Brittle fracture evaluation of a fine grain cement mortar in combined tensile-shear deformation

Different laboratory experiments are usually conducted to characterize the fracture behaviour and integrity properties of newly developed structural materials. However, the reported fracture tests data for an improved high strength cement mortar (HSCM) under combined tension-shear loading are not in agreement with theoretical predictions obtained from well-known fracture criteria. It is shown in this paper that the significant difference existing between the experimental and theoretical results is due to ignoring the effect of  T -stress on the processes of crack growth in the HSCM test specimens. A modified fracture model is then used to show that the theoretical predictions can be corroborated by the experimental results when the effect of  T -stress is taken into account.     

The fracture behavior of functionally graded piezoelectric materials with dielectric cracks

This article provides a comprehensive investigation on the fracture behavior of cracked functionally graded piezoelectric materials (FGPMs). To account for the effect of dielectric medium inside the crack upon the fracture behavior of FGPMs, a dielectric crack model is used in this work, in which the electric boundary condition along crack surfaces is deformation-dependent and is nonlinear. The analytical formulations are developed using Fourier transform technique and solving the nonlinear singular equations using Chebyshev polynomials. A solution technique is developed to determine the desired deformation mode of the crack. Numerical simulations are given to show the effects of material gradient and the dielectric medium filling the crack upon the fracture behavior of FGPMs. The results obtained from this dielectric crack model clearly demonstrate how the transition between electrically impermeable and permeable crack models occurs with the change of crack opening displacement in response to the applied electromechanical loads. It is also observed that a critical state for the applied electromechanical loading exists for FGPMs that determines whether the impermeable (or permeable) crack model serves as the upper or lower bound for the dielectric crack model considering the effect of dielectric medium filling the crack.     

Experiment and analysis of unidirectional CFRP with a hole and crack as sandwich-form inhomogeneous composite

In modern mechanical industry, many designs are incorporating lightweight materials. Known as the future’s lightweight material; Carbon Fiber Reinforced Plastic (CFRP) contains a fiber structure and therefore is vulnerable to cracks and holes that may be generated during machining. However, in a mechanical structure, it requires bonding the multiple parts using bolts, rivets, etc. In other words, it inevitably requires generating holes in the mechanical structure. Because of such problems, CFRP has been attached with a normally used structural steel, to lighten the weight. It is quite difficult to understand the mechanical features of inhomogeneous composites. In order to increase the usage of inhomogeneous composites that contain multiple light weight mechanical parts, there is a necessity of mechanical basic data. This study used CFRP and SM45C that has the stacking angle [0/60/–60/0]. It has a form of a sandwich in which 5 mm thick SM45C acts as the core, with 2 mm thick unidirectional CFRP attached on both sides of it. Using inhomogeneous composites of such sandwich form, Compact Tension (CT) test and analysis took place. In the test, considering the actual material being used in mechanical industry, a hole was made on the edge of the crack and compared the results according to the location of the hole. SIMADZU corporation’s Universal Testing Machine was used in the test. Setting the test as the basis, we constructed a model for the analysis. In the analysis, like we did in the experiment, we kept in mind the fiber structure of unidirectional CFRP and the conditions of the analysis were identical to the test. As the test result, the magnitude was at its optimal when there was no hole. But if a hole had to be made, it is at its best when placed at the spot where the center of the hole is 20 mm from the edge of the crack. A model with no hole had the reaction force of 14 kN and the forced displacement was 4 mm. A model with a hole placed 20 mm from the edge of the crack had the reaction force of 11 kN with the forced displacement of 10 mm. Test results and analysis differ a lot, but the data after the section when the reaction force rapidly arises are reliable. In other words, in the process of destruction due to the crack propagation, the analysis is reliable.     

