Effect of fiber orientation on interfacial fracture toughness for adhesively bonded composite plates

In this study, interfacial fracture toughness was investigated experimentally and numerically in laminated composite plates with different fiber reinforcement angles bonded with adhesive. The composite plates are four-layered and the layer sequence is [0º/θ]_s. DCB test was applied to composite plates reinforced with epoxy resin matrix and unidirectional carbon fiber. The experimental sample model for the DCB test was made using the ANSYS finite element package program. In the numerical study, four layered composites were prepared in three dimensions. Under critical displacement value; mode I fracture toughness at the crack tip was calculated using VCC (virtual crack closure) technique. Numerical values consistent with experimental results have presented in graphical forms. At 60o and 75° the greatest fracture toughness was obtained. In addition, numerical results have shown that fiber orientation prevents the uniform distribution of stress on the interface crack tip and causes stress accumulation, especially at the edge of the plate.

     

平纹编织C/SiC复合材料层间断裂行为试验研究

通过双悬臂梁和端部切口弯曲试验分别对平纹编织C/SiC复合材料的Ⅰ型(张开型)和Ⅱ型(滑开型)层间断裂行为进行试验研究,得到该材料以临界能量释放率GⅠC和GⅡC表征的层间断裂韧度值.试验后利用光学显微镜对两组试验的试样断口进行显微观察,以分析其破坏机理.结果表明:Ⅰ型层间断裂韧度值GⅠC和Ⅱ型层间断裂韧度值GⅡC分别为(737.2±57) J/m2和(1082.7±90) J/m2;Ⅰ型开裂为层间SiC基体沿初始裂纹方向的断裂破坏;Ⅱ型开裂与Ⅰ型开裂相似,但裂纹上下表面包裹碳纤维束的SiC基体发生脱落,并且出现碳纤维束中部分碳纤维剪切破坏.     

Interlaminar fracture toughness of CFRP laminates with silk fibers interleave

Interlaminar fracture toughness of CFRP laminates with silk fibers interleave was evaluated in this paper. Silk fibers were obtained from silkworm cocoon. Long silk fibers were wound around a bobbin and cut into specimen size. Resin films were bonded on both sides of a sheet of silk fibers. Silk fibers with resin films were put between [0₁₂] and [0₁₂] and cured by following the curing process. Evaluation of mode I and mode II interlaminar fracture toughness was accomplished by DCB and ENF test, respectively. Mode I interlaminar fracture toughness of CFRP laminates with silk fibers interleave was 59% higher than that of CFRP. Mode II interlaminar fracture toughness of CFRP laminates with silk fibers interleave was 44% higher than that of CFRP. It seems that CFRP laminates with silk fibers interleave will be useful to structures which need high interlaminar fracture toughness.     

Moving finite element analyses for fast crack propagation in sheet metal

The conventional fracture mechanics parameters K_IC and/or J_IC are used as fracture toughness criteria necessary for the start of crack propagation under plane strain conditions. These criteria are defined only for small-scale yielding or infinitesimal deformation, though actual fractures involve large plastic deformation. Hence, measurement of fracture resistance during crack propagation is difficult with the conventional parameters.Estimating the mechanical conditions around the propagating crack tip is very useful for reducing damage during accidental fracture. Therefore, establishing a criterion for crack propagation with large-scale yielding is very important for not only science fields but also some industrial fields. For fractures with large-scale yielding, micro- or mesoscale damage processes in the crack tip vicinity have to be considered.In this study, Gurson's constitutive model for void occurrence and growth was introduced into the finite element method to discuss failure behavior in the crack tip vicinity. Fast crack propagation behavior under high-speed deformation was simulated using the moving finite element method based on the Delaunay automatic triangulation. The excellent far-field integral path independence of the T* integral was verified for pure mode I fast crack propagation and non-straight crack propagation under mixed mode conditions. The void growth conditions near the crack propagation path were evaluated.     

