CHAPTER 4 EXPERIMENTAL ANALYSIS FOR A 2 IN, SCHEDULE 10 PIPE

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1 CHAPTER 4 EXPERIMENTAL ANALYSIS FOR A 2 IN, SCHEDULE 10 PIPE To validate the results of theoretical and numerical analysis, three cases of experimental analysis were conducted. Two types of pipe-bend test were performed including empty and pressurized pipe bend. In the case of empty pipe, there were three test configurations. The three configurations were in plane opening bending, in-plane closing bending and out-of-plane bending, respectively. For the case of pressurized pipe, in-plane opening bending was carried out by combining internal pressure and opening moment. Some previous studies encountered the problem of disagreement between numerical analysis results and the experimental results. The authors claimed the inconsistency between the material properties of pipe bends used in numerical analysis and the properties of actual pipe bend was the cause of the problem. Therefore, in this study, tension test of pipe bends were performed to get the actual properties of pipe material for introducing to numerical analysis Tension Test Tension test is probably the most fundamental type of mechanical test that can be performed on material. Tension tests are simple, relatively inexpensive, and fully standardized. Generally, there are two types of loading systems, mechanical (screw power) and hydraulic, used in tension test. According to available test rig in the laboratory, the hydraulic loading systems were used. Loading is a function of griping or holding device of the testing machine to transmit the load from the heads of the machine to the specimen under test. The center of action of the grips was set in alignment to keep the load transmitted axially and to minimize bending or KS

2 twisting of specimen. By performing the tension test, we can obtain tensile strength, yield point, yield strength, elongation, etc. The specimens were directly cut from unused portion of 2 inch, schedule 10 pipe. The material of pipe is mild-steel, which is widely used in industry, factory, etc. According to ASTM guideline, the specimens were thoroughly machined to obtain the general shape, called coupon. Following the ASTM guideline, the general shapes of coupons and its dimension are shown in Fig The thickness of the coupons is 2.77mm. The end of each coupon was flattened in order to fit in the grips available test rig as shown in Fig Fig Tension test equipment KS

3 Fig Location of longitudinal tension test specimens in rings cut from pipe and guideline dimension of coupon (standard test methods and definitions for mechanical testing of steel product, A 370) Before run the test rig, every unit of equipment should be setup. The load unit control should be setup as unloading. Test tar, which is transfer data from material test system (MTS) to PC computer, must be checked. Setup the test ware, which control displacement, specimen alignment to make sure that the load transmitted axially. The dimension of specimens and the tension testing setup is showed in Fig and Fig.5.1.4, respectively. Figure shown the progress of tension test. The test was performed in room temperature with the rate of axial displacement, approximately 0.02 mm/second. From tension test, stress-strain and load-displacement diagrams can be obtained. Figure is represented the results of tension test and Fig through Fig are showed the stress-strain and load-displacement curves for specimen 1, 2,and 3, respectively. The comparison results of three specimens are showed in Fig and KS

4 1 2 3 Fig Dimension of specimen used for tension test according to ASTM guideline Fig Tension test setup KS

5 Fig Tension test process Fig Tension test results KS

6 TRUE stress TRUE strain Load, N displacement, mm Fig Stress-strain and load-displacement curve obtained from tension test of specimen 1 KS

7 Load, N Displacement, mm Load, N Displacement, mm Fig Stress-strain and load-displacement curve obtained from tension test of specimen 2 KS

8 TRUE stress TRUE strain Load, N Displacement, mm Fig Stress-strain and load-displacement curve obtained from tension test of specimen 3 KS

9 Stress Test 1 Test 2 Test Strain Fig Comparison of Stress-Strain diagram obtained from three-tension test of mild-steel Load, N Test 1 Test 2 Test Displacement, mm Fig Comparison of Load-Displacement diagram obtained from three-tension test of mild-steel KS

10 4.2. Experiment and Equipment Selection Equipments used in the experiment are strain gauges, bridge boxes, load cell, strain amplifiers, data acquisition, and a computer. The functions of each device and theirs selection are described below. 1. Strain Gage Strain gage is one of the most important tools of the electrical measurement technique applied to the measurement of mechanical quantities. As their name indicates, they are used for the measurement of strain. Technically, strain consists of tensile and compressive strain that can be distinguished by positive or negative sign. Thus, strain gage can be used to pick up expansion as well as contraction. There are several types of strain gages depending on the proportional variance of electrical resistance to strain such as the piezoresistive or semi-conductor gage, the carbonresistive gage, the bonded metallic wire, and the foil resistance gages. Based on the four principles of selecting strain gages: 1. Suitability for the operating and environmental conditions 2. Accurate and reliable measurements 3. Easy to install 4. Minimum cost In addition, strain gage parameters are also important for selecting the strain gage such as gauge length, gage pattern, gage resistant, gage materials, and optional features. 1. Gage length is the strain sensitive length of the strain gage. Normally the strain gage lengths are ranged from 0.2 mm to 100 mm, but a length of 3 mm to 6 mm is generally recommended KS

