Steel Fastener Research Articles

Specified pretension values and installation parameters for 2205 and 17–4 PH condition H1150 stainless steel fastener assemblies

In many areas of the United States, corrosion is a common cause of steel bridge deterioration. This has caused some state agencies to use stainless steel for bridges in very corrosive environments. However, most steel bridges in the United States contain numerous slip-critical bolted connections, and stainless steel bolts have not historically been used in slip-critical connections. Therefore, there is limited or no guidance on the specified pretension to which stainless steel bolts can be installed. There is also limited or no guidance on the installation parameters, such as the nut rotation for the turn-of-nut method or initial torque and nut rotation for the combined method of installation. In this project, torqued tension tests were conducted on 128 ASTM A1082 UNS S32205 (2205) and 128 ASTM A1082 UNS S17400 Type 630 Condition H1150 (17–4-H1150) stainless steel fastener assemblies. All bolts tests were 7/8 in. in diameter and ranged from 2 to 5.5 in. in length. Multiple types of lubricants were used in the testing to evaluate their effectiveness. Test results were used to improve upon an existing method to determine the specified pretension and installation parameters of each fastener assembly type, depending on strength, ductility, and lubricant criteria. Analysis determined that 2205 fastener assemblies can be installed to a specified pretension of 80% of their ultimate strength while 17–4-H1150 fastener assemblies can be installed at their yield strength. Nut rotation values from snug for the turn-of-nut installation method and initial torque range and nut rotation values for the combined installation method are also provided in the paper.

Stress-Affected ORR on Stainless Steel

Oxygen reduction reaction (ORR) is an important cathodic reaction that often drives galvanic corrosion. In atmospheric environments, this reaction is responsible for accelerated corrosion from enhanced transport associated with discontinuous electrolytes. In load bearing structures, galvanic couples are subject to mechanical stresses that often necessitates these dissimilar metal unions. Greater understanding of mechanical stress effects on fundamental reactions of galvanic corrosion, such as ORR, is important from a forecasting perspective for improved prediction of component lifetimes and service requirements. Furthermore, extension of this behavior to confined, concentrated electrolytes is important to understand stress effects in atmospheric environments.This work reports the impact of tensile stress upon oxygen reduction reaction (ORR) for stainless steel fastener materials. Chronoamperometry was used to observe and quantify the influence of tensile stress profiles on ORR under NaCl droplet electrolytes. NaCl was utilized as a seawater proxy at concentrations of 0.6 M NaCl and 4.6 M NaCl, the latter corresponding to 80% relative humidity, which is near the deliquescence point for NaCl. The ORR current was shown to increase with tensile stress and displayed reversible behavior with simple stress profiles. Deoxygenated experiments were performed to isolate ORR contributions from oxide dynamics. Details of current transients during strain ramping are used to demonstrate these features are distinct from the ORR current shift observed during static tensile stress. Polarization experiments were performed during static tensile holds in the elastic and plastic regime for greater insight of stress effects on reduction kinetics. EIS and Mott-Schottky experiments were conducted to gain insight into the oxide behavior.

Head Pull-Through strength of pneumatically driven steel pins in wood structural panel sheathing

Wood structural panels may be attached to cold-formed steel framing with screws, pins, or staples. When subjected to uplift loads, induced fastener tension may result in connection failure due to pull-out from the steel framing or fastener head pull through the wood structural panel. For steel pins, no codified design provisions exist for the pin pull-through limit state in CFS construction. In earlier published work, an equation that accounted for pin head bearing and shank shear friction was proposed to estimate pull-through strength. In the study reported here, 102 rod punch-through tests were conducted to investigate head bearing strength of pins in wood structural panels, and manufacturer data was reviewed to propose adjustments for knurl pattern. Based on the data, a previously proposed equation was revised/updated. Predictions based on the revised equation were compared to test data and the comparison showed that the revised equation gave reasonably reliable strength estimates.

