Assessment of impact mechanical behaviors of rock-like materials heated at 1,000°C

2.1 Apparatus

The Φ 100 mm split Hopkinson pressure bar (SHPB) testing device is the core of the testing system (see Figure 1), which is an experimental technique commonly used in the study of the constitutive laws of materials at high strain rates. It consists of three components.

1. The main equipment: The striker bar, incident bar and transmission bar were 500, 4,500 and 2,500 mm long, respectively, and both of the bars were 100 mm in diameter.

2. The energy system, made up of air compressor, high pressure air bag and piping.

3. The testing system, made up of strain gauges and ultra-dynamic apparatus as well as the dynamic test and analysis equipment.

Figure 1

Apparatus of 100 mm diameter SHPB.

A projectile impacts the free end of the incident bar, thus developing a compressive longitudinal incident wave. Once this wave reaches the bar–specimen interface, part of it is reflected, whereas the other part travels through the specimen and develops the transmitted wave in the incident bar. With the strain gauges that are glued on the incident bar and transmission bars, these three basic waves are recorded. These strains are then used to compute the strain rate, strain and stress in the specimen as follows [25,26]:

where E is Young’s modulus of bars; c is wave velocity in bars; A and A_s are cross-sectional areas of bars and specimen, respectively; l_s is original length of specimen; ε_i, ε_r and ε_t are incident, reflected and transmitted strain, respectively; and τ₁ and τ₂ are time delays of reflected and transmitted pulses, respectively.

A high-temperature heating experimental setup is outlined in Figure 2. It consists of three components:

1. RX3-20-12 box-type resistance furnace: the device has six built-in thermocouples used for heating, with silicon carbon rod elements used in heating, high-performance fiber used in insulation.

2. The temperature control box: automatic in temperature control and heating, with the design maximum operating temperature up to 1,200°C.

3. A rail: to provide convenience for taking or placing specimens at high temperatures.

Figure 2

High-temperature heating experimental setup.

2.2 Samples and heating process

The Qinling Mountains stretch across central China, 1,500 km from east to west and 100–150 km from north to south. In this study, the rock samples were cored from fresh intact rock blocks belonging to an excavation of 50–150 m underground from the Qinling Mountains. The surface of the rock sample is complete without obvious defects such as holes and cracks. The rock samples are air dried in the natural state at room temperature. Three cylindrical specimens of 100 mm diameter rock samples of marble, sandstone and granite were used in this study. By using a high-speed rotary saw, the drilled specimens were cut into lengths of 50 mm with the ratio of length to diameter maintained at 0.5. An end face grinder was then used to grind the specimens, and inspections were conducted to confirm evenness and perpendicularity with respect to the vertical axis [27]. No bedding plane was observed in the specimens. The average density of marble, sandstone and granite samples are 2,600, 2,750 and 2,650 kg/m³, respectively, and complete mineral composition of the three rock types are listed in Table 1. The specimens were subjected to longitudinal wave velocity testing utilizing a RSM-SY5N nonmetal ultrasonic testing analyzer after dimensions were measured. The specimens were then heated in the heating setup at the rate of 10°C/min until 1,000°C and were kept at constant temperature for 3 h. The 1,000°C high-temperature specimens were then placed in the heating setup to cool to room temperature. Each temperature group is equipped with no less than three specimens. Three rock types exposed to room temperature and 1,000°C treatment are shown in Figures 3 and 4.

Figure 3

Three rock samples exposed to room temperature.

Figure 4

Three rock samples exposed to 1,000°C treatment.

4.1 DCS

Variation characteristics of DCS of three rock types exposed to room temperature and 1,000°C treatment under different strain rates are presented in Figure 12. DCS exposed to room temperature and 1,000°C treatment exhibit concentration in two different regions with nearly no overlap, indicating that under the same impact velocity, two different temperature conditions exert an exceptional influence on DCS and strain rate of the three rock types. The DCS sharply reduces, and the strain rates significantly increase following 1,000°C treatment, indicating that 1,000°C treatment produces severe degradations in DCS and strain rate. Specific test and analysis results are as follows: strain rate-enhanced performance on DCS of three rock types at room temperature is obvious with influences of strain rate on marble and granite much greater than on sandstone. Under the same impact velocity, the DCS of granite is greatest, followed by marble, with DCS of sandstone the least. Alterations in the strain rate-enhanced performance on DCS occur following 1,000°C treatment as DCS of marble and granite experience increases, with the increase significantly less than at room temperature, while DCS of sandstone experiences little change. In addition, unlike room temperature conditions, under the same impact velocity, DCS of granite is greatest, while DCS of marble and sandstone is minimal. But, in general, the DCS of sandstone is slightly greater than marble, showing that the weakening effect due to 1,000°C on marble DCS is most serious.

Figure 12

Dynamic compression strength of three rock types exposed to room temperature and 1,000°C treatment.

Dynamic compressive increase factor (DCIF) is the ratio of the dynamic and static compressive strength of rock material, reflecting the increased amplitude in DCS of rock under impact loading. Variation characteristics of DCIF of three rock types exposed to room temperature and 1,000°C treatment under different strain rates impact loadings are presented in Figure 13. Compared with the static compressive strength, DCIF of the three rock types increases partially independently of temperature conditions as DCIF is generally greater than 1. Strain rate-enhanced performance on DCIF is apparent at room temperature with the influence of strain rate on marble and granite much greater than sandstone. DCIF of marble is greatest under the same impact velocity followed by sandstone with DCIF of granite the least, indicating that the increased amplitude in DCS of marble is the greatest. Modifications occur in strain rate-enhanced performance on DCIF following 1,000°C treatment. DCIF of marble and granite exhibit slight increases with the increase of strain rate although significantly less than at room temperature, whereas DCIF of sandstone remains nearly unchanged.

