A Supplemental Laboratory for Strength of Materials Courses

Bolt Failure Fractography: A Supplemental Laboratory for Strength of Materials Courses

Temple, Richard

Abstract

The study of fractured surfaces has existed for centuries. As far back as 1540, blacksmiths would break quenched iron into fragments to determine the degree to which it had turned into steel. Fractography was developed in the 1940s as the science of studying fracture surfaces by means of eye examination, light microscopy, transmissionelectron microscopes, and scanning electron microscopes. The low-magnification study of fracture surfaces can be accomplished with simple equipment and moderate training, and it can implemented quite easily in typical statics and strength of materials courses found in mechanical engineering technology programs. If mechanical engineering technology graduates are to successfully design products for reliability, they must understand how machine components fail under load. This paper presents elements of low-magnification fractography for bolt failures as a supplement to strength of materials courses. Field failures were analyzed to determine loading conditions and cause of breakage. Proprietary issues prohibit the use of photographs of actual bolt failures, so typical bolt failure patterns are presented along with laboratory materials and fractography procedures.

Introduction

A brief history of fracture surface analysis is noteworthy. In 1627, fracture surfaces for the quality control of large bells were examined for composition adjustment to reduce cracking. In 1722, R. A. F. deReaumur was the first to use a microscope to document seven different types of fracture surfaces. In 1856, the U.S. Army Ordinance Corps used fractography and mechanical testing to study why cannon barrels ruptured during firing. In 1865, B. Kirsh described the classic cup-and-conc tensile fracture surface. In 1885, J. A. Brinell published a work on fracture surfaces that were composed of .52% carbon, describing these surfaces in detail under various treatments of cooling, heating, and quenching.1 Today fracture analysis is a notable science, fulfilling needs in product design, quality control, reliability, and product safety.

Bolts fail due to fatigue, stress corrosion cracking, elevated temperature, improper assembly, improper preload, and improper bolt joints.2 Failed bolts leave a track record of events leading to their failure via beach marks etched in their cross section. Beach marks (synonymous with sand lines on an ocean beach) are cause-effect evidence that can be examined to determine which type of loading created the failure. The result of a fracture failure is an image or footprint on the fracture surface, a three-dimensional historical record of the events leading to failure.3

Tools that can be used to analyze images of fracture surfaces include light microscopy, transmission-electron microscopes, and scanning electron microscopes, but often this equipment is beyond the budget of an engineering technology department.1 However, simple, low-cost, low-magnification inspection techniques can help students investigate fracture surface features and provide them with a basic understanding of failure modes. Although the field of fracture mechanics is a science, low-magnification analysis provides students with a general understanding of fractography that can increase student interest in proper component design and enhance strength of materials (SOM) courses.

Rationale

The understanding and interpretation of basic fractography for failed bolts is a useful body of knowledge for engineers and technicians working with machinery, pumps, and other mechanical devices subject to various bending, torsion, axial, or cyclic loads. Investigating fatigue failures and fractography as supplemental material for a SOM course enhances the ability of graduates to design components for reliable life. Incorporating hands-on activities, a form of active learning, in a SOM course can increase student learning and comprehension of material. Other examples of active learning strategies include projects, multimedia exercises, Java applets, student presentation of researched material, discussion groups, preclass mini tests, and group problem-solving activities. Research reveals that, when comparing active learning instruction with traditional teaching methods in a thermodynamics course, over the long run “the active learning section and the discovery/design section scored significantly higher” compared to traditional methods.4 Additional benefits of active learning include more appealing courses, better class attendance, increased student discussions and participation, a better grasp of material, increased performance on quizzes and exams, and a broader range of covered material.5 Active learning techniques have been shown to produce positive responses from students and help them grasp difficult concepts better than the traditional lecture format.6 Thus, a handson approach to investigating field failures enhances student knowledge of properly designed components.

