MIL-HDBK-17-3F Volume 3.Chapter 6 Structural Behavior of Joints CHAPTER 6 STRUCTURAL BEHAVIOR OF JOINTS 6.1 INTRODUCTION It would be difficult to conceive of a structure that did not involve some type of joint.Joints often oc- cur in transitions between major composite parts and a metal feature or fitting.In aircraft,such a situation is represented by articulated fittings on control surfaces as well as on wing and tail components which require the ability to pivot the element during various stages of operation.Tubular elements such as power shafting often use metal end fittings for connections to power sources or for articulation where changes in direction are needed.In addition,assembly of the structure from its constituent parts will in- volve either bonded or mechanically fastened joints or both. Joints represent one of the greatest challenges in the design of structures in general and in compos- ite structures in particular.The reason for this is that joints entail interruptions of the geometry of the structure and often,material discontinuities,which almost always produce local highly stressed areas, except for certain idealized types of adhesive joint such as scarf joints between similar materials.Stress concentrations in mechanically fastened joints are particularly severe because the load transfer between elements of the joint have to take place over a fraction of the available area.For mechanically fastened joints in metal structures,local yielding.which has the effect of eliminating stress peaks as the load in- creases,can usually be depended on;such joints can be designed to some extent by the "P over A"ap- proach,i.e.,by assuming that the load is evenly distributed over load bearing sections so that the total load(the"P")divided by the available area(the"A")represents the stress that controls the strength of the joint.In organic matrix composites,such a stress reduction effect is realized only to a minor extent,and stress peaks predicted to occur by elastic stress analysis have to be accounted for.especially for one- time monotonic loading.In the case of composite adherends,the intensity of the stress peaks varies with the orthotropy of the adherend in addition to various other material and dimensional parameters which affect the behavior of the joint for isotropic adherends. In principle.adhesive joints are structurally more efficient than mechanically fastened joints because they provide better opportunities for eliminating stress concentrations;for example,advantage can be taken of ductile response of the adhesive to reduce stress peaks.Mechanically fastened joints tend to use the available material inefficiently.Sizeable regions exist where the material near the fastener is nearly unloaded,which must be compensated for by regions of high stress to achieve a particular re- quired average load.As mentioned above,certain types of adhesive joints,namely scarf joints between components of similar stiffness,can achieve a nearly uniform stress state throughout the region of the ioint. In many cases,however,mechanically fastened joints can not be avoided because of requirements for disassembly of the joint for replacement of damaged structure or to achieve access to underlying structure.In addition,adhesive joints tend to lack structural redundancy,and are highly sensitive to manufacturing deficiencies,including poor bonding technique,poor fit of mating parts and sensitivity of the adhesive to temperature and environmental effects such as moisture.Assurance of bond quality has been a continuing problem in adhesive joints;while ultrasonic and X-ray inspection may reveal gaps in the bond,there is no present technique which can guarantee that a bond which appears to be intact does, in fact,have adequate load transfer capability.Surface preparation and bonding techniques have been well developed,but the possibility that lack of attention to detail in the bonding operation may lead to such deficiencies needs constant alertness on the part of fabricators.Thus mechanical fastening tends to be preferred over bonded construction in highly critical and safety rated applications such as primary aircraft structural components,especially in large commercial transports,since assurance of the required level of structural integrity is easier to guarantee in mechanically fastened assemblies.Bonded construction tends to be more prevalent in smaller aircraft.For non-aircraft applications as well as in non-flight critical aircraft components,bonding is likewise frequently used. 6-1MIL-HDBK-17-3F Volume 3, Chapter 6 Structural Behavior of Joints 6-1 CHAPTER 6 STRUCTURAL BEHAVIOR OF JOINTS 6.1 INTRODUCTION It would be difficult to conceive of a structure that did not involve some type of joint. Joints often oc￾cur in transitions between major composite parts and a metal feature or fitting. In aircraft, such a situation is represented by articulated fittings on control surfaces as well as on wing and tail components which require the ability to pivot the element during various stages of operation. Tubular elements such as power shafting often use metal end fittings for connections to power sources or for articulation where changes in direction are needed. In addition, assembly of the structure from its constituent parts will in￾volve either bonded or mechanically fastened joints or both. Joints represent one of the greatest challenges in the design of structures in general and in compos￾ite structures in particular. The reason for this is that joints entail interruptions of the geometry of the structure and often, material discontinuities, which almost always produce local highly stressed areas, except for certain idealized types of adhesive joint such as scarf joints between similar materials. Stress concentrations in mechanically fastened joints are particularly severe because the load transfer between elements of the joint have to take place over a fraction of the available area. For mechanically fastened joints in metal structures, local yielding, which has the effect of eliminating stress peaks as the load in￾creases, can usually be depended on; such joints can be designed to some extent by the "P over A" ap￾proach, i.e., by assuming that the load is evenly distributed over load bearing sections so that the total load (the "P") divided by the available area (the "A") represents the stress that controls the strength of the joint. In organic matrix composites, such a stress reduction effect is realized only to a minor extent, and stress peaks predicted to occur by elastic stress analysis have to be accounted for, especially for one￾time monotonic loading. In the case of composite adherends, the intensity of the stress peaks varies with the orthotropy of the adherend in addition to various other material and dimensional parameters which affect the behavior of the joint for isotropic adherends. In principle, adhesive joints are structurally more efficient than mechanically fastened joints because they provide better opportunities for eliminating stress concentrations; for example, advantage can be taken of ductile response of the adhesive to reduce stress peaks. Mechanically fastened joints tend to use the available material inefficiently. Sizeable regions exist where the material near the fastener is nearly unloaded, which must be compensated for by regions of high stress to achieve a particular re￾quired average load. As mentioned above, certain types of adhesive joints, namely scarf joints between components of similar stiffness, can achieve a nearly uniform stress state throughout the region of the joint. In many cases, however, mechanically fastened joints can not be avoided because of requirements for disassembly of the joint for replacement of damaged structure or to achieve access to underlying structure. In addition, adhesive joints tend to lack structural redundancy, and are highly sensitive to manufacturing deficiencies, including poor bonding technique, poor fit of mating parts and sensitivity of the adhesive to temperature and environmental effects such as moisture. Assurance of bond quality has been a continuing problem in adhesive joints; while ultrasonic and X-ray inspection may reveal gaps in the bond, there is no present technique which can guarantee that a bond which appears to be intact does, in fact, have adequate load transfer capability. Surface preparation and bonding techniques have been well developed, but the possibility that lack of attention to detail in the bonding operation may lead to such deficiencies needs constant alertness on the part of fabricators. Thus mechanical fastening tends to be preferred over bonded construction in highly critical and safety rated applications such as primary aircraft structural components, especially in large commercial transports, since assurance of the required level of structural integrity is easier to guarantee in mechanically fastened assemblies. Bonded construction tends to be more prevalent in smaller aircraft. For non-aircraft applications as well as in non-flight critical aircraft components, bonding is likewise frequently used