Joining Technologies for Composites and Dissimilar Materials, Volume 10

A coupler has been developed to prevent windshield wiper systems from being damaged by excessive loads that can occur when the normal wiping pattern is restricted. Unlike the traditional steel coupler used in wiper systems, the composite coupler will buckle at a prescribed compressive load threshold and become extremely compliant. As a result, the peak loading of the coupler and the entire wiper system can be greatly reduced. The coupler is composed of a pultruded composite rod with injection-molded plastic spherical sockets attached at either end. The sockets are used to attach the coupler to the crank and rocker of the windshield wiper linkage. Because the loads exerted on a coupler vary in magnitude and direction during a wiping cycle, the joint between the sockets and the pultruded composite rod must be robust. The paradigm for attaching sockets to steel couplers (i.e. over-molding the sockets around holes stamped into the ends of traditional steel couplers) was tested and found to produce inadequate joint strength. This paper details the methodology that was employed to produce and optimize an acceptable means to join the injection-molded sockets to the fiber glass pultruded rods. Specifically, a designed experiment based on the Robust Design Strategy of Taguchi was used to identify the process, processing parameters, and materials that yield a sufficiently strong joint at a reasonable manufacturing cost without damaging the integrity of the underlying composite structure.

An analysis of composite Pi/T-joint was performed through the comparison of a finite element simulation and experimental results obtained using digital image correlation. The finite element simulation was performed in ABAQUS using a realistically modeled adhesive layer consisting of cohesive zone elements. The composite Pi/T-joints were manufactured using vacuum assisted resin transfer molding (VARTM). The resulting Pi/T-joints were loaded in out-of-plane (web pull-out) until failure, during which three-dimensional digital image correlation (3D-DIC) was used to measure the in-plane displacements and strains.

From the experiments, it was found that the average peak pull-out force was 12.56 ± 0.82 kN, which was 10 % less than that from the FE simulation at 13.77 kN. However, this does not give much information about whether the stress and strain distribution has been accurately predicted by the simulation. To better understand this, the experimental data obtained using DIC was compared to that from the FE simulation using image decomposition. The image decomposition process reduces the large, full-field data maps to a feature vector of 130 or so elements which can be compared much more easily. In this case, there was good agreement between the experiment and simulation, when taking into account the experimental uncertainty.

Sensitization can occur in 5xxx aluminum alloy utilized in welded ship structures. Cumulative exposure of plating to elevated temperatures causes magnesium in the aluminum to migrate to the grain boundaries and develop a corrosion cell. As a result, the sensitized plate is susceptible to intergranular corrosion, which when exposed to salt water and stress leads to stress-corrosion cracking (SCC), typically at stress levels below nominal plate strength. SCC typically occurs at a level of sensitization that renders welded repairs impractical, requiring an alternative methodology. One alternative to traditional welded repair of marine structures is the application of a composite patch. Composite patch repairs have seen significant use by the aerospace community on thin aluminum aircraft skins to address stress related cracking while only limited use on thicker marine structures. Patches for marine structures have demonstrated their ability to mitigate crack growth, inhibit water ingress, and reinforce damaged or degraded structure for commercial and military vessels. The use of composite patches for repair of SCC onboard U.S. Navy ships began in December of 2010. Patch design and installation are supported by ASTM material testing, environmental conditioning, large center crack tension fracture mechanics specimens, and experience with a full scale plate ductile tearing test. Patch repairs patches are installed in-situ with vacuum consolidated hand laminations and have demonstrated the ability to increase static plate strength, fatigue life, large displacement capacity of the crack plate, and durability for in-service applications.

Aiming at an increase in failure resistance and damage tolerance of composite T-joints, a novel reinforcement technique in through-thickness direction using metallic arrow-pins has been proposed by the authors in previous studies. In a recent investigation, different options for further improvement of the T-pull performance have been assessed. These include optimized arrow-pin configurations, filler and ply materials and thermoplastic interleaf layers. T-specimens have been manufactured and tested under quasi-static and high-rate dynamic loading conditions to quantify the influence of these measures. Additionally, FE models have been developed in LS-Dyna to predict the performance numerically. Model validation was conducted step by step using material coupon test data, dedicated single-pin pull-out tests and T-pull tests.

