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In my two previous columns, I discussed the lower and intermediate levels of the building block approach for crashworthiness testing and analysis of composite structures. I focused on the commercial aviation industry, for which a building block exercise has being performed and documented by the Composite Materials Handbook-17 (CMH-17) Crashworthiness Working Group¹.

This building block approach features a multilevel process for designing composite structures with crashworthiness requirements. The overall goal is to validate the analysis methods through testing and then certify the structure by analysis. Therefore, the analysis methods selected for use must produce realistic simulations of test articles. A “building block” approach of progressively larger test articles is followed, in which the complexity of testing and analysis increases and the number of tests decreases. Although not the primary focus of this column, the use of numerical analysis methods for predicting the crashworthiness of composite structures is a key element in the building block approach.

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The building block pyramid, shown in Fig. 1, illustrates the levels of crashworthiness testing and analysis associated with a composite transport aircraft. Each level of the pyramid addresses varying aspects of structural response, progressing from simple material coupons to increasingly complex subcomponent, component and full-scale test articles.

My initial November 2024 column focused on the coupon-level crush testing used in the recent CMH-17 building block exercise. Flat-coupon crush testing was used to identify carbon fiber/epoxy laminates that produce high crush stress and energy absorption values. An initial assessment of numerical analysis methods was also possible through the prediction of the flat-coupon crush behavior and resulting energy absorption. These activities lead to further testing and analysis at the element level, focusing on composite C-channel stanchions used to support the lower cargo floor in a transport aircraft fuselage. In addi­tion to serving as structural members during normal aircraft operations, these stanchions also serve as primary energy-absorbing structural elements in an emergency landing event.

As discussed in my subsequent February 2025 column, a total of 11 different finite element-based progressive damage and failure analysis (PDFA) methods were assessed. The analysis teams were provided with flat-coupon crush test results as well as the C-channel test specimen geometry and drop-weight crush test conditions. Each team predicted the crush behavior using their unique numerical analysis approach and submitted their predicted crush behavior and resulting energy absorption for each of the C-channel laminates. After predictions were received from all analysis teams, the experimental crush test results were distributed to all participants².

In this third and final column of the series, my co-author Dr. Mostafa Rassaian and I will discuss the upper levels of the crashworthiness building block. We will continue to focus on transport aircraft using the building block approach documented in detail in the Composite Materials Handbook-17. Additionally, we’ll focus on the important roles of numerical analyses at the upper levels of the building block.

In general, the higher building block levels focus on test articles and loadings that produce multiple failure modes.

Although the crush testing at the lower levels of the building block pyramid typically produce a single, dominant failure mode, the complex assemblies at the higher building block levels tend to produce multiple failure modes that occur simultaneously or in separate stages within the composite test article. At the subcomponent building block level, testing and numerical analyses focus on an assembly of element-level structures that includes the primary energy-absorbing C-channel stanchions from the previous building block level as shown in the “element/subcomponent” level of Fig. 1.

An additional focus of subcomponent-level testing and analysis is the mechanical connections and the resulting interaction during crushing of structural elements that form the subcomponent. Results from subcomponent-level crush testing are used to ensure that the numerical analyses are properly predicting the sequence modes of failure, load redistribution and resulting structural element crush behavior observed during crush testing. As a result, crush testing is typically performed using multiple candidate subcomponent designs when developing subcomponent-level structures for crashworthiness. The overall energy-absorbing capability of these subcomponents is directly related to the materials used in the elements as well as their positioning within the subcomponent. As a result, extensive subcomponent-level crush testing and analysis typically is required within a building block approach to design crashworthy structures that will perform as intended.

At the component level, a larger assembly of elements and subcomponents that represents a significant portion of the primary crush structure are tested and analyzed. In the CMH-17 building block approach, a candidate component-level test article would include a portion of the aircraft fuselage barrel as shown in the “component” level of Fig. 1. In addition to further assessing the effectiveness of element- and subcomponent-level energy-absorbing features, component-level crush testing is used to assess the connections between adjacent elements and subcomponents. In general, component-level crush testing produces multiple failure modes due to multi-structural interactions under variable loading conditions and impact velocities during the crash event, as well as the complexity of the possible failure mechanisms within the assembled component.

As is the case for other levels of the building block, component-level testing is also used to validate the numerical modeling approach. This includes the material modeling parameters and crush-related properties for predicting energy absorption. In general, the higher building block levels focus on test articles and loadings that produce multiple failure modes due to interacting failure mechanisms. This can occur simultaneously or in separate stages within complex components, as well as variability in impact loading during the crash event.

Full-scale crashworthiness testing is often performed as a final validation test. For transport aircraft, full-scale crush testing is typically performed using a full-barrel section of the complete aircraft fuselage. Depending on the crashworthiness test program, however, a subassembly such as the half-barrel section shown in Fig. 1 may be used in place of the full test article for validation of all modeling parameters.

For more information regarding the building block approach for composites crashworthiness, an extensive write-up is available in Revision H of CMH–17¹. Details of the recent crashworthiness building block exercise are also available in a soon-to-be-published NASA report².

References

¹Composite Materials Handbook - 17 (CMH-17), Volume 3, Chapter 16: “Crashworthiness and Energy Management,” SAE International, Rev. H, 2025. Materials Usage, Design and Analysis.

²Rassaian, M., Pereira, J. M., et al., “Progressive Damage and Failure Analysis Methods Applications for Aircraft Crashworthiness and Impact Energy Management,” NASA/TM-20250002545, Spring 2025.

About the Author

 

Mostafa Rassaian

Dr. Mostafa Rassaian is a leading expert in computational structural mechanics, impact dynamics. An AIAA Fellow, he pioneered model-based structural mechanics to advance lightweight, damage-tolerant aerospace designs. A former Boeing Technical Fellow, he spent over 30 years at Boeing. There, he led certification by analysis supported by smart testing for the B787-8, using the building block approach to predict damage initiation and propagation across various loading conditions and crash scenarios, ultimately leading to FAA type certification. He holds 31 U.S. patents in computational structural mechanics and has published over 100 technical papers. As chair of the Crashworthiness Working Group for FAA-sponsored CMH-17, he leads PDFA predictive capability modeling best practices for composite structures. He is the lead author in the development and establishment of the PDFA evaluation framework, to be published in a NASA TM assessing computational methods for aircraft crashworthiness in spring 2025. He consults on material modeling for dynamic event-based simulation. mostafa@rassaianllc.com

About the Author

 

Dan Adams

Dr. Daniel O. Adams is president of Wyoming Test Fixtures Inc. (Salt Lake City, Utah, U.S.) and an emeritus professor of mechanical engineering at the University of Utah, where for 23 years he directed the Composite Mechanics Laboratory. He holds a B.S. in mechanical engineering and an M.S. and Ph.D. in engineering mechanics. Adams has a combined 45 years of academic/industry experience in the composite materials field. He has published more than 120 technical papers, is chair of ASTM Committee D30 on Composite Materials and co-chair of the Testing Working Group for the Composite Materials Handbook (CMH-17). He regularly provides testing seminars and consulting services to the composites industry. Dan@WyomingTestFixtures.com