Steel Rivets Research Articles

Safety of Aircraft Structures in the Context of Composite Element Connection

Recently, there has been a growing trend towards an ever greater use of composite materials in aircraft structures. This necessitates a search for effective ways of joining these materials. The desirable characteristics of such joints are predominantly high strength and low mass, both of which are features offered by rivet nuts. The study was conducted on one type of glass fabric material, from which two types of composite specimens were prepared. Each specimen used the same type of fibre of the same number of 3 layers, yet of different lay-up. Specimens were joined with aluminium alloy and steel rivet nuts to produce single and multi-row parallel models of riveted joints for further testing and analysis. The specimens were clamped in the grips of tensile machine and subjected to a tensile strength test. In the work, the course of the forces in the rivet nuts joint and the failure modes and effects of joint failure were analysed. The distribution of vertical displacement was examined according to the joint material and two fibre orientation variants. To determine the distribution of displacements, the FE (Finite Element) method was used. Upon completion of the experimental stage, the obtained results were subjected to comparative analysis.

Mechanical and Corrosion Assessment of Friction Self-Piercing Rivet Joint of Carbon Fiber-Reinforced Polymer and Magnesium Alloy AZ31B

Abstract In the present work, thermoset carbon fiber–reinforced polymer (CFRP) was spot joined to magnesium alloy AZ31B by a friction self-piercing riveting (F-SPR) process. Lap shear tensile and cross-tension testing were used to evaluate the mechanical joint performance. An average lap shear tensile load of 5.18 kN was achieved, while an average of 2.81 kN was found from cross-tension testing. All F-SPR samples showed a pullout of AZ31B after mechanical testing, indicating good mechanical interlocking between the steel rivet and AZ31B. Corrosion potential was measured for each material to establish the galvanic corrosion characteristics. As expected, AZ31B was found to be the most active, while thermoset CFRP was the most noble. The steel rivet fell between the AZ31B (active) and the thermoset CFRP (noble). Salt fog corrosion testing (ASTM B-117) was performed to evaluate the corrosion performance of the uncoated F-SPR joint. With up to 200 h of exposure, the post-corroded F-SPR joint integrity retained 81.2% of the pre-exposure F-SPR joint strength with AZ31B pullout failure mode. From cross-sectional analysis of the F-SPR joint, extensive corrosion of AZ31B was observed at the joint and other exposure areas. However, steel rivet was not significantly corroded due to sacrificial anode effect by which AZ31B corroded first in the galvanic couple.

Resistance rivet welding of magnesium/steel dissimilar materials

Welding of magnesium (Mg)/steel faces huge challenges due to the extremely low mutual solid solubility. In this paper, the resistance rivet welding (RRW) was used to join Mg alloy to steel for the first time, and a metallurgical-mechanical hybrid joint was formed among the semi-tubular rivet, Mg alloy and sheet steel. A steel hollow sphere weld was formed by fusion welding between steel rivet and lower steel sheet, with re-solidified Mg alloy maintained inside it. The microstructure of the hollow sphere weld consisted of lath-like martensite. The welds present both interfacial fracture (IF) mode and button pullout fracture (BPF) mode in tensile shear tests and the strength reaches the maximum of 4381 kN at 7 kA-200 ms with BPF mode. The failure mechanism of the hollow sphere weld for IF shows the ductile characteristic, and the fracture characteristics of Mg sheet for BPF show the mixture of ductility and brittleness. This study provided an alternative technology for welding Mg/steel immiscible materials.

Joint quality of self-piercing riveting (SPR) and mechanical behavior under the frictional effect of various rivet coatings

