SN curve is known a priori. In this paper, a modified Locati method is presented in which the initial approximation of the SN curve is based on the tensile strength and pseudo-fatigue limit of aluminum alloy 2618-T851. The first natural frequency is likely to be excited during the operation of an axial compressor, according to finite element (FE) modal analysis. A total of five axial compressor blades were tested by vibration exciter. The blade was excited at its first natural frequency until it failed due to a fatigue crack. After the natural frequency of the damaged blade dropped by more than 10%, failure was assumed. The fractured blade surface showed two fatigue crack areas, which occurred at the trailing edge of the blade and at the center of the convex blade surface as predicted by FE harmonic response analysis. The proposed method was successfully applied to the quality control of axial compressor blades in production.

Fatigue behaviour of discontinuous carbon-fibre reinforced specimens and structural parts

Discontinuous fibre reinforced polymers are increasingly used for structural components in automotive applications. In this paper, specimens made from a carbon-fibre sheet moulding compound are tested in constant amplitude fully reversed cyclic fatigue loading. The obtained fatigue lifetime curves are compared against component fatigue test results. In addition, the evolution of modulus in the specimen is compared to the structural stiffness in the component. A power law is used to describe both, degradations of specimen modulus and component stiffness. The exponent of the power law is proportional to the load amplitude, potentially indicating the existence of a fatigue limit.

An Update on the Impact of Forging Residual Stress in Airframe Component Design

Abstract In a recently concluded study, the effects of forging process–induced bulk residual stresses on fatigue life were evaluated at both the coupon and large component level. During this program, it was demonstrated that the extraction of confounding residual stress effects from material property data (especially fatigue crack growth rate data), coupled with the explicit inclusion of forging residual stresses in subsequent fatigue analyses, resulted in a significant improvement in the fidelity of those analyses. In the first phase of the program, coupon-level tests were carefully designed, executed, and analyzed, and it was shown that the newly developed methods resulted in analysis versus test life correlation ratios that were either within the United States Air Force (USAF) required scatter factor of 2, or were conservative. This is in contrast to a much broader scatter band (5x) for calculations made using traditional (non–residual-stress-informed) methods. The explicit residual stress method was incorporated into an integrated structural design/analysis tool suite that allows zoning of parts into residual stress–specific regions, automated generation of location-specific fatigue spectra, automated execution of fatigue crack initiation, fatigue crack growth, and residual strength analyses, and automated generation of fatigue-based design, allowable stresses, and margins of safety. In the second phase of the program, the residual stress design procedure was applied (to the furthest extent possible) to the design and manufacture of a large, fighter aircraft bulkhead. The objective of this phase of the program was to demonstrate, by way of two large component fatigue tests, that the technology would scale to the structural level, and that its use would result in components that are either lighter or more durable (or both) than their traditionally designed counterparts. In this article, we compare and contrast the traditional versus residual stress–informed design procedures, and we describe in detail the resulting baseline and “optimized” test articles. A full description of the fatigue tests, along with comparisons between detailed fatigue calculations and test findings, are given.

TSA infrared measurements for stress distribution on car elements

Abstract. Because of the continuous evolution of the market in terms of quality and performance, the car production industry is being subjected to more and more pressing technological challenges. In this framework the use of an advanced measurement technique such as thermoelasticity allows the engineers to have a fast and reliable tool for experimental investigation, optimization and validation of the finite element method (FEM) of those critical parts, such as parts of car-frame tables (Marsili and Garinei, 2013; Ju et al., 1997). In this work it is shown how the thermoelastic measurement technique can be used to optimize a Ferrari car frame, as a method of experimental investigation and as a technique of validation of numerical models.The measurement technique developed for this purpose is described together with the calibration method used in the test benches normally used for fatigue testing and qualification of this car's components. The results obtained show a very good agreement with FEM models and also the possibility of experimentally identifying the concentration levels of stress in critical parts with a very high spatial resolution and testing the effective geometry and material structure.

Four-point bending cycling: The alternative for thermal cycling solder fatigue testing of electronic components

This paper deals with an alternative testing approach for quantifying the life time of board level solder joint reliability of components. This approach consists of applying a relative shear displacement between component and Printed Circuit Board (PCB) through cyclic board bending. During the cycling, the temperature is kept constant, preferably at elevated temperature in order to accelerate the creep deformation of the solder joint. This is done in a four-point bending setup which allows to apply an equal loading on all components lying between the inner bars. The scope of the paper is, firstly, to evaluate if the four point bending testing generates the same fatigue fracture as in thermal cycling; secondly, that the measured life times can be also predicted through finite element simulations; and thirdly if the technique can finally accelerate the cycling frequency to reduce the testing time.

