SESSION ETD 422 FAILURE ANALYSIS STUDIES IN COLLABORATION WITH INDUSTRIES FOR

STUDENT DESIGN PROJECTS

K.V. Sudhakar1 and Tadeusz Majewski

2

Abstract

This paper discusses two case studies of student design projects on failure analysis of medical and automotive components, respectively, carried out in collaboration with local industries. The two student design projects are described here presenting failure analysis investigation. In the first case, vitallium 2000 alloy medical implant was analyzed using a scanning electron microscope at a local industry to carry out a detailed fracture analysis of the medical component. It was concluded that the fracture of the broken component had occurred at the interface between vitallium 2000 screws and the plate suggesting that the interface region was the potential site for corrosion fatigue. The mechanism of fracture in vitallium 2000 orthopedic plate was clearly corrosion fatigue based on SEM fractography. In the second case, the automotive bearing was studied in detail to determine the cause for its premature failure. Various processes were studied for making the bearing at the local automotive ancillary industry and the investigation work was carried out at the university and another local industry. It was established that the failure/cracking of the bimetallic bearing was due to improper sintering time and temperature that did not allow the diffusion process to complete. The presence of undesirable elements those were uncontrolled during production process caused embrittlement in the copper alloy layer resulting in its cracking. Index terms- Motor bearing; Failure mechanisms; Scanning electron microscopy; Vitallium 2000 implant; Microhardness test; Corrosion fatigue

1

Department of Mechanical Engineering, Universidad de las Americas-Puebla, Santa Catarina Martir, Puebla 72820, Mexico Email: sudhakar_vadiraja@yahoo.com,

2Department of Mechanical Engineering, Universidad de las Americas-Puebla, Santa Catarina Martir, Puebla 72820, Mexico,

Email: tadeusz.majewski@udlap.mx Case Study #1: Investigation of a Failure in Motor Bearings

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Introduction

Bearings reduce friction by providing smooth metal balls or rollers, and a smooth inner and outer metal surface for the balls to roll against. These balls or rollers "bear" the load, allowing the device to spin smoothly. Bearings typically have to deal with two kinds of loading, radial and thrust. Depending on where the bearing is being used, it may see all radial loading, all thrust loading or a combination of both. The bearings in the motor face only a radial load. In this case, most of the load comes from the tension in the belt connecting the two pulleys. The typical bimetal bearing used in a motor is shown in Fig. 1. Bimetal bearings are constructed of two layers. The backing is generally steel to which a layer of bearing metal (copper alloy in the present case) is bonded. In this type of construction the steel back provides rigidity and allows higher levels of press fit or crush for better retention. The bearing lining however, must provide all of the bearing properties from a single layer. This requires that some properties are to be compromised in favor of others. Bimetal bearings are typically used for light or medium loading [1-2]. In the present case, a few motor bearings having defects were noticed during the production cycle. Typical production process cycle that involves different stages for producing bimetallic bearing in the present investigation is depicted in Fig. 2.

F

Fig. 1 Bearing used in a motor

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Fig. 2 Production process Cycle The objective of the present investigation is to determine the root causes/failure mechanisms responsible for cracking in motor bearings.

Material and methods

Preparation of metal powders

The raw material of the bearing consists of steel (as a base material), copper, lead and tin. The steel (in a tape rolled form) is subjected to a process of lamination, sanding and cleaning to eliminate contaminants attached to the strip sent by the supplier. The top layer of the bearing viz., copper and lead are received in ingots and tin in bars.

Sintering

The process of sintering involves application of uniform heat to the steel strip and then the metal powders of copper, lead and tin are sprayed on the surface of strip. Alloys of copper, lead and tin (as a top layer) are diffusion bonded to steel that has relatively a high density and low porosity. The sintering temperature was in the range 800-870 degrees that involves partial melting of lead and tin during the diffusion process [3].

Forming

Forming is the process wherein the bimetallic bearing strip is cut and bent on a specific mandrel to the required final geometry.

