Fracture toughness measuring device in stone material and concrete considering lateral load

Abstract

Loading on materials and structures such as gravel dams, railway ballast, and oil or gas tanks, applied lateral loads in addition to the vertical load, will play an essential role in determining the fracture toughness of the modes I and II and composite modes of materials. This issue is important in predicting the failure behavior of brittle stone materials and the precise design of these types of structures. The device for measuring fracture toughness in stone and concrete materials was designed and built in terms of lateral load. Using this device makes it possible to obtain the fracture toughness of mode I (tensile) and mode II (shear) plus composite mode of brittle stone and concrete materials under confinement load. It performs tests on samples with sizes 4.5 to 11 cm. Also, the use of screw connections in the entire device has facilitated its installation and movement.

Description

Fracture Toughness Measuring Device in Stone Material and Concrete Considering Lateral Load

[0001] This invention is related to the fields of civil engineering, mechanical engineering, and material engineering, which can obtain the fracture toughness and if needed, other mechanical parameters of the fracture of brittle objects such as the radius of the crushing zone in the conditions of lateral confinement.

[0002] The fracture toughness of brittle stone and concrete materials has typically been assessed under uniaxial applied loads. However, in real-world scenarios such as gravel dams, railway ballast, and oil and gas tanks, the presence of lateral loads, in addition to vertical loads, significantly influences the determination of failure toughness. Considering these factors is crucial for accurately predicting the fracture behavior of brittle stone materials and for the precise design of such structures. To address this, a specialized device has been developed and constructed specifically for measuring fracture toughness in stone and concrete materials under lateral load conditions. This device is capable of providing measurements for mode I (tensile), mode II (shear), and composite mode fracture toughness under confinement load.

[0003] G01B 3 / 00

[0004] US6588283B2

[0005] Fracture toughness determination using spiral-grooved cylindrical specimen and pure torsional loading

[0006] A method for determining fracture toughness KIC of materials ranging from metallic alloys, brittle ceramics and their composites, and weldments. A cylindrical specimen having a helical V-groove with a 45° pitch is subjected to pure torsion. This loading configuration creates a uniform tensile-stress crack-opening mode, and a transverse plane-strain state along the helical groove. The full length of the spiral groove is equivalent to the thickness of a conventional compact-type specimen. KIC values are determined from the fracture torque and crack length measured from the test specimen using a 3-D finite element program (TOR3D-KIC) developed for the purpose. In addition, a mixed mode (combined tensile and shear stress mode) fracture toughness value can be determined by varying the pitch of the helical groove. Since the key information needed for determining the KIC value is condensed in the vicinity of the crack tip, the specimen can be significantly miniaturized without the loss of generality.

[0007] This mentioned invention covers to measure the fracture toughness of mode I in stone materials which are the tensile while our device covers both mode I and mode II which means shear materials too.

[0008] US4116049A

[0009] Method for measuring plane strain fracture toughness

[0010] The present invention consists of a method for accurately measuring the plane strain fracture toughness of ductile and brittle materials, the method to be practiced on a specimen formed such that, when the specimen is appropriately loaded, a crack will initiate at a predetermined point or line, with such crack formed thereat being intrinsically stable such that the load executed on the specimen must be continually increased in order to further advance the crack along a predetermined path through the specimen, until, at some critical location along the predetermined crack path, the load necessary to further advance the crack reaches a maximum, thereafter decrasing as the crack advances beyond the critical location along the remainder of its predetermined path through the specimen, that critical location being essentially independent of the specimen material, it being determined by the specimen geometry alone as long as elastic plane strain conditions prevail in the specimen sufficiently close to the crack tip. The method of the present invention involves taking the specimen through an appropriate loading sequence and determining the critical (or maximum) load that occurs when the crack passes through the critical position. With that data, the plane strain critical stress intensity factor, KIC, or fracture toughness for the specimen material, can be calculated mathematically considering the critical load, the specimen size and geometry, taking into account the Poisson's ratio of the specimen material, but without necessitating reference to any other standard specimen or fracture toughness test results thereon.