Low-Magnification Fractography

Low-magnification fractography requires only a high-power magnifying glass. Low-magnification tools such as this allow the interpretation of basic fractography features that are caused by various kinds of loading. The following sections illustrate common breakage patterns associated with different types of bolt loading. Note, however, that the described loading types are not inclusive. Covering these basic failure modes in a one- to two-hour lecture provides student with a general understanding of loading types. A more detailed investigation beyond commonly identified loading patterns requires higher-powered magnification tools such as a scanning electron microscope as well as additional lecture time.

TENSILE FRACTURE SURFACE MARKS

The most basic type of loading investigated in SOM textbooks is tensile. Pure tensile loading and breakage of an unnotched specimen leaves a well-defined macroscopic appearance consisting of three distinct features. As shown in figure 1, these tensile-fracture surface characteristics are known as fibrous, radial, and shear-lip zones. Cracks originate at the center of the fibrous zone, which contains fibrous ridges that are random or perpendicular to the crack origin. The radial zone develops as the crack changes from slow growth in the fibrous zone to a more rapid mode of growth. Radial marks coincide with the direction of crack development and actually point to the crack center, and their appearance varies with the strength and ductility of the material. For bolts that fail from tensile fracture, the radial marks run outward from and perpendicular to the crack origin. The shear-lip zone is produced by the final fracture of the bolt, and it is characterized by a very smooth surface appearance indicative of the final break.1

Low- AND HIGH-CYCLE FATIGUE

Low-cycle fatigue exists under conditions of extreme cyclic loads where plastic strain is induced during each cycle, resulting in short bolt life.7 Low-cycle fatigue is most common at 50,000 or more cycles. Figure 2 shows a typical low-cycle fatigue pattern. Under these conditions, cracks typically begin at the lower edge of the crescent-shaped area, with radial surface marks extending outward from the crack origin. High-cycle fatigue occurs at lower cyclic loads. Here, strain cycles are limited to the elastic range, resulting in more cycles to failure. Figure 3 shows a typical high-cycle fatigue fracture pattern.

Failure Analysis Methodology

Failure analysis is the precise examination of the characteristics and causes of component failure, extending beyond a basic investigation into the why and how of a failure. Failure analysis attempts to establish or identify cause-effect relationships, sequencing of events, interphysical relationships due to mechanical loads, structural and material failure aspects, and fracture locations.8 Material evaluation can involve environmental testing, mechanical testing, chemical analysis, metallography, and fractography. Proper failure analysis can provide evidence about the specific use of a material in a design.9 Finding out why components fail can help prevent future failures on similarly designed parts as well as improve current designs, safety, and reliability. Random attacks to solving engineering problems often leads to incorrect conclusions, and students should understand that there is a structured approach for failure analysis. Several investigative steps in failure analysis are presented below. Only those steps relevant to low-magnification examination are reported.

COLLECTION OP BACKGROUND DATA

The collection of background information is a fact-finding event concerned with the appearance and condition of the failure site, physical evidence, and related documentation.8 Collecting pertinent background data requires gathering all the design and operating details related to the failure. Such data should include the current design criteria of the failure, including static and dynamic loads, safety factors, and load source. Interviews with machine operators, design engineers, and assemblers about operating conditions can provide key information in failure analysis. Interview questions should investigate fabrication history, loading history, assembly sequence and specifications, design materials and specifications, torque specifications, and vendor quality data.9 A review of literature is suggested to determine if similar investigations under similar circumstances have been performed. Service records should be obtained, and should include operator log sheets that reflect abnormal loading, accidental overloads, cyclic loads, temperature extremes, corrosive environments, maintenance history, and vibration levels. A detailed knowledge of the failed component production history is beneficial. If possible, information should include part numbers, lot numbers, serial numbers, and date of production. Information on the history of previous failures is also important, specifically, whether this was a one-time failure or whether other failure occurred with the same design. If there were multiple failures, were the environmental and loading conditions similar? Was corrective action taken on past failures? Were there any recent design changes or operating parameter changes, and is the component being used according to design criteria? Have supply vendors been changed?9,10

