FRACTOGRAPHY OF METALS AND PLASTICS

FRACTOGRAPHY OF METALS AND PLASTICSRonald J. Parrington, P.E.IMR Test Labs131 Woodsedge DriveLansing, NY 14882AbstractFractography is critical to failure analysis of metals andplastics. Fractography of plastics is a relatively new fieldwith many similarities to metals. Utilizing case histories,various aspects of failure analysis and fractography arecompared and contrasted.Common failure modes include ductile overload, brittlefracture, impact and fatigue. Analogies can also be drawnbetween stress corrosion cracking (SCC)/stress cracking,corrosion/chemical aging, dealloying/scission, residualstress/frozen-in stress, and welds/knit lines. Stress raisers,microstructure, material defects, and thermo -mechanicalhistory play important roles in both cases.Keyfractographic features for metals and plastics are described.behavior; (2) use thermal spalling to detach bedrock fromthe working core; and (3) shape stone by pressure flaking.Fractography as we know it today, developed in the 16thcentury as a quality control practice employed for ferrousand nonferrous metal working. “De La Pirotechnia”published by Vannoccio Biringuccio (1) in 1540 is one ofthe first documents to detail fractographic techniques.Historical PerspectiveInvention of the optical microscope in 1600 provided asignificant new tool for fractography. Yet it was notutilized extensively by metallurgists until the 18th century.In 1722, R.A. de Réaumur (2) published a book withengravings depicting macroscopic and microscopicfracture surfaces of iron and steel. Interestingly, thecategories of macroscopic features developed by deRéaumur have remained essentially unchanged through thecenturies.Plastics have been in existence for approximately 130years.John Hyatt patented nitrocellulose, the firstcommercial plastic, in 1869.However, full-scaledevelopment and use of plastics is only about 50 years old.In contrast, metals have been in use for many hundreds ofyears.Partly due to the development of metallographictechniques for examining cross sections of metals, interestin microfractography waned during the 19th century. Metalworkers continued to utilize fractographic techniques forquality assurance purposes but, for the most part,researchers and publications ignored fractography.The application of engineering materials is unavoidablyaccompanied by the occurrence of failures, many of whichhave been catastrophic. The consequences of materialfailures; including deaths, financial losses and legalramifications; have encouraged the development ofeffective failure analysis methods. Although the cost offailure analysis may exceed the value of the part, the costof service failures usually far exceeds the cost of failureanalysis. Many of the techniques utilized over the yearsfor the evaluation of metals have been successfully appliedto plastics with only minor modifications.Fractography is arguably the most valuable tool availableto the failure analyst. Fractography, a term coined in 1944to describe the science of examining fracture surfaces, hasactually been utilized for centuries as part of the field ofmetallurgy. Even before that, however, Stone Age manpossessed a working knowledge of fracture. Archeologicalfindings of lithic implements, weapons and tools shapedfrom stone by controlled fracture, indicate that prehistoricman knew how to: (1) select rocks with favorable fractureSeveral technological developments in the 20th centuryrevitalized interest in fractography. Carl A. Zapffe (3)developed and extensively utilized fractographictechniques to study the hydrogen embrittlement of steels.His work lead to the discovery of techniques forphotographing fracture surfaces at high magnifications.The first fractographs were published by Zapffe in 1943.An even more revolutionary development was theinvention of the scanning electron microscope (SEM). Thefirst SEM appeared in 1943. Unlike the transmissionelectron microscope, developed a few years earlier, it couldbe used for fracture surface examination. An SEM with aguaranteed resolution of approximately 500 angstromsbecame commercially available in 1965. Compared to theoptical microscope, the SEM expands resolution by morethan one order of magnitude and increases the depth offocus by more than two orders of magnitude. The tools formodern fractography were essentially in place beforeplastics achieved widespread use.

