Metal Fasteners Research Articles

Galvanic Corrosion Between Coated Al Alloy Plate and Stainless Steel Fasteners, Part 1: FEM Model Development and Validation

Aerospace structures often involve dissimilar materials to optimize structural performance and cost. These materials can then lead to the formation of galvanic couples when moisture is present. Specifically, noble metal fasteners (such as SS316) are often used in aluminum alloy load-bearing structures, which can lead to accelerated, localized corrosion attack of the aluminum alloy due to the cathodic current supplied by the SS316 fastener. This localized attack is difficult to predict, and tests are often expensive, so modeling of these galvanic couples could be of great utility. The work reported here focuses on the galvanic coupling between fasteners installed in a panel test assembly, and the resultant corrosion damage down the fastener holes. This arrangement is a common assembly geometry in aerospace applications. A specific sol-gel coating was applied to the fasteners, to determine its effectiveness on mitigating galvanic corrosion; bare fasteners were also tested, to investigate a worst-case scenario. Geometric constraints in the model were made to match those of an experimental test panel, which was exposed to ASTM B117 salt fog for 504 h. The electrochemical boundary conditions were generated in solutions appropriate to the material and environment to which it would be exposed. Anodic charge passed during exposure was calculated from image analyses of the corrosion damage in the experimental test, and the results were compared with the model. The Laplacian-based model provides a very good first approximation for predicting the damage within the fastener hole. Validation was provided by both experimental results generated in this study as well as comparison to results in the literature that used similar, but not identical, conditions.

Numerical Model and Simulation Analysis of Composite Stringer Docking Structures

Due to the manufacturing restrictions or design requirements, composite structures are usually connected in the form of butt joint near the wing rib or fuselage frame to solve the problem of discontinuity of load transfer. In this paper, the finite element model of butt joint structure is established by simulating the engineering model of metal fasteners and common joint connected by beam element under the action of in-plane load, and the mechanical simulation analysis of typical butt joint configuration is carried out. In the engineering model, the allowable stress value is used to predict the failure of composite material and the classical strength theory is used to predict the fracture failure of metal fasteners. The result show that the strength prediction obtained by the engineering model is in good agreement with the test results, and the application of this method can improve the calculation efficiency of connection strength with complicated multi-fastened composite structures.

Computer Modeling of the Eddy Current Losses of Metal Fasteners in Rotor Slots of a Large Nuclear Steam Turbine Generator Based on Finite-Element Method and Deep Gaussian Process Regression

Eddy current analysis is a key issue for large turbine generators. The finite-element method (FEM) is a computational tool for obtaining the electromagnetic characteristics of electrical machines. In this article, we propose a computer model of the eddy current losses of metal fasteners in the rotor slots of a large turbine generator. The electromagnetic properties of the rotor fasteners and the outer diameter of the rotor are taken as the input, and the eddy current loss of the rotor fasteners is taken as the output. A prediction model is constructed using the FEM and deep learning. The analysis results show that compared with the independent finite-element analysis, this method reduces the design cycle time and improves the design efficiency for a large-capacity turbine generator. Compared with other machine learning models, the error is smaller and the accuracy is higher. This method provides a new way to accurately predict the eddy current loss of a generator under complex nonlinear conditions.

Dynamics of heat and moisture transfer in wooden structures tied with metallic fasteners

The article confers the influence of cyclically varying temperature and humidity of the environment on the behavior of wooden truss structures with metallic fasteners. The purpose of this work was to establish a mathematical model for the dynamics of temperature and humidity fields in the wood with metallic joints that accounts for changes in the environmental parameters, such as temperature and humidity, as well as to perform experimental study of the heat and mass transfer processes in wooden structures. The boundary problems of heat and moisture transfer in the components of wooden structures tied with dowels are formulated and analytically solved using the method of integral Laplace transform. The results of experimental studies on the processes of heat and moisture transfer through the components of wood truss structures are presented as graphs for temperature and humidity distribution in the dowel joints of wooden structures. The results of this study open up prospects for further research and development of recommendations for monitoring the condition of the components of wooden truss structures.

