Fracture R-curve characterization of toughened epoxy adhesives

The R-curve behavior of two different rubber-toughened epoxy adhesives was measured as a function of the mode ratio. A bilinear model was used to characterize the fracture resistance (R-curve) behavior from crack initiation to steady-state crack propagation. Experiments showed that the model parameters depended strongly on the loading mode ratio and the adhesive bondline thickness, but were largely independent of the crack initiation geometry. The results are relevant to the prediction of the crack initiation load and ultimate strength of adhesive joints having relatively short overlap lengths such that a steady-state damage zone cannot develop prior to rupture.     

Evaluation of R-curve behavior of ceramic-metal functionally graded materials by stable crack growth

This paper deals with the fracture toughness and R-curve behavior of ceramic-metal functionally graded materials (FGMs). A possibility of stable crack growth in a three-point-bending specimen is examined based on the driving force and resistance for crack growth in FGMs, and the distribution of fracture toughness or R-curve behavior is evaluated on FGMs fabricated by powder metallurgy using partially stabilized zirconia (PSZ) and stainless steel (SUS 304). The materials have a functionally graded surface layer (FGM layer) with a thickness of 1 mm or 2 mm on a SUS 304 substrate. Three-point-bending tests are carried out on a rectangular specimen with a very short crack in the ceramics surface. On the three-point-bending test, a crack is initiated from a short pre-crack in unstable manner, and then it propagates in stable manner through the FGM layer with an increase in the applied load. From the relationship between applied load and crack length during the stable crack growth in the FGM layer, the fracture toughness is evaluated. The fracture toughness increases with an increase in a volume fraction of SUS 304 phase.     

Effect of geometry and electrical boundary conditions on R-curves for lead zirconate titanate ceramics

R-curve measurements on PZT poled in thickness direction were carried out on CT specimens under different electric boundary conditions. The effect of specimen geometry was evaluated by measuring R-curves in CT specimens of different thickness and comparing these with R-curves in bend bars. A low coercive stress is responsible for the development of a large switching zone. This switching zone is of high relevance for the computation of the actual stress intensity factor at the crack tip and for the R-curve calculation.     

Influence of specimen geometry and size-effect on the <Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="Italic">R</Emphasis></Subscript>-curve based on the cohesive stress in concrete

Comparative study for determining the K _R -curves associated with the cohesive stress distribution for complete fracture process for two standard specimen geometries i.e., three-point bending test and compact tension test specimen geometries of concrete using analytical method and weight function approach is reported in the paper. The laboratory size specimen (100 ≤  D  ≤  400 mm) with initial-notch length/depth ratios 0.3 and 0.5 are considered in the investigation. The load-crack opening displacement curves for these specimens are obtained using well known version of Fictitious Crack Model (FCM). It is found from the numerical results that the weight function method improves computational efficiency without any appreciable error. The stability analysis on the K _R -curves and the influence of specimen geometry and the size-effect on the K _R -curves, the CTOD-curves and the process zone length during crack propagation of complete fracture process are also described.     

Debonding analysis of fiber-reinforced-polymer strengthened beams: Cohesive zone modeling versus a linear elastic fracture mechanics approach

A linear elastic fracture mechanics (LEFM) approach and a cohesive interface (cohesive zone) modeling approach to the debonding analysis of concrete beams strengthened with externally bonded fiber-reinforced-polymer (FRP) strips are studied and compared. The analytical models that are based on the two approaches are presented and discussed. The cohesive interface model is formulated using a potential function and it takes into account the shear effects, the effect of the peeling stresses, and the coupling of the shear and the peeling effects. This model takes the form of a set on nonlinear differential equations. The LEFM model combines stress analysis using the high order theory and fracture analysis using the concepts of the energy release rate and the J-integral. In addition, an algorithm that converts the results of the LEFM model into the equilibrium path of the debonding process is developed. The main advantages and disadvantages of the two approaches are also discussed. The two approaches are compared in terms of their applicability to quantify and describe the debonding process in various cases that include a single shear test, an edge peeling test, and a beam specimen strengthened with FRP.     