Fatigue of titanium weldments: S-N testing and analysis for data transferability among different joint types

Fatigue testing of both TIG and MIG welded titanium components were first carried out using fillet welded cruciform specimens. A mesh-insensitive traction structural stress method is then introduced for establishing data transferability from one joint geometry to another and one base plate thickness to another. In addition, available test data from literature are also analyzed using the traction stress parameter proposed. Major findings are:(a) There exist no noticeable differences in S-N curve behaviors between Ti-CP and Ti-6-4 weldments.(b) The proposed traction stress parameter with a thickness correction term can be used to effectively correlate lab specimen test data in the form of a single S-N curve with a narrow scatter band regardless of joint types and base plate thicknesses, which has been demonstrated for both MIG and TIG welded specimens.(c) By using both the S-N curve and traction stress parameter developed, satisfactory fatigue life estimates for a series of full-scale MIG-welded structural component fatigue tests have been achieved.

Assessment of thermal fatigue life for 316L and P91 pipe components at elevated temperatures

This paper presents an analysis of thermal fatigue tests of pipe components with 14mm wall thickness made of 316L austenitic steel or P91 ferritic-martensitic steels. The tests are done by heating the pipes to 550°C by induction heating and then cyclic cooling by water cooling resulting in secondary thermal stresses and an axial constant with a constant primary stress between 0 and 100MPa. The damage evolution and crack depth is measured by X-ray tomography and time-of-flight-diffraction. A very complex cracking is observed for the cases with no axial primary load, and with increasing primary load dominant circumferential cracks are observed. The test are analysed by an engineering approach that included a thermal analysis, a cyclic-plasticity analysis and crack propagation. The formation of a 1mm crack is estimated by fatigue curves. The propagation from a stipulated 1mm axial or circumferential crack is done by plasticity corrected stress intensity factors in conjunction with Paris law. The fatigue life predictions are compared with the experimental observations and in general very good agreement is reached, in particular for the cases with a large primary load. The proposed approach can be used in an engineering analysis to predict the thermal fatigue life of a component.

Reducing Reliability Uncertainties for Marine Renewable Energy

Technology Readiness Levels (TRLs) are a widely used metric of technology maturity and risk for marine renewable energy (MRE) devices. To-date, a large number of device concepts have been proposed which have reached the early validation stages of development (TRLs 1–3). Only a handful of mature designs have attained pre-commercial development status following prototype sea trials (TRLs 7–8). In order to navigate through the aptly named “valley of death” (TRLs 4–6) towards commercial realisation, it is necessary for new technologies to be de-risked in terms of component durability and reliability. In this paper the scope of the reliability assessment module of the DTOcean Design Tool is outlined including aspects of Tool integration, data provision and how prediction uncertainties are accounted for. In addition, two case studies are reported of mooring component fatigue testing providing insight into long-term component use and system design for MRE devices. The case studies are used to highlight how test data could be utilised to improve the prediction capabilities of statistical reliability assessment approaches, such as the bottom–up statistical method.

Understanding the “blue spot”

Abstract During hot component fatigue tests there have been two cases of low life crack initiation of gas turbine rotating parts manufactured from the Titanium alloy Ti-6246. Both exhibited a small (~ 0.1 mm) elliptical ‘blue spot’ at the origin. Through validated striation count work and fracture mechanics it was established that fatigue had propagated with a near-nil initiation life. Early investigation suggested that the ‘blue spot’ was possibly a region of stage 1 fatigue growth, and was therefore a material behaviour concern with potential implications for service. During an investigation of a later cracking incident in this alloy, subsequently shown to have resulted from stress-corrosion cracking (SCC), near-identical fractographic characteristics to that seen in the “blue spot” were found that subtly differentiated it from stage 1 fatigue. Also, similar ‘blue spots’ have since been identified on Ti6246 Laboratory hot LCF test specimens and found to have been due to contamination by NaCl, through the application of focussed long-term EDX examination and other novel chemical analyses techniques. By the application of those techniques, fractography, and comparison against these specimens, Rolls-Royce and Imperial College London have collaborated to show that the original two component ‘blue spots’ were subtle examples of NaCl-related Hot Salt Stress-Corrosion Cracking (HSSCC). Such cracking has not been found to occur in service components, due to air pressure within the engine, and the effect is therefore confined to Laboratory and component tests at near-atmospheric pressure or below.

Fatigue analysis of multiaxially loaded crane runway structures including welding residual stress effects

The paper addresses a fatigue evaluation procedure to assess the influence of both multiaxial stress state caused by a traveling crane wheel load and the welding residual stresses on the lifetime of crane runway girders. The behavior of the residual stresses during cyclic loading was investigated separately. Numerical calculation of welding residual stresses was conducted within a first step. Cyclic stress–strain curves of specimens both from the weld region and the base material were evaluated in strain-controlled uniaxial cyclic specimen tests as a basis for the fatigue assessment. Numerical investigations by means of elastic–plastic finite element analyses were carried out to estimate the local stress conditions acting in the weld notches. The local stresses and strains were post-processed within the MPA AIM-Life concept using critical plane approach and advanced fatigue parameters. The lifetime of crane runways was calculated both with and without residual stress effects. Finally, the estimated lifetimes were compared to experimental results of component fatigue tests on full-scale crane runway girder tests with traveling wheel load.

