MIL-HDBK-17-3F Volume 3.Chapter 6 Structural Behavior of Joints suspect,since some peel plies leave a residue on the bonding surfaces that makes adhesion poor. (However,some manufacturers have obtained satisfactory results from surface preparation consisting only of peel ply removal.)Low pressure grit blasting (Reference 6.2.2.6(b))is preferable over hand sand- ing as a means of eliminating such residues and mechanically conditioning the bonding surfaces. For joints which are designed to ensure that the adherends rather than the bond layer are the critical elements,tolerance to the presence of porosity and other types of defect is considerable (Reference 6.2.1(t)).Porosity(Reference 6.2.1(z))is usually associated with over-thickened areas of the bond,which tend to occur away from the edges of the joint where most of the load transfer takes place,and thus is a relatively benign effect,especially if peel stresses are minimized by adherend tapering.Reference 6.2.1(z)indicates that in such cases,porosity can be represented by a modification of the assumed stress-strain properties of the adhesive as determined from thick-adherend tests,allowing a straightfor- ward analysis of the effect of such porosity on joint strength as in the A4EI computer code.If peel stresses are significant,as in the case of over-thick adherends,porosity may grow catastrophically and lead to non-damage-tolerant joint performance. In the case of bond thickness variations(Reference 6.2.1(aa)),these usually take place in the form of thinning due to excess resin bleed at the joint edges,leading to overstressing of the adhesive in the vicin- ity of the edges.Inside tapering of the adherends at the joint edges can be used to compensate for this condition;other compensating techniques are also discussed in Reference 6.2.1(aa).Bond thicknesses per se should be limited to ranges of 0.005-0.01 in.(0.12-0.24 mm)to prevent significant porosity from developing,although greater thicknesses may be acceptable if full periphery damming or high minimum viscosity paste adhesives are used.Common practice involves the use of film adhesives containing scrim cloth,some forms of which help to maintain bond thicknesses.It is also common practice to use mat car- riers of chopped fibers to prevent a direct path for access by moisture to the interior of the bond. 6.2.2.7 Durability of adhesive joints Two major considerations in the joint design philosophy of Hart-smith are:(1)either limiting the ad- herend thickness or making use of more sophisticated joint configurations such as scarf and step lap joints,to insure that adherend failure takes precedence over bond failure;(2)designing to minimize peel stresses,either by keeping the adherends excessively thin or,for intermediate adherend thicknesses,by tapering the adherends (see discussion of effects of adherend tapering,Section 6.2.2.2 and 6.2.3.5.).In addition,it is essential that good surface treatment practices(Section 6.2.2.6)be maintained to insure that the bond between the adhesive and adherends does not fail.When these conditions are met,reliable per- formance of the joint can be expected for the most part,except for environmental extremes,i.e.,hot-wet conditions.The Hart-Smith approach focuses primarily on creep failure associated with slow cyclic load- ing (i.e.,1 cycle in several minutes to an hour)under hot wet conditions;this corresponds,for example,to cyclic pressurization of aircraft fuselages.In the PABST program,References 6.2.1(n)-(q)(see also Ref- erence 6.2.1(v)),18 thick adherend specimens,when tested at high cycling rates(30 Hz)were able to sustain more than 10 million loading cycles without damage,while tests conducted at the same loads at one cycle per hour produced failures within a few hundred cycles.Similar conclusions regarding the ef- fects of cycling rate were presented in Reference 6.2.2.7(a).On the other hand,specimens representative of structural joints,which have a nonuniform shear stress distribution that peaks at the ends of the joint and is essentially zero in the middle (see Section 6.2.3.4.3 on ductile response of joints and Figure 6.2.3.4.3(b),part(B)in particular)are able to sustain hot-wet conditions even at low cycling rates if (e, the length of the region of elastic response in the bond layer,is sufficient.Based on experience of the PABST program,the Hart-Smith criterion for avoidance of creep failure requires that blmin,the minimum shear stress along the bond length,be no greater than one tenth the yield stress of the adhesive.But the stress analysis for the elastic-plastic case (Section 6.2.3.4.3)using a bilinear adhesive response model leads to an expression for the minimum shear stress in double lap joints with identical adherends given by Tp Tblmin= 6.2.2.7(a sinh Bpdle/2to 6-9MIL-HDBK-17-3F Volume 3, Chapter 6 Structural