Abstracts:Ceria stabilized tetragonal zirconia polycrystal (CeO2-TZP) is one of the zirconia-based materials which is reported to have high fracture toughness and strong resistance to low temperature aging degradation (LTAD). Owing to the brittle behavior of CeO2-TZP, its fracture properties can be estimated using the concept of linear elastic fracture mechanics (LEFM). In this paper, the generalized maximum tangential strain (GMTSN) criterion is applied to predict the fracture initiation angles and the onset of fractures of CeO2-TZP disk specimens under mixed mode I/II conditions. It is found that the inclusion of T-term, the first non-singular term in strain solution, in the GMTSN criterion would yield significantly improved predictions of the experimental data obtained for CeO2-TZP disk specimens and reported in the literature.

Abstracts:Two-dimensional three-point bending and four-point bending of two-layer finite element models for a ceramic layer on a metallic substrate are developed to study the damage and fracture characteristics of two-layer systems by introducing an interface cohesive zone model. The damage evolution and fracture modes of ceramic layers of different thicknesses, with loading on the metallic substrates, are compared under different loading conditions based on simulation results. Multiple surface cracks, vertical to the interface between the ceramic and metallic layers, appear in all ceramic layers under four-point bending loading and only in the thinner ceramic layers under three-point bending. For the thicker ceramic layer systems under three-point bending loading, the interface fracture between the ceramic and metallic layers is the main failure mode, agreeing with previous experimental observations. Damage and damage rate, defined by the simulated crack evolution, are found to obey a power law relation with loading and to be consistent with the theoretical predictions based on a mathematical damage model. The damage coefficient, a parameter reflecting the damage rate, is found to be size-dependent based on the simulation and experimental results, and its energy mechanism is discussed. The damage is slower for the thinner ceramic layers with a smaller damage coefficient than that for the thick ceramic layers under three-point and four-point bending loading, and the damage of the ceramic layer systems is faster under three-point bending than under four-point bending, resulted from different crack distributions, damage localization degrees, and energy dissipation. Moreover, the damage is slower for the nanostructured ceramic layers with the stronger interface strength or toughness between two layers.

Abstracts:Pipelines and pressure vessels made of carbon and low alloy steels have suffered from Hydrogen Induced Cracking (HIC) in wet hydrogen sulfide environment in the oil & gas industry. Hydrogen which is produced at cathode due to corrosion reaction, diffuses into the steel and result in cracking in wet hydrogen sulfide environment. Hydrogen assisted cracking usually manifests in carbon and low alloy steels with unique crack initiation and propagation characteristics. The origin and morphology of cracks are dependent on various factors viz., mechanical properties & composition of the material, manufacturing process including heat treatment, applicable stresses etc. Hydrogen assisted cracking is commonly classified into three categories based on initiation, morphology and stress requirement in cracking as, Hydrogen Induced Cracking (HIC), Sulfide Stress Cracking (SSC) and Stress Oriented Hydrogen Induced Cracking (SOHIC).

Abstracts:Unloading compliance technique in fracture toughness tests according to ASTM E1820 standard is used to obtain the resistance curve (R-curve) of metallic materials in air tests. Even in this situation, there are aspects inherent to work hardening around the crack-tip that could affect the unloading compliance technique and that are not well discussed. Another important aspect is the application of this methodology in conditions involving corrosive environments. Indeed, the presence of hydrogen around the crack-tip can promote significant changes in the strain and stress around the process zone, leading to the phenomenon called subcritical crack growth. This phenomenon can affect the compliance increasing the divergence between the crack sizes obtained by the unloading compliance technique and the correct crack sizes measured after the test. The aim of this paper is to analyse the influence of subcritical crack growth and to present a methodology to correct crack sizes and the influence of work hardening during fracture.

Abstracts:The application of self-compacting concrete (SCC) has recently been more common in practice. SCC is a highly flowable type of concrete that spreads into the formwork itself without any mechanical vibration. In addition, fracture characteristics and brittleness of SCC can be modified by application of supplementary cementitious material (SCM) due to concrete internal structure densification. One of the highly reactive SCM is rice husk ash (RHA).

