Physical Modeling Investigation On Failure Mechanism of Layered Surrounding Rock And Deformation Characteristics Based On Digital Speckle Correlation Method

In rock mass engineering such as underground coal mine roadway, underground tunnel and water conservancy and hydropower chamber construction, layered rock mass is a type of surrounding rock mass that is often encountered. Due to the influence of bedding structural plane, its deformation process and failure mode are obviously different from that of intact enclosed rock mass. For the layered rock structure of roadway, the previous studies mainly focused on the situation of no support or single bolt support, but there is still a lack of experimental studies on the mechanical properties and failure laws of surrounding rock under different support methods. In this paper, the deep roadway and surrounding rock environment with a buried depth of 742-877m are taken as the engineering background, which is located in Anhui Province, China. Based on the similarity theorem and similarity criterion, river sand is used as aggregate, gypsum and cement are used as cementing material. According to the test results, 5:0.7:0.3, 6:0.3:0.7 and 4:0.5:0.5 of river sand, gypsum and cement were selected as the similar materials to simulate the surrounding rock of the roadway, and the bedding plane structure of the surrounding rock of the roof and floor near the roadway was designed by using the self-developed similar material physical model test bed. Digital Speckle Correlation Method (DSCM) is used to analyze the deformation evolution process of roadway surrounding rock under three supporting schemes: no-support scheme, bolt support scheme, and synergistic support of bolt and anchor cable. Based on this, the failure mode and instability mechanism of deep layered surrounding rock are further discussed. The results show that the tensile and shear resistance of laminated roof surrounding rock is weak, and the laminated roof is easily separated from each other at the bedding plane without support, resulting in bending deformation and bed separation. After adopting effective support, the laminated roof surrounding rock is transformed into a composite beam bearing structure, and the stability of surrounding rock increases. The research results have certain theoretical guiding significance and engineering application value for practical engineering.

Residual stress analysis by digital spectral correlation

It is essential to determine residual stress, as it can have a significant influence on the fatigue lives of engineering components, and as it can be induced unintentionally as well as intentionally. A classical method of measuring residual stress is to use hole drilling in combination with strain gauges, which themselves have inherent disadvantages. The Digital Speckle Correlation method (DSC) is a computerized method which determines displacement and strains and consequently calculates stress. This project demonstrates the viability of hole drilling in combination with DSC.

Residual stress analysis by digital spectral correlation

It is essential to determine residual stress, as it can have a significant influence on the fatigue lives of engineering components, and as it can be induced unintentionally as well as intentionally. A classical method of measuring residual stress is to use hole drilling in combination with strain gauges, which themselves have inherent disadvantages. The Digital Speckle Correlation method (DSC) is a computerized method which determines displacement and strains and consequently calculates stress. This project demonstrates the viability of hole drilling in combination with DSC.

Tear strength of a laminated fabric for stratospheric airship under uniaxial and biaxial tests

Envelopes are main structures of stratospheric airships. They are usually made of laminated fabrics and prone to tearing, so it is significant to study their tear strengths. This paper aims to investigate the tear strength of an envelope for stratospheric airships by uniaxial and biaxial tear tests. Three uniaxial tear specimens and 15 biaxial tear specimens under five different stress ratios (warp stress vs weft stress) were tested, and their tear strengths were measured. Two-dimensional digital speckle correlation method was used to obtain specimens’ strain contours. The test results show that the average tear strength of uniaxial specimens is 32.99 N/mm, over 20% higher than those of biaxial specimens, ranging from 25.50 N/mm to 27.25 N/mm. It reveals that weft stress reduces the tear strength; nevertheless, the stress ratios slightly affect the tear strength. Besides, the strain contours clearly show three zones in each specimen – the low-strain zone, the high-strain zone, and the medium-strain zone. Depending on the strain contours and previous research on imperfect composite materials, we inferred that the crack-tip stress concentration factor of a uniaxial specimen is lower than that of a biaxial specimen. It explains the difference in tear strength between uniaxial and biaxial specimens. These findings suggest using biaxial tear test to measure the tear strength of an SSA’s envelope.

Experimental Study on Structural Mechanics of Asphalt Pavement by Digital Speckle Correlation Methods

The experiments on the structural mechanics of asphalt pavement were carried out by the digital speckle correlation method (DSCM). The digital speckle correlation method is used to collect experimental data. The displacement and strain values of each layer of the asphalt pavement structure are analyzed. The research showed that (1) the digital speckle correlation methods could accurately observe the displacement value and the strain value of the pavement structure. (2) The stress state of the pavement structure was greatly influenced by the interlayer effect. The vertical displacement changed suddenly between layers. The concentration phenomenon of the tension-compression strain and the shear strain appeared between layers. (3) The overloading situation seriously worsened the overall stress state of the pavement structure, especially at the bottom of the asphalt top layer and lower layer and at the top of the cement-stabilized crushed stone. The research results are of great significance to the structural design of heavy-duty roads.

Experimental analysis of hysteretic behavior and strain field of external diaphragm joints between steel beams and CFDST columns

External diaphragm joints with high structural reliability are used widely in composite steel and concrete structures. In this study, the external diaphragm joint was improved to be a steel beam-to-concrete filled double steel tubular column connection. The digital speckle correlation method was used to measure and investigate the strain field in the panel zone and relative beam-to-column rotation in a low-cycle reciprocating loading test. The obtained hysteresis curves of moment-rotation ( M-θ) and shear force-deformation ( V-[Formula: see text]) showed that the external diaphragm joints had higher strength, higher ductility, and better energy dissipation capacity. Decreasing the axial compression ratio resulted in the deterioration of initial rotational stiffness. Wider external diaphragm produced better ductility and larger initial shear stiffness. The ribbed anchorage web was effective to increase the bending resistance by 10%. Beam-to-column bending stiffness ratio can not only influence the bending resistance and energy dissipation capacity significantly but also affect the shear deformation capacity in the joint core. The magnitude of the shear strain in the panel zone was large, especially for the specimens under column failure mode, and shear deformation in the panel zone should not be neglected for it accounted for 30%–40% of the beam-to-column rotation. Beam-to-column rotation and shear deformation obtained by the digital speckle correlation method offered better predictions to analyze the mechanical behavior of external diaphragm joints in the concrete filled double steel tubular structures.