Edward H. Glaessgen

Debonding Failure of Sandwich-Composite Cryogenic Fuel Tank with Internal Core Pressure

A summary of the failure analyses and testing that were conducted to determine the cause of the X-33 liquidhydrogen tank failure is presented. Ply-level stress analyses were conducted to explain the formation of microcracks in the plies of the inner and outer facesheet laminates of the honeycomb sandwich walls of the tank under known thermal and mechanical loads. The microcracks allowed the ingression of liquid- and gaseous-hydrogen and gaseous-nitrogen purge gas that produced higher than expected sandwich core pressures in the tank. Single cantilever beam tests were used to determine the toughness of the interface between the facesheets and honeycomb core. Fracture mechanics analyses were developed to determine strain-energy release rates for known foreign object debris shapes and sizes and known and statistically possible core internal pressures. The fracture mechanics analyses were validated by comparing with results of blowoff tests that were fabricated from undamaged tank sandwich material. Strain-energy release rates from the validated analyses were then compared with known and statistically possible values of toughness determined from the single cantilever beam tests. These analyses and tests were then used to substantiate a scenario for failure of the X-33 liquid-hydrogen tank that includes microcracking of the inner facesheets and ensuing ingression of hydrogen and nitrogen, a low bondline strength and toughness, and the presence of foreign object debris. Nomenclature ¯ A −1 jk =i nverse of the laminate stiffness matrix transformed to the local coordinates a, b = dimensions of debond Eii = elastic modulus in the ith principal material

Recent Advances in Durability and Damage Tolerance Methodology at NASA Langley Research Center

Durability and damage tolerance (D&DT) issues are critical to the development of lighter, safer and more efficient aerospace vehicles. 1 Durability is largely an economic life-cycle design consideration whereas damage tolerance directly addresses the structural airworthiness (safety) of the vehicle. Both D&DT methodologies must address the deleterious effects of changes in material properties and the initiation and growth of damage that may occur during the vehicle’s service lifetime. The result of unanticipated D&DT response is often manifested in the form of catastrophic and potentially fatal accidents. As such, durability and damage tolerance requirements must be rigorously addressed for commercial transport aircraft and NASA spacecraft systems. This paper presents an overview of the recent and planned future research in durability and damage tolerance analytical and experimental methods for both metallic and composite aerospace structures at NASA Langley Research Center (LaRC).

NASA Structural Analysis Report on the American Airlines Flight 587 Accident- Local Analysis of the Right Rear Lug

A detailed finite element analysis of the right rear lug of the American Airlines Flight 587 - Airbus A300-600R was performed as part of the National Transportation Safety Board’s failure investigation of the accident that occurred on November 12, 2001. The loads experienced by the right rear lug are evaluated using global models of the vertical tail, local models near the right rear lug, and a global -local analysis procedure. The right rear lug was analyzed using two modeling approaches. In the first approach, solid-shell type modeling is used, and in the second approach, layered-shell type modeling is used. The solid-shell and the layered-shell modeling approaches were used in progressive failure analyses (PFA) to determine the load, mode, and location of failure in the right rear lug under loading representative of an Airbus certification test conducted in 1985 (the 1985-certification test). Both analyses were in excellent agreement with each other on the predicted failure loads, failure mode, and location of failure. The solid-shell type modeling was then used to analyze both a subcomponent test conducted by Airbus in 2003 (the 2003-subcomponent test) and the accident condition. Excellent agreement was observed between the analyses and the observed failures in both cases. From the analyses conducted and presented in this paper, the following conclusions were drawn. The moment, Mx (moment about the fuselage longitudinal axis), has significant effect on the failure load of the lugs. Higher absolute values of Mx give lower failure loads. The predicted load, mode, and location of the failure of the 1985-certification test, 2003-subcomponent test, and the accident condition are in very good agreement. This agreement suggests that the 1985-certification and 2003subcomponent tests represent the accident condition accurately. The failure mode of the right rear lug for the 1985-certification test, 2003-subcomponent test, and the accident load case is identified as a cleavage-type failure. For the accident case, the predicted failure load for the right rear lug from the PFA is greater than 1.98 times the limit load of the lugs.

