MIL-HDBK-17-3F Volume 3,Chapter 12-Lessons Learned In general,if these stress risers are properly considered in design/analysis of laminated parts,fatigue loadings will not be critical. Another unique characteristic of composite material elastic response is its orthotropy.When metals are extended in one direction,they contract in the perpendicular direction in an amount equal to the Pois- son's ratio times the longitudinal strain.This is true regardless of which direction is extended.In compos- ites,an extension in the longitudinal(1 or x)direction produces a contraction in the transverse direction(2 or y)equal to the "major"Poisson's ratio,vxy,times the longitudinal extension.If this is reversed,an ex- tension in the transverse direction produces a much lower contraction in the longitudinal direction.In fiber dominated laminates,Poisson's ratio can vary from <0.1 to >0.5. The most unusual characteristic of composites is the response produced when the lay-up is unbal- anced and/or unsymmetric.Such a laminate exhibits anisotropic warping characteristics.In this condition an extension in one direction can produce an in-plane shear deformation.It can also cause an out-of-plane bending or torsional response.All these effects are sometimes observed in one laminate. This type of response is generally undesirable because of warping or built-in stresses that occur.Hence, most laminate configurations are balanced and symmetric. Classical lamination theory is used to combine the individual lamina properties to predict the linear elastic behavior of arbitrary laminates.Lamination theory requires the definition of lamina elastic proper- ties,their orientation within the laminate,and their stacking position.The process assumes plane sec- tions remain-plane and enforces equilibrium.Lamination theory will solve for the loads/stresses/strains for each lamina within the laminate at a given location for a given set of applied loads.This combined with appropriate failure theory will predict the strength of the laminate(empirically modified input ply prop- erties are often necessary). 12.2.2 Tailored properties and out-of-plane loads The properties of a composite laminate depend on the orientation of the individual plies.This pro- vides the engineer with the ability to tailor a laminate to fit a particular requirement.For high axial loads predominantly in one direction,the laminate should have a majority of its plies oriented parallel to that loading direction.If the laminate is loaded mostly in shear,there should be a high percent of t45 pairs. For loads in multi-directions,the laminate should be quasi-isotropic.An all 0 laminate represents the maximum strength and stiffness that can be attained in any given direction,but is impractical for most ap- plications since the transverse properties are so weak that machining and handling can cause damage. Fiber-dominated,balanced and symmetric,laminate designs that have a minimum of 10%of the plies in each of the0°，+45°，-45°，and90°directions are most commonly used. Tailoring also means an engineer is not able to cite a strength or stiffness value for a composite lami- nate until he knows the laminate's ply percentages in each direction.Carpet plots of various properties vs.the percent of plies in each direction are commonly used for balanced and symmetric laminates.An example for stiffness is shown in Figure 12.2.2.Similar plots for strength can also be developed. Out-of-plane loads can also be troublesome for composites.These loads cause interlaminar shear and tension in the laminate.Interlaminar shear stress can cause failure of the matrix or the fiber-matrix interphase region.Interlaminar shear and tensile stresses can delaminate or disbond a laminate.Such loading should be avoided if possible.Design situations that tend to create interlaminar shear loading include high out-of-plane loads(such as fuel pressure),buckling,abrupt changes in cross-section(such as stiffener terminations),ply drop-offs,and in some cases laminate ply orientations that cause unbal- anced or unsymmetric lay-ups.Interlaminar stresses will arise at any free edge.Interlaminar stresses will arise between plies of dissimilar orientation wherever there is a gradient in the components of in-plane stress. 12-2MIL-HDBK-17-3F Volume 3, Chapter 12 - Lessons Learned 12-2 In general, if these stress risers are properly considered in design/analysis of laminated parts, fatigue loadings will not be critical. Another unique characteristic of composite material elastic response is its orthotropy. When metals are extended in one direction, they contract in the perpendicular direction in an amount equal to the Pois￾son's ratio times the longitudinal strain. This is true regardless of which direction is extended. In compos￾ites, an extension in the longitudinal (1 or x) direction produces a contraction in the transverse direction (2 or y) equal to the "major" Poisson's ratio, ν xy , times the longitudinal extension. If this is reversed, an ex￾tension in the transverse direction produces a much lower contraction in the longitudinal direction. In fiber dominated laminates, Poisson's ratio can vary from <0.1 to >0.5. The most unusual characteristic of composites is the response produced when the lay-up is unbal￾anced and/or unsymmetric. Such a laminate exhibits anisotropic warping characteristics. In this condition an extension in one direction can produce an in-plane shear deformation. It can also cause an out-of-plane bending or torsional response. All these effects are sometimes observed in one laminate. This type of response is generally undesirable because of warping or built-in stresses that occur. Hence, most laminate configurations are balanced and symmetric. Classical lamination theory is used to combine the individual lamina properties to predict the linear elastic behavior of arbitrary laminates. Lamination theory requires the definition of lamina elastic proper￾ties, their orientation within the laminate, and their stacking position. The process assumes plane sec￾tions remain-plane and enforces equilibrium. Lamination theory will solve for the loads/stresses/strains for each lamina within the laminate at a given location for a given set of applied loads. This combined with appropriate failure theory will predict the strength of the laminate (empirically modified input ply prop￾erties are often necessary). 12.2.2 Tailored properties and out-of-plane loads The properties of a composite laminate depend on the orientation of the individual plies. This pro￾vides the engineer with the ability to tailor a laminate to fit a particular requirement. For high axial loads predominantly in one direction, the laminate should have a majority of its plies oriented parallel to that loading direction. If the laminate is loaded mostly in shear, there should be a high percent of ±45° pairs. For loads in multi-directions, the laminate should be quasi-isotropic. An all 0° laminate represents the maximum strength and stiffness that can be attained in any given direction, but is impractical for most ap￾plications since the transverse properties are so weak that machining and handling can cause damage. Fiber-dominated, balanced and symmetric, laminate designs that have a minimum of 10% of the plies in each of the 0°, +45°, -45°, and 90° directions are most commonly used. Tailoring also means an engineer is not able to cite a strength or stiffness value for a composite lami￾nate until he knows the laminate's ply percentages in each direction. Carpet plots of various properties vs. the percent of plies in each direction are commonly used for balanced and symmetric laminates. An example for stiffness is shown in Figure 12.2.2. Similar plots for strength can also be developed. Out-of-plane loads can also be troublesome for composites. These loads cause interlaminar shear and tension in the laminate. Interlaminar shear stress can cause failure of the matrix or the fiber-matrix interphase region. Interlaminar shear and tensile stresses can delaminate or disbond a laminate. Such loading should be avoided if possible. Design situations that tend to create interlaminar shear loading include high out-of-plane loads (such as fuel pressure), buckling, abrupt changes in cross-section (such as stiffener terminations), ply drop-offs, and in some cases laminate ply orientations that cause unbal￾anced or unsymmetric lay-ups. Interlaminar stresses will arise at any free edge. Interlaminar stresses will arise between plies of dissimilar orientation wherever there is a gradient in the components of in-plane stress