Fatigue assessment of full-scale welded crane runway girders*

Abstract This publication is focused on a critical plane approach to assess the influence of the multiaxial stress state on the lifetime of welded crane runway girders cyclically stressed by travelling wheel loads. Cyclic stress-strain curves of specimens both from the weld region and the base material of the girder are evaluated in strain-controlled reversible fatigue tests as a basis for the assessment of the lifetime. Finite element analyses are performed to estimate the multiaxial and non-proportional local stress condition acting on sharp notches of the girder, i. e. the weld toes and the weld root. The MPA AIM-life concept is used to evaluate the lifetime of crane runways on the basis of advanced fatigue parameters and the cyclic material properties. Furthermore, the notch stress approach according to IIW guideline [1] is applied to investigate the fatigue behavior. The estimated lifetimes are compared to experimental results of component fatigue tests conducted with full-scale welded crane runway girders.

Mechanical Characterisation of the Effect of Various Forming Processes Applied to Commercially Pure Titanium

This paper illustrates the effect of three forming processes viz. laser forming, mechanical forming and a combination process consisting of a laser forming step followed by a mechanical forming step, on the mechanical properties of commercially pure titanium. All three processes resulted in samples having a similar final radius of curvature. Evaluation of the formed samples includes residual stress measurements, fatigue testing, Charpy impact testing, microhardness measurements and microstructure evaluation. All results were compared to the various forming methods in order to determine which forming method is more suitable for which type of loading condition i.e. impact loading or cyclic loading. Fatigue testing of components produced by the various forming methods revealed that the laser forming process provided the best results when a high load was applied whereas at lower applied loads, the mechanical forming process showed the highest number of cycles to failure. Charpy impact testing done at room temperature and at a sub-zero temperature of −40°C revealed that the laser forming process negatively affected the toughness of the material. Residual stress measurements showed that the laser forming process resulte'd in the highest value of surface relieved residual stress values compared to the other two processes.

Study on Measurement of Six-DOF Force/Torque for Loading Experiment on Heavy Load Equipment

According to the requirements of fatigue test of aircraft cabin, large engine and mining machinery for heavy-duty six dimensional generalized force sensors, a new method of using piezoelectric quartz as force-sensitive element on the basis of the theory of heavy force division and equivalent capacity compression is proposed in this study. Furthermore, in order to minimize the force-coupling for the six-dimension force sensor, the optimization objective function is devised and the calculation method for decoupling is also proposed, on the basis of which, the generalized force sensor with high sensitivity, high stiffness, high linearity but light in its weight is designed. At last, the static calibration experiment is carried out the result shows that the sensor has a good ability of heavy force division and capacity for equivalent compression it can be well used in large tonnage six component force loading device and fatigue test system.

Fatigue Strength Of Thin- Plated Block Joints With Typical Shipbuilding Imperfections

During design of block joints in ship structures it is assumed that the permissible tolerances for axial and angular misalignment given in shipbuilding standards are not exceeded. This is particularly important for fatigue assessments based on the nominal stress approach which implicitly incorporates stress magnification due to imperfections. Preliminary investigations of thin-plated ship structures (t = 4 mm) showed that (a) it is very difficult to keep the thickness-dependent tolerances for thin plates, and (b) even if the tolerances are kept, the fatigue strength might not be sufficient as the stress magnification incorporated in the nominal stress approach is far exceeded. The present paper highlights the problem in the beginning. The main part describes fatigue tests of components performed in a joint industry project. Stiffened panels were fabricated by different shipyards under realistic shipbuilding constraints. The partly large imperfections (buckles, axial and angular misalignment) were extensively recorded using photogrammetric technique in order to obtain the characteristics of typical pre-deformations. The component tests establishing the link to the fatigue strength were carried out with a stress ratio R = −1 which is typical for naval vessels. Concurrent nonlinear finite element calculations containing the whole test set-up were carried out to obtain relations between pre-deformation and stress increase. After description of the test results, a procedure for the assessment of imperfections is discussed which are measured in the usual way in shipbuilding.

The lead fatigue crack concept for aircraft structural integrity

Over many years of quantitative fractographic examination of fatigue cracking from in-service and full-scale fatigue tests of metallic airframe components, it has been consistently observed that the largest cracks formed have grown in an approximately exponential manner. These crack growth observations range from the initiation of cracks and their early growth from a few micrometers through to many millimetres in length. It appears that these lead cracks commence growing shortly after the airframe is introduced to the loading environment. Furthermore these cracks usually initiate from production-induced or, less frequently, inherent material discontinuities. Based on these two observations, an aircraft lifing methodology that is based on the results of fatigue testing programs utilising the lead crack concept has been developed and implemented as an additional tool in the determination of aircraft component fatigue lives in several Royal Australian Air Force (RAAF) fleet types. In this paper the lead crack concept is developed and its strengths and weaknesses are discussed. Examples of crack growth behaviour that are considered typical and representative of lead cracks are presented.