Vibroacoustic response of steel–timber composite elements

With the increasing demand for sustainable construction and, at the same time, for larger spans in buildings and high-rise buildings, composite timber structures are gaining more attention, and steel–timber hybrid elements offer a promising approach. Therefore, this study analyzes a novel steel–timber composite floor structure experimentally and numerically. The investigations focus on the vibrational behavior of the structures while considering nonlinearities due to frictional effects in joints. First, eigenfrequencies, damping properties, and consequently, the vibrational characteristics of the systems are determined. Then, detection of nonlinearity follows, investigating amplitude-dependent stiffness and damping and the occurrence of higher harmonics. Results reveal nonlinearities indicated by the occurrence of higher-order harmonics and slight amplitude-dependent damping. Furthermore, the influence of parameters, such as steel thickness and fastener type, on the vibrational behavior of the structures is investigated, comparing specimens of various configurations. Finally, the experimental results validate a linear Finite Element model using the commercial software ANSYS as a first approach despite the nonlinearities that occurred during the measurements. Nonetheless, further investigations could deal with nonlinear models to get more detailed insight.

Reverse Manufacturing and 3D Inspection of Mechanical Fasteners Fabricated Using Photopolymer Jetting Technology

Polyjet additive manufacturing is gaining attention owing to its ability to manufacture intricate parts with microscopic resolution. This study investigates the spatial precision of mechanical fasteners utilizing the 3D scanning technology followed by reverse engineering to obtain the accuracy of polyjet fabricated components. Numerous M8 bolts, as well as nuts, are additively manufactured to replace metal fasteners in various industrial fields such as aerospace, medical, electronics, automotive, and food packaging. M8 mild steel fastener was 3D scanned using Carl Zeiss COMET L3D2 3D scanner, and precise details of the products were gathered. Three-dimensional scanning captures a large number of surface points, resulting in a more accurate representation of the object. The gathered data were used to fabricate numerous nuts and bolts utilizing the Object 260 Connex 3 printer utilizing Vero thermoplastic polymer resins. The stability of the 3D printed specimens was investigated using Carl Zeiss COMET L3D2 3D scanner. The fabricated M8 fasteners’ dimensional error is investigated in all orientations and every axes, and the respective dimensional variations are shown in detail. The high-quality production of fasteners through photopolymer jetting met all accuracy criteria and fit within the IT 06 transition fit grade.

The Effect of Corrosion Location Relative to Local Stresses on the Fatigue Life of Geometrically Complex, Galvanically Corroded AA7075-T6

Aluminum components used in aerospace structures are commonly coupled with stainless steel fasteners. These through-hole geometries on the aluminum substrate cause a concentrated stress field. The high-stresses at the fastener sites can preferentially initiate coating damage allowing for moisture ingress, which can lead to the formation of a galvanic couple between the aluminum alloy and the stainless steel fastener. Corrosion damage is known to cause early initiation of fatigue cracks, thus severely reducing the total life of the component. This work aims to understand the relative impact and interaction of fastener hole geometry-induced stress concentrations and corrosion damage on the fatigue crack initiation behavior and total fatigue life. Specifically, by imparting various levels of corrosion severities at different locations within the macro-scale stress field, the relative impact of each on the fatigue process can be determined. This work demonstrated a dominant role of the macro-scale stress field on the ability of corrosion morphologies to initiate fatigue cracks. Specifically, crack formation was found to preferentially occur at high-stress regions in lieu of forming at lower-stress regions, regardless of corrosion severity, and corrosion severity in the through-hole had a significant, but nonpredictive, correlation with the total fatigue life. Critically, the findings of this work will inform the means by which coatings are evaluated and will serve as a controlled validation of experiments for fracture mechanics modeling.

