Structural Hot Spot Research Articles

Effect of Load Sequence and Weather Seasonality on Fatigue Crack Growth for Monopile-based Offshore Wind Turbines

Offshore wind turbines are subjected to variable amplitude loading, but the impact of load sequence is commonly neglected in fatigue analysis. This paper presents an initial investigation if load sequence and weather seasonality influence fatigue crack growth for monopile-based offshore wind turbines. Focus is on the load sequence effect introduced by the non-linearity of crack propagation. Fatigue crack growth at two structural hot spots was analyzed with a fracture mechanics model applying Paris’ law. The model was calibrated to yield an identical lifetime as a SN-curve analysis. Input into the fracture mechanics model are structural stresses due to environmental and operational loading. Weather seasonality was simulated with a Markov model. Results show that loading sequence has only a negligible effect on crack sizes under the assumption made in this study. This makes fatigue lifetime predictions independent of weather seasonality. However, it becomes relevant for the prediction of future propagation of detected fatigue cracks throughout the year.

Numerical implementation for fatigue assessment of butt joint improved by high frequency mechanical impact treatment: A structural hot spot stress approach

In the study, three-dimensional finite element modelling of high frequency mechanical impact (HFMI) treatment is presented for butt joints of four strength grade materials (Q235B, AISI 1006, 45 Steel and AISI 2205). The fatigue assessment of butt joints by HFMI is implemented by structural hot spot stress approaches, i.e., linear surface extrapolation (LSE) and through thickness at the weld toe (TTWT). The effective structural hot spot stress concentration factors for HFMI-treated joints are determined, and the dependence of HFMI improvement on material strength and external stress is well captured by TTWT method. Following structural hot spot stress S-N curves for as-welded joints recommended by IIW, the structural hot spot stress S-N curves for HFMI-treated joints are suggested if the slope m of S-N curves for HFMI-treated joints is designated to vary from 5 to 10, inversely proportional to material strength. At final, the characteristic fatigue strength (FAT) has been determined and verified with the available experimental data.

Fatigue strength assessment of ship structures accounting for a coating life and corrosion degradation

PurposeFatigue strength and reliability assessment of complex double hull oil tanker structures, based on different local structural finite element approaches, is performed accounting for the uncertainties originating from load, nominal stresses, hot spot stress calculations, weld quality estimations and misalignments and fatigue S-N parameters including the correlation between load cases and the coating life and corrosion degradation.Design/methodology/approachShip hull wave-induced vertical and horizontal bending moments and pressure are considered in the analysis. Stress analyses are performed based on the nominal, local hot spot and notch stress approaches. A linear elastic finite element analysis is used to determine the stress distribution around the welded details and to estimate structural stresses of all critical locations. Fatigue damage is estimated by employing the Palmgren-Miner approach. The importance of the contribution of each random variable to the uncertainty of the fatigue limit state function is also estimated. The probability of fatigue damage of hot spots is evaluated taking into account random coating life and corrosion wastage. Fatigue reliability, during the service life, is modelled as a system of correlated events.FindingsThe fatigue analysis showed that the fatigue damage at the hotspot, located at the flange of the stiffener close to the cut-out, is always highest in the cases of the structural hot spot stress and effective notch stress approaches, except for the one of the nominal stress approach. The sensitivities of the fatigue limit state function with respect to changes in the random variables were demonstrated showing that the uncertainty in the fatigue stress estimation and fatigue damage are the most important. Fatigue reliability, modelled as a parallel system of structural hot spots and as a serial system of correlated events (load cases) was evaluated based on the Ditlevsen bounds. As a result of the performed analysis, reliability and Beta reliability indexes of lower and upper bounds were estimated, which are very similar to the ones adopted for ultimate strength collapse as reported in literature.Originality/valueThis paper develops a very complex fatigue strength and reliability assessment model for analysing a double hull oil tanker structure using different local structural finite element approaches accounting for the associated uncertainties and the correlation between load cases and the coating life and corrosion degradation. The developed model is flexible enough to be applied for analysing different structural failure modes.

