Key Engineering Materials Vols. 324-325

Abstract: A series of uniaxial-compression tests were conducted on some representative brittle rock specimens, such as granite, marble and dolerite. A multi-channel, high-speed AE signal acquiring and analyzing system was employed to acquire and record the characteristics of AE events and demonstrate the temporal and spatial distribution of these events during the rupture-brewing process. The test result showed that in the primary stage, many low amplitude AE events were developed rapidly and distributed randomly throughout the entire specimens. In the second stage, the number of AE increased much slower than that in the first stage, while the amplitude of most AE events became greater. Contrarily to the primary stage, AE events clustered in the middle area of the specimen and distributed vertically conformed to the orientation of compression. The most distinct characteristic of this stage was a vacant gap formed approximately in the central part of the specimen. In the last stage, the number of AE events increased sharply and their magnitude increased accordingly. The final failure location coincidently inhabited the aforementioned gap. The main conclusion is that most macrocracks are developed from the surrounding microcracks existed earlier and their positions occupy the earlier formed gaps, and the AE activity usually becomes quite acute before the main rupture occurs.

Abstract: A 3D anisotropic elastoplastic-damage model was presented based on continuum damage mechanics theory. In this model, the tensor decomposition technique is employed. Combined with the plastic yield rule and damage evolution, the stress tensor in incremental format is obtained. The derivate eigenmodes in the proposed model are assumed to be related with the uniaxial behavior of the rock material. Each eigenmode has a corresponding damage variable due to the fact that damage is a function of the magnitude of the eigenstrain. Within an eigenmodes, different damage evolution can be used for tensile and compressive loadings. This model was also developed into finite element code in explicit format, and the code was integrated into the well-known computational environment ABAQUS using the ABAQUS/Explicit Solver. Numerical simulation of an uniaxial compressive test for a rock sample is used to examine the performance of the proposed model, and the progressive failure process of the rock sample is unveiled.

Abstract: A damage constitutive model of dry/water-saturated granite is proposed within the framework of continuum mechanics and mixture theory, and the model allows for the simulation of the effects of micro-cracks, micro-pores and saturated water. By implementing the model into wave propagation code, one dimensional stain waves in dry granite induced by a rectangular impulsive loading and spherical waves in water-saturated granite due to underground explosion are studied. The simulation results demonstrate the main features of the model, and it is shown that the theoretical model developed is valid for study on the characteristics of stress waves in rock medium.

Abstract: Fracture of bonded cement based materials is complicated due to not only bonding itself but also heterogeneous genuine nature of cement-based materials. This study investigates the fracture parameters that obtained from the mechanical fracture tests and the post-peak behavior of bonded cement-based materials. Fracture parameters were analyzed such as the critical stress intensity factor, the critical crack length and the critical crack tip opening displacement. In addition, this study defines a new fracture parameter, the critical crack opening angle, which describes a crack opening resistance. In order to evaluate the fracture energy of quasi-brittle materials, it is typical to use the non-linear elastic fracture mechanics approaches. From the test results, however, it is known that the toughening action at the fracture process zone of the bonded interface has been significantly diminished because of the brittle fracture and the pre-determined weak crack path. Therefore, the post-peak behavior could be successfully estimated by using the suggested model that considered only the elastic deformations.

Abstract: Traditional concrete structures with steel reinforcing bars shall gradually deteriorate owing to external loadings and environmental attacks. Fiber reinforced polymer (FRP) is one of the most attractive alternative material for steel since it provides excellent tensile strength and much higher corrosion resistance as well as lower self-weight. This study utilized a newly developed FRP rebar that uses glass fibers in core and chopped glass fibers to make rips on the surface of rebar. Flexural test was performed on concrete beam specimens reinforced with the developed GFRP rebar at various reinforcement ratios. The stiffness of the beams reinforced with GFRP was lower than those with steel rebar. The ultimate strength, however, was improved by using the GFRP rather than the steel rebar. The rip-shaped surface provided better bonding between the GFRP rods and concrete and no significant slip/debonding was observed. In addition, the load and deflection increased gradually until the complete failure without apparent yielding. The current equations for estimating the ultimate moment was too conservative by underestimating values. However, as the reinforcement ratio increased, the difference calculated values became closer to the measured.

