Composite Structures Durability Design and Substantiation

<div class="section abstract"><div class="htmlview paragraph">In this paper, it is explained how composite structures, made of continuous fibers embedded in a polymer matrix, are designed and analyzed in the aerospace industry. The strategy is based on the building block approach, in which the knowledge on the composite material and structure is built step by step from the coupon level up to the final full scale structure. Damage is then discussed, as it can't be ignored when composites are concerned. The approach available in the SAMCEF finite element code is then described. It is based on the continuum mechanics approach, and allows studying the progressive failure of composites in the plies and at their interface (so considering delamination). The material models are described, and their use is illustrated at the coupon level. The identification procedure for this damage models is also discussed. It is therefore shown how this information of the material behavior can be used at the upper stages of the building block approach and so applied to larger scale structures. It is advocated here that this approach can be used for the automotive applications, leading to a transfer of technology from aerospace to automotive.</div></div>

Full-scale aircraft structural fatigue tests are extremely complex, not only in terms of the test definition and implementation, but also from the control systems viewpoint. The load spectra that these tests are subjectedto are also becoming more complex as manufacturers and certifying authorities strive for more realistic loading conditions. At the same time, there is pressure to complete the test more quickly or at least in the same time as previous tests that were simpler and contained fewer cycles. This can only be achieved by optimizing the test configuration and control system parameters or in some cases implementing new control strategies. This is difficult on a full-scale test because of all the complex interactions on these multi-channel tests. Methods that may work well on smaller tests can sometimes result in poorer performance. Thus there is a need for a greater understanding of the system and some predictive capability. This can be achieved with computer modeling. This paper suggests an implementation scheme in terms of the introduction and acceptance of the concepts of modeling to a full-scale structural test laboratory and provides examples of the use of modeling at various stages in its implementation. Examples are presented from the very basic level of education, using models to understand sources of problems on simple tests, to evaluating new control strategies and predicting the impulse response of a test system (structure/ actuators/control system). Finally, the possibility of predicting system behavior with sufficient accuracy to optimize the test for given test and control system hardware is discussed.

The curing distortion is a critical determinant of the quality of integrally manufactured composite structures, playing a pivotal role in the design and fabrication of composite. This paper presents two simplified methods in predicting the curing distortion for large-scale composite aircraft structures manufactured through the autoclave process. Firstly, the refined finite element models of the two simplified methods were developed. Then, it was utilized to predict the curing distortion of Ω-shaped composite laminates. The comparative study between the experimental data and numerical results shows that the proposed second simplified method balanced the prediction accuracy and efficiency, which is urgently needed in practice. Finally, using the second simplified method, predictions were conducted for the curing distortion of practical large-scale composite skin structures. The results were in good agreement with the corresponding experiments. This study provides a new solution for the rapid iterative design of large-scale composite structures, as well as the efficient design of frame molds for their manufacturing.

The smart sandwich structures have been widely used in the aerospace, automobile, marine, and civil engineering applications. A typical smart sandwich structure is usually comprised of two stiff face skins separated by a thick core with variety of embedded sensors to monitor the performance of the structures. In this study, the smart composite sandwich structure (CSS) samples are fabricated with glass microballoons syntactic foam core and resin infused glass-fiber face skins (with piezoelectric fiber composite sensors (PFCS) embedded inside the resin infused glass-fiber face skins). One of the main concerns associated with embedding sensors inside composite structures is the structural continuity, compatibility, and interface stress concentrations caused by the significant differences in material property between sensor and host structures. PFCS are highly flexible, easily embeddable, highly compatible with composite structures and their manufacturing processes, which makes them ideal for composite health monitoring applications. In this study, in-plane tensile, tension–tension fatigue, short beam shear, and flexural tests are performed to evaluate the effect on strengths/behavior of the CSS samples due to embedded PFCS. Then carefully planned experiments are conducted to investigate the ability of the embedded PFCS to monitor the stress/strain levels and detect damages in CSS using modal analysis technique. The tensile tests show that both the average ultimate strength and the modulus of elasticity of the tested laminate with or without embedded PFCS are within 7% of each other. The stress–life (S-N) curves obtained from fatigue tests indicates that the fatigue lives and strengths with and without the PFCS are close to each other as well. From short beam and flexural test results, it is observed that embedded PFCS leads to a reduction of 5.4% in the short beam strength and 3.6% in flexural strength. Embedded PFCS’s voltage output response under tension–tension fatigue loading conditions has been recorded simultaneously to study their ability to detect the changes in input loading conditions. A linear relationship has been observed between the changes in the output voltage response of the sensor and changes in the input stress amplitude. This means that by constantly monitoring the output response of the embedded PFCS, one could effectively monitor the magnitude of stress/strain acting on the structure. Experiments are also performed to explore the ability of the embedded PFCS to detect the damages in the structures using modal analysis technique. Results from these experiments show that the PFCS are effective in detecting the initiations of damages like delamination inside these composite sandwich structures through changes in natural frequency modes. Hence embedded PFCS could be an effective method to monitor the health of the composite sandwich structures’ in-service conditions.