Three-dimensional numerical investigation of brittle bond fracture

A fracture mechanics based analysis of interface bond failure is presented. The bond edge is regarded as an interface crack front loaded under combined mode 1, 2 and 3 loading, and results are obtained for the critical stress for initiation of bond failure and the location along the bond edge where failure is initiated. A numerical procedure is formulated to study the propagation of the interface crack following initiation. Assuming that the crack propagates at the interface, a criterion for propagation is formulated, and it is shown that the crack front shape predicted is consistent with the basic interface fracture mechanics assuming quasi-static crack propagation. Results for the bond strength are presented for different fracture criteria and different bond shapes.     

Mixed Bending-Tension (MBT) test for mode I interlaminar and intralaminar fracture

A new Mixed Bending-Tension (MBT) test is proposed for mode I fracture of laminated composites. The MBT specimen consists of a relatively small pre-cracked laminate adhesively bonded to pin-loaded steel beams. This design reduces significantly the bending stresses that prevent successful application of DCB tests to certain laminates. The MBT was here applied to carbon/epoxy unidirectional [0°]₂₆ and [90°]₂₆ laminates with starter delaminations. Interlaminar initiation G_IC values of [0°]₂₆ laminates agreed well with previous DCB test results, while [90°]₂₆ laminates exhibited 50% higher values. Significant lengths of fairly planar intralaminar crack propagation were seen in the latter laminates. The results showed a fibre bridging related R-curve, which was more pronounced in [0°]₂₆ laminates. The consistency of the present results indicates that the MBT opens new possibilities for the interlaminar and intralaminar mode I fracture.     

Bridging effect on mode I fatigue delamination behavior in composite laminates

This paper discusses the bridging effect of fibres on mode I fatigue delamination growth in unidirectional and multidirectional polymer composite laminates based on a series of double cantilever beam (DCB) tests. From the results, there is sufficient evidence that fibre bridging can decrease the crack growth rate da/dN significantly, and using only one fatigue resistance curve to determine the delamination behavior in composite materials with large-scale fibre bridging may be inadequate. The bridging created in fatigue delamination is different from that of quasi-static delamination at the same crack length. So it is incorrect to use the resistance curve (R-curve) from quasi-static delamination tests to normalize fatigue delamination results.     

Thermal fracture analysis of a functionally graded orthotropic strip with a crack

This paper is concerned with the thermal fracture problem of a functionally graded orthotropic strip, where the crack is situated parallel to the free edges. All the material properties are assumed to be dependent only on the coordinate y (perpendicular to the crack surfaces). By using Fourier transform, the thermoelastic problem is reduced to those that involve a system of singular integral equations. Numerical results are presented to show the effects of the crack position and the material distribution on the thermal stress intensity factors.     

Electrochemical behavior of a lead-free SnAg solder alloy affected by the microstructure array

The aim of this study is to evaluate the electrochemical corrosion behavior of a Sn–Ag solder alloy in a 0.5 M NaCl solution at 25 °C as a function of microstructural characteristics. Different microstructure morphologies, which can be found in Sn–Ag solder joints and that are imposed by the local solidification cooling rate, are evaluated and correlated to the resulting scale of the dendritic matrix and the morphology of the Ag₃Sn intermetallic compound. Cylindrical metallic molds at two different initial temperatures were employed permitting the effect of 0.15 °C/s and 0.02 °C/s cooling rates on the microstructure pattern to be experimentally examined. Electrochemical impedance spectroscopy (EIS) diagrams, potentiodynamic polarization curves and an equivalent circuit analysis were used to evaluate the electrochemical parameters. It was found that higher cooling rates during solidification are associated with fine dendritic arrays and a mixture of spheroids and fiber-like Ag₃Sn particles which result in better corrosion resistance than coarse dendrite arrays associated with a mixture of fibers and plate-like Ag₃Sn morphologies which result from very slow cooling rates.     