3D-printed thermoplastic composite fasteners for single lap joint reinforcement

This study presents findings for the strength and failure mechanism of a 3D-printed Continuous Carbon Fibre reinforced Onyx (CCF/Onyx) Thermo-Plastic Composite Fastener (TPCF) and a single lap-joint (SLJ) made of fibre/polymer composite reinforced by the TPCF. The study was carried out by numerical analysis and experiment methods including test sample design, manufacturing process and mechanical test. The 3D-printed fasteners were manufactured and tested in shear mode for two types of joining arrangement: fastened and hybrid bonded/fastened joints. Firstly, experiment was carried out for the TPCF fastened SLJ and the results show that addition of CCF in the Onyx matrix and post heat-treatment process could significant enhance the TPCF strength. The results was then benchmarked against a SLJ with steel fastening. The shear failure load of the SLJ reinforced by heat-treated CCF/Onyx TPCF of 8 mm diameter was 36% lower than a SLJ reinforced by a steel bolt of the same size. Numerical model for progressive damage simulation was also created based on the failure theory from Puck and Schürmann achieving good correlation with the experimental data. Secondly, the TPCF fasteners were manufactured with two types of heat-treated countersunk head and pan head forming and used to reinforce bonded SLJ. The test results show that the bonded SLJ reinforced by the TPCF fastener of countersunk head is of 11.7% higher strength and an increase in ultimate deformation by 9.1% compared to a bonded SLJ reinforced by steel fastener of 5 mm diameter. From the numerical and experimental study, it was noted that this was attributed to countersunk configuration to reduce out-out-plane bending and provide better crack arresting for the joint bonding.

Optimization of TIG Process Parameter in Improving Mechanical Properties of S304 Stainless Steel Using a Grey Based Taguchi Method

Tungsten inert gas welding is popular known welding technique for ferrous & nonferrous. Stainless steel grade 3HQ (S30430) is a specialized wire grade with very wide usage for manufacturer of stainless steel fastener. It has now totally replaced Grade 384 and 305 for heading application. The stable austenitic structure makes 302HQ nonmagnetic, even after substantial cold work, and also results in excellent toughness, even down to cryogenic temperatures. This paper attempts in optimizing the Tungsten Inert Gas (TIG) welding process parameter. The effect of various parameters and their influence is important to determine the strength of welded joint. To obtain a good quality weld, it is therefore, essential to control the input welding parameters. Therefore appropriate selection of input welding parameter is necessary in order to obtain a good quality weld and subsequently increase the productivity of manufacturing industry. This paper present multi objective optimization using grey relation analysis (GRA) for S30430 with TIG process to determine the suitable selection of parameters Experiment were conducted according to Taguchi's design of experiments (DOE) with orthogonal array L9 is used, mathematical model was developed using parameters such as speed (mm/min), current (Amp), voltage (V), depth of penetration (mm). After conducting experiment and collecting data, signal to noise ratio were determined by using Minitab18 and it is used to obtain optimum level for every input parameter.

Numerical modelling of power-actuated fastener connections joining high-strength steel sheet to mild steel plate subjected to monotonic shear

This paper explores the use of the finite element software package ABAQUS/Explicit to simulate the behaviour and strength of power-actuated fastener (PAF) connections joining cold-formed steel sheet to hot-rolled steel plate subjected to monotonic shear loading. The finite element (FE) models are developed for the simulation of the entire process including both installation and loading stages. The fastener is firstly driven into the steel materials to consider the initial plastic deformations around the contacts between steel plates and fastener caused by dynamic impact. Subsequently, monotonic shear loading is applied to the PAF connections until failure by a displacement control at a low speed. Various aspects of the FE models such as fracture model of the materials, loading rate, mesh sensitivity are calibrated to optimise the computing time and improve the accuracy of the FE models. The simulations are validated against experiments conducted in a separate study by the authors, and good agreement between the test and numerical results is achieved. The accurate and reliable FE models in this study are able to simulate and provide insights into the shear behaviour of the power-actuated fastener connections in terms of initial stiffness, maximum shear capacity, failure modes as observed in the experiments (i.e., bearing, shear fracture, and pull-out of the fastener), ductility, and damage patterns of the connectors. It is demonstrated that FE analysis can therefore be used to extend experimental data in a parametric study and to optimise the design of PAF connections in shear.