Abstract This study investigates the frictional influence of different rivet coatings on the joint quality and strength of self-piercing riveted aluminum alloys. A mechanically plated Zn-Sn-Al (Almac®) and an electroplated Zn–Ni coating were applied to the steel rivets to examine the frictional effect of rivet coatings. The friction properties of the rivet coatings were evaluated using a micro-mechanical surface tester. The results demonstrated that the friction properties of the rivet coatings remarkably influenced both the joint quality after riveting and the mechanical behavior of the joints. Under a similar riveting condition, owing to the lower friction coefficient, the Zn-Ni-coated rivets experienced deeper rivet head penetration and larger interlocking in terms of joint quality compared to those of Almac®-coated rivets with a higher friction coefficient. However, under a similar joint quality with respect to the appropriate riveting parameters, the joints with Almac®-coated rivets exhibited higher lap-shear and cross-tensile loads owing to the higher frictional force at the rivet/sheet interface compared to the joints with Zn-Ni-coated rivets. The surface profiles of the coatings after applying an external load using a micro-mechanical surface tester revealed a larger plastic deformation and subsequently higher delamination of the Almac® coating during the riveting process, which altered the friction properties at the rivet/sheet interface and thereby contributed to the higher lap-shear and cross-tensile loads. A numerical analysis also elucidated a significant effect of the friction coefficient at the rivet/sheet interface on the joint quality and lap-shear behavior, which is consistent with the experimental results.

Review: Frank Furness: Architecture in the Age of the Great Machines, by George E. Thomas

Review: <i>Frank Furness: Architecture in the Age of the Great Machines</i>, by George E. Thomas

Development of Bonded/Riveted Steel Anchorages of Prestressed CFRP Strips for Concrete Strengthening.

CFRP (carbon fiber reinforced polymer) strips are currently often used to strengthen reinforced concrete structures in flexure. In order to ensure effective strengthening, proper connection between FRP material and concrete structure is needed. CFRP strips can be applied passively (only by bonding to the concrete surface) or actively (by prestressing before bonding). In the case of passive strengthening, CFRP strips connecting by bonding to the surface along the strengthened element are usually sufficient. However, active (prestressing) CFRP strips should be additionally anchored at their ends. Anchoring of unidirectional CFRP strips to the reinforced concrete is difficult because of their weak properties in transverse directions. The paper presents a development of mechanical steel anchorages used in an active CFRP flexural strengthening system for reinforced concrete structures. The anchorages were made of steel plates connected to CFRP strips with steel rivets and epoxy adhesive. They were developed within series of tests on specimens from small-scale to full-scale tested in an axial tensile scheme. The paper describes successive modifications of the anchorages as well as the results of full-scale tests. The final version of the anchorage developed during the research had a tensile failure force of 185 kN, which is sufficient value for CFRP strengthening purposes.

Shock absorber with inward-folding composite tube and its application in legged landing gear

A shock absorber based on the progressive failure of a composite tube was developed. The composite tube is subjected to axial compression from the pressing cap at one end and folds inward at the other end under the effect of the crush cap without debris overflow. The crushed composite is compacted into a hollow tube and increases the resisting force once it reaches the pressing cap. For larger strokes without a load increase, a hole is made in the pressing cap for the crushed composite. Three crush caps with different curvatures of the inward-folding surface and two composite tubes were evaluated through static and dynamic tests. The crush cap lowered the initial peak load and varied the specific energy absorption (SEA). We also investigated the effects of connection rivets. Aluminum rivets were severed and had little effect on the load curve while steel rivets did not shear but cut the composite tube into strips, resulting in a lower sustained load because failure was induced. A set of shock absorbers with larger strokes was tested for use in the legged landing gear of a drone assumed to crash with a speed of 10 m/s, energy of 5000 J, and overload of less than 25 g. The maximum SEA of the absorbers, including the two caps, was 20 kJ/kg.

Mechanical Performance and Microstructure of Resistance Element Welds of Dissimilar Metals Created with a Headless Rivet

Resistance Element Welding (REW) is a novel processing technology developed to join dissimilar materials such as aluminum and steel. It uses an auxiliary steel rivet (element) inserted into an aluminum sheet that forms a resistance spot weld with the steel sheet, and the aluminum work piece is mechanically joined by the interlocking rivet. The versatility and agility of REW is advantageous, especially in the automotive industry. However, the head of the rivet often extends out externally over the work piece, and the misalignment of the electrode with the head can be problematic since inserting the head and the weld are performed in separate stages. In the present paper, we performed REW using a headless rivet which has minimal to no overhanging part above the aluminum surface. The lap-shear strength of REW is higher than self-piercing rivets, which makes REW a promising solution for extending applications of multi-material structures. The microstructure of the joint involves a fusion zone which mainly consists of martensite, and the failure occurs near the heat affected zone. This study finds that the electrode-rivet alignment needs to be optimized to manage the optimal nugget size and to avoid current arcing through the aluminum work piece. (Received August 12, 2019; Accepted September 30, 2019)