Effect of a novel thixoforming process on the microstructure and fracture behavior of A356 aluminum alloy

In the present study, a novel thixoforming process for semi-solid deformation of A356 aluminum alloy is introduced using a continuous hot deformation process to the temperature being lower than the eutectic temperature of the alloy. A new hypothesis was introduced and the deformation mechanism of the alloy was investigated using the presented hypothesis. Microstructure and fracture surfaces of thixoformed samples were investigated using image analyzing technique and scanning electron microscopy. Obtained results indicated that this novel thixoforming process produces fine and compact silicon particles, dispersed uniformly in the microstructure of the alloy, compared to those produced by conventional thixoforming and gravity-cast processes with large and integrated morphology for silicon particles. The production stages of these silicon particles in this process were well documented by mentioned hypothesis. In order to investigate the effect of this novel process on mechanical properties of A356 alloy, tensile tests were conducted on produced samples. It was found that morphological changes of silicon particles as well as increasing the density ratio of samples in this process have a remarkable effect on enhancing the mechanical properties of produced alloy in comparison with other production routes. A new combination parameter, i.e. silicon density ratio (SDR) index was introduced. This parameter correlates the mechanical properties of samples to morphological properties of silicon particles and density ratio of them. Results of the study also indicated that samples with low SDR index have superior mechanical properties and consequently intergranular fracture mode.     

Acoustic emission and fracture behavior of GFRP woven laminates at cryogenic temperatures

This paper describes an experimental and analytical study on fracture and damage behavior of GFRP woven laminates at cryogenic temperatures. CT (compact tension) tests were carried out at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) to evaluate the critical values of the fracture mechanics parameters. During the CT tests, AE (acoustic emission) method was implemented. AE signals can identify the critical load at which gross failure occurs. A FEA (finite element analysis) was also applied to calculate the fracture mechanics parameters. The failure criteria (Hoffman criterion and maximum strain criterion) or the damage variable based on the continuum damage mechanics was incorporated into the model to interpret the experimental measurements and to study the damage distributions within the specimen. Several methods of calculating J-integral are discussed.     

Behaviors of a cracked lapped joint under mixed mode loading

Fatigue failure of steel connections is a common failure mechanism, especially for structures which sustain heavy cyclic loads like steel bridges. In this paper, lapped bolted joints were modeled numerically to study the effect of a crack on the ultimate response of the joint. The site of crack initiation was located under different mixed mode loading in single and multiple bolts joints. The effects of axial to transverse loading ratio or load mixity (LM = F_x/F_y), friction coefficient (μ), and bolt diameter were analyzed. For a single bolt pin-joint, by increasing LM, the crack initiation site angle (γ) increased up to a certain value at which it became constant (γ_f) independent of LM. This value γ_f depended only on the coefficient of friction and the bolt hole diameter. Stress intensity factor and crack path of a propagated crack emanating from the predicted crack initiation site were analyzed in the lapped joints under either mode I or mixed mode loading. It was found that, for multiple-bolt joints, loaded with mixed mode loading, the crack path remained approximately horizontal like that for mode I loading. For pin joints, the crack path remained at the direction of the crack initiation. The numerical model developed was validated using existing experimental results for the initial stiffness of the bolted joint and using theoretical prediction of the stress intensity factor. A parametric study for different bolt diameters and numbers was developed to study the behavior of these connections under double and single side cracks. It was found that the crack advancement in a specific bolt hole may cause crack to initiate in other bolt holes, due to the increase of the stress concentration factor (SCF), K_t.     

Effects of microstructural through‐thickness non‐uniformity and crack size on fatigue crack propagation and fracture of rolled Al‐7075 alloy

Some of the fatigue tests performed using the standard compact tension (CT) and a non‐standard specimen made of rolled 7075 aluminium alloy exhibit fatigue crack growth (FCG) lagging in a small region along the crack front. Through‐thickness microstructural evaluation shows that material grains in this region did not flatten as much as other regions. In the non‐standard specimen, surface cracks are either grown under fatigue loading or broken under monotonically increasing quasi‐static loads at different crack sizes. The aforementioned lagging also exists in a narrow region of 3‐D FCG for specimens with microstructural through‐thickness non‐uniformity. A more important feature for this type of specimen with surface crack is the deflection of fast fracture direction into the grain interfaces, namely from L‐T orientation to S‐L and S‐T directions. It is proved that this is due to significant levels of second principal stresses near the free surface for small cracks and lower fracture toughness of the material in S‐L and S‐T directions.     

