Features Are Critical to Failure Analysis of Metals and Plastics

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FRACTO-Fractographic features are critical to failure analysis of metals and plastics. GRAPHIC Ronald J. Parrington* IMR Test Labs Lansing, New York FEATURES in Metals and Plastics ractography is the science of examining fracture surfaces. When material failure involves actual breakage, fractography can identify the fracture origin, direction of Fcrack propagation, failure mechanism, material defects, environmental interaction, and the nature of stresses. Fractography of plastics is a relatively new field, with many similarities to metals. Common failure modes include ductile overload, brittle fracture, impact, and fatigue. Analogies can also be drawn between stress corrosion cracking (SCC) in metals/ stress cracking in plastics, corrosion/chemical aging, de-alloying/scission, residual stress/frozen- in stress, and welds/knit lines. Stress raisers, microstructure, material defects, and thermo- mechanical history play important roles in both materials. This article describes some of the macroscopic and microscopic features considered by the failure analyst to evaluate fracture surfaces of metals and plastics. It discusses several case histories in order to explain key fractographic features for metals and Fig. 1— This image shows the hollowing out of a polyacetal hinge due to acid plastics, and to compare and contrast various as- catalyzed hydrolysis, a form of scission. pects of failure analysis and fractography. • High residual stresses can result from metal Causes of failure forming, heat treatment, welding, and machining. The primary objective of a failure analysis is to Similarly, high frozen-in stresses contained by in- determine the root cause. Whether dealing with jection molded plastic parts often contribute to metallic or nonmetallic materials, the root cause can failure. normally be assigned to one of four categories: de- • Porosity and voids are common to metal cast- sign, manufacturing, service, or material. In prac- ings and plastic molded parts. These serve as stress tice, several adverse conditions frequently con- raisers and reduce load carrying capability. tribute to the part failure. • Adverse thermo-mechanical history, poor mi- Improper material selection and stress concen- crostructure, and contamination are among other trations are examples of design-related problems manufacturing- and material-related problems that that can lead to premature failure. Material selec- may lead to failure. tion must take into account environmental sensi- Many service-related causes are also found to be tivities as well as requisite mechanical properties. the reason for failures. Environmental degradation Fractures frequently originate at stress raisers, par- is one of the most important service-related causes ticularly in fatigue. These include thread roots, of failure for both metals and plastics. Others in- sharp radii of curvature, through-holes, and sur- clude excessive wear, impact, overloading, and elec- face discontinuities such as gate marks in molded trical discharge. plastic parts. Likewise, many manufacturing and material Failure mechanisms problems found in metals are also observed or have Another key objective of failure analysis is to a corollary in plastics. identify the failure mechanisms. Once again, some • Weldments are a troublesome area for metals, failure mechanisms are identical for metals and as are weld lines or knit lines in molded plastics. plastics. These include ductile overload, brittle frac- *Member of ASM International ture, impact, fatigue, wear, and erosion. ADVANCED MATERIALS & PROCESSES/AUGUST 2003 37 fract.qxd 7/15/03 7:27 PM Page 2

