MIL-HDBK-17-3F Volume 3.Chapter 6 Structural Behavior of Joints This chapter describes design procedures and analytical methods for determining stresses and de- formations in structural joints for composite structures.Section 6.2 which follows deals with adhesive joints.(Mechanically fastened joints will be the subject of a future revision of the Handbook.) In the case of adhesive joints,design considerations which are discussed include:effects of adherend thickness as a means of ensuring adherend failure rather than bond failure;the use of adherend tapering to minimize peel stresses;effects of adhesive ductility;special considerations regarding composite ad- herends;effects of bond layer defects,including surface preparations defects,porosity and thickness variations;and,considerations relating to long term durability of adhesive joints.In addition to design considerations,aspects of joint behavior which control stresses and deformations in the bond layer are described,including both shear stresses and transverse normal stresses which are customarily referred to as "peel"stresses when they are tensile.Finally,some principles for finite element analysis of bonded joints are described. Related information on joints in composite structures which is described elsewhere in this handbook includes Volume 1.Chapter 7.Section 7.5(Mechanically Fastened Joints)and 6.3(Bonded Joints)to- gether with Volume 3,Chapter 2,Section 2.7.8 on Adhesive Bonding. 6.2 ADHESIVE JOINTS 6.2.1 Introduction Adhesive joints are capable of high structural efficiency and constitute a resource for structural weight saving because of the potential for elimination of stress concentrations which cannot be achieved with mechanically fastened joints.Unfortunately,because of a lack of reliable inspection methods and a re- quirement for close dimensional tolerances in fabrication,aircraft designers have generally avoided bonded construction in primary structure.Some notable exceptions include:bonded step lap joints used in attachments for the F-14 and F-15 horizontal stabilizers as well as the F-18 wing root fitting,and a ma- jority of the airframe components of the Lear Fan and the Beech Starship. While a number of issues related to adhesive joint design were considered in the earlier literature cited in References 6.2.1(a)-6.2.1(h),much of the methodology currently used in the design and analysis of adhesive joints in composite structures is based on the approaches evolved by L.J.Hart-Smith in a series of NASA/Langley-sponsored contracts of the early 70's (References 6.2.1(i)-6.2.1(n))as well as from the Air Force's Primary Adhesively Bonded Structures Technology (PABST)program (References 6.2.1(o)-6.2.1(r))of the mid-70's.The most recent such work developed three computer codes for bonded and bolted joints,designated A4EG,A4EI and A4EK(References 6.2.1(s)-6.2.1(u)),under Air Force contract.The results of these efforts have also appeared in a number of open literature publica- tions (Reference 6.2.1(v)-(z)).In addition,such approaches found application in some of the efforts tak- ing place under the NASA Advanced Composite Energy Efficient Aircraft(ACEE)program of the early to mid 80's (Reference 6.2.1(x)and 6.2.1(y)). Some of the key principles on which these efforts were based include:(1)the use of simple 1-dimensional stress analyses of generic composite joints wherever possible;(2)the need to select the joint design so as to ensure failure in the adherend rather than the adhesive,so that the adhesive is never the weak link;(3)recognition that the ductility of aerospace adhesives is beneficial in reducing stress peaks in the adhesive;(4)careful use of such factors as adherend tapering to reduce or eliminate peel stresses from the joint;and(5)recognition of slow cyclic loading,corresponding to such phenomena as cabin pressurization in aircraft,as a major factor controlling durability of adhesive joints,and the need to avoid the worst effects of this type of loading by providing sufficient overlap length to ensure that some of the adhesive is so lightly loaded that creep cannot occur there,under the most severe extremes of humid- ity and temperature for which the component is to be used. Much of the discussion to follow will retain the analysis philosophy of Hart-Smith,since it is consid- ered to represent a major contribution to practical bonded joint design in both composite and metallic 6-2MIL-HDBK-17-3F Volume 3, Chapter 6 Structural Behavior of Joints 6-2 This chapter describes design procedures and analytical methods for determining stresses and de￾formations in structural joints for composite structures. Section 6.2 which follows deals with adhesive joints. (Mechanically fastened joints will be the subject of a future revision of the Handbook.) In the case of adhesive joints, design considerations which are discussed include: effects of adherend thickness as a means of ensuring adherend failure rather than bond failure; the use of adherend tapering to minimize peel stresses; effects of adhesive ductility; special considerations regarding composite ad￾herends; effects of bond layer defects, including surface preparations defects, porosity and thickness variations; and, considerations relating to long term durability of adhesive joints. In addition to design considerations, aspects of joint behavior which control stresses and deformations in the bond layer are described, including both shear stresses and transverse normal stresses which are customarily referred to as "peel" stresses when they are tensile. Finally, some principles for finite element analysis of bonded joints are described. Related information on joints in composite structures which is described elsewhere in this handbook includes Volume 1, Chapter 7, Section 7.5 (Mechanically Fastened Joints) and 6.3 (Bonded Joints) to￾gether with Volume 3, Chapter 2, Section 2.7.8 on Adhesive Bonding. 6.2 ADHESIVE JOINTS 6.2.1 Introduction Adhesive joints are capable of high structural efficiency and constitute a resource for structural weight saving because of the potential for elimination of stress concentrations which cannot be achieved with mechanically fastened joints. Unfortunately, because of a lack of reliable inspection methods and a re￾quirement for close dimensional tolerances in fabrication, aircraft designers have generally avoided bonded construction in primary structure. Some notable exceptions include: bonded step lap joints used in attachments for the F-14 and F-15 horizontal stabilizers as well as the F-18 wing root fitting, and a ma￾jority of the airframe components of the Lear Fan and the Beech Starship. While a number of issues related to adhesive joint design were considered in the earlier literature cited in References 6.2.1(a)- 6.2.1(h), much of the methodology currently used in the design and analysis of adhesive joints in composite structures is based on the approaches evolved by L.J. Hart-Smith in a series of NASA/Langley-sponsored contracts of the early 70's (References 6.2.1(i) - 6.2.1(n)) as well as from the Air Force's Primary Adhesively Bonded Structures Technology (PABST) program (References 6.2.1(o) - 6.2.1(r)) of the mid-70's. The most recent such work developed three computer codes for bonded and bolted joints, designated A4EG, A4EI and A4EK (References 6.2.1(s) - 6.2.1(u)), under Air Force contract. The results of these efforts have also appeared in a number of open literature publica￾tions (Reference 6.2.1(v) - (z)). In addition, such approaches found application in some of the efforts tak￾ing place under the NASA Advanced Composite Energy Efficient Aircraft (ACEE) program of the early to mid 80's (Reference 6.2.1(x) and 6.2.1(y)). Some of the key principles on which these efforts were based include: (1) the use of simple 1-dimensional stress analyses of generic composite joints wherever possible; (2) the need to select the joint design so as to ensure failure in the adherend rather than the adhesive, so that the adhesive is never the weak link; (3) recognition that the ductility of aerospace adhesives is beneficial in reducing stress peaks in the adhesive; (4) careful use of such factors as adherend tapering to reduce or eliminate peel stresses from the joint; and (5) recognition of slow cyclic loading, corresponding to such phenomena as cabin pressurization in aircraft, as a major factor controlling durability of adhesive joints, and the need to avoid the worst effects of this type of loading by providing sufficient overlap length to ensure that some of the adhesive is so lightly loaded that creep cannot occur there, under the most severe extremes of humid￾ity and temperature for which the component is to be used. Much of the discussion to follow will retain the analysis philosophy of Hart-Smith, since it is consid￾ered to represent a major contribution to practical bonded joint design in both composite and metallic