A CAD Tool for Rapid analysis of Repairs

The objective of this project is to develop and validate a CAD tool for the rapid design of mechanically fastened repairs, bonded repairs, and hybrid repairs involving both adhesive bonding and mechanical fasteners. To allow use by non-specialists, the CAD tool will replace the current process of manual construction of joint finite element models with CAD tinker toy assembly using parametric parts. With parametric parts, the global finite element model of the aircraft can serve as a construction aid for the drag and place construction of the repair from parametric parts, i.e., skin, angle, doubler, fastener pattern, channel, cut-out, adhesive patch, etc. These parametric parts will define the external and internal boundaries and the interior points for the finite element mesh for each member. Virtual fastener elements will allow layers to be connected with a special line element which is an extension of the Barrois beam-on-elastic foundation model of the fastener to n-layers. The virtual fastener elements, along with the parametric parts, will allow repairs to both the skin and to the sub-structure to be easily synthesized. Once the CAD model of the repair has been constructed, a divide and conquer automeshing routine which can maintain nodal compatibility between the layers at the fastener pierce points and at the adhesive spring attach points will be used to generate the finite element mesh for the local model of the joint on a layer-by-layer basis. The tool will have the capability to perform static strength, fatigue and damage tolerance analyses of both metal and composite members. A scenario modeler and a multi-dimensional table data base management system will permit users to customize the methodology and the data used in the substantiation of the repair. A 2 ½ D prototype tool will be constructed in Phase I, and used to study repairs to a damaged composite skin which is mechanically attached to aluminum substructure. The geometry is chosen because it is typical of composite construction for fighter aircraft and also because it highlights the need for a repair tool to account for the interaction between the repair doubler, the skin and the sub-structure. Analyses performed in the Phase I proof-of-concept demo will include:(1) Static strength analysis of a external doubler mechanically attached to the skin, (2) Static strength analyses of a bonded scarfed doubler, (3) Damage tolerance analysis of a partially delaminated bond-line for the scarfed doubler, and (4) Fatigue analysis of the metal substructure. The proof-of-concept demo will verify that the proposed approach has the ability to substantiate--from a single CAD environment-- the structural integrity of bolted, bonded and hybrid bolted/bonded repairs to DOD and FAA airworthiness requirements for static strength, durability, and damage tolerance. BENEFITS: The expected result of the Phase I and II effort is an improved joint design methodology which will replace the current process of using separate stand alone programs for laminate analysis, joint analysis, strength analysis, and fatigue/damage tolerance analysis with a single integrated environment in which labor intensive operations such as meshing of a finite element model, or extraction of laminate ply properties, are performed automatically. Because it integrates into one platform the tasks now performed by multiple, non-compatible programs, and because the parametric technology will allow the construction and storage of parametric models of "reusable" structure, dramatic improvements in productivity will be possible.