Fig. 2— Hydrogen damage of an in- Fig. 3— Explosive decompression fractures Fig. 4 — Beach and radial marks are visible duction hardened steel piston rod displays of rubber O-rings are characterized by fisheye- on this torsional fatigue fracture of a six-inch di- “fisheyes.” like patterns. ameter shaft made of 4340 steel. Analogies can also be drawn between metals and Many macroscopically visible fractographic fea- plastics with regard to environmental degradation. tures serve to identify the fracture origin and di- Whereas metals corrode by an electrochemical rection of crack propagation. Fractographic features process, plastics are vulnerable to chemical changes common to metals and plastics are radial marks from aging or weathering. Stress corrosion cracking, and chevron patterns. Radial marks (Fig. 4) are lines a specific form of metallic corrosion, is similar in on a fracture surface that radiate outward from the many ways to stress cracking of plastics. Both re- origin and are formed by the intersection of brittle sult in brittle fracture due to the combined effects fractures propagating at different levels. Chevron of tensile stress and a material-specific aggressive patterns or herringbone patterns are radial marks environment. resembling nested letters “V,” and pointing towards Likewise, corrosion pitting and de-alloying or the origin. selective leaching in metals, defined as the prefer- Fatigue failures in metals display beach marks ential removal of one element from an alloy by cor- and ratchet marks that serve to identify the origin rosion, are somewhat similar to scission. This is a and the failure mode. Beach marks (Fig. 4) are form of aging that can cause chemical changes by macroscopically visible semi-elliptical lines run- selectively cutting molecular bonds in polymers ning perpendicular to the overall direction of fa- (Fig. 1). tigue crack propagation and marking successive Analogies can also be drawn between metals and positions of the advancing crack front. rubber, which is another type of polymer. The pre- Ratchet marks are macroscopically visible lines cipitation of internal hydrogen in steels can lead to running parallel to the overall direction of crack hydrogen damage, which is characterized by lo- propagation. They are formed by the intersection calized brittle areas of high reflectivity known as of fatigue cracks propagating from multiple ori- flakes or fisheyes on otherwise ductile fracture sur- gins. faces (Fig. 2). Brittle fractures in plastics exhibit characteristic Similarly, explosive decompression in rubber features, several of which are macroscopically vis- O-rings produces fisheye-like ovular patterns on ible (Fig. 5). These may include a mirror zone at the the fracture surfaces (Fig. 3). Explosive decom- origin, mist region, and rib marks. The mirror zone pression is the formation of small ruptures or em- is a flat, featureless region surrounding the origin bolisms that develop when an elastomeric seal, and associated with the slow crack growth phase saturated with high pressure gas, is subjected to of fracture. The mist region is located immediately an abrupt pressure reduction. adjacent to the mirror zone and displays a misty appearance. This is a transition zone from slow to Macroscopic fractures fast crack growth. Rib marks are semi-elliptical lines On a macroscopic scale, all fractures (metals and resembling the beach marks in metallic fatigue plastics) fall into one of two categories: ductile and fractures. brittle. Ductile fractures are characterized by material Microscopic features tearing, and they exhibit gross plastic deformation. On a microscopic scale, ductile fracture in metals Brittle fractures display little or no macroscopically (Fig. 6) displays a dimpled surface appearance cre- visible plastic deformation, and require less energy ated by microvoid coalescence. Ductile fracture in to form. plastics (Fig. 7) is characterized by material Ductile fractures result when stresses exceed the stretching related to the fibrillar nature of the material yield or flow stress. Brittle fractures gen- polymer’s response to stress. Although a part may erally develop well below the material yield stress. fail in a brittle manner, ductile fracture morphology In practice, ductile fractures result from over- is frequently observed away from the origin, if the loading or under-designing. They are rarely the final fast fracture was caused by ductile overload subject of a failure analysis, which usually involves (e.g., the “shear lip” in metal failures). The extent the unexpected brittle failure of normally ductile of this overload region is an indication of the stress materials. level. 38 ADVANCED MATERIALS & PROCESSES/AUGUST 2003 fract.qxd 7/22/03 8:32 AM Page 3

100 microns 100 microns Fig. 5 — Brittle fracture of an epoxy layer Fig. 6 — Dimpled appearance typical of duc- Fig. 7 — Fracture of a polyethylene tensile displays a mirror zone, rib marks, and hackles. tile fracture of metallic materials. test specimen exhibits material stretching.

100 microns 100 microns 20 microns Fig. 8 — Brittle fracture of a powder-forged Fig. 9 — Intergranular fracture of an em- Fig. 10 — Fatigue striations are visible on steel connecting rod displays cleavage facets. brittled cast steel pneumatic wrench. this Type 302 stainless steel spring fracture.

Brittle fracture of metallic ma- erials may result from a variety of failure mechanisms, but only a few basic micro-fractographic features clearly indicate the failure mechanism: cleavage facets (Fig. 8); intergranular facets (Fig. 9); and striations (Fig. 10). • Cleavage facets form in body-cen- tered cubic (BCC) and hexagonal close-packed (HCP) metals when the crack path follows a well-defined transgranular crystallographic plane Fig. 11 — Fatigue striations emanate from the Fig. 12 — SEM photomicrograph of the fa- (e.g., the {100} planes in BCC metals). fracture origin of this polycarbonate latch handle. tigue striations shown in Figure 11. Cleavage is characteristic of transgranular brittle fracture. on fatigue fracture surfaces (Fig. 11 and 12). Stria- • Intergranular fracture, recognizable by its “rock tions in plastics are typically observable at much candy” appearance, develops when the crack path lower magnifications (50 to 200X), although local follows grain boundaries. Intergranular fracture is softening and melting due to hysteretic heating can typical of many forms of SCC, hydrogen embrit- obliterate fatigue striations in less-rigid plastics. tlement, and temper-embrittled steel Alos, care must be taken to differentiate fatigue stri- • Striations are semi-elliptical lines on a fatigue ations in plastics from Wallner lines (see discussion fracture surface that emanate outward from the below). origin and mark the crack-front position with each In addition to mirror zones, mist regions, and rib successive stress cycle. Fatigue failures of many marks, which are normally visible without the aid metals exhibit striations at high magnifications (nor- of a microscope, brittle fracture of plastics may dis- mally magnifications of 500 to 2500X are required). play hackles, Wallner lines, and conic marks. The spacing of fatigue striations is usually very uni- Hackles (Fig. 5) are divergent lines radiating out- form and can be used to calculate the crack growth ward from the fracture origin. They resemble river rate if the cyclic stress frequency is known. Stria- patterns observed on the cleavage facets of trans- tions differ from striation-like artifacts on the frac- granular brittle fractures of metals. Wallner lines ture surface in that true fatigue striations never cross are faint striation-like markings formed by the in- or intersect one another. teraction of stress waves reflected from physical Plastics do not display cleavage or intergranular boundaries with the advancing crack front. Conic fracture. However, as in metals, striations are found marks are parabolic-shaped lines pointing back to- ADVANCED MATERIALS & PROCESSES/AUGUST 2003 39 fract.qxd 7/15/03 7:28 PM Page 4