Riveting–welding hybrid bonding of high-strength steel and aluminum alloy

ABSTRACTBased on laser-induced tungsten inert gas hybrid welding source, the riveting–welding hybrid joining technology was applied to join Q460 high-strength steel and 6061 Al alloy. The mechanism of the action of hybrid welding source was discussed. The effect of welding penetration of the Al alloy sheet on mechanical properties and element distribution of hybrid joints was investigated. In addition, the formation mechanism of Al-steel interface was also explored. Under the action of laser-induced arc hybrid welding source, three load-bearing areas were generated in the joint, which were, respectively, the steel-steel welding joint, the Al-steel mixing area, and the Al/steel interface formed by the sidewalls of Al alloy plate and steel rivet. When the penetration of Al alloy “H” was between 300 and 400 μm, the tensile shear load of the hybrid joint was evaluated to 5.3 ± 0.2 kN, which was 90% of the strength of Q460 steel. In addition, the properties of joints were improved by the bonding effect of the Al alloy plate and steel rivet.

Aluminum-steel dissimilar robotic arc spot welding with auxiliary insert

The authors have developed the novel process for joining dissimilar metals, which uses gas metal arc welding and an auxiliary insert or element. In this process, the element or the flanged steel rivet that has a hollow center is inserted into the pre-hole prepared in an aluminum alloy sheet for the first step. Second, the hollow center is filled with steel weld metal by arc spot welding to join the element and a steel sheet at the fixing point, and thereby the element and the steel sheet are welded strongly enough to fasten the aluminum alloy sheet between the flange of the element and the steel sheet. This new joining process is called element arc spot welding (EASW). EASW has such advantages that it can easily be applied to a dissimilar metal joint with ultra-high-strength steel; it does not need to pinch the joining metals from the upper and lower sides of the joint with a clamping tool; and the resultant shear and peel strengths of the joint are equal to or greater than those by other dissimilar metal joining processes. The prototype robot system for automating the EASW process has also been developed. In this robot system, the position of the pre-hole in the aluminum alloy sheet is detected by an image sensor, and then the tool at the tip of the robot arm moves to the detected pre-hole position to insert the element correctly into the hole; in the next step, low-spatter arc welding is carried out to fasten the aluminum alloy sheet between the element and the steel sheet.

Synthetic Magnetic Field Imaging With Triangle Excitation Coil for Inspection of Any Orientation Defect

Detection of any orientation defect that is deeply embedded under rivet head in multilayer riveted structure is a challenging task for eddy current testing (ECT). In this paper, a novel probe with right-angle triangle excitation coil and two tunneling magnetoresistance (TMR) sensor arrays is proposed to address this problem. The two TMR sensor arrays, which are capable of obtaining magnetic field images with spatial resolution of 0.5 mm, are placed on top of the two right-angle sides of the excitation coil measuring the normal component magnetic field. The probe moves along the direction of the long side of the triangular coil during the inspection. Therefore, the two sensor arrays scan the structure under test sequentially and two images are recorded. An image processing algorithm including data shifting, rotating, and multifrequency mixing is developed to fuse the two images, which results in a synthetic image. A prototype probe is built and tested to demonstrate the feasibility of the design. Three-layer aluminum samples with carbon steel rivet and defect located in the second and third layers are evaluated by the prototype probe. Defects with different sizes and orientations are imaged. Experiment results show that the proposed probe is capable of detecting defects in any direction. The signal processing algorithm highlights the defect information. The orientation angle of the defect can be estimated from the synthetic image.

