Detailed Analysis of a Complex Bonded Joint

Rousseau, C.Q., Ferrie, C., Hoyt, D.M., Ward, S.H.

Presented at the American Helicopter Society (AHS) Hampton Roads Chapter Structures Specialists’ Meeting, Williamsburg, VA, November 2001.

The motivation for this research was the discovery that strength of material analyses of bonded joints was overly conservative. The objective of this paper was to evaluate both stress- and interlaminar-fracture-based analysis methods based on detailed finite-element (FE) models for static and fatigue loading. It was found that detailed FE analysis generally yielded lower percentage errors, suggesting either (a) that predicted stresses from a detailed FE analysis were either lower than the typical strength-of-materials-based approach for the same location and failure mode; (b) different, more localized, failure modes and locations were interrogated in the detailed analyses; or (c) both. However, detailed local modeling techniques do seem to be quite sensitive to small changes in input data, modeling approach, or both.

A great deal of research over the years has focused on development of bonded joint technology, particularly stress analysis methods. One such effort was a recent U.S. Army contract titled Primary Structure Composite Joining (PSCJ) (Ref. 1), in which a variety of analytical approaches were evaluated for structural bonded joint static and fatigue strength prediction. A summary of the overall project has previously been presented in Ref. 2. The focus of this paper is solely on the detailed interlaminar-fracture-based static and fatigue analysis. This analysis was initiated after high levels of conservatism in the standard strength-of-materials-based analyses were discovered. Two additional analytical efforts (in-house at Bell and subcontracted to NSE Composites) were initiated using finite-element analyses and advanced stress- and interlaminar-fracture-based failure criteria. The relatively complex bulkhead-hat-tee-sidebody joint under various static and fatigue loading conditions was the focus of these efforts (see Figs. 1 and 2). For the Bell analysis, it is assumed that the adhesive behaves in a rigid-perfectly plastic manner, allowing a much simpler shear stress calculation. Peel stress checks and complex shear loadings are typically done using simple hand- or spreadsheet-calculated strength-of-materials approximations.

In the past few years, local finite-element models (FEM) have become a much more popular way to determine complex bonded joint stress fields. On the PSCJ program, the NASA-Grumman SUBLAM code was used, in one instance, to obtain shear and peel stress distributions in a honeycomb skin-stiffener detail. An emerging bonded joint stress analysis method is that which uses an interlaminar fracture-based, rather than stress-based, failure criterion. The advantages of this type of failure criterion are that it accurately reflects the physical nature of interlaminar failure of laminated media. Avoids problems associated with picking appropriate values from stress distributions with high gradients. Is amenable to damage tolerance analyses, where discrete flaws must be assumed to exist in part. Is amenable to fatigue delamination growth-based life prediction. The main disadvantage to fracture-based methods is the amount of manpower and computational effort required to perform the necessary global and local finite-element analyses and associated pre- and post-processing. Also, the analysis results are sensitive to input data and modeling approach.

The objective of this paper is to present the detailed analysis results, compare them to both static and fatigue experimental data, and identify advantages and shortcomings of the detailed finite-element modeling/interlaminarfracture-based approach. First, the test data will be summarized; the next section will discuss the details of both the Bell and NSE modeling efforts; the correlation between test and analysis will be presented; and a final section will summarize the findings.