Fastener Technology International - December 2007/January 2008 - (Page 30)

Comparing Preparation and Calibration Techniques for Ultrasonic Bolt Tension Measurement – Part 2 by: Dave Archer, President Archetype Joint, LLC 140 Engelwood Dr., Suite D Orion, MI 48359 USA www.archetypejoint.com This is the second and final installment of this examination of bolt preparation and calibration for ultrasonic bolt measurement. The first part, published in the last issue of this magazine, provided background on the subject and covered bolt preparation. Comparison of Std. Errors of the Estimate Std. Err. Est. Test 2 - Calibration Linearity and Repeatability The bolt elongation required for calibration can be created 0.18 with either tensile or torque inputs. The torque backers point 0.16 to the fact that since torsion will be applied to the bolt in the 0.14 actual joint, it only follows that it be calibrated in the same manner. While the tension crowd generally agrees that’s true 0.12 in theory, they will counter that the quantitative benefit is 0.10 unclear and unless the actual nut member and the bearing surface/finish are used in the calibration, the simulation ben0.08 efit is reduced. Also, as the under-head and thread surfaces 0.06 are disturbed, the calibrated bolts are generally not used for 1 2 3 4 5 6 7 8 9 testing, thus increasing the customer’s cost. Test ID Tensile bolt calibrations are nearly always performed in a Fig. 1 — Standard errors of the estimate for the nine test groups. universal test machine (i.e., tensile tester or UTM). Torquebased calibration can also be performed in a UTM although Range of Maximum Error the fixturing must be altered to provide access for the tool ap2.5 Max Error + 2.0 plying the torque and for resisting rotation. Another means of Max Error + 1.5 torque calibration, and the one that will used in this test, is in 1.0 a tension load cell designed for fastener testing. Independent 0.5 of what means of calibration is used, the grip length of the test 0.0 setup must simulate the actual joint as this has a direct impact -0.5 on the relationship between load and elongation. -1.0 Table 1, introduced in Part 1 of this article and shown again -1.5 below, summarizes the test plan developed for the M10 P.C. -2.0 10.9 hex head cap screws tested. -2.5 1 2 3 4 5 6 7 8 9 In this test, the piezo sensor was always applied to the Test ID tail of the bolt. While the head is generally not a preferred tightening location when there is an option, in this case it was Fig. 2 — Maximum individual error readings of each test group. done in an attempt to maximize torsional stress applied to the calibration. That was done on each of the tensile calibrations bolt, as this is one of the reasons sited for differences between in this test. As opposed to standard practice, the results of the preliminary tensioning were recorded and a calibration tensile and torque-based calibration. Figure 1 and Figure 2 summarize the linearity and repeat- file created so that the calibration of the first run could be ability of the individual calibration plots by comparing them to compared to the second to get insight into the effectiveness of the best fit line established by linear regression. The standard this practice. The results show that this preliminary tensioning error of the estimate used as the comparative basis of Figure 1 does appear to be of value, providing an average 12% decrease is the square root of the sum of the errors between the individual in Standard Error. The tensile calibration files used in all test trace and the best-fit line. The maximum error is the maximum comparisons were second runs. excursion of any individual trace from the best fit line. It is common before performing a tensile test to “exercise” Test 3 – Tension Measurement Error While Test 1 and Test 2 looked at the impact of preparathe setup, by tensioning it prior to performing the test or tion and calibration techniques on calibration accuracy, this TABLE 1— TEST PLAN test will measure the impact of those variables on the tension Objective Test ID Cal Method Grip Finish QTY measurement accuracy, the real test deliverable. Effect of finish/angle 1 Tensile med fine 10 If the accuracy of a measurement is to be determined it Calibration linearity 2 Tensile med extra fine 10 must be compared against a standard. For this test, determining what that standard should be used was not obvious. Because 3 Tensile med ground 10 the tooling available for the UTM was not configured to allow torque-based elongation, the tensile and torque calibrated bolts Calibration linearity 4 Tensile short ground 5 were calibrated to two different load standards—a UTM and a 5 Tensile long ground 5 tension load cell. It was decided that all tension measurement 6 Torque short ground 5 readings for this test would be taken on the tension load cell 7 Torque long ground 5 used for creating the torque-based calibrations. Tension accuracy 8 Tensile short extra fine 5 Each of the bolts in all test groups were placed in the load 9 Tensile long extra fine 5 cell and bolt was turned manually until the desired load cell Grip Lengths (mm) - Short: 23.4, Med: 31.4, Long: 39.4 tension was reached. The tension readings from both the 30 Fastener Technology International/December 2007 Maximum Error, % http://www.archetypejoint.com

Table of Contents Feed for the Digital Edition of Fastener Technology International - December 2007/January 2008

Fastener Technology International - December 2007/January 2008 Contents Editorial Calendar Industry News 5S – Turbocharged Lean House Cleaning: Principles and Practices for Creating an Efficient Workspace Corporate News Mechanical Properties – Part 7: Strength & Ductility in Bending FTI Technical Achievement Awards — Class of 2007 The Heat Treatment of Fasteners — Practice Melt Shop Provides More Efficient Titanium Wire & Rod Production Spotlight On Simulation Comparing Preparation and Calibration Techniques for Ultrasonic Bolt Tension Measurement – Part 2 Future-Oriented Testing Equipment Wire Handling Roundup What’s Happening at the IFI? Product Showcases Machinery Product Showcases Quality Education & Training in 2008 Fastener Focus Quality Checks on Retaining Rings: Your Recipe for Success Straight From The Show: NIFS/WEST Spotlight on Plastic Fasteners Competence in Stainless Steel Pick More Orders in Less Time With Slotting New Fastener, Drive Systems & Insert Join a Proven Product Line ASTM Update on Fastener Test Methods – The Bolt Doctor Column Advertisers’ Index Classified Advertisements Editorial Index

Fastener Technology International - December 2007/January 2008

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