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Author's personal copy

Failure analysis and redesign of the evaporator tubing

in a Kimchi refrigerator

Seong-woo Woo a,*, Dennis L. ONeal b, Michael Pecht c

a SAMSUNG Electronics Co., Ltd., 272, Oseon-Dong, Gwangsan-Gu, Gwangju-City 506-723, South Koreab Department of Mechanical Engineering, Texas A&M University, College Station, TX, USAc Center for Advanced Life Cycle Engineering (CALCE), University of Maryland, College Park, MD, USA

a r t i c l e i n f o

Article history:

Received 31 July 2009

Accepted 17 August 2009

Available online 21 August 2009

Keywords:

Fitting corrosion

Robustness

Parameter design

Accelerated life testing

a b s t r a c t

A failure analysis was conducted on a pitted aluminum tubes in the evaporator used to

cool, a Kimchi refrigerator. The root cause of the failure was a pitting corrosion which

was traced to chlorine in a cotton adhesive tape used on the tubes. To reproduce the failure

modes and mechanisms causing the tube pitting corrosions, a tailored set of accelerated

life tests was applied to the evaporator tubing. Using chemical loads, the key noise param-

eters in the assembly, including a variety of chemical reaction formula, were analyzed. The

failure modes and mechanisms found experimentally were identical to those of the failed

sample. To correct the problem, the cotton tape in the cooling evaporator was replaced by a

generic transparent tape. The B1 life of the new design is now guaranteed to be over

10 years with a yearly failure rate of 0.1%.

Published by Elsevier Ltd.

1. Introduction

Fig. 1 shows the Kimchi refrigerator with the newly designed cooling aluminum evaporator tubing. When a consumer

stores the food in the refrigerator, the refrigerant flows through the evaporator tubing in the cooling enclosure to maintain

a constant temperature and preserve the freshness of the food. To perform this function, the tube in the evaporator need to

be designed to reliably work under the operating conditions it is subjected to by the consumers who purchase and use the

Kimchi refrigerator. The evaporator tube assembly in the cooling enclosure consists of an inner case (1), evaporator tubing

(2), Lokring (3), and adhesive tape (4), as shown in Fig. 1b.

In the field, the evaporator tubing in the refrigerators had been pitting, causing loss of the refrigerant in the system andresulting in the loss of cooling in the refrigerator. The data on the failed products in the field were important for understand-

ing the usage environment of consumers and pinpointing design changes that needed to be made to the product.

Robust design techniques, including statistical design of experiment (SDE) and the Taguchi methods [1], have been devel-

oped by statisticians for use in improving designs in products. The Taguchi methods describe the robustness of a system for

evaluation and design improvement also known as called quality engineering [2,3] or robust engineering [4]. Robust design

processes include concept design, parameter design, and tolerance design [5].

Taguchis robust design method places the design in a position where random noise does not cause failure and helps to

determine the proper design parameters [6]. The basic idea of parameter design is to identify, through exploiting interactions

between control factors and noise factors, appropriate settings of control factors that make a systems performance robust in

1350-6307/$ - see front matter Published by Elsevier Ltd.

doi:10.1016/j.engfailanal.2009.08.003

* Corresponding author. Tel.: +82 62 950 6933; fax: +82 62 950 6807.

E-mail address: [email protected] (S.-w. Woo).

Engineering Failure Analysis 17 (2010) 369379

Contents lists available at ScienceDirect

Engineering Failure Analysis

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Fig. 1. Kimchi refrigerator (a) and the cooling evaporator assembly (b).

Nomenclature

AF acceleration factorBX durability indexC1 length of contraction tubeC2 type of adhesive tapeCl% chlorine concentration, PPMF(t) unreliabilityF corrosive force, NF1 corrosive force under accelerated stress conditionsF0 corrosive force under normal conditionsh testing times (or cycles)h* non-dimensional testing cycles, h

h=LB P 1

k Boltzmanns constant, 8.62 105 eV/degKCP key control parameterKNP key noise parameterLB target BX life and x = 0.01, on the condition that x6 0.2n number of test samplesN1 corrosive force of evaporator tubing under customer usage pattern

r pitted numbers of the evaporator tubing in the testS stressS1 mechanical stress under accelerated stress conditionsS0 mechanical stress under normal conditionsti test time for each sample, hTf time to failure, hx required target, x = 0.01, on condition that x 6 0.2.

