The effect of impact on the initiation and propagation of debonds in sandwich structures

Sandwich structures are increasingly used in many engineering applications due to their superior stiffness/weight ratio and strength/weight ratio. However, these structures are susceptible to impact damage, which can lead to undetectable internal damage and even debond failure if subjected to fatigue loading. The impact damage and fatigue behavior of the sandwich structures are widely studied separately. However, how does the interface debond nucleate from the impact damage? What about the effect of Arctic temperature? These questions remain unexplored and are crucial for the sandwich design.

The evolution of fatigue damage in the impacted specimens was initially examined by the experimental test. Three sandwich configurations (one honeycombcored and two foamcored sandwiches) and two temperature environments (room temperature and 55 ◦C) were considered. The damage inspection of both barely visible impact damage (BVID) and the fourpoint bending (4PB) fatigue damage was performed with the Xray Computed Tomography (XCT) method. Due to the evidence of interface debonding of the foamcored specimens, the following study focuses on the characterization of face/core interface fracture toughness.

The mode-mixity of the impacted foamcored sandwiches was numerically studied and results show the necessity of implementing the mixedmode III characterization of the foamcored specimens. Two foamcored sandwiches and two test temperatures were considered by using the MixedMode Bending (MMB) test method. The estimated fracture toughness in terms of the phase angle (mode-mixity) was given. This study provides the input for the finite element (FE) simulation of the foamcored specimens.

To fully characterize the debonding behavior, experimental characterization of the mode III fracture toughness of the foamcored sandwiches was studied using the constrained ShearTosionBending (STB) specimen. A 3D Digital Image Correlation (DIC) method was utilized to identify the critical force. The results of the PVCcored specimens show an apparent modeIII crack propagation, however, with longitudinal 45◦ tension cracks just below the face/core interface. This finding indicates the tension failure of the core materials is the “path of least resistance” when competing with the interface modeIII fracture toughness. This fact shows the mode III fracture toughness might be much higher than the other failure modes and thus its effect is disregarded from the FE model.

Based on the observation of the fatigue damage of the impacted specimens, combining the findings from mixedmode III and mode III tests, FE simulations including the macroscopic homogeneous foam model and detailed honeycomb core model, were further proposed. The emphasis is placed on validating the models using the fatigue models implemented in Abaqus. These FE simulations tend to capture the damage evolution observed in the 4PB fatigue test.