Influence of weld quality on the fatigue strength of thin normal and high strength steel butt joints

For building more energy-efficient and eco-friendly structures, new lightweight solutions are needed. Utilization of higher strength steels and smaller plate thicknesses together with modern welding technologies holds a huge potential for weight reduction. However, due to lack of knowledge about the fatigue resistance, current rules do not allow using smaller plate thicknesses than 5 mm nor benefiting from higher strength steels in large welded structures such as in ships. One of the main concerns in addition to larger welding-induced distortions is the weld quality. This paper compares the fatigue strength of 4 mm thick normal and high strength steel butt joints produced by different welding methods leading to different weld shape and quality. In order to eliminate the effect of secondary bending due to initial distortions and bring out the influence of weld quality, the fatigue test results are analyzed using structural hot spot stress approach with measured specimen geometries. The results show considerable yield-limit-dependent increase in the fatigue strength of high quality welds, while no benefit from higher base material strength was observed for low quality butt joints. In addition, the suitability of existing weld quality recommendations for thin welded joints was evaluated and future research needs discussed.

Structural hot spots for the solubility of globular proteins.

Natural selection shapes protein solubility to physiological requirements and recombinant applications that require higher protein concentrations are often problematic. This raises the question whether the solubility of natural protein sequences can be improved. We here show an anti-correlation between the number of aggregation prone regions (APRs) in a protein sequence and its solubility, suggesting that mutational suppression of APRs provides a simple strategy to increase protein solubility. We show that mutations at specific positions within a protein structure can act as APR suppressors without affecting protein stability. These hot spots for protein solubility are both structure and sequence dependent but can be computationally predicted. We demonstrate this by reducing the aggregation of human α-galactosidase and protective antigen of Bacillus anthracis through mutation. Our results indicate that many proteins possess hot spots allowing to adapt protein solubility independently of structure and function.

Chapter Two - Heterotrimeric G Protein Ubiquitination as a Regulator of G Protein Signaling.

Ubiquitin-mediated regulation of G proteins has been known for over 20 years as a result of discoveries made independently in yeast and vertebrate model systems for pheromone and photoreception, respectively. Since that time, several details underlying the cause and effect of G protein ubiquitination have been determined-including the initiating signals, responsible enzymes, trafficking pathways, and their effects on protein structure, function, interactions, and cell signaling. The collective body of evidence suggests that Gα subunits are the primary targets of ubiquitination. However, longstanding and recent results suggest that Gβ and Gγ subunits are also ubiquitinated, in some cases impacting cell polarization-a process essential for chemotaxis and polarized cell growth. More recently, evidence from mass spectrometry (MS)-based proteomics coupled with advances in PTM bioinformatics have revealed that protein families representing G protein subunits contain several structural hotspots for ubiquitination-most of which have not been investigated for a functional role in signal transduction. Taken together, our knowledge and understanding of heterotrimeric G protein ubiquitination as a regulator of G protein signaling-despite 20 years of research-is still emerging.

Nanoscale shaping and focusing of visible light in planar metal–oxide–silicon waveguides

Focusing light at the nanoscale has become a key factor in super-resolution applications. Dynamic control of this focusing can open new avenues in nanoelectronics and bioimaging, but it requires a platform that merges electronics with super-resolution capabilities. We present a planar metal–oxide–silicon (MOS) platform that allows us to shape, tune, and focus visible light at the nanoscale by compressing the wavelength of light fourfold, resulting in a scaled diffraction limit of 65 nm. We exemplify the control and flexibility by demonstrating nanovortex beams and short-wavelength super-oscillations of light that further enhance resolution toward 35 nm. Our platform achieves focusing strength similar to nanoantennas but without structural hotspots; hence it is possible to scan the focus via optical wavefront-shaping techniques. Super-resolution scanning without mechanical translations in a MOS platform can provide a building block for bioimaging, nanolithography, and lab-on-a-chip applications.

Fatigue evaluation for longitudinal-to-transverse rib connection of orthotropic steel deck by using structural hot spot stress

Orthotropic steel decks have serious fatigue problems especially on the connections between longitudinal and transverse ribs. In this paper, experimental and analytical works were conducted to evaluate the fatigue performance of connections between longitudinal U-ribs and transverse ribs with and without the slit on the transverse rib web. Finite element analyses of the specimen for all possible loading cases clarified the influence surfaces for structural hot spot stress (SHSS) working on weld toes of the connections. Fatigue tests with the loading case that generates the maximum and minimum SHSS on the connections were conducted. Fatigue test results for the slit connections evaluated by SHSS show agreement with previous fatigue data of similar connection details. No fatigue crack was initiated on the connection without the slit after 4.6 million cycles compared to fatigue failure of the slit connection at 0.7 million cycles.

Outward Rectification of Voltage-Gated K+ Channels Evolved at Least Twice in Life History.

