Vladimir M. Shkolnikov, Gabriel J. Hostetter, David K. McNamara, Joseph R. Pickens, Stanley P. Turcheck, Jr.
Concurrent Technologies Corporation (CTC)
Johnstown, Pennsylvania 15904, U.S.A.
Bruce G. I. Dance
Paper presented at ASME 28th International Conference on Ocean, Offshore and Arctic Engineering, OMAE 2009, Honolulu, Hawaii, 31 May - 5 June 2009. Paper #79769.
The paper summarizes results of an engineering investigation on advanced joining technology for hybrid (composite-metal) structures. Polymer Matrix Composites (PMC) used in structural applications are known to reduce structure weight, lower life-cycle cost and, in case of a floating platform, improve the deadweight/displacement ratio.
While beneficial, PMC applications for large hull structures have certain limitations in size and volume of seamless structural component without using joints. A hybrid hull that consists of both metal and composite structural members potentially enables desirable enhancements of structural efficiency, but robust joining between those heterogeneous structures must be employed.
A recently completed feasibility study has been performed involving a novel hybrid joining concept technology based on a combination of conventional adhesive bonding with novel metal surface preparation. Computer simulation of the joint structural behavior and failure, development of a material processing procedure based on adaptation of Vacuum-Assisted Resin Transfer Molding (VARTM) process to manufacturing of a large hybrid structure, fabrication of pilot joint test articles, and tensile testing of those to failure, have been performed as part of the feasibility study.
Two sets of the hybrid joint were tested, the novel joint being developed and its conventionally bonded analogue without the novel surface preparation considered as a base-line joint. The tests resulted with 48%-increase of load-bearing capability of the novel joint and a good match between generated computed and experimental data.
Existing joining techniques
Several design concepts which exploit a ship hybrid hull idea have been proposed by other authors.[1-3] Essentially, these represent a combination of a main metal hull with PMC light-weight structural components, such as hull panels, decks, platforms, bulkheads, deckhouse, and/or foundations for machinery and equipment. These design concepts potentially enable the benefits associated with a PMC application. However, robust, reliable and structurally/weight effective joining between those heterogeneous structures must be employed.
Typically, a properly designed joint of a large metal/composite structure possesses considerably lower load-bearing capability than a neighboring regular monolithic structure does. The strength reduction is due to several causes inherent to application of laminate PMC:
- Discontinuous fiber reinforcements within a joint
- Stress concentration attributable to both a joint's geometry and the abrupt change of the material stiffness within the joint
- Relatively low out-of-plane properties associated with the laminate composite architecture
- Lowered material performance as a result of secondary (post-cured) bonding and/or material processing.
In general, a transversely reinforced hybrid joint is capable of substantial increasing the joint's structural efficiency. The shipbuilding industry has experience in the design and application of fastened joints that implements this notion for assorted structures, including large, severely loaded hybrid hull structures. However, while these joints are providing the required load-bearing capability, they involve relatively heavy and labor-intensive construction.
The lowered fatigue performance and intensive corrosion of metal details within the joint are also among the factors that negatively affect performance of the fastened joints.
Along with mechanically fastened joints, two other basic techniques are commonly used for joining of hybrid metal-composite structures: adhesive bonding and combined bonding-fastening.
Adhesively bonded joints have a long history of development and application for a variety of composite and hybrid structures, including auxiliary ship structures. The significant shortcomings that are typically inherent to bonded-only joints include relatively low reliability and controllability of their structural performance. The guidelines for adhesive joining of ship structures summarizes the existing experience in adhesive bonding, primary relevant to aluminum-aluminum/steel and aluminum-composite joints of lightweight high-speed craft and passenger ships, providing invaluable contribution to the successful resolution of the problem. Along with that, the guidance emphasizes the pressing demand and engineering complexity of the composite structure bonding problem.
