Strength predictions of bonded joints using the critical CTOA criterion
Abstract
The critical CTOA criterion was applied to predict the strength of DCB bonded joints and single-lap bonded joints for specimens manufactured with a variety of adhesive thicknesses. Both specimen types were bonded with a rapid curing structural acrylic adhesive, Loctite® SPEEDBONDER H3000. Characterization tests showed that the adhesive exhibited significant non-linear stress-strain behavior. The critical CTOA of the adhesive was experimentally measured for a range of distances behind the crack tip and was shown to be independent of adhesive thickness. Significant crack tip blunting and crack tunneling was observed during the DCB tests, so a method for measuring the critical CTOA in reference to the location of the original crack tip was developed to eliminate the variability in the critical CTOA measurements due to the irregular nature of the crack blunting. The non-linear behavior of the adhesive coupled with the high degree of constraint in the adhesive bondline generated difficulties in utilizing the FEM to model the DCB specimens and to predict failure with the critical CTOA criterion. A technique was developed to account for the adhesive non-linearity and yielding behavior in a linear elastic analysis using Irwin’s extended crack concept. Predictions of DCB specimen strength using the critical CTOA criterion in conjunction with the extended crack concept agreed very well with experimental results for a variety of adhesive thicknesses. Single-lap specimens were tested and an attempt was made to directly correlate the critical CTOA measured from the DCB specimens to the prediction of strength of the single-lap specimens. The mixed mode loading in the single-lap specimen bondline together with the non-linear behavior of the adhesive and the high degree of constraint in the adhesive bondline prevented making any direct correlations between the critical CTOA measured from the DCB specimens and the strength of the single-lap joints. A method was developed to account for the high degree of constraint on the suppression of yield in the adhesive using the extended crack concept. The critical CTOA from mode I loading conditions was utilized to predict failure of the single-lap joints in conjunction with the extended crack concept and a calibrated model was developed to predict failure of the single-lap joints for a range of adhesive thicknesses. Strength predictions made with the calibrated model agreed well with experimental results, but the nature of the calibration and the range of adhesive thicknesses for which the calibration is valid make the model of limited utility.
Degree
M.S.E.
Advisors
Pipes, Purdue University.
Subject Area
Mechanics|Aerospace engineering|Mechanical engineering
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