Comparative study on tensile fracture behavior of monofilament and bundle C/C composites

Strength controlling factors in C/C composites were experimentally examined using monofilament fiber reinforced C/C composites and those reinforced by one carbon fiber bundle. Tensile strength of the monofilament C/C composites was almost the same level with that of the carbon fiber. This result indicated that carbon fibers in the C/C composites were intact even after the processing. On the other hand, remarkable reduction was observed in the bundle C/C composites. It was indicated that the fracture of the C/C composite is dominated by the brittle fracture of the sub-bundles, in which the fiber/matrix interface is bonded well.     

Influence of shear parameters on mixed–mode fracture of concrete

The influence of the mode II fracture parameters on the mixed mode fracture experimental tests of quasibrittle materials is studied. The study is based on experimental results and numerical analyses. For the numerical study, a procedure for mixed mode fracture of quasibrittle materials is presented. The numerical procedure is based on the cohesive crack approach, and extends it to mixed mode fracture. Four experimental sets of mixed mode fracture were modelled, one from Arrea and Ingraffea and another from a nonproportional loading by the authors, both with bending concrete beams. Two other sets of experimental fracture were modelled, based on double-edge notched testing; in these tests an important mode II is beforehand expected. The numerical results agree quite well with experimental records. The influence of the main parameters for mode II fracture on the mixed mode fracture is studied for the four experimental set of tests and compared with these results. In all them, large changes in the mode II fracture energy hardly modify the numerical results. The tangential and normal stresses along the crack path during the loading proccess are obtained, also with different values of the mode II fracture energy. For the studied experimental tests it is concluded that the crack is initiated under mixed mode but propagated under predominant mode I. This allows a development of mixed mode fracture models, mainly based on standard properties of the material measured by standard methods, avoiding the problems associated with the measurement of mode II fracture parameters, such as mode II fracture energy and cohesion.     

Mode-I,mode-II and mixed-mode I+II fracture behavior of composite bonded joints: Experimental characterization and numerical simulation

The fracture behavior of composite bonded joints subjected to mode-I, mode-II and mixed-mode I + II loading conditions was characterized by mechanical testing and numerical simulation. The composite adherents were bonded using two different epoxy adhesives; namely, the EA 9695 film adhesive and the mixed EA 9395-EA 9396 paste adhesive. The fracture toughness of the joints was evaluated in terms of the critical energy release rate. Mode-I tests were conducted using the double-cantilever beam specimen, mode-II tests using the end-notch flexure specimen and mixed-mode tests (three mixity ratios) using a combination of the two aforementioned specimens. The fracture behavior of the bonded joints was also simulated using the cohesive zone modeling method aiming to evaluate the method and point out its strengths and weaknesses. The simulations were performed using the explicit FE code LS-DYNA. The experimental results show a considerable scatter which is common for fracture toughness tests. The joints attained with the film adhesive have much larger fracture toughness (by 30–60%) than the joints with the paste adhesive, which exhibited a rather brittle behavior. The simulation results revealed that the cohesive zone modeling method performs well for mode-I load-cases while for mode-II and mixed-mode load-cases, modifications of the input parameters and the traction-separation law are needed in order for the method to effectively simulate the fracture behavior of the joints.     