As rationally designed, a combination of bonding and fastening is potentially capable of substantially mitigating shortcomings inherent to the separate application of each method. Ideally, the adhesive film bears most of the applied load so that fasteners don't experience substantial loading. Concurrently, the fasteners protect the adhesive film from premature peeling. 
In addition to high structural efficiency, the combined bonded-fastened joint demonstrates good watertight integrity and corrosion protection, both maintained in a natural way by filling all gaps and cavities with polymer matrix in the areas inaccessible for inspection. Another benefit is the reduced stress concentration near the bolt holes that levels the stress distribution. Altogether, these features considerably increase serviceability and reliability of the hybrid structure and allow reduction of the structure weight, production cost and maintenance expenses.
The combined joints also have a long history of successful application for hull structures starting from the 1960s. This includes naval vessels, both surface vessels and submarines, but primarily for the joining of external hybrid hull structures that are utilized for several classes of Russian/Soviet subs. The experience collected over the decades regarding design, fabrication and repair of the combined joints is summarized in the Shipbuilding Standard. 
While the benefits of combined bonding-fastening are recognized, it is difficult to achieve them while executing a routine design. Neither stress determination, nor strength reconciliation of the combined joints is a trivial computational task. Because of this, existing designs typically follow the conservative practice where each joining agent of a combined joint (bonding or fastening) is designed to carry the entire applied load, thereby ensuring full structural redundancy. This redundancy, however, unavoidably affects structural efficiency, weight, and cost of the hybrid structure.
Eventually, combined joining is somewhat labor-intensive because of the combined efforts required for both fastening and merely bonding the joints.
The joining option studied in this investigation is based on a revolutionary new material processing technology, known as Surfi-Sculpt®, developed by TWI, UK.[7,8] Using an electron beam (EB), a material can be processed to give a number of different types of surface topography. EB can reshape materials precisely, 'growing' protrusions that rise from the surface of the material. Such transversely oriented protrusions are capable of improving the mechanical performance of the bonded joints.
Figure 1 illustrates the protrusion pattern selected and employed for the novel hybrid joints.
Fig.1. Image of protruded metal surface
Based on Surfi-Sculpt material processing, TWI developed a joining system called Comeld® [9,10], which combines a composite material to be bonded or cocured to a protruded surface, thereby potentially augmenting the adhesive bond strength by a mechanical strength component.
In the original Comeld configuration, the EB-protruded metal is embraced by the composite overlap so that the protruded metal features (pins) penetrate into the composite laminate, adding mechanical resistance to the load-bearing of the bonded joint. The data, generated to date, demonstrates Comeld's capability of increasing the joint load-bearing up to 70%, while also improving energy absorption under dynamic loading as compared to the plain bonded joints.
However, the original Comeld configuration is not optimum in transverse bending, both static and dynamic, which represents most primary load cases for a hull structure.
A refined Comeld concept applicable to prospective hybrid hull structures has been delineated and explored during the reported investigation. Specifically, the original Comeld concept has been reconfigured so that metal 'catch' plates embrace the composite detail. Forming a bonded-pinned metal-composite sandwich that is stiffer and stronger with increased load-bearing capability under transverse bending.
In addition, the metal catch plates protect the penetrated composite from a direct impact, improving the joint impact damage resistance.
Figure 2 represents the selected joint configuration referred henceforth as 'Comeld-2' or 'bonded-pinned joint' to avoid any confusion.
Fig.2. Comeld-2 bonded-pinned joint configuration
Results & discussion
To date, several significant steps related to the selected Comeld-2 joining design/technology development have been completed, namely:
- Finite element (FE) modeling of the joint subjected to tensile loading in a plane stress state that simulates pulling the composite details out from the metal overlaps
- Development of an algorithm to post-process computed data that can characterize the ultimate stress state of a heterogeneous joint structure
- Design of a principle material processing procedure for manufacturing large hybrid structures
- Fabrication of two sets of pilot joint test articles, one with Comeld-2 joint and one with the baseline adhesive bonding in an outwardly identical joint configuration.