Examination of the 4-ENF test for measuring the mode III R-curve of wood

The four-point bend end-notched flexure (4-ENF) test, which was originally developed for measuring the mode II R-curve, is thought to be applicable for measuring the mode III R-curve. In this study, a 4-ENF fracture test of spruce was conducted for obtaining the mode III R-curve, and the test method was numerically and experimentally analyzed. In the numerical analysis, three-dimensional finite element calculations were conducted to determine the distribution of the strain energy release rate along the delamination front by the virtual crack closure technique (VCCT). In the experimental analysis, the mode III R-curve was examined by the modified beam theory and compliance calibration methods of data reduction, which have been conventionally used for analyzing the mode I or mode II R-curve. In addition to these conventional data reduction methods, the strain at each loading point was measured, as was the loading-line displacement and critical load for crack propagation, and the R-curve was obtained by the combination of loading-line compliance, load-longitudinal strain compliance, and critical load for crack propagation, which is named the “compliance combination method”. The finite element analyses suggested that the pure mode III fracture state existed in the mid-section of the specimen in spite of the existence of a small mode II component at the free edges of the delamination front, and the mode III strain energy release rate component calculated by the VCCT coincided well with those obtained by the data reduction methods examined here. The actual R-curve obtained by the compliance combination method coincided well with those by the modified beam theory and compliance calibration methods when the strain was appropriately measured. From these results, therefore, the 4-ENF fracture test is a promising means for obtaining the mode III R-curve of wood.     

An asymptotic approach to size effect on fracture toughness and fracture energy of composites

Size effect on fracture toughness and fracture energy of composites is investigated by a simple asymptotic approach. This asymptotic analysis based on the elastic/plastic fracture transition of a large plate with a small edge crack is extended to study fracture of composite. A reference crack length, a^*, is used in the model, which indicates an ideal elastic/plastic fracture transition defined by the yield strength and plane strain fracture toughness criteria. Experimental results of cementitious materials available in literature are analyzed and compared. It is shown that the common K_R-curves can also be obtained by the current asymptotic model. Furthermore, a local fracture energy distribution concept is also discussed and compared with the present asymptotic approach.     

The jump-like crack growth model, the estimation of fracture energy and JR curve

In this paper the jump-like crack growth model for monotonic loading is applied to re-examine both the onset of crack growth and process of stable crack growth. In the former case the fracture energy associated with a new surface creation is estimated and the in-plane constraint influence on this quantity is examined using the J-A₂ approach. In the later case the formula to compute the J-resistance curve is re-examined and compared with the one known from the standards. In the analysis the plane strain model of a structural element made of elastic-plastic material is assumed.     

Study of the tensile properties of stinging nettle fibres (Urtica dioica)

Developing new natural fibre composites is the focus of many studies today. Indeed, they are made out of renewable resources and, therefore, have a lower environmental impact in comparison to mineral fibre composites. The mechanical performances of stinging nettle fibres are measured and compared to flax and other lignocellulosic fibres. The stress/strain curve of stinging nettle fibres (Urtica dioica) shows they have a linear behaviour. The average tensile properties are a Young's modulus equal to 87 GPa, a tensile strength equal to 1594 MPa, and a strain at failure equal to 2.11%.     

Estimation of the transient creep parameter C(t) under combined mechanical and thermal stresses

This paper reports finite element analyses for an axially loaded cylinder with three different types of thermal stress. The magnitudes of the thermal and mechanical loadings are varied and creep crack tip parameters are evaluated for part-through-wall and fully penetrating defects with a range of sizes. The relaxation rate for the creep crack tip parameter is examined in detail and is found to depend on the magnitudes of the mechanical and thermal loads and on whether or not there is plasticity on initial loading. Simplified formulae are developed for describing the rate of relaxation by a modified redistribution time.     