Scanning electron microscopy

The fracture features are studied using a microprocessor controlled Scanning Electron Microscope (SEM), JEOL Model 5910 LV. The fractured surfaces were cleaned thoroughly using acetone in an ultrasonic stirrer before examination.

Results and discussion

Visual examination

Visual examination of the defective bearings indicated detachment of the copper alloy layer from the steel base material. In some cases, cracking of the copper alloy layer was also noticed in addition to detachment. But for these observations, the surfaces of the bimetallic bearing appeared fairly smooth all its geometry.

Optical microscopy

Optical microscopy of the fractured surfaces (of copper alloy layer and the steel base) was carried out to assess the quality of basic

SESSION ETD 422 microstructure [4]. The microstructures were observed as identical (in terms of grain size, phase distribution etc.,) to the defect-free samples of bimetallic bearings.

Chemical composition of the layer containing copper, lead and tin

The standard chemical composition of the copper alloy used in the top layer of the bearing is shown in Table 1.

Table 1. Chemical Composition of Copper alloy layer used in bearing

Designation Composition (%) Copper alloy Cu: 75

Pb: 24 Sn: 1

Table 2. EDAX analysis of the Copper alloy of the bearing and the corresponding spectrum

Element Weight% Atomic% C K 7.35 26.10 O K 6.65 17.72 Al K 3.80 6.01 Cu K 71.22 47.82 Sn L 0.62 0.22 Pb M 10.36 2.13 Total 100.00

SEM-EDAX analysis was carried out to determine the basic chemical composition of the top layer (copper alloy) of the bimetallic bearing that cracked during the manufacturing stages. The results are shown in Table 2 and the corresponding EDAX analysis spectra. The notable feature is the presence of oxygen, carbon and aluminum at significant levels that facilitated premature cracking due to embrittlement [5].

Quality of the interface and fracture morphology

The detachment of the copper alloy layer from the base steel material is shown in Fig. 3. Another typical defect involving cracking of the copper alloy layer is depicted in Fig. 4.

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Fig. 3 The defect of detach is shown by arrow marks

Fig. 4 Presence of cracks (on copper alloy region) in the bearing Detailed examination of the interface between steel and copper alloy was carried out to assess the quality of the interface. To understand the basic differences more specifically, defective as well as non-defective specimens were chosen for investigation. Figs. 5 and 6 demonstrate clean interfaces for a defect-free sample.

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Fig. 5 Fractograph showing the a clean interface

Fig. 6 Demonstration of the clean interface (without any defects) at a higher magnification

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Fig.7 Cracked (decohesion) interface

Fig. 8 Cracked interface at a higher magnification

However, the presence of cracking (non-bonding) is clearly evident in Figs.7 and 8, corresponding to a typical defective sample that cracked during manufacturing stages. The observation of cracking is attributed mainly to the presence of foreign elements namely, carbon, oxygen and aluminum.

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Fig. 9 Improper bonding of lead, tin powders to copper material

SESSION ETD 422 Fig. 10 Improper bonding due to insufficient diffusion during sintering cycle

The contact area between the particles for diffusion is related directly to the parameters that control the diffusion in this case are the temperature, speed, time and pressure of compaction. The quality of metallurgical bonding in copper alloy powder is demonstrated in Fig. 9 and 10. Fig. 9 depicts intergranular cracking (opening) especially at the lead/tin phases in copper matrix. Fig. 10 reveals incomplete/improper fusion of powder particles in copper alloy layer. This is clearly due to inadequate sintering time and temperature that resulted in incomplete diffusion for the powder particles to bond together [5-7].

Conclusions

• The failure/cracking (detachment of copper alloy layer) of the bimetallic bearing was due to improper sintering time and temperature that did not allow the diffusion process to complete.

• The presence of undesirable elements those were uncontrolled during production process, caused embrittlement in the copper alloy layer resulting in its cracking.

