Shear Debonding Research Articles

Effects of grinding mode on grinding performance of anisotropic CF/PEEK composites

AbstractGrinding could be used to enhance accuracy and quality of carbon fiber‐reinforced poly‐ether‐ether‐ketone (CF/PEEK) composites. However, the grinding mode, namely up‐grinding (UG) or down‐grinding (DG), is usually neglected while unbefitting grinding mode choice may cause inferior surface quality. To investigate effects of grinding modes on grinding performance of anisotropic CF/PEEK, grinding experiments in UG and DG were carried out under different fiber orientation angles θ. Experimental results indicated that maximum grinding temperatures and normal grinding forces in UG were lower than DG regardless of the abrasive grit sizes. Material removal mechanisms of CF/PEEK affected by grinding mode were revealed by analyzing morphologies of surface, chip and grinding wheel. When θ = 0°, the dominant mechanisms in DG were extrusion fracture and fiber deformation, while they were interface shear debonding and bending fracture in UG. When θ = 90°, the dominant mechanisms in DG included interface layer separation and bending fracture, while they were shear and extrusion fracture of well‐supported fibers in UG. Thus, UG was more favorable for material removal, obtaining lower surface roughness values, fewer machining defects and less wheel wear than DG. This study will provide technical guidance for high‐quality grinding of carbon fiber‐reinforced thermoplastic composites.Highlights The effects of grinding modes (DG and UG) on the grinding performance of anisotropic CF/PEEK composites were investigated. Material removal mechanisms of CF/PEEK affected by grinding mode were revealed under different fiber orientation angles. The maximum grinding temperatures and normal grinding forces in UG were lower than DG. UG was more favorable for material removal in grinding, and could produce better ground surface quality than DG.

Mechanical simulation and debonding risk analysis of OLED panels with optically clear adhesives

This paper presents a mechanical simulation model and analysis method for OLED panels. The functional layers within the panel are bonded using optically clear adhesives (OCAs), which are pressure-sensitive adhesives (PSAs), resulting in a soft interface bonding problem. It was discovered that the normal tensile debonding strength of OCAs was significantly lower than the shear debonding strength. To address this issue, a three-layer structure simulation model of “hard + soft + hard” is established, which allows for analyzing the screen folding problem of any radius. Additionally, a suitable element mesh generation strategy for multi-layer structure screens is proposed. A debonding risk index based on pressure is introduced by comparing the interlaminar normal and maximum principal stress. The influence of thickness, bending radius, and Young’s modulus on longitudinal stress and debonding risk index is examined. It is observed that the influence of thickness and Young’s modulus on debonding risk is non-monotonic, indicating the presence of an optimal solution. The high debonding risk area is located at the beginning of the folding mechanism and screen bonding. The proposed debonding risk index is not only applicable to OCAs but also to all PSA with similar structures.

Enhancing interfacial shear debonding resistance by mechanical mismatch

Layered systems of similar/dissimilar soft materials are widely used in advanced device applications, including bioelectronics, soft robotics, and energy generators. Device designers strive to minimize the mechanical mismatch between the soft material layers, claiming that the mismatch could be associated with low interfacial debonding resistance and then cause device malfunction. Although strong interfacial bonding has been repeatedly achieved based on the principle of minimizing-mechanical-mismatch, the specific question of whether mechanical matching is the optimal solution for high debonding resistance has never been solidly answered, leaving ambiguity in the further optimization of the layered soft material systems. In this paper, the influence of the stiffness ratio between the soft material layers on the interfacial shear debonding resistance is systematically analyzed through theoretical modeling and finite element simulation approaches. Monotonic improvement of the shear debonding resistance with increasing the loading layer stiffness, even when exceeding that of the substrate layer, is analytically derived by establishing a shear debonding theory of a hyperelastic bilayer. This mechanical-mismatch induced improvement is due to the effective consumption of the external work by the strain energy storage of the loading layer and the large increase in the shear lag length. The theoretical predictions of the debonding resistance agree well with the finite element simulation results, which implies the local non-uniform deformation in the loading and substrate layers near the interfacial crack front causes insignificant influence on the shear debonding. Increasing the modulus or thickness of the loading layer or adding a stiff backing can equivalently improve the shear debonding resistance, which can be quantitatively predicted by the established shear debonding theory. Reasonable applications of the proposed debonding resistance enhancement strategy are also discussed. The findings provide good guidance for the safe service and further optimization of the layered soft material systems to improve the device reliability.

