MIL-HDBK-17-3F Volume 3,Chapter 12-Lessons Learned been able to distinguish between bonded joints which will fail in service and those which will not.In addi- tion,most traveler specimens do not represent the same cure conditions as experienced by adjacent large parts and,therefore,mechanical testing also often fails to identify defective bonding.One must de- pend on process control of techniques which can be relied upon 100 percent of the time and on thorough validation of the processes before committing them to production. Consider a surface,created by stripping off a peel ply,which is then bonded as part of an adhesive joint.The resulting adhesive bond "sticks"well enough to pass all inspections;however,may fail prema- turely at the interface between the laminate and the adhesive.All premature bond failures,other than those caused by incomplete cure,occur at the interface between the adhesive and the resin in the lami- nate.Structurally sound bonds either fail outside the joint area,cohesively within the layer of adhesive,or interlaminarly in the resin matrix between the surface fibers and the adhesive layer.These premature failures can occur either when uncured adhesive is bonded to precured laminates,or when uncured pre- preg is cured against cured adhesive films used to stabilize honeycomb cores and the like. There are several ways in which peel plies can create surfaces on which reliable durable bonds are not possible. The peel ply can be coated with a release agent,which transfers to the cured laminate when the peel ply is stripped off. The surface of the peel ply's fibers must be sufficiently inert that the ply can be removed without damaging the laminate.The grooves left in the laminate (or glue layer)by stripping off the peel ply may retain enough inert surface that the resin which is subsequently cured onto it may simply fail to adhere.Adhesion requires more than cleanliness:surface tension is also critical.In the absence of cohesion at the interface,a bonded joint relies only on mechanical interlocking,which is far weaker in peel than it is in shear. The peel-ply surface in the laminate consists of innumerable short grooves separated by sharp edges where the resin between the filaments in the peel ply fractured as the peel ply was stripped off.Moisture on (or in)the adhesive or the laminate can be trapped in these grooves.If this moisture cannot escape during the curing of the adhesive (or of a co-cured face sheet),the trapped moisture will result in a slick bond when examined microscopically after failure. It should be noted that the latter two mechanisms function without any contamination. One aircraft company's process specification has,for decades,required that any peel-ply surface to be bonded must first be thoroughly abraded to remove all traces of the texture of the peel ply.In the ab- sence of the ridges between the grooves,it is presumed that moisture could escape,as it turned to steam during cure,unless the part was too large and too poorly ventilated.Using these requirements,this air- craft company has had no disbonds in those secondarily bonded composite structures which were grit blasted before bonding.The same cannot be claimed for bonds made to unabraded (or only scuff- sanded)peel-ply surfaces.In two instances,on different aircraft types,disbonds were traced to transfer of release agents from silicone-coated peel plies,the use of which is now banned throughout all docu- ments,not only the approved materials lists. On another aircraft type,interfacial failures on peel-ply surfaces appear to be the result of prebond moisture,the exact origin of which has yet to be established.An accident with one test panel during pro- cess qualification by a supplier revealed the consequences of condensate on adhesive film (the roll of adhesive had not been properly sealed when returned to the freezer after the previous use).There was absolutely no adhesion between the resin and the adhesive,even though the lap-shear numbers seemed to be acceptable.Microscopic examination of the surfaces clearly showed perfect imprints of the peel ply texture on both surfaces,with the surface in all grooves as smooth as glass and all of the resin on one surface and all of the adhesive on the other.However,with thicker-than-normal(0.123 inch(3.2 mm)) adherends of the same unidirectional carbon/epoxy,bonds made with the same nonreleased peel ply and the same kind of adhesive achieved cohesive failure of the bond at the same strength level attained by 12-7MIL-HDBK-17-3F Volume 3, Chapter 12 - Lessons Learned 12-7 been able to distinguish between bonded joints which will fail in service and those which will not. In addi￾tion, most traveler specimens do not represent the same cure conditions as experienced by adjacent large parts and, therefore, mechanical testing also often fails to identify defective bonding. One must de￾pend on process control of techniques which can be relied upon 100 percent of the time and on thorough validation of the processes before committing them to production. Consider a surface, created by stripping off a peel ply, which is then bonded as part of an adhesive joint. The resulting adhesive bond “sticks” well enough to pass all inspections; however, may fail prema￾turely at the interface between the laminate and the adhesive. All premature bond failures, other than those caused by incomplete cure, occur at the interface between the adhesive and the resin in the lami￾nate. Structurally sound bonds either fail outside the joint area, cohesively within the layer of adhesive, or interlaminarly in the resin matrix between the surface fibers and the adhesive layer. These premature failures can occur either when uncured adhesive is bonded to precured laminates, or when uncured pre￾preg is cured against cured adhesive films used to stabilize honeycomb cores and the like. There are several ways in which peel plies can create surfaces on which reliable durable bonds are not possible. • The peel ply can be coated with a release agent, which transfers to the cured laminate when the peel ply is stripped off. • The surface of the peel ply’s fibers must be sufficiently inert that the ply can be removed without damaging the laminate. The grooves left in the laminate (or glue layer) by stripping off the peel ply may retain enough inert surface that the resin which is subsequently cured onto it may simply fail to adhere. Adhesion requires more than cleanliness; surface tension is also critical. In the absence of cohesion at the interface, a bonded joint relies only on mechanical interlocking, which is far weaker in peel than it is in shear. • The peel-ply surface in the laminate consists of innumerable short grooves separated by sharp edges where the resin between the filaments in the peel ply fractured as the peel ply was stripped off. Moisture on (or in) the adhesive or the laminate can be trapped in these grooves. If this moisture cannot escape during the curing of the adhesive (or of a co-cured face sheet), the trapped moisture will result in a slick bond when examined microscopically after failure. It should be noted that the latter two mechanisms function without any contamination. One aircraft company’s process specification has, for decades, required that any peel-ply surface to be bonded must first be thoroughly abraded to remove all traces of the texture of the peel ply. In the ab￾sence of the ridges between the grooves, it is presumed that moisture could escape, as it turned to steam during cure, unless the part was too large and too poorly ventilated. Using these requirements, this air￾craft company has had no disbonds in those secondarily bonded composite structures which were grit blasted before bonding. The same cannot be claimed for bonds made to unabraded (or only scuff￾sanded) peel-ply surfaces. In two instances, on different aircraft types, disbonds were traced to transfer of release agents from silicone-coated peel plies, the use of which is now banned throughout all docu￾ments, not only the approved materials lists. On another aircraft type, interfacial failures on peel-ply surfaces appear to be the result of prebond moisture, the exact origin of which has yet to be established. An accident with one test panel during pro￾cess qualification by a supplier revealed the consequences of condensate on adhesive film (the roll of adhesive had not been properly sealed when returned to the freezer after the previous use). There was absolutely no adhesion between the resin and the adhesive, even though the lap-shear numbers seemed to be acceptable. Microscopic examination of the surfaces clearly showed perfect imprints of the peel ply texture on both surfaces, with the surface in all grooves as smooth as glass and all of the resin on one surface and all of the adhesive on the other. However, with thicker-than-normal (0.123 inch (3.2 mm)) adherends of the same unidirectional carbon/epoxy, bonds made with the same nonreleased peel ply and the same kind of adhesive achieved cohesive failure of the bond at the same strength level attained by