Abstract: Thermal barrier coatings (TBCs) have received increased attention for advanced gas turbine engine application. The oxidation damage plays an important role of the failure under high temperature load. An Y2O3 partially stabilized ZrO2 ceramic top coating was deposited a NiCrAlY bond coating by air plasma spray (APS). The substrate was directionally solidified superalloy (DZ40M). Isothermal oxidation has been performed at 700
and 1050
for 100h. The oxidation results in three transformations of the weight, the interface and the surface characters. All the specimens were characterized by highly precision balance, scanning electron microscopy (SEM) in cross-section, microscope with MFK2 visual measurement system on its surface. The curve of oxidation kinetics is obtained by weighting those specimens. The weight gain of the specimen under 700
is close to zero. Under 1050
the weight gain is very distinct. On the other hand, the interface cracks occur and extend at the interface between the ceramic and bond coating by SEM method. The thermal growth oxide (TGO) gradually appears on the coating surface under high temperature oxidation by MFK2. Last but not least, those observations will provide some ideas and damage parameters for the non-destructive inspection (NDI) for TBC under thermal load.

Abstract: Porous structure is an important component in C/C composites, which directly affects the fatigue behavior of materials. Therefore, it is necessary to discuss the evolution of porous structure in C/C composites under the fatigue loading. In present work, the character of porous structure in original C/C composites was summarized and the evolution of porous structure after fatigue loading was analyzed. The positive effect of the porous structure evolution after fatigue loading on the reinforcing behavior of fatigue was proposed as well, which could provide a basis for further studies on the fatigue mechanisms of C/C composites.

Abstract: The relation between material’s cyclic plastic behavior and fatigue crack growth is investigated. The present model is proposed on the dislocation-free zone (DFZ) theory. A cohesive zone theory is developed to determine the stress field of the DFZ and the value of J-integer under cyclic loading. The crack growth criterion is proposed based on J-integral. The calculated curve of fatigue crack growth rate da/dN is agreement with the general propagation pattern and the predicted threshold accords with the experiment threshold well. It is found that the near threshold characteristics are most determined by the cyclic deformation behavior of the material. The relation between fatigue crack growth threshold and material’s cyclic hardening behavior is discussed.

Abstract: Three corrosion degrees of reinforcement are chosen in this paper, which are considered to simulate minor, medium and severe degrees of corrosion in this field. A reinforcement mass loss about 5% is used to define a minor degree of corrosion, whereas mass losses about 10% and 20% are used to define medium and severe degree of corrosion, respectively. Fifteen specimens of the three corrosion degrees and seven uncorroded specimens are tested under fatigue. According to the results of the fatigue tests, using stress range as fatigue parameter and 50% as guaranteed probability, the fatigue S-N curves of the four different corrosion degrees of deformed reinforcement adaptable for fatigue reliability evaluation are put forward. These curves provide necessary principle for predicting the residual fatigue lives of aged existing reinforced concrete bridges.

Abstract: Application of Miner criterion, cumulate damage variable was estimated based on pseudo-static experiment study for reinforced concrete energy dissipation braced frame (EDBF) under low cyclic loads, accordingly, the constitutive relations about damage was established; the linear hook law turned into non-linear stress-strain relations; the dissipated-energy factor c β was determined based on following factors: the cumulate damage variable, hysteretic energy determined by load-displacement curve, maximum deformation and yield force of EDBF; it supplied a quantitative basis of dissipated-energy for EDBF; There were two reasons in energy dissipation for EDBF: one was energy dissipation equipment acting, the other was concrete damaged and cracked or low cycle fatigue failure in this structure, and the latter part of energy was associated with amounts of cracks and crack size; then the forced mechanism of EDBF was analyzed, and the reason caused cracks and crack type of EDBF columns, beam and braces were explained based on forced mechanism: the columns, beam and braces of EDBF were compressed or tensed under low cyclic loads, so most of cracks of columns, beam and braces belonged to mode I cracks
This study supplied a method for estimating energy of EDBF under earthquake ground motion, and the results showed: columns in EDBF are easily damaged under earthquake ground motion, so the structural elements must be designed strong column, weak beam and weak brace.