MIL-HDBK-17-3F Volume 3,Chapter 6 Structural Behavior of Joints plateau(i.e.elastic-perfectly plastic response)if the latter is adjusted to provide a strain energy density to failure equal to that of the actual stress-strain curve gives.Test methods for adhesives (see Volume 1, Section 7.6)should be aimed at providing data on this parameter.Once the equivalent elastic-perfectly plastic stress strain curve has been identified for the selected adhesive in the range of the most severe environmental conditions(temperature and humidity)of interest,the joint design can proceed through the use of relatively simple one-dimensional stress analysis,thus avoiding the need for elaborate finite ele- ment calculations.Even the most complicated of joints,the step lap joints designed for root-end wing and tail connections for the F-18 and other aircraft,have been successfully designed(Reference 6.2.1(t))and experimentally demonstrated using such approaches.Design procedures for such analyses which were developed under Government contract have been incorporated into the public domain in the form of the "A4EG","A4EI"and "A4EK"computer codes mentioned previously in Section 6.2.1 and are currently avail- able from the Air Force's Aerospace Structures Information and Analysis Center(ASIAC).Note that the A4EK code permits analysis of bonded joints in which local disbonds are repaired by mechanical fasten- ers. 6.2.2.5 Behavior of composite adherends Polymer matrix composite adherends are considerably more affected by interlaminar shear stresses than metals,so that there is a significant need to account for such effects in stress analyses of adhesively bonded composites.Transverse shear deformations of the adherends have an effect analogous to thick- ening of the bond layer and result in a lowering of both shear and peel stress peaks.(See Section 6.2.3.4.4). In addition,because the resins used for adherend matrices tend to be less ductile than typical adhe- sives,and are weakened by stress concentrations due to the presence of the fibers,the limiting element in the joint may be the interlaminar shear and transverse tensile strengths of the adherends rather than the bond strength(Figure 6.2.2.5(a)).In the case of single lap joints(Figure 6.2.2.5(a),part(A))bending failures of the adherends may occur because of high moments at the ends of the overlap.For metal ad- herends,bending failures take the form of plastic bending and hinge formation,while for composite ad- herends the bending failures are brittle in nature.In the case of double lap joints,peel stress build up in thicker adherends can cause the types of interlaminar failures in the adherends illustrated in Figure 6.2.2.5(a),part(B). The effect of the stacking sequence of the laminates making up the adherends in composite joints is significant.For example,90-degree layers placed adjacent to the bond layer theoretically act largely as additional thicknesses of bond material,leading to lower peak stresses,while 0-degree layers next to the bond layer give stiffer adherend response with higher stress peaks.In practice it has been observed that 90-degree layers next to the bond layer tend to seriously weaken the joint because of transverse cracking which develops in those layers,and advantage cannot be taken of the reduced peak stresses. Large differences in thermal expansion characteristics between metal and composite adherends can cause severe problems.(See Section 6.2.3.4.2)Adhesives with high curing temperatures may be unsuit- able for some low temperature applications because of large thermal stresses which develop as the joint cools down from the curing temperature In contrast with metal adherends,composite adherends are subject to moisture diffusion effects.As a result,moisture is more likely to be found over wide regions of the adhesive layer,as opposed to con- finement near the exposed edges of the joint in the case of metal adherends,and response of the adhe- sive to moisture may be an even more significant issue for composite joints than for joints between metallic adherends. 6-7MIL-HDBK-17-3F Volume 3, Chapter 6 Structural Behavior of Joints 6-7 plateau (i.e. elastic-perfectly plastic response) if the latter is adjusted to provide a strain energy density to failure equal to that of the actual stress-strain curve gives. Test methods for adhesives (see Volume 1, Section 7.6) should be aimed at providing data on this parameter . Once the equivalent elastic-perfectly plastic stress strain curve has been identified for the selected adhesive in the range of the most severe environmental conditions (temperature and humidity) of interest, the joint design can proceed through the use of relatively simple one-dimensional stress analysis, thus avoiding the need for elaborate finite ele￾ment calculations. Even the most complicated of joints, the step lap joints designed for root-end wing and tail connections for the F-18 and other aircraft, have been successfully designed (Reference 6.2.1(t)) and experimentally demonstrated using such approaches. Design procedures for such analyses which were developed under Government contract have been incorporated into the public domain in the form of the "A4EG", "A4EI" and "A4EK" computer codes mentioned previously in Section 6.2.1 and are currently avail￾able from the Air Force's Aerospace Structures Information and Analysis Center (ASIAC) . Note that the A4EK code permits analysis of bonded joints in which local disbonds are repaired by mechanical fasten￾ers. 6.2.2.5 Behavior of composite adherends Polymer matrix composite adherends are considerably more affected by interlaminar shear stresses than metals, so that there is a significant need to account for such effects in stress analyses of adhesively bonded composites. Transverse shear deformations of the adherends have an effect analogous to thick￾ening of the bond layer and result in a lowering of both shear and peel stress peaks. (See Section 6.2.3.4.4). In addition, because the resins used for adherend matrices tend to be less ductile than typical adhe￾sives, and are weakened by stress concentrations due to the presence of the fibers, the limiting element in the joint may be the interlaminar shear and transverse tensile strengths of the adherends rather than the bond strength (Figure 6.2.2.5(a)). In the case of single lap joints (Figure 6.2.2.5(a), part (A)) bending failures of the adherends may occur because of high moments at the ends of the overlap. For metal ad￾herends, bending failures take the form of plastic bending and hinge formation, while for composite ad￾herends the bending failures are brittle in nature. In the case of double lap joints, peel stress build up in thicker adherends can cause the types of interlaminar failures in the adherends illustrated in Figure 6.2.2.5(a), part (B). The effect of the stacking sequence of the laminates making up the adherends in composite joints is significant. For example, 90-degree layers placed adjacent to the bond layer theoretically act largely as additional thicknesses of bond material, leading to lower peak stresses, while 0-degree layers next to the bond layer give stiffer adherend response with higher stress peaks. In practice it has been observed that 90-degree layers next to the bond layer tend to seriously weaken the joint because of transverse cracking which develops in those layers, and advantage cannot be taken of the reduced peak stresses. Large differences in thermal expansion characteristics between metal and composite adherends can cause severe problems. (See Section 6.2.3.4.2) Adhesives with high curing temperatures may be unsuit￾able for some low temperature applications because of large thermal stresses which develop as the joint cools down from the curing temperature. In contrast with metal adherends, composite adherends are subject to moisture diffusion effects . As a result, moisture is more likely to be found over wide regions of the adhesive layer, as opposed to con￾finement near the exposed edges of the joint in the case of metal adherends, and response of the adhe￾sive to moisture may be an even more significant issue for composite joints than for joints between metallic adherends