In-service Loading Research Articles

Studies on ratcheting fatigue life prediction of 1Cr18Ni10Ti pressure pipe considering in-service loadings

Ratcheting behavior of aero-engine pressure pipe subjected to in-service loadings, including bolts preload, cyclic bending force, and internal fluid pressure, has been rarely incorporated in open experimental research. An optimized fatigue testing system was constructed to overcome the challenge caused by the non-standardized shape of the pipe. The objective of this paper is to investigate the ratcheting behavior of 1Cr18Ni10Ti pressure pipe subjected to in-service loadings and establish the ratcheting fatigue life prediction model based on the strain approach. Firstly, the effects of cyclic bending force amplitude and internal fluid pressure on the ratcheting behavior of the pressure pipe subjected to in-service loadings were analyzed. The results highlight that increasing force amplitude and internal fluid pressure lead to a remarkable ratcheting effect and shortened fatigue life. Then, the strain-based ratcheting fatigue life prediction model of the pressure pipe considering in-service loadings was developed based on finite element analysis (FEM). The main novelty of this method is that the model parameters were innovatively determined by incorporating the effects of cyclic bending force amplitude and internal fluid pressure. Lastly, the application boundary of the ratcheting fatigue life prediction method was compared by employing the KTA/ASME code, RCC-MR code, and C-TDF rule, confirming the effectiveness and feasibility of the proposed C-TDF rule. These findings advance our comprehensive understanding of the fatigue properties of the aero-engine pressure pipe considering in-service loadings.

A design method for continuous fiber-reinforced composite patches

The urgent need for lightweight and high-strength patches in the field of aircraft structural repair has propelled continuous fiber-reinforced thermoset composites into a research hotspot. However, constrained by manufacturing processes, the designs of composite repair structures primarily focus on external shape and ply layup. These approaches presently lack design methods for the internal fiber adaptability of patches driven by in-service loads. In this paper, we proposed a method for fiber distribution and orientation design based on maximum normal stress. The composite simulation model was established based on fracture toughness. The influence of tensile-shear coupling effects on the mechanical response of composites and the fiber orientation design were examined in the study. The orientation-density coupling effects on the enhancement of patch strength were also analyzed. Furthermore, additive manufacturing technology was employed to fabricate thermoset composite patches. The results show that the composite patches designed in this paper exhibit a 10.50 % improvement in strength compared to traditional unidirectional patches.

In-service Load Monitoring for an UAV Digital Twin

A load monitoring system plays an important role in recovering the actual load spectra of aeronautical structures, contributing to the online evolution of airframe digital twins. In scenarios where many aircrafts lack on-board strain sensors during the service phase, yet some strain data is available during the test flight phase, our innovative approach utilises deep learning-based flight-strain prediction and an inverse-direct approach for in-service load monitoring. Initially, a deep learning approach is employed during the test flight prior to service to establish a flight parameter-strain prediction model. This model, incorporating time series features of flight and strain da-ta, exhibits superior predictive accuracy compared to traditional regression methods. Moving into the subsequent service phase, the flight parameter-strain prediction model seamlessly integrates with an inverse-direct load monitoring method. This integrated approach facilitates real-time monitoring of full-field load distribution, relying solely on flight parameters. Validation of the approach utilises flight test data from an unmanned aerial vehicle, revealing better performance compared with the strain-measurement-based method. Notably, our method's efficacy extends across diverse aircraft types, as it does not rely on on-board strain sensors during the service phase.

In-situ computed tomography and transient dynamic analysis – failure analysis of a single-lap tensile-shear test with clinch points