Effects of temperature and stress on the oxidation behavior of a 3D C/SiC composite in a combustion wind tunnel

High-temperature oxidation of a 3D C/SiC composite has been conducted under various tensile creep loads in a combustion wind tunnel at 1200–1500 °C. The effects of temperature and stress on the oxidation behavior were evaluated according to length change, lifetime and morphology of the specimens. The damage mechanisms of the composite are changed from superficial oxidation to non-uniform even uniform oxidation by a tensile stress. The stressed oxidation process is controlled by a normalized threshold stress (NTS), which is increased with rising temperature. When the normalized stress (NS) is below the threshold value, the oxidation of carbon fibers is controlled by the in-crack diffusion, starts from the windward and develops region by region along the combustion gas flow. The specimen displays a multiple creep behavior because the applied tensile load is borne by several load-bearing regions in turn and each region manifests a typical creep behavior after the tensile load transferred from an oxidized region to it. When NS is above NTS, the oxidation of carbon fibers is limited by the boundary layer diffusion, and the specimen exhibits a typical creep behavior.     

An experimental analysis of the fracture behavior of composite bonded joints in terms of cohesive laws

Modeling adhesive joints by means of cohesive models relies on the definition of cohesive laws. Although cohesive laws are known to be dependent on the loading mode, there is a lack of experimental evidences to describe this dependence. At the same time, the adherend and adhesive thicknesses are known to affect the fracture toughness of the bond, but their effect on the cohesive law has not been clarified. In this work, an experimental characterization of an epoxy adhesive is presented. The effect that the mode mixity has on the bond toughness and its cohesive law is compared against the effect of the adhesive and adherend thicknesses. The impact of these two latest parameters is shown to be minor if compared to the influence of the mode mixity, which mainly defines the cohesive law shape. Finally, the implications of these experimental findings on the numerical simulation of adhesive joints are discussed.     

Impact of particle packing mix design method on fracture properties of natural and recycled aggregate concrete

The fracture properties of four types of concrete prepared using natural coarse aggregate and recycled coarse aggregate and conventional and particle packing method (PPM) of mix design approaches are studied. The three‐point bending (TPB) test is performed using three different sizes of single edge notched beam. The fracture energy is calculated from the load‐CMOD curve obtained in the TPB test, and in this process the load‐CMOD curve is curtailed at 2% of the depth of the beam. Based on CTOD_c and w₁ relationship, appropriate softening function is used to estimate the double‐K fracture parameters. The fracture energy and fracture toughness parameters of recycled aggregate concrete (RAC) is inferior to the natural aggregate concrete (NAC). The PPM mix design improves the fracture properties of concrete in comparison to the conventional mix design approach. The fracture properties of PPM mix designed RAC are comparable to that of NAC prepared using conventional method.     

Asymptotic analysis for a crack on interface of damaged materials

An asymptotic analysis for a crack lying on the interface of a damaged plastic material and a linear elastic material is presented in this paper. The present results show that the stress distributions along the crack tip are quite similar to those with HRR singularity field and the crack faces open obviously. Material constants n, μ and m0 are varied to examine their effects on the resulting stress distributions and displacement distributions in the damaged plastic region. It is found that the stress components σrr, σθ θ, σr θ and σe are slightly affected by the changes of material constants n, μ and m0, but the damaged plastic region are greatly disturbed by these material parameters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Investigation on the fatigue crack behavior of Zr702/TA2/Q345R composite plate with a crack normal to interface

In this paper, the effects of maximum load, load ratio, and average load on fatigue crack propagation of Zr702/TA2/Q345R composite plate with a crack normal to the interface are studied by experiment and finite element method. When crack propagates to the interface from the compliant material side, the crack growth rate decreases to the minimum at first. After crack penetrates through the interface, the fatigue crack growth rate accelerates continuously. When crack propagates to the interface from the stiff material side, the fatigue crack growth rate generally increases with the crack length. Regardless of the direction of crack growth, the increase of load ratio will weaken the difference of crack growth rate near the interface caused by material property mismatch. Finite element results show that elastic modulus mismatch significantly changes the variation of the stress intensity factor amplitude. All results demonstrate that crack growth rate is dependent on the competition of the stress intensity factor amplitude, the fatigue crack growth rate in the corresponding material, and the interface strength.