o l . 5, Issu E 2, May - o 2015 ISSN : 2249-5762 (Online

IJRMET Vol. 5, IssuE 2, May - ocT 2015 ISSN : 2249-5762 (Online) | ISSN : 2249-5770 (Print)

w w w . i j r m e t . c o m 44 InternatIonal Journal of research In MechanIcal engIneerIng & technology

Study of the Misalignment of Test Specimens Submitted to Fatigue Test Through Different Fastening Systems

1Lisiane Trevisan, 2Daniel Antonio Kapper Fabricio, 3Afonso Reguly1IFRS – Campus Farroupilha, Rio Grande do Sul, Brasil

2,3Laboratório de Metalurgia Física – LAMEF /UFRGS, Rio Grande do Sul, Brasil

AbstractOne of the measurement uncertainty sources in the fatigue test is the test specimen misalignment exercised in the fastening system of the universal testing machine. This work aims to evaluate misalignment of fatigue test specimens through the instrumentation of two samples, each of them with six displacement sensors (divided into two representative sets of upper and lower grip). In addition, we have tested two fastening systems: A fixing system through screw and a fastening system via hydraulic wedge grips. The strain measured with the use of screw fastening system found themselves questionable. The results showed the offset values calculated for the fastening system by hydraulic wedge grips are within the recommended by ASTM E1012: 2014.

KeywordsMechanical Testing, Fatigue, Misalignment, Instrumentation.

I. IntroductionFatigue is the process of progressive localized permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations [1].Every result of a measurement is only an estimation of the actual value, due to the influence of several sources of uncertainty [2]. The concept of uncertainty can also be described as a non-negative parameter characterizing the dispersion of the quantity values being attributed to a measurand, based on the information used, according to the International Vocabulary of Metrology - VIM [3]. Otherwise, the uncertainty of the result of a measurement reflects the lack of knowledge of the exact value of the measurand [4].In general, the lower the uncertainty in a measurement system, the greater the precision of the equipment and, consequently the greater the cost of performing this measurement [5]. In recent years, the industry has associated measurement uncertainty to the measured values due to requirements of international quality control standards [6].Table 1 describes some measurement uncertainty sources related to fatigue limit. For this, a mathematical proportion quantitatively related to each of these sources in relation to the fatigue life is presented [7].

Table 1: Ratio for Each One of the Different Variables Related to Fatigue Limit [7].

Source of Uncertainty Mathematical Proportion

Load Cell Calibration 1Load Cell Sensitivity 1Alignment of Testing Machine 1Dynamic Load Control 1Error (Drift) of Static Load 1Initial diameter of the test specimen 2Sensitivity of Measurement System 2

In addition to the factors described in Table 1, there is the influence of metallurgical parameters that alter the fatigue limit, such as grain size, alloying elements and heat treatment, in addition to operating conditions, including load history, imposed tension, temperature, etc. [8]. The elastic modulus, yield point, the tension modeling error and uncertainty of field carried loads are fundamental factors of a material that also alter the fatigue limit [9].The specimen alignment is critical to avoid generating undesirable efforts in other directions than the loading direction, and this problem in specimen alignment is called misalignment. Generally, the specimen is aligned with the fastening system of the testing machine when the application of the force passes through the center axis line of the test specimen axis. Misalignment can cause, among other problems, bending or deformation of the specimen, a non-uniform loading on the load cell, causing reading errors, and distinct efforts over the microstructure, among others [10].There are several ways to minimize misalignment of the specimen. First, there is the wear of the components of the testing machine. These should ensure concentricity of the system, because if this is not minimized, the wear can generate undesired tensions. Furthermore, it is found that the test specimen fastening system is also important in the alignment. The fastening systems, which ate called grips, should be subject to proper maintenance and alignment. Some of them may have stops so that the specimens are positioned accurately and reproducible [11].Mathematically, the amount of misalignment is related to the deformation arising in the folding calculated according to ASTM E1012: 2014 [11], multiplied by the modulus of elasticity of the material (Hooke’s Law). Hooke’s Law relates the stress and strain in the elastic region of the engineering stress-strain diagram through a linear relationship, for metals and alloys, as described in Equation (1) [12].