There is a great variety of joint types used in nature, from jaws, bones and tendons to root systems and tree branches. To understand how to optimise biomimetic-inspired engineering joints, first it is important to understand how biological joints work. In this paper, a review based on the functions of natural joint systems is presented. Emphasis was given to understanding natural joints from a mechanical point of view, so as to inspire engineers to find innovative methods of joining man-made structures. Nature has found many ingenious ways of joining dissimilar materials, with a transitional zone of stiffness at the insertion site commonly used. In engineering joints, one way to reduce the material stiffness mismatch is to gradually decrease the effective stiffness of the steel part of the joint by perforating it with holes. This paper investigates joining of flat perforated steel plates to a CFRP part by a co-infusion resin transfer moulding process. The joints are tested under static tensile loading. The perforated steel joints show a 175 % increase of joint strength comparing to non-perforated ones. Finite element analyses are used to interpret the experimental results.

The ultimate goal of this work was to create 3D polycarbonate sandwich structures fabricated utilizing ultrasonic spot welding (USSW). First, a study of weld strength vs. weld time, frequency, amplitude of vibration and depth of penetration was carried out. Lap shear and peel tests were used to quantitatively characterize weld strength, and from these results, a set of parameters for USSW of polycarbonate was determined. To create the sandwich structures, three layers of 1.59 mm polycarbonate sheet were stacked together, and using an alternating pattern of USSW, the middle layer was joined to each of the outer layers. Post-weld processing of the stacked layers involving heat and pressurization of the structure with air generated a 3D open cell geometry sandwich structure of polycarbonate. 3-point bend and drop impact tests were conducted to study the stiffness and the impact properties of the sandwich structures, and the results suggest that the fabricated polycarbonate sandwich structures might be suitable for applications requiring high strength and energy absorption while maintaining low weight. Additionally, the ability to easily incorporate different geometries in the structure allows the possibility to design structures with tailored mechanical properties.

Ultrasonic spot welding (USSW) is a widely used technique for joining thermoplastics where high frequency, low amplitude vibrations are applied through an ultrasonic horn resting on the polymer surface to create frictional heat, producing a solid state joint between polymer sheets. Advantages such as short weld cycle time, fewer moving components and reproducibility make this technique attractive for automation and industrial use. The goal of this work was to evaluate the feasibility and analyze the lap shear and impact strength of a composite material joint created using ultrasonic spot welding. The base material used for the joints was a composite consisting of a polycarbonate matrix with chopped glass fibers. The strength of the lap joints was determined through experimental lap shear and impact testing. A finite element analysis was conducted for more thorough insight into the stress patterns in the lap joints. Experiments showed that the ultrasonically spot welded joints tested in tensile lap shear loading carried a load 2.3 times higher than adhesive joints and the impact tested joints had an impact strength 3.5 times higher than adhesive joints. The results of this work suggest ultrasonic spot welding as a viable joining method for thermoplastic composite materials.

Drilling of holes in continuous fiber composites for fastening reduces the load carrying capacities, introduces delamination and creates sites for failure initiation and progression. Recent work on a novel hybrid fastening system, which introduces a structural resin insert through a channel in the bolt shaft, overcomes the effects of drilling, eliminates slip, reduces delamination and increases the load-carrying capacities. While the benefits of this system are experimentally observed, the phenomena that govern these improvements are not fully understood. In this work, numerical simulations and experimental tests of hybrid fastening systems comprised of glass fiber reinforced polymer (GFRP) composite substrates fastened using a ½″ Grade 5 bolt, and with a preload torque of 35 N-m were performed. The results were compared one to the other for stiffness and strength. The simulations captured the damage initiation and progress around the hole, and showed the reduction in bolt tilt ascribed to the incorporation of the adhesive insert. Overall, numerical simulations show promise in providing quantitative information about these complex phenomena, and such post-experimental validations can be used as powerful design tools.

The purpose of this study was to develop and validate a novel method for measuring shear deformation during the Thick Adherend Shear Test (TAST), to determine in situ mechanical properties of an adhesive under tensile shear loading, using two-dimensional (2D) Digital Image Correlation (DIC). Shear strains were optically measured using DIC from the bond line and also from adherend deformations; both were compared against measurements made using the ASTM D5656 standard (KGR-1 extensometers). The results from 17 TAST specimens showed that all techniques were in good agreement, however, both DIC techniques had significantly lower variance on measured mechanical properties compared to the KGR-1 extensometers. The use of correlated adherend deformations was the preferred technique for deriving plastic shear strains, and overall produced the lowest scatter on mechanical properties. This study demonstrates the potential for the use of 2D DIC as a more precise, and time-efficient alternative to the KGR-1 extensometers for room temperature in situ characterization of adhesives in shear.

