Cooperative Research and Development Agreement Final Report for Cooperative Research and Development Agreement Number ORNL93-0237 Adhesive Bonding Technologies for Automotive Structural Composites

In 1993, the Oak Ridge National Laboratory (ORNL) entered into a Cooperative Research and Development Agreement (CRADA) with the Automotive Composites Consortium (ACC) to conduct research and development that would overcome technological hurdles to the adhesive bonding of current and future automotive materials. This effort is part of a larger Department of Energy (DOE) program to promote the use of lighter weight materials in automotive structures for the purpose of increasing fuel efficiency and reducing environmental pollutant emissions. In accomplishing this mission, the bonding of similar and dissimilar materials was identified as being of primary importance to the automotive industry since this enabling technology would give designers the freedom to choose from an expanded menu of low mass materials for component weight reduction. The research undertaken under this CRADA addresses the following areas of importance: bulk material characterization, structural fracture mechanics, modeling/characterization, process control and nondestructive evaluation (PC/NDE), manufacturing demonstration, and advanced processing. For the bulk material characterization task, the individual material properties of the adherends and adhesives were characterized. This included generating a database of mechanical and physical properties, after identifying and developing standard test methods to obtain properties. The structural fracture mechanics task concentrated on test development to characterize the fracture toughness of adhesively bonded joints subjected to Mode I, Mode II and mixed-mode conditions. Standard test procedures for quantifying an adhesive/adherend system's resistance to crack growth were developed for use by industry. In the modeling/characterization task, fracture mechanics-based design guidelines and predictive methodologies have been developed which will facilitate iteration on design concepts for bonded joints while alleviating the need for extensive testing. Methods for nondestructive evaluation of adhesive bonds that can be used for process optimization, in-line process control and product validation were evaluated in the PC/NDE task. Promising NDE techniques were identified for additional development. In the advanced processing task, rapid-cure and advanced surface preparation processes were investigated with the goal of increasing the manufacturability and performance as well as reducing the costs of bonded composites. Demonstration that a ''designed for composites'' structure is manufacturable was undertaken in the manufacturability demonstration task. In addition to the aforementioned efforts, ancillary topics that were coordinated by the CRADA partners will be discussed briefly. These include the performance of bonded composite structures in crashes and fatigue durability of bonded and hybrid joints. This report covers the activities undertaken during the CRADA through February 1997

Interlaminar strains at the free edge of a hole in laminated composites: An experimental study

Free-edge effects in laminated composite materials were studied experimentally using high-sensitivity moire interferometry. Six laminates from two material systems were tested in uniaxial compression on an electro-mechanical testing machine. Interlaminar deformations were measured on a ply-by-ply basis at the straight free-edge and, for the first time, on the cylindrical surface of a hole. Strain distributions were determined with high fidelity for the hole surface and the straight free edge of the thick composite panels. Comparisons were made on a ply-by-ply basis for the transverse and tangential strains at the horizontal centerline of the hole (90{degree} location) and the corresponding plies at the straight boundaries

Fracture testing and analysis of adhesively bonded joints for automotive applications

In 1992, the Oak Ridge National Laboratory (ORNL) began a cooperative effort with the Automotive Composites Consortium (ACC) to conduct research and development that would overcome technological hurdles to the adhesive bonding of current and future automotive materials. This effort is part of a larger Department of Energy (DOE) program to promote the use of lighter weight materials in automotive structures for the purpose of increasing fuel efficiency and reducing environmental pollutant emissions. In accomplishing this mission, the bonding of similar and dissimilar materials was identified as being of primary importance to the automotive industry since this enabling technology would give designers the freedom to choose from an expanded menu of low mass materials for component weight reduction. This paper concentrates on the details of developing accurate fracture test methods for adhesively bonded joints in the automotive industry. The test methods being developed are highly standardized and automated so that industry suppliers will be able to pass on reliable data to automotive designers in a timely manner. Mode I fracture tests have been developed that are user friendly and automated for easy data acquisition, data analysis, test control and test repeatability. The development of this test is discussed. In addition, materials and manufacturing issues are addressed which are of particular importance when designing adhesive and composite material systems

Laser ablation assisted adhesive bonding of automotive structural composites

Laser ablation has been evaluated as a surface pretreatment prior to adhesive bonding. In prior experimental work, it was observed that when adhesively bonded, composite, single lap shear samples fail, the fracture often occurs at either the adhesive/adherend interface or in the resin rich surface layer of the composite. These two areas represent the weakest portion of the joint. Laser ablation pretreatment generates areas where the resin on the composite surface is selectively removed leaving behind exposed reinforcing fibers which are the major load bearing members of the composite. In a subsequent adhesive bonding operation, this allows portions of the fibers to be encapsulated in the adhesive while other portions of the fiber remain in the composite resin. This type of pretreatment permits fibers to bridge and reinforce the interface between adhesive and adherend. A secondary benefit is the removal of surface contaminantes by pyrolysis. Microscopic observation of laser ablated surfaces indicates a prominent, fiber rich area. Results of the mechanical evaluation indicated that the lap shear strength for laser ablated samples was significantly higher than specimens with no pretreatment or with solvent cleaning only, but were slightly lower than specimens that were mechanically roughened and cleaned with solvents prior to bonding

A Practical Test Method for Mode I Fracture Toughness of Adhesive Joints with Dissimilar Substrates

A practical test method for determining the mode I fracture toughness of adhesive joints with dissimilar substrates will be discussed. The test method is based on the familiar Double Cantilever Beam (DCB) specimen geometry, but overcomes limitations in existing techniques that preclude their use when testing joints with dissimilar substrates. The test method is applicable to adhesive joints where the two bonded substrates have different flexural rigidities due to geometric and/or material considerations. Two specific features discussed are the use of backing beams to prevent substrate damage and a compliance matching scheme to achieve symmetric loading conditions. The procedure is demonstrated on a modified DCB specimen comprised of SRIM composite and thin-section, e-coat steel substrates bonded with an epoxy adhesive. Results indicate that the test method provides a practical means of characterizing the mode I fracture toughness of joints with dissimilar substrates

Polymeric materials for impact and energy dissipation

Automotive plastic components are often required to withstand impact loadings and dissipate energy in automotive collisions, protecting occupants and pedestrians. The design of plastic components against impact loading is not a trivial engineering task, but still a challenging activity. The optimisation of the impact behaviour of plastic products requires a global approach involving material properties, processing methods and geometrical design solutions. This communication presents solutions to develop plastic components requiring high impact performance, based on a highly interrelated triad: polymeric material systems with improved impact toughness, processing methods for plastic products with enhanced toughening performance and design solutions for plastic components with superior impact behaviou