Strength Prediction And Fatigue De-Bond Growth In Bonded Joints In Metallic And Composite Structures
Sahoo, Pradeep Kumar
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Large scale structures such as those in aerospace flight vehicles are made in parts and assembled. Joints are inevitable in these systems and they are potential threats to the structural integrity of the flight vehicles. Fastener and bonded joints are the most commonly used methods of joining in these structures. Among these, adhesive bonding has become more popular with the advent of composite structures, due to the presence of less number of points of stress concentration and the resulting benefit for static strength and fatigue life. In modern aircraft in which maximum percentage of composite materials are being employed due to several benefits, designers are contemplating to replace discrete joints with adhesively bonded joints wherever possible. A detailed literature survey shows that the field of adhesively bonded joints has been extensively studied in the past. Initial publications appeared in late 1950’s and early 1960's, but many of the initial attempts were based on one dimensional (1-D) approximation of the adherents due to lack of computing power. With the current day emphasis on safety and damage tolerance, there is a definite need to study these joints with 2-D and 3-D idealization. In spite of valuable contributions in the literature from several researchers in past 4-5 decades, one finds that there are gaps to be filled, in particular, with reference to static strength prediction and de-bond growth to failure under fatigue loading. This thesis is intended as a modest contribution in this direction covering the methods of strength prediction and also correlations between de-bond growth and fracture parameters. Most commonly used bonded joints are single lap joints. The primary issue in their analysis is the geometric nonlinearity resulting in large deformations due to eccentricity of load path between the adherents. Further, adhesives have very low yield strength and plastic deformation in thin adhesives could affect the mechanics of load transfer. The current work is initiated by carrying out geometric and material nonlinear analysis of adhesively bonded single lap joints between metal-metal (aluminum-aluminum) adherents using standard NASTRAN finite element software. Modified Newton-Raphson iterative technique has been used to economize the computer time and also achieve fast convergence. A convergence study has been conducted to determine the order of mesh size required. Preliminary results are obtained on configurations analysed by earlier workers and the current results are compared with their results. Later, extensive experimental and numerical studies have been taken up on the numerical strength prediction of these joints correlating them to the experimental values. Cohesive failure along the centre line of the adhesive is assumed under both static and fatigue loading. The bonded joints are studied with both 2-D plane stress and plane strain nonlinear FE analysis. The issue in this type of analysis is the presence of theoretical elastic singularity at the ends of the lap length. The normally used maximum stress criterion can not be used in such circumstances. There were attempts in the past to use point stress or average stress criteria for this purpose. In point stress criterion the shear stress (or von-Mises stress) is picked at a characteristic distance away from the ends of the lap length and compared with the corresponding strength value to predict failure. In the average stress criterion the stresses are averaged over a characteristic distance from the ends of lap length and this is compared with the corresponding strength to predict failure. Determination of the characteristic distance in both the cases needs extensive experimental results on static strength of joints. The static strength data is to be correlated with numerical results to determine the characteristic distance in various specimens. In the current thesis a series of specimens with aluminum-aluminum, aluminum-CFRP composite and CFRP-CFRP composite adherents were tested to determine the static strength. In all the specimens the adhesive used was Redux 319 A. These experimental strength data was used to determine characteristic distance using point stress criterion. The consistency of estimates of the characteristic distances in all the specimens shows that the approach is capable of predicting the static strength. The above approaches are capable of predicting the strength of joints with linear material and nonlinear geometric analysis. But when the adhesive yield strength is low, a novel approach is required to predict the static strength. Numerical analysis is conducted using a combined material and geometric nonlinear analysis in NASTRAN software. The plastic zone size from the ends of the lap length is determined at different load levels. Combining the numerical results with experimental failure load data, a failure criterion based on plastic zone size (PZS) is proposed in this thesis and validated. It has been observed that the validation is with limited testing carried out and further experimental programs are required to complete the validation. To the best of the knowledge of the author PZS criterion is used for the first time for failure prediction of bonded joints. The structural integrity of the joints also requires a study of de-bond growth and damage tolerance assurances in the presence of de-bond type of defects. The first step in this direction is to estimate the fracture parameters at the tips of de-bond in the adhesive of lap joints between various adherents. Modified virtual crack closure integral (MVCCI) technique has been developed in the past for estimating Strain Energy Release Rates (SERR) in several crack problems. Large contributions for developing this technique have come from the group where the author has worked. This technique is simple and has the ability to estimate individual SERR components GI and GII in cases of mixed mode fracture. It is seen clearly that the de-bond growth in bonded joint is one of mixed mode. The mode-II component is because of shear stresses transferring the load across the adherents and mode-I component is due to peel stresses developed during the deformation. The mode I SERR component is primarily responsible for de-bond growth and the effect of mode II component on de-bond growth is insignificant. The mesh details for accurately estimating the SERR components are evaluated and those meshes are used to estimate these values for the cases of aluminum-aluminum, aluminum-CFRP composite and composite-composite joints. Obviously, when the adherents are dissimilar, mode I SERR components are the highest and assist faster de-bond growth. Painstaking fatigue de-bond growth experiments were conducted and de-bond growth rate with number of cycles of fatigue loading was determined. MVCCI method is used to estimate SERR components at maximum load and zero load in the fatigue cycle, to determine the SERR range in the fatigue cycle. Since the stress ratio, R of the loading cycle is -1, the minimum load for estimating SERR components is taken as zero. From the experimental data and numerical estimates, a Paris type of equation was developed for the de-bond growth. The thesis concludes with a summary of the achievements in the current work with respect to the structural integrity of adhesively bonded joints and also with suggestions for future work.
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