Multi-frequency fusion algorithm for detection of defects under fasteners with EC-GMR probe data

Rotating eddy current (EC) excitation with giant magnetoresistive (GMR) sensor pick up (EC-GMR) has proved to be a promising technique for detection of defects under fasteners in multi-layer riveted structures. Challenges in this application include arbitrary orientation of cracks, influence of dominant signal from steel fasteners and need for rapid inspection of parts. This paper presents a novel data fusion algorithm for suppressing signals from steel fastener and enhancing defect indications in multi-frequency EC-GMR measurements. The algorithm fuses transformed versions of EC-GMR probe data using a multi-dimensional constrained optimization procedure based on Sequential Least Square Programming (SLSQP). The optimal transformation parameters are determined by minimizing the field residue at a reference fastener site with no defects. Performance of the proposed algorithm is evaluated on experimental measurements acquired using EC-GMR sensor on a riveted lap-joint with steel fasteners and second layer EDM notches. Initial results demonstrate that bottom layer defects of different sizes and orientations can be successfully detected with this approach.

Corrosion resistance of composite coating used for fastener protection

Purpose The purpose of this paper is to examine the performance of a fastener composite coating system, sherardized (SD) coating/zinc-aluminum (ZA) coating whether it has good performance in marine environment. Design/methodology/approach In this paper, SD coating was fabricated on fastener surface by solid-diffusion method. ZA coating was fabricated by thermal sintering method. Corrosion behaviours of the composite coating were investigated with potentiodynamic polarization curves, open circuit potential and electrochemical impedance spectroscopy methods. Findings Neutral salt spray (NSS) and deep sea exposure tests revealed that the composite coating had excellent corrosion resistance. Polarization curve tests showed that corrosion current density of the sample with composite coating was significantly decreased, indicating an effective corrosion protection of the composite coating. OCP measurement of the sample in NaCl solution demonstrated that the composite coating had the best cathodic protection effect. The good corrosion resistance of the composite coating was obtained by the synergy of SD and ZA coating. Practical implications SD/ZA coating can be used in marine environment to prolong the life of carbon steel fastener. Social implications SD/ZA composite coating can reduce the risk and accident caused by failed fastener, avoid huge economic losses. Originality/value A new kind of composite coating was explored to protect the carbon steel fastener in marine environment. And the composite coating has the long-term anti-corrosion performance both in simulated and marine environment test.

Experimental study on bolted and anchored beam-to-column joints of prefabricated concrete frames

A new type of beam-to-column joint used in prefabricated concrete frames was proposed in this study. In this joint, the longitudinal bars at the top of the beam are anchored to the column using straight thread sleeves, and the bars at the bottom are welded to the steel fastener that is bolted to the column. Cyclic loading tests of three specimens, namely, two beam–column joints of this type and a cast-in-place beam–column joint, were conducted to study the seismic behavior and feasibility of this type of joint. The difference between the two prefabricated joints is the shape of the holes on the end plate. Failure modes of the specimens were observed and analyzed. The hysteretic curves, bearing capacities, stiffness degeneration, ductility, and energy-dissipating capacities of the specimens were compared and studied. Test results indicated that all beam–column joints exhibited beam hinge failure. No slippage was observed between the concrete and horizontal plates of the steel fasteners used in the new type of joint. The bearing capacity and initial stiffness of both prefabricated specimens compared with the cast-in-place ones were increased. The steel fastener could increase the distance between the plastic hinge and the side surface of the column while enlarging the length of the plastic hinge. The trend of energy dissipation and stiffness degeneration of the specimens were similar, and the ductility coefficient ranged from 2.7 to 4.91. The displacement angles of the joints exceeded 1/50 before the failure of the specimens. The mechanical behavior of both prefabricated joints was similar, but the joint with U-shaped holes on the end plate was convenient to create.