(1)

Where the symbol σ describes the tension to which the material is subjected and E represents the modulus of elasticity of the material. For steel, it is usual to assign the value of E = 210 GPa. Finally, the symbol ε describes the strain measured by the strain gauges.It is essential to ensure the alignment of both the upper and the lower grips at the initial time of test, and this alignment is not sufficient to ensure a null value during the test while applying a uniform stress [10]. In practice, the offset is not zero during the test, but should be less than 5%, as indicated by ASTM E467: 2008 [13].It is recommended to use strain gauges to measure the specimen misalignment, noting that the configuration of the installation of these should be interrelated with the magnitude of the tension used in the fatigue test. It is recommended to follow the manufacturer’s recommendations in their installation. Figure 1 shows the central positioning of the respective strain gages (position “A”) thus highlighting the problems related to each type of misalignment “B” (concentric misalignment) and “C” (angular misalignment) [11].

IJRMET Vol. 5, IssuE 2, May - ocT 2015

w w w . i j r m e t . c o m InternatIonal Journal of research In MechanIcal engIneerIng & technology 45

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Fig. 1: Positioning Strain Gauges and Misalignment of the Fastening System [11].

II. Methods usedThe metallic material used in this work is a stainless steel SAE 316 L, due to their qualities, such as dimensional stability and excellent resistance to corrosion. The SAE 316 L steel has a low carbon content (maximum of 0.03%) when compared to SAE 316 (maximum of 0.08%) [14]. The dimensions of the specimens are described in Fig. 2 [15], with tolerances described by NBR ISO 2768-1:2001 [16].

Fig. 2: Dimensions of Specimens (Millimeter).

Two test specimens were prepared under the same surface finish (sanded and polished with 6 µm diamond paste) to minimize the influence of surface finish on the behavior of the metallic material. Moreover, they were taken from separate rolled bars, but belonging to the same race in the steel and chemical composition within the standard recommended by ASTM A276/276M [17].The chemical composition of the test specimens was performed using a Optical Emission spectrometer. The values shown in Table 2 are the result of the arithmetic mean of 5 measurements for each specimen.

Table 2: Chemical Composition of the Test Specimens, Values Expressed in Percentage of MassChemical Element C Si Mn P S

CP A 0.0165 0.0400 1.762 0.0400 0.0215CP B 0.0217 0.3820 1.765 0.039 0.022Chemical Element Cr Mo Ni Co Cu

CP A 16.46 1.940 10.06 0.1461 0.4704CP B 16.38 1.905 10.06 0.1405 0.4646

According to the values shown in Table 2, the chemical compositionof the samples A and B are within the range stipulated by ASTM A276:2014.To develop this project, we used strain gauges brand VISHAY code 192911, VF452020 race number, with the following technical characteristics: resistance grade: %6,00,120 ± , calibration factor:

to 24°C and cross-sensitivity %2,04,1 ±+ [18]. All strain gauges used are from the same lot.It is recommended that for a useful length of the specimen greater than 12 mm, to use 2 or 4 lines of sensors (strain gauges). For a useful length shorter shelf length 12 mm is recommended to fix only one line of sensors [11].The clamping around the circumference was taken with 3 sensors distributed with a spacing of 120° between them [11]. Starting with a working length indicated by the letter A, the strain gauges must be separated by a minimum distance of 0,375A, equidistant from the longitudinal center of the specimen (1/2 L) [11].Deformation measurements were performed on a universal machine test MTS model 810 and 100 kN load capacity.The reading of the data generated by the strain gauges was undertaken by Spider 8 together with CatMan 4.0 software. The frequency of data acquisition was 50 Hz. The assessment of this assembly was held and passed with use of ASTM E 1942:2004 [19].The test specimens were instrumented with screw mounted as shown in Figure 3, where the thread of the test specimen is inserted into the threads of the fastening system. Importantly, the engagement between the two components should be as close as possible to avoid the decomposition of forces.

Fig. 3: Fixing System of the Specimens With Machined Thread

In this mounting system, the deformation measurements generated by the strain gauges through the hydraulic grip system were made as shown in Fig. 4. In this mounting system, the specimens were fixed directly on the hydraulic grip, without engaging in intermediate components, as with screw fastening system.

Fig. 4: Fixing System of the Specimens With the Use of Hydraulic Grips



Before any check, ASTM E467: 2008 [13] recommends that the maximum load is applied to the system so that hysteresis is minimized. By applying the maximum load, the system should be kept under this load until strain gauges make a deformation reading with minimum variation over time.

III. Results and ConclusionAfter the application of the maximum load to minimize hysteresis and verification of the acquisition system, different load values (4kN, 6kN and 8kN) have been implemented and, in parallel, the readings on each of the six strain gauges in each instrumented test specimen were performed. The readings of the gages were divided into 2 groups with each three sensors, the sensors being located at the top of the test specimen assigned to the reading of misalignment “upper grip”, and identically, the sensors located in the lower section “lower grip.

