skip to main content

Adhesively bonded composite joints can help reduce weight in structures and avoid material damage from fastener holes, but stress concentrations formed at the edges of the adhesive bond line are a main cause of failure. Stress concentrations within the adhesive can be reduced by lowering the stiffness at these edges and increasing the stiffness in the center of the joint. This may be achieved using a dual-cure adhesive system, where conventional curing is first used to bond a lap joint, after which high energy radiation is applied to the joint to induce additional crosslinking in specific regions. Anhydride-cured epoxy resins have been formulated to include a radiation sensitizer enabling the desired cure behavior. Tensile testing was performed on cured systems containing varying levels of radiation sensitizer in order to evaluate its effects on young’s modulus as a function of radiation dose.  more » « less
Award ID(s):
Author(s) / Creator(s):
; ;
Ozden, O.
Date Published:
Journal Name:
Proceedings of the American Society for Composites Technical Conference
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Adhesively bonded joints contain stress concentrations at geometric and material discontinuities within the joint, causing the joint to be inefficient. This study investigates a method to grade the material properties of an adhesive across the bondline to have a soft, flexible adhesive near the stress concentration and a stiff, strong adhesive elsewhere. Theoretical studies and a few experiemental studies have shown an that the load is distributed more evenly along the joint and strength is increased. Adhesive gradation is achieved through a secondary crosslinking system in the adhesive which is activated via radiation. After an adhesive is initially cured, the joint can be exposed to varying levels of radiation to grade the properties. Initial results demonstrate the ability to grade stiffness using radiation shielding, and final results will demonstrate the application in an adhesively bonded joint. 
    more » « less
  2. null (Ed.)
    This research presents an experimental program executed to understand the strength and stiffness properties of hollow built-up glass compression members that are intended for use in the modular construction of all glass, compression-dominant, shell-type structures. The proposed compression-dominant geometric form has been developed using the methods of form finding and three-dimensional graphical statics. This research takes the first steps towards a new construction methodology for glass structures where individual hollow glass units (HGU) are assembled using an interlocking system to form large, compression-dominant, shell-type structures, thereby exploiting the high compression strength of glass. In this study, an individual HGU has an elongated hexagonal prism shape and consists of two deck plates, two long side plates, and four short side plates, as is shown in Figure 1. Connections between glass plates are made using a two-sided transparent structural adhesive tape. The test matrix includes four HGUs, two each fabricated with 1 mm and 2 mm thick adhesive tape. All samples are dimensioned 64 cm on the long axis of symmetry, 51 cm on the short axis of symmetry, and are 10 cm in width. Glass plates are all 10 mm thick annealed float glass with geometric fabrication done using 5-axis abrasive water jet cutting. HGU assembly is accomplished using 3D printed truing clips and results in a rigid three-dimensional glass frame. Testing was done with the HGU oriented such that load was introduced on the short side edges of the two deck plates, resulting in an asymmetric load-support condition. A soft interface material was used between the HGU and steel plates of the hydraulic actuator and support for the purpose of avoiding premature cracking from local stress concentrations on the glass edges at the load and support locations. Force was applied in displacement control at 0.25 mm/minute with a full array of displacement and strain sensors. Test results for load vs. center deck plate transverse deflection are shown in Figure 2. All samples failed explosively by flexural buckling with no premature cracking on the load and support edges of the deck plates. Strain and deformation data clearly show the presence of second-order behavior resulting from bending deformation perpendicular to the plane of the deck plates. In general, linear axial behavior transitions to nonlinear second-order behavior, with increasing rates in deflection and strain growth, ultimately ending in glass fracture on the tension surfaces of the buckled deck plates. The failure resulted in near-complete disintegration of the deck plates, but with no observable cracking in any side plates and a secure connection on all adhesive tape. Results of the experimental program clearly demonstrate the feasibility of using HGUs for modular construction of compression dominant all-glass shell-type structures. This method of construction can significantly reduce the self-weight of the structure, and it will inspire the use of sustainable materials in the construction of efficient structures. 
    more » « less
  3. Functionally graded adhesive bondlines are currently being researched to relax stress concentrations at the re-entrant corner of bonded joints and improve the strength of joints. Bi-adhesive joints have been under development for some time, but lately adhesives with continuous gradation have been shown to theoretically enable more stress reductions and greater strength benefits. Several researchers have shown the potential to create a working adhesive gradation system with very promising results, but adhesive stability over long periods of time has proven difficult to realize. Nearly as important as adhesive development are analysis methods for functionally graded adhesive joints, since the gradation must be designed to yield beneficial results. Therefore, this work addresses the potential gains provided by design of functionally graded adhesive joints driven by finite element analysis. A parametric study on a strap joint with homogenous adhesive is conducted to highlight parameters which influence the global strength of an adhesively bonded joint. A statistical approach is used to identify significant correlations between strength and adhesive material parameters. Results from the statistical study are applied to drive strategies to create joints with optimized gradation and validated by failure analysis within the finite element model. A strap joint is analyzed as example of the potential gain of functionally graded joints. 
    more » « less
  4. Since the inception of carbon fiber-reinforced polymers (CFRPs) they have steadily gained in popularity due to their light weight, high tensile strength and modulus, and environmental toughness. However, curing of CFRPs of the thermosetting type generally must be performed within an autoclave, whose fixed, physical dimensions effectively limit the maximum size of the part. Alternative curing chemistries may potentially eliminate the requirement for an autoclave, which would allow creation of much larger panels. This project seeks to develop a thermoset composite matrix that is radiation-curable using the Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) click reaction. Previously, Storey et al.(1,2) reported that the azide-modified epoxy resin, di(3-azido-2-hydroxypropyl) ether of bisphenol-A (DAHP-BPA), could be cured by reaction with polyfunctional alkyne crosslinkers under mild conditions using Cu(I) catalysis. In the absence of reducing agents, Cu(II) compounds are catalytically inactive; however, upon exposure to ultraviolet light, they are reduced to Cu(I), which then catalyzes the reaction, allowing it to progress to a high degree of cure at room temperature. Herein, we report the kinetics of photo-induced CuAAC polymerization of the DAHP-BPA and several polyfunctional propargyl amine based crosslinkers, monitored by real-time FTIR as well as mechanical properties of fully cured materials. Polymerizations were studied as a function of Cu(II) compound type, Cu(II) concentration, UV light (365 nm) intensity, and duration of irradiation. 
    more » « less
  5. null (Ed.)
    Grading adhesive properties across a bondline can lead to more unniform stresses and increased strength without altering the geometry of the adherends. In this research, radiation sensitizers have been added to adhesives to create a secondary cross-linking possibility that is activated with radiation. In this way the adhesive stiffness and strength can be controlled by controlling the exposure to radition. In this paper, a system of grading adhesive properties is introduced and the double cantilver beam test results show that the gradation not only changes stiffness and strength, but also mode I fracture properties. Additionally, specimens were created with graded properties along the bondline and test results will be presented in the final paper. 
    more » « less