Figure 13

Dynamic compression strength increase the factor of three rock types exposed to room temperature and 1,000°C treatment.

4.2 Failure modes

Failure mode variation characteristics of the three rock types exposed to room temperature and 1,000°C treatment under different strain rates impact loadings are presented in Figures 14–16. Failure mode reflects the rock stress state as a result of interactions among rock micro-cracks. High temperature of 1,000°C and the strain rate exert significant influence on failure modes of the three rock types as shown in Figures 14–16. Failure modes of each rock type at room temperature, when subjected to impact loading exceeding the ultimate rock strength, demonstrate tensile splitting failure characterized by tensile fracturing in parallel to the axial direction rather than axial compression failure. Differences exist in failure modes for marble and granite as samples are typically broken into pieces with the increasing strain rate with the amounts of failure pieces gradually increasing and pieces tend to be uniform. Sandstone failure modes manifest typically as samples exhibiting an intact core and corroded edges. Sandstone samples will not be destroyed immediately after the axial tensile failure occurred and may still bear certain axial pressure in the shear plane formation throughout the sample. The failure of sandstone samples is significant with the increasing strain rate, often rapidly disintegrating after reaching the peak strength. Because the sample failure process lasts shorter, end effect limits the vertical development of splitting cracks, thus resulting in the failure modes characterized by edge avalanche and shear buckling in the central area of rock pillar treatment. Internal damage in rock, after 1,000°C treatment, is extensive and induces significant changes in failure modes of each rock type compared with room temperature. The failure mode for marble and sandstone is far more severe than with room temperature as failure pieces rapidly increase, and pieces tend to be in the powder form. Failure modes for granite exhibit tensile splitting failure similar to that of sandstone at room temperature with a typical intact middle core and edge corrosion.

Figure 14

Failure modes of marble exposed to room temperature and 1,000°C treatment. (a) 65.1 s⁻¹, (b) 95.0 s⁻¹, (c) 101.4 s⁻¹, (d) 115.2 s⁻¹ and (e) 123.8 s⁻¹. Room temperature: (a) 114.9 s⁻¹, (b) 175.6 s⁻¹, (c) 180.5 s⁻¹, (d) 186.1 s⁻¹ and (e) 191.4 s⁻¹.

Figure 15

Failure modes of sandstone exposed to room temperature and 1,000°C treatment. (a) 93.1 s⁻¹, (b) 123.5 s⁻¹, (c) 135.2 s⁻¹, (d) 154.9 s⁻¹ and (e) 166.0 s⁻¹. Room temperature: (a) 145.3 s⁻¹, (b)190.0 s⁻¹, (c) 224.1 s⁻¹, (d) 237.8 s⁻¹ and (e) 248.4 s⁻¹.

Figure 16

Failure modes of granite exposed to room temperature and 1,000°C treatment. (a) 65.5 s⁻¹, (b) 75.6 s⁻¹, (c) 98.6 s⁻¹, (d) 108 s⁻¹ and (e) 116.9 s⁻¹. Room temperature: (a) 168.6 s⁻¹, (b) 174.1 s⁻¹, (c) 184.8 s⁻¹, (d) 214.1 s⁻¹ and (e) 255.9 s⁻¹.

As a structural medium containing various defects, rock exhibits original and spontaneous defects in the deformation process leading to stress nonuniformity. Local tensile stress concentration occurs as a result in the sample rendering stress nonuniformity, which is the decisive factor in tension failure. Splitting failure mode is often then presented as tensile strength of the rock that is extremely low in the compression process.

The comparative analysis of the DCS and failure modes of rock types exposed to room temperature and 1,000°C treatment under different strain rate impact loadings reveals that the failure mode and strength correlations are nearly nonexistent for the same rock type subjected to different temperature treatments and strain rates and for different rock types subjected to the same external environment. Variation characteristics of failure modes and strength affected by temperature and strain rate are also revealed to be dissimilar.

4.3 Microstructure characteristics

The microstructure characteristics of rock directly determine its macroscopic mechanical response. Taking marble as an example, the typical scanning electron microscope images of fracture failure of rock samples exposed to room temperature and 1,000°C treatment under impact loading are shown in Figures 17 and 18.

Figure 17

Fracture morphology of marble exposed to room temperature.

Figure 18

Fracture morphology of marble exposed to 1,000°C treatment.

At room temperature, the fracture forms of marble under impact loading are very rich and contain a lot of information about the crystal structure and micro-crack changes in the rock. The failure form is mainly transgranular fracture. Temperature has a significant influence on the mineral composition and crystal structure of rocks, so the microstructure characteristics of rocks under the influence of high temperature are obviously different from those under normal temperature. After 1,000°C treatment, the fracture morphology of marble is dominated by obvious plastic fracture, with a large number of plastic fracture characteristics such as slip separation and dimples. This indicates that the marble has begun to undergo a brittle-plastic transition, which is also reflected in macroscopic mechanical tests.