Composite materials research and education program: The NASA-Virginia Tech composites program

Major areas of study include: (1) edge effects in finite width laminated composites subjected to mechanical, thermal and hygroscopic loading with temperature dependent material properties and the influence of edge effects on the initiation of failure; (2) shear and compression testing of composite materials at room and elevated temperatures; (3) optical techniques for precise measurement of coefficients of thermal expansion of composites; (4) models for the nonlinear behavior of composites including material nonlinearity and damage accumulation and verification of the models under biaxial loading; (5) compressive failure of graphite/epoxy plates with circular holes and the buckling of composite cylinders under combined compression and torsion; (6) nonlinear mechanical properties of borsic/aluminum, graphite/polyimide and boron/aluminum; (7) the strength characteristics of spliced sandwich panels; and (8) curved graphite/epoxy panels subjected to internal pressure

Assessment of Hashin’s failure criteria in finite element modelling of orthogonal cutting of fiber-reinforced composites

Abstract: Fiber-reinforced composite materials are widely used in aerospace structures because of their high stiffness, high strength and high fatigue properties. Yet, machining of this type of materials during manufacturing is still challenging because of the undesirable and unwanted damage around the machining area. To avoid that, finite element simulation paves the way to predict the induced damage and to customize the machining parameters. How to develop a robust finite element model for composite machining, however, is still an open question due to the complexity of the failure mechanisms of fiber-reinforced composite. Although in the previous research the simulations were experimentally validated, our literature review shows that there is still a research gap at the level of verification and convergence of the results. In this study, we built a 2D finite element model to predict the reaction force over the cutting tool during orthogonal cutting of glass fiber-reinforced composites. Verification was conducted in the context of mesh refinement and convergence study. Two finite element problems were solved, one material without any failure criteria and one with Hashin failure criteria were examined. The results show that the former converged for the element size of less than 0.008 mm and the later did not converge even at a very fine mesh with an element size of 0.004 mm. Adding a damage model to the contact simulation of orthogonal cutting of composite materials significantly amplified the discretization error. The predicted maximum cutting force decreased 97% when the element size was decreased from 0.01 mm to 0.004 mm. Hence, we believe more comprehensive research is needed on verification of existing material models for simulation of machining of composite.Communication présentée lors du congrès international tenu conjointement par Canadian Society for Mechanical Engineering (CSME) et Computational Fluid Dynamics Society of Canada (CFD Canada), à l’Université de Sherbrooke (Québec), du 28 au 31 mai 2023

Cohesive/Adhesive failure interaction in ductile adhesive joints Part I:A smeared-crack model for cohesive failure

AbstractThis paper proposes a new methodology for the finite element (FE) modelling of failure in adhesively bonded joint. Unlike current methods, cohesive and adhesive failures are treated separately. Initial results show the method׳s ability to give accurate prediction of failure of adhesive joints subjected to thickness-induced constraint and complex multi-axial loading using a single set of material parameters. The present paper (part I), focuses on the development of a smeared-crack model for cohesive failure. Model verification and validation are performed comparing the model predictions with experimental data from 3 point bending End Notched Flexure (3ENF) and Double Cantilever Beam (DCB) fracture tests conducted on adhesively bonded composite panels of different adhesive thicknesses

Design And Analysis Of Filament- Wound Compressed Natural Gas Carbon Fibre - Reinforced Composite Tank

First ply failure (FPF) strengths of laminated composite tank subjected to uniform internal pressure loads are studied via both analytical and finite element analysis approaches. The filament-wound CNG carbon fibre reinforced composite tanks are designed with a T6-6061 aluminium cylinder with elliptical end closures acts as the liner which is overwrapped with high modulus carbon fibre-reinforced epoxy composite. The objectives of this study are to optimize the composite layer thickness and to optimize fibre orientation configurations of carbon fibre laminate as to have a lightweight and high performance filament-wound CNG carbon fibre-reinforced composite tanks. In analytical approaches, in order to predict the first-ply failure (FPF) pressure of filament-wound CNG carbon fibre-reinforced composite tanks, the stresses and strains throughout the laminate were determined using the classical lamination theory which were then used in three most common composite failure theories, that are the maximum stress theory, maximum strain theory, and quadratic or Tsai-Wu failure theory. Optimal general design of fibre orientations were then used to carry out in lay-up optimization or arrangement of composite layer stage to be used for filament winding process in order to study the effect of fibre orientation angles using an equal thickness of composite layer on the tank performance. The range of helical angles used is in between 0° to 60°, which is based on the traditional theoretical optimal helical angles from classical lamination theory. The ratio of 2:1 hoop to helical angles is used to predict the maximum first-ply failure (FPF) pressure. The optimization results gave the optimal fibre orientations of the [( ) ] 11 24 30 /− 30 / 90 with b/a = 1.093 for CNG 1, b/a = 1.110 for CNG 2 and b/a = 1.128 for CNG 3 which obtained were then used for stress analysis in finite element analysis using ANSYS version 7.1 software. The accuracy of the theoretical and finite element analysis of first-ply failure (FPF) pressure is verified by a verification study where a similar finite element model from literature have been modelled and analysed using similar method used to design filament wound CNG carbon fibrereinforced composite tanks in order to verify a valid finite element method used. The results were then being compared literature study

Guidelines for Verification Strategies to Minimize RISK Based on Mission Environment, -Application and -Lifetime (MEAL)

There is a trend of compromising verification testing to address the cost and schedule constraints, which poses a high-risk posture for programs/projects. Current and emerging aerospace scientific and/or human exploration programs continue to pose new technological challenges. These technological challenges combined with finite budgets and truncated schedules are forcing designers, scientists, engineers, and managers to push technologies to their physical limits. In addition, budget and schedule pressures challenge how those technologies/missions are verified. A clear understanding of the different verification processes is needed to ensure the proper verification of the technology within the mission (i.e., capabilities, advantages, and limitations). The goal of verification is to prove through test, analysis, inspection, and/or demonstration that a product provides its required function while meeting the performance requirements. It is important that verification yield understanding of representative performance under worst-case conditions so that margins to failure can be evaluated for proposed applications. The capabilities, advantages, and limitations of the testing and inspection performed at each level are different, and the risk incurred by omitting a verification step depends on the level of integration as well as Mission, Environment, Application and Lifetime (MEAL). This paper focuses on verification processes. The goal of the verification process is to ensure the given avionics technology could be safely implemented on the given MEAL consistent with the program/project risk posture

Probabilistic Guarantees for Safe Deep Reinforcement Learning

Deep reinforcement learning has been successfully applied to many control tasks, but the application of such agents in safety-critical scenarios has been limited due to safety concerns. Rigorous testing of these controllers is challenging, particularly when they operate in probabilistic environments due to, for example, hardware faults or noisy sensors. We propose MOSAIC, an algorithm for measuring the safety of deep reinforcement learning agents in stochastic settings. Our approach is based on the iterative construction of a formal abstraction of a controller's execution in an environment, and leverages probabilistic model checking of Markov decision processes to produce probabilistic guarantees on safe behaviour over a finite time horizon. It produces bounds on the probability of safe operation of the controller for different initial configurations and identifies regions where correct behaviour can be guaranteed. We implement and evaluate our approach on agents trained for several benchmark control problems