TC4钛合金的超高周疲劳行为及其竞争失效模型构建

针对现有模型对TC4竞争失效预测的不准确性,建立了基于最大应力强度因子的竞争失效模型。在室温以及两种应力比下,针对TC4钛合金进行超高周疲劳试验,通过试验与最弱键竞争失效理论相结合的方法进行评估,研究其超高周疲劳性能。通过对试样断口形貌的观察,可将其失效模式分为如下两类:表面失效以及内部失效。对试样表面缺陷以及内部解理刻面尺寸进行测量,并评估其最大应力强度因子值。进一步通过正态分布得到最大应力强度因子的累计分布函数,基于两参数泊松分布建立了与最大应力强度因子有关的竞争失效模型。通过模型计算结果,可以得出在任一最大应力强度因子下试样发生各种失效模式的概率,且经分析对比,本文中TC4两种疲劳失效模式的失效概率评估结果与试验数据吻合较好,为分析TC4钛合金超高周疲劳状态下的疲劳失效模式提出了新的评估方法。     

Characterising the Correlations of Failure Events: A 2‐D Block‐and‐Springs Model

Abstract: To mimic observations from acoustic emission experiments for random systems, we used a block‐and‐springs model to investigate the effect that increasing strain has on the locations of microscopic failure events leading to macroscopic failure across the sample. Model results show that failure events, which are initially located randomly throughout the sample, begin to cluster as stress build‐up near earlier failure events. At failure, the system‐wide fracture network was found to have a fractal dimension, D_f ≈ 1.29. To quantify the observed clustering, we applied a number of different measures of this space‐time behaviour: (i) the stress–strain curve; (ii) the total number of broken bonds and the average energy released by the broken bonds, (iii) the number distribution of cracks with s broken bonds, N(s), and the number distribution of cracks with s broken bonds or more, N(≥s), both of which follow power‐laws agreeing with earlier predictions; and (iv) the number–number and energy–energy correlations at time t between a failure event at position (x′, y ′) and a failure event at (x′ + x, y ′ + y). Our results quantify the short‐range clustering, exhibiting quantitatively and qualitatively different behaviour from the long‐range clustering at failure; our results also show that the energy released outpaces the number of broken bonds.     

Molecular mechanics modeling of deformation and failure of super carbon nanotube networks

A generalized molecular structure mechanics (MSM) model is proposed to investigate the deformation and failure behaviors of super carbon nanotubes (SCNTs) within the quasi-static approximation. The failure mechanism of the SCNTs with Y-?and X-type junctions was examined by combining a failure criterion for the breakage of the carbon-carbon bonds in the CNT networks. The carbon-carbon bonds are modeled as elastic bars with equivalent stiffness and break as their elongation ratio reaches only 19%, which means that the broken carbon-carbon bonds are ineffective in terms of the Morse potential function. It is shown that the MSM method, combined with the failure criterion of the carbon-carbon bonds, is a powerful approach to simulate the deformation and failure of both Y junctions and X junctions with different chiralities and sizes. The deformation and failure modes of these junctions which involve rotation, bending and stretching of the CNT arms are predicted using the present model and the effects of various parameters of the junctions on their mechanical behaviors are discussed.     

Fracture Analysis of a Special Cracked Lap Shear (CLS) Specimen with Utilization of Virtual Crack Closure Technique (VCCT) by Finite Element Methods

Some of the most important characteristics due to a fracture investigation of a special specimen are taken into account. Debonding considerations for a composite/steel cracked lap shear (CLS) specimen by utilization of finite element methods (FEM) as well as a virtual crack closure technique (VCCT) approach have been investigated. Strain energy release rate, delamination load case and direct cycle fatigue analysis have taken into consideration in this study, and the corresponding simulations have been done by ABAQUS/Standard. Linear elastic fracture criteria are used for validation of numerical results from the simulation. For comparison of three different categories of analysis, some special characteristics such as effective energy release rate ratio, bond state, time at bond failure and opening behind crack tip at bond failure have been illustrated. In this work, a detailed analysis of a special CLS specimen debonding by using VCCT and FEM is presented and varied results for validation of this kind of combination are obtained and have been discussed.     

