بررسی تجربی و عددی الگوهای خرابی پیچ های پرمقاومت در روش های متداول آزمایش کشش (مقاله علمی وزارت علوم)
درجه علمی: نشریه علمی (وزارت علوم)
آرشیو
چکیده
استحکام کششی پیچ های پرمقاومت، با دو شیوه آزمایش تعیین می شود. در روش اول، مقاومت کششی پیچ با استفاده از مهره، تعیین می گردد. اما در روش دوم، به جای مهره از ابزار معرفی شده در استاندارد ایزو استفاده می شود. در پیچ های مورد آزمایش، دو الگوی خرابی مشاهده می شود، لهیدگی رزوه ی پیچ و گسیختگی پیچ از ناحیه کاهش یافته رزوه . در این تحقیق علاوه برانجام آزمایش، شبیه سازی عددی الاستیک _ پلاستیک الگوی خرابی پیچ ها، در نرم افزار اجزاء محدود، نیز انجام شده است. نتایج مطالعه حاظر نشان می دهد، فراوانی الگوی خرابی لهیدگی رزوه، در آزمایش کشش پیچ با ابزار ایزو، چهار درصد و در آزمایش کشش پیچ با مهره 90 درصد است. در ضمن استحکام کششی حاصل از آزمایش کشش پیچ با مهره، کمتر از روش آزمایش کشش پیچ با ابزار ایزو است. با توجه به درصد فراوانی الگوی خرابی در هر روش آزمایش معلوم می گردد، آزمایش کشش پیچ با ابزار ایزو منجر به تعیین استحکام کششی مصالح پیچ و آزمایش کشش پیچ با مهره، منجر به ارزیابی استحکام کششی مجموعه پیچ و مهره می گردد. به دلیل مطابقت عملکرد پیچ و مهره در طراحی و اجرای سازه های فولادی، این تحقیق انجام آزمایش کشش پیچ با مهره را، علاوه بر انجام سایر آزمایش ها پیشنهاد می نماید.Experimental and Numerical Investigation of Failure Patterns of High-Strength Bolts in Conventional Tensile Test Methods
The tensile strength of high-strength bolts is determined by two test methods. In the first method, the tensile strength of the bolt is determined using a nut. The second method uses the device introduced in ISO instead of the nut. In the tested bolts, two failure patterns are observed,the bolt thread stripping and fracture in the threaded length of bolt. In this research, in addition to the experiment, the elastic-plastic numerical simulation of the bolt failure pattern was also performed in Finite element software. The results of the study show that the frequency of the thread stripping failure pattern is 4% in the bolt tension test with the ISO device and 90% in the bolt tension test with the nut. In addition, the tensile strength obtained from the bolt tension test with a nut is lower than the bolt tension test method with the ISO device. According to the frequency of the failure pattern in each method, the bolt tension test with the ISO device leads to the determination of the tensile strength of the bolt material, and the bolt tension test with the nut leads to the evaluation of the tensile strength of the bolt and nut assembly. Due to the compatibility of bolt and nut performance in steel structure, this research suggests performing tensile test of bolt and nut, in addition to other tests.Keywords:Bolt Failure Pattern, Thread Stripping, High-Strength Bolt, Bolt Tensile Test, Tensile Strength.Introductionbolts are one of the most important components of a bolted connections, and its failure will be more destructive than the failure of other components, and even the failure caused by the weakness of the bolts in the connections can be progressive. Therefore, the importance of bolt inspection reveals the need for more accuracy and study in this field. In case of inaccuracy in the selection of bolts, contrary to expectation, a lower level of safety will be achieved. Therefore, not paying attention to any part of the inspection process of bolts will cause errors in the test results, and ultimately this will cause wrong approval to be issued for the use of rejected bolts in the structure. The tensile strength test of bolts is done in two common ways in laboratories. In this study, while performing both common methods of testing the tensile strength of Bolts, the frequency of failure patterns in each method was investigated. In this regard, the previous related studies and the statistics and frequency of each pattern were examined in the test methods. Numerical simulation was also performed for both tensile test methods.2. Methodology2.1. Prerequisite tests for testing the tensile strength of boltsIn this study, before performing the tensile test, Decarbonization test (ISO 898-1, 2013), Charpy impact test at -20 degrees Celsius (EN ISO 148-1, 2016), Proof load on the nut (ISO 898-2, 2012), was conducted on both specimens. The reason for conducting these three tests was that the results of the experimental failure pattern belonged to the bolts according to the standard, and possibly the results of the non-standard bolts should not be relied upon, and thus the health of the bolt and nut threads was examined as much as possible. In all three mentioned tests, all the specimens confirmed the standard conditions.2.2. Common tests to determine the tensile strength of boltsThe tensile strength of high-strength Bolts is determined by two test methods. At first method, the tensile test is doing with bolt and nut and but by second method, ISO 898-1 (2013) device fastended instead of nut on the bolt. On the Bolts fracture, observed two damage Pattern, the thread striping and fracture in the threaded length of bolt (Fig 1).3. Results and discussionTo evaluate the frequency of the failure pattern in the two test methods, the statistics of the failure pattern in the two test methods were collected and analyzed from several laboratories (fig 2).4. ConclusionIn this research, according to the results obtained from the laboratory investigation, the frequency of the failure pattern and the numerical simulation, we obtain the following important results:• Bolt performance in joints is more conformity with bolt-to-nut tensile testing.• The tensile strength of the bolt with nut is lower than the tensile strength of the bolt with the ISO 898-1 device, and it will not be correct to refer to the number obtained from the tensile test with the ISO 898-1 device as the tensile strength of the bolt and nut set.• Pre-tensioning or over-tightening will cause the thread stripping and the strength of the bolt to decrease significantly. This defect does not have any visible symptoms and therefore it can be frequent.• The shortness of the bolt length can reduce the tensile strength of the bolt and nut and thread stripping when pre-tensioning the bolt.Based on the results of this research, the tensile test of bolt with nut is suggested, in addition to other test methods.5. ReferencesISO 898-1: 2013 (E), 2013. Mechanical Properties of Fasteners Made of Carbon Steel and Alloy Steel.ISO 148-1: 2016 (E), 2016. Metallic materials — Charpy pendulum impact test —ISO 898-2: 2012 (E), 2012. Part 2: Nuts with specified property classes —Coarse thread and fine pitch thread.
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