Grey Based Taguchi Method for Optimization of TIG Process Parameter in Improving Tensile Strength of S30430 Stainless Steel

Tungsten inert gas welding is popular known welding technique for ferrous & nonferrous. Stainless steel grade 3HQ (S30430) is a specialized wire grade with very wide usage for manufacturer of stainless steel fastener. It has now totally replaced Grade 384 and 305 for heading application. The stable austenitic structure makes 302HQ nonmagnetic, even after substantial cold work, and also results in excellent toughness, even down to cryogenic temperatures. This paper attempts in optimizing the Tungsten Inert Gas (TIG) welding process parameter. The effect of various parameters and their influence is important to determine the strength of welded joint. To obtain a good quality weld, it is therefore, essential to control the input welding parameters. Therefore appropriate selection of input welding parameter is necessary in order to obtain a good quality weld and subsequently increase the productivity of manufacturing industry. This paper present multi objective optimization using grey relation analysis (GRA) for S30430 with TIG process to determine the suitable selection of parameters Experiment were conducted according to Taguchi’s design of experiments (DOE) with orthogonal array L9 is used, mathematical model was developed using parameters such as speed (mm/min), current (Amp), voltage (V), depth of penetration (mm). After conducting experiment and collecting data, signal to noise ratio were determined by using Minitab18 and it is used to obtain optimum level for every input parameter.

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

Localized Corrosion Damage Morphology and Corrosion Electrochemistry for Al-Zn-Mg-Cu and Stainless Steel 316 Fastener Galvanic Couples in Simulated Marine Environments

Introduction: Aluminum alloy 7050-T7451 (Al-Zn-Mg-Cu) is often used for aerospace applications because it provides an advantageous combination of strength, stress corrosion cracking (SCC) resistance, corrosion resistance, and fracture toughness1. Grain boundary precipitation of MgZn2- η influences the corrosion behavior by locally depleting Zn, Mg, and Cu, especially in the T6 temper2, which can enable intergranular corrosion (IGC), intergranular stress corrosion cracking (IGSCC) and exfoliation corrosion 3-4. Cu content is important, as the composition of both the matrix and η changes with aging, decreasing the potential window between Mg(Cu)Zn2 dissolution and the matrix pitting or repassivation potential; this decreases IGC and exfoliation susceptibility6. However, incongruent dissolution of Cu-rich phases may leave a Cu-rich surface that is cathodic to the matrix7-9. Defects in the corrosion protection system are commonly due to the harsh environment operating conditions of aircraft and are enhanced at complex joining/fastener locations which trap electrolyte into tight crevices leading to an occluded local environments where a galvanic cell is established between the aluminum alloy and a high strength steel fastener.12 While the danger of galvanic corrosion has been recognized, little work has been done to measure the extent of corrosion damage and characterization of the damage morphology of AA7050 and SS.1-13 However, there is a lack of literature on the unique factors and environments present in confined spaces such as a rivet hole and furthermore how these factors and environments effect the overall damage morphology. Previous work limits our understanding of damage morphologies to a few specific test environments such as marine seacoast atmospheric exposures, lab accelerated test environments, full immersion environment such as in 0.6 M NaCl, EXCO solution and 0.6 M NaCl + H2O2. There has been very little detailed investigation of crevice and rivet environments formed in a marine or simulated marine atmospheric solutions. The goal of this work is to utilize a fastener configuration to interrogate the electrochemical, microstructural, and physical factors that govern galvanic induced local corrosion pit morphology development at both the meso- and micro-length scales. Experimental: Multiple techniques were used to understand the macro-scale and mirco-scale corrosion including utilizing a microelectrode array instrumented in a fastener geometry, synchrotron x-ray tomography, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and other diagnostic electrochemical methods in full immersion and under thin film conditions to elucidate key kinetic factors responsible for corrosion behavior. Results: Synchrotron x-ray tomography (XCT) was used to monitor real time operando corrosion measurements on a simulated rivet geometry under a NaCl droplet to enable 3D assessment of corrosion damage and the associated anodic charge. A horizontal slice through an X-ray tomograph is shown in Figure 1 (a) for the NaCl exposure. Scanning electron microscopy (Figure 1 (c)) and the reconstructed 3-D corrosion model (Figure 1 (d)) showed that the fissures in the simulated fastener were intragranular. This was further investigated using electron backscatter diffraction (Figure 1 (b)). The areas of corrosion for each individual fissure developed in the simulated rivet were tracked over time and depth enabling determination of damage volume and anodic charge. This enabled a comparison of the theoretical maximum available cathodic charge associated with the cathodic reactions on the stainless steel and other sources over the range of galvanic couple potentials found in potential distribution modeling. The stainless steel pin and exposed constituent particles, uncovered during corrosion, both supported and enabled the growth of multiple fissures. This suggests that cathodically limited fissure growth is not so severe of a constraint as to confine corrosion to the rivet hole mouth as well as the growth of a single fissure during the exposure. To test this hypothesis a multi-electrode array galvanically coupled was utilized to investigate sources of cathodic reactions and anodic sites continuously. Acknowledgments: This work was funded by ONR under the contract ONR: N00014-14-1-0012 with William Nickerson as contract manager. X-ray data were collected with assistance from Sarah Glanvill, Andrew du Plessis and Weichen Xu of the University of Birmingham and Aaron Parsons and Trevor Rayment of Diamond Light Source.