Greek symbolsg characteristic life

Superscriptsb shape parameter in a Weibull distributionn the stress dependence, n

@lnTf

@lnSh i

T

Subscripts0 normal stress conditions1 accelerated stress conditions

370 S.-w. Woo et al. / Engineering Failure Analysis 17 (2010) 369379
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relation to changes in noise factors. Thus, the control factors are assigned to an inner array in an orthogonal array, and the

noise factors are assigned to an outer array.

However, a large number of experimental trials array may be required for the Taguchi method because the noise array is

repeated for every row in the control array. Alternative methods, such as the combined array approach, have been proposed

[7,8]. However, for a simple mechanical structure, a lot of design parameters should be considered in the Taguchi methods

robust design process. Products with the missing or improper minor design parameters may result in recalls and loss of

brand name value.Based on an analysis of failed products in the field, accelerated life testing (ALT) with the new concept (BX) and sample

size, an alternative parameter study can be considered [9,10,11]. Failure analysis of the field data helps pinpoint the missing

key control parameters in the design process. ALT may help identify the missing key control parameters of the newly de-

signed mechanical system and the proper choices for its levels.

The key control parameters precipitated by ALT may not represent those occurring in the field because of inconsistencies

in the types and magnitudes of the loads applied during testing. Moreover, the numbers of test samples and the test dura-

tions are usually insufficient to uncover some occasional failure modes. ALT should be performed with sufficient samples and

testing time, and with equipment designed to match expected product loads.

The steps for the robust design of chemical / mechanical systems can be summarized as follows: (1) analysis of the prob-

lems identified in field samples; (2) analysis of the loads in the dynamic system; (3) ALT to identify the key control param-

eters; and (4) formulation of a corrective action plan to determine the level of the key control parameters.

The robust design of a newly designed evaporator tubing system in the cooling enclosure of a Kimchi refrigerator was

investigated in this study. Because the evaporator assembly is a simple mechanical heat exchanger system including watercondensation on the evaporator tubing and their corrosions, it is important to model the load on the basis of thermodynam-

ics, heat transfer, and chemical reaction formula. ALT equipment can then be fabricated on the basis of load analysis. First,

uncontrolled corrosive load conditions on the system were analyzed using design schematics and chemical equations. The

analysis then proceeded to new robust methodologies for parameter designs. After a sequence of ALTs and corrective action

plans, key control parameters and their levels were summarized in the BX life. Finally, the effectiveness of these methodol-

ogies was demonstrated by developing a robust design for the evaporator assembly.

2. Field design problems

In the field, the refrigerant tubes in the evaporator assembly in a Kimchi refrigerator were experiencing pitting and the

refrigerant was leaking out of the tubes (Fig. 2). The specific usage conditions of the refrigerators by consumers in the field

were unknown. Field data indicated that the damaged products might have had design flaws. The design flaws combined

with the repetitive loads could cause failure [12,13]. The pitted surfaces of a failed specimen from the field were character-ized by a scanning electron microscopy (SEM) and EDX spectrum ( Fig. 3). We found a concentration of the chlorine in the

pitted surface (Table 1). When Ion Liquid Chromatography (ILC) was used to measure the chlorine concentration, the result

for the tubing having had the cotton adhesive tape was 14 PPM. In contrast, the chlorine concentration for tubing having had

the generic transparent tape was 1.33 PPM. It was theorized that the high chlorine concentration found on the surface must

have come from the cotton adhesive tape.

3. Load analysis

The evaporator tubing assembly in the cooling enclosure of the Kimchi refrigerator consists of many mechanical parts.

Depending on the consumer usage conditions, the evaporator tubing experienced repetitive thermal duty loads due to the

normal on/off cycling of the compressor to satisfy the thermal load in the refrigerator. Because the refrigerant temperatures

are often below the dew point temperature of the air, condensation can also form on the external surface of the tubing.

Fig. 4 shows a robust design schematic overview of the cooling evaporator system. Fig. 5 shows the failure mechanism ofthe crevice (or pitting) corrosion that occurs because of the reaction between the cotton adhesive tape and the aluminum

evaporator tubing. As a Kimchi refrigerator operates, water acts as an electrolyte and will condense between the cotton

adhesive tape and the aluminum tubing and the crevice (or pitting) corrosion will begin.

The number of Kimchi refrigerator operation cycles is influenced by specific consumer usage conditions. In the Korean

domestic market, the compressor can be expected to cycle on and off 2298 times a day to maintain the proper temperature

inside the refrigerator.