Permeability invariance transformation for inspection of magnetized steel rivets

Permeability invariance transformation for inspection of magnetized steel rivets

Fatigue life prediction of hot riveted double covered butt joints

Many existing steel bridges are constructed by using hot-driven steel rivets as fasteners. It is known that the fatigue strength of these joints depends on their geometry, since it affects the load transfer mechanism and, therefore, the severity of the stress concentration. Attempts have been taken in the past to provide the characteristic fatigue strength of riveted joints based on test results, i.e. to provide a detail classification. However, the limited number of experimental results per geometry and their large scatter jeopardises the accuracy of the classification, which has resulted into different classifications by different researchers. This paper presents a theoretical fatigue strength prediction model for hot riveted double covered butt joints. The model results agree very well with fatigue test data and the model provides a fundamental understanding of the empirical observations. The model reveals that the classification of joints as proposed in the past is inaccurate for many cases. As an example, the plate width over rivet diameter ratio and the surface finish have a much larger influence on the strength than what has always been considered, whereas the ultimate tensile strength of the plate material has limited influence.

A Comparative Study on the Corrosion Properties of Self-Piercing Riveted CFRP/Steel and CFRP/Al Joints

This work aims to investigate the corrosion properties of CFRP/Steel (carbon fiber reinforced plastic) and CFRP/Al SPR (self-piercing riveting) joints by an Almac®-coated steel rivet. The corrosion behavior of all samples was evaluated by open-circuit potential and potentio-dynamic polarization tests in 3.5 wt% NaCl solution. Meanwhile, the samples were exposed to continuous a salt spray test up to 90 days. Furthermore, single-lap shear tests were carried out in order to assess the mechanical performance of these SPR joints before and after the salt spray test. The results show that the lap shear strength of the CFRP/Steel SPR joints was decreased drastically as a function of increasing exposure time, whereas the strength of CFRP/Al SPR joints remained relatively unaffected for the entire exposure time. It is also observed that the Almac®-coated steel rivet in the CFRP/Steel SPR joint was affected by severe galvanic and/or crevice corrosion as a result of the rivet head deterioration as the exposure time increased. On the other hand, the Al sheet of CFRP/Al SPR joint experienced noticeable pitting corrosion.

Numerical simulation and experimental validation of self-piercing riveting (SPR) of 6xxx aluminium alloys for automotive applications

Two-dimensional axisymmetric models of the self-piercing riveting (SPR) process consisting of punch, blank holder, die, rivet, upper blank and lower blank were built using the finite element software package simufact.forming™. Three different joint configurations were modelled. An optical measurement system was used for capturing the actual geometries of both the die and the rivet which were then introduced into the simulation model. The stiffness of the riveting pliers was modelled as numerical spring. A combined model of Coulomb friction and shear friction was applied. Both friction coefficients were determined by inverse modelling. The deformation behaviour of the 6xxx aluminium alloy blanks to be joined was described with flow curves extrapolated from results of uniaxial tensile tests. In order to validate the results of the simulations self-piercing riveting experiments using 6xxx aluminium alloy blanks and coated steel rivets were conducted. Force-displacement curves of the punch were captured during the riveting process. Characteristic geometrical features of cross-sections of these joints including horizontal undercut of the rivet, bottom thickness of the lower blank and rivet head overlap were investigated using optical microscopy. The cross-sections of the self-piercing riveting joints obtained from simulations and experiments showed very good geometrical agreement, and just slight differences were observed between the force-displacement curves.

Influence of Corrosion of Self-Piercing Riveted High Strength Aluminium Alloy Joints with Button Cracks on the Mechanical Strength

For high strength aluminium alloys, such as AA6008T61 and AA6111 PFHT, when they are joined as the bottom material by self-piercing riveting (SPR), they tend to crack at the joint buttons. These cracks, especially those penetrated to the rivets, may cause galvanic corrosion problem with the steel rivet. In this paper, the mechanical strength of four stack/die combinations with different joint button cracking severity was studied before and after salt-spray corrosion test. The results showed that corrosion on the severely cracked joints was worse than that on the joints with small cracks and the corrosion on the stack with the same top and bottom material was less than that on the stack with different top and bottom materials. For all joints the static lap shear strength after corrosion was improved, but the static T peel strength after corrosion was slightly reduced or did not have obvious change. The results also showed that after corrosion the lap shear fatigue strength of the specimens with severe cracks did not have obvious change, but the lap shear fatigue strength of the specimens with small cracks slightly reduced.

