Characterizations on fracture process zone of plain concrete

The fracture property of concrete is essential for the safety and durability analysis of concrete structures. Investigating the characteristics of the fracture process zone (FPZ) is of great significance to clarify the nonlinear fracture behaviour of concrete. Experimental and numerical investigations on the FPZ of plain concrete in pre-notched beams subjected to three-point bending were carried out. Electronic speckle pattern interferometry (ESPI) technique was used to observe crack evolution and measure the full-field deformation of the beams. The development of the FPZ were evaluated qualitatively and quantitatively based on the in-plane strain contours and displacement field measured by ESPI, respectively. By integrating the cohesive crack model and finite element (FE) model, various tension softening curves (TSCs) were employed to simulate the fracture response of concrete beams. By comparing the deformation obtained by FE simulation and experiments, the TSCs of plain concrete were evaluated and most suitable TSCs of concrete were recommended

Mechanical properties of magnesium-based wood-like material subjected to splitting tensile tests

To investigate the splitting tensile characteristics of a new building material, namely magnesium-based wood-like material (MWM), the cubic splitting tensile tests were carried out at a loading rate of 200 N/s. Full-field displacements and crack behaviors were measured using Digital Image Correlation, and the splitting tensile strength is 1.79 MPa. The elastic Young modulus, Poisson’s ratio, and axial compressive strength were measured as 2.21 GPa, 0.21, 8.76 MPa respectively. In the splitting tensile tests, primary cracks were observed to initiate from the geometric centre of the specimen and then extend to the loading ends where secondary cracks appeared. A new method for identifying cracking modes showed the secondary cracks were mainly caused by shear and tensile-shear failure, whereas the primary cracks were caused by tensile failure. An accurate method for estimating the elastic Young modulus, simultaneously with the determination of the splitting tensile strength of MWM cubes is proposed

Fracture toughness evaluation of a nuclear graphite with non-linear elastic properties by 3D imaging and inverse finite element analysis

Effective small specimen tests are needed to obtain fracture toughness and elastic properties as the limited availability of irradiated graphite restricts the quantity and dimensions of test specimens. Both properties have evaluated simultaneously in a crack propagation test with the double cleavage drilled compression (DCDC) specimen geometry of a fine-grained graphite (SNG742) that has non-linear elastic properties in the unirradiated condition. Three-dimensional displacement fields were obtained by digital volume correlation of in situ laboratory X-ray computed tomographs, and the 3D crack geometry (crack tip position, crack opening displacements and angle) was determined objectively by a wavelet variance method. The tensile softening of the Young modulus was determined by inverse analysis of the strain field using the finite element model updating (FEMU) method. The strain energy release rate of the quasi-static propagating crack was calculated using the contour integral method in a finite element model with the derived non-linear elastic properties and the measured displacements as boundary conditions. The critical strain energy release rate was constant with crack length (118 ± 14 J m−2) and equivalent to a fracture toughness of 1.13 ± 0.07 MPa m1/2

Incremental displacement collocation method for the determination of fracture properties of quasi-brittle materials