In situ observation of crack propagation in ESP (engineered stress profile) glass

Important features of the ESP (engineered stress profile) glasses are the crack arrest and multiple cracking phenomena that occur even in an unstable stress field. In this work a detailed “in situ” observation of crack observation and analysis was performed with the aim to examine crack propagation in detail and relate it to the residual stress field produced by ion exchange and to the final mechanical performances of the material. The results showed that the peculiar residual stress field with a maximum below the surface is responsible for the formation of a multitude of stable cracks on the tensile surface of the glass that evolved into through-thickness flaws. The propagation within the material is limited by the increasing compressive residual stress, which also leads to kinking of the cracks in a direction parallel to the surface. The observed fracture phenomena are also responsible for a shielding effect that makes the measured failure resistance of ESP glass larger than predicted by simplistic single crack models.     

On assumptions associated with ΔKeff and their implications on FCG predictions

In this paper, an assessment of commonly used assumptions associated with ΔK_eff and their implications on FCG predictions in light of existing experimental and numerical data is presented. In particular, the following assumptions are examined: (1). ΔK_eff fully describes cyclic stresses and strains at the crack-tip vicinity. (2). K_op can be determined experimentally or numerically with certain accuracy. (3). Overload alters K_op but not K_max and associated σ_max at the crack-tip ‘process zone’. (4). Contact of crack faces curtails the crack driving force in terms of ΔK_eff.The analysis indicates that there is insufficient support to justify the above assumptions. In contrary, the analysis demonstrates that a two-parameter fatigue crack driving force in terms of ΔK and K_max, which accounts for both applied and the internal stresses should be used in FCG analyses and predictions.     

Comparison of creep crack growth rate in heat affected zone of welded joint for 9%Cr ferritic heat resistant steel based on C, dδ/dt, K and Q parameters

Creep crack growth behavior is very sensitive to the materials’ micro-structures such as the heat affected zone of a weld joint. This is a main issue to be clarified for 9%Cr ferritic heat resistant steel for their application in structural components. In this paper, high temperature creep crack growth tests were conducted on CT specimens with cracks in the heat affected zone of weld joints of W added 9%Cr ferritic heat resistant steel, ASME grade P92. The creep crack growth behavior in the heat affected zone of welded joint was investigated using the Q^∗ concept following which the algorithm of predicting the life of creep crack growth has been proposed. Furthermore, three-dimensional elastic-plastic creep FEM analyses were conducted and the effect of stress multiaxiality of welded joint on creep crack growth rate was discussed as compared with that of base metal.     

Preparation of (ZnO)1−x(GaN)x:Mn powder and its photoluminescence properties

(ZnO)_1−x(GaN)_x:Mn²⁺ powder was prepared by a conventional solid-state reaction under an NH₃ gas flow. The sample preparation conditions including the mixing ratio of the raw materials, the annealing temperature, and the annealing time were varied. The crystallinity and the photoluminescence (PL) intensity of this fluorescent material were improved by increasing the amount of ZnO and by increasing the annealing time, and no changes was observed in the PL wavelength. The crystallinity of the samples was enhanced and the PL intensity increased markedly at annealing temperatures of 700 °C and 800 °C, respectively. Moreover, it was clarified that the sample could be synthesized at annealing temperatures of above about 650 °C.     

Effects of deposition parameters on SiCln (n=0-2) densities in SiCl4 glow discharge plasma

The relative densities of SiCl_n (n=0-2) in SiCl₄ radio frequency (rf) glow discharge plasma are measured by mass spectrometry. The effects of discharge parameters, including rf power, discharge pressure, substrate temperature and SiCl₄ flow rate on the relative densities of SiCl_n (n=0-2) are investigated in detail. The experimental results demonstrate that the relative densities of SiCl_n (n=0-2) in SiCl₄ plasma are dependent strongly on these discharge parameters. An optimum configuration of discharge parameters (low rf power, high discharge pressure, low substrate temperature and low flow rate), which enhanced the formation of SiCl_n (n=0-2) radicals, was searched. Further, researching of SiCl_n (n=0-2) spatial distribution for seeking a suitable deposition condition is beneficial for understanding the deposition mechanism of thin films.     