Initially, the system was submitted to the step 1 described in Table 3 to check the stability of the value of percent bending (% PB) function of time. This characteristic is represented by the force reading over time, i.e., verifying the ability of strain gauges maintain a constant reading of the deformation.

Table 3: Description of steps for Measuring Deformations Stage Load1 Maximum Load – 5%*(Ampl.load)2 Maximum Load3 Maximum Load + 5%*(Ampl.load)4 Minimum Load + 5%*(Ampl.load)5 Minimum Load6 Minimum Load - 5%*(Ampl.load)

With the load Amplitude described by Equation (1).

(1)

Upon the application of step 1 with strain rate of 0.5 mm / min and load of 4kN in threaded specimens, it was found that the strain does not stabilize with time, thus the misalignment amount (% PB) goes beyond the recommendations of the standard (5%) as shown in Fig. 4 -5 for the specimens A and B, respectively.

Fig. 4: Misalignment Calculated for Specimen A Threaded

Fig. 5: Misalignment Calculated for B Specimen With Thread

Since there is no deformation stability with time, there may likely be a problem in thread engagement with the fastening system of the universal testing machine. In other words, when the machine subjects the specimen to a deformation, there occurs a decomposition of the strength in triaxial components (X, Y and Z), and this is an undesired effect, considering the fatigue test should be uniaxial.

To prevent any changes in microstructure or problems in connecting sensors, the same screwed test specimens were machined on a lathe. This machining procedure was carried out carefully to avoid the occurrence of problems in the setting of the sensors, changes in microstructure or roughening of the clamping area of the specimens.

The average offset values (% PB) measured average for the same specimens fixed with hydraulic grips system are shown in Fig. 6-7.

Fig. 6: Misalignment Calculated for Threadless “A” Specimens, Fixed Via Hydraulic Grips.

Fig. 7: Misalignment Calculated for Threadless “B” Specimens, Fixed Via Hydraulic Grips

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As shown in Fig. 6-7, there is a stability of the misalignment (% PB) value as a function of time, furthermore the calculated values are smaller than the stipulated by the standard (5%). Mathematically, the calculation of misalignment was carried out as described in ASTM E1012: 2014 [11] item 10.2.1, described in Table 4, average for all steps outlined in Table 3 for loads of 4kN, 6kN and 8kN.

Table 4: Misalignment (PB%) of the Specimens Measured with use of Hydraulic Grips

Specimen A

Load Upper grip Lower grip Mean 4 kN 0.76 2.97 1.866 kN 2.13 4.14 3.138 kN 0.91 3.43 2.17Specimen BLoad Upper grip Lower grip Mean 4 kN 1.08 2.07 1.576 kN 1.23 2.13 1.688 kN 1.28 2.73 2.00

The results presented in Table 4 show that there are different values of the offset value to both the representative strain gauges as the upper grip of the lower grip. However, these values are within the maximum value stipulated in ASTM E466: 2007 [19], which as reference maximum acceptable folding 5% of the applied load.Considering the mean values between the upper and lower grips, for each of maximum loads, no mathematical relationship between the load on the specimens and the amount of misalignment has been found, that is, there is no relationship between the increase in the load values when increasing the misalignment.For future works, it is recommended to conduct tests on threads obtained through different machining processes such as machining using computer numerical control, in order to minimize machining problems or plug in other mechanical components. In addition, there should be tested loads used daily by industry.

References[1] ASTM INTERNATIONAL,“ASTM E1823: Standard

Terminology Relating to Fatigue and Fracture Testing”, Pennsylvania, 2013.

[2] JOINT COMMITTEE FOR GUIDES IN METROLOGY; “Evaluation of measurement data: Guide to the expression of uncertainty in measurement (GUM)”, JCGM 100, 1. ed. Geneva: BIPM, 2008.

[3] JOINT COMMITTEE FOR GUIDES IN METROLOGY; “International vocabulary of metrology (VIM): basic and general concepts and associated terms”, JCGM 200, 3. ed. Geneva: BIPM, 2012.

[4] ARENCIBIA, R. V.; RIBEIRO, J. R. S.,“Incerteza na medição da largura de cordões de solda”, Soldagem Inspeção, São Paulo, Vol. 14, No. 3, pp. 263-269, Jul/set 2009.

[5] Rede Metrológica RS – Procedimento do Sistema de Gestão da Qualidade,“RM 68 – Incerteza de Medição: Guia prático do Avaliador de Laboratórios”, Revisão 05. Abril/2013.