In recent years, composite structures have been used in a large number of applications in aerospace, machinery, marine, and civil engineering. However, there are inevitably many uncertainties in the whole life cycle of composite structures, which can easily lead to structural damage and failure. Therefore, it is important to analyze the reliability and sensitivity of composite structures. At present, most of the contributions use the first-order reliability method (FORM) and the second-order reliability method (SORM) to study the reliability of composite structures and compare them with the results of the Monte Carlo simulation. However, both methods have their limitations. FORM cannot guarantee the calculation accuracy for the highly nonlinear limit state equation, and the calculation efficiency of SORM is too low. Therefore, this paper proposes to use importance sampling (IS) and backpropagation neural network-based Monte Carlo (MC-BPNN) to study the reliability, sensitivity, and dispersion of delamination growth of composite laminates. The results show that compared with FORM and SORM, IS and MC-BPNN have higher calculation accuracy and efficiency and can more accurately evaluate the failure degree of composite structures and ensure their safe operation in the field of aerospace equipment. The universality of this method can make it being widely used in the reliability and sensitivity analysis of different composite materials as well as dispersion analysis.

The aircraft and space industry strives for significantly reduced development and operating costs. Reduction of structural weight at safe design is one possibility to reach this objective which is aimed by the following two running research projects: the EC project "COCOMAT" and the ESA study "Probabilistic Aspects of Buckling Knock Down Factors". These projects develop improved concepts and tools for a fast and reliable simulation of the buckling and the postbuckling behavior of thin-walled structures up to collapse, respectively, which allow the exploitation of considerable reserves in primary fibre composite structures in aerospace applications. For the validation of the concepts and tools, a sound database of experiments is needed which is also performed within these projects. This paper focuses on the experimental activities within these projects performed at the buckling test facility of the Institute of Composite Structures and Adaptive Systems (DLR). It presents an overview about the DLR buckling, postbuckling and collapse tests which are already finished and gives an outlook to the results which are expected until the end of the running projects. This paper explains the working of the buckling test facility, the advanced measurement systems, which are running in parallel to the tests, and gives exemplarily two test results. The structures considered are unstiffened cylinders (ESA study) as well as panels, which are understood as sections of cylinders, stiffened by stringers (COCOMAT project). The unstiffened cylinders are more related to space applications (e.g. Ariane busters or parts of the international space station ISS) and the stiffened panels focus more on aircraft structures (e.g. fuselage). The load case considered for all investigations presented in this paper is axial compression under static loading although the test facility is also ready to apply torsion and internal pressure, as well as dynamic axial impact.

A review of full-scale structural testing of wind turbine blades

Principles for determining material allowable and design allowable values of composite aircraft structures

The strength theory design accuracy of carbon fiber reinforced plastics (CFRP) composite structures are integral to CFRP composite structures product weights along with production costs. In order to further improve the CFRP composite structure strength theory design accuracy in this paper, it developed an ATM/SIFT analysis method by combining strain invariant failure theory (SIFT) and Accelerated testing methodology (ATM). The developed implementation scheme of ATM/SIFT includes: three parts of accelerating material strength test, component strength main curve characterization, structures life prediction, among which the material life prediction is achieved by using Python code to customize user material subroutines (VUMAT), through secondary development on the ABAQUS platform. Based on the developed ATM/SIFT method, it analyzed the compressive gradual failure mechanism and life of IM600/TDE-85 composite laminated plates open pore structures. Finally, it predicted the long-term static load compression life of IM600/TDE-85 composite laminate open-hole structures by using ATM/SIFT method and compared this prediction results with test results.