Voltage-gated potassium (K+) channels are present in all living systems. Despite high structural similarities in the transmembrane domains (TMD), this K+ channel type segregates into at least two main functional categories—hyperpolarization-activated, inward-rectifying (Kin) and depolarization-activated, outward-rectifying (Kout) channels. Voltage-gated K+ channels sense the membrane voltage via a voltage-sensing domain that is connected to the conduction pathway of the channel. It has been shown that the voltage-sensing mechanism is the same in Kin and Kout channels, but its performance results in opposite pore conformations. It is not known how the different coupling of voltage-sensor and pore is implemented. Here, we studied sequence and structural data of voltage-gated K+ channels from animals and plants with emphasis on the property of opposite rectification. We identified structural hotspots that alone allow already the distinction between Kin and Kout channels. Among them is a loop between TMD S5 and the pore that is very short in animal Kout, longer in plant and animal Kin and the longest in plant Kout channels. In combination with further structural and phylogenetic analyses this finding suggests that outward-rectification evolved twice and independently in the animal and plant kingdom.

Research on rib-to-diaphragm welded connection by means of hot spot stress approach

The cutout hole locating at the place of rib-to-diaphragm welded connection is adopted to minimize the restraint, which is caused by the floor-beam web to rib rotation at the support due to the unsymmetrical loads in orthotropic deck. In practice, an inevitable problem is that there is a large number of welding joint's cracks formed at the edge of cutout hole. In this study, a comparative experiment is carried out with two types of cutout hole, the circular arc transition and the vertical transition. The fatigue life estimation of specimens is investigated with the application of the structural hot spot stress approach by finite element analyses. The results are compared with the ones of the fatigue tests which are carried out on these full-scale specimens. Factors affecting the stress range are also studied.

Molecular dynamics modelling of local 3D structural differences in genes when carcinogens are bound at mutation hotspots – The link between structural change and mutation hotspot status

Molecular dynamics modelling of local 3D structural differences in genes when carcinogens are bound at mutation hotspots – The link between structural change and mutation hotspot status

Lamb wave line sensing for crack detection in a welded stiffener.

This paper proposes a novel Lamb wave line sensing technique for crack detection in a welded stiffener. The proposed technique overcomes one of the biggest technical challenges of Lamb wave crack detection for real structure applications: crack-induced Lamb waves are often mixed with multiple reflections from complex waveguides. In particular, crack detection in a welded joint, one of the structural hot spots due to stress concentration, is accompanied by reflections from the welded joint as well as a crack. Extracting and highlighting crack-induced Lamb wave modes from Lamb wave responses measured at multi-spatial points along a single line can be accomplished through a frequency-wavenumber domain analysis. The advantages of the proposed technique enable us not only to enhance the crack detectability in the welded joint but also to minimize false alarms caused by environmental and operational variations by avoiding the direct comparison with the baseline data previously accumulated from the pristine condition of a target structure. The proposed technique is experimentally and numerically validated in vertically stiffened metallic structures, revealing that it successfully identifies and localizes subsurface cracks, regardless of the coexistence with the vertical stiffener.

Hot spot stress analysis on rib–deck welded joint in orthotropic steel decks

Orthotropic steel decks are used in beams and cable-supported bridges. Fatigue cracks of the vertical rib–deck welded joint have been found in some of the bridges. In this paper, the structural hot spot stress (SHSS) approach is applied to evaluate the rib–deck fatigue. Refined solid models are built using a multi-sub-model technique. Stress around the weld tip is analyzed and effects of the weld profile, the weld toe radius and mesh size are discussed. The SHSS is analyzed using the surface stress extrapolation method, the stress linearization method and the 1mm stress method. Fatigue strength of the joint based on the SHSS is proposed. Results of this study show that the refined multi-sub-model considering the weld detail can reflect the mechanical behavior of the rib–deck joint. Variation of the SHSS by the three methods decreases to less than 10% and a convergent SHSS is achieved using the refined models. The derived fatigue strength for the rib–deck joint using the SHSS of the refined models is close to FAT100. A more precise fatigue strength prediction can be achieved using the refined model while the coarse models result in a conservative design.

