Digital photoelasticity has rapidly progressed in the last few years and has matured into an industry-friendly technique. This review thematically classifies all the developments in digital photoelasticity and highlights the relative merits and drawbacks of the various techniques. The overall objective is to provide enough information and guidance to allow an end-user to make an informed choice on the type of technique to be used in a particular situation.

Digital photoelasticity – A comprehensive review

A brief review and prospect of experimental solid mechanics in China

STATEMENT OF PROBLEM
The outcomes from load-to-failure tests may not be applicable to clinical situations.


PURPOSE
The purpose of this study was to critically evaluate the efficacy of load-to-failure tests in the investigation of the fracture load and pattern of metal-free crowns.


MATERIAL AND METHODS
Four groups were formed from 128 bovine roots restored with metal posts, resin cores, and feldspathic, leucite, or lithium disilicate ceramic systems or polymer crowns. Each group was divided into 4 (n=8) according to the cement: zinc phosphate, self-adhesive resin, autopolymerizing resin, and glass ionomer. Mean fracture loads from compressive tests were submitted to ANOVA and Tukey HSD test. Finite element and fractographic analyses were performed and associated with the fracture load and pattern.


RESULTS
Significantly higher fracture load values were obtained for the lithium disilicate ceramic, but finite element and fractographic analyses showed that the cement effect could not be determined. The finite element analysis showed the cement likely affected the fracture pattern, confirmed that stresses in the cements were little affected by the crown materials, and found that the stressed conditions were lowest in the lithium disilicate compared with other crowns for all cement combinations. The stressed conditions in the crowns depended more on the adhesive properties than on the elastic modulus of the cement materials. The level of the stressed condition in the crowns at the occlusal surface was about the same or higher than along their cement interface, consistent with the fractography, which indicated fractures starting at the load point. Higher stress levels in the crowns corresponded with a lower number of catastrophic fractures, and higher stresses in the cements seemed to reduce the number of catastrophic fracture patterns. The highest stressed conditions occurred along the occlusal surface for crown materials with a low elastic modulus or in combination with adhesive cements.


CONCLUSIONS
The method used was not appropriate either for investigating the crowns' fracture load and pattern or for stating the role of the cements within the crown-cement-tooth interaction.

Crown Fracture: Failure Load, Stress Distribution, And Fractographic Analysis

Branch cutting algorithm for unwrapping photoelastic phase map with isotropic point

The physical visualisation of a three-dimension (3D) stress field is a promising method for quantitatively analysing and revealing the stress distribution and evolution of a porous solid, and it significantly contributes to the understanding of the governing effects of stress fields on the mechanical behaviours of complex porous solids. However, experimental limitations regarding the manufacture of complex porous models and the extraction of the stress distributions in matrices inhibit the accurate visualisation of the 3D stress fields of porous structures. This paper presents a method of experimentally visualising and elucidating the 3D structures and stress fields of porous solids using photopolymer materials, 3D printing, the frozen-stress method, and photoelastic tests. Transparent thick discs containing various randomly distributed pores were produced using photopolymer materials and 3D printing technology. Experimental measures, including the frozen-stress method, photoelastic testing, and the phase-shifting method, were applied to quantitatively characterise the 3D stress fields distributed throughout the porous discs under radial-direction compressive loads. The temperature for ‘freezing’ stresses in the photopolymer materials was experimentally determined. The effects of pore distribution and population on the stress-field characteristics were investigated. The experimental results were used to validate the numerical analysis of the stress-field characteristics of the porous models. The visualisation test results agreed well with those of the numerical simulations. The proposed method can be used to visually quantify the characteristics and evolution of the 3D stress fields of porous solids.

Experimental Visualisation Methods for Three-Dimensional Stress Fields of Porous Solids

https://www.umbc.edu/lamp/pubwork/Zhang_et_al_optics_and_lasers_in_engineering_2007.pdf

Automatic determination of parameters in photoelasticity

Photoelasticity is an effective method for evaluating the stress and its spatial variations within a stressed body. In the present study, a method to determine the stress distribution by means of phase shifting and a modified shear-difference is proposed. First, the orientation of the first principal stress and the retardation between the principal stresses are determined in the full-field through phase shifting. Then, through bicubic interpolation and derivation of a modified shear-difference method, the internal stress is calculated from the point with a free boundary along its normal direction. A method to reduce integration error in the shear difference scheme is proposed and compared to the existing methods; the integration error is reduced when using theoretical photoelastic parameters to calculate the stress component with the same points. Results show that when the value of Δx/Δy approaches one, the error is minimum, and although the interpolation error is inevitable, it has limited influence on the accuracy of the result. Finally, examples are presented for determining the stresses in a circular plate and ring subjected to diametric loading. Results show that the proposed approach provides a complete solution for determining the full-field stresses in photoelastic models.