This study investigates the viability of implementing multifunctional structural power composites in a four-seater air taxi, the CityAirbus. For a given specific energy of the power source, the cruise endurance can be approximately doubled by using structural power composites as opposed to conventional batteries. Replacing all the eligible composite mass and batteries with structural power composites can reduce the CityAirbus weight by 25%. To achieve the current design performance, the minimum required elastic modulus, strength, specific energy and power for the structural power composite are 54 GPa, 203 MPa, 74 Wh/kg and 376 W/kg, respectively: current state-of-the-art structural power composites are now approaching this level of performance. Hence, structural power composites are considered feasible for adoption in the urban air mobility sector and have the potential to improve endurance and facilitate commercialization. This paper also discusses several key challenges that must be addressed to realize the adoption of structural power composites in future electric air taxis.

Lawrence Livermore National Laboratory and Boeing Defense and Space Group worked together to advance the understanding and aerospace application of the Resin Transfer Molding (RTM) process. Future aircraft designs will require the use of lightweight fiber composites in primary structures. RTM is the prime manufacturing process to replace existing labor and cost-intensive procedures. RTM's unique processing capabilities are that it is not labor intensive and that it provides speed and near net shape forming to complex aircraft parts. The qualities could reduce the cost of tomorrow's airline programs by several million dollars each.

To expand further the composites in aircraft structures a consortium of leading aerospace organisations, led by British Aerospace (BAe), is currently engaged in the second phase of the Affordable Manufacture of Composite Primary Structures (AMCAPS II) research and development programme. The aim of this programme is to investigate innovative design, material and manufacturing concepts and technologies with the potential of providing cost‐effective composite primary wing structures. In order to support AMCAPS II tasks within BAe, process time estimating models are being developed for each of the main manufacturing processes being considered for part manufacture and assembly. The outputs from these models form the basic data from which process costs are established. The objective of this paper is to describe, using examples from the automated tape laying process, the approach to model development being applied within the AMCAPS II programme.

Performance-based durability design has gained considerable attention in the construction industry for the service life-based designs of new constructions. Currently, prescriptive-type specifications are used in durability design to meet the service life requirements which prescribe the constituent materials with limiting values of the concrete mix proportion and cover to reinforcement under a given exposure condition. In this context, performance requirements are assumed to be satisfied during the life span of the structure. Prescriptive specifications are valid within a selected range of the cement composition and exposure conditions of concrete, and therefore, these descriptive requirements are often not applicable in the rapidly changing concrete industry. Contrarily, in the case of performance-based durability design approach, there is more flexibility in selection and proportioning of constituent material in producing concrete to satisfy a particular exposure condition. In order to change from prescriptive specifications to performance-based design for durability of concrete, it is essential to find relevant durability indicators with appropriate test methods to evaluate those indicators. In this research, the main focus was given to evaluate two durability-related tests which are commonly used in the construction industry such as surface resistivity test and initial surface absorption test (ISAT) for concrete with the addition of fly ash as a supplementary cementitious material as a partial replacement of Ordinary Portland cement. Experimental investigations were carried out by varying the fly ash percentage, water-to-cement (w/c) ratio and curing condition. It was found that initial surface absorption and surface resistivity are having higher sensitivity toward the variation of factors affecting durability such as w/c ratio, fly ash content and curing condition and can be used as reliable concrete durability indicators.KeywordsConcrete durabilityISATSurface resistivityFly ashW/C ratioCuring condition

Reliability-based Design Optimization of Laminated Composite Structures under Delamination and Material Property Uncertainties

In the last 20 years NASA has worked in collaboration with industry to develop enabling technologies needed to make aircraft safer and more affordable, extend their lifetime, improve their reliability, better understand their behavior, and reduce their weight. To support these efforts, research programs starting with ideas and culminating in full-scale structural testing were conducted at the NASA Langley Research Center. Each program contained development efforts that (a) started with selecting the material system and manufacturing approach; (b) moved on to experimentation and analysis of small samples to characterize the system and quantify behavior in the presence of defects like damage and imperfections; (c) progressed on to examining larger structures to examine buckling behavior, combined loadings, and built-up structures; and (d) finally moved to complicated subcomponents and full-scale components. Each step along the way was supported by detailed analysis, including tool development, to prove that the behavior of these structures was well-understood and predictable. This approach for developing technology became known as the approach. In the Advanced Composites Technology Program and the High Speed Research Program the building-block approach was used to develop a true understanding of the response of the structures involved through experimentation and analysis. The philosophy that if the structural response couldn't be accurately predicted, it wasn't really understood, was critical to the progression of these programs. To this end, analytical techniques including closed-form and finite elements were employed and experimentation used to verify assumptions at each step along the way. This paper presents a discussion of the utilization of the building-block approach described previously in structural mechanics research and development programs at NASA Langley Research Center. Specific examples that illustrate the use of this approach are included from recent research and development programs for both subsonic and supersonic transports.