Structural and magnetic properties of CoFe2O4 thin films deposited by laser ablation on Si (001) substrates

Cobalt ferrite (CoFe₂O₄) thin films have been deposited on Si (001) substrates, with different substrate temperatures (T_dep = 25 °C − 600 °C). The films were prepared by pulsed laser ablation with a KrF excimer laser (wavelength λ = 248 nm). The oxygen pressure during deposition was 2 × 10⁻² mbar. The films structure was studied by X-ray diffraction (XRD) and their surface was examined by scanning electron microscopy (SEM). The magnetic properties were measured with a vibrating sample magnetometer (VSM). For low deposition temperatures, the films presented a mixture of a CoFe₂O₄ phase, with the cubic spinel structure, and cobalt and iron antiferromagnet oxides with CoO and FeO stoichiometries. As the deposition temperature increased, the CoO and FeO relative content strongly decreased, so that for T_dep = 600 °C the films were composed mainly by polycrystalline CoFe₂O₄. The magnetic hysteresis cycles measured in the films were horizontally shifted due to an exchange coupling field (H_exch) originated by the presence of the antiferromagnetic phases. The exchange field decreased with increasing deposition temperature, and was accompanied by a corresponding increase of the coercivity and remanence ratio of the cycles. This behavior was due to the strong reduction of the CoO and FeO content, and to the corresponding dominance of the CoFe₂O₄ phase on the magnetic properties of the thin films.     

J-Resistance curve testing of HY80 steel using SE(B) specimens and normalization method

The normalization method is adopted for standard and nonstandard specimens in this paper to develop J-R curves for HY80 steel directly from load versus load-line displacement records without use of automatic crack length measurement. The standard specimens usually contain high crack-tip constraints, while the nonstandard specimens involve low crack-tip constraints. To obtain J-R curves with different constraints, a series of single edge notched bend (SE(B)) specimens with different crack lengths for an HY80 steel are tested in accordance with ASTM standard E1820. The normalization method is then used for determining crack extension and J-R curves for these SE(B) specimens.To validate the normalization method, the J-R curves determined using the normalization method are compared with those obtained by the elastic unloading compliance method for the SE(B) specimens. The comparison shows that good agreements exist between the two methods, and the normalization method is a viable tool to be used to determine J-R curves of the HY80 steel for the standard as well as nonstandard SE(B) specimens. In the J-integral calculations, the resistance curve test method, the basic test method and the modified basic test method specified in ASTM E1820 are evaluated. The results indicate that the modified basic method can be equivalent to the resistance curve method.     

Influence of Se on the electron mobility in thermal evaporated Bi2(Te1−xSex)3 thin films

Thermoelectric solid solutions of Bi₂ (Te_1−xSe_x)₃ with x = 0, 0.2, 0.4, 0.6, 0.8 and 1 were grown using the Bridgman technique. Thin films of these materials of different compositions were prepared by conventional thermal evaporation of the prepared bulk materials. The temperature dependence of the electrical conductivity σ, free carriers concentration n, mobility μ_H, and seebeck coefficient S, of the as-deposited and films annealed at different temperatures, have been studied at temperature ranging from 300 to 500 K. The temperature dependence of σ revealed an intrinsic conduction mechanism above 400 K, while for temperatures less than 400 K an extrinsic conduction is dominant.The activation energy, ΔE, and the energy gap, E_g, were found to increase with increasing Se content. The variation of S with temperature revealed that the samples with different compositions x are degenerate semiconductors with n-type conduction. Both, the annealing and composition effects on Hall constant, R_H, density of electron carriers, n, Hall mobility, μ_H, and the effective mass, m^∗/m₀ are studied in the above temperature range.     

新型非线性光学晶体Ca4RO(BO3)3的研究进展

作为新型非线性光学材料,钙－稀土硼酸盐晶体Ｃａ４ＲＯ（ＢＯ３）３（Ｒ－Ｌａ＾３＋、Ｎｄ＾３＋、Ｓｍ＾３＋、Ｇｄ＾３＋、Ｙ＾３＋、Ｅｒ＾３＋、Ｔｂ＾３＋、Ｌｕ＾３＋）近来引起了广泛的重视,该系列晶体属非中心对称的单斜晶系,空间群Ｃｍ。本文综术字上述晶体的结构和生长研究的进展,总结了晶体的线性光学／非线性光学性能以及要晶体的激光自倍频性能,指出这些晶体在非线性光学领域潜在的应用前景。