Fracture characterisation of green-glued polyurethane adhesive bonds in Mode I

Unseasoned (green) spruce timber side boards of size 25 × 120 × 600 mm were flatwise-glued with a one-component PUR adhesive. Glued pairs of boards were then kiln-dried to 12 % moisture content. A special small-scale specimen for testing the fracture properties of the adhesive bond in Mode I was developed in order to evaluate the adhesive bond properties. The complete force versus deformation curve, including both the ascending and the descending parts, could be obtained with these small-scale specimens, enabling the strength and fracture energy of the bond line to be calculated. In addition, the fractured specimens were examined by scanning electron microscope. Results show that both the tensile strength and the fracture energy of the green glued PUR adhesive bonds were equal to those of the dry glued bonds. The methodology developed and used in the present study gives new possibilities for analysis of the mechanical behaviour of wood adhesive bonds, and particularly of their brittleness and its correlation with the type of fracture path. This is in sharp contrast to the use of standardised test methods (e.g. EN 302, ASTM D905) with specimens having relatively large glued areas. Using such types of specimens, it is not possible to obtain the complete force versus deformation response of the bond. In addition, when using such test methods, failure takes place in the wood or in the fibres near the bond, thus making it impossible to obtain detailed information about the bond line characteristics.     

A study of the failure mechanisms in Al/Al2O3 and Al/SiC composites through quantitative microscopy

The failure mechanisms in tension and fatigue of three Al alloys reinforced with ceramic particulates were studied by means of optical and scanning electron microscopy. Damage was concentrated in the reinforcements, which failed in a brittle fashion during deformation, leading to the specimen fracture when a critical fraction of broken particulates was reached in a given section of the specimen. This critical fraction was measured on polished longitudinal sections of broken specimens for each composite, temper, and loading condition, and was mainly dependent on the matrix alloy. It was also found that the reinforcement fracture probability was controlled by the particulate size and aspect ratio: large and elongated particulates oriented in the loading direction were more prone to fail than small, equiaxed ones. Finally, a significant fraction of the reinforcements in the cast materials was broken prior to testing. They were shattered during extrusion rather than fractured, and associated with clusters of particulates formed during solidification.     

Probability distribution of energetic-statistical size effect in quasibrittle fracture

The physical sources of randomness in quasibrittle fracture described by the cohesive crack model are discussed and theoretical arguments for the basic form of the probability distribution are presented. The probability distribution of the size effect on the nominal strength of structures made of heterogeneous quasibrittle materials is derived, under certain simplifying assumptions, from the nonlocal generalization of Weibull theory. Attention is limited to structures of positive geometry failing at the initiation of macroscopic crack growth from a zone of distributed cracking. It is shown that, for small structures, which do not dwarf the fracture process zone (FPZ), the mean size effect is deterministic, agreeing with the energetic size effect theory, which describes the size effect due to stress redistribution and the associated energy release caused by finite size of the FPZ formed before failure. Material randomness governs the statistical distribution of the nominal strength of structure and, for very large structure sizes, also the mean. The large-size and small-size asymptotic properties of size effect are determined, and the reasons for the existence of intermediate asymptotics are pointed out. Asymptotic matching is then used to obtain an approximate closed-form analytical expression for the probability distribution of failure load for any structure size. For large sizes, the probability distribution converges to the Weibull distribution for the weakest link model, and for small sizes, it converges to the Gaussian distribution justified by Daniels' fiber bundle model. Comparisons with experimental data on the size-dependence of the modulus of rupture of concrete and laminates are shown. Monte Carlo simulations with finite elements are the subject of ongoing studies by Pang at Northwestern University to be reported later.     

Modelling curved S–N curves

The fatigue limit distribution is estimated using fatigue data and under the assumption that the fatigue limit is random. The stress levels for the broken and unbroken specimens are used. For the broken specimen the number of cycles to failure is also used. By combining the finite life and fatigue limit distribution it is possible to get the probability of not surviving a certain life. This probability is used to estimate a curved S–N curve by using the method of likelihood. The whole S–N curve is estimated at the same time. These curves show the predictive life given a certain stress level. The life and the quantile of the fatigue limit distribution are also predicted by using profile predictive likelihood. In this way the scatter around the S–N curve as well as the uncertainty of the S–N curve are taken into account.     

