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Bamboo and engineered bamboo products have gathered increasing scientific and technological interests for its promising applications in sustainable engineering purpose. Bamboo-based fiber composites , a main example of engineered bamboo products, is fabricated from processing the raw bamboo bundles treated with water-soluble resin into a bamboo fiber reinforced bio-composite. The present work investigates the transverse mode-I fracture criterion of bamboo-based fiber composites: the tensile strength ft and fracture toughness KIC . The fiber bundles are taken as the “aggregates” of this material, and the average grain size G is determined as 0.4 mm. By the experimental curve between the applied load and the mid-span deflection from the three-point-bending test on single edge notched specimens , the fracture process as well as the crack development includes three stages: linear, softening and failure stages. The quasi-brittle fracture criterion is calculated with the experimental peak load Pmax according to the boundary effect model in non-linear elastic fracture mechanics. Also, the normal distribution analysis is applied to cover all the experimental scatters with desired reliability. Furthermore, the method in this work on bamboo-based fiber composites can be extended to study other fiber reinforced composites .

Subsurface micro-cracks in rock, detrimental to long-term structural integrity due to potential moisture or water penetration and stress concentration, should be carefully assessed and repaired in service. In this study, we propose a resin pre-coating (RPC) technique for sealing the sub-surface micro-cracks that normally cannot be accessed by adhesive bonding. The subsurface micro-cracks with narrow openings are penetrated by a specially prepared solution of resin (without hardener) and acetone through capillary action. After evaporation of acetone, the resin is left to fill the micro-cracks. The normal resin and hardener mixture is then applied. The diffusion process will grow the adhesive joint deep into those micro-cracks leading to stronger adhesive bonding. Notched three-point-bending (3-p-b) granite specimens were first tested to fracture, and then bonded together using epoxy adhesive after RPC. Short Aramid fibers (SAF) were also used to strengthen the brittle epoxy adhesive joint. After removing potential premature failure sites by sealing those sub-surface micro-cracks, the peak loads of repaired samples were 35% higher than those measured from the original granite samples, and the total fracture energy consumption was 118% higher than the original value.

Pervious concrete is one of essential components in construction of sponge city, and the current study mainly focuses on the fracture performance of such promising material. Combined with three-point-bending (3-p-b) tests, a new method is proposed to determine the fracture properties of pervious concrete on the basis of Boundary Effect Model (BEM). The effects of material micro-structures (pore structure, aggregate structure) and specimen geometries on the fracture of pervious concrete are incorporated into the proposed formula in the form of fictitious crack. Then four groups of 3-p-b test results are collected to verify the proposed methods. The results show that the fracture properties obtained are independent of the specimen geometries, but are significantly influenced by the micro-structure of pervious concrete. The pervious concrete is strengthened with the increase of aggregate sizes, but is weakened with the increase of voids contents. Taking into account all the conclusions offered above, the fracture properties are strongly suggested to be carefully considered during the design of pervious concrete structures.

This study presents a design methodology for concrete fracture properties, linking the average aggregate size dav to the tensile strength ft and fracture toughness KIC for a given cement grade and water/cement (W/C) ratio. Two different concrete mixes with dav = 3.5 and 7 mm were tested and analyzed. Similar peak load Pmax values were obtained under three-point-bending conditions, but with different “ductility”. Variations of ft & KIC with dav were established by a simple design formula, which can be used to tailor bulk concrete fracture properties for specific purposes. A statistical method was introduced to estimate the effective dav if only the maximum aggregate size dmax was given. Comprehensive fracture data in literature were analyzed to demonstrate the estimation of dav from dmax. The purposely designed concrete mixes and data in literature show the significance of dav design even after the cement grade and W/C ratio are fixed.

The fracture properties (e.g. tensile strength, fracture toughness) are regarded as the material constants, which could be used to predict the fracture behavior of concrete. However, the scattered distribution and size effect of test results make the prediction in conventional methods difficult. In this study, the artificial intelligence (AI) approaches and the Boundary Effect Model (BEM) closed-form solution were tried to analyze such complex relations between the fracture test results and specimen geometries in the 3 PB tests. Firstly, a cluster of data were collected and divided into the training set and testing set. Then, based on the training data, the ensemble algorithm (Random Forests) and the Particle Swarm Optimization (PSO) were combined to establish the hybrid AI predictive model, and the fracture properties and predictive domain were determined with the BEM closed-form solution and normal distribution analysis. After that, the testing data were used to evaluate the behavior of these two predictive methods. The performance of the AI predictive model was quantified with R2 = 0.947, and the unknown data in testing set all fell into the predicted domain which was determined by using the BEM predictive model. The merits of the two predictive methods in predicting the fracture performance of concrete specimens were compared and expected to integrate together in the future work.

Notched three-point-bending (3 PB) tests of small concrete samples with a fixed depth/size but various span/depth ratios can be performed in almost every laboratory. Additional usefulness is added to the flexibility of those fixed-depth 3 PB tests in this study by providing a simple closed-form solution for determination of the depth and span independent tensile strength ft and fracture toughness KIC measurable from 3 PB samples with any span-depth ratio. Material constants, ft and KIC, only existed asymptotically for large concrete structures are determined by the simple fracture mechanics model regardless the span-depth ratio. Recent results of notched 3 PB tests with 6 span-depth ratios (2, 2.5, 3, 4, 5 and 6) were analysed again by the closed-form model and compared with the effective fracture toughness in the previous study (without ft). A good agreement in KIC values was obtained. That is the two different fracture models have affirmed the usefulness of small notched 3 PB tests of a fixed depth/size but with flexible choices of the span/depth ratio. Moreover, the weight of specimens is suggested to be considered when the span/depth ratio is greater than 4.0 for 3 PB tests. Finally, fracture statistics analysis, not considered by the previous method, was included in this study, which provided the 95% reliability band besides the mean ft and KIC values.

