MIL-HDBK-17-3F Volume 3.Chapter 12-Lessons Learned metal-to-metal bonding (6,000 psi(40 MPa)or so).With normal thickness composite adherends,only half this strength was reached,because the resin between the surface fibers and adhesive layer then failed in peel,leaving resin clearly covering both surfaces.This problem can be minimized by maintaining very tight time limits between making parts and bonding them together,with a requirement to thoroughly dry everything before bonding if the time constraints are exceeded.Careful scheduling can avoid this added drying step.The same high-strength cohesive bond failures had previously been achieved by an- other supplier of composite structures using grit-blasted surfaces and 0.080 inch(2.0 mm)thick unidirec- tional laminates. In considering adhesive bond strength,it is vital to note that the specimen testing validates the proc- ess,NOT the part.There is no requirement for the specimen to look like the actual part.Indeed.in a properly designed bonded joint,the bond will not fail first.Consequently,the use of specimens which are "similar"to the part and which are evaluated in terms of the "adequacy"of the load carried in relation to the stresses in the part,is not sufficient to ensure the integrity of the bonded composite structure.This issue is complicated because,only with unidirectional tape laminates is it possible to develop sufficient load to fail a high-strength adhesive bond cohesively.Therefore,only such specimens can provide any assurance that the part they are intended to substantiate has been bonded properly.However,in real parts made from woven-fabric laminates,failures within bundles of fibers at 90 to the applied load will trigger interlaminate failures before such bond strengths can be attained. In all cases,the one condition which can be detected visually on test specimens and failed parts alike which is a guaranteed indicator of a defective bond is an interfacial failure with all of the resin on one side and all of the adhesive on the other,with a clear imprint of the peel ply texture on both surfaces. 12.2.7 Design The design of composite structure is complicated by the fact that every ply must be defined.Draw- ings or design packages must describe the ply orientation,its position within the stack,and its boundaries. This is straightforward for a simple,constant thickness laminate.For complex parts with tapered thick- nesses and ply build-ups around joints and cutouts,this can become extremely complex.The need to maintain relative balance and symmetry throughout the structure increases the difficulty. Composites can not be designed without concurrence.Design details depend on tooling and proc- essing as does assembly and inspection.Parts and processes are so interdependent it could be disas- trous to attempt sequential design and manufacturing phasing. Another factor approached differently in composite design is the accommodation of thickness toler- ances at interfaces.If a composite part must fit into a space between two other parts or between a sub- structure and an outer mold line,the thickness requires special tolerances.The composite part thickness is controlled by the number of plies and the per-ply-thickness.Each ply has a range of possible thick- nesses.When these are layed up to form the laminate they may not match the space available for as- sembly within other constraints.This discrepancy can be handled by using shims or by adding "sacrifi- cial"plies to the laminate(for subsequent machining to a closer tolerance than is possible with nominal per-ply-thickness variations).The use of shims has design implications regarding load eccentricities. Another approach is to use closed die molding at the fit-up edges to mold to exact thickness needed. The anisotropy of special laminates,while more complicated,enables a designer to tailor a structure for desired deflection characteristics.This has been applied to some extent for aeroelastic tailoring of wing skins. Composites are most efficient when used in large,relatively uninterrupted structures.The cost is also related to the number of detail parts and the number of fasteners required.These two factors drive de- signs towards integration of features into large cocured structures.The nature of composites enables this possibility.Well designed,high quality tooling will reduce manufacturing and inspection cost and rejection rate and result in high quality parts. 12-8MIL-HDBK-17-3F Volume 3, Chapter 12 - Lessons Learned 12-8 metal-to-metal bonding (6,000 psi (40 MPa) or so). With normal thickness composite adherends, only half this strength was reached, because the resin between the surface fibers and adhesive layer then failed in peel, leaving resin clearly covering both surfaces. This problem can be minimized by maintaining very tight time limits between making parts and bonding them together, with a requirement to thoroughly dry everything before bonding if the time constraints are exceeded. Careful scheduling can avoid this added drying step. The same high-strength cohesive bond failures had previously been achieved by an￾other supplier of composite structures using grit-blasted surfaces and 0.080 inch (2.0 mm) thick unidirec￾tional laminates. In considering adhesive bond strength, it is vital to note that the specimen testing validates the proc￾ess, NOT the part. There is no requirement for the specimen to look like the actual part. Indeed, in a properly designed bonded joint, the bond will not fail first. Consequently, the use of specimens which are “similar” to the part and which are evaluated in terms of the “adequacy” of the load carried in relation to the stresses in the part, is not sufficient to ensure the integrity of the bonded composite structure. This issue is complicated because, only with unidirectional tape laminates is it possible to develop sufficient load to fail a high-strength adhesive bond cohesively. Therefore, only such specimens can provide any assurance that the part they are intended to substantiate has been bonded properly. However, in real parts made from woven-fabric laminates, failures within bundles of fibers at 90° to the applied load will trigger interlaminate failures before such bond strengths can be attained. In all cases, the one condition which can be detected visually on test specimens and failed parts alike which is a guaranteed indicator of a defective bond is an interfacial failure with all of the resin on one side and all of the adhesive on the other, with a clear imprint of the peel ply texture on both surfaces. 12.2.7 Design The design of composite structure is complicated by the fact that every ply must be defined. Draw￾ings or design packages must describe the ply orientation, its position within the stack, and its boundaries. This is straightforward for a simple, constant thickness laminate. For complex parts with tapered thick￾nesses and ply build-ups around joints and cutouts, this can become extremely complex. The need to maintain relative balance and symmetry throughout the structure increases the difficulty. Composites can not be designed without concurrence. Design details depend on tooling and proc￾essing as does assembly and inspection. Parts and processes are so interdependent it could be disas￾trous to attempt sequential design and manufacturing phasing. Another factor approached differently in composite design is the accommodation of thickness toler￾ances at interfaces. If a composite part must fit into a space between two other parts or between a sub￾structure and an outer mold line, the thickness requires special tolerances. The composite part thickness is controlled by the number of plies and the per-ply-thickness. Each ply has a range of possible thick￾nesses. When these are layed up to form the laminate they may not match the space available for as￾sembly within other constraints. This discrepancy can be handled by using shims or by adding "sacrifi￾cial" plies to the laminate (for subsequent machining to a closer tolerance than is possible with nominal per-ply-thickness variations). The use of shims has design implications regarding load eccentricities. Another approach is to use closed die molding at the fit-up edges to mold to exact thickness needed. The anisotropy of special laminates, while more complicated, enables a designer to tailor a structure for desired deflection characteristics. This has been applied to some extent for aeroelastic tailoring of wing skins. Composites are most efficient when used in large, relatively uninterrupted structures. The cost is also related to the number of detail parts and the number of fasteners required. These two factors drive de￾signs towards integration of features into large cocured structures. The nature of composites enables this possibility. Well designed, high quality tooling will reduce manufacturing and inspection cost and rejection rate and result in high quality parts