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Fastener Failure Analysis: Oil Field Case Study
By: Admin on June 13, 2012

Fastener Failure Analysis, Case Study: Oil Field Case Study was published in the June/July 2012 publication of Fastener Technology International.

In this case we will examine a failure analysis for a customer involved in the very active oilfield drilling and hydraulic fracturing industry. Being a fastener in this industry is tough rigorous duty.

In this case our failure analysis engineers were provided with eight fractured stud bolts with a request to determine the cause of the fractures. The bolts failed after some time in service as threaded into the fracture head. Failure reportedly occurred at 15,042 psi. The bolt material was reportedly ASTM A193 Grade B7 steel fasteners. Bolts are shown in the as received condition in Figure 1 and were labeled alphabetically for reference by the analyst.

SUMMARY OF EXAMINATION CONDUCTED

Visual Examination with Stereoscopic Microscope - The bolts all exhibited what is referred to as necking (reduction in their diameter, similar to the reduction in thickness when you stretch a rubber band) consistent with ductility and stretching of the bolt during fracture due to slow strain rates. The fractures exhibited flat initial separation in the transverse orientation along the thread root edges with final separation by ductile and shear overload on the remaining material. See Figure 2.  Bolt A was further examined as the representative sample since all bolts exhibited similar features. A strong detergent was used to clean the fracture surface.

Metals chemistry - Elements examined conformed to the chemical requirements for ASTM A193 Grade B7.

Rockwell Hardness -  Tests per ASTM E8-09b were noted as HRC 36 which conforms to ASTM A193 Grade B7 requirements.

Tensile - Tensile test (Round) per ASTM E8-09. - Orientation - parallel to length of specimen, diameter = 0.352", tensile strength =166,000 psi, yield strength at 0.2% offset = 157,000, elongation in 1.4 inch = 16%, reduction in area = 49%.  All of which are typical of this grade of bolt.

Scanning Electron Microscopy - SEM per ASTM E 1508-98 (2003). The fracture surface was examined using a Scanning Electron Microscope (SEM) equipped with Energy-Dispersive Spectroscopy (EDS) for chemical analysis. The EDS microprobe was capable of detecting elements illuminated by the electron beam whose atomic numbers are smaller than ten. At low magnification, widely-spaced yield strain indications were observed. At higher magnification, the fracture exhibited ductile dimple and micro-void coalescence, which suggested that fracture occurred by stresses simply pulling the fastener apart. See Figure 3. The final fracture area exhibited further evidence of ductile dimples and shear dimples, which were consistent with material ductility during fracture.

Micro - ASTM E 3-01 (2007) - Metallographic specimens were prepared to show the microstructure in cross section to the fracture initiation site at the thread root radius. See Figure 4.  In the as-polished condition, the cross-section exhibited the flat separation adjacent to the thread root. Non-metallic inclusions were oriented in the longitudinal direction and were consistent with A193 material. The bolt exhibited secondary separations at other thread roots remote from the fracture which was consistent with ductile fracture during yielding. The separations exhibited dimensional deformation. The specimens were etched with Nital to reveal microstructure which consisted of tempered martensite, as would be expected for this grade of material. Microstructure deformation at the threads was consistent with rolled threads. Tempered martensite was observed in the core material along the bolt centerline, as the microstructure was uniform throughout the cross-section. No anomalies that played a role in the failure were observed in the microstructure.

Discussion

The necking observed in the bolt sample was indicative of ductile tensile overload with a strain rate which facilitated material deformation and yielding. The yield indications on the fracture surface observed by SEM was indicative of yielding and ductile fracture propagation as the loading pattern changed across the remaining bolts until final separation of the remaining material. The process of ductile overload fracture, i.e. necking and micro-void coalescence, likely occurred over a short time (seconds) before final separation.

The amount of necking was similar on each of the eight bolts, which indicated that the bolts failed virtually simultaneously, although slightly different orientations of fracture and different amounts of shear occurred as the loading pattern changed upon yielding. It could not be determined from this investigation whether the bolts had been installed properly with the appropriate torque load, although no evidence of installation damage or imbalance in fastening pattern was observed.

Conclusion

The bolts failed coincidentally due to ductile overload fracture. The material yielded briefly before final separation by ductile and shear fracture. The necking observed at each bolt fracture indicated a strain rate which yielded material at multiple thread root surfaces due to tensile stresses normal to the bolt length.

No fatigue cracks or pre-existing defects were observed at the thread roots. Chemistry, mechanical properties, and microstructure were consistent with ASTM A193 Grade B7 steel fasteners in the quenched-and-tempered condition with rolled threads.

Additional comments

During the course of conducting failure analysis, the evidence suggests that simply too much stress was applied to the part, as was the case in today’s Case Study.  Sometimes a professional Failure Analyst closes doors by answering a puzzling question and sometimes answers derived open doors to further investigation. In the case we reviewed today the later is true. The failure mode was determined, as requested. However when failure occurs in a material that conforms to all required specifications still fails, and no apparent adverse service conditions are reported, the analysis can open the door to a need to carefully examine the suitability of the materials, the application and process involved in order to eliminate continued or future failures.

The next step in resolving the issue and prevent future failures is to include the design and field service teams to determine if an underestimate of the loads and stresses that the bolts would experience in service was made, or if unanticipated field conditions contributed to the failure.   This latter step would include determining whether or not the loads and stresses applied were greater than is typically experienced by this sort of equipment in the given application. Then, assess why, and possibly modify the material or the process.

We welcome the opportunity to provide you a quote for our services or discuss how we can help.

News Tags: failure analysis failure investigation failure analysis lab failure analysis services fastener failure analysis SEM-EDS analysis oil and gas industry drilling and exploration industry bolt failure analysis ductile overload fracture failure