Effects of thermal treated on the dynamic facture properties using a semi-circular bend technique (Dynamic Behavior of Materials)

ABSTRACT

Dynamic fracture toughness of Laurentian granite (LG) subjected to heat treatment was tested by means of a notched core-based semi-circular bend (SCB) specimen with a modified split Hopkinson pressure bar (SHPB) apparatus. The samples for testing the fracture toughness were manufactured and heat treatment up to 850 °C according to the requirements of experiment. The relationship between the fracture toughness and temperature were investigated of rocks subjected to thermal treatment. The experimental results showed that fracture toughness decreases with increasing temperature. Furthermore, the fracture surfaces of rocks in thermal treated were observed by means of the scanning electronic microscope (SEM). These results provide the theoretic direction for engineering application in thermal treatment.

Keywords: Dynamic fracture toughness; thermal treated; NSCB; SHPB

INTRODUCTION

Temperature plays an important role in many engineering practices, such as rock drilling, ore crushing, deep petroleum boring, geothermal energy extraction, and deep burial of nuclear wastes [1]. So far many researchers have investigated the effect of temperature on mechanical properties of rocks under static loading [1-3]. Nasseri, et al. reported that with the increasing of the temperature of thermal treatment (250 °C to 850 °C), the number of fractures of rock sample becomes more and the opening of cracks becomes wider; and then the values of P-wave velocity and fracture toughness of Westerly Granite decrease [2]. Yavuz, et al. measured the bulk density, P-wave velocity, and effective porosity of carbonate rocks suffered thermal damages introduced by various temperatures from 100 °C to 500 °C [3]. They found that the P-wave velocity of measured rocks is higher than initial values at 100 °C, than the velocity decreases sharply with the rising of the temperature. Based on above researches, a conclusion can be formed that with the rising of the temperature, the mechanical properties of rocks will be diminished due to the thermal induced fractures.

Some researchers also considered the effects of the temperature on dynamic mechanical properties of rocks [4, 5]. Lindholm et al. utilized the split Hopkinson pressure bar to measure the uniaxial compressive strength of Dresser basalt with up to 527 °C [4]. They obtained a relationship between temperature, strain rate, and rock flow stress for Dresser basalt. Zhang et al. measured the dynamic fracture toughness of Fangshan gabbro and Fangshan marble under high temperature, by employing a short-rod (SR) specimen performed on the SHPB system [5]. Zhang et al. considered the axial pressure on the rock samples during tests in order to eliminate the difference between the measured fracture toughness and the true toughness. But it will cost lots of time to prepare the SR specimen for dynamic fracture toughness due to the complex sharp of samples. Dai et al. employed a notched semi-circular bend (NSCB) method to measure the dynamic fracture of toughness [6]. This NSCB sample, compared with SR, chevron bending (CB) [7], and cracked chevron notched Brazilian disc (CCNBD) [8], has little time consuming and can works for small segment of cores. It has an attraction for the weaker core pieces, which are too thin or fragmented to be used in short rod or uniaxial compressive strength [9].

The objective of this study is to examine the correlation between dynamic fracture toughness and various temperatures for heat treatment under dynamic loading for Laurentian granite. The notched semi-circular bend (NSCB) specimen with a split Hopkinson pressure bar (SHPB) apparatus is introduced in this work [6]. The recovered fracture surfaces of rocks were examined using the scanning electronic microscope (SEM). DYNAMIC NOTCHED SEMI-CIRCULAR BEND FRACTURE TEST Sample preparation

In this study, Laurentian granite (LG) is adopted to perform dynamic fracture test, which is taken from the Laurentian region of Grenville province of the Precambrian Canadian Shield, north of St. Lawrence and north-west of Quebec City, Canada. The mineralogical composition is obtained using X-ray diffraction technique: feldspar 60%, quartz 33%, biotite3-5%>. Rock cores with a nominal diameter of 40mm were drilled from the same block of Laurentian granite, and then sliced to obtain discs with an average thickness of 18 mm. The SCB samples were subsequently made from the discs by diametrical cutting. A notch with approximately 1 mm thickness is then machined using a rotary diamond- impregnated saw from the center of the disc perpendicular to the diametrical cut. Then a 1 mm wide, 5 mm length of notch was cut in the semicircular rock disc and the tip was sharpened with a diamond wire saw to achieve a tip diameter of 0.5 mm shown in Fig. 1b. The average grain size is about 0.59 mm [10, 11], so the diameter of the crack tip is similar to the thickness of naturally formed cracks. This will ensure accurate measurements of fracture toughness.

Fig. 1 Schematics of the straight through notched semi-circular bend fracture test in (a) the material testing machine and (b) the SHPB system

Here, Groups of ten samples were heat-treated at 100 °C, 250 °C, 450 °C, 600 °C and 850 °C, respectively, with ten samples remained untreated at room temperature as a contrast. The heat treatment was carried out in an electrical furnace with a 2 C/min heating/cooling speed, which was sufficiently slow to avoid crack due to thermal shock [2].

Split Hopkinson pressure bar system

The dynamic test was carried out on a 25 mm diameter split Hopkinson pressure bar (SHPB) system (Fig. 2). A typical SHPB system consists of a striker bar, an incident bar, a transmitted bar, and specimen is located between the incident and transmitted bars[12]. The lengths of the striker bar, incident bar and transmitted bar are 200mm, 1500mm and 1200mm, respectively, which are made from maraging steel, with high yield strength of 2.5GPa. Strain gauges are mounted at 733 and 655 mm away from the bar-specimen interfaces on the incident bar and transmitted bar respectively. The data processing unit used an eight-channel Sigma digital oscilloscope by Nicolet, to record strain gauge signals collected from the Wheatstone bridge circuits after amplification, and another channel to collect the signal from the strain gauge cemented near the crack tip on the sample surface.