11 for common applications. To select gage length, parameters below are considered Accuracy requirement Maximum strain (elongation) Heat dissipation Strain gradients Installation (space and easiness) Fig gage length of strain 2. Gage pattern refers to the number of the grid (uniaxial/multiaxial) and the layout of the grid (planar/stacked). Gage pattern consists of Uniaxial, biaxial, or multi-axial Heat dissipation Strain gradients Installation 3. Gage resistance. The electrical resistance of a strain gage is directly related to its sensitivity. Higher resistance gage have higher sensitivity than lower resistance gage. However, higher sensitivity of strain gage or higher resistance gages also higher cost. So, to minimize the cost, the suitability to the applications KS

12 should be considered. In the present work, a lower resistance gage (120 Ω) was chosen. The following parameters are included in gage resistance: Signal noise ratio Lead-wire desensitization Heat dissipation 4. Gage materials. The material construction of the wire directly affects the sensitivity of the strain gage. Based on the applications are mainly static strains and may encounter plastic deformation (large elongation), constantan alloy wire is selected. Gage materials selection should consider: Gage wire sensitivity Adhesive Backing (carrier) Self-temperature-compensation 5. Optional features. The main purpose is to reduce the installation time and effort and/or achieve certain performance characteristic. The common optional features includes: Built-in solder dots Preattached leadwire cables Integral terminals Individual furnished resistance values Encapsulation According to the principles above, strain gage type FLA-6-11 was used in this experimental analysis. KS

13 2. Bridge In order to measure strain with a bonded resistance strain gage, it must be connected to an electric circuit that is capable of measuring the minute changes in resistance corresponding to strain. Strain gage transducers usually employ four strain gage elements electrically connected to form a Wheatstone bridge circuit. A Wheatstone bridge is a divided bridge circuit used for the measurement of static or dynamic electrical resistance. The outputt voltage of the Wheatstone bridge is expressed in millivolt output per volt input. Fig Wheatstone bridge schematic Fig Bridge box 3. Load Cell According to the type of output signal generated (pneumatic, hydraulic, electric) or according to the way they detect weight (bending, shear, compression, tension, etc.), there are several types of load cell, which KS

14 have different capacity depend on the application. In this work the strain - gage load cell LCHD 5k (capacity 5,000 lb, compression) was used. Straingage load cell convert the load acting on them into electrical signals. The gauges themselves are bonded onto a beam or structural member that deforms when the weight is applied. In most cases, four strain gages are used to obtain maximum sensitivity and temperature compensation. Two of the gage are usually in tension, and two in compression, and are wired with compensation adjustments (Fig ). When weight is applied, the strain changes the electrical resistance of the gauges in proportion to the load. Fig Wiring to the bridge box KS

15 Fig Wheatstone circuit with compensation 4. Strain Amplifier Strain amplifier is the instruments that connected with bridge box, the bridge box transducer the data into strain amplifier. This instrument provided two output, OUTPUT-V (voltage output) and OUTPUT-I (current output). Both of them can be selected depend on the application. In the present of experimental, voltage output (OUTPUT-V) was used. 5. Data Acquisition There are several series of National Instruments-Data Acquisition (NI- DAQ) such as USB-6210, USB-6211, USB-6215, USB All of series are almost the same, the different only analog input (AI), analog output (AO), digital input (DI), and digital output (DO). For this work, the USB was chosen. USB-6211 devices feature up to 32 analog input (AI) channels, up to two analog output (AO) channels, 8 lines of digital input (DI), 8 lines of digital output (DO), and two counters. The USB-6211 and its components shown in Fig KS

16 Fig Strain amplifier DPM-611 auto-balancing type KS

17 Fig NI-USB 6211 and its components Fig USB-6211 Block diagram KS

18 4.3. Load Cell Calibration Before conducting the experiment, load cell calibration is important and necessary because the main purpose of using load cell is to know the amount of load that applied to the specimen. The output data from the load cell are in the form of strains not in form of load directly. Therefore, to obtain accurate results load cell calibration is needed Load Cell Calibration Setup The equipment that used in the load cell calibration are power hydraulic with displacement controller, bridge box, strain amplifier and multi-meter. The installation of load cell was carefully checked to make sure that load cell is perpendicular to the horizontal axis to assure that the applied load is only compression. Load cell was then wired to strain amplifier by bridge box, and multi-meter was used to read the strain values from the amplifier. Finally, the initial strain value was set to zero, as the load cell was unloaded. In this step, it was quite difficult and took long time because the hydraulic power is controlled by the displacement. Figure shows the setup of load cell calibration. To obtain accurate results, small displacement increment step was applied to increase the applied load. The maximum displacement was 4.6 mm and the displacement interval was 0.2 mm. The applied load can be recorded from control panel of power hydraulic controller. As the displacement is increased, the load and the output current from strain amplifier are also increased. To illustrate the results of load cell calibration, the strain plotted against load is showed in Fig KS