PRELlMlNARY EXAMINATION

Preliminary examination is visual inspection with the unaided eye, a powerful investigative tool that can detect even slight changes of color and texture. The investigator should look for crack propagation paths, beach marks, scratch marks, and material pits.9

MACROSCOPIC OR LOW-MAGNIFICATION EXAMINATION

“The amount of information that can be obtained from the examination of a fracture surface at low power magnification is surprisingly extensive.”9 The use of low-power magnification can reveal the type of loading exerted on the component. The most noticeable types of failure visible with low-magnification examination (LME) are tensile, fatigue, or bending failures. LME can determine the direction of crack growth and origin of failure, and show regions with different textures that indicate fibrous, radial, or shear failures. A logbook of each failed part is required, and should include sketches of special crack features, crack development, surface texture, crack origin, and beach marks in relation to crack progression.10

MECHANICAL TESTING

Mechanical testing is a relatively inexpensive measure of material properties, and many engineering technology programs have basic hardness testing machines that can be incorporated into a failure analysis laboratory course. Several tests can be conducted with these machines, such as hardness testing of failed and nonfailed bolts from the same production lot. Hardness testing is conducted to determine if the bolts measure up to their design properties according to a machinery or engineering handbook. Hardness testing can reveal if the part was heat treated properly, can provide an approximation of the tensile strength of the steel, and can be used to detect the work hardening or softening of sleel caused by overheating. Tensile tests can be performed to determine the ultimate strength, yield strength, elongation, and modulus of elasticity for comparison against established material standards.9

Supplemental Lecture and Laboratory

This paper provides a supplemental laboratory outline using actual field failures. Proprietary issues restrict printing photographs of these failures and part design, but this outline also can be used to develop a similar course that employs case studies. Table I contains websites of case studies of common field failures, including photographs, and table 2 lists texts containing case studies that can be used in laboratory assignments.

LECTURE OUTLINE USING ACTUAL FIELD FAILURES

An overview of fractography analysis, a review of common bolt failure modes, and a brief overview of bolt design loading are presented during one or two laboratory class sessions. Lecture topics include:

* Review of bolt behavior and clamping configurations

* Overview of preloaded joints

* Effective stiffness

* Bolt strength

* Common bolt failure modes

* Fatigue

* Examples of high-cycle failures

* Examples of low-cycle failures

* Details of the field failure, including loading, prints, and application

* History of the failure

* Examination of the print for joint configuration, actuator location, sources of loading

* Fractography methodology

* Overview of fractography investigation methodology

LABORATORY STRUCTURE

The class is divided into three or four groups for a failure analysis competition, with each group representing a fictitious consulting firm. Each group is required to present its failure analysis to the class for comparison with other groups. The instructor provides some of the research material in print form, including those references listed below. Networked computers are available for students to access photographs of common fatigue failures. Laboratory periods consist of three one and one-half hour sessions plus additional time outside of class for report and presentation preparation. During presentations, students are required to justify their results based on prints, historical data, and low-magnification examinations.

LABORATORY OBJECTIVES

The objectives of the laboratory course are to help students:

* Increase their understanding of common boll failure modes visible by low-magnification examination

* Acquire and understand a methodology for boll failure analysis

* Develop an appreciation for properly designed components and systems for safety and reliability

SAMPLE LABORATORY HANDOUT

The following handout can be used to introduce students to the methodology of fracture analysis.

Introduction to Case Study

Cardinal Equipment, a supplier of animal feeder equipment, has provided ten bolt field failures from one of their vibrator feeders, and it is up you to determine the type of loading that caused the failure. The bolts are from a hydraulic feeder mounting support bracket. As we cannot strain gauge the bolts to collect specific loading data at each location. Cardinal Equipment is only interested in the type of loading that caused the failure. There is a dispute between the company and a customer as to the cause of the failure: the customer says that it is a design fatigue problem, and the company maintains that it is an incorrect application problem and as such they are not liable for expenses occurred due to the failure. The bolts are used with lock nuts in a clamping unit with 12 mm-diameter through holes in both flanges. The metric bolts arc class 8.8 and have a 10 mm by 40 mm thread.

