The algorithms for Digital image correlation (DIC) in subpixel determination have been well developed regarding the accuracy and efficiency. In this paper, an efficient integer-pixel search scheme with combination of an improved particle swarm optimization (PSO) algorithm and the block-based gradient descent search (BBGDS) algorithm has been proposed. Incorporated with the inverse compositional Gauss-Newton (IC-GN) algorithm for subpixel registration and the parallel computing technology, a real-time DIC algorithm for displacement or strain measurement for dynamic tests has been achieved. Numerical simulation and the experimental results showed that the proposed method could reach a rate of 60 fps (frames per second) for strain measurement under cyclic loading with equivalent accuracy compared with the conventional DIC algorithm.

•A modified NR algorithm was proposed which was useful in case of large rotation.•A synchronized stereo high-speed camera system was established.•This technique could be potentially applied to on-line monitoring the work condition of the wind turbine.Non-contact and accurate motion measurement of the rotary objects is crucial in engineering applications. A modified Newton–Raphson algorithm, which is capable of positioning marks with large rotation, has been proposed. A stereo imaging system with a pair of synchronized digital high-speed cameras was developed and achieved full-field displacement measurement based on 3D image correlation photogrammetry for rotary objects. This system has been applied to measuring the 3D motion of a wind turbine blade model. The displacement components of the rotary blade were presented, and the corresponding frequency spectra were investigated. The experimental results demonstrated that the proposed system could measure the 3D motion of rotary blades precisely, and it also provided an alternative potential non-contact diagnosis means for large wind turbine blades.

Biological materials exhibiting non-self-similar hierarchical structures possess desirable mechanical properties. Motivated by their penetration resistance and fracture toughness, the mechanical performance of model materials with non-self-similar hierarchical structures was explored and the distinct advantages were identified. A numerical model was developed, based on microscopic observation of enamel prisms. Computational simulations showed that the systems with non-self-similar hierarchy displayed lateral expansion when subjected to longitudinal tensile loading, which reflected negative Poisson׳s ratio and potential for greater volume strain energies when compared with conventional materials with positive Poisson׳s ratio. Employing the non-self-similar hierarchical design, the capability of resilience can be improved. Additionally, the non-self-similar hierarchical structure exhibited larger toughness, resulting from the large pull-out work of the reinforcements. The findings of this study not only elucidate the deformation mechanisms of biological materials with non-self-similar hierarchical structure, but also provide a new path for bio-inspired materials design.Materials with non-self-similar hierarchical structures exhibit high toughness which is represented with pull-out work (Fig. 1). Under appropriate design principles artificial materials with negative Poisson’s ratio can be achieved (Fig. 2).

The purpose of this study was to investigate how cyclic loading influenced the fracture toughness of hot-press lithium disilicate and zirconia core materials and whether there was an increase in the propensity for crown failure. Two types of all-ceramic crowns including the IPS e.max Press system (n=24) and the Lava zirconia system (n=24), were selected. Sectioned specimens were subjected to cyclic loading with the maximum magnitude of 200 N (R=0.1) until two million cycles. The material properties including Young's modulus (E) and hardness (H) and the fracture toughness (KIC) of the core materials were evaluated using indentation methods (n=12 each). The load-bearing capacities of the specimens were examined by means of monotonic load to fracture (n=12 each). It was found that the material properties, including E, H and KIC, of the two types of dental ceramics, were reduced. Statistical analysis indicated that there were no significant influences of fatigue loading on material properties E and H for both types of dental ceramics or KIC for zirconia, while for the IPS e.max Press core, KIC, which was parallel to the direction of the lithium disilicate crystals, was significantly reduced (P=0.001). A conclusion was drawn that zirconia possesses high mechanical reliability and sustainable capacity to resist fatigue loading, while fatigue loading remarkably degraded the anisotropic mechanical behaviour of hot-press lithium disilicate ceramics.

The purpose of this study was to investigate how water aging of the resin cement influences the stress distribution in all-ceramic crowns and if there is an increase in the propensity for crown failure. The failure of all-ceramic crowns attributed to cement degradation was explored using a combination of experimental and numerical methods. Sectioned all-ceramic crown specimens were fabricated of IPS e.max Ceram/e.max Press (CP) and Vita VM9/Cercon zirconia (VZ), and then stored in either air or distilled water for 30 days. Monotonic contact loads were applied to fracture near the buccal cusp ridge of each sample. Deformation within the crown layers during loading was analyzed by means of Digital Image Correlation (DIC). A 3D finite element model of the restoration including veneer, core, cement and tooth substrate was developed to evaluate the stress distribution in the crowns before and after cement degradation. There was a significant decrease (p<0.001) in the critical fracture load and a change in the fracture mode after cement water absorption in the CP crowns. In contrast, there was no significant influence of cement aging on fracture modes and fracture loads (p>0.05) in the VZ crowns. Finite element analysis showed that regardless of the crown types, the stress distribution is identical by degradation in Young's modulus of the cement. However, core/substrate debonding results in a change of the stress distribution and a significant increase in the magnitude. Water aging causes reduction of stiffness and bonding strength of cement agents. Degradation in bonding strength and stiffness could potentially lead to stress redistribution in the restored crown and reduce the load-bearing capacity of all-ceramic restorations after years of service.Highlights► Water aging results in mechanical degradation of cements. ► Degradation of cements increases stress level in restored crowns. ► Mechanical degradation of cements includes two factors. ► Loss of bond strength is detrimental to dental ceramics with low flexural strength.

