On the validity of linear elastic fracture mechanics methods to measure the fracture toughness of adhesive joints

Abstract

The analysis of large-scale fracture processes, such as those involved in the fracture of adhesive joints, falls outside the scope of Linear Elastic Fracture Mechanics (LEFM). However, experimental data produced in testing adhesive joints are usually reduced with LEFM methods. The consequent error has not yet been evaluated. In this work, an experimental characterization under pure-mode loading of an FM-300 epoxy film adhesive is presented for different adhesive and adherend thicknesses. The experimental data is analyzed using both LEFM-based and J-integral-based data reduction methods in order to study their suitability to analyze adhesive joints. LEFM-based data reduction methods are shown to entail a relevant deviation in the fracture toughness results that heavily depends on the size of the fracture process zone. It is concluded that LEFM methods are not suitable to characterize adhesive joints and that their use is restricted, at best, to the measurement of initiation values. The effect that the adhesive and the adherend thicknesses have on the fracture toughness and the R-curve of the material is studied. Adhesive and adherend thicknesses are shown to have a significant influence on the bond fracture toughness and the source of such influence is discussedThe first author would like to acknowledge the FPU programgrant AP20100977 from the Spanish Ministerio de Educación,Cultura y Deporte. The authors also acknowledge the support of the Spanish government through the Ministerio de Economía y Competitividad under the contracts DPI2012-34465 and MAT2012-37552-C03-0

Mode I Interlaminar Fracture Toughness Analysis of Co-Bonded and Secondary Bonded Carbon Fiber Reinforced Composites Joints

“…Additionally to composite material itself, the study of alternative bonding techniques has also become a topic of relevance in the industry, taking place of the conventional screws and rivets, which not only increase weight but also deteriorate the components structural capacity because they actuate like a stress concentrator contributing to severe delamination problems 3,4 . In this manner, co-cure, co-bonding and secondary bonding techniques have received special attention in recent years, with emphasis on the last two for allowing more varied project designs, sizes and simpler manufacture 3 , besides lowering or suppressing the stress concentration 5 . The difference between each of them is that in co-cure, two uncured laminates are cured simultaneously, in the presence of an adhesive or not.…”

Long-term durability of bismaleimide composite in marine environments

“…(a) Load-displacement curves (b) rotation angles vs. crosshead displacement at loading point and (c) Mode I interlaminar fracture toughness results (d) observed and calculated delamination lengthHowever, larger deviations were observed in the measurements for the wet specimens, especially for propagation values. Similar results were also reported and discussed in a recent study of Sarrado at el [145]…”

“…They also compared the use of the LEFM-based method and the J-integral method for analyzing adhesive joints under mode I and mode II loadings. The results showed that the LEFM-based method was not suitable for the analysis of adhesive joints fracture, as it depends heavily on the size of the FPZ [145].…”

Measurement of interlaminar fracture properties of composites using the J-integral method

“…All the specimens experienced a sudden failure as the loading curves dropped abruptly after the peak values. The total energy release rate (G T ) was calculated by equations (15), (16) and (17), and the ratio mixity given by equation (18). The J MMB was evaluated using equation (9).…”