Bondline thickness effects

Delale F, Erdogan F, Aydinoglu MN (1981) Stresses in adhesively bonded joints-a closed form solution. J Compos Mater 15 249-271 Fernlund G, Spelt JK (1991) Failure load prediction of structural adhesive joints, Part 1 analytical method. Int J Adhes Adhes ll(4) 213-220 Fernlund G, Papini M, McGammond D, Spelt JK (1994) Fracture load predictions for adhesive joints. Compos Sci Technol 51(4) 587-600 Goland M, Reissner E (1944) The stresses in cemented joints. JAppl Mech 11 A17-A27 Gleich DM, Van Tooren MJL, Beukers A (2001) Analysis and evaluation of bondline thickness effects on failure load in adhesively bonded structures. J Adhes Sci Technol 15(9) 1091-1101... 

Gleich DM, van Tooren MJL, Beukers A (2001) Analysis and evaluation of bondline thickness effects on failure load in adhesively bonded structures. J Adhes Sci Technol 15 1091... 

Figure 13 Effect of bondline thickness on the cleavage crack intiation energy and cleavage crack arrest energy Gj of hard maple specimens bonded with rigid thermosetting PRF adhesive. (From Ref. 3.)...

Ji, G., Ouyang, Z., and Li, G. (2011) Effects of bondline thickness on Mode-II interfacial laws of bonded laminated composite plate. International Journal of Fracture, 168, 197-207. 

Scarf joint. The importance of testing adhesive fracture under mixed-mode stress conditions has been noted. In the scarf joint (Fig. 4.14(e)), the applied load is resolved in the bondline to Mode I and Mode II components and their ratio changes with the scarf angle, < >. Bascom et al. 26, 70) investigated the effects of bondline thickness and test temperature, and G,i. n)c for < ) = 45° was calculated from the failure load and crack length using a finite-element analysis. A complex behaviour pattern emerged, as discussed... 

There are numerous examples of the application of fracture mechanics to structural adhesive systems. Most notable are those of Mostovoy and his coworkers which have already been mentioned. " Bascom and coworkers have made significant contributions to the understanding of the effect of bondline thickness on fracture toughness. Kinloch and Shaw extend the work of Bascom to include rate effects and to develop mathematical models of the fracture resistance of adhesives. Hunston et al have used these methods to study viscoelastic behavior in the fracture process of structural adhesives.Mostovoy and Ripling used these techniques to determine the flaw tolerance of several adhesives,while Bascom and Cottington have studied the effect of flaws caused by air entrapment in structural adhesives." Finally it must be mentioned that one of the most simple, most widely used tests for strucural adhesives, the peel test, is actually a version of the double cantilever beam test. 

The use of phenolic resins for wood bonding has been reviewed by Pizzi, Dinwoodie, Kollmann and coworkers, " the U.S. Department of Agriculture, and Barth. Tapered double cantilever beam specimens have been recently used to test the effect of bondline thickness and cure time on the fracture energy of phenolic-wood adhesive joints. ... 

Table VIII. Effect of Bondline Thickness on OLS Bond Strength (Primed CRS)...

Tong L, Steven GP (1999) Analysis and design of structural bonded joints. Kluwer, Boston Van Tooren MJL (2004) Experimental verification of a stress singularity model to predict the effect of bondline thickness on joint strength. J Adhes Sci Technol 18(4) 395-412... 

The effect of the bondline thickness on single-lap joints is well-documented in the literature. Most of the results are for typical structural adhesives and show that the lap-joint strength decreases as the bondline increases (da Silva et al. 2006 Adams and Peppiatt 1974). Experimental results show that for structural adhesives, the optimum joint strength is obtained with thin bondlines, in the range of 0.1-0.2 mm. However, the classical analytical models such as those of Volkersen (1938) or Goland and Reissner (1944) predict the opposite. There are many theories that attempt to explain this fact and this subject is still controversial. Adams and Peppiatt (1974) explained that an increase in the bondline thickness increases the probability of having internal imperfection in the joint (voids and microcracks), which will lead to premature... 

Al-Samhann and Darwish (2003) demonstrated with the FE method that the stress peaks typical of adhesive joints can be reduced by the inclusion of a weld spot in the middle of the overlap. They studied later (Darwish and Al-Samhann 2004) the effect of adhesive modulus and thickness and concluded that for rational design of weld-bonded joints, adhesives of low Young s modulus are recommended with the largest possible bondline thickness. Darwish (2004) also investigated weld-bonded joints between dissimilar materials and found that the introduction of an adhesive layer resulted in an increased joint strength and a better stress distribution. 

To determine adhesive failure, it was necessary to apply appropriate algorithms to the data For quantitative analysis data were imported to a spreadsheet, smoothed to remove noise from the LVDTs, and then sorted to remove edge effects. Because there was considerable warp in all specimens due to the durability test, a parabolic function was fit to this distortion and subtracted from the raw data to produce a flat bondline. The data were again sorted (in ascending order) to produce a cumulative frequency distribution of surface irregularities (wood failure). Conceptually, a thickness tolerance could then be specified to define the bondline region as well as a depth tolerance for shallow wood failure. The relative population of data within these regions represented the percent e of adhesive, shallow, and deep wood failure. 

Ideally, this method would be fairly simple to implement on flat specimens. However, all the specimens tested contained significant degrees of warp due to the effects of a vacuum-pressure exposure test. In many specimens, the magnitude of this distortion was several times that of the surface irregularities. Therefore, it was necessaiy to flatten the bondline by fitting a parabolic function to the cup and then subtract the fit from the raw data. Next, the data were sorted in ascending order to produce a cumulative distribution for all measured surface displacements. A thickness tolerance could then be specified to define the bondline region as well as a depth... 

Hart-Smith (references 5.25, 5.26, 5.30 and 5.31) has conducted extensive studies of bonded joints using the elastic—plastic model for the adhesive. He has covered the analysis of lap, strap, scarf and step-lap joints. He has modified the load eccentricity induced peel stress approach by using a modified bending stiffness. He has studied the effects of non-uniform adhesive thickness, adhesive non-uniform moisture absorbtion and defects in the bondline. He has also included thermal stresses in his models. 

Bondline effects have been seen immediately on appUcalion (when any evaporative cooling was just beginning). It is not known whether this was due to different temperatures across the part (e.g., from the primer bake) or because of electrostatic effects. Bondline readout is greatly affected by the application process (type of gun or bell, number of passes), flash time (longer flash reduces the effect), the thickness of the primer, the design of the part, and the temperature of the oven. Bondline effects that occur on apphcation have been known to diminish or disappear with higher bake temperatures. 

It was shown that RH, temperature, and strain rate greatly affect tensile properties. To properly account for their effects, the tiiree parameters must be carefiilly controlled. Most important is the need to allow enough time for the moisture content in a bulk adhesive specimen to equilibrate with the RH of the environmoit before the test is started. To this end, specimens of0.025 mm or less thickness—typical of bondlines — should be conditioned for at least 48 hours. 

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