- Tensile testing of the elemental joint articles aimed at experimental verification of the concept feasibility and validity of the computer model.
Specifically, two similar heterogeneous parametric FE models comprising second order solid elements, both isotropic and orthotropic relevant to the joint components were built, debugged, and run employing ANSYS FE software. One model represents the base-line bonded-only joint and another has protruding features on the contact metal surface which penetrate the composite.
The protrusion was modeled with idealized cylindrical pins, the main dimensions of which correspond to those of the real protrusion. Parameters of the model were varied to reflect design features of the studied joints and to assess the influence of these factors on a joint's performance.
Due to the structural and loading symmetry, only half of a joint model was subjected to the computation. A model free from lateral constraint was used to reflect the boundary conditions intrinsic to the intended tensile testing.
Assorted failure modes were applied to heterogeneous structural assemblies. Hybrid joints undergo a variety of loading exposures in-service. Depending upon the joint design, each of a joint's components may represent a weak link able to initiate a failure of the entire joint structure.
In general, a joint's failure onset might occur as a result of a rupture or delamination of the composite detail, its debonding from the metal, fiber fracture due to stress concentration within a zone of structural irregularity (e.g., at the tip of the metal overlap and/or nearby a pin insertion), metal cracking within weld and/or other stress concentration areas.
Therefore, an integral strength criterion had to be employed to reconcile individual strengths of joint components against the given design and operational requirements. The condition incorporates non-dimensional stress indices relevant to the joint structural components: composite (C), metal (M), and bonding film (B). Here ΨCΨ MΨB are the indexes characterizing stress intensity of each component relatively to either specified allowable design stress or ultimate strength of the utilized materials.
The criterion (1) accompanied with utilization of various failure criteria relevant to the assortment of component materials and possible failure modes were applied to assess the joints performance under test loading.
Figure 3 illustrates the results of the FE simulation displaying variation of the maximal stress index Ψmax over the joint longitudinal cross section.
Fig.3. Failure index: a) Bonded-only joint b) Bonded-pinned joint
As can be seen, the metallic catch plates within the bonded-pinned joint experience notably higher loading intensity than those in the bonded-only joint. Presumably, that is due to the pinning, which helps to intensify the load-sharing between the metal and composite structural components. In fact, this is the main advantage of the bonded-pinned joint option over the bonded-only joint which enables the sought increase of the joint's load-bearing capability.
Overall, the computer simulations provided useful information on the joint's structural response to the tensile loading and helped to identify the joint's critical areas prone to crack initiations. To verify the model validity and obtain the actual data on feasibility and structural efficiency of the selected Comeld-2 hybrid joint concept, mechanical testing of pilot joint articles were performed.
The joint articles were fabricated in CTC utilizing materials certified for marine structural application. PMC laminates were constructed with E-Glass 24-oz, woven roving with vinyl ester resin (Derakane 8084). The fabric plies were altered in the fiber orientation in a proportion: ¼, ¼, ¼, ¼ (0°, 45°, 90°, -45°) per laminate thickness. The metal details were made with the shipbuilding EH-36 steel alloy (ASTM A945 Grade 65).
Two metal cutch plates were furnished with protruded pins (roughly 3 mm high, 0.5 mm in diameter and 3-5 mm spacing) over contact surfaces with composite. See photo in Figure 1 above that illustrates the selected protrusion pattern.
CTC has performed a variety of manufacturing trials to implement the selected PMC material processing. An adaptation of VARTM, known as the most advantageous close mold material process for the construction of large ship structures, was applied to Comeld-2 manufacture.
The advantage of VARTM in combination with Comeld-2 include:
- The opportunity to minimize fit up problems between the large metal and composite details being assembled into a hybrid structure
- The elimination of secondary bonding from the joint, reducing labor operations and avoiding lowered structure performance
- Enhancing repeatability and predictability of the manufacturing process.