The superior mechanical properties of enamel, such as excellent penetration and crack resistance, are believed to be related to the unique microscopic structure. In this study, the effects of hydroxyapatite (HAP) crystallite orientation on the mechanical behavior of enamel have been investigated through a series of multiscale numerical simulations. A micromechanical model, which considers the HAP crystal arrangement in enamel prisms, the hierarchical structure of HAP crystals and the inelastic mechanical behavior of protein, has been developed. Numerical simulations revealed that, under compressive loading, plastic deformation progression took place in enamel prisms, which is responsible for the experimentally observed post-yield strain hardening. By comparing the mechanical responses for the uniform and non-uniform arrangement of HAP crystals within enamel prisms, it was found that the stiffness for the two cases was identical, while much greater energy dissipation was observed in the enamel with the non-uniform arrangement. Based on these results, we propose an important mechanism whereby the non-uniform arrangement of crystals in enamel rods enhances energy dissipation while maintaining sufficient stiffness to promote fracture toughness, mitigation of fracture and resistance to penetration deformation. Further simulations indicated that the non-uniform arrangement of the HAP crystals is a key factor responsible for the unique mechanical behavior of enamel, while the change in the nanostructure of nanocomposites could dictate the Young’s modulus and yield strength of the biocomposite.Comparison of the densities of plastic energy dissipation with the non-uniform and uniform arrangement of crystallites in enamel prisms. Comparison of the plastic deformation distribution with the non-uniform and uniform arrangement of crystallites in enamel prisms.

In this study, the mechanical design principles of human enamel were evaluated using a hybrid experimental and computational approach. Nanoindentation was applied to evaluate the load-depth response of human enamel, and Vickers indentations were used to assess the damage behavior. An elastic–plastic numerical model was then developed to analyze the stress and strain distribution about the indentations, and to characterize energy dissipation about indents in three locations including inner, middle and outer enamel. Results confirm that enamel exhibits a gradient in its mechanical behavior. Outer enamel has a limited potential for energy dissipation by inelastic deformation, indicating that the ability of outer enamel to resist fracture is low. While inner enamel, the region close Dentin Enamel Junction (DEJ), possesses less resistance to penetration deformation, it has a much higher capacity to dissipate energy by inelastic deformation than outer enamel. The computational simulations identified that the gradients in mechanical properties of human enamel promote resistance to penetration, energy dissipation and mitigation of fracture, all critical performance requirements of human teeth.

Digital image correlation (DIC) has been widely conducted in many engineering applications. This paper describes a dual-camera system which is mounted on a stereo light microscope to achieve 3D displacement measurement at microscale. A glass plate etched with precision grids was used as the calibration plate and a translation calibration procedure was introduced to obtain the intrinsic and extrinsic parameters of the cameras as well as the aberration of the imaging system. Two main error sources, including grid positioning and stage translation, were discussed. It was found that the subpixel positioning errors had limited influences on displacement measurement, while the incorrect grid positioning can be avoided by analyzing the standard deviation between the grid spacing. The systematic translation error of the stage must be eliminated to achieve accurate displacement measurement. Based on the above analysis, a precisely controlled motorized calibration stage was developed to fulfill fully automatic calibration for the microscopic dual-camera system. An application for measuring the surface texture of the human incisor has been presented. It is concluded that the microscopic dual-camera system is an economic, precise system for 3D profilometry and deformation measurement.Highlights► A stereo microscope camera system incorporated with the motorized translation stage and the Tsai's calibration method was presented to achieve 3D profile measurement. ► The error introduced by the motorized stage in calibration step could cause severe errors in displacement measurement. ► The ratio of f to Tz can be used to determine if the camera calibration is precise, since it should be equal to the magnification factor of the microscopic system.

