Periodic Reporting for period 1 - DamBuckler (Material damage and buckling instability: towards a unifying general theory)
Periodo di rendicontazione: 2019-09-01 al 2021-08-31
In general, structural failure comprises a combination of the material failing due to overstressing, which results in permanent deformation (plasticity) or fracture (cracks or delaminations), and the geometry being or becoming unsuitable, which results in structural instability (buckling) that may be triggered while the material is in the low-stress, elastic, range. A general structural stability theory that considers damage initiation and its growth has been practically non-existent. The DamBuckler project addresses this issue successfully by developing a novel structural stability formalism that enables the consideration of material damage within a structural stability analysis. Composite structures, for instance as used in the aircraft industry, are a prominent application example where buckling instability and material damage need to be accounted for in the design process. With the DamBuckler theory, such structures – vulnerable to simultaneous buckling instability and material damage – can be efficiently analysed. In the project, models predicting the compressive behaviour of composite panels, while considering various damage mechanisms, are developed. Such models can be used to determine damage allowable loads, i.e. applied loads causing damage growth and/or failure, as well as to study the stability behaviour under the presence of specific damage mechanisms. Thus, the DamBuckler project lays the foundation to contribute towards a more sustainable design of engineering structures by fully exploiting their load carrying capacity.
In the DamBuckler project, a novel analytical framework for a structural stability analysis of structures vulnerable to buckling instability and material damage has been developed. The framework comprises the derivation of an extended total potential energy accounting for elastic and inelastic deformation, and conditions that enable general damage growth behaviour to be accounted comprising transitions from active to passive damage mechanisms, loading—unloading regimes and multiple active damage mechanisms, as well as enforcing the second law of thermodynamics. Moreover, a solution algorithm has been developed, such that the research conducted within the DamBuckler project can be practically implemented. The new framework has been successfully applied to the problem of composite panels with multiple types of damage loaded under in-plane compression. The developed models constitute an initial step towards efficient analyses of composite structures with barely visible impact damage, which is a critically important problem within the aerospace industry. The models enable the precise prediction of damage allowable loads, i.e. applied loads causing damage growth and/or failure, as well as the structural stability behaviour of composite panels considering various damage mechanisms (e.g. delamination and matrix crack growth).
With the results arising from the DamBuckler project, the current state of the art of structural stability analysis is advanced. The developed analytical framework enables the consideration of material damage within the context of structural stability. It may therefore be regarded as an extension to elastic stability theories employing a discrete coordinate approach such as the seminal work by Thompson and Hunt “A general elastic stability theory” published as a monograph in 1973. With the novel theory, efficient models of structures vulnerable to buckling instability and material damage can be formulated, which has been demonstrated in the DamBuckler project. This is of particular importance for slender structures made from (fibre reinforced) composite materials, where buckling instability often goes hand-in-hand with various damage mechanisms. Owing to the complexity of such problems, current modelling approaches are excessively expensive in terms of computation time, which impedes their suitability for design practice. This issue can be mitigated with the results from the DamBuckler project, where efficient predictions of the damage allowable load input, the ultimate strength and the stability behaviour of such structures can be facilitated. As a consequence, the results of the DamBuckler project makes a contribution to the current societal need for designing more sustainable engineering structures by exploiting their full load bearing capacity, which minimises material and resource use thereby reducing its intrinsic embodied energy.