Experimental Study on Shear-Peeling Debonding Behavior of BFRP Sheet-to-Steel Interfaces.

In order to study the failure mode and debonding behavior of the interface between BFRP (basalt fiber reinforced polymer) sheet and structural steel under mixed-mode loading conditions, eighteen specimens with different initial angles were tested in this study. The specimens were designed with different initial angles to ensure that the interface performed under mixed-mode loading conditions. The relations between the bond strengths, failure modes, and initial angles were investigated. A new evaluation method to predict the interfacial bond strength under shear-peeling loading mode was proposed. The test results show that specimens with a smaller initial angle are more likely to exhibit a shear debonding failure at the interface between the steel plate and adhesive. In contrast, specimens with a larger initial angle are more likely to exhibit peeling of the interface. The ultimate tensile strength of the specimen is higher with a smaller initial angle. The results predicted by the proposed method are in good agreement with the experimental results.

Development of a local bond shear stress-slip model of RC beams externally strengthened with FRP materials

This paper presents an analysis of the numerical investigation of RC beams, strengthened with CFRP and tested in flexure, using the ANSYS© numerical analysis code. In the first part of this paper, finite-element models of RC beams (control and CFRP-strengthened), subjected to four-point bending, are developed based on experimental tests as reported in the literature. Then, a 3D nonlinear finite-element analysis (NLFEAs) with perfect bonding, along with 12 cohesive zone material (CZM) models proposed by researchers in literature, are explored to simulate the behaviour of CFRP-strengthened RC beams. In the second part of this work, the compressive strength of the concrete and the elastic modulus of the epoxy resin are examined to study their effect on the same CFRP-strengthened RC beam. On the basis of those two studies, a bilinear CZM model is modified to predict the behaviour of RC beams strengthened with FRP materials, and interfacial shear debonding phenomena. The modified CZM model is verified against a database of experimental results that includes 23 RC beams, strengthened with FRP tested in flexure, with different concrete compressive strength, FRP modulus of elasticity and resin elastic modulus. The experimental and predicted ultimate loads are compared to verify the accuracy of the modified CZM model. The level of fitness between the modified CZM model and experimental results is elaborated with statistical metrics such as integral of the absolute error (IAE), root mean square error (RMSE) and coefficient of determination (R2). The results show good agreement between the experimental and predicted ultimate loads.

The temperature-dependent shear strength of ice-filled joints in rock mass considering the effect of joint roughness, opening and shear rates

Abstract. Global warming causes many rockfall activities in alpine mountains, especially when ice-filled joints in the rock mass become thawed. The warming and thawing of frozen soils and intact rocks have been widely studied in the past several decades; however, the variation of shear strengths of ice-filled joints is not fully understood. In this study, a series of compression–shear experiments were conducted to investigate the shear strength of ice-filled rock joints by considering the effects of joint roughness, temperature, opening, shear rates and normal stress. The joint roughness can improve the shear strength of ice-filled joints. However, the contribution of joint roughness is controlled by some noticeable bulges instead of the joint roughness coefficient (JRC) index. The peak shear strength of the ice-filled joint linearly increases with the increasing aggregation of rupture ice area before these noticeable bulges. As the joint opening increases, the effect of joint roughness decreases, and the shear strength of ice-filled joints tends to be equal to the shear strength of pure ice. In addition, the shear strength quickly reduces with increasing temperature from −15 to −0.5 ∘C. The shear failure mode changes from shear cracking of joint ice to the shear debonding of the ice–rock interface above −1 ∘C. Increasing shear rate decreases the shear strength of ice-filled joints because the joint ice displays the brittle failure phenomenon at a high shear rate. The peak shear strength of the ice-filled joint linearly increases with the increase in normal stress. Moreover, it is also proved that the Mohr–Coulomb criterion can be used to characterize the shear strength of ice-filled joints under different normal stresses. This research can provide a better understanding of the warming degradation mechanism of ice-filled joints by considering the abovementioned important influencing factors.