Interfacial crack propagation under various mode-mixes

Initiation and propagation of interfacial crack along bimaterial interface are considered in this study. A biaxial loading device for a single specimen is used for obtaining a wide range of mode-mixities. The specimen is an edge-cracked bimaterial strip of glass and epoxy; the biaxial loading device, being capable of controlling displacements in two perpendicular directions, is developed. A series of interfacial crack initiation and propagation experiments are conducted using the biaxial loading device for various mixed modes. Normal crack opening displacement (NCOD) is measured near crack front by a crack opening interferometry and used for extracting fracture parameters. From mixed mode interfacial crack initiation experiments, large increase in toughness with shear components is observed. The behavior of interfacial crack propagation analyzed as a function of mode-mix shows that initial crack propagation is delayed with increase of mode-mixity, and its velocity is increased with positive mode-mixity but decreased with negative case. However, it is found that crack propagation is less accelerated with positive mode-mixity than the negative mode-mixity, which may be caused by contact and/or effects of friction between far field and near-tip field along the interfacial crack.     

Fatigue crack propagation behavior in STS304 under mixed-mode loading

The use of fracture mechanics has traditionally concentrated on crack growth under an opening mechanism. However, many service failures occur from cracks subjected to mixed-mode loading. Hence, it is necessary to evaluate the fatigue behavior under mixed-mode loading. Under mixed-mode loading, not only the fatigue crack propagation rate is of importance, but also the crack propagation direction. In modified range 0.3≤a/W≤0.5, the stress intensity factors (SIFs) of mode I and mode II for the compact tension shear (CTS) specimen were calculated by using elastic finite element analysis. The propagation behavior of the fatigue cracks of cold rolled stainless steels (STS304) under mixed-mode conditions was evaluated by using K_I and K_II(SIFs of mode I and mode II). The maximum tangential stress (MTS) criterion and stress intensity factor were applied to predict the crack propagation direction and the propagation behavior of fatigue cracks.     

Investigation of mechanical and tribological properties of amorphous diamond-like carbon coatings

We investigated the mechanical and tribological properties of amorphous diamond-like carbon (DLC) coatings deposited on Si(100) by a pulsed bias deposition technique. Tribological studies were performed using a pin-on-disc (POD) apparatus under a normal load of 6.25 N and at 10% relative humidity, with a ruby pin as a slider. Hardness measurements were performed using a nanoindenter and apparent fracture toughness using indentation techniques. We studied the influence of residual stresses on apparent fracture toughness. The data revealed that the thickness, hardness and compressive stress of the coating play different roles in the apparent fracture toughness. Crack initiation is influenced by the thickness and hardness of the coating, whereas crack propagation is influenced by the compressive stress in the film. The apparent fracture toughness of DLC coatings increased with coating hardness.     

Subsurface crack mechanisms under indentation loading

A linear elastic fracture mechanics analysis of plane-strain indentation of a homogeneous half-space with a subsurface horizontal crack was performed using the finite element method. Stress intensity factor results obtained for an infinite plate with a central crack subjected to far-field tension and a half-space with a frictionless subsurface horizontal crack under a moving surface point load are shown to be in good agreement with corresponding analytical results. Crack mechanism maps illustrating the occurrence of separation, forward and backward slip, stick, and separation at the crack interface are presented for different indentation load positions and crack face friction coefficients. Results for the stresses in the vicinity of the crack tips and the mode I and mode II stress intensity factors are given for different indentation positions, crack face friction coefficients, and both concentrated and distributed surface normal tractions. Although indentation produces a predominantly shear and compressive stress field, mode I loading conditions are shown to occur for certain indentation positions. However, the magnitude of the mode I stress intensity factor is significantly smaller than that of mode II, suggesting that in-plane shear mode crack growth is most likely to occur in the absence of microstructural defects. The significance of crack face friction and sharpness of the indenter on the subsurface shear mode crack propagation rate is interpreted in terms of the mode II stress intensity factor range and material behavior.     

10CrNiMo结构钢悬臂弯曲加载低周疲劳表面裂纹的扩展特性

通过COD位移规控制悬臂弯曲加载试样表面裂纹前缘的应变,采用逐级递增应变的试验方法,研究了10CrNiMo结构钢的低周疲劳表面裂纹的扩展特性,并用扫描电镜对断口形貌进行了观察。结果表明:表面裂纹扩展速率与裂纹前缘总应变范围之间满足良好的幂律关系;裂纹源附近出现轮胎花样,裂纹扩展初期挤压严重,扩展中期发现较多的二次裂纹,扩展后期疲劳辉纹清晰可见,最后断裂区具有典型的韧窝;裂纹按照锐化-钝化机制扩展,最后断裂模式为韧性断裂。     

Investigation of the fracture characteristics of the interfacial bond between bone and cement: experimental and finite element approaches

