The composite structural materials testing method is a step-by-step process that serves as a framework for designing composite structures while reducing risk and cost.
Figure 1Pyramid of structural testing.
In the context of composite testing, the building block approach is a series of increasingly complex mechanical tests, coupled with analysis at each step, as a framework for designing composite structures. While most references to the building block approach to composite design are focused on the aviation industry, which was originally developed, many other end markets served by the composites industry also follow a generic approach. Not surprisingly, the number and type of composite testing varies widely, depending on the application and complexity of the composite structure.
A generic test pyramid is shown in Figure 1 illustrating the level of testing performed. In the building block approach, each level or "block" serves a specific purpose and is processed sequentially from the bottom. The tests associated with each level can be described in terms of the complexity of the test item and the use of the test results. In this column, I'll focus on the lowest level of the building block pyramid that corresponds to the initial stages of the design process.
The datum of the test pyramid, known as the test piece level, is where most test piece level tests are performed. These tests are used for a variety of purposes, and depending on the structure being designed, the use can vary greatly. The first purpose of testing is to screen candidate composites. If the candidate material is not yet available, screening tests are performed to determine its basic stiffness and strength characteristics.
In addition, it is common practice to select additional types of screening tests to obtain key characteristics for a particular application or purpose. For example, the short beam shear test (ASTM D 23441) can be used for material selection because the measured short beam strength is highly sensitive to issues of porosity, fiber-to-matrix adhesion, and interlaminar strength within composite laminates. In addition, the open-pore compression test (ASTM D64842) and the post-impact compression test (AS***71373) are used in many aerospace applications because both are used as screening tests for notched susceptibility, with the former being used for notched laminate design tolerances, which are often more critical than compressive strength. Since a specific multi-directional laminate has not yet been selected for use, quasi-isotropic laminates (25% 0° layer, 50% 45° layer and 25% 90° layer) are typically used for these trials. In addition, these screening tests are sometimes performed under critical environmental conditions required for the application, such as samples with humidity regulation at high temperatures. Candidate composites can also undergo manufacturing and repair trials before making final selections.
While this testing requires considerable time and expense, the aim is to reduce the number of further sub-assembly and component-level tests on the pyramid.
Please note that if a composite of an existing database is selected, then a significant portion of these screening tests may be cancelled. Although composites vendors typically offer only a limited set of properties for their materials, public databases are available for a growing number of composites. For example, the experimental database of advanced general air transport, developed in the 90s of the 20th century4, includes the properties of thin layers of materials of carbon fiber and glass fiber reinforced composites with woven fabrics and unidirectional fibers. In addition, the NCAMP-National Center for Advanced Materials Performance database5 includes thin layer and laminate performance data for a variety of composite materials, with a focus on the aerospace industry. Finally, data on the mechanical properties of various composites of general interest can be obtained in Volume 2 of the CMH-17-Composite Materials Handbook - 17)6.
Once candidate composites have been selected for a particular application and the key mechanical properties to support the design have been identified, a robust test board manufacturing process must be developed that can continue to build more complex test pieces to form the final structure. With the development of process specifications, application-critical mechanical properties can be tested on multiple production batches of materials and used to determine preliminary material tolerances – material performance values derived from the test data. For this purpose, testing is often carried out using materials produced in multiple production runs. Tests can also be conducted to address the effects of non-ambient environments. For polymer matrix composites (PMCs), the most common environment of concern is hot and wet conditions, under which the matrix material loses its stiffness and strength, and at low temperatures, the matrix material loses ductility.
Additional specimen level testing is performed at this point in the design process to obtain material properties that are significantly more difficult to produce through analysis. Examples include notch-sensitive (bare and filled) tests, bearing tests, and damage tolerance (compression after impact) tests. These tests are typically performed using specific multi-directional composite laminates of interest for a specific application. To limit the amount of test required, the number of laminates is usually reduced by limiting the number of fiber directions, such as 0°, 45°, and 90° laminates, and changing the percentage of each group of laminates in the laminates. For aerospace applications, common laminate tests include quasi-isotropic laminates (25% 0° layer, 50% 45° layer, 25% 90° layer) as well as other "hard" laminates (40-50% 0° layer). "Soft" laminates (8-10% 0° layers) are also sometimes tested. Even if the number of candidate laminates is limited, the number of tests can increase rapidly.
In summary, a relatively large number of specimen level tests are typically performed at the basic level of the building block approach. While this testing requires considerable time and expense, the aim is to control material and process variability and reduce the number of further component and component-level tests up the pyramid, thereby reducing risk while reducing overall costs. However, as I mentioned at the outset, there is considerable variability in the build process in different end markets and in the different complexities of composite structures. Much of what is said about the building block approach is focused on aerospace, especially commercial aerospace. Currently, the Building Blocks approach is presented in the most extensive way in Volume 3, Chapter 4 of the Handbook of Composites (CMH-17)7.
References:
1 ASTM D2344 D2344M-16, "Standard Test Method for Strength of Polymer Matrix Composites and Their Laminate Short Beams", ASTM International (West Conshchucken, PA, USA), 2016 (first published in 1965).
2 ASTM D6484 D684M-20, "Open-Cell Compressive Strength of Polymer Matrix Composite Laminates," ASTM International (West Conshchoken, PA, USA), 2020 (first published in 1999).
3 ASTM D7137 D7137M-17, "Compressive Residual Strength Properties of Damaged Polymer Matrix Composite Sheets," ASTM International (West Conshchoken, PA), 2017 (originally published in 2005).
4 Advanced General Air Transport Experiment (AGATE) Database, wwwniar.wichita.edu/agate/
5 National Center for Advanced Materials Performance (NCAMP) database, wwwniar.wichita.edu/coe/ncamp.asp
6 Handbook of Composites Materials-17 (CMH-17), Volume 2: Material Properties, SAE International, Rev. H, 2018.
7 Handbook of Composites-17 (CMH-17), Volume 3: Materials Use, Design and Analysis, SAE International, 2012 G edition.
-END --Note:The original article is available in Composites Testing as Part of a Building Block Approach, Part 1: Coupon-Level Testing, 20217.28
Chaofan Yang 202312.8