Remote Monitoring of Fatigue Cracks Growth in the Aircraft Structure Based on Active Piezosensor Network during the Full Scale Fatigue Test

One of the major issues from a structural integrity point of view of the aircraft structure is an appropriate health monitoring technology delivery for the damage tolerant philosophy. This paper presents a development of a system for fatigue crack growth monitoring and early damage detection in the PZL 130 ORLIK TC II turbo-prop military trainer aft structure. The maintenance system of the aircraft shifts from the safe-life to the hard-time. The aircraft started Full Scale Fatigue Test (FSFT) which will continue up to 2013. In the article a built block approach for the system design, signal modeling, sensing and signal processing as well as damage detection is presented. Taking into the consideration a previous experience of AGH as well as AFIT, a network of PZT transducers was deployed in the aircraft structure hot-spots. The system components are: remote monitoring unit, signal analysis, graphical user interface, sensor self-diagnostic tools, and data classification model. Description of damage detection capabilities are delivered in the paper. In particular some issues concerning the proposed damage indices and its application to crack growth estimation models are discussed. Fishers Linear Discriminant is used as a method to obtain effective crack growth predictors and one of the self-diagnostic tools used in the system. The results of the data collected from specimen fatigue tests are delivered and cross-validation technique is used to evaluate a classification model based on the damage indices derived.

Finite element simulation of road tanks for ADR and fatigue assessments

Purpose – The paper aims to illustrate the application of a state-of-the-art fatigue prediction concept (IIW recommendations) to the ADR calculation requirements, in order to include failure-critical weld details to the strength assessment of road tanks. The level of calculation accuracy and, therefore, safety of such structures transporting dangerous goods can be substantially increased. Design/methodology/approach – A case study of a 2-compartment aluminum alloy LBGF tank used for the transportation of gasoline/petroleum products is conducted adopting the meshing directives of the structural hot spot stress concept (SHSSC) of the IIW recommendations for the weld details of the structure. The stresses on the prescribed Hot Spots are extracted from the solutions of each load case described in the ADR legislation and assessed both in terms of static strength according to EN 14286 and fatigue strength using the standardized IIW assessment curves (FAT curves). Findings – The detailed analysis of the road tank has successfully allocated the failure critical points of the structure. Additionally it has illustrated the ability of the embedded fatigue concept to accurately calculate the stresses which can later be used for the durability evaluation, providing that at least some operational load spectra become available. Research limitations/implications – The use of a global meshing and calculation concept offers a relatively limited accuracy level considering any individual weld detail. On the other hand, it offers a sufficient overview of the whole structure with minimum effort and costs. Practical implications – Time-consuming integrity inspections of road tanks can be minimized, as the failure critical locations are identified and therefore easily monitored. Furthermore, the transformation of the fatigue life prediction issued by the proposed calculation into actual operational time may be used to determine realistic time intervals between service pauses required for the safe operation of such structures. Originality/value – The present study constitutes the first approach of embedding the SHSSC into the calculation of large-scale structures such as road tanks and verifies the applicability of this concept beyond simple geometries and load cases.

Fatigue strength improvement of steel structures by high-frequency mechanical impact: proposed fatigue assessment guidelines

In the past decade, high-frequency mechanical impact (HFMI) has significantly developed as a reliable, effective, and user-friendly method for post-weld fatigue strength improvement technique for welded structures. During this time, period 46 documents on HFMI technology or fatigue improvements have been presented within Commission XIII of the International Institute of Welding. This paper presents one possible approach to fatigue assessment for HFMI-improved joints. Stress analysis methods based on nominal stress, structural hot spot stress, and effective notch stress are all discussed. The document considered the observed extra benefit that has been experimentally observed for HFMI-treated high-strength steels. Some observations and proposals on the effect of loading conditions like high mean stress fatigue cycles, variable amplitude loading, and large amplitude/low cycle fatigue cycles are given. Special considerations for low stress concentration details are also given. Several fatigue assessment examples are provided in an appendix. A companion paper has also been prepared concerning HFMI equipment, proper procedures, safety, training, quality control measures, and documentation has also been prepared. It is hoped that these guidelines will provide stimulus to researchers working in the field to test and constructively criticize the proposals made with the goal of developing international guidelines relevant to a variety of HFMI technologies and applicable to many industrial sectors. The proposal can also be used as a means of verifying the effectiveness of new equipment as it comes to the market.

