Investigation of fracture behavior of cement-bonded corundum castables using wedge splitting test and digital image correlation method

To investigate the fracture behavior of cement-bonded corundum castables, various cement contents and pre-treating temperatures have been comparatively studied using the wedge splitting method and the digital image correlation technique. The results show that the microstructure enhances the mechanical properties, so the fracture energy and the maximum load as well as the fracture modes are affected correspondingly. The castables demonstrate the highest fracture energy and maximum load at 1600 °C with cement content of 10 wt% due to an appropriate amount of CA6. At the temperatures of 110 and 1100 °C, the crack propagation within the matrix and along the interface are dominated whereas within the aggregates significantly increased at 1600 °C, leading to the brittleness of materials. However, increasing the cement content can reduce their brittleness, caused by the maximum strain in thex-direction, largest length of the main crack, and high ratio of crack propagation in the matrix.     

R-curves determination of ordinary refractory ceramics assisted by digital image correlation method

As a figure-of-merit, the rising ratio of crack propagation resistance to fracture initiation resistance indicates a reduction of the brittleness and enhances the thermal shock resistance of ordinary refractory ceramics. The significant nonlinear fracture behaviour is related to the development of a fracture process zone (FPZ). The universal dimensionless load–displacement diagram method is applied as a promising graphical method for the determination of R-curves for magnesia refractories showing different brittleness. By applying digital image correlation (DIC) together with the graphical method, the problems arisen with accurate determination of the fracture initiation resistance and the crack length are overcome. Meanwhile, the R-curve is subdivided with respect to the fracture processes, viz the fracture initiation, the development of FPZ and the onset of traction free macro-crack. With the simultaneous crack lengths evaluated from DIC, the contribution of each fracture process to the crack propagation resistance at certain loading stage is quantitatively presented.     

Determination of the fracture behaviour of MgO-refractories using multi-cycle wedge splitting test and digital image correlation

Refractories with reduced brittleness show a pronounced deviation from linear elastic behaviour and an enhanced thermal shock resistance. This paper aims to study the influence of microstructure on the fracture behaviour of magnesia refractories. The wedge splitting test(WST), which enables stable crack propagation for quasi-brittle materials, was used to identify the fracture behaviour and evaluate the energy dissipation. The evaluation of the crack lengths of the magnesia and magnesia spinel materials during the entire cyclic WST is based on the localized strain evaluated using the digital image correlation (DIC). A significant fracture process zone develops in the magnesia spinel material. The relationship between the dissipated energy and the actual crack length, which was used to characterize the crack growth resistance, was determined. The refractory materials that showed reduced brittleness consume a small amount of energy for fracture initiation but a large amount of energy for further crack propagation.     

Assessment of local strain field in adhesive layer of an unsymmetrically repaired CFRP panel using digital image correlation

The present study focuses on experimental investigation of through the thickness displacement and strain field in thin adhesive layer in single sided (unsymmetrical) patch repaired CFRP (carbon fiber reinforced polymer) panel under tensile load. Digital image correlation (DIC) technique is employed to acquire the displacement and strain (longitudinal, peel and shear) field. Experimental determination of shear transfer length based on shear strain field obtained from DIC is introduced to estimate the optimum overlap length which is an essential parameter in patch design for the repair of CFRP structures. Further, DIC experiment with magnified optics is performed to get an insight into complex and localized strain field over thin adhesive layer especially at critical zones leading to damage initiation. The failure mechanism, load displacement behavior, damage initiation and propagation are closely monitored using DIC. The influence of patch edge tapering on strain distribution in adhesive layer is also investigated. The DIC successfully captures the global and localized strain field at critical zones over thin adhesive layer and further helps in monitoring the damage based on strain anomalies. Strains are found to have maximum magnitude at the patch overlap edge and the shear strain level in adhesive layer is higher than the peel strain. Normal tapering increases the peel strain and has negligible influence on shear strain level in adhesive layer. The recommended overlap length is found to be consistent with the recommendation in the literature. Whole field strain pattern and the overlap length obtained from experiment are further compared with the finite element analysis results and they appear to be in good coherence.     

Damage and failure analysis of a SiCf/SiC ceramic matrix composite using digital image correlation and acoustic emission

The analysis of failure behaviors of continuous fiber-reinforced ceramic matrix composites (CMCs) requires the characterization of the damage evolution process. In service environments, CMCs exhibit complex damage mechanisms and failure modes, which are affected by constituent materials, meso architecture, inherent defects, and loading conditions. In this paper, the in-plane tensile mechanical behavior of a plain woven SiC_f/SiC CMC was investigated, and damage evolution and failure process were studied in detail by digital image correlation (DIC) and acoustic emission (AE) methods. The results show that: the initiation of macro-matrix cracks have obvious local characteristic, and the propagation paths are periodically distributed on the material surface; different damage modes (matrix cracking and fiber fracture) would affect the AE energy signal and can be observed in real-time; the significant increase of AE accumulated energy indicates that serious damage occurs inside the material, and the macroscopic mechanical behavior exhibits nonlinear characteristic, which corresponds to the proportional limit stress (PLS) of the material.     