This thesis presents experimental and numerical investigations on the fracture properties of quasi-brittle materials, including mortar, concrete and graphite. Fracture toughness in terms of the critical stress intensity factor K_IC and fracture energy G_F of the materials were determined through three-point bending tests on centre-notched beams. Furthermore, full-field displacement of the beams subjected to bend was obtained using Electronic Speckle Pattern Interferometry (ESPI) technique. In order to verify the accuracy of the displacement data measured using the ESPI technique and to obtain reliable deformation information, the displacement and strain errors induced by the rigid-body motions of the specimen were quantified. This study found that the displacement errors were negligible whereas the strain errors were notable and must be eliminated. The influence of different rigid-body motions was analyzed. It was found that the out-of-plane movement of the specimen was critical and affected considerably the accuracy of strain data. Thus the experimental setup was improved accordingly to eliminate the influence of critical rigid-body motions. Quasi-brittle materials have a finite stress region near the crack tip, known as the fracture process zone (FPZ). The materials exhibit nonlinear fracture behaviour in the FPZ. The cohesive crack model (CCM) is widely used to characterize the nonlinear fracture behaviour of quasi-brittle materials. According to the CCM, all the nonlinear behaviours in the FPZ can be represented by a cohesive crack, and the crack propagation is controlled by the relationship between the cohesive stress and crack opening, namely, the tension softening curve (TSC). Thus an accurate estimation of the TSC is essential. In order to determine the TSC of quasi-brittle materials, an incremental displacement collocation method (IDCM) was originally developed in this study. In the IDCM, the deformation data measured by the ESPI sensor was analyzed to obtain the crack opening displacement (COD) of the notched specimens. The experimental COD profiles together with the CCM were then integrated into a finite element model to simulate the nonlinear fracture response of the specimen. By minimizing the difference between the computed and measured displacements at selected collocation points, the cohesive stress corresponding to certain crack opening was determined. The entire TSC was constructed in a step-by-step manner following the loading steps. The IDCM was first applied to estimate the TSC of mortar. By using the estimated TSC, the displacements of the specimen under certain loading levels were computed. By comparing the computed displacements with the experimental data, the reliability of the IDCM and the accuracy of the estimated TSC were verified. The application of the IDCM was further extended to the determination of the TSCs of concrete and graphite. The parameters used to define the shape of the TSC of the materials were determined using regression analysis. The applicability of those parameters was verified by comparing the TSCs estimated in the present study with those derived by other researchers. Recommendations were put forward to choose appropriate tensile properties of quasi-brittle materials in the numerical simulations. Furthermore, by using the ESPI technique, fracture phenomena of quasi-brittle materials were observed and reported. Such records can greatly enhance the understanding of crack initiation, growth and arrest in quasi-brittle materials, and lead to improvements to the existing fracture models.published_or_final_versionCivil EngineeringDoctoralDoctor of Philosoph

Determination of tensile strength and fracture toughness of nuclear graphite and prediction of its structural failures

The fracture properties of nuclear graphite and their effects on the structural failure process threaten the integrity and safety of High Temperature Gas-cooled Reactor structures. This study aims to develop close-form solutions to determine the size-independent tensile strength and fracture toughness of graphite from small-sized specimens, and thus to predict its structural failure behaviors. Firstly, the scale coefficient and dispersion coefficient are introduced to reflect the heterogeneity of graphite; these are estimated to be 21.0 and 16.8, respectively, in accordance with the meso-structural characteristics and macro-mechanical responses of the graphite. The fracture parameters of geometrically non-similar IG-11 graphite specimens are statistically analyzed with a normal distribution to determine the true fracture toughness and tensile strength, in the form of analytical solutions. Thus, a complete fracture failure curve is constructed to evaluate its fracture behavior. Then, the proposed method is employed to obtain the fracture responses of geometrically similar IG-11 graphite specimens. Good consistency in the calculation results of two types of specimens are observed, verifying the accuracy and size-independence (including absolute and relative sizes) of the proposed method for determining the fracture parameters of graphite. Finally, the structural failure loads of IG-11 and IG-110U graphite specimens with different sizes are predicted within an error margin of 15%. The minimum size requirements of graphite specimens satisfying the condition of linear elastic fracture mechanics are discussed, and the relationship between the fracture behavior of small-sized graphite specimens in the laboratory and the structural failure of actual large-sized graphite members is established

Study on fracture characteristics of steel fiber reinforced manufactured sand concrete using DIC technique