Abstracts:Thermal and mechanical responses of a cracked transparent conducting film to laser irradiation were investigated. A representative coin-shaped crack was modeled in the transparent conducting film and the case of vertical incidence of laser beam considered. Firstly, the multiple reflections, transmissions as well as absorptions are formulated in the quasi-multilayer media due to the effect of inner crack. Then, the temperature characteristics generated by the dissipated light energy are computed for the film of typical thermal boundary conditions. Finally, the thermal stress and stress intensity factors around the crack tip are particularly analyzed. The effects of the coin-shaped crack are discussed on the multi-physical responses of the film to laser irradiation.

Abstracts:A new damage model is proposed in the bond-based peridynamics (BBPD) frame to quantitatively investigate mode-I crack propagation in concrete, which is established based on the pattern of bilinear softening curve commonly used in the discrete cohesive zone model (CZM). This new damage model can be obtained completely based on material properties, that is, once experimental bilinear softening curve, the uniaxial tensile strength, the elastic modulus and fracture energy of concrete are given, this corresponding new damage model will be exclusively determined. And then, the whole fracture processes of concrete specimens with different strengths, dimensions and crack-depth ratios under quasi-static three-point-bending (TPB) are simulated using BBPD to verify this proposed new damage model. The predicted load-crack mouth opening displacement (P-CMOD) curves are in a good agreement with the experimental ones. Finally, in the BBPD frame, a strategy to determine the initial cracking load of TPB beams, the influence of different definitions for computing fracture energy and different damage models on numerical results are discussed respectively. It can be found that different from brittle materials, fracture energy is mainly consumed in the softening stage rather than the elastic stage for concrete. Besides, this new damage model based on bilinear softening curve is more appropriate to describe the damage mechanism of concrete, compared with other damage models in BBPD.

Abstracts:In this work, viscoelasticity has been introduced into lattice spring model by utilizing van der Pol elements obeying the Boltzmann superposition principle to investigate the time-dependent mechanical behavior and damage evolution of viscoelastic particle filled composites (PFCs). Stress relaxation, crack mouth opening displacement test and uniaxial tension were simulated. Influence of filler content, filler dispersion state, strain rate and interfacial strength on the tensile response of PFCs were investigated. It is shown that (1) viscoelasticity of microscopic elements can be reflected to the global behavior of composites; (2) cluster effects exists when particles are closely packed and it will cause incipient damage under low extent of deformation; (3) increase in strain rate strengthens both the stiffness and strength of composites and the increment is proportional to the logarithm of strain rate; (4) well-dispersed PFCs can be severely weakened when adequate interfacial bonding is not provided.

Abstracts:A Grand Canonical Monte Carlo Approach (GCMC) is proposed for the fracture analysis of solids discretized as mass points and bond interactions. In contrast to classical load-driven fracture processes, the GCMC approach introduces an auxiliary field, the bond rupture potential $\mu$, to which the system is subjected; in addition to changes in volume V and temperature T. In this $\mu$VT-ensemble, bond isotherms that link the average number of bonds to the bond rupture potential ($〈N_k〉-\mu$) are obtained that carry critical information for fracture analysis. Specifically, the slope of the bond isotherm reflects bond fluctuations, permitting identification of (1) a fluctuation-based damage variable, and (2) the competition in energy fluctuations between the redistribution of strain energy induced by bond rupture, and the dissipation of the groundstate energy. Based on these fluctuations, it is shown that the GCMC-approach allows the identification of a critical bond energy release rate of material samples, when strain energy fluctuations equal groundstate energy fluctuations – much akin to Griffith’s 1921 stationarity postulate to “predicting the breaking loads of elastic solids”. This is illustrated by means of thermodynamic integration of bond isotherms to determine force-displacement curves, for both notched and unnotched homogeneous samples discretized by regular 2-D lattices with bonds exhibiting harmonic potentials.

Abstracts:Advanced non-linear finite element models are currently available to perform the design of concrete structures. The development and calibration of the safety factors for the next generation of design guidelines will require a deep understanding of the uncertainty associated with such models. This paper presents a first study focusing on the assessment of the uncertainty of discrete and smeared crack models applied to the simulation of the behaviour of RC beams under flexural loads. The discrete strong discontinuity approach (DSDA) and the smeared crack model available in ATENA were chosen for this purpose. Experimental data for mean and maximum crack widths, and average crack spacing for four beams were used as reference. The mesh size dependency and its relation with the uncertainty in the prediction of deflections, crack openings and spacing was investigated. The model uncertainty of mean and maximum crack widths was evaluated for progressive load stages of serviceability conditions for the two crack models considering subsets of data based on the reinforcement ratios and concrete cover.