Papers by Keyword: Crack Propagation

Abstract: Metals belonging to groups 4-6 of periodic table of elements form with carbon compounds named metal-like carbides (WC, TiC1-x, NbC, TaC, CryCz). It was established that is possible to achieve dense sintered bodies of this carbides at temperatures near 2000oC using only carbon powder as an additive. Its role consists in reduction of oxide impurities. Presented paper summarized investigations on sintering of metal-like carbides with carbon. Dense WC, TiC1-x, NbC, TaC and CryCz bodies were characterized by KIc factor value. A special care was taken on microstructure and the way of fracture description in relation to the critical stress intensity factor (KIc). It was established that the way of fracture and the finally fracture toughness depend on type of microstructure and degree of nonstoichiometry.

Abstract: Layered ceramics are foreseen as possible substitutes for monolithic ceramics due to their attractive mechanical properties in terms of strength reliability and toughness. The different loading conditions to which ceramic materials may be subjected in service encourage the design of tailored layered structures as function of their application. The use of residual stresses generated during cooling due to the different thermal strain of adjacent layers has been the keystone for the improvement of the fracture response of many layered ceramic systems, e.g. alumina-zirconia, alumina-mullite, silicon nitride-titanium nitride, etc. In this work, the fracture features of layered ceramics are addressed analysing two multilayered structures, based on the alumina-zirconia system, designed with tailored compressive residual stresses either in the external or internal layers. Contact strength and indentation strength tests have been performed to investigate the response of both designs to crack propagation. The experimental findings show a different response in terms of strength and crack growth resistance of both designs. While layered structures with compressive stresses at the surface provide a better response against contact damage compared to monoliths, a flaw tolerant design in terms of strength and an improved toughness through energy release mechanisms is achieved with internal compressive stresses. The use of layered architectures for automotive or biomedical applications as substitutes for alumina-based ceramics should be regarded in the near future, where reliable ceramic designs are needed.

Abstract: Interface damage characterization and interlaminar failure of sandwich specimens with two initial interlaminar defects (inserts) is done by using the digital image correlation method. Mode I tests reveal interesting particularities on damage localization and unstable crack propagation. After analyzing the experimentally obtained results, we propose as a failure parameter the local strain at the crack tip or, alternatively, the crack tip opening displacement which quantifies the non-linear phenomena.

Abstract: Modelling of polycrystalline ceramics built up of different phases has been done within mesomechanical approach. Porous ceramics consists of matrix material and second phase – pores. Such material has non-linear and complex overall response to applied load. Modelling of porous polycrystalline material was performed by multiscale approach describing different phenomena occuring at different scales: micro- meso- and macro- with application of averaging procedure over the Representative Surface Element (RSE).

Abstract: The paper presents results of studies into the effect of repetitive low-energy impacting (known as impact fatigue) on reliability and crack growth in adhesively bonded joints. This type of loading is compared to the standard tensile fatigue in order to assess severity of such loading regime. Another loading type studied is a combination of a small portion of repetitive impacts with tensile fatigue. Crack propagation in a joint exposed to these types of loading is studied experimentally and numerically (with finite elements). This analysis is accompanied by microstructural studies of various damage processes, active at different stages of the crack growth process.

Abstract: Effects of atmosphere and specimen geometry on thermal fatigue (TF) crack initiation and propagation in a low Si content hot work tool steel X38CrMoV5-47HRC were investigated. The TF specimen’s geometry enhances the uni-axial TF loading conditions. A high frequency induction heating (3 to 4 MHz) is used. A new TF rig, working under air and/or inert atmosphere with reduced PO2 has been set up. The reduction of PO2 results in localized oxidation sites. Whatever geometry and atmosphere conditions, TF cracks initiate exclusively in the oxide layers. Damage mechanisms are environment dependant. Under laboratory air, parallel macroscopic cracks initiate perpendicular to the hoop stress. Under argon and nitrogen, SEM surface observations show that initiated cracks coalesce by zigzagging along crystallographic paths between non-oxidized zones. In-depth crack propagation mechanism is mainly trans-granular. TF crack initiation life under air and in presence of Fe-Al intermetalllics is decreased in comparison to inert atmosphere.

Abstract: In this paper, molecular dynamics method has been employed to model mode I crack propagation in body center cubic (BCC) single iron crystal. To maximize the simulation efficiency the parallel computing was performed. Six cases with different lattice orientations have been simulated to investigate the crack propagation behaviors at atomic level. The strain distributions have been calculated to indicate the density of dislocation. It has been found that the lattice orientation significantly affects the propagation behaviors. The crack in BCC iron propagates more readily along the direction <111> on the plane {1-10}.

Abstract: Fatigue properties of a squeeze cast Al-Si-Mg alloy were examined under rotating bending and the effect of humidity on the crack initiation and propagation behavior was investigated in controlled humidity. The fatigue strength was decreased in high humidity (RH85%) in comparison with the one in low humidity (RH25%). Cracks initiated at the early stage of fatigue life from slip bands and eutectic Si particles in many cases and partially casting defects, and most of fatigue life was occupied by the growth life of a small crack regardless of humidity. The propagation of a crack were promoted by humidity, especially the growth rate of a crack larger than about 100 µm, which propagated in tensile mode, was strongly accelerated.

Abstract: The main objective of this study is to evaluate the flexural toughness and crack strength of a cement-based composite under the effect of using recycled polypropylene (PP) fiber and recycled glass. This research study proves the feasibility of the use of glass as aggregates replacement and PP fibers for composite reinforcement.

Abstract: In order to investigate the influence of grain size on notch sensitivities in fatigue of a fine-grained carbon steel, rotating bending fatigue tests were carried out using specimens with a V-grooved circumferential notch of commercial fine-grained carbon steel with grain size of 6.5µm. The results were compared with those of a larger grain sized carbon steel (grain size: 20 µm) and the notch sensitivities were evaluated based on Linear notch mechanics proposed by Nisitani. Notch sensitivities for both of fatigue limits for a crack initiation and its propagation of the fine grained steel were high. The results were discussed from the view points of the size of area related to crack initiation and the crack growth resistance.