[0011] The mentioned method is able to measure the fracture toughness of mode I stone materials and not both mode I and mode II like our claimed device and also its method is different.

[0012] US4152941A

[0013] Process for measuring the fracture toughness of rock under simulated down-hole stress conditions

[0014] The present invention relates to a method for measuring the fracture toughness of rock as it would exhibit in its natural down-hole setting. Practicing the method of the present invention involves internally pressurizing a rock specimen while simultaneously exerting an increasing external horizontal stress thereon, raising simultaneously the internal pressure to maintain a desired relationship of exterior to interior pressures until the specimen fractures, which pressure, in relationship to the stress intensity factor and specimen geometry, provides the specimen fracture toughness as it would exist in a down-hole situation.

[0015] The mentioned process is only limited to measuring the fracture toughness of rocks and not all the stone materials, and also its method is different.

[0016] JP1995225180

[0017] MEASURING METHOD OF FRACTURE TOUGHNESS

[0018] To simply measure fracture toughness over a wide range in a pure mode I, a pure mode II and a mixed mode by a method wherein a crack is introduced into the central part on the surface of a flat-board test piece, a saddle-shaped stress is loaded onto the flat-board test piece, the strength of the test piece and the size of the crack are measured and an angle by the normal line of the crack and by a tensile stress is adjusted.

[0019] The mentioned method is an old version of measuring fracture toughness and it is simple while we designed a wholesome device with keys and a vast progress.

[0020] CN106289990

[0021] DEVICE AND METHOD FOR MEASURING FRACTURE TOUGHNESS OF SOFT CLAY UNDER ANY CONFINING PRESSURE

[0022] The invention discloses a device and a method for measuring the fracture toughness of soft clay under any confining pressure. The device mainly comprises a metal pedestal, a hydraulic jack, a pressure tank, a transparent tempered glass reaction plate, two connecting rods, a digital camera, a steel sheet, an annular porous stone, two independent servo systems and the like. The existing device for measuring the fracture toughness of materials, such as concrete and metal, cannot apply confining pressure to a test sample and cannot be used for testing the fracture parameter of the soft clay, because the fracture toughness of the soft clay is determined by the confining pressure, but the fracture parameters of the materials, such as concrete and metal, are irrelative to the confining pressure. The device and the method for measuring the fracture toughness of soft clay under any confining pressure can provide direct mechanical parameters (namely the fracture toughness of a soft clay seabed) for predicting critical crack initiation conditions required by petroleum and natural gas exploitation by a hydraulic fracturing method and under different depths of the seabed.

[0023] This mentioned device has similar components to our designed device but it is for measuring the fracture toughness of the clay while our claimed device is for all the stone materials.

[0024] A device for measuring the fracture toughness of stone and concrete materials. It can create biaxial loading conditions on samples with variable dimensions and extract desired mechanical failure parameters. The device includes a metal chassis, jack and hydraulic pump, dynamometer and pressure manometer, displacement gauge, data logger, loading jaws, pressure control valve, and an index for determining the crack angle relative to the direction of loading. It allows tests on disk samples with diameters of 4.5 to 11 cm and can apply any intended confinement ratio to the sample. The results can be used to determine the fracture toughness of mode I, mode II, and combined mode.

[0025] Fracture mechanics is based on the realistic assumption that all objects have cracks-like defects and that these defects are the reason for the initiation of failure in the object. These defects, which are often modeled as cracks, can exist as holes or other defects. In the mass of rock, due to the existence of structural problems such as joints, holes, and fractures, and in concrete due to the existence of holes and cracks, the concepts of fracture mechanics are true, and therefore its stability and resistance against static and dynamic loads should be applied using be evaluated from the concepts of failure mechanics. Considering that the most important cause of breakage in stone and concrete materials that have brittle behavior is crack development, calculating the fracture toughness (the resistance of the material to crack growth) is very important to understand the behavior of stone and concrete structures. Among the factors that affect the fracture toughness of stone and concrete materials is the lateral compressive load applied to the material. Stone materials according to their placement in structures such as gravel dams and railway lines, concrete materials in enclosed conditions such as columns enclosed with reinforced fiber polymers, as well as concrete foundations and piles in addition to Vertical loads are also subjected to lateral loads. Of course, this problem also applies to the rock masses in the deep earth, which are naturally exposed to horizontal stresses (oil and gas reservoirs). Therefore, it is very important to check the fracture toughness of brittle stone and concrete materials based on the state of applied stresses to that material. Based on this, the purpose of building this device is to measure the fracture toughness of mode I, mode II, and composite mode in stone and concrete materials in terms of lateral load.Solution of Problem

