© 2002 by CRC Press LLC For this reason, the adhesive supplier seldom reports the tensile strength of the adhesive. The data reported by adhesive suppliers is usually the shear strength obtained from a single lap joint test. An exception to this is that the tensile tests are conducted by sandwich panel manufacturers to calculate the bond strength between skin and honeycomb core materials. This test is per- formed for quality control purposes or for selecting the correct adhesive for a sandwich structure. In honeycomb-cored sandwich structures, the bonded surface area between the core and skin material is much less than the surface area of the skin and therefore failure usually takes place at the interface. The most common test method for shear testing is the single lap joint test (ASTM D 1002). The lap shear test measures the strength of an adhesive in most extent in shear. In this test, the specimen is prepared as shown in Figure 9.4. Substrate materials are cut into 1 ¥ 4-in. test coupons from a large, flat rectangular sheet. The substrate materials are then joined in a 0.5-in. overlap area with the desired adhesive. Once the adhesive is cured, the two ends of the substrate materials are pulled under tension as shown in Figure 9.3. Because shear at the bonded surface during the lap-shear test is created by applying tensile load on the substrate material, this test is also called the tensile-shear test. This test method is used extensively because of its simplicity and low cost for evaluation purposes. However, due to non- uniform stress at the joint, the strength values obtained by this test method are of little use for engineering design purposes. However, this test method can be used to see the effects of lap length, adhesive thickness, adhesive material, etc. on bonded joints. When the bond strength test is performed for two dissimilar materials (e.g., glass/epoxy and aluminum) the thickness of the substrates is maintained such that the stiffness values of the adherend materials are the same (i.e., E 1 t 1 = E 2 t 2 , where E 1 , E 2 , t 1 , t 2 are the stiffnesses and thicknesses of the adherend materials 1 and 2, respectively). Mazumdar and Mallick 1 conducted a series of tests on SMC-SMC and SRIM- SRIM joints to determine the effects of lap length, bond thickness, and joint FIGURE 9.4 Standard lap shear test specimen. 0.0064" Adhesive Substrate 2 Substrate 1 4.0" 1.0" 5.5" 0.5"0.5" 1.0" 7.5" © 2002 by CRC Press LLC For this reason, the adhesive supplier seldom reports the tensile strength of the adhesive. The data reported by adhesive suppliers is usually the shear strength obtained from a single lap joint test. An exception to this is that the tensile tests are conducted by sandwich panel manufacturers to calculate the bond strength between skin and honeycomb core materials. This test is per- formed for quality control purposes or for selecting the correct adhesive for a sandwich structure. In honeycomb-cored sandwich structures, the bonded surface area between the core and skin material is much less than the surface area of the skin and therefore failure usually takes place at the interface. The most common test method for shear testing is the single lap joint test (ASTM D 1002). The lap shear test measures the strength of an adhesive in most extent in shear. In this test, the specimen is prepared as shown in Figure 9.4. Substrate materials are cut into 1 ¥ 4-in. test coupons from a large, flat rectangular sheet. The substrate materials are then joined in a 0.5-in. overlap area with the desired adhesive. Once the adhesive is cured, the two ends of the substrate materials are pulled under tension as shown in Figure 9.3. Because shear at the bonded surface during the lap-shear test is created by applying tensile load on the substrate material, this test is also called the tensile-shear test. This test method is used extensively because of its simplicity and low cost for evaluation purposes. However, due to non- uniform stress at the joint, the strength values obtained by this test method are of little use for engineering design purposes. However, this test method can be used to see the effects of lap length, adhesive thickness, adhesive material, etc. on bonded joints. When the bond strength test is performed for two dissimilar materials (e.g., glass/epoxy and aluminum) the thickness of the substrates is maintained such that the stiffness values of the adherend materials are the same (i.e., E 1 t 1 = E 2 t 2 , where E 1 , E 2 , t 1 , t 2 are the stiffnesses and thicknesses of the adherend materials 1 and 2, respectively). Mazumdar and Mallick 1 conducted a series of tests on SMC-SMC and SRIM- SRIM joints to determine the effects of lap length, bond thickness, and joint FIGURE 9.4 Standard lap shear test specimen. 0.0064" Adhesive Substrate 2 Substrate 1 4.0" 1.0" 5.5" 0.5"0.5" 1.0" 7.5" © 2002 by CRC Press LLC 10 Machining and Cutting of Composites 10.1 Introduction Composite materials offer the benefits of part integration and thus minimize the requirement for machining operations. However, machining operations cannot be completely avoided and most of the components have some degree of machining. Machining operations are extensively used in the aerospace industry. In a typical aerospace application, assembly and sub-assembly labor costs account for as much as 50% of the total manufacturing costs of current airframes. 1 A fighter plane has between 250,000 and 400,000 holes and a bomber or transport has between 1,000,000 and 2,000,000 holes; there- fore, machining cost has become major production cost factor in aerospace applications. 1 A typical wing on an aircraft may have as many as 5000 holes. 2 There are several types of machining operations, such as cutting, drilling, routing, trimming, sanding, milling, etc., performed to achieve various objec- tives. The majority of these machining processes are similar to metal machin- ing. The objectives of these machining operations are discussed below. 10.2 Objectives/Purposes of Machining Machining of composites is done to fulfill the following objectives. 1. To create holes, slots, and other features that are not possible to obtain during manufacturing of the part. For example, if a pul- truded part needs holes and other features as shown in Figure 10.1 then machining of the part is unavoidable. 2. Machining is done to create the desired tolerance in the component. For example, if a filament wound part requires the outside diameter to have a tolerance of 0.002 in., then centerless grinding is done to get that tolerance on the outer surface. . 2 are the stiffnesses and thicknesses of the adherend materials 1 and 2, respectively). Mazumdar and Mallick 1 conducted a series of tests on SMC-SMC and SRIM- SRIM joints to determine. 2 are the stiffnesses and thicknesses of the adherend materials 1 and 2, respectively). Mazumdar and Mallick 1 conducted a series of tests on SMC-SMC and SRIM- SRIM joints to determine. adhesive for a sandwich structure. In honeycomb-cored sandwich structures, the bonded surface area between the core and skin material is much less than the surface area of the skin and therefore