The objective of this investigation is to explore the region-dependent damage behavior of enamel, as well as to develop a good understanding of the deformation mechanisms of enamel with numerical modeling. Nanoindentation experiments have been performed to investigate the load-penetration depth responses for outer and inner enamel. Results show that the unloading curve does not follow the loading curve, and degradation of stiffness in the unloading curve is observed. Based on the experimental data, a physical quantity, the chain density in protein, has been introduced to the Drucker-Prager plastic model. Numerical simulations show that the simulated load-penetration depth curves agree with the experiments, and the stiffness degradation behaviors of outer and inner enamel are captured by the numerical model. The region-dependent damage behavior of enamel could be revealed by the numerical model. The micro damage affected area at inner enamel is larger than that at outer enamel, indicating that the inner enamel experiences more micro damage than the outer one. Compared with its outer counterpart, the inner enamel which is rich in organic protein could break more internal protein chains to dissipate energy and to enhance its resistance to fracture accordingly.

In this investigation, the crack propagation mechanisms contributing to the toughness of cortical bone were studied using a combination of experimental and numerical approaches. Compact tension (CT) specimens were prepared from bovine cortical bones to achieve crack propagation in the longitudinal and transverse directions. Stable crack extension experiments were conducted to distinguish the crack growth resistance curves, and virtual multidimensional internal bond (VMIB) modeling was adopted to simulate the fracture responses. Results from experiments indicated that cortical bone exhibited rising resistance curves (R-curves) for crack extension parallel and perpendicular to the bone axis; the transverse fracture toughness was significantly larger, indicating that the fracture properties of cortical bone are substantially anisotropic. Microscopic observations showed that the toughening mechanisms in the longitudinal and transverse directions were different. When the crack grew in the transverse direction, the crack deflected significantly, and crack bifurcations were found at the crack wake, while, in the longitudinal direction, the crack was straight and uncracked ligaments were observed. Numerical simulations also revealed that the fracture resistance in the transverse direction was greater than that in the longitudinal direction.

This paper presents an automatic approach for the evaluation of isochromatics and isoclinics in photoelasticity using complementary phase shifting. First, the phase values of the isoclinics are obtained from four images in the plane polarizer arrangement by rotating the polarizer and analyzer simultaneously. Then, through use of the isoclinics, a full-field description of the first principal stress orientation with respect to the horizontal axis is determined. Phase maps for the isochromatics are then achieved at eight discrete orientations through sequential analyzer rotations. With the first principal stress orientation known from the isoclinic phase map, a whole-field description of the phase values of the isochromatics is then constructed. The proposed method was validated for the problem of a ring under diametrical load.

•An optical extensometer is used for dynamic strain measurement in fatigue strain-life test.•Sequential images can be recorded at 10 Hz using a common imaging system.•A complete strain-life curve has been obtained for thin sheet steel.Characterizing the strain-life fatigue behavior of thin sheet metals is often challenging since the required specimens have short gauge lengths to avoid buckling, thereby preventing the use of conventional mechanical extensometers. To overcome this obstacle a microscopic optical imaging system has been developed to measure the strain amplitude during fatigue testing using Digital Image Correlation (DIC). A strategy for rapidly recording images is utilized to enable sequential image sampling rates of at least 10 frames per second (fps) using a general digital camera. An example of a complete strain-life fatigue test for thin sheet steel under constant displacement control is presented in which the corresponding strain within the gage section of the specimen is measured using the proposed imaging system. The precision in strain measurement is assessed and methods for improving the image sampling rates in dynamic testing are discussed.

The mechanisms contributing to failure of full dental ceramic crowns under occlusal loads were studied using a unique optical approach. Model specimens comprising triple-layered crowns (veneer, core and substrate) were developed with both flat and curved occlusal surfaces and then subjected to simulated quasi-static occlusal loading using a spherical indenter. Deformation within the specimens during loading was analyzed by means of digital image correlation (DIC). Finite element models were also developed and used to examine the mechanics of contact. Results of the experiments with flat dental crowns indicated three typical modes of failure, i.e. cone cracks, plastic yielding and radial cracks. Fracture of the specimens with curved dental crowns was complicated by contributions from competing and multiple modes of failure. Both experimental and numerical results conclude that the dominant fracture mode in the full-ceramic crowns was radial cracking in the core beneath the contact area. However, displacement fields obtained using DIC showed that debonding developed near the shoulder of the crown, particularly during off-axis loading, and initiated under substantially lower occlusal loads than those required for crack initiation.

In this study, a theoretical framework for simulation of fracture of bone and bone-like materials is provided. An expanded cohesive zone model with thermodynamically consistent framework has been proposed and used to investigate the crack growth resistance of bone and bone-like materials. The reversible elastic deformation, irreversible plastic deformation caused by large deformation of soft protein matrix, and damage evidenced by the material separation and crack nucleation in the cohesive zone, were all taken into account in the model. Furthermore, the key mechanisms in deformation of biocomposites consisting of mineral platelets and protein interfacial layers were incorporated in the fracture process zone in this model, thereby overcoming the limitations of previous cohesive zone modeling of bone fracture. Finally, applications to fracture of cortical bone and human dentin were presented, which showed good agreement between numerical simulation and reported experiments and substantiated the effectiveness of the model in investigating the fracture behavior of bone-like materials.