Residual Stress Management in Machined Parts

Distortion is a prevalent problem in airframe parts machined from aluminum plate and forgings. Stock removal alters grain structures and disturbs the balance of the materials bulk residual stress. The resulting imbalance is manifested as distortion, which can be as severe as several inches.

The current industry practice to control distortion uses a series of sizing operations to maintain in-process stability. However, the proper sequence always requires expensive iterations to develop and is rarely 100% reliable. The result is a 50% increase in process development costs and recurring quality problems on 10% to 20% of components produced.

Proposed Solution/Approach

MSC Corporation, with the participation of Sikorsky, has developed a software module to MSC.MARC that can predict the resulting distortion after machining a structure from a plate or billet with a known residual stress profile. The software merges the volume machined away with the materials beginning volume, recalculating the stresses and the related distortional effect.

The machined volume is calculated from CL files, a standard output of all N/C programming software tools. Cincinnati Machine has developed a feature based software tool that can quickly obtain the necessary CL files required by MSC.MARC. This project proposes the marriage of these two software tools in an expanded user community to validate and enhance both tools. In addition, the MARC code will be expanded to accommodate die forged materials.

For more information, please contact Tony Haynes, (734) 995-4930.

Fuel Tank Desealing and Resealing

The project team proposes a two part study to research aircraft fuel tank sealing, desealing, and resealing needs.  The first part is a study of the feasibility of developing the chemistry to extend sealant life and performance characteristics of existing Mil Spec sealants.  The second part is a study of the properties and performance of new commercially available sealers, sealant removers and their related cleaners, etc. for the eventual purpose of modifying existing fuel tank sealing, desealing and resealing specifications to allow their use on commercial and military aircraft, if appropriate.

Rejuvenation of sealant compounds in-situ – Resealing aircraft fuel cells is expensive, highly imprecise and time consuming.  If a sealant could be rejuvenated or ‘periodically boosted’ to extend sealing performance, considerable maintenance dollars could be saved.  By boosting or recharging the sealant, performance degradation can be retarded and sealant life extended until time for the next maintenance interval. 

Evaluation of new commercially available sealants and sealant remover systems - If a sealant needs to be removed locally or in sections, then the use of new methods and new products must be explored and certified for use.  Mechanical sealant removal methods degrade the structure under the sealant and create potential future fuel leak failures.  Since most mechanical removal methods used currently are destructive, abrasive and labor intensive, new chemically based sealant removal systems would be assessed by this project.  By breaking down the sealant adhesive barrier, sealants can be removed like banana skins and any remaining residuals removed with minor scraping (less likely to damage structure).  When a desealed area needs resealing, the application of new adhesive cleaners / boosters / primers may extend sealant performance, thus reducing leaks in the future.  A robust application process would allow the sealant to be used in areas where the surface may not be as pristine as desired, but the surface and sealant adhesion can still maintain sealant performance and durability. 

New chemistries, new products and new processes must be explored to assess total ownership cost and performance feasibility. 

If you are interested in more information about this project please contact Constance J.S. Philips, 734-995-7051.

Thermoplastics in Overhaul & Repair; Thermoplastics for Material Substitution

Cyclics Corporation’s low viscosity cyclic butylene terephthalate (CBT™) thermoplastic material processes like a thermoset and has post processing properties that make it attractive as a material substitute for trade name materials like Valox and Crastin.  CBT can be used in low pressure composite applications in overhaul and repair operations.

Cyclics CBT material can be processed using vacuum bag molding, Resin Film Infusion (RFI), and Vacuum Assisted Resin Transfer Molding (VARTM).  CBT polymerizes into a tough thermoplastic polyester “PBT” (poly butylene terephthalate) having up to 300% elongation, very low moisture absorption (less than 1% in 24 hr. soak) and good hot wet properties.  As a substitute material CBT could replace many epoxies.  CBT has a low water-like viscosity for easy processing, which then polymerizes into conventional PBT.  Normal PBT has high viscosity when melted and is not suited for VARTM or RFI processes.

As a laminating material for overhaul and repair, CBT would be tougher and more damage tolerant and may offer advantages for quick repair (heat, melt, repair, cool).  In addition, the resulting PBT material is easy to bond with conventional epoxy adhesives thereby yielding a more damage resistant repair, but also allowing current repair procedures to be used.  Laminates have been built up to 2.5 inches thick. Thicker laminates are possible.  No exotherm during cure of the CBT into PBT occurs, so rapid curing without fear of burning the laminate is possible. 

• CBT’s low viscosity allows thorough wetting of the fibers.

• Fiber volumes of 55% are typically achieved with VARTM and RFI processes using woven or stitch bonded fabrics.

• Extremely large parts such as 12 metric tonne wind blades can be fabricated in a single molding operation.

• Secondary thermoforming is possible.

• Good chemical resistance and heat performance (HDT of 215C with fiber).

• Can be made fire retardant.

• Is paintable/bondable/machinable.

Interested participants should contact Manish Mehta, 734-995-4938.

Sheet Metal Fabrication – Precision Metal Origami (PMO)

A Sheet Metal Fabrication – Precision Metal Origami (PMO) project is currently emerging.  This project will focus on Industrial Origami's Precision Fold Technology ™ (PFT) for designing and manufacturing components from sheet materials.

DoD maintenance facilities along with commercial firms throughout the U.S. make extensive use of sheet metal forming and welding process to fabricate and/or repair components.  In most cases the designs for these components are conceived, designed and rendered using precision 3 dimensional (3D) Computer Aided Design (CAD) software.  The components are subsequently manufactured using equally precise Computer Numeric Controlled (CNC) machines.  In the case of sheet metal, precision CNC cutting of 2 dimensional parts can be achieved with a high degree of accuracy and reproducibility.  Once cut, those close tolerances are lost in the subsequent bending and welding processes currently available to fabricate the final 3D structures.  Considerable waste is suffered when close tolerance 3D structures are pursued or close tolerances are abandoned altogether because they are not achievable.  In the case of welded structures, heat warping and unpredictable fatigue failure keep weld-fabricated 3D structures from achieving their ideal as conceived in the original 3D CAD design.

Industrial Origami, Inc. (IOI) is developing a technology to help solve many of these problems. In demonstration projects IOI has shown that structures fabricated using PFT are stronger, more rigid, and constructed to dimensional tolerances that are not achievable by traditional sheet metal fabrication methods.  Additional testing has demonstrated that PFT can be used in a wide range of materials, from very thin sheets to thick plates. These initial demonstrations effectively show that PFT can improve the performance of many components fabricated from sheet and plate materials and at the same time reduce the costs and time to manufacture the components.  IOI's PFT technology takes advantage of existing materials, existing capital equipment, in this case lasers and water jet cutters, to achieve reduced labor and material costs of fabrication.  For the designer, PFT is the first example of "What You See Is What You Get" in the precision sheet metal and welded plate fabrication processes.

If you would like additional information please contact Gary Burkart, 612 839-4567.