Behavior of Joints 6-9 suspect, since some peel plies leave a residue on the bonding surfaces that makes adhesion poor. (However, some manufacturers have obtained satisfactory results from surface preparation consisting only of peel ply removal.) Low pressure grit blasting (Reference 6.2.2.6(b)) is preferable over hand sand￾ing as a means of eliminating such residues and mechanically conditioning the bonding surfaces. For joints which are designed to ensure that the adherends rather than the bond layer are the critical elements, tolerance to the presence of porosity and other types of defect is considerable (Reference 6.2.1(t)). Porosity (Reference 6.2.1(z)) is usually associated with over-thickened areas of the bond, which tend to occur away from the edges of the joint where most of the load transfer takes place, and thus is a relatively benign effect, especially if peel stresses are minimized by adherend tapering. Reference 6.2.1(z) indicates that in such cases, porosity can be represented by a modification of the assumed stress-strain properties of the adhesive as determined from thick-adherend tests, allowing a straightfor￾ward analysis of the effect of such porosity on joint strength as in the A4EI computer code. If peel stresses are significant, as in the case of over-thick adherends, porosity may grow catastrophically and lead to non-damage-tolerant joint performance. In the case of bond thickness variations (Reference 6.2.1(aa)), these usually take place in the form of thinning due to excess resin bleed at the joint edges, leading to overstressing of the adhesive in the vicin￾ity of the edges. Inside tapering of the adherends at the joint edges can be used to compensate for this condition; other compensating techniques are also discussed in Reference 6.2.1(aa). Bond thicknesses per se should be limited to ranges of 0.005-0.01 in. (0.12-0.24 mm) to prevent significant porosity from developing, although greater thicknesses may be acceptable if full periphery damming or high minimum viscosity paste adhesives are used. Common practice involves the use of film adhesives containing scrim cloth, some forms of which help to maintain bond thicknesses. It is also common practice to use mat car￾riers of chopped fibers to prevent a direct path for access by moisture to the interior of the bond. 6.2.2.7 Durability of adhesive joints Two major considerations in the joint design philosophy of Hart-smith are: (1) either limiting the ad￾herend thickness or making use of more sophisticated joint configurations such as scarf and step lap joints, to insure that adherend failure takes precedence over bond failure; (2) designing to minimize peel stresses, either by keeping the adherends excessively thin or, for intermediate adherend thicknesses, by tapering the adherends (see discussion of effects of adherend tapering, Section 6.2.2.2 and 6.2.3.5.). In addition, it is essential that good surface treatment practices (Section 6.2.2.6) be maintained to insure that the bond between the adhesive and adherends does not fail. When these conditions are met, reliable per￾formance of the joint can be expected for the most part, except for environmental extremes, i.e., hot-wet conditions. The Hart-Smith approach focuses primarily on creep failure associated with slow cyclic load￾ing (i.e., 1 cycle in several minutes to an hour) under hot wet conditions; this corresponds, for example, to cyclic pressurization of aircraft fuselages. In the PABST program, References 6.2.1(n)-(q) (see also Ref￾erence 6.2.1(v)), 18 thick adherend specimens, when tested at high cycling rates (30 Hz) were able to sustain more than 10 million loading cycles without damage, while tests conducted at the same loads at one cycle per hour produced failures within a few hundred cycles. Similar conclusions regarding the ef￾fects of cycling rate were presented in Reference 6.2.2.7(a). On the other hand, specimens representative of structural joints, which have a nonuniform shear stress distribution that peaks at the ends of the joint and is essentially zero in the middle (see Section 6.2.3.4.3 on ductile response of joints and Figure 6.2.3.4.3(b), part (B) in particular) are able to sustain hot-wet conditions even at low cycling rates if Ae , the length of the region of elastic response in the bond layer, is sufficient. Based on experience of the PABST program, the Hart-Smith criterion for avoidance of creep failure requires that τ b | min , the minimum shear stress along the bond length, be no greater than one tenth the yield stress of the adhesive. But the stress analysis for the elastic-plastic case (Section 6.2.3.4.3) using a bilinear adhesive response model leads to an expression for the minimum shear stress in double lap joints with identical adherends given by b min p bd o | = sinh / 2 t τ τ β Ae 6.2.2.7(a)