Key Takeaways & Executive Findings
- •• Dynamic tensile strength of composite rocks increases with higher strength ratios and strain rates, indicating a strain-rate dependent behavior. • Failure mode transitions from center-splitting to tension-shear combined and back as the incident angle varies from 0° to 90°, highlighting the role of bedding plane orientation. • The effect of incident wave order on strength diminishes at high strain rates (≥400 s−1), with differences below 5%, simplifying dynamic loading assessments. • Microfracture distribution is influenced by strength ratio: cracks concentrate on the softer side for ratios 1.5 and 2.0, but distribute on both sides of the bedding plane for ratio 1.2, providing insights into failure mechanisms.
Abstract
Composite rock layers are widely present in mining and tunnel construction projects, and are prone to dynamic tensile failure along bedding planes under dynamic disturbances. To ensure engineering safety, it is necessary to conduct research on the dynamic tensile characteristics under different working conditions. Considering the difficulty of on-site sampling, composite rock samples were prepared with cement mortar, and dynamic Brazilian splitting tests were carried out using split Hopkinson pressure bar (SHPB) equipment, a high-speed camera, and PFC2D numerical software to explore their dynamic tensile properties under dynamic disturbance under different strength ratios and other factors. The results show that the dynamic tensile strength of samples exhibits a rising trend with the strength ratio and strain rate growth. As the incident angle increases from 0° to 90°, the stress contour deflects transform from center-splitting failure to tension-shear combined failure and back again. The influence of the incident order in different lithology rocks on the dynamic tensile strength of composite samples is controlled by strain rate, and when the strain rate increases to 400 s−1, the difference in strength due to the sequence of incident stress waves is within 5%. Based on PFC2D, the strength ratio of composite samples has a certain influence on the distribution of microfractures. With strength ratios equaling 1.5 or 2.0, the cracks are mainly concentrated on the softer material side, while a large number of cracks are distributed on both sides of the bedding plane with a strength ratio equal to 1.2.
1. Introduction
In China, composite rock strata with various inter-layer properties frequently occur in the construction of transportation, water conservation, hydro-power, energy mining, and other projects. Composite rock strata, such as coal-rock composite strata and sandstone-slate composite strata, commonly contain defects like bedding, holes, and other defects. And their mechanical properties are more complex than those of homogeneous rocks [1−4]. In the composite rock layers, the mechanical characteristics of the rock strata vary considerably, with the dynamic failure mechanism being unknown, and the engineering issues caused by dynamic disturbance are prominent, which significantly affects the security and efficiency of the construction. Engineering accidents and natural disasters caused by the uncertainty of the stability and load-bearing capacity of the composite rock formation have been common in recent years [5−7].
Rock is a brittle substance with a significantly lower tensile strength than compressive strength. In the construction of underground projects accompanied by blasting, impact, and other dynamic loads, when dynamic loads operate upon the rock, tensile damage is highly likely to occur. To ensure construction efficiency and engineering safety, more investigation is required into the causes of dynamic tensile failure in composite rock strata. Previous studies have mainly focused on the dynamic loading behavior of layered or homogeneous rocks, and systematically evaluated the tensile, compressive, and shear properties of rock materials under impact conditions through experimental methods such as split Hopkinson pressure bar (SHPB) [8−12]. Among them, factors such as bedding angle, loading rate, and interface roughness have been proven to significantly affect rock failure modes and macroscopic and microscopic mechanical properties [13−16]. The introduction of visualization techniques such as digital image correlation (DIC) and high-speed photography [17−19] further reveals the crack propagation path and failure evolution law.
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WEN Sen, SONG Ruo-tong, ZHANG Chun-shun, LI Sheng, KONG Qing-mei (2025). Tensile mechanical behavior of composite rocks under stress waves: A focus on strength variation between rock layers. Journal of Central South University. https://doi.org/10.1007/s11771-025-6122-0
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Frequently Asked Questions
What is the main focus of this study?
The study investigates the dynamic tensile mechanical behavior of composite rocks under stress waves, specifically focusing on the effect of strength variation between rock layers.
How were the composite rock samples prepared?
Due to the difficulty of on-site sampling, composite rock samples were prepared using cement mortar.
What experimental and numerical methods were used?
Dynamic Brazilian splitting tests were conducted using split Hopkinson pressure bar (SHPB) equipment, a high-speed camera, and PFC2D numerical software.
What are the key findings regarding dynamic tensile strength?
Dynamic tensile strength increases with higher strength ratios and strain rates. The failure mode transitions from center-splitting to tension-shear combined and back as the incident angle varies from 0° to 90°.
How does the incident order of stress waves affect the strength?
The influence of incident order on dynamic tensile strength is controlled by strain rate; at strain rates above 400 s−1, the strength difference due to wave sequence is within 5%.
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