Gefu Ji

On Approximately Realizing and Characterizing Pure Mode-I Interface Fracture Between Bonded Dissimilar Materials

Bimaterial systems in which two dissimilar materials are adhesively joined by a thin adhesive interlayer have been widely used in a variety of modern industries and engineering structures. It is well known that interfacial fracture is the most common failure mode for these bimaterial systems. Particularly, the interface fracture is a mixed mode in nature mode-I (pure peel) and mode-II (pure shear) due to the disrupted symmetry by the bimaterial configuration. Obviously, characterizing individual fracture modes, especially mode-I fracture, is essential in understanding and modeling the complex mixed mode fracture problems. Meanwhile, the J-integral is a highly preferred means to characterize the interfacial fracture behaviors of a bimaterial system because it cannot only capture more accurate toughness value, but also facilitate an experimental characterization of interfacial traction-separation laws (cohesive laws). Motivated by these important issues, a novel idea is proposed in the present work to realize and characterize the pure mode-I nonlinear interface fracture between bonded dissimilar materials. First, a nearly pure mode-I fracture test can be simply realized for a wide range of bimaterial systems by almost eliminating the mode-II component based on a special and simple configuration obtained in this work. Then, the concise forms of the J-integral are derived and used to characterize the interfacial fracture behaviors associated with classical and shear deformation beam theories. The proposed approach may be considered as a promising candidate for the future standard mode-I test method of bimaterial systems due to its obvious accuracy, simplicity, and applicability, as demonstrated by the numerical and experimental results.

Effects of Adhesive Thickness on Global and Local Mixed Mode I/II Interfacial Fracture of Bonded Steel Joints

Interfacial toughness and interfacial strength, as two critical parameters in an interfacial traction-separation law, have important effect on the fracture behaviors of bonded joints. In this work, the global and local tests are employed to investigate the effect of adhesive thickness on interfacial energy release rate, interfacial strength, and shapes of the interfacial traction-separation laws. Basically, the measured laws in this work reflect the equivalent and lumped interfacial fracture behaviors which include the cohesive fracture, damage and plasticity. The experimentally determined interfacial traction-separation laws may provide valuable baseline data for the parameter calibrations in numerical models. The current experimental results may also facilitate the understanding of adhesive thickness-dependent interface fracture of bonded joints.Copyright

Novel Impact/Debonding Tolerant Sandwich Panel With Millitubes Grid Stiffened Foam Core

Sandwich construction has been extensively used in various fields. However, sandwich panels have not been fully exploited in critical structural applications due to damage tolerance and safety concern. A major problem of sandwich panels is the debonding at or near the core/face sheet interface, especially under impact loading, which can lead to a sudden loss of structural integrity and cause catastrophic consequences. In order to improve the debonding resistance and energy absorption of sandwich panel under impact loadings, a new foam core is proposed which is a hybrid core consisting of grid stiffened hollow metallic millitubes reinforced polymer matrix. The objective of this study was to characterize its dynamic performances. The core consisted of polymer resin reinforced by grid stiffened continuous metallic millitubes. Low velocity impact test demonstrated that new core panel may be considered a promising option for critical structural applications featured by debonding and multiple impact tolerance.Copyright

Multifunctional MWCNT Reinforced Self-Healing System

Multi-Wall Carbon Nano Tube (MWCNT) reinforced composite structure has been widely researched. However, there are a few studies report the MWCNT as a sensor and an actuator integrated in the same composite structure to form a multifunctional composite. The composite used in this study was based on commercially available thermosetting epoxy polymer, dispersed with thermoplastic particles, and MWCNT. Multifunctional MWCNT reinforced epoxy polymer composite was fabricated and tested. The specimens were damaged by the three point bending load. The sensor and actuator system automatically detected the damage and triggered the out electric power to heal the damages. The healing efficiency was evaluated by measuring resistant of composite.Copyright

Shape Memory Polymer With Aluminum Tube Reinforced Impact Resistant Sandwich Core

Sandwich construction has been extensively used in various fields. However, sandwich panels have not been fully exploited in critical structural applications due to damage tolerance and safety concern. A major problem of sandwich panels is the debonding at or near the core/face sheet interface, especially under impact loading, which can lead to a sudden loss of structural integrity and cause catastrophic consequences. In order to improve the debonding resistance and energy absorption of sandwich panel under impact loadings, a new sandwich core is proposed which is a hybrid core consisting of hollow metallic millitubes reinforced Shape Memory Polymer matrix. The objective of this study was to characterize its dynamic performances. The core consisted of programmed shape memory polymer resin. Low velocity (4m/s) impact tests demonstrated that new core panel may be considered a promising option for critical structural applications featured by debonding and multiple impact tolerance.Copyright

