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Nuclear graphite is widely used as a moderator, reflector, and core structure material in very/high temperature gas-cooled reactors. However, because graphite may experience impacts with medium-low velocities during its transport and installation or under extreme conditions such as severe earthquakes, it is necessary to evaluate the dynamic fracture toughness of graphite to assess the safety of the reactors. The ASTM standard recommends a formula to calculate the fracture toughness; however, this formula includes the crack length parameter, which cannot be measured accurately in a dynamic process. To enable the accurate measurement of the dynamic crack propagation length of graphite and subsequently obtain the fracture toughness, a virtual extensometer method based on digital image correlation was established in this study. Three-point bending experiments were conducted using pre-cracked specimens to obtain the dynamic fracture toughness of graphite IG11 under medium-low impact velocities. The results indicated that the dynamic fracture toughness of graphite IG11 increased with the increase in the impact velocities, and its value was substantially higher than the static fracture toughness in certain cases. The proposed approach involves a simple operation, yields a high accuracy, and can enable automatic calculation. Moreover, this approach can be used to evaluate the dynamic fracture performance of other brittle or quasi-brittle materials.

A new photogrammetry based measurement technique for contact areas in line contact structures is developed. From the data collected by this technique, a method for measuring the contact strength of contact structures in high temperature applications is proposed. The contact strength of a line contact structure of polyimide at different temperatures is measured; the results show that the increase in temperature decreases the contact strength of the structure. The effect of temperature on the contact strength is studied by investigating the occurrence and evolution of yielding core within the structure at different temperatures. The decrease in the yield strength and elastic modulus of the polyimide material at high temperature induces core yielding at lower loads and eases the propagation of core yielding to the surface.

Subset-based digital image correlation (DIC) has limitations in subset size selection for heterogeneous deformation field measurement. To maintain good spatial resolution, selecting sufficiently small subsets is necessary, but the measurement resolution is reduced because of poor matching information, inducing large errors in the correlation matching procedure. Spatial-temporal subset digital image correlation (STS-DIC) incorporates the temporal continuity of displacement in the deformation analysis. This can increase the matching information of subset in the temporal dimension. Here, the advantages of STS-DIC for the measurement of heterogeneous deformation fields were investigated through simulations and experiments. The results demonstrated that the measurement resolution was improved while ensuring a good spatial resolution for heterogeneous deformation field measurement by using STS-DIC. Finally, STS-DIC was applied to calculate the displacement field near a precast crack, and the stress intensity factors were calculated accurately from high accuracy displacement fields.

The yielding formation and propagation of a ductile polycarbonate cylinder subjected to line loading, which corresponds to the global behavior of the structure, was experimentally studied and evaluated by measuring the strain fields on the end of the cylinder using Digital Image Correlation. The global behavior of the structure is expressed by a relationship between the average stress (load divided by contact area) and the equivalent strain (ratio of half width of contact area to radius of the cylinder); the contact area was measured in the same experiment by analyzing the images of the compressed pressure film captured through a transparent plate below the cylinder. A correspondence between the yielding state and the nonlinearity of the global behavior was constructed and, as loading was increased, the cylinder was found to first yield at a specific point after which a yielding core formed and propagated. Before the yielding core propagated to the surface of the cylinder, the global behavior of the structure remained linear, although the cylinder was no longer homogeneous and the material was nonlinear. After the yielding core propagated to the surface of the cylinder, the propagation was accelerated and the global behavior became nonlinear. The correspondence constructed in the paper will be helpful to understand the failure process and to evaluate the carrying capacity of a ductile polycarbonate cylinder subjected to line loading.

Compared to other approaches to improving digital image correlation (DIC) measurement accuracy, improving image quality is becoming increasingly effective. Image noise level, which is the main indicator of image quality, is very sensitive to the configurations of an imaging system and affects DIC measurement accuracy. However, the mechanism by which the imaging configurations affect accuracy is unclear. In this study, the influence of imaging configuration on DIC measurement accuracy was investigated using the common factor of noise. Firstly, the type and levels of noise introduced into actual images by different imaging system configurations were investigated. Next, the effect of noise on DIC measurement accuracy was determined using simulated images. Finally, the effects of imaging configuration on DIC measurement accuracy were experimentally investigated using actual measurements. The results show that both camera-related and lens-related configurations affect DIC measurement accuracy. In particular, lengthening the exposure time and reducing the f-number of the lens achieve similar improvement in DIC measurement accuracy, and a prime lens with high light transmittance is recommended for full-field optical measurements. In contrast, the imaging gain does not affect the accuracy of DIC measurements.

The systematic strain measurement error in Digital Image Correlation (DIC) induced by the environment temperature variation around the digital camera was extensively studied. The temperature variation of different camera components along with the changes of the environment temperature is experimentally studied and the motions of different components are then analyzed. The strain error in DIC is then analyzed according to a physical model to express the imaging geometry changes. Finally, the DIC measurement error caused by environment temperature variation was experimentally verified.

A simple uniaxial oedometric system is developed to test the elastic modulus of granular materials. The stress–strain relationship is first measured under conditions of uniaxial compression with additional lateral stress and strain, then the elastic modulus of the material is determined by the linear fitting method. It is found that the modulus is positively correlated to the grain size and negatively correlated to the container size. Arching and dragging are revealed to be the mechanism of such correlations by using the digital image correlation method and the pressure film technology based on the statistical method.

In this work, a pre-cracked semi-circular shaped explosive simulant was loaded using a split Hopkinson pressure bar (SHPB). A high-speed camera was used to capture the deformation and fracture process of the specimen in situ. The digital images were processed using the digital image correlation (DIC) method. Next, full displacement and strain fields were obtained. The displacement vector field shows that the specimen fractured under tensile stress action. The strain field can be used to predict the crack propagation. Results show that the method of combined DIC and SHPB is effective to study the dynamic deformation and fracture behavior of explosive simulants. In addition, the specimen was loaded using a drop weight. The fracture toughness of the specimen was preliminary measured.