Analysis of interfacial cracks in a TBC/superalloy system under thermal loading

Thermal barrier coatings (TBCs) provide thermal insulation to high temperature superalloys. Residual stresses develop in TBCs during cool down from processing temperatures and subsequent thermal cyclic loading due to the thermal expansion mismatch between the different layers (substrate, bond coat, and TBC). These residual stresses can initiate microcracks at the bond coat/TBC interface and can lead to debonding at the bond coat/TBC interface. The highest residual stresses occur at the interfaces. The effect of voids or crack like flaws at the interface can be responsible for initiating debonding and accelerate the oxidation process. The effect of interfacial microcracks has been investigated using the fracture mechanics approach. In particular, J-integral and the energy release rate G, for both mode I and mode II using the virtual crack extension method were evaluated. Two types of specimens were studied. The specimens were cooled down from processing temperature of 1000°C to 0°C. The variation of the properties as a function of temperature were used for the analysis. It was found that the use of temperature dependent properties in contrast to constant properties provide significantly different values of J-integral and G. For the stepped-disc specimen with an edge crack, crack growth is only due to mode II, while for the cylinder specimen with an internal crack, crack growth is due to mixed-mode loading. An important implication of this result is that edge delaminations in a disk specimen may only grow due to mode II conditions under pure thermal loading. Shear fracture characteristics of interfacial crack thus become important in the failure of the TBC.     

The random walk theory of crack propagation

Crack propagation is considered as a random walk process of consecutive atomic bond breaking and healing steps. The total number of steps is proportional to the propagation time, and the difference of the number of breaking and healing steps is proportional to the location of the crack tip at that time. The analysis of the probability and of the number of distinct sequences led to a rigorous expression of a crack size probability distribution function in terms of time, mechanical work, bond free energy or surface energy, and temperature. It is shown that the shape of the probability distribution function is proportional to the sum of the breaking and healing rate constants, and the average crack size is determined by the difference of these two rate constants and by the propagation time.     

Three-dimensional fracture analysis of CT specimens with a ductile damage model based on endochronic plasticity theory

For many fracture problems of practical engineering importance, the three-dimensional effects are significant and a three-dimensional analysis for the problems is thus required. In this paper, an endochronic theory coupled with anisotropic damage is first established, which is actually an elasto-plastic damage theory coupled with isotropic-nonlinear kinematic hardening. The ductile damage evolution equation is derived from the orthogonality rule with a new intrinsic time scale introduced especially for damage evolution. Then, a three-dimensional finite element program incorporating the endochronic damage model is formulated and emploved to analyze the widely used CT fracture specimen. Two failure criteria are proposed for the prediction of crack initiation direction and crack initiation load. From the analysis, significant three-dimensional effects are observed and the crack is estimated to initiate first at the middle of the crack front line. Experiments have been conducted to verify the proposed theory and the results are found to compare well with the theoretical values.     

Tensile fracture process of Strain Hardening Cementitious Composites by means of three-dimensional meso-scale analysis

There are a wide variety of short fiber reinforced cement composites. Among these materials are Strain Hardening Cementitious Composites (SHCC) that exhibit strain hardening and multiple cracking in tension. Quantitative material design methods considering the properties of matrix, fiber and their interface should be established. In addition, numerical models to simulate the fracture process including crack width and crack distribution for the material are needed.This paper introduces a numerical model for three-dimensional analysis of SHCC fracture, in which the salient features of the material meso-scale (i.e. matrix, fibers and their interface) are discretized. The fibers are randomly arranged within the specimen models. Load test simulations are conducted and compared with experimental results. It is seen that the proposed model can well simulate the tensile failure of Ultra High Performance-Strain Hardening Cementitious Composites (UHP-SHCC) including strain-hardening behavior and crack patterns. The effects of matrix strength, its probability distribution inside the specimen and fiber distribution on the tensile fracture are numerically investigated. Consideration of the probability distributions of material properties, such as matrix strength, appears to be essential for predicting the fracture process of SHCC.     