Clinching is a mechanical joining technology, in which a mainly form-fit joint is created by means of local cold forming. To characterize the load-bearing behavior of such joints, they are typically analyzed destructively, for example by tensile-shear tests in combination with metallographic sections. However, both the initiation and progress of failure can only be described to a limited extent by this method. Furthermore, these tests allow only limited conclusions about clinch points under in-service loading. More purposefully, clinch points can be analyzed nondestructively by combining in-situ computed tomography (CT) and transient dynamic analysis (TDA). The TDA continuously measures the dynamic behavior of the specimen and indicates failure events like crack initiation, which then can be evaluated thoroughly by stopping the test and performing a CT scan. To qualify the TDA for this task, it is necessary to link the observed damage behavior with specific dynamic characteristics. In this work, the complementation of in-situ CT and TDA is investigated by testing a clinched single-lap tensile-shear specimen made of aluminum. The testing procedure is stepwise: at certain displacement levels, the specimen is investigated by in-situ CT and TDA. While the in-situ CT provides the location, extent, and development of the failure phenomena, the TDA uses this information to evaluate the dynamic signal and detect relevant frequency ranges, which indicate damage events. The results demonstrate, that failure initiation and progression can be analyzed efficiently by combining both measuring systems. The TDA reliably detects relevant signal changes in the monitored frequency band. By means of in-situ computed tomography, the corresponding failure phenomena can be described in detail, enhancing the understanding of the load-bearing and deformation behavior of clinch points. The concatenation of characteristic signal changes and observed failure phenomena can henceforth be transferred to analyze complex structures during operation nondestructively by TDA.

Influence of build orientation and support structure on additive manufacturing of human knee replacements: a computational study.

Developing patient-specific implants has an increasing interest in the application of emerging additive manufacturing (AM) technologies. On the other hand, despite advances in total knee replacement (TKR), studies suggest that up to 20% of patients with elective TKR are dissatisfied with the outcome. By creating 3D objects from digital models, AM enables the production of patient-specific implants with complex geometries, such as those required for knee replacements. Previous studies have highlighted concerns regarding the risk of residual stresses and shape distortions in AM parts, which could lead to structural failure or other complications. This article presents a computational framework that uses CT images to create patient-specific finite element models for optimizing AM knee replacements. The workflow includes image processing in the open-source software 3DSlicer and MeshLab and AM process simulations in the commercial platform 3DEXPERIENCE. The approach is demonstrated on a distal femur replacement for a 50-year-old male patient from the open-access Natural Knee Data. The results show that build orientations have a significant impact on both shape distortions and residual stresses. Support structures have a marginal effect on residual stresses but strongly influence shape distortions, whereas conical support exhibits a maximum distortion of 18.5 mm. Future research can explore how these factors affect the functionality of AM knee replacements under in-service loading.

Cyclic plasticity behavior and deformation mechanism of 1Cr18Ni10Ti pressure pipe subjected to in-service loadings

Fatigue performance and durability application have always been the key issues impeding the understanding of aero-engine pressure pipe due to its non-standardized shape, making tests more challenging. This paper constructs an optimized testing system to elucidate the plastic shakedown and ratcheting behavior of 1Cr18Ni10Ti pressure pipe subjected to in-service loadings. Moreover, microstructural evolution's role in regulating the cyclic plasticity behavior is qualitatively established. Experimental results indicate that the pipe deformation exhibits compressive ratcheting behavior in plastic shakedown state, and the initial cyclic hardening followed by cyclic saturation occurs during the whole cyclic response. This corresponds to a decreasing ratcheting displacement rate followed by a steady state appears. Except for the first two similar deformation stages mentioned above, the abrupt cyclic softening and the resultant increasing ratcheting displacement rate are obtained at the final fracture stage of the pipe deformation in ratcheting state. A series of detailed microstructural characterizations, including optical microscopy (OM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and acoustic emission (AE) are combined to reveal the underlying microstructural mechanisms during cyclic deformation. The volume fraction of austenite and ferrite and the average grain size change during different stages of plastic shakedown and ratcheting deformation. The synergistic effects of dislocation structure multiplication, destruction, and dissolution at different stages of plastic shakedown and ratcheting deformation dominate the cyclic plasticity behavior.

