Abstract: The strengths of crack initiation and damage are the important stress thresholds for their significance in analyzing the progressive failure process of rock, which can be applied in the prediction of brittle failure in tunnels. Firstly, the uniaxial and triaxial compression tests are conducted on two sets of hard sandstones, while their strain and acoustic emission (AE) characteristics are analyzed to study the evolution process of crack. The measured lateral dilation strains and the AE counts of rock samples can characterize their crack accumulation accurately according to the test results. And also, there is a consistent segmented transformation law in these two figures as the inner micro-cracks are accumulated. Thus the strengths of crack initiation and damage of the sandstone samples can be determined based on a detailed analysis on stage boundaries of the above curves. The average strength of crack initiation of green sandstone samples is 0.42 of its peak strength, while that of red sandstone samples is 0.48 of its peak strength. Furthermore the confinement influence on evolution process of crack is discussed by comparing different test results under varying confinements. It is found that the initial internal friction angle (φ₀) of sandstone samples at the crack initiation stage is less than its ultimate value (φ) when the peak load is applied. And the calculated initial φ₀ is about 1/2 of the ultimate φ. As a result, the initial internal friction level φ₀/φ of rock is defined, which has an inverse relation with the ratio of uniaxial compressive strength to tensile strength σ_c /σ_t , and it can also reflect the brittleness of rock. And the linear strength threshold of crack initiation is established, while the slope of curve represents the influence of friction effects.