[6] SCHWENKE, H. et al.,“Assessment of uncertainties in dimensional metrology by Monte Carlo Simulation: Proposal of a Modular and Visual Software”, Annual of the CIRP. Vol. 49/1/2000.

[7] STANDARDS MEASUREMENT & TEST

MEASUREMENTS (SM&T); “UNCERT CoP 01 – Manual of Codes of Practice for Determination of Uncertainties in Mechanical Tests on Metallic Material nº 01 - The Determination of Uncertainties in High Cycle Fatigue Testing (for plain and notch-sensitive specimens)”, 2000.

[8] YE, D.; et al.,“Effects of low-cycle fatigue on static mechanical properties, microstructures and fracture behavior of 304 stainless steel”, Materials Science and Engineering A. 527. 2010, pp. 4092-4102.

[9] BENGTSSON, A; RYCHLIK, I,“Uncertainty in fatigue life prediction of structures subject to Gaussian loads”, Probabilistic Engineering Mechanics, 24. 2009, pp. 224-235.

[10] ASTM INTERNATIONAL; “Special Technical Publication STP Nº 566: Handbook of fatigue testing”, 1st ed , Philadelphia, 1974.

[11] ASTM INTERNATIONAL; “ASTM E1012: Standard Practice for Verification of Specimen Alignment under Tensile Loading”, Pennsylvania, 2014.

[12] SMITH, W. F.; HASHEMI, J.,“Fundamentos de Engenharia e Ciência dos Materiais”, Mc Graw Hill. Porto Alegre. 5 ed. 2012.

[13] ASTM INTERNATIONAL,“ASTM E467: Standard Practice for verification of constant amplitude dynamic forces in an axial fatigue testing system”, Pennsylvania, 2008.

[14] CALLISTER, W.,“Materials Science and Engineering”, New York: John Wiley & Sons Inc, 7 ed. 2007.

[15] COSTA, L.;,“Avaliação da incerteza de medição no levantamento de curva de fadiga S-N de materiais metálicos a temperatura ambiente”, Federal University of Rio Grande do Sul. Porto Alegre. 2010.

[16] BRAZILIAN ASSOCIATION OF TECHNICAL STANDARDS (ABNT); “NBR ISO 2768-1 - General tolerances - Part 1: Tolerances for linear and angular dimensions individual without tolerance indications”, Rio de Janeiro, Brazil, 2001.

[17] ASTM INTERNATIONAL,“ASTM A276/A276M: Standard Specification for Stainless Steel Bars and Shapes”, Pennsylvania, 2015.

[18] [Online] Available: http://www.vishay.com/ref/straingages accessed 19 Aug 2013.

[19] ASTM INTERNATIONAL; “ASTM E1942: Standard Guide for Evaluating data acquisition systems used in cyclic fatigue and fracture mechanics testing”, Pennsylvania, 2004.

[20] ASTM INTERNATIONAL; “ASTM E466: Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials”, Pennsylvania, 2007.

Lisiane Trevisan received degree in Metallurgical Engineering from the Federal University of Rio Grande do Sul - UFRGS (2005), Master in science in PPGE3M/UFRGS (2008). PhD student at the Graduate Program in Mining, Metallurgical and Materials -PPGE3M/UFRGS. She is currently Professor of Basic, Technical and Technological Education at the Federal

Institute of Education, Science and Technology of Rio Grande do Sul - IFRS. She has experience in Materials and Metallurgical Engineering area with emphasis on casting, mechanical testing and metrology.



Daniel Antonio Kapper Fabricio is Industrial Engineer by Federal University of Rio Grande do Sul (2013) and Master in Science and Technology of Materials in PPGE3M/UFRGS (2015), working with Quality Management at Physical Metallurgy Laboratory. Currently pursuing PhD in Science and Technology of Materials by PPGE3M/UFRGS.

AfonsoReguly holds a degree in Metallurgical Engineering from the UFRGS (1992), master’s degree in Mining, Metallurgical and Materials Engineering at PPGE3M/UFRGS (1994), doctorate in Mining, Metallurgical and Materials Engineering by PPGE3M/UFRGS (1999) and post-doctorate at Colorado School of Mines (2012). He is currently Associate Professor at the Federal

University of Rio Grande do Sul - UFRGS. He has experience in the area of Materials and Metallurgical Engineering, with emphasis on Transformation Metallurgy.

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o l . 5, Issu E 2, May - o 2015 ISSN : 2249-5762 (Online