Mechanical integrity of photovoltaic panels under hailstorms: Mono vs. poly-crystalline comparison

The performance of Photovoltaic (PV) modules heavily relies on their structural strength, manufacturing methods, and materials. Damage induced during their lifecycle leads to degradation, reduced power generation and efficiency. Mechanical stresses, originating from manufacturing, transportation, and operational phases impose significant loads on PV modules. These in-service loads encompass various environmental forces such as wind, snow, dust, hail, rain, and heat. In-service loads encompass static and dynamic forces such as wind, snow, dust, hail, rain, and heat. Among these factors, the mechanical loads from hail impacts play a crucial role in PV module performance and require a comprehensive investigation. This research focuses on evaluating the impact of hail loads on different PV modules, following international standards like ASTM 1038-10 and IEC-61215-2. The developed simulator effectively assesses the reliability of PV modules. The number of busbars within a PV module was identified as a key factor influencing the module's resilience to hail impacts. Notably, mono-crystalline PV modules exhibited better resistance to hail loads compared to their poly-crystalline counterparts. The PV modules experience micro-cracking due to hail impacts, leading to an efficiency reduction of 4.15% in mono-crystalline modules and 12.59% in poly-crystalline modules. Similarly, the generated power output decreased by 3.3% and 12.5%, respectively, in these module types.

Inspection of antennas embedded in smart composite structures using microwave NDT methods and X-ray computed tomography

Antennas embedded in composites are key components in communication, radar, and sensing systems aboard aircrafts. The manufacturing complexities and in-service loading could compromise the integrity of the composite structure through defects within the antenna which undermine the structure utility. Therefore, Non-Destructive Testing (NDT) is needed to ensure antenna structure integrity pre- and post- commissioning. While X-Ray computed tomography (CT) is the gold standard method for these inspections as it provides high sensitivity and resolution, it is relatively expensive, slow, and inapplicable for in-situ inspection. This paper devises various Microwave NDT methods towards composite-embedded antenna inspection, unprecedentedly. By direct benchmarking against X-Ray CT, it will be demonstrated that a low-frequency resonant microwave probe provides a viable alternative to X-Ray CT with similar detection sensitivity (85 % defect detection in inspected samples). Moreover, the resonant microwave probe provides multifold enhancement in the sensitivity (>10x) compared to waveguide probes used conventionally for microwave NDT.

Fatigue strength of adhesively bonded deep drawn sheet specimens under multiaxial loading with variable amplitudes

Adhesive bonding is a frequently used joining technology in the manufacturing of vehicle bodies. In comparison to conventional spot-welded designs, adhesive bonding provides a higher stiffness due to a planar load transfer. Furthermore, there are advantages in damping behavior and chemical separation in multi-material designs.Up to now, there exists still quite some uncertainty in the fatigue assessment of adhesively bonded joints due to a lack of reliable assessment approaches. This is especially true for the case of more complex in-service loading conditions as variable amplitude loading, due to the sparse database of experimental fatigue results, which are needed for the development of new methods. For this reason, the results of such tests are presented in this paper on component-like specimens, so called bowl specimens, where even uniaxial loading results in multiaxial stress state due to the complexity of the geometry. These specimens consist of a deep drawn sheet that is bonded with a thermosetting structural adhesive to a planar sheet and are subjected under cyclic loading. They were loaded uniaxially and multiaxially, with (φ = 90°) and without phase shift (φ = 0°).As a complex shaped and stressed sheet metal specimen, the bowl specimen closely resembles components of e.g. vehicle bodies, which is why the conducted tests are presented for a better understanding of such vehicle structures. It is shown that a phase shift leads to an increase of the fatigue life. Furthermore, the cyclic stiffness degradation is discussed, which often serves as a failure relevant parameter in real structures.

Effect of Loading Frequency on the Fatigue Response of Adhesive Joints up to the VHCF Range

Modern structures are designed to withstand in-service loads over a broad frequency spectrum. Nonetheless, mechanical properties in numerical codes are assumed to be frequency-independent to simplify calculations or due to a lack of experimental data, and this approach could lead to overdesign or failures. This study aims to quantify the frequency effects in the fatigue applications of a bi-material adhesive joint through analytical, numerical, and experimental procedures. Analytical and finite element models allowed the specimen design, whereas the frequency effects were investigated through a conventional servo-hydraulic apparatus at 5, 25, and 50 Hz and with an ultrasonic fatigue testing machine at 20 kHz. Experimentally, the fatigue life increases with the applied test frequency. Run-out stress data at 109 cycles follow the same trend: at 25 Hz and 50 Hz, the run-out data were found at 10 MPa, increasing to 15 MPa at 20 kHz. The P–S–N curves showed that frequency effects have a minor impact on the experimental variability and that standard deviation values lie in the range of 0.3038–0.7691 between 5 Hz and 20 kHz. Finally, the trend of fatigue strengths at 2·106 cycles with the applied loading frequency for selected probability levels was estimated.