Failure Analysis of NbVC Coating Formation onto AISI D2 Tool Steel during Thermo Reactive Diffusion by Pack Method

Threads of metal fasteners can be created by thread rolling process using specific thread dies. The quality of the thread rolling die is determined by their wear-resistant and hardness properties. In order to increase the hardness and the wear resistance of the dies, thermo reactive diffusion can be applied. The TRD process is carried out by powder-pack method in a sealed steel container containing powder mixture of niobium and vanadium as master alloy, alumina (Al 2 O 3 ) as inert filler and ammonium chloride (NH 4 Cl) as activator. The samples were covered with powder mixture and then heated in the electric resistance furnace at temperature of 950, 1000 and 1050 ˚C for 6 hours. Upon finishing the process, the sample is allowed to cool in the open air. The coating cross section was analyzed using optical microscope. The hardness measurement also performed on the same sample. XRD was utilized to investigate the coating chemical composition. The wear of the coatings are assessed using revolving tribological test under dry sliding condition. The XRD and microstructure test results did not find any niobium-vanadium carbide coating on AISI D2 substrate. In untreated and untreated specimens found chromium carbide (Cr 7 C 3 ). Untreated specimen has hardness number of 277.8 HV. After TRD treatment, the hardness number is increase. For heating temperature variation of 950°C, 1000°C, and 1050°C have hardness number of 798.21HV, 787.3HV, 774.2HV respectively. The value of hardness rises due to the hardening of the AISI D2 specimen. The highest wear resistance owned by temperature variation of 950°C and the lowest wear resistance owned by the untreated specimen.

Experimental investigations on adhesive free laminated oak timber beams and timber-to-timber joints assembled using thermo-mechanically compressed wood dowels

This paper presents an experimental investigation on a novel adhesive free engineered Wood Products (AFEWPs) as an alternative to the conventional glued EWPs which have a high degree of petrochemicals. The research outcomes demonstrate the feasibility of using compressed wood dowels (CWD) as a joint element, to connect timber laminates and members as well as to substitute for adhesives and metallic fasteners. The paper describes the production of thermo-mechanically compressed wood dowels as well as the manufacturing of the dowelled timber specimens. The strength and stiffness properties of CWD are characterized based on three-point bending tests and compared to the values obtained from uncompressed wood specimens. After that, the paper discusses the relative mechanical performances of three-layer dowelled oak beams subjected to four-point bending, in comparison to their conventional glued counterparts. Finally, the paper shows results obtained from double shear push-out tests under both monotonic and cyclic loadings, using two wood species, namely spruce and oak. The obtained results show clearly the potential of thermo-mechanically compressed wood dowel, as a joint fastener, for load bearing capacity.

Manufacturing CFRP-Parts Using a Modified Co-Curing Method – Fundamental Experimental Analysis

With the increasing utilization of fiber-reinforced polymers in various areas of application such as aviation and automotive, the need for efficient production methods appropriate for these materials also gains importance. The widespread use of metallic fasteners in conjunction with differential design leads to stress concentrations when applied to FRPs and prevents the full utilization of their mechanical properties. In an effort to develop an appropriate joining technology that also contributes to reducing tooling costs, co-curing of thermoset FRP laminates is analyzed in the present work. To gain insight into the feasibility of the use of pre-cured laminates in co-curing processes, laminates of varying degree of cure are joined with uncured laminates in a co-curing process and the mechanical properties of the resulting laminates are investigated experimentally. Based on these experimental results, an interdependence between bonding properties and the degree of cure of the pre-cured laminate can be shown.

Corrosion of Metallic Fasteners in Timber–Concrete Composite Structures

While Timber–Concrete Composite (TCC) construction has been intensively studied in terms of structural behaviour, there is little research on the risk of corrosion of the used metallic shear fasteners in different exposure environments. Due to the marked differences in chemical environments of wood and concrete, however, it can be expected that the fasteners are subject to galvanic corrosion. This was experimentally studied in the present work. TCC specimens were fabricated for electrochemical testing and exposed to two different exposure environments, that is, 95% relative humidity and immersion in water. The moisture content in the wood, the difference in electrochemical potentials and the galvanic current were monitored over 2 months. Two types of fasteners were tested, namely zinc galvanized steel fasteners as well as browned fasteners that have been widely used in the TCC construction industry since 2013. It was found that the fastener parts in the wood generally act as an anode, while the remaining parts in the concrete act as a cathode. However, significant galvanic currents were only observed upon immersion in water. For practical applications in structural engineering, it is concluded that the mass loss of metal of the studied fasteners cannot present a severe risk for structures, especially not at the service classes 1 and 2 according to EN 1995-1-1.