Fatigue strength improvement of steel structures by high-frequency mechanical impact: proposed procedures and quality assurance guidelines

High-frequency mechanical impact (HFMI) has emerged as a reliable, effective, and user-friendly method for post-weld fatigue strength improvement technique for welded structures. During the past decade, 46 documents on HFMI technology for fatigue improvements have been presented within Commission XIII of the International Institute of Welding (IIW). This paper presents an overview of the lessons learned concerning appropriate HFMI procedures and quality assurance measures. Due to differences in HFMI tools and the wide variety of potential applications, certain details of proper treatment procedures and quantitative quality control measures are presented generally. Specific details should be documented in an HFMI procedure specification for each structure being treated. It is hoped that this guideline will provide a stimulus to researchers working in the field to test and constructively criticize the proposals made with the goal of developing international guidelines relevant to a variety of HFMI technologies and applicable to many industrial sectors. A companion document presents a fatigue design proposal for HFMI treatment of welded steel structures. The proposal is considered to apply to steel structures of plate thicknesses of 5 to 50 mm and for yield strengths ranging from 235 to 960 MPa. Stress assessment may be based on nominal stress, structural hot spot stress, or effective notch stress.

An Enhanced Experimental Facility for Durability and Performance Characterisation of Piezoelectric Transducers for Structural Health Monitoring

With any structural health monitoring (SHM) system, verification of the health of the sensing elements is essential in ensuring confidence in the measurements furnished by the system. In particular, SHM systems utilised for structural hot spot monitoring applications will conceivably require transducers to operate reliably after sustained exposure to severe mechanical loading. Consequently, a good understanding of the long term mechanical durability performance of structurally integrated piezoelectric transducers is vital when designing and implementing robust SHM systems. An experimental facility has been developed at the Australian Defence Science and Technology Organisation (DSTO) capable of performing an autonomous long-term mechanical durability test on piezoceramic transducers. The Autonomous Mechanical Durability Experimentation and Analysis System (AMeDEAS) incorporates a general purpose data acquisition program controlling up to three 8-channel relay multiplexers and two instruments. AMeDEAS is highly flexible, allowing user-specified channel configurations and automatic interrogation of selected instruments. The system also interfaces with the uni-axial mechanical testing machine to provide control of the load sequence allowing transducer elements to be interrogated under stable load-free conditions after being subject to a predefined loading regime. AMeDEAS was used to investigate the fatigue characteristics of a low-profile layered piezoceramic transducer package developed by DSTO. A total of 16 transducers were tested under tension-dominated cyclic loading with peak-to-peak strain amplitude increasing from 400 με to a maximum of 3000 με, with periodic acoustic transduction efficiency and electromechanical impedance measurements taken throughout the test. This paper details the AMeDEAS and includes preliminary results which confirm the efficacy of the new facility.

Fatigue Test and Simulation Research of Rib-to-Diaphragm Welded Connection

Binhua Wang, Pengmin Lu, Yuhong Shao Key Laboratory for Highway Construction Technology and Equipment of Ministry of Education, Chang’an University, Xi’an 710064, Shaanxi, China Email: wangbh@chd.edu.cn Abstract In order to minimize the restraint provided by the floor-beam web to rib rotation at support due to unsymmetrical loading in orthotropic decks, the structure of cutout holes are adopted at the place of rib-to-diaphragm welded connection. However, there is a large number of welding joint’s cracks formed at the edge of cutout holes. In this study, the comparative specimens with two types of cutout holes are designed, i.e. the circular arc transition structure and the vertical transition structure. And the fatigue life estimation of specimens is investigated with the application of the structural hot spot stress approach by use of finite element analyses. The results of the finite element calculations are compared with the results of the fatigue tests which are carried out on these full-scale specimens.

Fatigue assessment of high frequency mechanical impact (HFMI)-improved fillet welds by local approaches

Local fatigue assessment methods like the structural hot spot stress and effective notch stress methods as defined by the International Institute of Welding are widely used by design engineers and researchers to assess the fatigue strength of welded components. This paper provides a comprehensive evaluation of published data for welded joints which had been improved using high frequency mechanical impact (HFMI) treatment. All of the published data for HFMI-treated welds are presented in terms of nominal stress. The goal of the current paper is to establish local fatigue assessment procedures for improved fillet welds. In total, 160 published experimental results for longitudinal and cruciform welds subjected to R=0.1 axial loading are evaluated. Local stress quantities for each joint were assessed based on the finite element analyses and reported nominal stress values. A correction procedure for yield strength that was previously verified for nominal stress-based fatigue assessment is also applied to the local stress methods studied in this paper. For both the structural hot spot stress and effective notch stress methods, sets of characteristic fatigue strength curves as functions of yield strength are proposed and verified. The structural hot spot stress method includes one set of fatigue strength curves for load-carrying welds and a second set for non-load carrying welds. The effective notch stress method includes a single set of curves for all welds. All of the design curves proposed in this study are conservative with respect to available fatigue test data.