The role of internal defects on anisotropic tensile failure of L-PBF AlSi10Mg alloys

This paper investigates the effects of defects on tensile failure of additive manufactured AlSi10Mg alloy focusing particularly on the role of large pancake shaped loss of fusion (LOF) defects lying perpendicular to the build direction (BD). Time-lapse in situ synchrotron radiation X-ray micro-computed tomography during straining reveals how, when tested parallel to the BD, the LOF defects extend laterally with straining connecting to other defects and giving rise to low plasticity and an essentially brittle failure mode. When they are aligned edge-on to the straining direction, failure is characterised by a ductile cup-cone failure with significant elongation of the defects axially and extensive necking prior to failure. The soft fish-scale melt pool boundaries were also found to affect the fracture path. These results highlight the anisotropic effect of loss of fusion defects in controlling tensile ductility and the need to minimize their size and aspect ratio. In cases where these cannot be fully eliminated the component should be fabricated such that the BD is not aligned with the dominant in-service loading direction.

In-service load calculation surrogate models for high-pressure turbine blade life digital twin

Abstract There are developed methods for high-pressure turbine (HPT) blade loads and remaining useful life (RUL) prediction; however, they are ineffective and time-consuming for in-service HPT blades under actual operating conditions. Hence, it is necessary to use an acceptable computational effort to predict the HPT blade’s load and in-service lifetime. Drawing from the idea of the usage-based life (UBL) prediction method, this paper first proposes a framework for the life digital twin (LDT) to characterize and track the in-service life consumption of the HPT blades under actual operating conditions. The second work mainly focuses on developing the steady-state and transient load calculation surrogate models for the HPT blade’s LDT. Using the developed surrogate models, it can now calculate the steady-state and transient loads of the HPT blade in an acceptable time with the necessary accuracy. The proposed approach is demonstrated on an HPT blade of a typical commercial turbofan engine. Because the operating load of the HPT blade severely affects its in-service lifetime, the application of this approach enables the construction of an LDT of the HPT blade. It can reduce the uncertainty and variability associated with the in-service life prediction of the HPT blade under actual operating conditions.

A numerical study on dynamic interaction and wear of flange tip lift crossings in tramlines

Flange tip lift crossings are widely utilized in modern tramlines to replace traditional crossings to enhance commuters’ comfort and prevent crossing nose failures. However, due to altered contact conditions between wheel flanges and ramps of crossings, the use of flange tip lift crossing has led to the critical issue of ramp wear. To resolve this issue, this paper investigated the dynamic contact response of a ramp during in-service loading, as well as the resultant wear of the ramp. With a dynamic finite element model, the local contact pressure and local sliding distance were computed and subsequently applied in a local contact-based wear model to evaluate the nominal wear of the ramp. Our findings indicate that under typical in-service loading conditions, the ramp exhibited up to 120 times more wear than the rail head, owing to increased contact pressure and reduced contact area during the ramp contact. To mitigate this issue, a design change of a gentler ramp inclination from the default 1:100 ramp to a 1:150 ramp could provide approximately 10% wear reduction. In addition, increasing the flange radius by 1 mm could reduce the wear by up to 13%. Conversely, increasing the speed to 45 km/h from the recommended 15 km/h speed may cause up to 5 times more wear. Furthermore, an empty tram showed 56% less wear on the ramp when compared to the condition of the maximum allowable axle load. These results offer an insight into the detrimental wear of ramps used in flange tip lift crossings of tramlines and can assist in the development of new designs and operating guidelines to mitigate ramp wear.

