Abstract
The focus of our study is to identify physical properties of different impact-resistant/toughened structural adhesives and identify/develop an elastic-viscoelastic-plastic model as a function of loading rate by using Ludwik-type equations to be able to predict adhesive behavior at higher loading rates and to make cars more crashworthy. For this purpose, we first characterized eight different commercial toughened epoxy structural adhesives to provide detailed information about their constituents using X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA), scanning electron microscope (SEM) and energy dispersive x-ray spectrometer (EDS). Most (but not all) of the model adhesives contained organic tougheners in the form of carboxyl terminated butadiene acrylonitrile (CTBN) copolymer, as well as polyurethane adducts. The main crystalline inorganic phases were found as calcite (CaCO3), wollastonite (CaSiO3) or calcium silicate (CaSiO3), talc (Mg3Si4O10 (OH)2), zeolite which is an alumina silicate based mineral and has many different elements in its composition (M2/nO·Al2O3·xSiO2·yH2O, M can be Mg, Na, Ca, K, Li). The total amount of inorganic fillers was found to be different in each adhesive. Material behavior of the model adhesives were determined via tensile tests and Single Lap Joint (SLJ) tests in shear. Split Hopkinson pressure bar (SHPB) was also used to measure the strain and stress values at higher strain rates in the order of 102 s−1, which is generally encountered in impact related loading situations. Toughness values in the range ~0.5 to ~1.35 MJ/m3 were observed with the model adhesives tested in tensile mode within the ~3 × 10−3 to 0.18 m/m/s strain rate range. The softening behavior of the elastic moduli at higher strain rates observed during tensile testing was also observed with SHPB testing. It is remarkable that, overall, the modulus magnitudes seem to be similar between the tensile test and SHPB specimens within this softening range of the initial bilinear elastic behavior observed. When the results from bulk (tensile) and bonded (shear) specimens were compared, it was clearly seen that the toughness responses of the adhesives to (tensile/shear) strain rates in the bulk and bonded forms, respectively, were different, with the bonded shear toughness values in the ~25 to ~120 MJ/m3 range within ~1.25 to ~25 mm/mm/s shear strain range. The model adhesive which included just inorganic fillers had the lowest tensile toughness at the lowest tensile strain rate, but the highest slope in its tensile toughness regression line, exhibited the second highest bonded shear toughness. When tested at the extension rates of 25 mm/min and 100 mm/min in bonded lap shear, the same adhesive exhibited limited interfacial failure areas, however the dominant failure mode was cohesive failure. When the extension rate increased further, transition to interfacial (adhesive) failure was observed revealing that interfacial failures do not necessarily diminish adhesive bond toughness. Our observations point to the fact that cohesive deformation/failure processes indicating interfacial separations, inter-particle interactions as well as polymer matrix deformation in high deformation loading scenario as in bonded shear loadings may provide the highest toughness. Apparently, a large inorganic filler weight fraction is not necessary to obtain high shear toughness in bonded form since the highest bonded shear toughness was obtained with the adhesive which had the least amount of inorganic fillers among the model adhesives with 14.72 wt %.
Highlights
Structural adhesives have been widely used in many industrial sectors such as aerospace, marine, aeronautics and automotive industry due to their energy absorption capability, high fatigue strength, flexibility, toughness and lightness of weight.The importance of weight reduction in vehicles is increasing day-by-day in automobile industry due to the requirement of lower CO2 emission and fuel consumption requirements
The important common observation from all differential thermal analysis (DTA) curves of adhesives 1–4 is the fact that a large amount of energy is absorbed beyond mid six-hundred degrees centigrade through processes involving decomposition of weakly crystalline calcite followed by the decomposition of well crystallized calcite
Adhesive materials were characterized by X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA), scanning electron microscope (SEM) and energy dispersive x-ray spectrometer (EDS)
Summary
Structural adhesives have been widely used in many industrial sectors such as aerospace, marine, aeronautics and automotive industry due to their energy absorption capability, high fatigue strength, flexibility, toughness and lightness of weight. Much research is being done to reach US corporate average fuel efficiency (CAFE) requirements which target to increase mileage to 54.5 miles per gallon by the year 2025 for passenger cars and light trucks [1] Structural adhesives serve these targets by enabling the bonding to similar, and dissimilar substrates including aluminum, advanced high strength steels, magnesium, carbon fiber, plastics, wood and composites. By using load bearing structural adhesives, high-strength and tough structures can be incorporated into automotive bodies to create impact resistant automobiles with improved crashworthiness to lower the number of deaths in traffic accidents For this purpose, there are mainly two different classes of structural adhesives that have been used in automotive industry; one part and two-part epoxies and two-part polyurethanes. Researchers have been trying to get a better understanding of mechanical response of these toughened adhesives in order to develop predictive modeling, especially, for vehicle crashworthiness analysis and to provide safer conditions for the driver and passengers against the crash effects in automobile accidents
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
