非对称荷载下含中心孔洞煤岩变形损伤特性与本构模型
Deformation damage characteristics and constitutive model of raw coal and briquette with central hole under asymmetric loading
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摘要: 受地势起伏或工程开挖影响,隧道与地下开采巷道常受到不同程度的非对称荷载作用。 为 了分析非对称荷载对煤岩变形损伤规律的影响,以含中心孔洞原煤与型煤为研究对象,采用数字图 像相关( DIC) 的全场非接触应变测量方法,实时评估了不同程度非对称荷载单轴压缩下煤岩表面 应变场局部化演化进程,分析了不同加载方式下煤岩的损伤破坏过程;基于统计指标定义了损伤变 量,建立了能够同时反映原煤与型煤全应力应变特征的损伤本构模型。 研究结果表明:非对称荷载 不会改变原煤与型煤试样应力应变曲线的阶段性特征,两者应力应变曲线仍表现为初始压密阶段、 线性变形阶段、塑性屈服阶段与破坏后阶段;原煤与型煤试样的峰值应力、局部化启动应力均随着 荷载非对称程度的增加呈线性函数规律降低,荷载非对称程度越大,煤样越容易发生破坏;均布荷 载压缩时,中心孔洞可看做煤样的初始损伤,压密阶段原煤主要表现为原生裂隙受压闭合与中心孔 洞的变形,高应变值呈全域散乱分布,型煤试样主要表现为内部颗粒相对错动导致的颗粒间孔隙减 少与中心孔洞的变形,高应变值在中心孔洞集中,均布荷载压缩时含孔原煤主要发生近 X 型剪切 破坏,表现出硬脆性特征;型煤试样主要发生拉-剪复合破坏,表现出软塑性特征;非对称荷载作用 时,由于加载区与非加载区交界面剪切作用带的存在,原煤与型煤均发生宏观剪切破坏,应变局部 化带能够很好反映表面主控裂纹位置,应变局部化带与表面主控裂纹的形态受加载区与中心孔洞 的相对位置影响,且中心孔洞的存在可引导两者的演化方向,其引导效果受控于煤岩强度,煤岩强 度越大,中心孔洞对两者形态影响越小;建立的损伤本构模型能够较好地反映室内试验原煤与型煤 应力应变曲线各阶段特征。Abstract: Under the influence of topographic fluctuation or engineering excavation, some tunnels and mining roadways are often subjected to different degree of asymmetric loading. In order to analyze the influence of asymmetric load on the deformation and damage law of coal and rock, the raw coal and briquette coal with central holes were used as the research object, the full⁃field non⁃contact strain measurement approach of Digital Image Correlation (DIC) was employed for the real⁃time evaluation of the progression of strain⁃field localization and damage evolution of the coal specimens, and the damage and failure process of coal and rock under different loading modes was analyzed. The dam⁃ age variables were defined based on the statistical index, and the damage con⁃stitutive model was established to re⁃ flect the full stress and strain characteristics of coal and rock. The results show that the unsymmetrical load will not change the phased characteristics of the stress⁃strain curves of raw coal and briquette coal samples and the stress⁃strain curves of them still show the initial compaction stage, linear deformation stage, plastic yield stage and post failure stage. The peak stress and localized starting stress of raw coal and briquette coal samples decrease linearly with the increase of the load asymmetry degree. The greater the degree of load asymmetry, the more likely the coal sample is to be damaged. During uniform load compression, the central hole can be regarded as the initial damage of the coal sample. In the compaction stage, the raw coal is mainly characterized by the compression closure of the pri⁃ mary fracture and the deformation of the central hole, and the high strain value is scattered in the whole area. The bri⁃ quette sample is mainly characterized by the reduction of inter particle pores and the deformation of central hole caused by the relative dislocation of internal particles, and the high strain value is concentrated in the central hole. Under uniform load compression, the porous raw coal mainly occurs near⁃x⁃type shear failure, showing the hard brittle characteristics. The briquette samples are mainly subjected to tensile⁃shear composite failure, showing soft plastic characteristics. Under the action of asymmetric load, the existence of the shear action zone at the interface between loading area and non⁃loading area causes the macro shear failure of raw coal and briquette coal. The strain localization zone can well reflect the location of the main control crack on the surface. The forms of the strain lo⁃ calization band and the main control crack are affected by the relative position of the loading area and the central hole, the existence of the central hole will guide the evolution direction of the strain localization zone, and the guiding effect is controlled by the strength of coal and rock. The greater the strength of coal and rock, the smaller the influence of central hole on the forms of the strain localization band and the main control crack. The estab⁃ lished damage constitutive model can better reflect the characteristics of each stage of stress⁃strain curve of raw coal and briquette coal in laboratory test.