Multispeckle diffusing wave spectroscopy as a tool to study heterogeneous mechanical behavior in soft solids

Multiple speckle diffusing wave spectroscopy (MSDWS) can be applied to measure spatially heterogeneous mechanical behavior in soft solids, with high sensitivity to deformation and both spatial and temporal resolution. In this paper, we discuss the mathematical approach behind the quantification of the deformation rate from MSDWS data and provide guidelines for optimizing the selection of experimental parameters in measurements. After validating the method in extensional tests on an elastomer, we provide an example of the potentiality of MSDWS by measuring the spatial distribution of the deformation rate during shear debonding of adhesive tapes. We quantitatively characterize the deformation rate distribution related to shearing and peeling under loading. A highly heterogeneous deformation rate distribution is observed, and time-dependent measurements reveal an increase in deformation localization hundreds of seconds before full debonding. This behavior, previously predicted by theory and simulation, is demonstrated here experimentally for the first time.

Experimental and Numerical Investigations of Cement Bonding Properties.

Well integrity is of high importance during the entire well life span especially when renewable energy resources such as geothermal are designed to cover the increasing world energy demand. Many studies have documented the importance of the casing–cement interfacial bonding to ensure critical well integrity achievements; however, laboratory experiments and field data are not always aligned. Furthermore, Finite Element Analysis shows relatively high discrepancies compared with the results of various scholarly published works. The limitations in the FEA are most probably generated by the casing–cement interaction modeling parameters. Typically, the contact between casing and cement is modeled using the so-called CZM method, which includes the shear debonding process into the FEA. Several setups have been used in the past to determine the interfacial casing–cement bonding shear strength. Some of these setups are briefly summarized herein. The novelty of this paper consists in the combination of a relatively simple experimental setup with the finite element modeling of the experiment itself to demonstrate that it is important to acquire accurate laboratory data for debonding simulations and, thus, to improve the well integrity prediction. The aim of this paper is to better understand the limitations of the finite element method when modeling shear bonding of the cement and, in the same, to verify that the proposed experimental setup can be modelled using numerical approaches. The successful numerical simulation can later be used for upscaled models. The results confirm the experimental push down setup and aid engineers to further understand and validate CZM models and optimize the well design to achieve maximum well integrity potential. Our results are within 1% error from the average field data.

Prediction of shear debonding strength of concrete structure with high-performance fiber reinforced concrete

Debonding of the fiber-reinforced concrete reinforcement is counted as significant matter in concrete design because of the shear stresses. The main issue is the potential of brittle debonding failures that could highly reduce the effectiveness of strengthening application. Shear bond strength and the governing variables have been empirically analyzed several times; however, these experiments couldn’t provide accurate predictions because of the complexity of debonding process. In this study was analyzed debonding behavior of concrete structure with high-performance fiber reinforced concrete by adaptive neuro fuzzy inference system (ANFIS). High-performance fiber reinforced concrete could be used as repairing material for normal concrete structures. In this study the concrete structure with high-performance fiber reinforced concrete was subjected to shear loadings and corresponding data samples has been acquired for ANFIS analyzing. Mechanical surface treatment with and without chemical substitute was used as bonding strategies for fabrication of samples. Finite element method is used for data samples extraction. ANFIS methodology was used for data samples analyzing based on prediction accuracy of shear debonding strength. Influence of parameters on the shear debonding strength were investigated by ANFIS approach. Obtained results could be used for further improvement of the high-performance concrete structure.

A Rate-dependent Continuum Damage Model for Dynamic Shear Debonding of CFRP-concrete Interface

Dynamic shear debonding behaviour in fibre reinforced polymer (FRP) strengthened concrete structures has been studied by some researchers through experiments and finite element (FE) simulations. However, there is still a lack of a continuum damage model to simulate dynamic debonding behaviour of CFRP-concrete interface using a method derived from nonlinear fracture mechanics (NLFM). To gain deeper insight into the dynamic fracture mechanics of carbon fibre reinforced polymer (CFRP)-concrete interface, a novel continuum damage model was proposed in this paper. Model analyses and regression test data were used to formulate the relationship between model coefficients and material parameters of tested specimens in literature. It was found that the dynamic enhancing effect on interfacial bond properties became more significant when a specimen has a larger CFRP load capacity or a lower concrete load capacity. The proposed damage model was verified through comparing numerical predictions of load-displacement curve and CFRP strain distribution along the bond length with published test results. Finally, based on a comparison with experimental study of specimens using basalt FRP and glass FRP, the proposed model was also found to be applicable to different types of FRP.

