A recent composite technique, namely Resin Pre-Coating (RPC), has demonstrated remarkably high effectiveness in the repair of Carbon Fiber-Reinforced Polymer (CFRP) composites. Compared to widely used scarf repair and injection repair, this non-destructive method offers advantages in addressing subsurface damages from the millimeter to micron scale, such as edge delaminations that frequently occur due to machining or low-energy impacts. The acetone-rich RPC solution can spontaneously transport sticky resin and other toughening agents into defects through capillary action. In this study, we further improved the solution by adopting a self-curing resin (i.e., SC-RPC), reducing the repair duration from the initial 2–3 months to merely a few hours. Using this modified solution, the CFRP specimens prepared containing delamination cracks were largely restored, reaching up to 94.9% of the original compressive strength. With the additional incorporation of carbon nanotubes (CNTs), full restoration was achieved, as is evidenced by load-bearing capacities and overload failure modes comparable to those of pristine specimens. The findings of this study may help alleviate concerns regarding substandard post-repair performance and prolonged repair durations, which are frequently criticized in real-world CFRP maintenance projects. The preparation of two new formulations, SC-RPC and SC-RPC+CNT, along with the optimization of key parameters, was carefully detailed in the manuscript to ensure experimental reproducibility.

Surface scratches represent a common defect encountered in the operation of carbon fiber reinforced polymer (CFRP) composite structures. However, predicting the residual flexural properties of scratched CFRP structures remains an engineering challenge, as shallow scratches (20-100 mu m) can induce varied effects on CFRP structures contingent upon their sizes and geometries. To investigate the issue, comprehensive flexural tests using 150 notched CFRP laminate specimens with diverse span lengths and thicknesses were performed in this study. For experimental analysis, a closed-form model for determining the bulk tensile strength and fracture toughness of heterogeneous composites has been adopted and modified. It highlights that the ply thickness of CFRP plays a pivotal role in governing its (brittle or quasi-brittle) failure behavior and thereby should be considered as a microstructural parameter in the modeling. Based on that, the strength results from different test groups exhibit good consistency, shedding light on the key interactions between crack growth and specimen thickness, as well as between crack growth and specimen deformation (influenced by span-to-thickness ratio). These findings hold promises for aiding engineers in evaluating non-standard CFRP structures and in calculating the residual capacity of damaged CFRP structures.

Size effect experiments using geometrically similar specimens of different sizes overlap boundary effect experiments using specimens of single size but different notches if sufficient sample designs are tested. This study shows the interchangeability and relevance of size and boundary effect experiments through in-depth analysis of comprehensive size effect experiments. Initially-designed five groups of geometrically similar concrete specimens with notch/size ratios of 0, 0.025, 0.075, 0.15 and 0.3 are rearranged into four groups with constant sizes of 40, 93, 215 and 500 mm. Thus, total nine groups of experimental results, containing abundant information on quasi-brittle fracture of concrete and associated size effect phenomenon, are analysed by three size effect laws (SELs) and a linearized boundary effect model (BEM). It is found that fracture behaviours revealed from comprehensive size effect experiments (using large and small geometrically similar specimens required by SEL) can also be confirmed by simple boundary effect experiments (using specimens of single convenient size). Discussion on SEL and BEM based on principles of mathematical fitting and data processing is provided for more objective quasi-brittle fracture experiments and modelling.

Edge-delamination of carbon fiber reinforced plastics (CFRP) has been successfully repaired using a novel technique named resin pre-coating (RPC). The acetone-diluted resin does not only adhere to the surface but penetrates deeply into sub-surfaces through capillary action. In the current study, Carbon Nanotube (CNT) was tentatively added into RPC to further strengthen the bonding. DSC analysis and FTIR results showed no chemical effect on resin curing through adding CNT. Likewise, using CNT as additives has not changed the surface properties of RPC solution with 5 wt% resin and 95 wt% acetone. Compressive testing was performed to validate the effect of CNT incorporation (with four concentrations: 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%) on RPC repair. The results show that the 5% RPC solution mixing 1.5 wt% of CNT works best with a 22% increase over only RPC in repair effectiveness. SEM revealed CNT bridging at the fracture surface of the RPC/CNT-repaired sample.

The wedge splitting (WS) test geometry, suitable for testing large concrete specimens up to a few meters in size because of the self-weight supports, will find more applications if a closed-form model is available. This study presented a linearized boundary effect model (BEM) on quasi-brittle fracture of large WS mass concrete specimens containing aggregates up to 100 mm. Two separate small WS specimens containing aggregates around 10 mm and 20 mm were also analyzed to show the versatility of the model and the necessity to include the aggregate size in modelling. Modelling concrete as a large particle composite led to a simple closed-form solution, and both the fracture toughness K-IC and tensile strength f(t) for the formation of the crack/notch-tip fracture process zone (FPZ) were determined from the maximum splitting force Pmax-h of WS specimens. The material properties K-IC and f(t) from the current WS model were also confirmed by three-point-bending (3-p-b) tests of the same concrete. Comparison between the linear BEM and the well-known size effect law (SEL) was provided for purposes of experimental data analysis and mathematical fitting principles.