This study integrated the finite element method, fracture mechanics, and three-point bending test to investigate the fracture characteristics of the interfacial bond between bone and cement. The fracture tests indicated that the interfacial fracture toughness of the bone/cement specimens was 0.34 MN/m^3/2, with a standard deviation of 0.11 MN/m^3/2, which was in good agreement with the experimental data available in the literature. A finite element model of the experimental testing specimen was used to predict the critical stress intensity factor (SIF) at the fracture load by the proposed fracture analysis method. The critical SIF of the opening mode of the interface crack was 0.392 MN/m^3/2, which was slightly higher than the fracture toughness obtained in the experiment. Additionally, considering the coupled effects of the crack opening mode and shearing mode, the critical effective SIF was 0.411 MN/m^3/2, with a phase angle of 17.2°. Comparisons of the results obtained from the bending test and numerical analysis made it obvious that the fracture characteristics of the bonded interface between the bone and cement could be accurately predicted by the proposed model. With this analysis model, a realistic investigation on the debonding behavior of cemented artificial prosthetic components is highly expected.     

Numerical study on thermal stress cutting of silicon wafers using two-line laser beams

We propose a method of cleaving silicon wafers using two-line laser beams. The base principle is separating the silicon wafer using crack propagation caused by laser-induced thermal stress. Specifically, this method uses two-line laser beams parallel to the cutting line such that the movements of the laser beam along the cutting line can be omitted, which is necessary when using a point beam. To demonstrate the proposed method, 3D numerical analysis of a heat transfer and thermo-elasticity model was performed. Crack propagation was evaluated by comparing the stress intensity factor (SIF) at the crack tip with the fracture toughness of silicon, where crack propagation is assumed begin when the SIF exceeds the fracture toughness. The influences of laser power, line beam width, and distance between two laser beams were also investigated. The simulation results showed that the proposed method is appropriate for cleaving silicon wafers without any thermal damage.

     

微合金化元素钛对中碳非调质钢韧性的影响

探讨了钛对非调质钢的显微组织和变形过程的影响。测试了钢在平面应变状态下的断裂韧性,计算了冲击和拉伸状态下的裂纹形成能和扩展能。用SEM研究了断口形貌。结果表明,钢中加入钛可使先共析钛素体量增多,在冲击状态下,加入钛对韧性影响不大,钒钢和钒钛钢均为准解理断裂,但在静态拉伸时,钒钛钢为剪切断裂,钒钢为准解理断裂。     

Cohesive and continuum damage models applied to fracture characterization of bonded joints

In this work, two different methods for simulating damage propagation are presented and applied to fracture characterization of bonded joints in pure modes I and II. The cohesive damage model is based on a special developed interface finite element including a linear softening damage process. In the continuum damage model the softening process is performed by including a characteristic length associated with a given Gauss point. The models were applied to the simulation of “double cantilever beam” (DCB) and “end notched flexure” (ENF) tests used to obtain the critical strain release rates in mode I and II of bonded joints. In mode I it was observed, under certain conditions, a good agreement between the results obtained by the two models with the reference value of critical strain energy release rate in mode I (G_Ic), which is an inputted parameter. However, in mode II some discrepancies on the obtained G_IIc values were observed between the two models. These inaccuracies can be explained by the simplifying assumptions inherent to the cohesive model. Better results were achieved considering the crack equivalent concept.     

Finite element modelling of brittle fracture of thick coatings under normal and tangential loading

This paper addresses coating fracture in hard brittle coatings subjected to combined normal and tangential loads through a finite element based methodology. The coating is modelled as an elastic layer perfectly bonded to an elastic substrate with a pre-microcrack, assumed to initiate at the contact trailing edge due to high tensile stress. The predicted results are consistent with previously published coating fracture results. The model predicts a significant effect of coating thickness on crack propagation for coatings with large elastic mismatch and the final propagated crack profile is predicted to depend on friction coefficient, coating fracture toughness and sliding displacement.     