https://iopscience.iop.org/article/10.1088/1361-6501/aaa7ae/pdf

Full-field stress determination in photoelasticity with phase shifting technique

The objective of this study was to compare the resistance to failure of three types of all-ceramic crowns under static and cyclic contact loading. Three types of crown specimens were fabricated (N = 14 per group) including: i) sintered bi-layer crowns (SBC) made of IPS e.max CAD veneers fused on Lava zirconia core ceramics, ii) adhesive bi-layer crowns (ABC) made of IPS e.max CAD veneers bonded onto Lava zirconia core ceramics, and iii) monolith ceramics crown (MCC) made of IPS e.max CAD. All were bonded onto a compliant substrate of resin composite with commercial adhesive. Eight specimens were selected from each group for monotonic loading to failure via 6 mm diameter ball to obtain the initial load capacity (Pinitial) of the crowns. The remaining six crown specimens were subjected to cyclic contact loading to 5 million cycles, and then loaded monotonically to fracture to obtain residual load capacity after fatigue (Presidual). A finite element analysis (FEA) was used to analyze the stress distributions within the three types of crowns and estimate the crack origins from the maximum stress. The initial load to failure (Pinitial) for the SBC, ABC, and MCC groups were 1120 ± 170 N, 970 ± 150 N, and 950 ± 60 N, respectively, and significantly different (p = 0.027). The residual load to failure after fatigue (Presidual) for the crown types were 890 ± 240 N, 680 ± 240 N, and 1050 ± 120 N, respectively, and also significantly different (p = 0.012). For SBC and MCC, failure occurred predominantly by bulk fracture, whereas the ABC failed primarily by chipping. The FEA suggested that failure of SBC and ABC would originate in the cement layers, and MCC has a more reasonable stress allocation than that in bi-layer crowns (SBC and ABC), which agreed with the observed failure modes. The SBC crowns had superior load to failure, whereas the bi-layer systems (SBC and ABC) had inferior fatigue resistance compared with the monolith crowns (MCC). The cement interface was the weak-link of the bi-layer systems, especially for ABC. Considering their use in clinical practice, the ranking in resistance to failure is SBC>MCC>ABC.

Bearing capacity of ceramic crowns before and after cyclic loading: An in vitro study

Photoelasticity has served as a valuable experimental method for performing stress analysis for decades. Since its inception, it has been applied in solving a tremendous number of engineering problems. This paper presents an automatic approach for the evaluation of isochromatics and isoclinics in photoelasticity using complimentary phase shifting. The phase map of the isoclinics is obtained from 4 images acquired in the plane polarizer arrangement by rotating the polarizer and analyzer simultaneously, as shown in Fig. l(a). Ambiguity exists due to the isoclinic phase values are wrapped in a range of (0,π/2), which indicate the angle made by either σ1 or σ2 with horizontal axis. We present a novel method to extend the range of isoclinic phase value to (- π/2, π/2), which represent the orientation of σ1 only, as shown in Fig. l(b). In circular polariscope, phase shifting can be applied to isochromatic fringe patterns if the orientation of σ1 is given analogous to Tardy compensation. Phase maps for the isochromatics are then achieved at 8 specifically selected discrete orientations through sequential analyzer rotations. With the knowledge of σ1 orientation, a whole-field description of the phase values of the isochromatics is then constructed, as shown in Fig. 2(a). The absolute phase value of isochromatics can be obtained through an unweighted leastsquares phase unwrapping processing, as shown in Fig. 2(b). Sophisticated software has been developed for automatic photoelastic data processing from image acquirement to stress analysis. In an effort to validate the process and illustrate application, an example of the proposed method is presented using a plate loaded in diametrical compression. Stress components at three horizontal sections of the circular plate are calculated with the shear stress differential method (Fig. 3). Results show good agreement with the theoretical solution and the method is easy to conduct and applicable to engineering problems.

Yongsheng Han

Papers

Automatic determination of parameters in photoelasticity

Full-field stress determination in photoelasticity with phase shifting technique

Bearing capacity of ceramic crowns before and after cyclic loading: An in vitro study

Stress Separation by Means of Phase Shifting in Photoelasticity