作为土工构筑物中常见的安全隐患，裂缝与土体的强度和结构稳定性有着密切联系。为了探究裂缝所引起的土体断裂破坏特征，本文借助断裂力学试验方法测试了不同角度预设裂缝（0°、20°、40°和60°）和不同干密度（1.4 g/cm³、1.5 g/cm³、1.6 g/cm³、1.7 g/cm³）半圆形黄土试样的Ⅰ型和Ⅱ型断裂强度，通过断裂加载系统采集了力-位移数据，分析了预设裂缝黄土的断裂破坏能量、最大承载力和断裂韧度变化规律；配合高速摄影捕捉到的黄土断裂破坏过程，厘清了黄土的裂缝发育规律；运用数字图像技术探究了黄土断裂破坏过程中的位移和应变特征；利用声发射技术监测了试样断裂破坏过程中的振铃计数、声发射能量、上升时间、持续时间和幅度值等声发射信号参数的变化特征，结合RA和AF之间的特征关系，判别了黄土的断裂破坏模式，得到如下结论：

（1）随着预设裂缝角度和干密度的增加，黄土的最大承载力和断裂能量不断增大。当干密度逐渐增大时，Ⅰ型、Ⅱ型和有效断裂韧度均不断增大，峰值出现在干密度1.7 g/cm³处。预设裂缝角度增大导致了Ⅰ型应力集中因子不断降低且Ⅱ型应力集中因子先增大后减小，使得在预设裂缝角度增大过程中，Ⅰ型断裂韧度和有效断裂韧度不断降低，Ⅱ型断裂韧度则先增大后减小，40°预设裂缝角度附近Ⅱ型断裂韧度出现峰值。

（2）黄土的断裂破坏实质上是新生裂缝从预设裂缝尖端出现并沿着轨迹线发育至上部加载点处的过程。在黄土断裂破坏过程中，随着预设裂缝角度的增加，新生裂缝轨迹线与预设裂缝方向线之间的夹角不断减小。在新生裂缝发育并贯通试样的过程中，试样中心应力集中区域的颗粒摩擦和位置错动为新生裂缝发育提供了帮助。在数字图像云图中可以清晰地观察到，加载过程中新生裂缝两侧明显的颗粒位置错动最终导致了宏观裂缝的形成。

（3）颗粒的胶结断裂、摩擦和位置错动所产生的声发射信号参数（振铃计数、声发射能量等）随着加载的持续而不断增大，在黄土最终断裂失效的瞬间出现峰值。通过对比分析RA和AF之间的特征关系发现，新生裂缝发育过程中剪切破坏模式占据了主导，拉张破坏模式对于裂缝扩张起到了促进作用。

（4）颗粒之间的摩擦移动、胶结断裂和位置错动诱发了声发射信号的产生，局部应力集中所导致的颗粒明显错动逐渐发育为新生裂缝，当裂缝贯通试样后，黄土的承载力达到最大值，即黄土抗断裂能力消失。

As a widespread safety issue in geotechnical constructions, cracks are closely linked to the strength and structural stability of the soil. In order to study the fracture failure characteristics of soil, the type I and type Ⅱ fracture strength of semi-circular loess samples with different angles of prefabricated cracks (0°, 20°, 40° and 60°) and different dry density (1.4 g/cm³, 1.5 g/cm³, 1.6 g/cm³, 1.7 g/cm³) are tested by means of fracture mechanics test method. Combined with the force displacement monitoring system, the variation laws of fracture energy, maximum bearing capacity and fracture toughness in the fracture failure of prefabricated cracks loess are analyzed. With the help of the loess fracture failure process captured by high-speed photography technology, the crack development law of loess is analyzed, and the displacement and strain characteristics in the loess fracture failure process are explored by using digital image technology. Using acoustic emission technology, the variation laws of acoustic emission signal parameters including ring count, acoustic emission energy, rise time and duration in the whole failure process are analyzed. Combined with the characteristic relationship between RA and AF, the fracture failure mode of loess is identified. The main conclusions are as follows:

(1) With the increase of prefabricated cracks angle and dry density, the maximum bearing capacity and fracture energy of loess are increasing. When the dry density increases gradually, the type I, type Ⅱ and effective fracture toughness increase continuously, the peak values occur at a dry of density of 1.7 g/cm³. The increase of prefabricated cracks angle leads to the continuous decrease of type I stress concentration factor, and the type Ⅱ stress concentration factor first increases and then decreases, so that in the process of increasing precast crack angle, type I fracture toughness and effective fracture toughness continue to decrease, while type Ⅱ fracture toughness first increases and then decreases. The peak values of the type Ⅱ fracture toughness occurs at an angle of 40°.

(2) The fracture failure process of loess is essentially a phenomenon that new cracks appear from the tip of prefabricated cracks and develop along a certain trajectory to the upper loading point. In the process of loess fracture, with the increase of prefabricated cracks angle, the angle between the trajectory line of new cracks and the direction line of prefabricated cracks decreases. In the process of new cracks developing and penetrating the sample, the particle friction and position dislocation in the central stress concentration area of the sample provide help for the development of new cracks. Through the digital image cloud image, it can be clearly observed that the obvious particle position dislocation on both sides of the new crack during the loading process finally leads to the formation of macro crack.

(3) The acoustic emission signal parameters (ring count, acoustic emission energy, etc.) produced by the cementation fracture, friction and position dislocation of particles increase with the progress of loading, and the peak appears at the moment of the final fracture failure of loess. By comparing and analyzing the characteristic relationship between RA and AF, it is found that the shear failure mode is dominant in the development of new cracks, and the tensile failure mode promotes the expansion of cracks.

(4) The friction movement, cemented fracture and position dislocation between particles induce the generation of acoustic emission signals. The obvious dislocation of particles caused by local stress concentration gradually develops into new cracks. When the cracks pass through the sample, the bearing capacity of loess reaches the maximum, that is, the fracture resistance of loess disappears.