Reduced order modelling using neural networks for predictive modelling of 3d-magneto-mechanical problems with application to magnetic resonance imaging scanners

The design of magnets for magnetic resonance imaging (MRI) scanners requires the numerical simulation of a coupled magneto-mechanical system where the effects that different material parameters and in-service loading conditions have on both imaging and MRI performance are key to aid with the design and the manufacturing process. To correctly capture the complex physics, and to obtain accurate solutions, finite element simulations with dense meshes and high order elements are needed. Reduced order model approaches, based on the established proper orthogonal decomposition (POD) approach, are attractive as they can rapidly predict the numerical simulations needed under changing parameters or conditions. However, the projected (PODP) approach has an invasive computational implementation, whilst the interpolated (PODI) approach presents challenges when the dimension of the space of parameters to be investigated becomes large. As an alternative, we investigate a POD technique based on using a neural network regression, which is not as invasive as PODP, but has superior approximation properties compared to PODI. We apply this to the coupled magneto-mechanical system to understand three pressing industrial problems: firstly, the accurate and rapid computation of the resonant frequencies associated with this coupled magneto-mechanical system, secondly, the effects of magnet motion on the Ohmic power and kinetic energy curves, and, thirdly, the prediction of the uncertainty in Ohmic power and kinetic energy curves as a function of exciting frequency for uncertain material parameters.

Vibration-induced crack tip flipping: A closer look at an unfamiliar phenomenon in pipeline failure analysis

Fracture propagation control is an essential strategy to avoid a catastrophic event involving both economic losses and environmental damage. The Dynamic Tensile Tear Test (DT3) was introduced as an alternative tool to characterise dynamic fracture behaviour of high-strength pipeline steels. Mimicking the in-service loading conditions by imposing a dynamic tensile load, the obtained fracture surfaces closely resemble those observed in full-scale pipeline burst tests. A phenomenon called Crack Tip Flipping (CTF) could also be observed in combination with the formation of Arrowhead Markings (AHMs). The mechanisms provoking these phenomena and their effect on the fracture resistance of the pipeline material have not yet been investigated with respect to pipeline failure. Therefore, a high-speed stereo DIC setup was used to study the fracture behaviour of ×70 grade pipeline steel. The DIC setup identified a mechanical out-of-plane vibration that resulted in an additional torsional loading component. Numerical models were constructed with implementation of the Modified Bai-Wierzbicki (MBW) damage model to validate the experimental observations. Imposing a vibration-induced out-of-plane component resulted in a nearly identical fracture behaviour as observed during experiments. Both the crack path and the final fracture surface could be reproduced. Based on the obtained data, it is suggested that this oscillation causes a rotation of the stress tensor controlled by the through-thickness shear component. As a result, the stress state at the crack front is oriented towards the plane of maximum shear, forming a slanted fracture surface. Notably, the additional torsion load did not affect the load-displacement curve, as the value of the effective stress remains unaffected. Consequently, the fracture resistance, computed from energy values extracted from the load-displacement curve, are not influenced by the occurrence of CTF or the formation of AHMs. Conversely, based on the discussed mechanism of CTF, its presence could indicate the conditions under which pipeline failure occurred.

Geotechnical considerations for assessing the life extension of offshore wind foundations

This paper describes the geotechnical considerations in assessing the fatigue lives of offshore wind foundations over their lifetime, including the potential for life extension.A case study is presented where a new analysis tool was used to reassess the fatigue lives of monopile foundations at a wind farm experiencing significant scour. Previous assessments using conventional modelling techniques for foundation loads and soil structure interaction predicted several locations with unacceptable fatigue lives.The fatigue reassessment was based on in-service loads calculated from an innovative monitoring system. The latest industry guidance for the geotechnical design of monopiles, as recommended by the PISA project, was used to predict the sub-mudline stiffness. Life reassessment was completed for all primary foundation steel circumferential welds in the wind farm including critical sub-mudline locations.The use of advanced soil structure modelling techniques, coupled with in-service load data and explicit modelling of scour effects, has enabled a more accurate assessment of fatigue lives. Fatigue life was demonstrated to be acceptable at all locations, with the potential for life extension. The relative effect on fatigue life of the improved geotechnical assessment and monitored loads was studied. This showed that both components were necessary to achieve the fatigue life targets.

Loading test on the oil tank ground settlement performance monitored by an optical parallel scheme.