The strength of traditional and self-pierced riveted joints

The objective of this study is to compare the strength of riveted joints fabricated by traditional riveting (with pre-drilled holes) and self-piercing riveting (SPR) for different types of joints. Riveted joints were produced using steel and aluminum alloy rivets and two types of sheet material: 235JR steel sheet and EN AW 6060 aluminum alloy sheet with the following dimensions: length l = 100 ± 1 mm, width b = 50 ± 1 mm and thickness g = 2 mm. For all tested types of riveted joints (pre-drilled and SPR), 5 sets of joints were fabricated, each set containing 6 samples. The sets of joints differed with respect to the number of rivets (1, 2, 3, 4 and 6 rivets), joint type (single-, three- and four-riveted joints) and lap length. For all tested joints, the highest load capacity was obtained for self-pierced riveted joints, while the lowest - for pre-drilled joints with aluminum alloy rivets. In addition, it was found that the shear strength of self-pierced riveted joints is higher than that of aluminum and steel blind rivets.

Fatigue test of a fiberglass based composite panel. Increasing the lifetime of freight wagon

In the XXI century transportation of goods plays a key role in the economy. Due to a good logistics the economy is able to grow fluently. Although land transportation is carried out mainly through trucks for the last several years there has been noted an increase in the percentage share of rail transport in the freight transport. The main goods transported by railways are mineral fuels, mining and quarrying products. They constitute the greater part of 70% of total transported goods. Transportation of material of such high weight, high hardness and with different shapes involves increased and accelerated wear and tear of the cargo space of the wagon. This process is also magnified by substances used to prevent overheating or goods theft. Usually they are in the form of chemical compounds powder, eg. Calcium. A very large impact on the wear of the freight wagons hull is made because of mechanical damage. Their source comes mostly from loading cargo with impetus and using heavy machines during unloading. A large number of cycles of loading and unloading during the working period causes abrasion of body and as a result after several years a wagon car qualifies for a major maintenance. Possibility of application composite panels in the process of renovating the wagons body could reduce the weight of whole train and prolong the service life between mandatory technical inspection. The Paper "Fatigue test of a fiberglass based composite panel. Increasing the lifetime of freight wagon" presents the research process and the results of the endurance test of the composite panel samples fixed to a metal plate. As a fixing method a stainless steel rivet nut and a stainless steel button head socket screws were chosen. Cyclic and multiple load were applied to test samples using a pneumatic cylinder. Such a methodology simulated the forces resulting from loading and unloading of the wagon and movement of the cargo during transport. In the study a dedicated stand equipped with a pneumatic cylinder and a strain gauge sensors was used. A logic controller to control pneumatic valves was used. Tests were carried out for different number of cycles. The paper contains the results and conclusions which will be useful for the actual application of composite panels on the hull of a freight wagon.

Resistance Element Welding of 6061 Aluminum Alloy to Uncoated 22MnMoB Boron Steel

A novel resistance element welding technology was applied to join 6061 Al alloy and uncoated 22MnMoB boron steel. To conduct the resistance element welding process, a technological hole was drilled in the Al sheet into which a Q235 steel rivet was inserted. Resistance spot welding was carried out at the rivet. The mechanical properties, fracture morphology, nugget formation process, dynamic resistance, microstructure, and hardness distribution of the resistance element welding were investigated. Traditional resistance spot weld joints were also prepared for comparison. Resistance spot welding could barely join Al 6061 and boron steel, and had a maximum tensile shear force of less than 1000 N. Novel resistance element welding could join the metals reliably with a maximum tensile shear force of over 7000 N and a relatively high toughness. Nugget formed at the interface of rivet and steel acted as loading position, and IMC interlayer connected rivet and aluminum.

Residual stress profiles in riveted joints of steel sheets

The residual stress profiles in two different self-piercing riveted (SPR) joints were characterised using the neutron diffraction technique. The joints were produced using semi-tubular steel rivets and carbon steel sheets of different thicknesses and hardnesses. The residual stress in the sheet material inside the bore of the rivet was compressive at the centre and the stress became tensile away from the centre. The stresses found in the semi-tubular rivet were compressive, with a lower magnitude in the rivet head compared with those in the rivet leg. For the SPR joints examined, the compressive residual stress in the rivet leg was greater for the thin joint than the thick joint, and this was due to the higher force gradient encountered during the rivet flaring stage. The originality of this work was to relate the residual stress profile to the physical event that occurred during the SPR process.