A comparative evaluation of dental luting cements by fracture toughness tests and fractography

In recent years there has been a shift from traditional methods of investigating dental materials to a fracture mechanics approach. Fracture toughness (KIC) is an intrinsic material property which can be considered to be a measure of a material's resistance to crack propagation. Glass-ionomer cements are biocompatible and bioactive dental restorative materials, but they suffer from poor fracture toughness and are extremely susceptible to dehydration. The main objective of this study was to evaluate the fracture toughness of three types of commercially available dental cements (polyacid-modified composite resin, resin-modified and conventional glass ionomer) using a short-rod chevron-notch test and to investigate and interpret the results by means of fractography using scanning electron microscopy. Ten specimens of each cement were fabricated according to manufacturers' instructions, coated in varnish, and stored at ambient laboratory humidity, 100 per cent relative humidity, or in water at 37 degrees C for 7 days prior to preparation for testing. Results indicated that significant differences existed between each group of materials and that the fracture toughness ranged from 0.27 to 0.72 MN/m3/2. It was concluded that the resin-modified glass-ionomer cement demonstrated the highest resistance to crack propagation. Fractographs clearly showed areas of stable and unstable crack growth along the fractured surfaces for the three materials examined.     

常用压力容器钢断裂行为研究

利用三点弯曲试验,测试了16MnR、20R和Q235B三种常用压力容器钢的断裂行为和断裂韧度,并用扫描电镜观察了试样的断口形貌。结果表明：三种材料常温下均呈现韧性断裂,所有试样断口均存在裂纹分层现象,其中以16MnR裂纹扩展区分层现象最不明显,而Q235B裂纹扩展区分层现象最明显;当裂纹扩展方向与分层方向不一致时,分层现象的存在阻止了裂纹的扩展,从而提高材料的强度与韧度。     

Mode I fracture toughness analysis of a single-layer grapheme sheet

To predict the fracture toughness of a single-layer graphene sheet (SLGS), analytical formulations were devised for the hexagonal honeycomb lattice using a linkage equivalent discrete frame structure. Broken bonds were identified by a sharp increase in the position of the atoms. As crack propagation progressed, the crack tip position and crack path were updated from broken bonds in the molecular dynamics (MD) model. At each step in the simulation, the atomic model was centered on the crack tip to adaptively follow its path. A new formula was derived analytically from the deformation and bending mechanism of solid-state carbon-carbon bonds so as to describe the mode I fracture of SLGS. The fracture toughness of single-layer graphene is governed by a competition between bond breaking and bond rotation at a crack tip. K-field based displacements were applied on the boundary of the micromechanical model, and FEM results were obtained and compared with theoretical findings. The critical stress intensity factor for a graphene sheet was found to be K _IC = 2.63 ~ 3.2MPa \(\sqrt m \) for the case of a zigzag crack.     

Dynamic crack growth in TDCB specimens

Dynamic crack propagation in tapered double cantilever beam (TDCB) specimens is analysed via beam theory and the finite element method. Steady state and transient solutions of the energy release rate G are given for various load conditions. Finite element analysis is performed to obtain the dynamic G at given crack speed or the crack history for a given fracture toughness. The stress wave effects on the dynamic G are discussed. The beam solutions are compared with the finite element results and some experimental phenomena are explained.     

Electron microscopy of crack/particle interactions in Al2O3/SiC nanocomposites

Crack/particle interactions in alumina/silicon carbide nanocomposites have been investigated by scanning electron microscopy and transmission electron microscopy, with cracks induced by Vickers microindentation. Intergranular cracks are frequently deflected into grains by SiC particles on grain boundaries inclined to the average direction of crack propagation. This mechanism is proposed to explain the change in the fracture mode from intergranular fracture for monolithic alumina to predominantly transgranular fracture for the nanocomposites. Neither stress-induced microcracking around SiC particles nor significant crack deflection by intragranular particles was found to occur in the nanocomposites. It is argued that an addition of nanoparticles may not be a promising approach for increasing the toughness of alumina.     

Experimental and numerical determination of critical stress intensity factor of aluminum curved thin sheets under tensile stress

We determined the fracture toughness of aluminum curved thin sheets using tensile stress tests and finite element method. We applied Linear elastic fracture mechanics (LEFM) and Feddersen procedure to evaluate stress intensity factor of the samples with central wire-cut cracks and fatigue cracks with different lengths to investigate the notch radius effect. Special fixture design was utilized to establish uniform stress distribution at the crack zone. Less than 9 % difference was found between the wire-cut and the fatigue cracked samples. Since generating central fatigue crack with different lengths required so much effort, wire-cut cracked samples were used to determine critical stress intensity factor. Finite element analysis was also performed on one-quarter of the specimen using both the singular Borsum elements and the regular isoparametric elements to further investigate fracture toughness of the samples. It was observed that the singular elements presented better results than the isoparametric ones. A slight difference was also found between the results obtained from finite element method using singular elements and the experimental results.