To prevent the steel surface from rusting before the resin infusion, the metal plates were grit-blasted using medium-size grit. After grit-blasting, the strips were airblown to remove excess grit. Methanol was rinsed over the pinned area which was then lightly coated with a primer that promotes a longer-lasting bond over years of temperature alteration and exposure to harsh environments.
The VARTM-ed hybrid panels were post-cured at 180°F for 4 hours and then cleaned and cut to form individual test articles, 2½-in. (63.5-mm) wide. The thickness of the PMC laminate was ½ in. (12.7 mm). The thickness of the metal overlapping catch plates was ¼ in. (6.35 mm). The metal-composite contact surface was 2½×4 in. (63.5×101.6 mm).
Static tensile tests to failure were carried out to determine load-bearing capability of the selected Comeld-2 joints under short-term single loading events in a comparison with that of the outwardly identical base-line joints without metal profiling.
A Tinius Olsen 120 kip (534 kN) Universal Test Machine was employed for the tests. The articles were placed in wedge grips and monotonically loaded in displacement control until separation of the metal and composite details. Figure 4 illustrates the tensile test set-up.
Fig.4. Tensile test set-up
Ultimate loads were determined for both joint configurations. The base-lane bonded only joints failed under force 11.1±0.2 kip per an inch of the articles width (1,944±35 kN/m). The bonded-pinned joints failed under 16.3±1.1 kip per an inch (2,854±193 kN/m). Hence, the novel Comeld-2 joints are 48%-stronger under static tensile loading than the base-line bonded-only joints.
The coefficients of variation of the test results for the bonded only and bonded-pinned joints were CVB = 2.2% and CVP = 6.8%, respectively, which are within a typical range for PMC coupon testing.
Outwardly, all tested joints appeared to have a similar failure mode: double shear of the composite being pulled out of the metal fork formed by the capture plates. However, fractographic analysis revealed that the two tested joint configurations (bonded-pinned vs. bonded-only) had slightly different failure modes.
Figure 5 and Figure 6 illustrate this regarding bonded-only and bonded-pinned joints, respectively.
As shown in Figure 5, the bonded-only joint exhibits a mainly cohesive failure typical of well prepared composite-to-metal.
Fig.5. Bonded-only joint failure a) Separated composite and metal details b) Composite surface c) Metal surface
Fig.6. Bonded-pinned joint failure a) Composite surface b) Metal surface
The failure of the bonded-pinned joints (Figure 6) is also cohesive in nature, but this is accompanied by partially sheared off and bent and plastically deformed pins. The composite beyond the pins shows no signs of cohesive failure which implies lowered contribution of this area to the load-bearing of the overall joint.
Overall, the test results substantiate the feasibility of the novel joining technology along with its structural superiority compared to its base-line analogue.
It should be emphasized that the selected and employed pilot configuration of the Comeld-2 joint is not optimized to date and there might be some room for a further improvement of this novel joint design over the achieved notable 48%-increase of load-bearing capability.
The performed tests also testify to the relevance of the selected FE model to a real hybrid joint structure and its capability to further the understanding of structural behavior and failure of the Comeld-2 hybrid joints. The modeling and simulations are still insufficient to completely characterize the load-bearing capability and/or failure of the Comeld-2 joint. This is due to the multiple uncertainties intrinsic to the employed model relevant to a multi-component intricate-geometry hybrid joint structure.
Among the primary uncertainties are the properties of the adhesive film placed between the metal and composite details, the film's thickness variation caused by imperfection of the contact between the metal and composite components, the impossibility of accurate representation of the real geometry of both protruded pins and cavities formed in the metal surface by EB treatment among others. For these reasons, the computer modeling must be accompanied with an experimental verification of the joints structural performance.
It should be also noted that the Surfi-Sculpt process is presently relatively expensive. This raises a concern about its affordability for an industrial application that may represent a considerable obstacle for the technology implementation.