Case stdy #2: investigation of a failed vitallium 2000 medical implant

Introduction

There are basically two types of vitallium 2000 (cobalt-chromium) alloys used in implants. One is the Co-Cr-Mo alloy, which is usually used to cast a product and the other is the Co-Ni-Cr-Mo alloy, which is usually wrought by (hot) forging. The castable Co-Cr-Mo alloy has been used for many decades in dentistry and recently, in making artificial joints. The wrought Co-Ni-Cr-Mo alloy is used for making the stems of prosthesis for heavily loaded joints such as the knee and hip. Cobalt-based alloys are highly resist ant to corrosion and especially to attack by chloride within crevice . As in all highly alloyed metals in the body environment, galvanic corrosion can occur, but to a lesser extent than in the iron-based alloys. Cobalt-based alloys are quite resistant to fatigue and to cracking caused by corrosion, and they are not brittle, since they have a minimum of 8% elongation [8-11]. However, as is true of other alloys, cobalt based alloys may fail because of fatigue fracture (but less often than stainless steel stems). The abrasive wear properties of the wrought Co-Ni-Cr-Mo alloy are similar to the cast Co-Cr-Mo alloy. The superior fatigue and ultimate tensile strength of the wrought Co-Ni-Cr-Mo alloy make it suitable for the applications which require long service without fracture or stress fatigue. Such is the case for the stems of the hip joint prosthesis. Both the cast and wrought alloys have excellent corrosion resist ance. However, in the modern applications of implants, forged alloys are preferred to cast alloys due to their superior fatigue/fracture toughness properties. The modulus of elasticity for the Cr-Co alloys does not change with the changes in their ultimate tensile strength. The values are higher than other materials such as stainless steels [12-14]. This may have some implications of different load transfer modes to the bone in artificial joint replacements, although the effect of the increased modulus on the fixation and longevity of the implants is not clear.

SESSION ETD 422 Material and methods

The cracked vitallium 2000 (67Co-32Cr-1Mo forged alloy) bone plate was investigated to determine the failure mechanism/s responsible for its fracture. The fractured piece was analyzed for determining the mechanism for failure. Typical vitallium plate and screws used in fixation device are shown in Fig. 1.

Fig. 1 Vitallium 2000 plain pattern bone plate and screws (arrow mark indicating the location of fracture) The dimensions of implant plate and screws are given below: Dimensions of a vitallium plate Length (L): 77.2 mm Width (W): 10.2 mm Diameter of holes (d): 3.8 mm Dimensions of vitallium screws Length (L): 28.6 mm Diameter (D): 4.0 mm

1X

Microhardness test Microhardness test was carried out on the failed samples of vitallium plate and screws to confirm whether or not they conformed to the specifications. From the hardness measurements, the vickers hardness (at 100 g load) of both screws and plate was determined as 41 (average of 10 readings).

Tensile test

Non-standard specimens (due to the small size of vitallium 2000 bone plate) were prepared from the vitallium 2000 plate to determine the tensile parameters. The experimentally determined values for tensile strength, 0.2% proof stress and the % elongation were 824 MPa, 612 MPa and 15.2%, respectively. Both the tensile test parameters and vickers hardness values matched with the specifications provided for the implant material.

Scanning electron microscopy

The fractured samples were cleaned using acetone to remove any grease, dirt or artifacts on the fracture surface and also to have a better clarity of the fracture features. Fig. 2 depicts the fracture surface features of the plate at low magnification.

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Fig. 2 Fractured surface of vitallium 2000 medical implant

The presence of typical fatigue striations and also the corrosion pits are seen in Fig. 3. Figs. 4 and 5 demonstrate the fracture surface features related to fatigue crack propagation. The evidence of corrosion in particular and corrosion fatigue in general can be seen in the fracture surface features presented in Fig. 6.

Discussions on the basic mechanisms of fracture

Fatigue fracture

In many applications in the body, implants are subjected to varying loads over a period of time. The load on a weight-bearing joint may vary from near zero to as much as 1000 lb during walking, and the joint (and implant) may have to withstand several million cycles per year. It is not surprising, therefore, that fatigue failures of medical implants occur [14]. In the present case, one of the mechanisms of fracture was clearly fatigue as evident by the presence of fatigue

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Fig. 3 Demonstration of typical striations in a fatigue fracture including the presence of corrosion pits

Fig. 4 Presence of a long fatigue crack across the slip bands

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Fig. 5 Fatigue crack (more severe than in Fig. 4) including secondary cracks

Fig. 6 Fracture surface affected by corrosion and fatigue

striations in Fig. 3. Also, the evidence of fatigue cracks propagating perpendicular to the direction of the slip bands is well demonstrated in Figs. 4 and 5.