19 Fig Load cell calibration setup From Fig it can be seen that, the strain and load have a direct relationship, as they presented in the linearity form. From the result, the factor or the slop can be obtained which will be used to calculate the load applied to the test rigs of pipe bends. It should be notified that the strains obtained in this diagram are resulted from load cell calibration in the form of millivolt (mv), but the values here are already converted to the strain. KS

20 y = 21149x + 5E 14 Load, kn Strain Fig Load-Strain behavior of load cell obtained from the load cell calibration 4.4. Test Rigs of Pipe Bends Spacemen Modelling The specimens were made from two equal lengths of straight pipes combined by an elbow. Each of straight pipes was connected with each ends of elbow by welding method. At its ends, pipes were welded to the flanges. Fig showed the pipe bend specimen which used through this experimental analysis. KS

21 Flange 500 Flange Fig Geometry and dimension of specimen, unit in mm Non-Pressurized Pipes Test Out-of-Plane Bending Moment The first step of the test was to design and fabricate the necessary components. The test set up is shown in Fig The pipe bend specimen would lie in a horizontal plane, one end fixed with supporter and other one attached with adapter, which is connected to load cell and hydraulic power (hydraulic jack). Both ends of the pipe were welded with the flanges to close the ends of the pipe in case of pressurized pipe. Four strain gages were stuck at the bend area of the specimen (Fig ) to observe the plastic behavior of pipe bend, and then connected to the bridges. The KS

22 bridges will transfer dataa from the strain gages to the strain amplifier, whichh are contained two options of output such as ampere (current) and voltage. In this experimental analysis, voltage was chosen as an output. The voltage outpu from strain amplifier will go to the data acquisition. Finally, the data acquisition will transfer the data to computer and then by using Lab View, the results of experiment can be obtained. The force was created by increasing the pressure of the hydraulic jack continuously until it reached the plastic collapse load. Fig Out-of-plane bending moment experimenta al setup The test was performed at room temperature with no internal pressure. The test was load controlled; the maximum load is 5,0000 kn. The location of strain gages showed in Fig , and the out-of-plane bending moment experimental setup is shown in Fig KS

23 Fig Photo of out-of-plane bending moment test setup Fig Straingauges location KS

24 Figure is showed the load plotted with displacement curve resulted from experimental analysis, in case of out-of-plane bending moment Load, N Experimental Numerical Displacement, mm Fig Load Displacement curve in case of out-of-plane bending moment In-Plane Opening Bending Moment This test was conducted on the same dimension of specimen, 2 inch, schedule 10 of pipe. The schematic setup of the test is showed in Fig , and Fig is the photo of this test setup. The pipe bend specimen would lie on a vertical plane, one end fixed with supporter and other one is pulled by adapter, which is conducted from load cell. The test setup procedure is the same as out-of-plane bending moment test. KS

25 Strain Pin Load cell Hydraulic jack Hydraulic jack Fig In-plane opening bending moment experimental setup Fig Photoo of experimental setup KS

26 The results of experiment are shown in load displacement curve, Fig In the elasticity region of the test is very good results compared with numerical analysis but in plasticity region the results not so good Load, N Experimental Numerical Displacement, mm Fig Load Displacement curve in case of opening bending moment In-Plane Closing Bending Moment In this case is similar with the in-plane opening bending moment but the load applied at the one end of pipe bend specimen is become compression loading. The experimental setups are all the same with the previous test, different only hydraulic jack could not be applied in this test because there are some problems with the setup. Therefore, dummy loads were used. The loads are applied by adding the dummy (increase the weight) until the specimen reaches to the plastic deformation. Unfortunately, the dummy loads were not enough so, the plastic deformation can not KS

27 be obtained in this test. The test setup is shown in Fig and the photo of test setup is shown in Fig Strain gage Dummy Fig Test setup in case of in-plane closing bending moment Fig Photo of in-plane closing test setup KS

28 Figure is shown the results of this test. The results represented here can see only linearity or elasticity region because there were not enough of dummy load. Load, N Expeerimental Numerical Displacement, mm Fig Load Displacement curve in case of closing bending moment KS