A loading test of the ground settlement (GS) performance of the oil tank must be examined before beginning its commercial service. This test requires the sensors to be installed around the oil tank, and the GS is measured while water is being filled in, where the liquid level is read with an ultrasonic radar equipment, etc., to indicate the applied water loads. During the service of the oil tank, loading and unloading corresponding to the oil inlet and outlet are the critical factors to cause the oil tank destruction in a fatigue way. Thus, a regular in-service loading test is the means of evaluating the tank base health condition. However, the sensors for GS measurement of the oil tank are mostly based on a liquid hydraulic sensor, which is an intrinsically static sensor determined by the fluidity of the measurement liquid. In order to meet the instantaneous requirement of the loading test, first, the configuration of the optical GS sensor was designed to suit the simultaneous measurement. Secondly, a data acquisition system was designed by combining the digital signal processing with a field programmable gate array to carry out a parallel multiple channel data collection. This ensures that the GS sensors are interrogated simultaneously to snapshot a GS status of the oil tank, even if its load was changed slowly. A practical oil inlet process was recorded with an ultrasonic radar oil level measurement, and the results of oil tank GS were verified with a manual measurement by using the Electronic Total Station. The effectiveness of our sensor monitoring of the oil tank GS performance during the loading test has been proven.

The 2022 Pacheco Silva lecture: The influence of residual loads on pile foundation behavior

Residual loads can affect the load transfer and the settlement-induced in-service loadings, although they do not alter the bearing capacity. When residual loads are present and not measured or evaluated, the settlement estimate is greater than predicted if these loads are known. Residual loads can be measured when the pile instrumentation is nullified before pile installation, in the case of displacement piles, or before the first loading in non-displacement piles, such as bored cast-in-place piles, continuous flight auger piles, and micro-piles. In the case of underpinning foundation and piled raft, when the loading transfer is shared by the original and new foundation, or by the piles and the raft, it is essential to know the stiffness of each foundation element to estimate the load partition. If residual loads are present, pile stiffness is greater than when not considered in the design. The paper revisits this theme of practical relevance. A historical review of the most relevant research involving pile residual load measurements, pile loading tests including the interpretation of residual loads locked at a pile toe, and a new procedure for residual loads prediction are provided. A comparison is made of the experimental residual loads observed in some of the instrumented cases and the values estimated with the suggested procedure. The development of residual loads at the pile toe as a function of the toe resistance to total capacity ratio is very similar to the variation of the soil density as a function of soil moisture content in soil compaction.

Uncertainty analysis for prediction of elastic properties of unidirectional bamboo fiber-reinforced composites

This paper presents a virtual framework to obtain the elastic response of unidirectional (UD) continuous bamboo fiber-reinforced composites based on the micromechanical modeling approach. The virtual framework involves the development of an algorithm for the generation of stochastic Representative Volume Elements (RVEs), taking into account the micro-structural uncertainty and variability of the bio-based bamboo fibers and their composites. The developed algorithm and model are validated with experimental characterizations and theoretical models. Different statistical micro-structural realizations with random fiber packing, fiber geometries, and variable RVE sizes are compared to investigate the effects of these parameters on the accuracy of the prediction of the elastic properties of bamboo fiber-reinforced composites. All models and loading scenarios are applied automatically through Python scripting in the Abaqus finite element (FE) solver. The uncertainty analysis of input parameters is carried out through parametric studies. This virtual micromechanical framework can be further linked in the future to macroscale models of bamboo or other natural fiber composites for multiscale prediction of the behavior of biocomposite structures under in-service loading.

Mechanical behaviour of fabric-reinforced plastic sandwich structures: A state-of-the-art review

The use of fibre-reinforced plastics (FRPs) in sandwich structures increased for various industrial applications thanks to their strength-to-weight ratio which provides designers with advanced options for modern structures. FRP Sandwich Structures (FRPSS) are often used in aerospace, biomedical, defence, and marine products, where their high structural performance is required to sustain complex in-service loads and withstand varying environmental conditions. Progressive degradation of FRPSS under such circumstances has been a subject of interest for researchers owing to safety requirements for products with FRP. This paper reviews the state-of-the-art of the mechanical behaviour of FRPSS subjected to various loading regimes. It highlights the variation in structural performance, viscoelastic properties, damage resistance, and sequence of environmental degradation of FRPSS. Numerical methods and damage algorithms used to predict failures are also presented to provide sufficient knowledge for the design of FRPSS. This review contributes to further research on characterizing the properties of FRPSS under quasi-static and dynamic loading conditions.