Operando Assessment of Galvanic Corrosion Between Al-Zn-Mg-Cu Alloy and a Stainless Steel Fastener Using X-ray Tomography

Operando x-ray tomography was used to study the galvanic corrosion between aluminum alloy 7050-T7451 and Type 304 stainless steel using a simulated fastener covered with droplets of either 4 M NaCl or 2 M MgCl2. The simulated rivet was an x-ray transparent cylindrical 1 mm pin with a 250 μm diameter rivet hole in the center. The rivet hole was embedded with a 500 μm, in length, stainless steel wire. The rivet hole formed an alkaline crevice between the stainless steel and AA7050-T7451 owing to the proximity of anodic and cathodic sites. The corrosion fissures followed an intragranular path and did not follow bands of intermetallic particles. The x-ray tomography indicated that multiple corrosion fissures propagated over the galvanic couple potential range at all depths examined suggesting the presence of multiple strong proximate cathodes. The volume loss was converted to anodic charge using Faraday’s Law. Cathodic reaction rate studies were conducted on Type 316 stainless steel, copper replated on AA7050, pure Cu, and other matrix secondary phases in simulated crevice environments to assess the ability of each to support fissure corrosion. The substrates for fast cathodic reactions capable of supporting the growth of these fissures were found to be the stainless steel fastener, copper replating on the AA7050-T7451 surface, and dealloyed S-phase. Moreover, removal of the stainless steel fastener stifled fissure growth. The damage morphology assessed with x-ray tomography was compared to laboratory-produced accelerated exposures, as well as field data, and were found to have similar morphologies.

Evaluation of Stainless Steel Fasteners for Bolted Field Splice Connections of ASTM A1010 Corrosion-Resistant Steel Plate Girders

The Virginia Department of Transportation, Richmond, initiated this study to compare the mechanical properties, availability, and costs of stainless steel fastener materials for use with ASTM A1010 stainless steel plate. The investigation focused on fastener materials included in ASTM A193 and compared them with ASTM A325 bolts. The ASTM A193 bolts tested were the B6, B8, and B8M. Test results indicated that the ASTM A193 B8 fasteners provided the most economic combination of mechanical strength, corrosion resistance, and cost. Uniaxial tension tests and Skidmore–Wilhelm rotational capacity tests revealed that the B6 fasteners had high strength but lower ductility, whereas the B8M fasteners had lower strength but higher ductility. The B8 fastener had an ideal combination of strength and ductility. The mechanical performance of the ASTM A193 fasteners was improved further by the use of hardened washers. Because the bolts are hot forged, sensitization, which can reduce corrosion resistance, was of concern. Standard testing indicated that the B8 and B8M as-received bolts were unsensitized. The cost of the B8 fasteners compared favorably with that of other stainless steel fasteners. Further, the B8 fasteners are available from manufacturers that comply with Buy America requirements. The cost of stainless steel fasteners is higher than that of ASTM A325 steel fasteners. As their use increases, however, larger orders may help reduce prices. The initial research indicates that stainless fasteners could be a structurally sound option to use to build reduced maintenance bridges.