Due to this concern, a close look at the cost of Surfi-Sculpt processing regarding its application for a serial construction of hybrid hulls was undertaken.
A notional large ship with maximum beam Bmax= 70.0 ft. (21.3m) and depth H = 40.0 ft. (12.2m) was studied to assess the total length of the joint seam between metal and composite components of the hybrid hull. This was presumed to include bow and stern sections with 2 decks, 2 platforms and 3 longitudinal bulkheads as well as a deck house. The estimate of the total length of the seam is LS = 1,520 ft. (463m).
Since a sandwich design is employed for composite hull sections, the total length of the required EB-treated metal strip associated with application of Comeld-2 joining (with two metal plates overlapping a composite laminate skin) is to be roughly LEB = 4LS = 6,080 ft. (1,853m).
The cost of EB-metal surface treatment is to comprise of:
- Capital cost of EB-equipment
- Non-recurring engineering to develop a Surfi-Sculpt pattern per given requirements
- Processing cost of Surfi-Sculpt treatment.
In a preliminary assessment, the capital cost of an EB system capable of processing metal plates up to 9 ft. (3m) long is $235K. Non-recurring engineering costs are estimated at $14.5K and the EB processing for each 4-in. (0.1-m) wide × 9-ft. (3-m) long metal plate is $520 per plate.
The period of the capital expenses amortization is assumed here to be 7 years during which construction of 10 ships is presumed.
Altogether the cost estimation for the required EB-metal treatment needed for metal treatment of Comeld-2 joints for the hybrid hull structures is about $378K per ship.
Hence, the cost of the EB-treatment of one linear foot of a 4-in. wide metal strip will be $62.2 or $1.30 per square inch.
While the estimated dollar figures are commensurate with other metalworking processes used in shipbuilding, the benefits associated with increased joint performance, weight saving, reduction of construction labor (in comparison with a fastened joint) and lowered maintenance expenses should be taken into account when the cost of the advanced Comeld-2 joining is accurately analyzed.
During performance of this investigation, several critical aspects of the novel bonded-pinned joints were studied. These include design of an advanced hybrid joint configuration, selection of a Surfi-Sculpt pattern, computer modeling and simulation of the joint structural behavior and failure, design and implementation of a manufacturing process to fabricate pilot joint articles, tensile testing of those to acquire dependable test data on the joints load-bearing capability and demonstration of the validity of the developed computer model.
The preparatory metal treatment technique and a version of the VARTM composite material processing were devised and demonstrated during the fabrication of pilot hybrid joint test articles. These allowed elimination of secondary bonding from the joint and substantiated the enhanced performance of the novel joint. The mean value of the ultimate load of the novel hybrid joint under tensile loading compared to the baseline bonded-only joint is 48% higher with moderate variability of the test results.
The test data demonstrate general agreement with results of computer simulation employing the selected FE model.
On the whole, the analytical and experimental data generated during this investigation convincingly demonstrate the feasibility of the selected novel joining technology concept and provides a solid basis for its further advancement and implementation.
A number of research, design, and manufacturing issues still need to be addressed to ensure high efficiency and affordability of the bonded-pinned joints for a hybrid hull construction. Mostly, these relate to provision of a methodological basis for bonded-pinned joint design and optimization, finalization of the manufacturing process relevant to a large hybrid construction and deployment of an EB facility sufficient to provide metal protrusion at an industrial scale and fatigue testing to examine long term effectiveness of the joining technology.
This paper is based upon work supported by the U.S. Office of Naval Research under contracts No. N00014-06-D-0048 and N00014-00-C-0544. The sponsorship and guidance for this work were provided by Dr. Roshdy Barsoum, Program Officer (Project Monitor) of the Office of Naval Research and Naval Surface Warfare Center, Carderock Division personnel: Mr. Gerard (Jeff) Mercier, Mr. Paul Coffin, and Mr. Loc B. Nguyen.
The authors appreciate the engagement of these individuals and their constructive suggestions throughout the project performance.
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