Safety and efficacy of spider anchoring device for apical repair in laparoscopic sacrocolpopexy: Retrospective cohort study

ObjectiveThe aim of the study was to assess safety and efficacy of apical support when using the soft-tissue metal fasteners of EndoFast Reliant™ SCP during laparoscopic sacrocolpopexy (LSCP). Study designThis is a single-center, retrospective cohort study including all patients which underwent LSCP between 2015 and 2017. Safety outcome included the procedure and the device related complications. Effectiveness outcomes included the anatomical success of apical repair and the functional results as were obtained from the clinic and from a Quality of life questionnaire (PFDI-20). ResultsEighty-four patients, 54% with grade III and IV prolapse were included in the study. Uterus was preserved in 68% of patients with uterus. Mean follow-up period for the clinic visit was 4.5 ± 4 months. Short term anatomical success was achieved for 98.8% of the patients. Marked improvement was observed in the related symptoms including urinary incontinence. Three intra-operative complications occurred: 1 case of bladder injury, 1 case of intra-abdominal bleeding, and 1 case of post-operative vaginal bleeding. Later complications included only two patients (2.3%) complained of chronic abdominal pain. There were no cases of mesh erosion or de-novo dyspareunia. The mean follow-up period for the PFDI-20 questionnaire was 15.6 months. 75% of patients had no bulge symptoms at all. Quarter of the patients had some bulge symptoms, bothers them between somewhat to moderately according to the questionnaire grading. ConclusionsThe use of Endofast SCP fasteners for laparoscopic promonto-fixation is a safe and effective surgery. Longer follow-up and larger studies should be performed to establish the results.

Copper distribution and oxidation states near corroded fasteners in treated wood

Metal fasteners are used to hold wood structures together. In outdoor applications, these fasteners are subject to corrosion when the wood is treated with certain preservative treatments. Typically, these treatments contain copper. Prior work has hypothesized that the mechanism of corrosion in treated wood involves reduction of copper ions from the wood treatments. However, copper was rarely detected in the corrosion products of metals embedded in treated wood, which contradicts the hypothesized mechanism. This present work utilizes synchrotron based X-ray fluorescence microscopy (XFM) and X-ray absorption near edge spectroscopy to examine the corrosion mechanism in treated wood by looking at the spatial distribution and oxidation states of copper in the treated wood near the fastener and in the corrosion products removed from the fastener surface. The samples were obtained after a 1-year corrosion test. In the wood cell walls, the oxidation state of the copper treatment did not change in the immediate vicinity of the fastener, although there was a depletion of copper near the fastener. Furthermore, copper was detected in the corrosion products in trace amounts using XFM. Together, these techniques confirm that the corrosion mechanism involves transport of the cupric ions to the fastener surface, where they are reduced and suggest that previous attempts to detect copper were unsuccessful because the concentration of copper in the corrosion products was below the level of detection of the previously used techniques.

Comparison of the use of angle brackets in timber joints with eurocode 5

The aim of this study is to describe the design of steel angle bracket in timber joints. The design has insufficient support in the existing standards. The load-bearing capacity of these connections can be calculated in accordance with EN 1995-1-1 Eurocode 5, Chapter 8 Connections with metallic fasteners. The main drawback is that this manual only focuses on the metallic fasteners itself. The other disadvantage is that it does not look into the question of stiffness of the joint, its deformation, which is often a problem with these connections. This issue was confirmed by experiments in this study. Eight different angle brackets have been tested in the load direction where the part of the metallic fasteners is stressed in the pull and the second part in the shear. In addition, three different angle brackets were tested in the load direction where all the metallic fasteners are stressed in shear. These experiments were compared with computational models and the calculations according to EC 5. The computational models were performed in software RFEM5. The angle brackets were modeled, nails and wood were replaced by point or flat supports. The results from computational models are comparable to experiments. However, the values given by the calculations according to EC 5 are considerably higher than the real load-bearing capacities obtained from the experiments. Therefore the use of EC 5 for this type of connections can be dangerous in practice. The differences between experiments, models and calculations indicate the difficulty of designing timber joints with three-dimensional nailing plates.

