Fracture behavior of an interface crack in a magnetoelectric sandwich structure under electric field: Effects of the poling directions

Magnetoelectric (ME) sandwich structure is a common form in device applications. Poling directions of component materials are essential for the improvement of ME device properties. In this paper, the effects of the electric and magnetic poling directions on the interface fracture of a ME sandwich structure are investigated by integral transform and singular integral equation techniques. The expressions of the normalized stress intensity factors (NSIFs) are derived, and some numerical examples are presented. It is found that the poling direction of active layer can greatly affect the interface cracking mode. And the crack propagation can be promoted or impeded by adjusting the applied field. The structure with a larger volume fraction of active material will be more likely to crack.

MULTISCALE DAMAGE IN CO-CURED COMPOSITES—PERSPECTIVES FROM EXPERIMENTS AND MODELLING

Bonded and co-cured composites are popular alternatives to structures joined with mechanical fasteners in aircraft but the complex and coupled damage mechanisms in the co-cured/bonded region are poorly understood, thus making the evaluation of their strength and durability difficult with current modelling strategies. This study explores the potential of interleaf inclusion in failure-prone, critical regions of co-cured composite specimens in improving the joint strength and interface fracture toughness and strives to advance the understanding of damage initiation in the co-cured region using an atomistic model. A two-pronged approach is pursued here with bench-scale experimental testing and molecular modelling in this study. Experiments are performed for mode I fracture toughness with double cantilever beam (DCB) on composite laminates with an epoxy interleaf layer. Two epoxy resins and three methods for interleaf inclusion are explored in this study; we supplement the results from DCB testing with insights from confocal microscopy on the crack tip and the interleaf layer pre- and post-testing. Molecular dynamic (MD) simulations capture the cohesive interactions at the threephase interface containing the carbon fiber, the prepreg epoxy, and the interleaf epoxy. Results highlight that an interleaf layer made from partially-cured and filmed epoxy, further consolidated in the composite lay-up is the most effective way to suppress void formation, improve dispersion, and maximize cohesive interactions at the interface of co-cured composites.

Interface Controlled Micro- and Macro-Mechanical Properties of Vibration Processed Carbon Fiber/Epoxy Composites

The fiber-resin interface is an important component that significantly affects mechanical properties of composites. Random vibration-assisted vacuum processing (RVAVP), a new method to improve the adhesion of the fiber-resin interface, was presented. The effects of different curing processes on mechanical properties were comprehensively assessed by combining the fiber push-out test, finite element model simulation, cure monitoring approach, and short-beam three-point bending test, and the correlation between fiber volume fraction and mechanical properties was quantified by a facile thermogravimetric analysis-based methodology. The results revealed that application of random vibration during the curing process can promote the impregnation of resin into fibers and impede the growth of interface defects while improving mechanical properties at the same time. For this reason, the laminates produced by RVAVP exhibited the average interfacial shear strength of 78.02 MPa and the average interface fracture toughness of 51.7 J/m2, which is obtained a 48.26% and 90.77% improvement compared with the 0 MPa autoclave process. With the large observed increase in micro-mechanical properties, the average interlaminar shear strength of 93.91 MPa showed a slight reduction of 5.07% compared with the 0.6 MPa autoclave process. Meanwhile, the mechanical properties tended to be stable at the fiber volume fraction of 65.5%.

Analysis of the bi-material interface cracks based on configurational forces

In this paper, an innovative interface fracture criterion is proposed based on the concept of configurational forces in material space. In this criterion, the crack tip configurational forces as driving force is introduced to describe the interface crack evolution under mixed mode loading conditions. And it assumes that the interface crack propagates due to the competition of resultant of configurational forces with interface fracture toughness. The analytical expression of the configurational forces are obtained by differentiating the elastic strain energy density and conservative integral for interface cracks. And the relation of interface crack tip configurational forces with classical complex intensity factors are obtained through strict mathematical deduction. The interface crack tip configurational forces are evaluated for a classic interface crack problem covering a wide range of bi-material oscillation index. The configurational forces based interface fracture criterion is validated through series interface fracture experiments. The proposed criterion may provide a novel framework for analysis of interface fracture under complex loading conditions.

Residual Stresses in Suspension Plasma Sprayed Electrolytes in Metal-Supported solid Oxide Fuel Cell

This study is aimed at identifying the change of residual stresses in suspension plasma sprayed (SPS) 8 mol% YSZ electrolytes on top of porous stainless steel substrate with varying processing parameters and temperatures. The residual stresses in the electrolyte layer are tensile with a value of approximately 90 MPa at room temperature. Porosity, microcracks and segmentation cracks are observed to form in the coating during post-deposition cooling. The decrease of residual stresses with increasing temperature is related to the changes in the Young’s modulus, thermal expansion mismatch, micro-defects and possible creeping of porous stainless steel substrate. The coating fabricated using a torch power of 133 kW and stand-off distance of 90 mm exhibits the highest residual stress due to the formation of a denser microstructure and less cracking. Furthermore, the fracture toughness and interface fracture toughness of the SPS YSZ coating at the optimized condition was determined and discussed.

Residual Stresses in Suspension Plasma Sprayed Electrolytes in Metal-Supported solid Oxide Fuel Cell

This study is aimed at identifying the change of residual stresses in suspension plasma sprayed (SPS) 8 mol% YSZ electrolytes on top of porous stainless steel substrate with varying processing parameters and temperatures. The residual stresses in the electrolyte layer are tensile with a value of approximately 90 MPa at room temperature. Porosity, microcracks and segmentation cracks are observed to form in the coating during post-deposition cooling. The decrease of residual stresses with increasing temperature is related to the changes in the Young’s modulus, thermal expansion mismatch, micro-defects and possible creeping of porous stainless steel substrate. The coating fabricated using a torch power of 133 kW and stand-off distance of 90 mm exhibits the highest residual stress due to the formation of a denser microstructure and less cracking. Furthermore, the fracture toughness and interface fracture toughness of the SPS YSZ coating at the optimized condition was determined and discussed.