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Failure Analysis: Excessive Tensile Loading Can Result in Necking, Thread Spreading and Ductile Overload Failure
By: Admin on January 31, 2013
Fastener Failure Analysis, Case Study: Excessive Tensile Loading Can Result in Necking, Thread Spreading and Ductile Overload Failure was published in the October/November 2012 publication of Fastener Technology International.
Today we will look at a case in which bolts were submitted for failure analysis by a Middle Eastern oilfield equipment manufacturer. The bolts were manufactured in the United States.
Two fractured hex-head, 5/16” x 24; UNF-2 Bolts were submitted for metallurgical failure analysis along with one “New” bolt for comparison.
The fractured bolts were labeled as Samples A and B. The new bolt measured 1.450" in total length with approximately 1" long threaded section.
The diameters of the three bolts’ threaded sections ranged 0.308" to 0.309". The failed bolts were similar in length, measuring approximately 0.95" total length. The hex head of the "New" bolt measured 1/2" across parallel sides. The material used in the manufacture of the bolts was reportedly Monel K-500, a nickel copper alloy. This is a precipitation-hardenable nickel-copper alloy that combines corrosion resistance of the Monel 400 alloy with greater strength and hardness. It has low permeability and is non-magnetic even in sub-zero conditions. This alloy is typically used for pump shafts, sucker rods, oil-well tools, doctor blades and scrapers, springs, valve trim fasteners, and marine propeller shafts.
A review of fastener specifications found that requirements for bolts manufactured from Monel K-500 is included within ASTM F486-01a and Military Specification MIL-DTL-1222J, among other documents. It should be noted that little or no information concerning the failure was provided including operating temperatures, operating conditions and stresses, total number of bolt failures, etc.
Summary of Failure Analysis
The bolts were examined both visually and with the aid of a stereomicroscope at magnifications of 10X to 70X. The as-received fractured surfaces of bolts A and B, as well as the threaded sections, were coated with a black, tar like substance. To better examine the surface fractures, the bolts were cleaned in alcohol and acetone, which removed most of the black substance from the fracture. The cleaned fractured surface of bolt A is shown in Figure 1. Both bolts fractured at approximately 45° angles. There were no obvious features indicative of fatigue. The threads adjacent to the fracture exhibited necking and stretching in the roots between crests as shown in Figure 2. Measurement of the distance between threaded crests as shown in Figure 3. document that crest-to-crest measurements at several threads away from the fracture averaged 0.046", while those adjacent to the fractures measured up to 0.063".
Room Temperature Tensile Testing
A sub-size tensile specimen was machined from the new bolt and tensile tested at room temperature. The specimen met the minimum ASTM F468-01a and MIL-DTL-1222J requirements for fasteners manufactured from Monel 500 K materials.
Alloy Chemical Analysis
- Combustion Method (CO), ASTM E 1019-11.
- Direct Current Plasma-Atomic Emission Spectrometry (DCP), ASTM E 1097-07 / CTP 3005.
The chemical composition of the failed bolt material conformed to ASTM F468-01a Alloy 500 specification (UNS Nickel Alloy Grade N05500).
Scanning Electron Microscopy/Microanalysis
The cleaned fractured and threaded surfaces of bolts A and B were examined with the Scanning Electron Microscope (SEM). At high magnification (1000X) the fractured region near the root exhibited ductile dimple morphology as seen in Figure 4. Examination in the core regions of the fracture also exhibited ductile dimple morphologies. These features are indicative of ductile tensile overload. Some shear was also present that were associated with threaded tips on one side of the failure. Examination of the threaded roots adjacent to the fracture confirmed that stretching and necking occurred in these regions prior to fracture. A typical root area is shown in Figure 5.
Longitudinal cross-sections were cut through the center of the fractured surfaces of bolts A and B. The cross sections were examined with the light microscope at magnifications ranging from 25X to 1000X in the as-polished condition and as etched with Ferritic Acid (5g FeCl3 + 50 ml HCL + 100 ml H2O).
The as-polished cross sections were generally featureless, however, the radii of the threaded root sections were measured for comparison. The threaded roots near the fracture averaged 0.016" compared to 0.007" outside of the stretched section.
The material was considered very clean with respect to nonmetallic inclusions, as only a few small inconsequential inclusions were present in the core of the bolts.
Conclusions and Discussion
The bolts failed as a consequence of excessive tensile loading that resulted in necking, thread spreading and ductile overload failure. The bolts conformed to material and mechanical requirements of ASTM F468-01a and were of acceptable metallurgical quality.
The two bolts fractured when an excessive tensile load was placed on the bolts. Excessive refers to that in excess of the strength of the bolt material not necessarily above what is expected in service. The tensile loads caused necking in the bolts as well as thread stretching until the bolts separated by ductile overload. These types of bolt failures are typically referred to as a 45° full-slant fractures. It should be noted that if additional bolts were part of this failure that it is not necessarily correct to assume that all failed by ductile overload. The first bolt to fail may have done so by a different failure mode, which would subsequently increase the tensile stress on the remaining bolts.
The bolt material, mechanical properties, response to heat treatment and micro-structural quality were typical of good and acceptable quality. In the absence of gross anomalies or embrittlement, these bolts would be expected to provide the requisite strength and performance typical for bolts of this size and type. It is recommended that the process conditions that imposed an excessive tensile stress on the bolts be investigated. Other contributing factors requiring review include any high temperature conditions, torque tightening requirements and use of proper thread locking compounds or lubricants.
As in this case, our analysts are frequently provided with failed materials for analysis with little or no information provided on the specifics of the application and its associated operating forces and environment. In these cases, failure mode can be determined but further investigation is often called for to determine corrective action.
In an upcoming column we will deal with this issue by providing a simple to follow guide and checklist for quality and R & D engineers to follow in submitting requests for failure analysis. This guide includes an easy to follow checklist format for submitting materials for analysis. The guide is designed to provide client engineers with the basic tools needed to reduce cost and turn times and for obtaining the most accurate and useful analysis from their submittals.
Learn more about failure analysis services we provide as well as our full service metallurgical testing services. We welcome the opportunity to provide you a quote for our services or discuss how we can help.
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