The incident wave, reflected wave and transmitted wave were denoted with subscripts i, r and t, respectively. The loading forces P1 and P2 on the two ends of the specimen induced by the SHPB are[13]:

Where E is Young’s modulus of the bar material, A is the cross-sectional area of the bar and e denotes strain.

Fig.2. Schematics of the notched semi-circular bend (NSCB) specimen in the split Hopkinson pressure bar (SHPB) system [6].

Static test is performed by an MTS hydraulic servo-control testing system (Fig. 1a).

Determination of Mode-I fracture toughness

Compared with the static test, it is hard to achieve the balance of the dynamic loading forces, thus the inertial effect will induce during dynamic test and complex calculation is required to measure the fracture toughness of rocks [14]. To achieve the stress equilibrium of the samples, pulse shaper technique is utilized [14, 15]. In this work a composite pulse shaper (a combination of a C11000 copper and a thin rubber shim) is utilized to shape the loading pulse. In a test the dynamic forces on both ends of the NSCB sample have been achieved, the inertial effect in the sample can be effectively minimized, and the quasi-static data reduction scheme can be utilized to determine the fracture toughness of rocks. We thus only need guarantee the balance of the time-resolved dynamic force on both ends of the NSCB sample. To do so, pulse shaping technique is employed for all the dynamic tests and the dynamic force balance on the two loading ends of the sample has been compared before data processing. Fig. 3 shows the forces on both ends of the sample in a typical test. From Eq. 1, the dynamic force P1 is proportional to the sum of the incident (In) and reflected (Re) stress waves, and the dynamic force on the other side P2 is proportional to the transmitted (Tr). It can be seen that the balance of dynamic forces on both end of the sample is clearly achieved.

Fig. 3. Dynamic force balance check for a typical dynamic punch test with pulse shaping

Based on the ASTM standard E399-06e2 for rectangular three-point bending sample, a equation for calculating the stress intensity factor (SIF) for mode-I fracture is proposed[13]:

Where K is the quasi-static Mode-I SIF, P is the time-varying loading force, and Y (a/R) is a function of the dimensionless crack length a/R, which can be calculated using the finite element software ANSYS [6]. Then the maximum value of K is the fracture toughness KIc of tested sample.

RESULTS

Effect of thermal treated on ultrasonic velocity

The P-wave velocity of rock samples was measured in a quality detector of rock and soil engineering .Ten sample sets were prepared from each rock and each set was subjected to one of temperature levels of 25, 100, 250, 450, 600, 850 C, respectively. The variation of P-wave velocity (Vp) with increasing temperature as shown in Fig.3. Experiments showed that P-wave velocity measured through dry rocks was very sensitive to thermal treated. The ultrasonic velocity of all rocks markedly decreased with increasing temperature (Fig. 4). This phenomenon can be explained as the thermally induced fractures increase with the temperature [2, 3]. At 100 C, the P-wave velocity is similar with that of rock sample at 25 C. Similar results also have been reported by many researchers [3]. The main reason is the new thermally induced microcracks are hardly found with this temperature.

Fig.4 Variation of P-wave velocity (Vp) with increasing temperature

Fig.5 Sample recovered was separated into two equal parts from the NSCB in a SHPB

Fracture toughness

Fig. 5 gives out a typical NSCB LG sample. After the test, the sample was separated into two equal parts through its main crack, as shown Fig. 5.

The relationship between fracture toughness and loading rate as a function temperature is shown in Fig. 6. The results indicate that the fracture toughness increase linearly with increasing loading rates at different temperature. In addition, the results show that, at lower loading rate (less than 60 GPam1/2/s) the fracture toughness values of rocks are close to each other at different temperature; then at higher loading rate, the slope of fracture toughness and loading rates decreased, that means fracture toughness decreased with increasing temperature. Further investigation is required to explain this phenomenon.

Fig. 6.The effect of loading rate on the fracture toughness as a function temperature Study of micro-cracks

This research was carried out by scanning electron microscope (SEM), which works on the basis of impingement of electron beam. Six measured samples at different temperatures were prepared for microcrack study. The surface of samples was polished and sprayed gold to observe microcracks. The microphotos of the samples with various temperatures are shown in Fig. 7, all of them are magnified 400 times. For temperature ranged started from 25 C to 250 C, it is hardly to observe microcracks in Fig. 7. When temperature rises to 450 C , the thermal induced microcracks are clearly observed, and the amount and the width of microcracks generally increase with the temperature. It can be referred to the fracture toughness testing results that the fracture toughness decreases with the rising of the temperature due to the induced microcracks. The result indicate that the pre-existing microcracks of samples fracture surfaces less than widening and development of new microcracks, thus the temperature effect is a fatal factor influence the fracture toughness of rocks.

CONCLUSION

In this study, the notched semi-circular bend method with split Hopkinson pressure bar system is utilized to measure the  dynamic fracture toughness of thermally treated Laurentian granite up to 850 C . A dynamic force equilibrium technique is introduced to eliminate the inertia effect during dynamic test, thus the static analysis can be used to calculate the dynamic fracture toughness of rock. The fracture toughness increases with increasing loading rates in various temperatures. The investigation of microcracks was performed by scanning electron microscope. The thermal induced microcracks are observed when the temperature increases to 450 °C, which is the main factor reducing the fracture toughness of rock in this study. Higher temperature, more and wider of microcracks can be observed. All above mentioned conclusions is an improved understanding of the effect of temperature on fracture properties of rocks which improves engineering practices.

Fig. 7. SEM photos of the section of Laurentian granite (LG) specimens as a function temperature