Subsurface micro-defects in rock and concrete-like materials, crucial for the integrity and durability of the epoxy repaired structures, which should be carefully treated during repair. In this study, a Resin Pre-coating (RPC) method was used to seal the subsurface micro-cracks by capillary action. The sandstone (the average grain size G = 0.5 mm) and granite (G = 2 mm) were selected to test the efficiency of RPC method. After the notched three-point-bending (3-p-b) specimens were loaded to fracture, the substrates were bonded together using epoxy adhesive or epoxy adhesive plus RPC. By comparing the peak loads, fracture energy, crack paths, micro-cracking activities before peak load and X-ray Microscopy (XRM) images of subsurface micro-structures of specimens repaired with different methods, the RPC method was proved to be effective for repairing the subsurface micro-cracks and strengthening the repaired structures, meanwhile the efficiency was confirmed to be relevant with the grain size.

Cubic Co9S8 fracture structure nanoshells and cubic Co3S4 nanocrystallines were prepared by an ultrasonic spray pyrolysis process using cobalt chloride (CoCl2·6H2O) and thiourea (CH4N2S) as the starting materials. The as-prepared Co9S8 nanoshells and Co3S4 nanocrystallines were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) with energy-disperse X-ray spectroscopy (EDX) attachment and Fourier-transform infrared (FTIR) spectrometer. It is found that the phase transition could be controlled by adjusting the reaction temperature, while morphologies of final products could be influenced by the molar ratio and reaction time of the precursor. And the possible mechanism for the formation of hollow and solid crystalline structures of the product was proposed.

Quasi-brittle fracture properties of a medium-grain sandstone with an average grain size G around 0.3–0.4 mm were investigated under three-point-bending (3-p-b) conditions. In total, 95 specimens were tested with the beam width W varying from 10 to 300 mm, or the specimen-size/grain-size ratio from 30 to 900. 45 medium-sized specimens (W = 30, 60, 100 mm) were tested first to determine the tensile strength ft, which was then used as a reference for tests of 8 large notched specimens (W = 300 mm) and 42 small un-notched specimens (W = 10 mm). Statistical fracture modelling, based on normal distributions and the characteristic microstructure measurement (the average grain size G in this study), was used to quantify the quasi-stable fictitious crack growth Δafic at the peak load Pmax and the characteristic crack length a ch * defined by the bulk toughness and strength properties. The statistics-assisted modelling has changed the previous curve-fitting boundary effect model (BEM) to a predictive closed-form solution, providing a useful option when large scatters in experimental data and reliability in design need to be focused. The well-known size effect law (SEL) proposed for geometrically similar specimens was also used to fit the sandstone results, and compared with the closed-form BEM with built-in statistical functions. Applications of SEL and BEM and their key differences were explained.

A thin layer of un-cured resin over metal substrates applied by using an acetone-diluted resin solution (without hardener) has been found to be beneficial to strong adhesive bonding. The resin pre-coating (RPC) solution can effectively seal sub-surface micro-cavities and increase the substrate wettability. This study examines possible aftermath effects of the acetone dilution on mechanical properties of epoxy through comparison of samples made from as-received resin and resins diluted once and twice by acetone. RPC can be accepted with confidence in substrate pre-treatments for strong adhesive bonding if no detrimental effect on epoxy properties is observed. Fourier transform infrared spectrum (FTIR) was conducted, showing the spectrogram of the resin previously diluted by acetone was the same as that of as-received resin, i.e. no change in epoxy molecular structures after complete evaporation of acetone. Strength and modulus of elasticity measured by flexural and compressive tests were compared using samples made from as-received resin, and resins diluted once and twice by acetone. Variations among results from the three groups were less than 2%, or negligible, affirming the RPC method can be used for substrate pre-treatments and stronger adhesive bonding.

Maximum fracture loads (Pmax) of small notched granite specimens under three-point-bending (3-p-b) conditions can be easily measured with any notch/size ratio. In this study, we report a simple closed-form solution of a non-Linear Elastic Fracture Mechanics (non-LEFM) model emphasizing the influence of average grain size G on quasi-brittle fracture of granite. This simple analytical solution containing the grain size G can be conveniently used to determine granite tensile strength ft and fracture toughness KIC from Pmax measurements of small notched 3-p-b specimens of geometry dissimilarity. The span/width (S/W) ratios of small 3-p-b specimens can vary, e.g. 2.5 or 4. The notch/width (a0/W) can also vary, e.g. the notch a0 can be as short as the average grain size (G), or close to width W. However, specimens with α-ratio (=a0/W) around 0.2 are recommended to minimize the boundary influence/effect from both the front and back specimen boundaries as proven by the Boundary Effect Model (BEM). Blue granite with the average grain size around 2 (mm) was selected to test the new method. Total 64 granite samples from four different groups (different 3-p-b sample designs) were tested, with W = 27, 40 and 70 (mm), S/W = 2.5, 4.0 and a0 = 4, 6 and 8 (mm). The tensile strength ft and fracture toughness KIC estimated from every group is fairly close to the values determined from the entire population of 64 tests. Therefore, tests from any specimen group of given geometry and size are sufficient. Estimations for G = 1.5 and 2.5 (mm) were also provided and compared with those for G = 2 (mm) to show the grain size influence. Advantages and disadvantages of BEM and well-known SEL (size effect law) are also discussed using the granite results.