214 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES shear at constant amplitude or under spectrum loading,and may be aimed at simulating the actual loading conditions in a particular application. Dynamic loads are used to measure the resistance of the materials to impact or ballistic conditions.These tests are also conducted under tension,compression, shear,or flexure,or they may be conducted using an impactor or penetrator of some type.In some tests,the impact event may occur while the specimen is under tensile or compressive loading.Typically,loading occurs over a 1-millisecond time interval. Testing may be conducted at different temperatures and levels of absorbed moisture.They may also include exposure to a range of other environmental conditions,such as UV and solvents. The specimens may be simple coupons or they may be structural details with representative stress-raisers such as holes,filled with a fastener or open.The coupons or details may include representative damage such as sharp notches or impact damage. Test machines consist of loading frames,one fixed and one moving crosshead, separated either by a simple electromechanical screw action or by a servo- hydraulic piston.For simple static testing,the screw-driven machines are simpler and less costly and there is less danger of overload caused by accidental rapid movement of the crosshead.However,for versatility in loading (e.g.spectrum loading in fatigue testing)and in load capacity,the servo-hydraulic machines are unmatched. 7.1.2 Special Requirements for Testing Composites During the early development of composites,many of the test techniques used for metals and other homogenous,isotropic materials were used to determine the properties of composite materials.It was soon recognized that anisotropic composite materials often required special consideration in terms of mechanical property determination.Much of the test method development was also undertaken within individual organizations,thus standardization was difficult and many methods developed were not adequate for the newer,emerging materials. Since those early days,there has been a great deal of effort devoted to the standardization of test methods,and there are a number of reference sources that can be used to identify the relevant techniques.Test standards have been published by the American Society for Testing and Materials(ASTM)2-and the Suppliers of Advanced Composite Materials Association (SACMA),10.11 together with other information sources such as the U.S.Department of Defense Military Handbook 17 (MIL-HDBK 17;Polymer Matrix Composites).MIL- HDBK 17 is specifically suited to composite materials for aerospace applications and is generally used for test method selection. The test techniques briefly described here are the ones most commonly used when measuring stress and strain in the tensile,compressive,flexural,and shear load states,but they are not the only techniques that can be used.The most critical214 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES shear at constant amplitude or under spectrum loading, and may be aimed at simulating the actual loading conditions in a particular application. Dynamic loads are used to measure the resistance of the materials to impact or ballistic conditions. These tests are also conducted under tension, compression, shear, or flexure, or they may be conducted using an impactor or penetrator of some type. In some tests, the impact event may occur while the specimen is under tensile or compressive loading. Typically, loading occurs over a 1-millisecond time interval. Testing may be conducted at different temperatures and levels of absorbed moisture. They may also include exposure to a range of other environmental conditions, such as UV and solvents. The specimens may be simple coupons or they may be structural details with representative stress-raisers such as holes, filled with a fastener or open. The coupons or details may include representative damage such as sharp notches or impact damage. Test machines consist of loading frames, one fixed and one moving crosshead, separated either by a simple electromechanical screw action or by a servo￾hydraulic piston. For simple static testing, the screw-driven machines are simpler and less costly and there is less danger of overload caused by accidental rapid movement of the crosshead. However, for versatility in loading (e.g. spectrum loading in fatigue testing) and in load capacity, the servo-hydraulic machines are unmatched. 7.1.2 Special Requirements for Testing Composites During the early development of composites, many of the test techniques used for metals and other homogenous, isotropic materials were used to determine the properties of composite materials. It was soon recognized that anisotropic composite materials often required special consideration in terms of mechanical property determination. Much of the test method development was also undertaken within individual organizations, thus standardization was difficult and many methods developed were not adequate for the newer, emerging materials. Since those early days, there has been a great deal of effort devoted to the standardization of test methods, and there are a number of reference sources that can be used to identify the relevant techniques. Test standards have been published by the American Society for Testing and Materials (ASTM) 2-9 and the Suppliers of Advanced Composite Materials Association (SACMA), 1°'ix together with other information sources such as the U.S. Department of Defense Military Handbook 17 (MIL-HDBK 17; Polymer Matrix Composites). MIL￾HDBK 17 is specifically suited to composite materials for aerospace applications and is generally used for test method selection. The test techniques briefly described here are the ones most commonly used when measuring stress and strain in the tensile, compressive, flexural, and shear load states, but they are not the only techniques that can be used. The most critical