MIL-HDBK-17-3F Volume 3.Chapter 12-Lessons Learned Other environmental effects worth noting include the effect of long term exposure to radiation.Ultra- violet rays from the sun can degrade epoxy resins.This is easily protected by a surface finish such as a coat of paint.Another factor is erosion or pitting caused by high speed impact with rain or dust particles. This is likely to occur on unprotected leading edges.There are surface finishes such as rain erosion coats and paints for preventing surface wear.Lightning strike is also a concern to composites.A direct strike can cause considerable damage to a laminate.Lightning strike protection in the form of conductive surfaces is applied in susceptible areas.In cases where substructure is also composite,the inside end of attachment bolts may need to be connected with each other and to ground by a conducting wire. 12.2.6 Joints 12.2.6.1 Mechanically-fastened joints Successful joint design relies on knowledge of potential failure modes.Failure modes depend on joint geometry and laminate lay-up for one given material.The type of fastener used can also influence the occurrence of a particular failure mode.Different materials will give different failure modes. Net-section tensile/compressive failures occur when the bolt diameter is a sufficiently large fraction of the strip width.For most successful designs,this fraction (D/W)is about one-quarter or more for near-isotropic lay-ups in carbon/epoxy systems that have a D/E of one-third or less. Shear-out and shear-out delamination failures occur because the bolt is too close to the edge of the laminate.Such a failure can be triggered when there is only a partial net-section tension or bearing fail- ure.D/t ratios should be 0.75 to 1.25. In some instances the bolt head may be pulled through the laminate after the bolt is bent and de- formed.This mode is frequently seen with countersunk fasteners and is highly dependent on the particu- lar fastener used. Bearing strength is a function of joint geometry,fastener and member stiffnesses.For a 0/45/90 family of laminates with 20-40%of 0 plies and 40-60%of t45plies,plus a minimum(10%)of 90 plies, the bearing strength is relatively constant.Fastener characteristics such as clamp-up force and head configuration have a significant effect.However,for a specific laminate family,a specific fastener,and equal thickness laminate joining members,the parameter with the greatest influence is D/t. Composite joints require smaller D/W and D/E ratios than do metals to get bearing failures. Composite joint strength characteristics differ from metals because the strength is influenced by the bypass load going around the joint.This occurs when two or more fasteners are arranged in a line to transfer the load through a joint.Since not all of the load is reacted by one fastener,some of the load by-passes it.The by-pass effects become prominent once the ratio of by-pass to fastener bearing load exceeds 20%. Titanium fasteners are the most common means of mechanical attachment in composites.This is because titanium is non-corrosive in the galvanic atmosphere created by the dissimilar materials.Tita- nium is closer to carbon on the cathodic scale. 12.2.6.2 Problems associated with adhesive bonding to peel-ply composite surfaces There are two schools of thought in regard to the adhesive bonding of fibrous composite laminates. One demands light but thorough mechanical abrasion,such as by low-pressure grit blasting,because the only such bonds never to fail prematurely were made to abraded surfaces on completely dry laminates. The other permits bonding directly to surfaces created by stripping off peel plies,with or without a drying requirement,using the justification that there is "adequate"initial strength,even though some of these joints have failed prematurely in service.It is also significant that no ultrasonic inspection technique has 12-6MIL-HDBK-17-3F Volume 3, Chapter 12 - Lessons Learned 12-6 Other environmental effects worth noting include the effect of long term exposure to radiation. Ultra￾violet rays from the sun can degrade epoxy resins. This is easily protected by a surface finish such as a coat of paint. Another factor is erosion or pitting caused by high speed impact with rain or dust particles. This is likely to occur on unprotected leading edges. There are surface finishes such as rain erosion coats and paints for preventing surface wear. Lightning strike is also a concern to composites. A direct strike can cause considerable damage to a laminate. Lightning strike protection in the form of conductive surfaces is applied in susceptible areas. In cases where substructure is also composite, the inside end of attachment bolts may need to be connected with each other and to ground by a conducting wire. 12.2.6 Joints 12.2.6.1 Mechanically-fastened joints Successful joint design relies on knowledge of potential failure modes. Failure modes depend on joint geometry and laminate lay-up for one given material. The type of fastener used can also influence the occurrence of a particular failure mode. Different materials will give different failure modes. Net-section tensile/compressive failures occur when the bolt diameter is a sufficiently large fraction of the strip width. For most successful designs, this fraction (D/W) is about one-quarter or more for near-isotropic lay-ups in carbon/epoxy systems that have a D/E of one-third or less. Shear-out and shear-out delamination failures occur because the bolt is too close to the edge of the laminate. Such a failure can be triggered when there is only a partial net-section tension or bearing fail￾ure. D/t ratios should be 0.75 to 1.25. In some instances the bolt head may be pulled through the laminate after the bolt is bent and de￾formed. This mode is frequently seen with countersunk fasteners and is highly dependent on the particu￾lar fastener used. Bearing strength is a function of joint geometry, fastener and member stiffnesses. For a 0/±45/90 family of laminates with 20-40% of 0° plies and 40-60% of ±45° plies, plus a minimum (10%) of 90° plies, the bearing strength is relatively constant. Fastener characteristics such as clamp-up force and head configuration have a significant effect. However, for a specific laminate family, a specific fastener, and equal thickness laminate joining members, the parameter with the greatest influence is D/t. Composite joints require smaller D/W and D/E ratios than do metals to get bearing failures. Composite joint strength characteristics differ from metals because the strength is influenced by the bypass load going around the joint. This occurs when two or more fasteners are arranged in a line to transfer the load through a joint. Since not all of the load is reacted by one fastener, some of the load by-passes it. The by-pass effects become prominent once the ratio of by-pass to fastener bearing load exceeds 20%. Titanium fasteners are the most common means of mechanical attachment in composites. This is because titanium is non-corrosive in the galvanic atmosphere created by the dissimilar materials. Tita￾nium is closer to carbon on the cathodic scale. 12.2.6.2 Problems associated with adhesive bonding to peel-ply composite surfaces There are two schools of thought in regard to the adhesive bonding of fibrous composite laminates. One demands light but thorough mechanical abrasion, such as by low-pressure grit blasting, because the only such bonds never to fail prematurely were made to abraded surfaces on completely dry laminates. The other permits bonding directly to surfaces created by stripping off peel plies, with or without a drying requirement, using the justification that there is “adequate” initial strength, even though some of these joints have failed prematurely in service. It is also significant that no ultrasonic inspection technique has