S. Azari

Effect of Surface Roughness on the Performance of Adhesive Joints Under Static and Cyclic Loading

The role of surface roughness on the fatigue and fracture behavior of a toughened epoxy adhesive system was investigated experimentally. Fatigue studies covered both the fatigue threshold strain energy release rate, G th , and fatigue crack growth rates, while the strain energy release rate for crack initiation, , and the steady-state value, , were measured under quasi-static loading. Mixed-mode fatigue results showed a significant dependency on surface roughness. G th increased with roughness, reached a plateau, and then decreased for very rough surfaces. This increase in G th was explained in terms of the increase in bonding and fracture surface area, crack growth retardation due to the microtopography of the substrate, and crack path deviation from the interface. The decrease in G th for very rough substrates was attributed to void formation and stress concentration at the tip of asperities. The effect of roughness on fatigue diminished as the applied strain energy release rate increased. This was a result of the crack path becoming more cohesive, moving away from the interface. Similarly, no effect of surface roughness was observed in the mode-I fatigue results and the mixed-mode fracture results, since the crack path in these cases was far enough from the interface.

Characterization and prediction of fracture properties in hygrothermally degraded adhesive joints: an open-faced approach

One of the challenges in the application of structural adhesive joints is the prediction of their long-term durability. During the service life, moisture diffuses into the adhesive layer and eventually degrades its fracture properties. Environmental degradation should thus be taken into consideration in the design and analysis of adhesive joints. This work first provides an overview, summarizing the recent efforts regarding the hygrothermal exposure of adhesive joints, accelerated aging methods, water diffusion modeling, and characterization of fracture properties in adhesively bonded joints. The second part presents a recent degradation methodology by which the fracture toughness evolution of adhesive joints can be predicted using fracture test data obtained using the accelerated open-faced degradation method.

Analysis and design of adhesively bonded joints for fatigue and fracture loading: a fracture-mechanics approach

An experimental–computational fracture-mechanics approach for the analysis and design of structural adhesive joints under static loading is demonstrated by predicting the ultimate fracture load of cracked lap shear and single lap shear aluminum and steel joints bonded using a highly toughened epoxy adhesive. The predictions are then compared with measured values. The effects of spew fillet, adhesive thickness, and surface roughness on the quasi-static strength of the joints are also discussed. This fracture-mechanics approach is extended to characterize the fatigue threshold and crack growth behavior of a toughened epoxy adhesive system for design purposes. The effects of the mode ratio of loading, adhesive thickness, substrate modulus, spew fillet, and surface roughness on the fatigue threshold and crack growth rates are considered. A finite element model is developed to both explain the experimental results and to predict how a change in an adhesive system affects the fatigue performance of the bonded joint.