Papers by Keyword: Sub Critical Crack Growth

Abstract: The lifetime under thermal cycling of a system consisting of an air plasma sprayed thermal barrier coating (TBC) deposited on a metallic bondcoat (BC) is determined by the subcritical growth of micro-cracks near the interface between both coatings. This growth mainly occurs during the cooling down phase, as shown by the acoustic emission monitoring during the thermal cycling. The factors controlling the stress level leading to the crack growth are the local curvature of the metallic-ceramic interface, the growth of an oxide scale (TGO) at such interface and the sintering of the TBC, the two last processes occurring during the high temperature cycle phase. Implementing all these factors, a model based on Finite Element Method (FEM) calculations is presented where growing cracks are incorporated by assigning soft properties to the FEM cells occupied by the cracks. Determining the growth direction for the maximum energy release rate at every cooling down step, the current crack extension during the cycling is tracked until it reaches a characteristic length corresponding to the TBC failure. The influence by the metallic-ceramic interface roughness and by the temperature gradient across the TBC is discussed.

Abstract: Abstract. The improvement of subcritical crack growth (SCG) resistance for alumina glass dental composites was explored in this study. The addition of nitrogen to the glass phases in the composite was found to increase the SCG resistance, where the SCG exponent n increases from 22 for the oxide glass composites to 30 for the composites containing 0.5 mol% nitrogen in the glass phases. The improvement was tentatively attributed to the nitrogen addition, which makes the glass network stronger through forming the non-flexible Si-N bonds and thus inhibits the hydrolysis reactions under the SCG conditions. Analyses demonstrated that the increase of the n value from 22 to 30 offers the potential to greatly extend the lifetime and improve the long-term reliability for the alumina glass dental composites.

Abstract: According to laboratory accelerated test data, stress corrosion cracking (SCC) in structural metal materials occurs by initiation and coalescence of micro cracks, subcritical crack growth, multiple large crack formation and final failure under the combination of materials, stress and corrosive environment. In this paper, a computer simulation model for the process of SCC has been proposed. The procedure is as follows: The possible number of crack initiations is set for a given space and the initiation times for all cracks are assigned by random numbers based on exponential distribution. The sites and length of the cracks are assigned by uniform random numbers and normal random numbers, respectively. The coalescence of cracks and the subcritical crack growth are determined based on the fracture mechanics. The simulation is terminated when the maximum crack length reaches a critical value or all of the possible number of cracks is initiated. The results obtained in this paper indicate the applicability of the present model to predict the SCC behavior in real structures based on the laboratory accelerated test data.

Abstract: The aim of this study is to demonstrate the influence of the stress parallel to the crack plane on subcritical crack growth in brittle materials by using a numerical code MFPA2D. The mechanism of this influence is also discussed. The curves of subcritical crack extension vs. strain of brittle materials under uniaxial and biaxial stress were obtained through numerical tests with acoustic emission consideration. The results showed that the tensile stress parallel to the crack plane has the effect on crack arrest, while the compressive stress parallel to the crack plane plays important role in crack opening process. The numerical results were consistent with experimental observed result, which shows the reliability of the numerical method, and provides theoretic foundation for failure analysis and life estimation of brittle materials.