A 64-pin Nanowire Surface Fastener Like a Ball Grid Array Applied for Room-temperature Electrical Bonding

Surface-mount techniques primarily depend on soldering. However, soldering techniques have encountered some challenges in recent years. These challenges include rare metal recycling, thermal problems, and Pb toxicity. We recently developed a metallic nanowire surface fastener (NSF) to resolve the abovementioned problems. This fastener can be used to connect electronic components on a substrate at room temperature using the van der Waals force between each nanowire. This study demonstrates a 64-pin NSF that behaves like a ball grid array (BGA) for application to actual electronic devices. The adhesion strength and electrical properties of the NSF were investigated by adjusting the nanowire parameters, such as diameter, length, density (number per area), preload, and shape. The shape control of the nanowires greatly contributed to the improvement of the properties. A maximum adhesion strength of 16.4 N/cm2 was achieved using a bent, hook-like NSF. This strength was 4–5 times the value of the straight NSF. The contact resistivity was 2.98 × 10−2 Ω∙cm2. The NSF fabricated through the simple template method showed the room temperature bonding ability and adaptability to a highly ordered electrode like the BGA.

Assessing variability in the hygrothermal performance of magnesium oxide (MgO) cladding products of the Australian market

Magnesium oxide (MgO) boards are relatively new cladding materials on the Australian construction market. They are chosen primarily for their fire resisting properties and are fast replacing traditional cladding materials in newer buildings. Recent reports of water staining on MgO boards is thought to be due to the effects of relative humidity promoting corrosion of metallic fasteners. Different commercially available MgO board samples were investigated by following their behaviour in a controlled relative humidity environment and when in contact with different metallic fasteners. It is observed that water absorption is different for different products and that staining varied with the composition of the metallic fasteners. Staining and water absorption was not a problem at a low relative humidity (RH) of 75% but was evident at a high humidity of 97% for all but one of the MgO board samples. The concentration of chloride is implicated and has its origin in the binder material. These results emphasise the need for standard characterisation of commercially available MgO boards for their use in high humidity environments.

Thermal bridges of metal fasteners for aerogel-enhanced blankets

The use of silica aerogel-enhanced blankets as a thermal insulation material is one of the most promising solutions to reduce the heat loss through the building envelope. Aerogel-enhanced blankets provide exceptionally high thermal resistance, thanks to a thermal conductivity value as low as 0.014 W/mK. However, a challenge in the use of aerogel-enhanced blankets is represented by the way to connect these highly flexible panels to the backing support. This paper describes the effect of mechanical fasteners over the thermal performance of a brick wall covered with an aerogel-enhanced blanket investigated. The effective thermal transmittance of the selected aerogel-enhanced blanket due to the presence of the metal fasteners is evaluated experimentally using a calibrated chamber. The laboratory measurements are also compared with theoretical calculations conducted both for metal and plastic fasteners. Finally, some conclusions about the ways to reduce the thermal bridging effects during the installation of aerogel-enhanced blankets are provided, and the effective thermal conductivity of aerogel blankets considering different materials (plastic or metal anchors) for their fastening are reported.

Corrosion of metal fasteners embedded in acetylated and untreated wood at different moisture contents

ABSTRACT Acetylated wood is now commercially available and designed to be used in certain outdoor applications as an alternative to preservative-treated wood. Fastener corrosion can be a concern in preservative treated wood when the wood remains wet for long periods. However, little data on the corrosiveness of acetylated wood exists beyond the product literature. Here we examine the corrosiveness of commercially obtained acetylated wood and compare it against unmodified (untreated) southern pine (Pinus spp.). Corrosion rates of plain carbon steel, hot dip galvanized steel, and stainless steel were calculated gravimetrically after a one year exposed in the wood. Four different moisture conditions were examined: 90% relative humidity (RH), 95% RH, 100% RH, and a fully water saturated condition. When compared to literature data on the corrosion of fasteners in preservative treated wood at 100% RH, the acetylated wood had much lower steel corrosion rates than all preservatives examined; the measured corrosion rates for galvanized steel were lower than all preservatives except chromated copper arsenate. These measured corrosion rates across a range of moisture conditions can be used to inform the selection of appropriate corrosion resistant fasteners when building with acetylated wood.

