MIL-HDBK-3F Volume 3,Chapter 4 Building Block Approach for Composite Structures the types of analyses or tests,as these may be highly dependent upon particular design details,loadings, and structural criticality. While it is certainly desirable to standardize the Building Block approach and to develop methods for assigning statistical reliability to the process,these goals are viewed as fairly long term,given the current body of work and diversity of individual approaches.The purpose of this chapter is to summarize the most prevalent and widely accepted methodology,and to present examples of Building Block programs for various applications and material forms. This section has presented an introduction to the concept of a building block approach.The rationale and assumptions required in developing a building block approach are described in Section 4.2.The general methodology of such an approach is described in Section 4.3.An example describing the use of the building block approach for EMD and production aircraft,processed using autoclave cure of prepreg. is presented in detail in Section 4.4.Section 4.5 includes the use of the building block approach for other applications with general descriptions and references to the more detailed example.The implications of using other types of processing and material forms are discussed in the final section. 4.2 RATIONALE AND ASSUMPTIONS The Building Block approach has been used in aircraft structures development programs long before the application of composites.However,this approach is more crucial for the certification of composite structures because of issues such as sensitivity to out-of-plane loads,their multiplicity of potential failure modes,and their sensitivity to operating environment.The combination of these issues and an inherent defect sensitivity of the composites,which are best classified as quasi-brittle,has resulted in a lack of analytical tools to predict the behavior of full-scale structure from the lowest level material properties. The multiplicity of potential failure modes is perhaps the main reason that the Building Block ap- proach is essential in the development of composite structural substantiation.The many failure modes in composite structures are mainly due to the defect,environmental and out-of-plane sensitivities of the materials. The low interlaminar strength of composites makes them sensitive to out-of-plane loads.Out-of- plane loads can arise directly or be induced from in-plane loads.The most difficult loads to design and analyze for are those loads which arise insidiously in full-scale built-up structures.Analysis tools currently available for structural engineers often assume these loads as secondary loads and they are usually simulated with lower degrees of accuracy.Therefore,it is very important to simulate all potential out-of- plane failure modes and obtain experimental data through a well planned Building Block testing program. Simulation of the correct failure modes plays an important role in a Building Block testing program. Since failure modes are frequently dependent on the test environment and defects present(manufactur- ing,bad design detail,or accidental damage),it is important to carefully select the correct test specimens that will simulate the desired failure modes.Special attention should be given to matrix sensitive failure modes.Following selection of the critical failure modes,a series of specimens is designed,each one to simulate a single failure mode.These specimens will generally be lower complexity specimens. Ideally,if structural analysis tools are fully developed and the failure criteria fully established,the structural behavior would be predictable from the constituent properties.Unfortunately,the capability of the state-of-the-art analysis methods are limited.Thus,lower level test data can not always be used to accurately predict the behavior of structural elements and components with higher levels of complexity. The accuracy of the analytical results are further complicated by the material property variability,the in- clusion of defects,and the structural scale-up effects.Therefore,step-by-step building block testings are required to: Uncover failure modes which do not occur at a lower level tests 4-4MIL-HDBK-3F Volume 3, Chapter 4 Building Block Approach for Composite Structures 4-4 the types of analyses or tests, as these may be highly dependent upon particular design details, loadings, and structural criticality. While it is certainly desirable to standardize the Building Block approach and to develop methods for assigning statistical reliability to the process, these goals are viewed as fairly long term, given the current body of work and diversity of individual approaches. The purpose of this chapter is to summarize the most prevalent and widely accepted methodology, and to present examples of Building Block programs for various applications and material forms. This section has presented an introduction to the concept of a building block approach. The rationale and assumptions required in developing a building block approach are described in Section 4.2. The general methodology of such an approach is described in Section 4.3. An example describing the use of the building block approach for EMD and production aircraft, processed using autoclave cure of prepreg, is presented in detail in Section 4.4. Section 4.5 includes the use of the building block approach for other applications with general descriptions and references to the more detailed example. The implications of using other types of processing and material forms are discussed in the final section. 4.2 RATIONALE AND ASSUMPTIONS The Building Block approach has been used in aircraft structures development programs long before the application of composites. However, this approach is more crucial for the certification of composite structures because of issues such as sensitivity to out-of-plane loads, their multiplicity of potential failure modes, and their sensitivity to operating environment. The combination of these issues and an inherent defect sensitivity of the composites, which are best classified as quasi-brittle, has resulted in a lack of analytical tools to predict the behavior of full-scale structure from the lowest level material properties. The multiplicity of potential failure modes is perhaps the main reason that the Building Block ap￾proach is essential in the development of composite structural substantiation. The many failure modes in composite structures are mainly due to the defect, environmental and out-of-plane sensitivities of the materials. The low interlaminar strength of composites makes them sensitive to out-of-plane loads. Out-of￾plane loads can arise directly or be induced from in-plane loads. The most difficult loads to design and analyze for are those loads which arise insidiously in full-scale built-up structures. Analysis tools currently available for structural engineers often assume these loads as secondary loads and they are usually simulated with lower degrees of accuracy. Therefore, it is very important to simulate all potential out-of￾plane failure modes and obtain experimental data through a well planned Building Block testing program. Simulation of the correct failure modes plays an important role in a Building Block testing program. Since failure modes are frequently dependent on the test environment and defects present (manufactur￾ing, bad design detail, or accidental damage), it is important to carefully select the correct test specimens that will simulate the desired failure modes. Special attention should be given to matrix sensitive failure modes. Following selection of the critical failure modes, a series of specimens is designed, each one to simulate a single failure mode. These specimens will generally be lower complexity specimens. Ideally, if structural analysis tools are fully developed and the failure criteria fully established, the structural behavior would be predictable from the constituent properties. Unfortunately, the capability of the state-of-the-art analysis methods are limited. Thus, lower level test data can not always be used to accurately predict the behavior of structural elements and components with higher levels of complexity. The accuracy of the analytical results are further complicated by the material property variability, the in￾clusion of defects, and the structural scale-up effects. Therefore, step-by-step building block testings are required to: 1 Uncover failure modes which do not occur at a lower level tests