MODELLING THE TENSILE FRACTURE BEHAVIOUR OF THE REINFORCING FIBRE YARNS IN CERAMIC MATRIX COMPOSITES

Abstract— Using experimentally determined data on fibre radius distributions, yarn geometry, matrix and fibre elastic moduli and frictional shear stress at the matrix/fibre interface (obtained by nano-indentation experiments), the failure probability of the composite fibre yarns (after matrix cracking) is estimated. Each fibre is divided into a fixed number of segments above and below the matrix crack. The failure probability on every segment of each fibre is computed using Weibull fibre strength statistics. A fibre is assumed to be broken when the cumulative failure probability for the complete yarn reaches a value of 0.5. The segment and fibre are then selected at "random", according to their individual failure probabilities. After fibre failure, the broken fibre can only carry the frictional load and the load drop is transferred to its neighbours according to their distances to the broken fibre. The remote stress is then modified to match again the cumulative failure probability of 0.5 and a new fibre is broken. This procedure is repeated until all the fibres are broken. In this way, it is possible to obtain the "characteristic" load carried by the yarn and its corresponding elongation. Fibre extraction and pull-out behaviour are also considered. The roles of different load-transfer laws (from global to highly localised) are examined. The model is applied to simulate the fracture tensile behaviour of individual yarns of SiC/SiC ceramic-matrix composites. The results are compared with those obtained from tensile experiments on SiC/SiC individual yarns. The computed fracture morphology, in terms of individual pull-out lengths, is also compared to the actual SEM fractography of a woven SiC/SiC composite.     

Composite-to-metal tubular lap joints: strength and fatigue resistance

The axial strength and fatigue resistance of thick-walled, adhesively bonded E-glass composite-to-aluminum tubular lap joints have been measured for tensile and compressive loadings. The joint specimen bonds a 63 mm OD aluminium tube within each end of a 300 mm long, 6 mm thick E-glass/epoxy tube. Untapered, 12.5 mm thick aluminium adherends were used in all but four of the joint specimens. The aluminum adherends in the remaining four specimens were tapered to a thickness of 1 mm at the inner bond end (the bond end where the aluminum adherend terminates). For all loadings, joint failure initiates at the inner bond end as a crack grows in the adhesive adjacent to the interface. Test results for a tension-tension fatigue loading indicate that fatigue can severely degrade joint performance. Interestingly, measured tensile strength and fatigue resistance for joints with untapered adherends is substantially greater than compressive strength and fatigue resistance.The joint specimen has been analyzed in two different ways: one approach models the adhesive as an uncracked, elastic-perfectly plastic material, while the other approach uses a linear elastic fracture mechanics methodology. Results for the uncracked, elastic-plastic adhesive model indicate that observed bond failure occurs in the region of highest calculated stresses, extensive bond yielding occurs at load levels well below that required to fail the joint, and a tensile peel stress is generated by a compressive joint loading when the aluminum adherends are untapered. This latter result is consistent with the observed joint tensile-compressive strength differential. Results of the linear elastic fracture mechanics analysis of a joint with untapered aluminum adherends are also consistent with the observed differential strength effect since a mode I crack loading is predicted for a compressive joint loading. Calculations and a limited number of tests suggest that it may be possible to selectively control the differential strength effect by tapering the aluminum adherends. The effect of adherend material and thickness on fracture mechanics parameters is also investigated. The paper concludes by examining the applicability of linear elastic fracture mechanics to the joints tested.     

Microstructure and fracture in three dimensions

A high resolution three dimensional (3D) scanning technique called X-ray microtomography was used to measure internal crack growth in small mortar cylinders under compressive loading. Tomographic scans were made at different load increments in the same specimen. 3D image analysis was used to measure internal crack growth during each load increment. Load–deformation curves were used to measure the corresponding work of the external load on the specimen. Fracture energy was calculated using a linear elastic fracture mechanics approach using the measured surface area of the internal cracks created. The measured fracture energy was of the same magnitude that is typically measured in concrete tensile fracture. A nominally bilinear incremental fracture energy curve was measured. Separate components for crack formation energy and secondary toughening mechanisms are proposed. The secondary toughening mechanisms were found to be about three times the initial crack formation energy.     