Failure Analysis OverviewThe general procedure for conducting a sound failureanalysis is similar for metallic and nonmetallic materials.The steps include: (1) information gathering; (2)preliminary, visual examination; (3) nondestructive testing;(4) characterization of material properties throughmechanical, chemical and thermal testing; (5) selection,preservation and cleaning of fracture surfaces; (6)macroscopic examination of fracture surfaces, secondarycracking and surface condition; (7) microscopicexamination; (8) selection, preparation and examination ofcross sections; (9) identification of failure mechanisms;(10) stress/fracture mechanics analysis; (11) testing tosimulate failure; and (12) data review, formulation ofconclusions and reporting.Although the basic steps of failure analysis are nearlyidentical, some differences exist between metals andplastics.Nondestructive testing of metals includesmagnetic particle, eddy current and radiographic inspectionmethods that are not applicable to plastics for obviousreasons.However, ultrasonic and acoustic emissiontechniques find applications for both materials.Likewise, different chemical test methods are necessary.Typical test methods for metals are optical emissionspectrometry (OES), inductively coupled plasma (ICP) andcombustion.Fourier transform infrared (FTIR)spectroscopy is extensively used to identify plastics bymolecular bonding and thermal testing, differentialscanning calorimetry (DSC) and thermo -gravimetricanalysis (TGA), is also very important for polymercharacterization. Energy dispersive X-ray spectroscopy(EDS), used in conjunction with the SEM, is a verypractical tool for elemental chemical analysis of metals andplastics. Also noteworthy, different chemical solutions arerequired for metals and plastics to clean fracture surfacesand to etch cross sections to reveal microstructure.Causes of FailureOf course, the primary objective of a materials failureanalysis is to determine the root cause of failure. Whetherdealing with metallic or nonmetallic materials, the rootcause can normally be assigned to one of four categories:design, manufacturing, service or material. Often times,several adverse conditions contribute to the part failure.Many of the potential root causes of failure are common tometallic and nonmetallic materials.Improper material selection, overly high stresses, andstress concentrations are examples of design-relatedproblems that can lead to premature failure. Materialselection must take into account environmental sensitivitiesas well as requisite mechanical properties. Stress raisersare frequently a preferred site for fracture origin,particularly in fatigue. These include thread roots (Figure1), sharp radii of curvature, through holes, and surfacediscontinuities (e.g., gate marks in molded plastic parts).Likewise, many manufacturing and material problemsfound in metals are also observed or have a corollary inplastics. Weldments are a trouble prone area for metals, asare weld lines or knit lines in molded plastics (Figure 2).High residual stresses can result from metal forming, heattreatment, welding and machining. Similarly, high frozenin stresses in injection molded plastic parts often contributeto failure. Porosity and voids are common to metalcastings and plastic molded parts (Figure 3). These serveas stress raisers and reduce load carrying capability. Othermanufacturing- and material-related problems that maylead to failure include adverse thermo -mechanical history,poor microstructure, material defects and contamination.Environmental degradation is one of the most importantservice-related causes of failure for metals and plastics.Others include excessive wear, impact, overloading, andelectrical discharge.Failure MechanismsAnother key objective of failure analysis is to identify thefailure mechanism(s). Once again, some failure modes areidentical for metals and plastics. These include ductileoverload, brittle fracture, impact, fatigue, wear and erosion.Analogies can also be drawn between metals and plasticswith regards to environmental degradation. Whereasmetals corrode by an electrochemical process, plastics arevulnerable to chemical changes from aging or weathering.Stress corrosion cracking, a specific form of metalliccorrosion, is similar in many ways to stress cracking ofplastics. Both result in brittle fracture due to the combinedeffects of tensile stress and a material specific aggressiveenvironment. Likewise, dealloying or selective leaching inmetals (Figure 4), the preferential removal of one elementfrom an alloy by corrosion, is somewhat similar to scissionof polymers (Figure 5), a form of aging which can causechemical changes by selectively cutting molecular bonds.Analogies can also be drawn between metals and anothertype of polymer, rubber. The precipitation of internalhydrogen in steels can lead to hydrogen damage, which ischaracterized by localized brittle areas of high reflectivityknown as flakes or fisheyes on otherwise ductile fracturesurfaces (Figure 6). Similarly, explosive decompression inrubber O-rings produces fisheye-like ovular patterns on thefracture surfaces (Figure 7). Explosive decompression isthe formation of sma ll ruptures or embolisms when anelastomeric seal, saturated with high pressure gas,experiences an abrupt pressure reduction. This failuremechanism is analogous to the “bends” that afflicts diversthat surface too quickly.