Study On The Stress Distribution Of Adhesively Bonded Joint With Nonstandard Lap Zone

The elasto-plastic finite element method (FEM) and experiment were used to investigate the stress distribution of recessing bonded single lap joint, recessing bonded double lap joint and recessing bonded cleavage joint, and the influence of the mechanical properties of adhesive and the figuration of adherend on the stress distribution in adhesively bonded aluminium single lap joint. The stress distribution of joint was optimized and the strength was improved through the analysis of the stress and strain distribution of bondline and the stress distribution of adherend, and the following conclusions have been drawn:1) The influence of recess on the peak stress of joint bonded with high elastic modulus adhesive was not evident, improving the actual strength of joint largely basing on the nominal strength not being reduced evidently, but recess caused the peak stress of joint bonded with low elastic modulus adhesive to be raised evidently, reducing the nominal strength and actual strength of joint markedly.2) All of the elastic modulus, Poisson ratio, yield strength and hardening modulus of adhesive influenced the stress distribution of single lap joint, but the influence of elastic modulus was greatest. For the joint bonded with high elastic modulus adhesive, the function of adhesive fillets was very large, but the function of adhesive fillets was very small as that in the one bonded with low elastic modulus adhesive. If adhesive fillets were adopted adhesive with high elastic modulus and high yield strength, the strength of single lap joint could be improved. Bi-adhesive bonding with fillets could optimize the stress distribution of joint and improve the strength.3) The influence of reasonable recess on the peak stress of double lap joint in the mid-bondline was inconspicuous, reducing the quantity of used adhesive and lightening joint, meanwhile, increasing the nominal strength great and actual strength decreased little. The influence of recess length on the nominal strength could be negligible, but increased the actual strength great. Since the bondline of recessed bonding joint was divided into several segments, then the restriction of bondline in the course of curing was smaller than the state without recess, so the inner stress was lower and benefit for the strength of joint.4) The influence of reasonable recess on the peak stress of cleavage joint was negligible, and it was not harmful to the nominal strength. The influence of the location of recess center on the peak stress was evident, but the influence of recess length on the stress peak was negligible. Each peak stress increased rapidly when there was not adhesive at the end of joint, and the nominal strength reduced markedly.5) The inner chamfer in the unload ends of adherends reduced the peak stress in the mid-bondline enormously. As the chamfer height increasing or the chamfer angle decreasing, the stress in the corner decreased first and then increased again, and the stress reached minimum whenθ=30°. For the joint bonded with high elastic modulus adhesive, the inner chamfer reduced the peak stress remarkably and the peak stress transferred from fillets to the bondline, so that the strength increased. For the joint bonded with low elastic modulus adhesive, the inner chamfer increased the peak stress instead, so it was not benefit for the joint.6) The peak stress of mid-bondline reduced for the joint with outer chamfer when the elastic modulus of adhesive decreased. For the joint bonded with high elastic modulus adhesive, the outer chamfer reduced the peak stress remarkably, while the function of outer chamfer could be ignored for the joint bonded with low elastic modulus adhesive.7) The step figuration of adherend decreased the peak stress of mid-bondline markedly. The stress transferred from the corner to the middle of lap zone, and the stress of middle increased. The influence of the step length on the peak stress was little. The stress of joint bonded with high elastic modulus adhesive decreased markedly due to the existence of step, but the function of step could be ignored for joint bonded with low elastic modulus adhesive.8) The peak stress of mid-bondline decreased markedly when there were a couple of notches located in the out side of the adherend corresponding to the middle part of the lap zone. The peak stress decreased firstly and then increased again as the length of notch increased. For the stress around the corner decreased gradually as the depth of notch increased, and the stress of middle increased gradually. The peak stress of mid-bondline increased when the elastic modulus of adhesive increased. The influence of notch on the stress distribution and peak stress of mid-bondline with low elastic modulus adhesive could be negligible, but for the joint bonded with high elastic modulus adhesive, the stress in the middle of joint where there was low stress originally increased evidently, bearing more load, meanwhile, the stress concentration around the corner and the peak stress decreased markedly.