Incorporation of steel fibers to improve the defects of manufactured sand concrete (MSC) has received a great deal of attention, while the fracture mechanism of the MSC by volume fraction of steel fibers (Vf) has been less studied. To investigate the effects of steel fibers on the fracture characteristics of the MSC, three-point bending tests were conducted on the MSC beams with different Vf (0, 0.5 %, 1.0 % and 1.5 %) using digital image correlation technique. The effects of Vf on the fracture parameters and crack propagation paths of steel fiber reinforced manufactured sand concrete (SFRMSC) were analyzed. In addition, fracture process zone (FPZ) of the SFRMSC was analyzed via the length of the FPZ (lFPZ) and area of the FPZ (AFPZ). The results indicated that the residual strength, initiation fracture toughness, unstable fracture toughness, fracture energy and characteristic length of the SFRMSC increased with the increase of Vf, whereas the crack propagation paths were not sensitive to the changes of Vf. The addition of steel fibers increased the interaction between the aggregate and the matrix, which led to an increase in the number of microcracks. Before approaching the Pc, both the lFPZ and AFPZ showed essentially stable changes. As Vf increased, the lFPZ and AFPZ developed faster to their corresponding maximum values. The AFPZ reached a maximum and increased with the Vf. At the Pc, the lFPZ and AFPZ changed in a similar trend when Vf ≤ 1.0 %. When Vf > 1.0 %, the lFPZ changed slightly while the AFPZ increases sharply

Combined evaluation of Young modulus and fracture toughness in small specimens of fine grained nuclear graphite using 3D image analysis

The fracture toughness of the fine-grained nuclear graphite SNG742 has been investigated by observation of stable crack propagation in double cleavage drilled compression specimens. The three-dimensional displacement fields were obtained by digital volume correlation (DVC) of in situ laboratory X-ray computed tomographs. The crack tip location and crack opening displacements were determined using an image edge detection algorithm based on the wavelet modulus maxima. The Young modulus was estimated by fitting a finite element model to DVC displacement field data measured before crack initiation. Using the 3D crack geometry and the surrounding full-field 3D displacement fields as boundary conditions, the elastic strain energy release rate J and the three-dimensional stress intensity factors KI to KIII were then evaluated via the contour integral method. Constant mode I critical stress intensity factor was obtained along the curved crack fronts, with negligible shearing modes. This method allows evaluation of the fracture toughness without prior knowledge of elastic properties, and has potential applications to assess the effects of high temperature, oxidation and irradiation in small specimens of nuclear graphite

Seismic Behaviour of Cast-In-Situ Phosphogypsum-Reinforced Concrete Grid Frame Composite Walls

This paper mainly studies the effect of cast-in-situ phosphogypsum on seismic behaviour of reinforced concrete grid frame. The mechanical behaviour of three reinforced concrete grid frames and four cast-in-situ phosphogypsum-reinforced concrete grid frame composite walls under low cycle alternating load was designed and tested. The test results show that the reinforced concrete grid frame has less bearing capacity and poor energy consumption. The addition of cast-in-situ phosphogypsum can effectively improve the seismic behaviour of the reinforced concrete grid frame. Compared with the reinforced concrete grid frame, the bearing capacity of the cast-in-situ phosphogypsum-reinforced concrete grid frame composite wall is increased by 2-3 times, the displacement ductility coefficient is increased by 0.95∼1.2 times, and the relative accumulative energy consumption is increased by 86%∼216%. This shows that the composite wall has better bearing capacity, ductility, and energy dissipation capacity

Fracture toughness evaluation of a nuclear graphite with non-linear elastic properties by 3D imaging and inverse finite element analysis

Effective small specimen tests are needed to obtain fracture toughness and elastic properties as the limited availability of irradiated graphite restricts the quantity and dimensions of test specimens. Both properties have evaluated simultaneously in a crack propagation test with the double cleavage drilled compression (DCDC) specimen geometry of a fine-grained graphite (SNG742) that has non-linear elastic properties in the unirradiated condition. Three-dimensional displacement fields were obtained by digital volume correlation of in situ laboratory X-ray computed tomographs, and the 3D crack geometry (crack tip position, crack opening displacements and angle) was determined objectively by a wavelet variance method. The tensile softening of the Young modulus was determined by inverse analysis of the strain field using the finite element model updating (FEMU) method. The strain energy release rate of the quasi-static propagating crack was calculated using the contour integral method in a finite element model with the derived non-linear elastic properties and the measured displacements as boundary conditions. The critical strain energy release rate was constant with crack length (118 ± 14 J m−2) and equivalent to a fracture toughness of 1.13 ± 0.07 MPa m1/2.</p