Effect of the different debonding strength of metal and ceramic brackets on the degree of enamel microcrack healing.

ABSTRACTObjective: This study aims to determine shear debonding strength of metal and ceramic brackets, and the degree of enamel crack healing. Material and Methods: Extracted human maxillary premolars were flattened on the buccal surface, and randomly separated into five groups (n = 15). In control groups (groups 1 and 2), metal and ceramic brackets were bonded on flat polished enamel, while in experimental groups (groups 3 and 4), metal and ceramic brackets were bonded on the surface with boundary where corner cracks were created. Additionally, fifteen specimens (group 5) were also prepared for an indentation procedure with no bracket installation. The degree of crack healing was measured. All brackets were detached with a universal testing machine, and the adhesive remnant index (ARI) was also identified. Healing degree and apparent fracture toughness were then calculated. Results: Between groups with similar bracket types, there was no statistically significant difference in debonding strength. Regarding bracket types, ceramic brackets provided significantly higher debonding strength than metal brackets. There was a significant difference in ARI scores between metal and ceramic brackets. The corner cracks showed signs of healing in both horizontal and vertical directions. No statistically significant difference in the healing rates among the groups was found and the apparent fracture toughness increased from the initial to the final measurement. Conclusions: Within the limitations of this study, even though ceramic brackets required significantly higher debonding force compared to metal brackets, debonding stress was limited to the bonding site and did not affect the surrounding cracks on enamel.

Shear Characteristics between FRP-Concrete Bonding Interface

At present, fiber reinforced composite materials (FRP) are widely used in the reinforcement of concrete structures. The bonding interface between FRP plates and concrete is the key part of the strengthening of concrete structures with FRP plates. The bonding performance directly determines the success or failure of structural reinforcement. Based on the self-developed test device, the development of FRP and concrete in direct shear debonding test specimens, with the aid of advanced digital image correlation DIC technology, accurate measurement of specimen strain distribution on the surface of the FRP plate, and the FRP plate surface strain along the plate long distribution rule and the bond strength of the specimens was well researched, it reveals that the stripping of FRP and concrete interface failure process, and provides technical guidance for the treatment of FRP-concrete bond interface in practical projects.

Interlaminar toughening of unidirectional CFRP with multilayers graphene and MWCNTs for Mode Ⅱ fracture

Carbon fiber reinforced polymers (CFRPs) often encounter impact load especially for flight vehicles. The invisible delamination raised by the impact leads to largely decreasing on the residual load capacity. The shear stress and delamination of the out-plane play the important role in the impact loading. Therefore, this paper focuses on enhancing the interlaminar shear fracture toughness through adding nano-fillers of multi-walled carbon nanotubes (MWCNTs) and multilayers graphene (mG) between the layers of CFRPs. MWCNT and mG nano-materials are respectively sprayed onto the surface of prepregs at density of 1.0 g/m2. The end notched flexure (ENF) testing is carried out for verifying the interlaminar toughening performance. MWCNT toughening strategy with 1.0 g/m2 harvests 154.47% improvement on GIIC (Mode II shear fracture toughness) with medium standard deviation. Multilayers graphene also obtain 105.80% improvement in spite of large data scatter. Scanning electron microscope (SEM) analysis reveals that MWCNT and mG nano-fillers make the shearing fracture appearance roughly in macro-scale, generate more cracks in micro-scale, and improve shear fracture toughness through the crack deflection namely MWCNT pull-out, mG surface peeling and shear debonding.

