Key Engineering Materials Vols. 452-453

Abstract: For most of structural failure in engineering structures , fracture often takes place due to a phenomenon called fatigue. Therefore, many studies about the effect of the various mode-mixities on fatigue characteristics have been performed. However, most of the former studies are about metal/metal interface or delamination of composite. In this study, the fatigue characteristics of a composite/metal interface are investigated. The fatigue tests were performed using single-leg bending (SLB) specimens bonded with composite and steel using co-cure bonding method. This paper focuses on the fatigue characteristics depending on different mode ratios (GⅡ/GT). The overall results obtained in this study show that the crack propagation rate increases with the mode Ⅱ loading component.

Abstract: The bi-material notch composed of two orthotropic parts is considered. The radial and tangential stresses and strain energy density is expressed using the Stroh-Eshelby-Lekhnitskii formalism for the plane elasticity. The potential direction of the crack initiation is determined from the maximum mean value of the tangential stresses and local minimum of the mean value of the generalized strain energy density factor in both materials. Matched asymptotic procedure is used to derive the change of potential energy for the debonding crack and the crack initiated in the determined direction.

Abstract: In the contribution the limits of the validity of classical linear elastic fracture mechanics are extended to problems connected with failure of composite structures. The work is focused mainly on the case of a crack touching the interface between two different materials, two different constituents. The approach suggested in the paper facilitates the answer to the question what is the influence of particle (in particulate composite) or layer (in laminates) on crack propagation through bimaterial interface. Different composite (bimaterial) structures are considered: layered composites and composites reinforced by particles. The presented approach follows the basic idea of linear elastic fracture mechanics, i.e. the validity of small scale yielding conditions is assumed, and has a phenomenological character.

Abstract: Conditions of damage initiation in bi-material structures are analysed in the paper. The considerations are derived from the knowledge of the stress state caused by the existence of the sharp notch and the bi-material interface. The step change of material properties and the geometrical discontinuity at the interface leads to a singular peak of stress that is similar to the stress singularity near the crack tip in homogeneous material. That fact urges to use generalized linear elastic fracture mechanics for assessment of conditions of crack initiation. In the paper crack initiation direction and quantification of the external load connected with crack initiation are discussed. Within the stability assessment, possible directions of crack initiation should be considered and tested in both material components and at their interface as well. Knowing the critical applied stresses, one can decide among the damage eventualities.

Abstract: The mixed-Mode interlaminar fracture toughness, GI/IIC, of z-pinned hybrid laminated composites is studied to investigate the effect of 3D-composites on the crack propagation resistance of delaminated composite structures. In this regard, the mixed-Mode interlaminar fracture toughness, GI/IIC, was measured using asymmetric double cantilever beam (ADCB) test method. The hybrid ADCB and z-pinned hybrid composite beams were laid-up from [G0/C0]4, [G0/C90]4, [G90/C0]4 and [G90/C90]4 to study the effect of z-pinning on the interlaminar fracture toughness. From the obtained results from test it was found that the resistance of z-pin fibres against the crack propagation in z-pinned hybrid composites can significantly increase the mixed-mode interlaminar fracture toughness.

Abstract: Polycrystalline Cubic Boron Nitride (PCBN) is a super-hard material used in some of the most demanding material removal operations today. These include turning of hardened steels, as well as the machining of highly abrasive alloys. In these applications the tools are subjected to high operating temperatures, abrasive and impact loading. This can lead to the brittle fracture of the tool. Accurate determination of the fracture toughness and mechanical properties of PCBN under a wide range of operating conditions is therefore essential in order to evaluate the performance of the tool under these highly demanding conditions. For this study, a laboratory scale three point bend test rig has been used for the fracture tests. The fracture toughness of two different grades of PCBN are measured at a range of loading rates and temperatures corresponding to the actual in-service conditions. The results show the measured properties of these materials vary with both loading rate and temperature. The fracture surfaces of the specimens are examined using scanning electron microscopy to determine dominant fracture mechanisms.

Abstract: Based on the nearly 5,000 building samples investigated on site, damage characteristics of different types of building structures which located in different intensities regions are described and the reasons are analyzed in this paper. Three kinds of structural damage characteristics have been summarized as follows: 1) fortified masonry structures and unfortified ones; 2) masonry buildings with bottom frame; 3) reinforced concrete frame structures. The damage ratio of these three kinds of buildings in different intensities is obtained and the vulnerability has been compared. It provides the reference for those people who work for earthquake engineering to know the earthquake damage of different types of structures with fortified or unfortified. Also it provides important reference for revising seismic design code.

Abstract: Shape memory polymer (SMP) is being used in many engineering fields due to its shape memory effect and the advantages of large recoverable strain, excellent manufacturability, light weight, low cost and bio-degradability. Therefore it is of practical interest to investigate its thermo-mechanical behaviors. In this paper, a numerical simulation method is supposed to simulate the thermo-mechanical behaviors of SMP using the finite element software of MSC Marc, which is good at simulating the thermo-mechanical behaviors of viscous elastic material. The comparisons between the numerical results and the experimental results by Tobushi et al. illustrate the supposed numerical simulation method well describes the thermo-mechanical behaviors of SMP during the thermo-mechanical processes realizing shape memory effect.

Abstract: Following the 1994 Northridge earthquake, widespread damages were discovered in welded steel moment frame buildings. In order to accurately simulate the typical seismic damage of welded steel moment frame structures, a new simplified model is proposed for performing seismic evaluation of welded steel moment frame structures. In this model, the slabs effect is considered, as well as the effects of the slip between slabs and steel beams, deformation of panel zone and connection fractures. Fracture toughness demands were evaluated in terms of the mode I stress intensity factor. The model was employed in simulation of seismic damage of Blue Cross Building which experienced fractured connections in the Northridge earthquake. It indicates that the model can accurately predict the earthquake response of welded steel moment frame structures and estimate the level of damage. The approach proposed in this paper has important meaning to the research on seismic damage of steel frame which may experience fractured connections.

Abstract: Brittle fracture was identified in many of prequalified weld joints in steel moment frames in the 1994 Nothridge earthquake. Then analyses of response and damage mechanism of beam-to-column connections under seismic load were widely studied in the world, but few people conduct the research on seismic-resistant behavior of beam-to-column web connections. To quantify the variation of stress intensity factor to weld root flaw sizes beam-to-column web connections with tapered beam flange plates, detailed 3D finite element analyses is used to study fracture toughness requirements in beam-to-column web connections, considering the large deformation, large strain, bolts pretension, bolt contact-slide, as well as material harden and soften. Fracture toughness demands are evaluated in terms of the mode I stress intensity factor. The stress intensity factor is calculated through a J-integral approach. The fracture toughness demands are studied for the flaw on the top of the beam flange and the bottom surface, respectively. Results indicate that the likelihood of top flange fractures is smaller than that of bottom flange fracture. Stress intensity factor is not uniform and is largest in the edge of beam flange. The fracture toughness in the edge of beam flange for web connections with step beam flange plates is 15% less than that for tapered beam flange plates.