Research on the Optimization Design of Purlin Column Connection Joints of Crossing Bracket Type Wooden Structure Building in Ba-yu Region

Crossing bracket type framework is widely applied in the wooden structure buildings in Ba-yu region. The purlin is often put in the groove on top of the chapiter. This connection format is not suitable in consideration of satisfying the security of modern architecture. This research focuses on the strengthening and optimization design of purline column connection joints and proposes a steel fastener of purlin column based on existing joint mode of purlin column to prevent the falling of purlin in the loading process. Finite element analysis and simulation software Ansys Workbench to build the model of wooden structure buildings and connection joints. The material properties are defined referring to the data result of wood experiment and the horizontal seismic load is applied along the longitudinal direction of purlin. Calculation analysis is conducted on the purlin column connection joint model after optimization. Through the contrastive analysis before and after the optimization, this steel fastener does not need to open up mortise and tenon joint on the wooden members. The effective bearing capacity of members has certain extent of enhancement while maintaining the integrity of wood, thus contributing to the enhancement of safety factor of the whole structure.

Galvanic Corrosion of AA7050-T7451 and 316L Stainless Steel in a Simulated Fastener

Introduction: Aluminum alloy (AA) 7050-T7451 (Al-Zn-Mg-Cu, UNS A97050) is often used for aerospace applications because it provides an advantageous combination of strength, stress corrosion cracking (SCC) resistance, corrosion resistance, and fracture toughness 1. Grain boundary precipitation of MgZn2- η influences the corrosion behavior by locally depleting Zn, Mg, and Cu, especially in the T6 temper2, which can enable intergranular corrosion (IGC), intergranular stress corrosion cracking (IGSCC) and exfoliation corrosion 3. The T74 temper has been overaged to suppress IGC and SCC on grain boundaries4 5. Cu content is important, as the composition of both the matrix and η changes with aging, decreasing the potential window between Mg(Cu)Zn2 dissolution and the matrix pitting or repassivation potential; this decreases IGC and exfoliation susceptibility6. However, incongruent dissolution of Cu-rich phases may leave a Cu-rich surface that is cathodic to the matrix7. This may increase general and local corrosion. 8 Fastener holes and complex joining locations are particularly vulnerable towards corrosion because their geometry inherently aids trapping/wicking of the electrolyte into tight crevices and produce thin films with high cathodic reaction rate9. Few studies have incorporated all factors present at a rivet hole that could affect corrosion damage morphology evolution and studied by operando observation including attempts to quantify sources of cathodic current and charge. The importance of micro-galvanic coupling, induced by the constituent particles compared to meso or macro-scale galvanic coupling provided by the stainless steel fastener towards the total accumulated corrosion damage observed is unclear. Such damage can transition to fatigue. 10 The goal of this work is to utilize a simulated fastener configuration to interrogate the electrochemical, microstructural, and physical factors that govern galvanic induced local corrosion pit morphology development at both the meso- and micro-length scales. The macro and micro topography of corrosion and controlling microstructural factors are examined in a separate study. Experimental: Synchrotron x-ray tomography (XCT) was used to monitor real time operando corrosion measurements on a simulated rivet geometry under a NaCl droplet to enable 3D assessment of corrosion damage and the associated anodic charge 11 12. Results: A horizontal slice through an X-ray tomograph is shown in Figure 1 for the NaCl exposure. Scanning electron microscopy and the reconstructed 