History of Failures

The failed bolts were installed two years ago on a feeder. No other bolts have failed in the twenty-year history of this design. However, in the past few months two out of three recently installed feeder units have experienced bolt failure. Other recent failures include two units out of a five-batch production run, all five of which were installed at the same site at the same time. Each unit has sixteen bolts, and both of the failed units had enough bolts fail simultaneously to cause the flanged unit to fall off the feeder. Torque requirements call for 50 ft-lbs on the 10 mm class 8.8 bolt, a value that is at the high end of the standard recommended torque values for this material.

Failure Scenarios

There are a number of circumstances that could have contributed to the bolt failure, including:

* Defective bolts: The engineer at Cardinal Equipment found four different brands of bolts during a site visit to the production factory. The factory says they buy in bulk and don’t have a brand preference.

* Impact loading: Peed material can collect or “bridge” within the storage shaft, then crash down onto the feeder at the shaft outlet. Heavy impact loads can exceed the rated capacity of the bolts in questions. This condition is difficult to verify, and indeed the customer has installed air cannons within the shaft to break up such bridging, but they insist that these impact loads are not the reason for the failures. Determining whether the failures are from static or dynamic loading is needed to determine the cause of failure.

* Insufficient design safety factor: The bolts have a total allowable load of 32 metric tons per the American Institute of Steel Construction (AlSC) Allowable Stress Design (ASD) method, and the clamping unit is rated for a maximum head load of 25 metric tons. Increasing the safety factor to a 3:1 minimum was suggested.

LABORATORY PROCEDURES

The following outline can be used in the three laboratory sessions:

Lab Session 1 (1.5 hours)

Instructor:

* Field failure problem and background

* Review of a structured approach to failure investigation

Students:

* Brainstorm and discuss within teams. Generate ten questions related to the failure

* Examine bolts with unaided eye

* Review fractography photos from metals handbook and web

* Research bolt grade, material composition, hardness, minimum proof load, and tensile strength

Lab Session 2 (1.5 hours)

Instructor:

* Brief review of crack propagation, loading types, and breakage patterns

Students:

* Low-magnification examination and interpretation

* Hardness testing and interpretation

* Examine and sketch the failure pattern in logbook

* Compare failure patterns with those found in literature

* Using technical terms, record in logbook what is seen

Example:

* Beach stress marks pointing toward the crack origin

* Dark crescent-shaped area at crack nucleus

* Break point at approximately same length as bolt 1, approximately .0625 inches

* Beach stress marks pointing toward the crack origin

* Dark crescent-shaped area at crack nucleus

* Crack origin looks like very uniform surface area

* As the crack progress, larger beach marks from the origin are present

Lab Session 3 (1.5 hours)

Instructor:

* Review of proper bolt design calculations for various load types

Students:

* Based on analysis, estimate the loading that caused the failure

* Research literature for proper bolt design for static and dynamically loaded joints

* Complete logbook and professional memo

* At next class meeting, team presentation of findings

Example:

* Bolts 1,4, 5

* Estimated loading type: Low-cycle fatigue

* Justification: Identical breakage patterns similar to photos in metal handbook:

* 4502: a threaded bolt of nickel-based alloy with 210 ksi tensile strength

* 4416: a threaded specimen of H1 1 tool steel with 296 ksi tensile strength

* 3917: a threaded specimen of alloy 718 loaded at 60% tensile strength

LABORATORY DELIVERABLES

Teams are required to present their findings and reports to the other class teams. The report, which is divided into introduction, purpose, data analysis, results, conclusions, and recommendations sections, must include the following data:

* A summary of background data on the failure, including preload conditions, bolt tensile strength, bolt grade, bolt material properties, current torque specifications, type of bolt loading (static/dynamic), and operating temperatures

* Identification of the crack initiation point, beach marks, and breakage Zones

* Hardness testing results and interpretation

The team presentation must include the following deliverables:

* A brief background and analysis of the bolt failures

* Photographs and sketches of these failures, noting crack initiation points and crack progression

* Details of the laboratory results presented in the introduction, purpose, data analysis, results, conclusions, and recommendations sections of the written report.