Improving adhesion properties and strength of thin films used in wide range of important application is the key factor in successfully manufacturing micro-electro-mechanical devices, multilayer micro-electronic and optical devices. Mechanical performance and reliably of thin film on a substrate depends on strength of interfacial adhesion. Understanding the role of process parameters like deposition condition and design parameters such as substrate thickness and film thickness, on mechanical performance of the layer is essential in optimizing manufacturing. However, there are limited techniques for characterizing intrinsic interfacial strength of thin films. The use of laser-generated stress pulses has shown the ability to provide quantitative evaluation of the adhesion strength of thin films. In the current work, we use the laser-spallation approach for the investigation of the adhesion strength of thin tin oxide films to alumina substrates.

Ultra-high temperatures ceramics (UHTCs) are the subject of intense worldwide research effort, and their stability in severe environments makes them candidates for aerospace, nuclear and solar energy applications. Widespread usage UHTCs requires the development of effective and reliable joining methods that facilitate the fabrication of large, complex-shaped, and potentially multimaterial components and devices. Joining of HfB₂ and ZrB₂, UHTC diborides, which exhibit outstanding thermo-mechanical and thermochemical properties and good erosion and corrosion resistance, was the focus of the present study. MoSi₂ is an effective sintering aid and a composite component for both HfB₂ and ZrB₂, resulting in dense bulk materials with excellent mechanical properties. HfB₂–10 vol.% MoSi₂ composites were joined at 1500 °C with a Ni/Nb/Ni interlayer that forms a thin liquid film. Joint-region characterization revealed well-bonded interfaces with interfacial reaction products with the MoSi₂. Well-bonded interfaces were also obtained for a ZrB₂–10 vol.% MoSi₂ composite bonded at 1500 °C with both Ti and Zr interlayers. It was found that the Ti interlayer exhibited more intensive interfacial reaction with ZrB₂ composite than the Zr interlayer. Additionally, well-bonded interfaces were also found for a ZrB₂–10 vol.% MoSi₂ composite bonded at 1500 °C with ZrB₂-X vol.% Ni (X = 20, and 40) powder-based interlayer. Joint-region characterization revealed well-bonded interfaces with microstructures strongly dependent on the Ni content.

Dissimilar material components in structural applications require advanced joining geometries and processes that permit the interface of these components to have mechanical behavior equivalent to the lesser of the base metal material behavior and the base composite material behavior. In volume-constrained applications, such as drivetrain parts, the interface must have the strength of the base materials, and the envelope of space of the current drivetrain componentry. The interface surfaces of the composite and metal are demonstrated here to achieve base material capability for the thickness, width, and length directions, resulting in conservation of mechanical properties throughout the interfaces, for all loading and deformation at material transitions. The method of material forming, modification, combining, and product operation is explained here, for optimization of materials usage and for processing equipment.

In designs to date, the metal to composite interface has been the weak link of the assembly, and product performance has been limited to this characteristic. This new method of interface build, and theory of base-to-joint equivalent strength, results in a new capability for drivetrain component design, and will start a new era of dissimilar materials combinations to offer the consumer improved product functionality with enhanced interface design, manufacture, envelope, and operational improvements.

Bolted fastening is one of the oldest joining techniques, and it is widely used in vast number of applications. Although such fasteners provide many advantages, bolted joint assemblies and in-service behaviors demonstrate complex phenomena and stress distributions. Among these factors, bolt tension affects joint reliability and residual lifetime most significantly. One of the major concerns in bolted joints with composite panels is the effect of creep in the through-the-thickness direction of the material, which leads to preload reduction and premature failure of the loosened bolted joint. Thus, it is essential to monitor the preload after joint assembly. In this work, a previously developed bolt tension monitor that is instrumented with a reusable optical sensor was used to evaluate the short-term relaxation of preload in similar and dissimilar composite bolted joints. A single-parameter model was used to fit experimental data and obtain a relaxation parameter, which was assumed to be an extensive property. Preload reductions in lap joints with steel and glass fiber reinforced plastic (GFRP), and GFRP with GFRP were measured for ca. 9 min after the application of two levels of initial preloads. It was found that, depending on the joint configuration, the preload reduction varied considerably; and an increase in initial preload tends to reduce the preload relaxation. Overall, this technique provides robust and cost-efficient health monitoring in composite bolted joints.