Crashworthy Composite Structures

The writing and publication of this book are extremely timely. The high level of interest in advanced air mobility (AAM) and the explosion in innovative vertical takeoff and landing (VTOL) design configurations has led to the creation of many new and innovative aircraft companies. As the vehicle designs created by these companies mature beyond the scale model, concept demonstration and prototype stages, many practical aspects of these designs will come under development and consideration, among which will be accommodating and protecting the occupants. While the book also covers automotive crashworthiness, this review focuses on aircraft and in particular VTOL aircraft.

The preferred strategy for occupant protection is always to eliminate mishaps; this strategy not only protects passengers, but protects bystanders, property and the vehicle itself. However, until a new technology’s ability to eliminate mishaps is demonstrated in practice, the incorporation of vehicle crashworthiness will be a prudent requirement. Thus, one can anticipate these new companies confronting the need to incorporate crashworthiness into their designs and consequently the need for crashworthiness expertise. Likewise, there will be an increased demand on regulatory agencies to evaluate and certify these aircraft. The book reviewed herein is an excellent first source for a new organization gaining knowledge in crashworthiness, particularly for composite structures. The book can also serve someone already experienced in crashworthiness and seeking greater knowledge in crashworthiness of composite structures.

Chapter 1 of Composite Crashworthy Structures confirms the success of crashworthy technology in reducing injuries and fatalities in aircraft mishaps, especially VTOL aircraft.

Chapter 2 presents a concise review of the principles of crashworthiness with an emphasis on energy absorption as the primary strategy for protecting the vehicle and its occupants. The discussion of the principles is enhanced by extensive references to the literature on crashworthiness requirements and solutions. Since much of this literature was originally created and published as military research, the specific documents may not be readily located by those new to the field. An entire section is devoted to rotorcraft with the important references to the relevant literature.

Chapter 3 describes the requirements that need to be satisfied in order to certify a vehicle’s crashworthiness. The section on rotorcraft effectively presents the advanced state of requirements for military vehicles compared to civil vehicles. The authors describe the documents available and how they apply. The aircraft portion of this chapter primarily addresses rotorcraft, but also includes some fixed-wing certification information. Of potential interest to the newly evolving AAM industry will be the discussion of seat certification by computer modeling and how that is being approached.

Chapter 4 contains a section entitled Special Features, which presents the topic of anthropomorphic test devices (ATDs) together with the simulation of ATDs and injury assessment criteria. The application of injury assessment criteria, the suitability of the various ATD types and their corresponding computer models is a critical topic in establishing a design’s certification as crashworthy. The section includes a comparison of lumbar injury criteria that are critical to vertical impact certification. This chapter is highly practical and is exemplary of the book’s usefulness to the user. The chapter describes both physical crash testing of aircraft (complete and segments) and computer simulation testing. The section on aircraft drop testing describes the facilities available as well as the challenges faced in conducting such tests. The discussion of computer crash simulation is quite comprehensive and very useful for a person new to the field. The section describes the two basic types of solvers and progresses to the critical details of how structural and material properties are put into the models. The chapter also demonstrates the extent of the work and level of detail required by presenting an example of a full-scale crash test along with the associated computer model including the occupants.

Chapter 5, Mechanics of Composite Energy Absorbers, begins with the principles of energy absorbing devices and progresses through the details of materials, physical designs, initiation, and finishes with several of the more subtle variables involved. The importance of initiating deformation in the energy absorbers and then maintaining a consistent deformation mechanism is emphasized. Besides delivering a sense of the diversity of this field, the chapter provides extensive references so that the reader can pursue specific designs back to the original research work. The chapter includes sections on energy absorber design testing by describing the various verification levels (i.e. coupon, element, component and full-scale), and the benefits and limitation of each level.

Chapter 6 recapitulates the fifth chapter by describing in detail the sequential development and testing of two composite energy absorbers. Section of this chapter cover the coupon level testing and simulation of the energy absorber through the element/component level testing and concluding with the full-scale aircraft drop test involving the energy absorber. This chapter also serves as a cautionary tale on limitations of simulation and testing.

The authors conclude the book with a series of recommendations for improving aircraft crashworthy design. These recommendations range from the future of test facilities to the improvements needed in ATDs. The chapter adds those involved in aircraft certification of the book’s potential readership.
Most engineers have had the experience of being handed a project or a responsibility in a new field, and of being tasked with getting themselves “up to speed.” For anyone who must get up to speed in the field of aircraft crashworthiness, this book will take you from the basic principles to the details and pitfalls of actual designs and their testing. Had this book been commissioned by certain technical societies, it might well have been entitled “Handbook for . . .”

Reviewed by Lance Labun, Ph.D., University of Texas at Austin, Department of Physics
As appeared in the January/February 2021 Issue of Vertiflite, published by The Vertical Flight Society