LABORATORY REFERENCES AVAIEABLE TO STUDENTS

Several references were chosen to help students research the properties and behavior of the bolts used in the case studies, namely,

* American Society for Metals. ASM Metals Handbook. 8th ed. Vol. 9. Materials Park, Ohio: ASM, 1974.

* Bickford, J. H. An Introduction to the Design and Behavior of Baited Joints. 3d ed. New York: Marcel Dekker, 1995.

* Collins, J. Failure of Materials in Mechanical Design. New York: Wiley & Sons, 1981.

* Hull, D. Fractography: Observing, Measuring and Interpreting Facture Surfaces. New York: Cambridge University Press, 1999.

* Rothbart, H. Mechanical Design Handbook. New York: McGraw-Hill, 1996.

* Shigley, J. and C. Mischke. Standard Handbook of Machine Design. New York: McGraw-Hill, 1986.

* Shigley, J. Mechanical Engineering Design. 6th ed. New York: McGraw-Hill, 2001.

These do not represent a complete list of available references, and as such can be modified according to program requirements.

Summary

A supplemental laboratory outline for a SOM course was developed with the objective of enhancing industry-required competencies, increasing student appreciation for SOM subject matter, and exposing students to the field of fractography. The supplemental laboratory course helps students develop their technical writing skills through the completion of a comprehensive technical report, and improve their critical thinking skills about product design through a methodology of collecting background data, applying design literature research, and drawing conclusions from the data. An understanding of and appreciation for SOM course content is enhanced through real-world failure investigation. The investigation process not only helps motivate students to learn, but also helps them develop a curiosity about failure modes and SOM topics in general.

References

1. American Society for Metals. ASM Metals Handbook. 8th cd. Vol. 9. Materials Park, Ohio: American Society for Metals. 1974.

2. Bickford, J. H. An Introduction to the Design and Behavior of Bolted Joints. 3d ed. New York: Marcel Dekker, 1995.

3. Hull, D. Fractography: Observing, Measuring and Interpreting Facture Surfaces. New York: Cambridge University Press. 1999.

4. Samples, J. W. “Becoming a Better Teacher: Adjusting from the Baseline.” Proceedings of the American Society for Engineering Education Annual Conference, june 23-26, 1996, Washington, D.C.: n.p.

5. Cartwright, A. N. “Cooperative Learning Environments foiEngineering Courses.” Proceedings of the American Society for Engineering Education Annual Conference, june 28 – july 1, 1998, Seattle: n.p.

6. Clough, D. E. “Bringing Active Learning into the Traditional Classroom: Teaching Process Control the Right Way.” Proceedings of the American Society for Engineering Education Annual Conference, june 28 – july 1, 1998, Seattle: n.p.

7. Collings, J. A. Failure of Materials in Mechanical Design: Analysis, Prediction, Prevention. New York: Wiley & Sons, 1981.

8. Whitherell, C. Mechanical Failure Avoidance. New York: McGraw-Hill, 1994.

9. American Society for Metals. ASM Metals Handbook. 8th ed. Vol. 10. Materials Park, Ohio: American Society for Metals, 1974.

10. Wershoven, Tijs. “How to Conduct Failure Analysis.” Advanced Materials and Processes 156, no. 5 (November 1999): 19-22.

Richard Temple is an assistant professor in the Department of Engineering Technology at Western Carolina University. He received degrees from Purdue University, Indiana State University, Vincennes University, Murray State University, and the University of Northern Iowa. he has nine years product testing experience at Caterpillar Inc. and the Allison Transmission Division of General Motors.

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