Numerical investigation of fracture behavior of nanostructured Cu with bimodal grain size distribution

Nanostructured metals with bimodal grain size distribution, composed of coarse grain (CG) and nanograin (NG) regions, have proved to have high strength and good ductility. Here, numerical investigation, based on the mechanism-based strain gradient plasticity theory and the Johnson–Cook failure model, focuses on effects of (1) distribution characteristics of the CG regions and (2) the constitutive relation of the NG with different grain sizes on fracture behavior in a center-cracked tension specimen of bimodal nanostructured Cu. High strain rate simulations show that both of them directly influence load response and energy history, and importantly, they are closely related to the fracture pattern. This study shows that both CG region bridging and crack deflection toughen the bimodal nanostructured Cu significantly, while debonding enhances the overall ductility moderately. Simulations also show that with volume fraction of the CG regions increasing, both structural strength and ductility of the bimodal nanostructured Cu specimen can be improved.     

Mechanical characterization of glass/polyimide microjoints fabricated using cw fiber and diode lasers

This paper discusses the laser-irradiated microjoints between glass and polyimide for applications in neural implants. To facilitate bonding between them, a thin titanium film with a thickness of approximately 0.2 μm was deposited on glass wafers using the physical vapor deposition (PVD) process. Two sets of samples were fabricated where the bonds were created using diode and fiber lasers. The samples were subjected to tension using a microtester for bond strength measurements. The failure strengths of the bonds generated using fiber laser are quite consistent, while a wide variation of failure strengths are observed for the bonds generated with diode laser. Few untested samples were sectioned and the microstructures near the bond areas were studied using an optical microscope. The images revealed the presence of a sharp crack in the glass substrate near the bond generated with the diode laser. However, no such crack was observed in the samples made using fiber laser. To investigate the reasons behind such discrepancy in bond quality further, uncoupled three-dimensional finite element analyses (FEA) were conducted only for the samples created using diode laser. First, the transient heat diffusion-based FEA was conducted by using the laser power intensity distribution as a time dependent heat source. This model calculates the temperature distribution within the substrates as a function of time. Next, the structural model predicts the amount of residual stresses developed in the joint system as it is cooled down to room temperature. The out-of-plane normal component of residual stresses was within the failure strength range of glass that may have caused fracture initiation in the substrate.     

Bonding to aged surfaces: A thermally-aged epoxy

It is common experience that aged surfaces are often difficult to bond to. We report an examination of bonding to thermally-aged epoxy surfaces, using as the adhesive the same epoxy as that of the aged surface. The cured and postcured epoxy was aged at 200 ° C, with the ageing time varying from 2 to 8 h. The fracture energy of the bond line was measured by mode I cleavage under conditions of relatively slow crack growth. The bondline fracture energy was found to decrease logarithmically with ageing time. The fracture energies for bonds to surfaces aged for 2, 4, and 8 h at 200 ° C were 0.077, 0.059, and 0.050 kJ M^–2, respectively. These compare to 0.13 kJ M^–2 for a bond to an unaged surface and 0.21 kJ m^–2 for bulk fracture. Fracture surfaces resulting from both slow and rapid fracture were examined by optical and scanning electron microscopy. Fracture features different from those arising from bulk fracture were found. Areas with good adhesion occurred amidst fields of featureless fracture surface; the frequency and size of these areas decreased with increased ageing time. Evidence of plastic deformation was found, always occurring on the new side of the bond: ridges parallel with crack propagation at high crack speeds and subsurface undulations perpendicular to crack propagation at low speeds. The bond has the effect of channelling the crack along the bondline, but fracture does not always remain exactly at the interface. Fracture often occurred a relatively constant distance away from the interface, suggesting that the presence of the interface was felt for some distance.     

Some basic aspects of electromagnetic radiation during crack propagation in metals

Measurements of the electromagnetic radiation (EMR) emitted during crack propagation and fracture and the effect of modes of fracture, physical properties and high temperature on the characteristics of emitted EMR from metals have been discussed. It has been observed that all the three modes of fracture give rise to EMR emission; however, the relative amplitude in tearing mode is very low. A linear variation of EMR peak voltage with bond energy has been observed while frequency varies parabolically with bond energy. Both these curves indicate that no EMR emission or negligible EMR emission is expected in metals having bond energy <270 kJ/mole. EMR characteristics decrease with increase in lattice parameter. Higher tensile strength metals emit stronger EMR signals. Experiments conducted at high temperatures validate the prediction of Molotskii that an increase in specimen temperature should decrease the EMR frequency. One additional but important observation has been that while the EMR peak amplitude decreases with increase in temperature in steel, it increases with increase in temperature in copper.