Failure Analysis and Prevention will be one of the featured programs at ASM Materials Solutions 2003, the major annual technical gathering for members of ASM International. More than 20 world-class technical programs will be presented at the David L. Lawrence Convention Center in Pittsburgh, October 13 - 15. The event will be co-located with American Society for Nondestructive Testing (ASNT). Visit www.asminternational.org/materialssolutions to view complete details. The sessions have been organized by D. S. MacKenzie, Houghton In- ternational, and A. B. Tanzer, Siemens Westinghouse Power Corpo- ration. They are sponsored by the Failure Analysis Committee of the Materials Testing, Analysis, and Instrumentation Critical Technologies Sector. Overview: The program features in-depth coverage of the practical aspects of failure analysis and real-world case histories. An exciting addition to the program is the application of metallurgy to the field of criminal forensics. The use of Non-Destructive Testing to detect failures and to understand failure mechanisms is described on a variety of materials. Experts in the field will be sharing their knowledge and experience in the analysis of fatigue, using many techniques such as Linear Elastic Fracture Mechanics (LEFM) and striation counting. Practical case studies of failed components are presented by experts in the field on topics ranging from the residue on a fork aiding a forensic investigation, to failure of motorcycle swing arms. An exchange of practical ideas and information will be encouraged at sessions covering the failure of everyday items. These and other failures to be discussed represent a broad range of engineering materials: metals, polymers, ceramics, and composites. The sessions cover: Non-Destructive Testing Forensic Investigations and Everyday Failures Fatigue Structures Here is a sampling of some of the paper titles: • Forensic Study of the Steel in the World Trade Center • Silicone Replication as a Non-Destructive Failure Analysis Tool • Radiographic Inspection as a Tool for Failure Analysis • Failure Analysis of Traffic Signal Mast Arms • Forensic Science and the Law • Failure Analysis of Main Reduction Gear from USS Arctic (AOE-8) • Metallurgical Examination of a Failed Engine Impeller • Low Cycle Fatigue and Failure of an AerMet 100 Arresting Hook Shank • Corrosion of a Titanium Water Pump for the International Space Station • Fractography of Plastics and Metals • The Hoan Bridge Failure: It Was Not the Steel Visit www.asminternational.org/materialssolutions to view complete details.

wards the origin. Hackles and Wallner lines may high filler content. Fractures of these materials are or may not be visible without the aid of a micro- too often dismissed as inherently lacking mean- scope. ingful fractographic features. Finally, an authorita- tive publication on fracture in plastics is definitely Fractographic features needed. ■ Fractography is an invaluable tool for the failure analyst. Metals and plastics share many of the same For more information: Ronald J. Parrington is the Vice- failure mechanisms, including ductile overload, President of Materials Engineering at IMR Test Labs, 131 brittle fracture, impact, and fatigue. Not surpris- Woodsedge Drive, Lansing, NY 14882; tel: 888/464-8422; ingly, some fractographic features cross over ma- fax: 607/533-9210; e-mail: [email protected]; terial categories. On the other hand, some failure Web site: www.imrtest.com. mechanisms and fracture morphologies are unique Acknowledgements to a material category or subcategory. The author gratefully acknowledges the contributions Fractographic techniques developed and applied of Dave Christie and Steve Ruoff of IMR Test Labs. to metal failures for centuries have been readily adapted to the fracture analysis of plastics since their emergence as a key engineering material over How useful did you find the information the last 50 years. However, more work remains to presented in this article? Very useful, Circle 282 be done to advance fractography of plastics. One Of general interest, Circle 283 notable area for research is fracture analysis of com- Not useful, Circle 284 posites, reinforced plastics, and plastics containing 40 ADVANCED MATERIALS & PROCESSES/AUGUST 2003