[0026] To improve the existing devices and methods to determine the fracture toughness of stone and concrete materials and remove the obstacles and limitations of the previous methods, a device with the ability to simultaneously apply vertical and lateral loads to simulate the actual fracture behavior of brittle materials and Obtaining the mechanical parameters required to analyze the results is made in such a way that by performing the fracture test of disc-cracked materials, it is possible to obtain the fracture toughness of mode I, mode II, composite mode, and in general the fracture mechanism of brittle materials under different lateral loading conditions.

[0027] To make this device, two flanges made of steel sheet with a width of 200 mm, a height of 180 mm, and a thickness of 15 mm with two holes embedded in it with a diameter of 16 mm and a length of 35 mm to close the jack. The lateral loads were prepared at the desired height. Each of the two flanges of the side jack bar is supported by two laths made of steel sheet with a width of 190 mm a height of 180 mm and a thickness of 15 mm behind it as a support on the plate made of steel sheet. With a length of 7500 mm, a width of 200 mm, and a thickness of 15 mm, it has been welded and formed the chassis so that the two lateral load jacks move back and forth in one axis and towards each other.

[0028] The 10-ton hydraulic jack installed in the center of the upper beam of the steel frame is responsible for vertical loading. Four number 18 studs connect the steel frame with a length of 910 mm and two number 18 studs with a length of 1000 mm by screws and nuts so that two 1000 mm studs as vertical members of the frame have three holes with a diameter of 16 mm at a distance of 56 mm from each other in each wing in the lower part and seven holes with a diameter of 16 mm at a distance of 56 mm from each other in each wing It is the upper part. Four 910 mm studs as horizontal members of this frame have 3 holes in the frame at both ends of each member. In the lower part, the two horizontal members become the vertical members of the screw and the nut. The advantage is that in the upper part, with seven holes in the wing of the vertical member, there is the ability to adjust the height of the vertical loading hydraulic jack.

[0029] To load the sample in the upright position, two loading jaws with a radius of 75 mm have been used. It should be noted that the Brazilian test jaws are made according to ASTM D3967 standard in such a way that firstly, the contact arc of the jack with the sample is not more than 15 degrees and secondly, the contact width of the jaw with the samples is not less than D / 6 (D diameter of the disc). The lower jaw is fixed and the upper jaw is reciprocating in the vertical direction. Also, between the upper jaw and the vertical hydraulic jack, a load cell is inserted to record the instantaneous force values.

[0030] To load the sample in horizontal mode, two loading jaws with a radius of 75 mm are used, and both left and right jaws are moving back and forth in the horizontal direction. It should be noted that the chassis of the device is designed in such a way (screw connections of the chassis and punch holes in support of the side jacks) that it is possible to perform tests on disk samples with diameters of 4.5 to 11 cm.

[0031] To control and keep the amount of lateral force constant against lateral deformation caused by the application of vertical load, a pressure control valve has been used in the circuit of the lateral load hydraulic system. Having the cross-section of lateral jacks (A) and corresponding pressure reading (P), the amount of lateral force is calculated using the formula F=PA.

[0032] A pressure gauge is used to see the pressure in the vertical load hydraulic circuit. Also, hydraulic pumps are used to feed hydraulic jacks. The displacement sensor installed on the upper jaw is for real-time recording of displacement. Instant information of force by a load cell and change of location by displacement sensor is transferred and recorded to a computer through a data logger.

