Fracture Characterization and Analysis of Debonded Sandwich Composites

Sandwich composites marked by their characteristic high stiffness to weight ratio have received wide attention from various industrial sectors for weight critical applications. Sandwich constructions invariably comprise of different materials with distinct material properties, and are prone to peculiar failure modes. A critical and most common damage mode is face/core debonding (or disbonding). Debonds can occur due to several reasons - insufficient wetting of face sheet and core during the production process, blunt body impacts, tool drop or by prolonged exposure to in-service loads. The presence of a debond compromises the safety of the structure, as lack of adhesion between face sheet and core in a sandwich undermines the integrity of the entire structure.
Nowadays, structures are pushed close to their performance limits leading to significant reduction of built-in reserve margins. Therefore, from the design and analysis perspective of sandwich structures, adequate tools are necessary for damage assessment. In order to assess the critical strain energy release rate of the face/core interface or fracture toughness, accurate methodologies need to be developed. The aim of this Thesis is to develop robust fracture mechanical based tools to characterize face/core debonds. Primarily, the focus was laid on fracture based test methods to assess the strength of the sandwich interface such as the Single Cantilever Beam (SCB) and the Double Cantilever Beam loaded with Uneven Bending Moments (DCB-UBM).
A parametric study is conducted to analyze the SCB sandwich specimen from a mode mixity perspective based on the numerical mode-mixity method - Crack Surface Displace-ment Extrapolation (CSDE) method. For a wide array of sandwich systems it was shown that despite conforming to the existing sizing study, mode-mixity deviate away from mode I conditions during a SCB test. Recommendations are laid out based on the modelled results to ensure that the debonding occurs under mode I conditions corresponding to a peel loading on the face sheet. The conclusions from this finite element based paramet-ric study can serve as input to the ASTM International draft standard currently being developed.
Analysis of a force loaded SCB sandwich specimen using the Winkler model is presented. This analysis is further extended to a moment loaded SCB sandwich specimen. A new foundation modulus expression is introduced and good agreement between analytical ex-pressions and finite element simulations are obtained for both moment and force loading configurations. The effect of shear contribution on the mode-mixity phase angle, , for both force and moment loaded SCB sandwich specimens is investigated. The phase angle is higher at short crack lengths and reaches a plateau as the crack length increases, which is in accordance with the decrease in shear component for a force loaded SCB sandwich specimen. The current foundation analysis for a moment loaded SCB specimen paves way for further development in the analysis of a moment loaded DCB sandwich specimen.
The Double Cantilever Beam specimen loaded with Unequal or Uneven Bending Moments (DCB-UBM) applied to sandwich composites is capable of achieving a constant mode-mixity condition throughout the test. Moreover, the DCB-UBM specimen is capable of performing fracture tests in mode I, mode II and mixed mode I/II conditions. Closed-form expressions for energy-release rate and mode-mixity phase angle for a symmetric DCB-UBM sandwich specimen reinforced with stiff layers are derived. The mode-mixity phase angle is expressed in terms of a single scalar parameter, ω which depends only on geometry of the sandwich system and is independent of the applied loading. The algebraic expressions are derived by analyzing the reinforced sandwich beam using the laminate beam theory and the J -integral. These derived expressions are an addition to the literature.
A novel DCB-UBM test rig capable of applying pure moments independently, is im-plemented in this Thesis. The presented rig is high-load and fatigue rated as well as overcomes many shortcomings of the traditional concept presented in the literature. A control algorithm based on Cascade control system is developed using a dedicated con-troller to perform static and fatigue testing. A data reduction procedure, based on the measured moments is deduced to obtain interface fracture toughness. Fracture testing of PVC foam and aerospace grade honeycomb core sandwich specimens are carried out using the newly developed test rig in mode I, mode II and mixed-mode conditions.
The theoretical, numerical and experimental fracture methodologies developed in this Thesis pave way to establish a framework in performing interface fracture toughness char-acterization of typical sandwich composites. The tools developed here have contributed to the international fracture standard development and have kindled interest of the com-munity in creation of a mixed-mode fracture testing standard based on the DCB-UBM test methodology.