Rate dependent shear debonding between a highly stretchable elastomer and a rigid substrate: Delayed debonding and pre-stretch effect

In this paper, the interfacial shear debonding characteristics and mechanism between a highly stretchable acrylic tape and a rigid polycarbonate strip are explored systematically with three types of in-plane loading. An effective approach is proposed based on the findings to improve interfacial adhesive strength. The debonding energy solely dissipated by breaking the interfacial bonds is found to be closely related to the loading rate, which can be explained by the Chaudhury’s model. Rate dependent delayed debonding is repeatedly observed under various stretch levels. The threshold level of the driving force of delayed debonding is correlated with the intrinsic fracture energy of the interfacial molecular chains and expected to be rate independent. Shear debonding is significantly affected by the pre-existed defects at the interfacial crack front. An effective and easy-to-operate cyclic pre-stretch approach with a moderate amplitude and rate is proposed, which is capable of improving the interfacial reliability by diminishing the pre-existed defects. A quantitative model is also constructed to characterize the pre-stretch effect on the interfacial adhesion.

Theoretical and numerical models for the influence of debonding on the interaction between hydraulic fracture and natural fracture

Purpose The interaction between hydraulic fracture (HF) and natural fracture (NF) in naturally fractured rocks is critical for hydraulic fracturing. This paper aims to focus on investigating the development of tensile and shear debonding zone on the NF caused by the stresses produced by HF, and the influence of NF’s debonding behavior on the interaction between HF and NF. Design/methodology/approach Theoretically, tensile and shear debonding modes of NF are considered, two dimensionless parameters are proposed to characterize the difficulty of tensile and shear failure of NF, respectively. Numerically, a finite element model combining the extended finite element method and cohesive zone method (CZM) is proposed to study NF’s debonding behavior and its influence on the interaction between HF and NF. Findings Both theoretical analysis and numerical simulation show the existence of two debonding modes. The numerical results also show that the HF can cross, offset or propagate along the NFs depending on the parameters’ value, resulting in different fracture network and stimulated reservoir volume. When they are large, the NF’s debonding area is small, HF tends to cross the NF and the fracture network is simple; when they are small, the NF’s debonding area is large, HF will propagate along the NF. In addition, HF is easier to propagate along with NF under tensile debonding mode while it is easier to pass through NF under shear debonding mode. Originality/value The theoretical and numerical considerations are taken into account in the influence of the debonding of NFs on the interaction between HFs and NFs and the influence on the formation of the fracture network.

High-performance fiber reinforced concrete as a repairing material to normal concrete structures: Experiments, numerical simulations and a machine learning-based prediction model

High-performance fiber reinforced concrete (HPFRC) has been reported as a repairing material to normal concrete (NC) structures due to its predominant mechanical performance. Here, we investigate the debonding behavior between HPFRC and NC subjected to direct shear loading. HPFRC specimens are fabricated and experimentally calibrated to determine the compressive and bending (i.e., flexural) strengths. HPFRC-NC samples are fabricated using two bonding strategies, i.e., mechanical surface treatments with and without chemical agent. Direct shear loading is applied to test the HPFRC-NC debonding behavior. A finite element (FE) model is developed to predict the direct shear debonding response. The FE model is validated by the experimental observations and then used to characterize the debonding behavior with various geometric and material parameters, as well as bonding interface treatments. Subsequently, a robust machine learning model is developed to formulate the shear debonding strength of HPFRC-NC with those influencing parameters. Design examples are presented to illustrate the efficiency of the proposed machine learning model in describing the debonding response of HPFRC-NC. A sensitivity analysis is further conducted to investigate the contribution of the chosen predictors to the debonding behavior of HPFRC-NC. The reported HPFRC and machine learning-based prediction model provide powerful tools to address repairing issues in various existing normal concrete structures.

Dynamic mechanical behavior, damage mode and mechanism of multi-scale high energy insensitive particulate reinforced composites of a new type of anti-missile sandwich wall structure

Multi-scale high energy insensitive particle reinforced composites (MSHEIPRC) are the most important part of the anti-missile sandwich wall structure. Its dynamic mechanical behavior and damage mode will directly affect the success of anti-missile capability. In this paper, based on a modified Split Hopkinson Pressure Bar (SHPB) experimental technique and the design of triaxial impact loading test device, combining with theoretical study and meso analysis, the dynamic mechanical response under high strain rates and damage characteristics of MSHEIPRC under high overload were carried out. The results show that the dynamic stress-strain curves of MSHEIPRC are highly non-linear and have obvious strain rate effect and density effect. Compared with the experimental results, the modified constitutive model can accurately describe its dynamic mechanical behavior at high strain rates. Based on the energy model and related parameters, the bond strength between the surface of crystal particles and the binder is calculated to be about 0.6 MPa. Therefore, the shear debonding damage mode between the surface of crystal particles and the binder is first found at the stage of small impact loading. The critical fracture strength of cracks with different sizes is calculated by Griffith method. With the increase of impact loading pressure, the fracture phenomenon gradually appeared on the surface of crystal particles, which was basically consistent with the theory model. Continuing to increase the impact loading pressure, sufficient shock wave energy forces crystal particles to produce transgranular fracture, which leads to crystal particles breaking.

