EFFECT OF MICROSTRUCTURE ON INITIATION OF DELAMINATION IN CROSS PLY LAMINATES FROM A TRANSVERSE CRACK

Transverse crack propagating towards a cross-ply interface is investigated in this study. The non-uniform fiber distribution near ply interface is modelled explicitly in order to study the effect of microstructure on crack path and initiation of delamination. The growth of fiber/matrix interfacial debond and debond kinking out of interface are analyzed based on a combination of energy and stress-based approach, which is convenient in predicting matrix crack path. Kinking of transverse crack tip when it approaches ply interface is investigated using an energy-based approach. It is found that predicted matrix crack path and crack tip kinking behavior near interface is strongly influenced by the local microstructure. The obtained results indicate that an induced symmetrical delamination, i.e., interface cracks of equal length on either side of the transverse ply crack, as often assumed in modeling studies, is not always a favorable damage mode.

Matrix Crack Networks in SiC/SiC Composites: In-Situ Characterisation and Metrics

Ceramic matrix composites can offer clear potential for a variety of engineering applications where the temperature capabilities of conventional metals are exceeded. Continued mechanical characterisation is essential to gain an understanding of their associated damage and failure mechanisms across a wide range of representative temperatures. The present paper will report ongoing research to characterize the initiation of matrix cracking at room temperature under tensile stress and subsequent damage development under fatigue loading in a SiCf/SiC composite. Imaging and mechanical property data were obtained via in-situ loading within a scanning electron microscope. The temporal nature of damage development was also recorded through the selective employment of acoustic emission. Metrics to describe the spatial distribution of cracks, crack lengths and crack opening displacement under load will be presented. The inspections also provided detailed evidence of the associated crack closure phenomena. The understanding of matrix crack saturation and matrix/fibre interfacial mechanics will be explored, together with the implications for the use of X-ray tomographic inspection of engineering components during service. The potential for these emergent techniques as a basis for future CMC characterization, via automated image recognition and machine learning, will be highlighted.

Micromechanical Modeling Tension-Compression Fatigue Hysteresis Loops Model of Fiber-Reinforced Ceramic-Matrix Composites Considering Fibers Failure

In this paper, a micromechanical tension-compression fatigue hysteresis loops model of fiber-reinforced ceramic-matrix composite (CMC) was developed considering fibers failure. Multiple fatigue damage mechanisms of fibers failure, interface debonding, slip and wear, and matrix fragmentation were considered and incorporated in the micromechanical fatigue hysteresis loops model. Upon unloading, the unloading stress-strain relationship was divided into three stages, including, (1) Unloading Stage I: the unloading interface counter slip stage and the unloading stress is between the tensile peak stress and the matrix crack closure stress; (2) Unloading Stage II: the unloading partial compressive stage and the unloading stress is between the matrix crack closure stress and the unloading complete compressive stress; and (3) Unloading Stage III: the unloading complete compressive stage and the unloading stress is between the unloading complete compressive stress and the compressive valley stress. Multiple micromechanical damage parameters of fibers failure probability, unloading/reloading transition stress, closure stress of the matrix cracking, compressive transition stress, complete compressive stress, unloading/reloading inverse tangent modulus (ITM), and interface counter slip/new slip ratio (ICSR/INSR) were adopted to characterize the tension-compression stress-strain hysteresis loops. Experimental tension-compression fatigue stress-strain hysteresis loops of unidirectional CMCs were predicted using the developed micromechanical models. The characteristics of the tension-compression fatigue hysteresis loops of unidirectional CMC are analyzed for different material properties, damage state, and tensile fatigue peak stress.

Crack Deflection Near a Ply Interface in a Composite Laminate

The deflection of a matrix crack near 0°/90° interface in a cross-ply laminate was studied numerically. In the finite element (FE) model, an initial matrix crack was introduced in the 90° layers away from the 0°/90° interface. The initial matrix crack could be initiated either at the middle of 90° layer or at one side of 0°/90° interface. The 0° layers and a part of the initial matrix crack were modeled using homogenized layer properties to simplify the model. The nonuniformly distributed fibers were modeled explicitly close to the 0°/90° interface in order to study the influence of this nonuniformity on the crack deflection process. The Energy Release Rate (ERR) of debond crack tip was calculated using Virtual Crack Closure Technique (VCCT) to study the debond growth. Maximum principal stress was then adopted to access the debond crack kinking qualitatively. It’s found that when a macro-size matrix crack forms and propagate towards ply interface, the subsequent debonding and debond cracking process in nearby intact fiber shows some distinct differences compared to the same processes at single isolated fiber without considering the interaction with nearby debonded fiber and existing matrix crack. Meanwhile, present analysis shows clear influence of microstructures on the crack deflection process by affecting the fiber/matrix debonding and debond kinking processes.