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Corrosion

For implant applications, corrosion rate of a metal or alloy must be much lower than 0.1 mil per year to avoid tissue irritation and associated effects. Standard measurements indicate that stainless steel and cobalt-chrome alloys corrode at less than 0.01 mil per year in body fluids. Cobalt-chrome alloys are usually resistant to corrosion but under some conditions can develop potentials in the transpassive region. This behavior is in agreement with clinical findings, with stainless steel corroding frequently and cobalt chrome alloys occasionally [14].

Corrosion fatigue

Corrosion process when combined with fatigue is known as corrosion fatigue. This normally occurs when an implant is subject to time-varying stresses in a corrosive liquid environment. The liquid environment may well decrease the fatigue strength of the implant. This possibility had been observed in the presently investigated vitallium 2000 implant material based on the fracture surface features presented in Figs. 3 and 6. Specifically, in Fig. 3, the presence of fatigue striations along with corrosion pits (fracture initiation site) at some locations can clearly be seen. Furthermore, the evidence of corrosion (and corrosion fatigue) can be seen in the fracture surface features presented in Fig. 6. In addition, the chemical analysis (EDAX) of the fracture surface revealed the presence of corrosion products. Based on these observations, it was determined that the failure mechanism in vitallium 2000 alloy was corrosion fatigue rather than by the individual mechanisms of corrosion and fatigue acting independently.

Conclusions

• The fracture occurred at the interface between vitallium 2000 screws and the plate suggested that the interface region was

the potential site for corrosion fatigue. • The mechanism of fracture in vitallium 2000 orthopedic plate was clearly corrosion fatigue based on SEM fractography. • Perhaps, improper insertion of the screws to the vitallium bone plate caused uneven fatigue stress that encouraged corrosion

fatigue mechanism.

References

[1]. Fritz, L, “Powder Metallurgy Principles and Applications”, Princeton, USA, 1980. [2]. ASM Handbook. Powder Metallurgy, American Society of Metals, Metals Park, OH, 1988. [3]. Yoichi, I, “Fundamentals of diffusion bonding”, Elsevier, New York, 1987. [4]. Vander Voort, “Metallography Principles and Practice”, McGraw Hill, USA, 1984. [5]. Stanley, P, “An Introduction to Grain Boundary Fracture in Metals”, Institute of Metals, London, 1991. [6]. Vito, C, and Heiser, FA, “Analysis of Metallurgical Failures”, John Wiley & Sons, USA, 1987. [7]. Brooks, C, and Choudhury, A, “Metallurgical Failure Analysis”, EUA: McGraw Hill, 1993. [8]. Jacobs, JJ, Gilbert, JL, and Urban, RM, “Corrosion of metal orthopedic implants”, J Bone Joint Surg Am, 80(2), 1998,

268-82. [9]. Galante, JO, “Causes of fractures of the femoral component in total hip replacement”, J Bone Joint Surg Am, 62(4),

1980, 670-3. [10]. Hallab, N, Merritt, K, Jacobs, JJ, “Metal sensitivity in patients with orthopedic implants”, J Bone Joint Surg Am, 83-

A(3), 2001, 428-36. [11]. Huo, MH, and Cook, SM, “What's New in Hip Arthroplasty?”, J Bone Joint Surg Am, 83-A(10), 2001,1598-610. [12]. Mason, R, “Biomaterials – The Science and Biology Behind Biomaterials Research”, Materials World, 5(1), 1997, 16-

17. [13]. Hallab, N. et al., “Metal sensitivity in patients with orthopedic implants”, J Bone Joint Surg-Am, 83, 2001, 428-33. [14]. ASM metals handbook, vol 11, 11

th ed. Failure analysis and prevention: Metals Park, OH, 2002.