Bolt is a kind of supporting body that transmits tensile load to the stable rock layer to improve the stability and strength of the rock medium, bolt has a lower cost in terms of material and manpower, so it has been widely used in mining, transportation, water conservancy and urban underground space engineering [1–4]. With the development of rock anchoring problems in the direction of "large, deep and difficult", sudden engineering disasters and major malignant accidents caused by nonlinear large deformation damage have occurred frequently in recent years. The reason for these problems is that the traditional supporting materials including traditional bolts, anchor cables, U-shaped steel brackets and other traditional supporting materials frequently fail [5–6]. One of the root causes is that the materials of these supporting bodies belong to the traditional Poisson's ratio materials, i.e. plastic hardening materials, which instantly reach the yield strength and lose the bearing capacity under a large load, leading to failure of the support [7–10].
The Poisson's ratio ν is first discovered and proposed by French scientist Poisson (Simon Denis Poisson, 1781-1840). Poisson’s ratio is also called the transverse deformation coefficient, which refers to the absolute value of the ratio of the transverse normal strain to the axial normal strain when the material is subjected to uniaxial tension or compression. It is the elastic constant reflecting the material transverse deformation. Expressed perpendicular to the section when they are compressed by external force. It is generally considered that almost all materials have a positive Poisson's ratio, such as the Poisson's ratio of rock is generally 0.2 to 0.3, wood (along grain) is 0.05, concrete is 0.16 to 0.18, and alloy steel is 0.25 to 0.30 .
According to thermodynamic theory, the Poisson's ratio of isotropic material ranges from − 1 to 1, so there are negative Poisson's ratio materials in theory [11, 12]. Lakes was the first to obtain a new type of material of a concave unit structure with a Poisson's ratio − 0.7 in a series of treatments of ordinary polyurethane foam in 1987 . This also means that when the material is subjected to axial tension, it will occur lateral expansion deformation (see Fig. 1b). This kind of auxetic behavior greatly improves the material mechanical properties such as shear modulus, fracture toughness, energy absorption and indentation resistance. In addition, negative Poisson's ratio materials have a good stress dispersion effect, a very high shear modulus and a very low bulk modulus [14–17]. Because negative Poisson's ratio materials have stronger physical and mechanical properties than positive Poisson's ratio materials, they can be identified as structural materials and functional materials at the same time. Therefore, they have broad application space in the fields of biomedicine, automobile industry, textile, human body protection and national defense technology [18–20].
Figure 1. The deformation of axial tension material
Because the physical and mechanical parameters of different experimental samples have a certain degree of discreteness, it is basically impossible to change only one physical and mechanical parameter of the experimental sample while keeping other physical and mechanical parameters the same in indoor and outdoor comparative experiments [21, 22]. However, the numerical experiment method can be used to quickly adjust the parameters, and load in various directions can be applied arbitrarily, so that only one parameter can be changed under the condition that other parameters remain unchanged, so as to ensure the accuracy of the comparison results. In addition, the numerical experiment method adopted in this paper can take into account the heterogeneity of the rock and the complex stress conditions, so this method has strong applicability.
Based on the special mechanical properties of negative Poisson's ratio material, as well as the advantages of numerical experiment method, we used RFPA software to study the failure characteristics of anchored rock with negative Poisson's ratio bolt. The similarities and differences of stress, displacement and acoustic emission (AE) of anchored rocks with positive Poisson's ratio bolt and negative Poisson's ratio bolt had been compared and analyzed.