[0033] This device can apply any chosen confinement ratio to the sample with variable dimensions. The fracture toughness of mode I, mode II, and combined mode can be determined based on the results.Advantage Effects of the Invention

[0034] - This device is capable of measuring the fracture toughness of modes I, II, and composite modes of stone materials and concrete.

[0035] - The loading jaws and chassis of the device are designed so that it is possible to test samples of 4.5 to 11 cm of different materials.

[0036] - Due to the geometry of the sample, which is a disk, the sample preparation process is easy.

[0037] - The use of screw connections throughout the device has facilitated its installation and movement.

[0038] - According to the preparation of the graduated plate and its attachment to the loading jaws, the change of the crack angle in the samples concerning the length of loading for analysis of fracture mechanics is done very quickly and easily.

[0039] shows a general view of the components of the device.

[0040] presents the details of the flanges connected to the side jacks.

[0041] shows the details and dimensions of the device’s loading jaws.

[0042] displays the details of the chassis connected to the side jacks.

[0043] is a general and two-dimensional view of the device.

[0044] Shows a general view of the device components which are:

[0045] 1- the plate

[0046] 2- four studs

[0047] 3- two studs

[0048] 4- laths

[0049] 5-screws

[0050] 6- nuts

[0051] 7- one of the flanges

[0052] presents the details of the flanges connected to the side jacks:

[0053] 7- one of the flanges

[0054] 8- another one of the flanges

[0055] shows the details and dimensions (in millimeters) of the device's loading jaws and their top and horizontal views:

[0056] 9. Upper jaw

[0057] 10. Lower jaw

[0058] 11. Loading jaw

[0059] shows the details of the chassis connected to the side jacks (dimensions are in millimeters) and their views of 3D, top, and horizontal:

[0060] 9- upper jaw

[0061] 10- lower jaw

[0062] 11- loading jaw

[0063] 12- chassis

[0064] 13- lateral load

[0065] 14- lateral load

[0066] 15- loading sample

[0067] shows the general and two-dimensional view of the device and its parts:

[0068] 16- hydraulic jack

[0069] 17- pressure gauge

[0070] 18- hydraulic pump

[0071] 19-hydraulic pump

[0072] 20- load cell

[0073] 21- displacement sensor

[0074] 22- pressure controller

[0075] 23- pressure gauge

[0076] 24- data logger

[0077] 25- the monitor of the computer

[0078] 26- steel frameExamples

[0079] To perform the test, first, a cracked disc sample with a certain ratio of crack length to disc diameter a / R and specific thickness B is placed inside the loading jaw. Due to the attachment of the progressed plate to the loading jaw, the crack direction of the sample can be easily controlled concerning the vertical extension. In this step, the lateral forces along with the vertical force are applied to the sample in certain steps and at a certain speed (less than 0.1 mm / sec). From the manometers connected to the horizontal jacks and the load cell connected to the vertical jack, the values of lateral and vertical forces are recorded and controlled by the data logger at every moment. It should be noted that displacement values in vertical and horizontal directions are also recorded and controlled by LVDT and displacement gauge. The simultaneous loading process continues until the desired lateral force is reached, and the pressure control valve connected to the circuit of the lateral jacks causes the lateral force to remain constant during the lateral deformation caused by the vertical load. Then the vertical force is increased and the loading process continues until the failure of the sample. From the values of vertical force and deformation as well as horizontal force and deformation, the load-displacement behavior of the sample can be drawn in both directions [Chart. 1]. Also, the fracture toughness of mode I, mode II, and composite mode can be calculated from the following formulas:

[0080] [Formula. 1]:

[0081]

[0082] [Formula. 2]:

[0083]

[0084] KI and KII are the stress intensity coefficients of mode I and II, R is the Brazilian disk radius and B is the disk thickness. F1 is the compressive load at the moment of failure and F3 is the confining force; a is half the length of the crack and YI and YII are dimensionless coefficients that depend on a / R and the angle of the crack concerning the loading length (β and β')

[0085] [Pic. 1] shows the loading mechanism of the sample:

[0086]

[0087] It should be noted that in the case where the crack is aligned with the vertical force, pure mode I will occur, and in the case where the crack has an angle of 23-27 degrees with the vertical force, pure mode II will occur. The value of this angle is related to the a / R of the sample as well as the ratio of vertical force to lateral force.