Effect of off-axis angle on mesoscale deformation and failure behavior of plain-woven C/SiC composites with digital image correlation

In this study, the deformation response and failure behavior of a plain-woven C/SiC composite were investigated under on-axis and off-axis tensile loading. Digital image correlation (DIC) was utilized to characterize the full-field deformation and mesoscale strain distribution. The test results indicate a strong influence of the woven architecture on the mechanical properties and strain distribution, and the materials exhibit failure modes dependent on the loading directions or off-axis angles: the fracture positions of different layers are the same under off-axial load, while for on-axil loading, the fracture positions of different layers do not affect each other. SEM results provide direct evidence that the width of the off-axis specimen has a great influence on the mechanical properties. The reduction of the modulus and strength of off-axis specimen, is not only due to the off-axis loading, but also due to the reduction of effective bearing area or effective bearing fiber.     

In situ characterization of high temperature elastic modulus and fracture toughness in air plasma sprayed thermal barrier coatings under bending by using digital image correlation

The evolutions of elastic modulus and fracture toughness are the key factors affecting the failure mechanism and durability of thermal barrier coatings (TBCs). Combined the high temperature three-point bending with the digital image correlation (DIC) method, the variations of high temperature elastic modulus and fracture toughness of air plasma sprayed TBCs with temperature are determined. The surface and interfacial cracking information can be monitored real-time by DIC system. The results show that when the temperature rises from 30 °C to 800 °C, the elastic modulus and fracture toughness of TC decrease from 20.3 GPa to 13.1 GPa and from 1.31 MPa m^1/2 to 1.16 MPa m^1/2, respectively. And the interfacial fracture toughness increases from 83.7 J/m² to 156.3 J/m². These results are consistent with the available values determined in literatures, which ensures the validity of this method.     

Mechanical characterization and damage assessment of thick adhesives for wind turbine blades using acoustic emission and digital image correlation techniques

Adhesives play a key role in the structural integrity of the Wind Turbine Blades as they are one of the main load carrying materials. A deep knowledge of the adhesives' mechanical behaviour in terms of failure mechanisms and damage processes enhances the attempt to optimize the blade design. Therefore, a comprehensive experimental programme was performed in order to determine the static mechanical properties of the adhesives. Ultimate tensile strength, ultimate compression strength, ultimate shear strength and the elastic properties of the adhesive specimens were determined through tensile and compression tests on dogbone specimens and single-lap bonded joints. The Acoustic Emission (AE) technique was used to relate the acoustic activity in the specimens to their damage state. More specifically, a frequency-based methodology, analysing the AE data, was used for the identification of the different damage mechanisms into the material during the loading. In addition, Digital Image Correlation technique, as a full-field technique, was used to measure displacements and deformations.     

Monitoring tensile fatigue crack growth and fiber failure around a notch in laminate SIC/SIC composites utilizing acoustic emission,electrical resistance,and digital image correlation

Acoustic emission, electrical resistance, and surface optical techniques were used to monitor matrix cracking and fiber-breakage during fatigue in tension for [0/90]2s SiC-based laminate composite single-notch specimens. Acoustic emission sensors were positioned in several locations including on the edge of the specimen which enabled location of events through the width and the location of internal tunnel-type cracks. Surface optical techniques, including digital image correlation, enabled the extent of surface crack growth. From these two sets of data a simple circuit could be constructed of the different damaged and undamaged regions in the region of the notch that was in good agreement with the change in electrical resistance, thus establishing a correlation with change in ER and damage development. The unique placement of AE sensors on the edge of the specimen also enabled the capture and location of what are believed to be fiber failure events prior to ultimate failure.     

Fracture mechanism of alumina-spinel castables containing ZrO2 by wedge splitting test and digital image correlation technique

To clarify the fracture mechanism of alumina-spinel castables, two kinds of alumina-spinel castables with or without fused zirconia-alumina (FZA) were prepared. The full-field strains and crack propagation process in the region of interest (ROI) of alumina-spinel castables were investigated by wedge splitting test (WST) and digital image correlation (DIC) technique. Fractographic methods were used to analyze the crack propagation path of the castables after the WST. The results indicated that the load-displacement curve of alumina-spinel castables containing FZA exhibits a non-linear fracture, demonstrated typical ductile fracture; while that of alumina-spinel castables without FZA is linear, showed typical brittle fracture. The characteristic length reaches to 258.9 mm in FZA containing castables, more than 4 times that of the castables without FZA. In contrast, castables containing FZA has longer and more tortuous crack propagation path, larger damage zone length, which results in the increase of the dissipated energy. Crack branching can be observed around the main crack in castables containing FZA, meaning that microcracks toughening is the main mechanism for flexibility improvement of the alumina-spinel castables containing FZA, formation of micro-cracks can be attributed to the martensitic transformation of zirconia.