| Περίληψη: | The present thesis deals with the issue of fracture analysis of generally layered beam-like structures with elastic couplings and hygrothermal stresses. In particular, it develops (i.e., formulates, validates, and implements) an analytical framework for the calculation of the fracture toughness of such non-conventional beams. In parallel, it investigates the fracture behavior of a metal-to-composite adhesive joint of interest to the aerospace industry.
The general problem concerns a beam structure that may feature several “peculiarities”: it may consist of multiple layers of dissimilar materials; it may have asymmetries in terms of layer thicknesses; it may feature elastic couplings (in particular, bending-extension coupling [BEC]); it may be loaded by arbitrary mechanical loads (i.e., concentrated forces and bending moments); and it may contain residual hygrothermal stresses (RHTS). To tackle this problem, we build a generic analytical model that determines the fracture toughness (i.e., energy release rate [ERR] and mode mixity [MM]) of beams with all the peculiarities just mentioned. Classical theories in the discipline of mechanics (e.g., beam theory, mechanics of composite materials, energetic methods) and important tools in the field of fracture mechanics (e.g., crack-tip element [CTE], crack closure integral, J-integral) are employed while developing this novel framework.
With reference to the state of the art, the proposed analyses and solutions extend the level of knowledge and enable us to study a variety of, for example, new hybrid material systems, geometries, and testing setups. All new solutions were validated through finite element analyses (FEAs), employing mainly the virtual crack closure technique (VCCT) and secondarily the cohesive zone modeling (CZM). The new solutions are compared with existing, simpler ones, highlighting the usefulness of the new solutions.
A plethora of analytical, numerical, and experimental case studies of the fracture toughness of several material systems (e.g., fiber metal laminates [FMLs], metal-to-composite adhesive joints, multidirectional [MD] composite laminates) and test configurations (e.g., double cantilever beam [DCB], end-notched flexure [ENF], double cantilever beam-uneven bending moments [DCB-UBM]) have been carried out.
As a parallel project, the thesis investigated the technological problem of the fracture analysis of a titanium-to-CFRP adhesive joint to be applied in the hybrid laminar flow control (HLFC) system of future aircraft. Extensive experimental, analytical, and numerical methods were combined to understand the fracture behavior and extract the fracture toughness of this joint under various conditions.
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