[0088] [Chart. 1] shows the load-vertical displacement chart for one of the samples:

[0089]

[0090] [Chart. 2] shows the values of the mode I stress intensity factor in different lateral load values for one of the samples:

[0091]

[0092] [Chart. 3] shows the values of mode II stress intensity factor in different lateral load values for one of the samples:

[0093]

[0094] [Chart. 4] shows the amount of changes in mode I fracture toughness according to different lateral confinement ratios:

[0095]

[0096] The application of this invention is in civil projects (construction), dam building, oil and gas, and railways, which can be used to determine the characteristics and failure parameters of materials.

Claims

A device for measuring the fracture toughness of mode I, mode II, and composite mode in stone and concrete materials in such a way that it is possible to create biaxial loading conditions on samples with variable dimensions and extract the desired mechanical failure parameters comprising:- Metal chassis- Jack and hydraulic pump- Dynamometer and pressure manometer- displacement gauge- Data logger- Loading jaws- Pressure control valve- The index for determining the angle of the crack relative to the direction of loadingAccording to claim 1, two flanges made of steel sheet with a width of 200 mm a height of 180 mm, and a thickness of 15 mm with two punch holes embedded in it with a diameter of 16 mm and a punch length of 35 mm to close the lateral load jacks at the height were prepared as desired.According to claim 2, each of the two lateral jack-bar flanges is supported by two steel sheet laches with a width of 190 mm and a height of 180 mm, and a thickness of 15 mm behind it as a support on a plate made of steel sheet with 7500 mm long, 200 mm wide, and 15 mm thick which welded and formed the chassis so that the two lateral load jacks move back and forth in one axis and towards each other.According to claim 3, the chassis of the device is designed (screw connections of the chassis and bean holes in support of the side jacks) to perform tests on disk samples with diameters of 4.5 to 11 cm.According to claim 4, the 10-ton hydraulic jack installed in the center of the upper beam of the steel frame is responsible for vertical loading.According to claim 5, hydraulic pumps are used to feed hydraulic jacks.According to claim 5, the steel frame is connected by four number 18 studs with a length of 910 mm and two number 18 studs with a length of 1000 mm by bolts and nuts.According to claim 7, two 1000 mm studs as vertical members of the frame have three holes with a diameter of 16 mm at a distance of 56 mm from each other in each wing in the lower part and seven holes with a diameter of 16 mm at a distance of 56 mm from each other.According to claim 8, four 910 mm studs as horizontal members of this frame have 3 holes at both ends of each member.According to claim 9, in the lower part, two horizontal members are bolted from the wing to the vertical member and in the upper part, with seven holes in the wing of the vertical member, the height of the vertical loading hydraulic jack is adjusted.According to claim 10, to load the sample in an upright position, two loading jaws with a radius of 75 mm have been used.According to claim 11, the lower jaw is fixed and the upper jaw is reciprocating in the vertical direction.According to claim 12, the real-time information of the force by the load cell between the upper jaw and the hydraulic jack and the change of location by the displacement sensor is transferred and recorded to the computer through the data logger.According to claim 12, to load the sample in horizontal mode, from two loading jaws It has a curvature with a radius of 75 mm, and both left and right jaws are movable in a horizontal direction.According to claim 14, to control and keep the amount of lateral force constant against the lateral deformation caused by the application of vertical load, a pressure control valve has been used in the circuit of the lateral load hydraulic system.According to claim 15, having the cross-section of the lateral jacks and the corresponding pressure reading, the amount of lateral force is calculated using the formula F=PA.According to claim 16, a pressure gauge is used to see the pressure in the vertical load hydraulic circuit.According to claim 17, by using this device, any looked-for confinement ratio can be applied to the sample with variable dimensions and the fracture toughness of mode I, mode II and combined mode can be determined based on the results.

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