3-D corrosion model showed that the fissures in the simulated fastener were intragranular (Figure 1). The areas of corrosion for each individual fissure developed in the simulated rivet were tracked over time and depth allowing determination of damage volume and anodic charge (Figure 2). This enabled a comparison of the theoretical maximum available cathodic charge associated with the cathodic reactions on the stainless steel and other sources over the range of galvanic couple potentials found in potential distribution modeling (Figure 3)13. The stainless steel pin and exposed constituent particles, uncovered during corrosion, both supported and enabled the growth of multiple fissures. This suggests that cathodically limited fissure growth is not so severe of a constraint as to confine corrosion to the rivet hole mouth as well as the growth of a single fissure during the exposure. Acknowledgement: This work was funded by the Office of Naval Research under the contract ONR: N00014-14-1-0012 with William Nickerson as contract manager. X-ray data were collected and analyzed with assistance from Sarah Glanvill, Andrew du Plessis and Weichen Xu of the University of Birmingham and Aaron Parsons and Trevor Rayment of Diamond Light Source. (1) Knight, S. P.; Birbilis, N.; Muddle, B. C.; Trueman, A. R.; Lynch, S. P. Corros Sci 2010, 52, 4073. (2) Buchheit, R.; Martinez, M.; Montes, L. J Electrochem Soc 2000, 147, 119. (3) Ramgopal, T.; Gouma, P. I.; Frankel, G. S. Corrosion 2002, 58, 687. (4) Birbilis, N.; Buchheit, R. G. J Electrochem Soc 2008, 155, C117. (5) Birbilis, N.; Buchheit, R. G. J Electrochem Soc 2005, 152, B140. (6) Ramgopal, T.; Schmutz, P.; Frankel, G. S. J Electrochem Soc 2001, 148, B348. (7) Vukmirovic, M.; Dimitrov, N.; Sieradzki, K. J Electrochem Soc 2002, 149, B428. (8) Holroyd, N. Environment-Induced Cracking of Metals (Houston, TX: NACE International, 1990) 1990, 311. (9) Davis, J. R. Corrosion of aluminum and aluminum alloys; ASM International: Materials Park, OH, 1999. (10) Burns, J. T.; Larsen, J. M.; Gangloff, R. P. Fatigue & Fracture of Engineering Materials & Structures 2011, 34, 745. (11) Rafla, V.; King, A. D.; Glanvill, S.; Parsons, A.; Davenport, A.; Scully, J. R. Corrosion 2015, 71, 1171. (12) Rafla, V.; Davenport, A.; Scully, J. R. Corrosion 2015, 71, 1300. (13) Liu, C.; Rafla, V.; Scully, J. R.; Kelly, R. G. In CORROSION/2015 C2015-5579 Dallas TX., 2015. Figure 1

Magnetoresistive Sensor With Magnetic Balance Measurement for Inspection of Defects Under Magnetically Permeable Fasteners

Magnetoresistive (MR) sensors in conjunction with low-frequency eddy current can be used for detecting subsurface defects under fasteners in aluminum multilayer airframe structures. Frequently, the fasteners are made of magnetically permeable material, e.g. steel, which can be magnetized resulting in a strong remanence. Strong residual magnetism can shift the work point of MR sensor outside of its linear range. The limited measureable range of MR sensor presents additional challenges, since the amplitude of the steel fastener signal is much greater than the defect signal. This paper shows that one can employ magnetic balance measurement (MBM) method to inspect defects under permeable fasteners using MR sensor. The MBM method is based on proportional–integral–derivative feedback control mechanism. The operational principle of MBM was numerically mimicked using a Simulink model. Calculation results show that the bias point of the MR sensor in MBM is kept invariant, so the effect of the strong remanence field is eliminated and the operating point of the sensor on the linear region of its characteristics stays constant. Furthermore, the measurement range of the MR sensor was extended 21 times with given parameters without sacrificing sensitivity. A prototype was built and tested on a two-layer aluminum sample with steel fasteners and machined subsurface notches. Experimental results show that MBM improves the performance of MR sensor for defect inspection around magnetized steel fastener.