The distinct element method (DEM) and the extended finite element method (XFEM) application for analysis of interaction between hydraulic and natural fractures

Hydraulic fracturing can be considered one of the most effective stimulation methods for fractured network formation. The interaction between induced and natural fractures greatly affects the success of the treatment. In this paper, the extended finite element method (XFEM) and distinct element method (DEM) have been combined to identify the propagation of hydraulic fractures in a porous medium with naturally fractured blocks. DEM is used to investigate hydraulic fractures in a natural-fracture network and evaluate the production flow rate under different apertures and lengths of hydraulic fracture. XFEM is used to simulate the deformation of the natural-fracture interface during the approaching phase of induced fractures as a no-remeshing tool. The results of DEM reveal that length of hydraulic fracturing has a significantly greater effect on production than aperture. Results from XFEM show that tensile and shear debonding of natural fractures change as a function of angle and distance from an induced fracture.

Green’s function for a line heat source acting on the surface of a coated isotropic thermoelastic material

Two-dimensional Green’s function, for a line heat source acting on the surface of a coated isotropic thermoelastic material, is investigated in this paper to improve the understanding of interface mechanisms of coating/substrate system. The coating and substrate are modeled as infinite layer and semi-infinite substrate respectively. They are perfectly bonded or are in smooth contact at the interface. Based on the two-dimensional general solution of isotropic thermoelastic materials expressed by harmonic functions, the corresponding harmonic functions with undetermined constants for coating and semi-infinite substrate are constructed, respectively. The thermoelastic field can be obtained by substituting the harmonic function into the general solution. The constants can be determined by the free surface boundary conditions and interface continuous conditions between the coating and the semi-infinite substrate. Numerical results are exhibited in the form of contours and some valuable conclusions for interface effect, interface shear debonding and coating tensile failure are presented.

The effect of orthodontic bracket pad shape on shear bond strength, an in vitro study on human enamel.

BackgroundTo evaluate the effect of bracket pad shape on shear bond strength (SBS) of orthodontic brackets bonded to human enamel.Material and MethodsOne hundred and five extracted human maxillary permanent molars were divided into 7 groups of 15 specimens per group (n=15). Each group of teeth was bonded with 6 different shaped WildSmiles® brackets (Star, Heart, Soccer ball, Football, Flower, and Diamond) and GAC® rectangle shaped brackets. Shear debonding force was measured with an Instron universal testing machine using a knife-edged chisel 24 hours after initial bonding. Descriptive statistics (mean, standard deviation, and range) for each bracket pad shape was calculated. Analysis of variance (ANOVA) using SPSS software version 24.0 was performed with P-value set at 0.05. Post-Hoc Tukey analysis was used to analyze differences among groups. Differences in Adhesive Remnant Index (ARI) scores among groups were analyzed using Chi-square test.ResultsDebonding force values (N ± SD) ranged from 205.51 ± 49.12 (Star) and 275.96 ± 69.05 (Soccer). SBS values (MPa ± SD) ranged from 13.34 ± 3.18 (Star) and 17.77 ± 6.94 (Rectangle). Even though intergroup comparison of SBS in Newtons revealed statistical significance (p = 0.014) between Star-Soccer and Star-Football group, it does not have any clinical significance since ranges of SBS of all groups are clinically acceptable. Analysis of ARI scores showed no significant differences in mode of bond failure among groups (P = 0.82).ConclusionsOrthodontic bracket pad shape has no effect on SBS and does not affect the mode of fracture pattern. Key words:Shear bond strength, orthodontic bracket, bracket pad shape, orthodontic bracket base shape, adhesive remnant index.