Complexity of Understanding the Failure of Aerospace Composite Structures
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EDITORIAL
Complexity of Understanding the Failure of Aerospace Composite Structures Daniel J. Thomas
Submitted: 19 June 2016 ASM International 2016
Due to its multifaceted nature, failure analysis is a complex field of engineering. This is particularly the case with aerospace failure analysis, due to the introduction of new materials, which operate within complex conditions and often with a combination of extreme environmental factors. Aerospace components are subjected to fluctuating stresses, often operating in extreme and varied environments around the world. Traditionally, these components fail by modes of fatigue fracture, corrosion, brittle fracture, ductile overload, high-temperature corrosion, corrosion fatigue, creep wear, abrasion, and erosion. But what challenges face the contemporary and future aerospace failure analysis professionals when we add the complexities of composite material families into the mix? Over the past decade, advanced engineering composites have become a mainstream family of materials in the aerospace sector, resulting in the significantly reduction of
D. J. Thomas (&) Creation Engineering Group, Llynfi Enterprise Centre, Maesteg, Bridgend, Wales, UK e-mail: [email protected]
weight of a range of aircraft structures. These advanced materials offer weight savings of 20% on average, compared to more conventional aluminum and metallic materials. New generations of commercial transport aircraft use composite materials extensively, with the structural elements of these new aircraft being similar to their metallic predecessors. Fiber reinforcement is added to resin systems to increase the tensile strength and stiffness of the finished component. The main types of fiber reinforcement used in the advanced composite industry include carbon, graphite, aramid, ceramic, glass fibers, and in the future carbon nanotubes. Failure of composite systems is a complex multistage process, which is completely different to traditional metallic systems. The interesting thing to keep in mind is that the failure of a composite can be initiated in one mode, but the propagation and final failure modes can be significantly different. Composite materials display two main types of failure: (1) failure of unidirectional layers and (2) failure of unidirectional layer due to longitudinal tension. In a large number of cases, composite failure is initiated internally at the microlevel (fiber or matrix). But it is only once failure has propagated that changes in material behavior and appearance are observed. The failure of a composite manifests as a breaking of fibers, development of microcracks in matrix, debonding between fibers and matrix, and delamination, in which there is a separation of different laminated layers. Therefore, before failure occurs at macroscale, the material response changes significantly at the microscale. This change in material response precedes component failure and stems from a large number of microfailures and subsequent nonlinear material behavior. For simple un
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