Experimental and analytical study of an CF-PEEK Fastener all composites single-lap shear joint under static and fatigue loading

This paper presents a detailed experimental study on carbon fibre (CF) polyether etherketone (PEEK) composite fasteners designed to join conventional high performance composites (CFRP). The failure mechanisms of two CF-PEEK fasteners with countersunk heads joining two laminate plates in a single-lap configuration were investigated under static (tensile) and cyclic loading (tension–tension). The failure process of the bolted joints is described in detail using acoustic emission and microscopic cut views. For comparison the CF-PEEK fasteners were replaced by metal fasteners (Titanium) under the corresponding conditions and loadings. The experimental results are in good agreement with the newly developed “closed-form” model up to the damage point of the joints. This enhanced analytical approach is a closed-form extension of the spring-based method, where bolts and laminates are represented by an arrangement of springs and masses. The model covers in all variables influencing the joint behaviour, such as fastener position, joint material, fastener type, hole diameter, joint thickness, bolt-hole clearance, and bolt torque.

Enhancing the performance of beech-timber concrete hybrids by a wood surface pre-treatment using sol-gel chemistry

Timber-concrete composites require reliable connections between both components, which are usually obtained by metal fasteners or slots in the wood. In this study, an alternative approach is presented based on a fully glued connection in combination with a primer treated wood surface, to enhance the compatibility and the adhesion properties at the interface between beech wood and concrete. Prior to the gluing and the concrete application in a wet-on-wet process, the wood surface was functionalised with a xerogel obtained by means of a sol-gel process, consisting of two layers of silane nanofilms, with different functional groups, which are capable of undergoing further chemical crosslinking reactions with the adhesive. The coating with its functionalities allows for reducing the penetration of the epoxy adhesives into the wood structure and an additional chemical connection to the adhesive can be established. The main objective of this study was to analyse the effect of the surface treatment on the mechanical properties of such composites in 3-point and 4-point bending tests as well as push-out-tests. The results showed that the pre-treatment can improve the load bearing capacity of the timber-concrete composites, but that a ductile behaviour cannot be achieved with the tested adhesives.

Continuous monitoring of tightening condition of single-lap bolted composite joints using intrinsic mode functions of acoustic emission signals: a proof-of-concept study

Intrinsic mode functions of acoustic emission signals, extracted from signals using empirical mode decomposition, were used to characterize the contact conditions of asperities (e.g. sliding friction or collision) in the mating parts of bolted composite joints undergone flexural vibration, whereby to evaluate the tightening condition of the joints quantitatively. Specifically, the sliding friction–related intrinsic mode functions, generated in the mating parts of the two joining composite components (termed as C-C contact), were ascertained from those generated from the contacts between the joining components and metallic fasteners (termed as M-C contact), via a Hilbert–Huang transform. Subsequently, the C-C contact–related intrinsic mode functions were linked to the contact behaviors of asperities at the joining interfaces, reflecting quantitatively the degree of the residual torque of the bolted joints. The fatigue performance of the joints was further evaluated according to the changes in the energy ratios of the C-C contact–related intrinsic mode functions. Experimental results have revealed that the gross energy of acoustic emission signals is capable of evaluating the residual torque of the joints within a limited range. Vibration loosening of composite joints was found to result in an increase in the energy ratios of C-C contact–related high-frequency intrinsic mode functions, on which basis the detectability of the acoustic emission–based structural health monitoring is further improved, making it possible to evaluate the tightening condition of a bolted joint when the joint undergoes vibration fatigue. This proof-of-concept study provides a promising solution to evaluate the contact conditions of bolted composite joints during assembly and to continuously monitor the tightening condition throughout the service life of the joints using the acoustic emission technique.