Limited bone sample size and number, coupled with hierarchical microstructures and non-linear damage, make credible assessment of bone quality difficult. This study presents a simple non-Linear Elastic Fracture Mechanics (non-LEFM) model, which can be easily adopted to measure bone strength and toughness using a limited number of small bone samples. The new closed-form model assumes that osteon, the fundamental functional unit of cortical bone, plays the dominant role in bulk bone properties. Different to Griffith theory [1] or LEFM [2] containing 0 microstructure, the new non-LEFM model contains 1 microstructure, the osteon diameter OD or the characteristic microstructure C-ch. The "0 to 1" leap in microstructure modelling is critical for the evaluation of the non-linear damage prior to a transverse fracture. And it leads to the fracture relation P-max = f(t) x A(e) (Fracture Load = Strength x Area), in which the longitudinal tensile strength f(t) is the slope of the linear P-max - A(e) relation while the equivalent area Ae contains sample dimensions as well as Cch. Fracture measurements from samples with different sizes or initial cracks are on one straight line through the origin (0, 0), i.e. f(t )can be determined from any sample group. Fracture toughness KIC can then be transferred from f(t )and C-ch. A statistical analysis based on normal distribution has been combined into the model so that the reliability band for bone strength and toughness can be specified. Bone data from literature are analyzed by the present model, and non-linear damage zones at P-max in dry and wet bones are estimated and compared.

Comprehensive tests using more than 200 CFRP laminate specimens with and without shallow surface scratches (20 mu m 3 mm) have been performed under three-point-bending (3-p-b) and direct tensile conditions. Critical flexural loads of scratched 3-p-b specimens have been linked to the tensile strength f(t) of CFRPs by a simple closed form solution of a non-Linear Elastic Fracture Mechanics (non-LEFM) model for composite failures, which models the crack tip damage using the characteristic composite microstructure Cch (ply thickness of CFRP). The average f(t) measured from 30 direct tensile tests is virtually identical to that from 170 different 3-p-b tests with only 2% relative error, indicating potential applications of the simple closed-form solution in structural integrity analysis for scratched CFRP components. The statistical reliability has been combined into the predictive model, which can be important when not only the mean strength but also the prediction interval is required.

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.

This study investigates the influence of pre-existing discontinuities on the roof breaking behaviors under Longwall (LW) mining condition. Taking the Meiyukou Coal Mine’s 82,002 working face as the engineering background, a series of similar experiments were conducted for the purpose. To be specific, mica powder was schematically set above the coal seam, simulating the angled discontinuities in the overlaying strata (α = 30° and 60°). Markers and gauges were employed to monitor the strata displacement and relevant stress variation. The results show that the prefabricated discontinuity leads to an early first weighting and frequent periodical weightings. Besides, discontinuities provide favorable conditions for the crack formation and propagation in the overlaying strata. As a result, the final roof caving height has almost doubled, which may aggravate the bearing pressure of the hydraulic pillars at the working surfaces if in the real situation. Hopefully, these information from the reduced-scale models could serve as a reference for the future practical designs of LW face supporting system.

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 f(t) and fracture toughness K-IC measurable from 3 PB samples with any span-depth ratio. Material constants, f(t) and K-IC, 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 f(t)). A good agreement in K-IC 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 f(t) and K-IC 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. (C) 2020 Elsevier Ltd. All rights reserved.

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.

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 Delta a(fic) at the peak load P-max 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.

As a green and natural engineering material, bamboo is going to be vastly used in constructions, as well as it needs to be bonded with other materials for composite application. This study applied a resin pre-coating (RPC) method on the epoxy adhesive joint between engineered bamboo and steel substrates and experimentally investigated the beneficial effect of the RPC method on the bonding property of epoxy joints. The diluted resin solution was made by dissolving resin in acetone, and three mixing concentrations by weight were prepared as 5wt%, 10wt% and 20wt%. The test results showed that RPC treatment obviously increases the shear strength and energy of the epoxy joint, while the degree of the effect depends on the resin solution concentration. Moreover, the RPC method also improves the failure mode of the epoxy joint from adhesive fracture along the steel bonding interface to mixed mode of structure fracture in engineered bamboo and adhesive fracture in both adhesive interfaces, as well as increases the failure surface roughness. The working mechanism of RPC treatment is that the resin solution could take sticky epoxy adhesive penetrate into the micro-openings along the bonding surface zone and remove the bonding defects, and thereby, it enhances the interlocking effect between substrate and adhesive as well as improves the rupture strength of engineered bamboo near the bonding interface zone. Finally, it improves the bonding property of epoxy adhesive joint between engineered bamboo and steel.

•Geometry dissimilar notched 3-p-b specimens can be used to study size effect.•Linear relation of closed-form solution is obtained for both LEFM and non-LEFM.•Only one group of tests are needed to establish the linear relation.•Normal distribution analysis of the new model removes the need of curve-fitting.