An Approach to Risk-Based Design Incorporating Damage Tolerance Analyses

Incorporating risk-based design as an integral part of spacecraft development is becoming more and more common. Assessment of uncertainties associated with design parameters and environmental aspects such as loading provides increased knowledge of the design and its performance. Results of such studies can contribute to mitigating risk through a system-level assessment. Understanding the risk of an event occurring, the probability of its occurrence, and the consequences of its occurrence can lead to robust, reliable designs. This paper describes an approach to risk-based structural design incorporating damage-tolerance analysis. The application of this approach to a candidate Earth-entry vehicle is described. The emphasis of the paper is on describing an approach for establishing damage-tolerant structural response inputs to a system-level probabilistic risk assessment.

AN OVERVIEW OF INNOVATIVE STRATEGIES FOR FRACTURE MECHANICS AT NASA LANGLEY RESEARCH CENTER

Engineering fracture mechanics has played a vital role in the development and certification of virtually every aerospace vehicle that has been developed since the mid-20th century. NASA Langley Research Center s Durability, Damage Tolerance and Reliability Branch has contributed to the development and implementation of many fracture mechanics methods aimed at predicting and characterizing damage in both metallic and composite materials. This paper presents a selection of computational, analytical and experimental strategies that have been developed by the branch for assessing damage growth under monotonic and cyclic loading and for characterizing the damage tolerance of aerospace structures

Lessons Learned from Recent Failure and Incident Investigations of Composite Structures

During the past few decades, NASA Langley Research Center (LaRC) has supported several large-scale failure and incident investigations and numerous requests for engineering consultations. Although various extenuating circumstances contributed to each of these incidents, in all cases, the failure resulted from accumulation and/or propagation of damage that reduced the load carrying capability of the structure to a level below that which was needed to sustain structural loads. A brief overview of various failure and incident investigations supported by LaRC, including some of the computational and experimental methodologies that have been applied, is presented. An important outcome of many of these failure and incident investigations is the development of an improved understanding of not only the state-of-the-art in experimental and analytical methods but also the state-of-the-art in the design and manufacturing processes that may contribute to such failures. In order to provide insight into such large-scale investigations, a series of lessons learned were captured. Awareness of these lessons learned is highly beneficial to engineers involved in similar investigations. Therefore, it is prudent that the lessons learned are disseminated such that they can be built upon in other investigations and in ensuing research and development activities.

Probabilistic risk assessment strategy for damage-tolerant composite spacecraft component structural design

Incorporating risk-based design as an integral part of spacecraft development is becoming more and more common. Assessment of uncertainties associated with design parameters and environmental aspects such as loading provides increased understanding of the design and its performance. Results of such studies can contribute to mitigating risk through a system-level assessment. Understanding the risk of an event occurring, the probability of its occurrence, and the consequence(s) of its occurrence can lead to robust, reliable designs. An approach to risk-based structural design incorporating damage-tolerance analysis is described. The application of this approach to a candidate Earth-entry vehicle is also described. The emphasis is on describing an approach for establishing damage-tolerant structural response inputs to a system-level probabilistic risk assessment.

Micromechanics Modeling of Fracture in Nanocrystalline Metals

Abstract Nanocrystalline metals have very high theoretical strength, but suffer from a lack of ductility and toughness. Therefore, it is critical to understand the mechanisms of deformation and fracture of these materials before their full potential can be achieved. Because classical fracture mechanics is based on the comparison of computed fracture parameters, such as stress intlmsity factors, to their empirically determined critical values, it does not adequately describe the fundamental physics of fracture required to predict the behavior of nanocrystalline metals. Thus, micromechanics-based techniques must be considered to quanti@ the physical processes of deformation and fracture within nanocrystalline metals. This paper discusses hndamental physics- based modeling strategies that may be useful for the prediction Iof deformation, crack formation and crack growth within nanocrystalline metals. Introduction Fracture processes in materials such