Investigation of Galvanic Coupling on AA7075-T6 Navair Plate inside Fastener Hole

AA7075-T6 is widely used in aerospace structures, and often SS316L fasteners are extensively used, especially for repairs because they are relatively cheap and easy to acquire. Separately, these materials are very efficient at performing their functions. However, when placed in intimate contact (such as in a fastener/hole configuration) and exposed to marine atmospheres, a galvanic couple forms which exacerbates the localized corrosion of the AA7075-T6. In service, the AA7075-T6 is coated with a primer and organic topcoat. Thus any defects in the coating serve as anodes in the galvanic couple with an extremely unfavorable cathode:anode area ratio. In addition, the interior of the hole is often poorly coated or suffers mechanical damage during operations. The overall goal of this research is to evaluate the efficiency of the mitigation strategy of coating the cathodic material with a sol-gel coating, thereby effectively ionically isolating the stainless steel fasteners from the AA7075-T6. Titanium fasteners are also under study, as they are expected to have very limited galvanic coupling with the AA7075-T6. This characteristic made them a positive baseline to which we continually compared the stainless steel corrosion. Experimentally the interactions between the SS316 and Ti-6Al-4V bolts, washers, and nuts and the coated AA7075-T6 were assessed using a configuration designed by NAVAIR [1]. It consists of a plate with eight holes and eight fasteners. Details can be found elsewhere [1]. In the present work, the fasteners were spray coated with low quality sol-gel coating to serve as a baseline of damage accumulation. Around four of the holes for the fasteners, two scribes were made through the coating in order to assess the effects of coating defects on the corrosion morphology. These NAVAIR plates underwent ASTM B117 [2] testing for 1,344 hours. Once the plates were taken out of the testing environment, they were stripped of the primer, coating, and the fasteners were removed. The plates were then cut into sections dividing the hemispheres of the holes. The exposed holes were polished down to a 1 micron mirror finish, and Keller’s Etchant was used to highlight the grains. Cross-sectional images of the holes were acquired using the Hirox RH 8800 optical microscope. The primary focus was assessment of damage within the holes as these are the locations of highest stress. Corrosion damage is known to serve as an initiation site for fatigue cracks [3]. After the samples were taken out at 1,344 hours, it was observed that the corrosion damage in the hole in which the stainless steel fastener had been was far more severe than that in the hole in which the titanium fastener had been (see Figure 1). The most intense pitting on all of the holes occurred near the head of the fastener, indicating that the head of the fastener is throwing cathodic current not only to the surface of the plate, but also inside the hole. In the case of the stainless steel fastener, the damage was primarily long intergranular fissures, whereas the Ti-6Al-4V caused more pitting with a smaller amount of intergranular attack. Using polarization curves generated in relevant simulants of the different localized sites as boundary conditions, FEM modeling results will be presented that rationalize the type, location, and extent of the damage observed for the different noble metal fasteners as well as the expected reduction in that damage if a high-quality sol-gel coating were applied to them. References Feng, Z., & Frankel, G. S. “Galvanic test panels for accelerated corrosion testing of coated al alloys: Part 1 – Concept” Corrosion 2014; 70(1), 95–106. https://doi.org/10.5006/0907ASTM B117, “Standard Practice for Operating Salt Spray (Fog) Apparatus” (West Conshohocken, PA: ASTM International, 1994)Burns JT, Larsen JM, Gangloff RP. “Driving forces for localized corrosion-to-fatigue crack transition in Al-Zn-Mg-Cu” Fatigue Fracture Engineering Material Structures 2011; 34:745–73. doi:10.1111/j.1460-2695.2011.x Figure 1

Automatic code compliance with multi-dimensional data fitting in a BIM context

BIM-based tools can contribute to addressing some of the challenges faced by structural engineering practitioners. A BIM-based framework for the development of components that deliver Automatic Code Compliance (ACC) is presented. The structural design problems that such components solve are categorised as simple, where ACC can be implemented directly, or complex, where more advanced approaches are needed. The mathematical process of Multi-Dimensional Data Fitting (MDDF) is introduced in order for the latter, enabling the compression of complex engineering calculations to a single equation that can be easily implemented into a BIM software engineering package. Proof-of-concept examples are given for both cases: offsite-manufactured structural joists are utilised as a non-recursive example, implementing the results obtained in the manufacturer’s literature; the axial capacity of metal fasteners in axially loaded timber-to-timber connections are utilised as an example of recursive problems. The MDDF analysis and its implementation in a BIM package of those problems are presented. Finally, the concept is generalised for non-structural aspects at a framework level, and the challenges, implications, and prospects of ACC in a BIM context are discussed.