Impact/Debonding Tolerant Sandwich Panel With Aluminum Tube Reinforced Foam Core

Sandwich construction has been extensively used in various fields. However, sandwich panels have not been fully exploited in critical structural applications due to damage tolerance and safety concern. A major problem of sandwich panels is the debonding at or near the core/face sheet interface, especially under impact loading, which can lead to a sudden loss of structural integrity and cause catastrophic consequences. In order to improve the debonding resistance and energy absorption of sandwich panel under impact loadings, a new foam core is proposed which is a hybrid core consisting of hollow metallic microtubes reinforced polymer matrix. The objective of this study was to characterize its static and dynamic performances. Two types of new hybrid cores were investigated in this work. One consisted of polymer resin reinforced by transversely aligned continuous metallic militubes, denoted as type-I sandwich panel. The other was made of polymer resin reinforced by aligned continuous in-plane metallic militubes, denoted as type-II sandwich panel. Additionally, the traditional sandwich panels with polymeric syntactic foam core were also prepared for comparisons. Static and impact tests demonstrated that interface debonding and subsequent shear failure in the core could be largely excluded from the type-II panel. Meanwhile, a significant transition to ductile failure was observed in type-II sandwich panel with dramatically enhanced load capacity and impact energy dissipation. The results indicated that type-II panel may be considered a promising option for critical structural applications featured by debonding and impact tolerance.Copyright

Healable and Repeatable Adhesively Bonded Joint

The adhesively bonded structure has to be replaced after the crack initiation and propagation. In a previous study, a biomimic two-step self-healing scheme (close-then-heal) by mimicking human skin has been proposed for self-healing structural-length scale damage. The adhesively bonded joint are prepared and to invest its feasibility and repeatability by fabricating a composite adhesive bonded joint with thermoplastic particles dispersed in a most commonly used epoxy based adhesive material. The fractured specimens were healed per the close-then-heal mechanism and tested again to fracture. This fracture-healing test lasted for 3 cycles.© 2011 ASME

Mechanical Properties of New Hybrid Materials: Metallic Foam Filled With Syntactic Foam

Syntactic polymer foam has received intensive attention and extensive application due to its remarkable low cost, lightweight, mechanical properties as well as its thermal, acoustic properties for multifunctional purpose. Electrically conductive polymers have the advantages of light weight, resistance to corrosion, good processability, and tunable conductivity. In a recent separated study, we proposed a novel conductive polymer which was based on the metallic foam filled with syntactic polymer foam. In this study, instead of focusing its unique multi-physical properties, we focus on characterizing the mechanical properties of this new conductive syntactic foam. Before the exploration of this new hybrid foam, an understanding of the mechanical properties is quite necessary. To this end, hybrid foams were prepared by varying the volume fractions of microballoons in the syntactic foam and types of microballoon materials: glass and polymer microballoons. The metallic foam adopted in this work was based on aluminum with an average relative density of 7% (the porosity is about 93%). Both compressive and bending tests were conducted. The current test results may provide the valuable baseline and also facilitate the further understanding of this hybrid foams as a core material in the advanced sandwiched pipe/pressure vessel structures featured by lightweight, impact tolerant, self-monitoring, thermal and acoustic insulation, and electromagnetic shielding.Copyright

Effect of Bondline Thickness on Interfacial Fracture of Laminated Composite Materials

The interfacial fracture of bonded structures is a critical issue for the extensive applications to a variety of modern industries. In the recent two decades, nonlinear fracture mechanics methods have been receiving intensive attentions for adhesively bonded joints. Extensive experimental efforts have been made to determine the toughness of adhesive joints. Several experimental studies have also been conducted to determine the interface cohesive law in bonded joints. However, very few studies investigated the effect of adhesive thickness on the interface cohesive laws. In the cohesive law, both fracture energy and the interfacial cohesive strength, as two critical parameters, have significant effect on the fracture behavior and joint’s structural capability. The present study presents the experimental investigation into how the adhesive’s thickness affect these two important parameters with the nonlinear fracture mechanics. The equivalent interface cohesive laws are experimentally determined for the bonded joints with various adhesive thicknesses. The experimental cohesive laws may provide valuable baseline data for simple analytical and numerical cohesive zone models. Based on the test results, the mechanism for the intrinsic fracture energy and plastic energy dissipation is discussed. Several other interesting conclusions are also obtained.Copyright

Effect of Adhesive Thickness on Interfacial Fracture of Bonded Steel Joints

The interfacial fracture of bonded structures is a critical issue for the extensive applications to a variety of modern industries. In the recent two decades, nonlinear fracture mechanics methods have been receiving intensive attentions for adhesively bonded joints. Extensive experimental efforts have been made to determine the toughness of adhesive joints. Several experimental studies have also been conducted to determine the interface cohesive law in bonded joints. However, very few studies investigated the effect of adhesive thickness on the interface cohesive laws. In the cohesive law, both fracture energy and the interfacial cohesive strength, as two critical parameters, have significant effect on the fracture behavior and joint’s structural capability. The present study presents the experimental investigation into how the adhesive’s thickness affect these two important parameters with the nonlinear fracture mechanics. At the mean time, the equivalent interface cohesive laws are experimentally determined for the bonded joints with various adhesive thicknesses. The experimental cohesive laws may provide valuable baseline data for simple analytical and numerical cohesive zone models. With the test results, the mechanism for the intrinsic fracture energy and plastic energy dissipation is discussed. Several other interesting conclusions are also obtained.Copyright