The crevice (or pitting) corrosion mechanism on the aluminum evaporator tubing can be summarized as: (1) passive film

breakdown by Cl attack; (2) rapid metal dissolution: Al ? Al+3 + 3e; (3) electro-migration of Cl into pit; (4) acidification

by hydrolysis reaction: Al+3 + 3H2O? Al(OH)3; + 3H+; (5) large cathode: external surface, small anode area: pit; (6) the large

voltage drop (i.e., IR drop, according to Ohms Law V= I R, where R is the equivalent path resistance and I is the average

current) between the pit and the external surface is the driving force for propagation of pitting ( Fig. 6).

Because the corrosion stress of the evaporator tubing depends on the corrosive load (F) that can be expressed as the con-

centration of the chlorine, the life-stress model (LS model) [14] can be modified as

Tf ASn

AFn

ACl%n

1

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The acceleration factor (AF) can be derived as

AFS1S0

n

F1F0

n

Cl1%

Cl0%

n2

4. BX life and sample size

The characteristic life g determined by maximum likelihood estimation can be defined as

gb

Ptbir

ffin h

b

r3

As the product (or part) reliability improves, the number of failures decreases and there may be no failures in the labo-

ratory. Thus, it is not appropriate to evaluate the characteristic life in Eq. (3). When the number of failed samples is below

four, it is assumed to follow the Poisson distribution [15]. At a 60% confidence level, the characteristic life can be redefined as

gb ffi1

r 1 n h

b4

To introduce the BX life, the characteristic life in the Weibull distribution can be modified as

Fig. 2. A damaged product after use.

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LbB ffi x gb

x

r 1 n h

b5

To assess the BX life with about a sixty percent confidence level, the number of test samples is derived from Eq. (5). That is,

n ffi

1

x r 1

1

h b

6

with the condition that the durability target is defined as, h

h=LB P 1.

Fig. 3. SEM fractography showing a pitting corrosion on the evaporator tube.

Table 1

Chemical composition of the no pitting and pitting surfaces.

No pitting Pitting

Weight% Atomic% Weight% Atomic%

O 11.95 18.65 25.82 37.38

Al 87.29 80.74 68.28 58.61Cl 0.33 0.23 3.69 2.41

Si 0.42 0.38 0.66 0.55

Ca 0.70 0.40

K 0.50 0.30

Na 0.34 0.34

Totals 100.00 100.00

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5. Laboratory experiments

Generally, the operating conditions for the evaporator tubing in a Kimchi refrigerator are approximately 0 $43 C with

relative humidity ranging from 0 $ 95%RH, and 0.2$ 0.24 gs of acceleration. The compressor in a Kimchi refrigerator is ex-

pected to cycle on average 2298 times per day. With a life cycle design point of 10 years, the Kimchi refrigerator incurs

358,000 cycles (Table 2).

The chlorine concentration of the cotton adhesive tape was 14 PPM. To accelerate the pitting of the evaporator tubing, the

chlorine concentration of the cotton tape was adjusted to approximately 140 PPM by adding some salt. Using a stress depen-

dence of 2.0, the acceleration factor was found to be approximately 100 in Eq. (2). The shape parameter was 6.41, and the test

cycles and the numbers of samples [15] used in the ALT were calculated as follows:

n ffi r 1 1

x

LBAF h

b7

Input OutputCooling

EnclosureSystem

pmeTdloCpmeTtoH

Key Noise ParametersN1: Customer usage & load conditionsN2: Environmental conditions

Key Control ParametersC1: Evaporator specifications

Fig. 4. Robust design schematic of a cooling enclosure system.

Fig. 5. An accelerating corrosion in the crevice due to low PH, high Cl concentration, depassivation and IR drop.

Fig. 6. Kimchi refrigerators in accelerated life testing.

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For B1 life, the required target x was 0.01. The test cycles and test sample numbers calculated in Eq. (7) were 4700 cycles

and 18 units, respectively. The ALT was designed to ensure a B1 of 10 years life with about a sixty percent level of confidence

that it would fail less than once during 5200 cycles. Fig. 6 shows the Kimchi refrigerators in accelerated life testing and an

evaporator tubing in the enclosure contained a 0.2 M NaCl water solution. Fig. 7 shows the duty cycles for the corrosive force

(F) due to the chlorine concentration.

6. Results and discussion

6.1. Validity of the accelerated life test and failure analysis

Fig. 8a and b shows the failed product from the field and from the accelerated life testing respectively. In the photos in

Fig. 8, the shape and location of the failure in the ALT were similar to those seen in the field. Fig. 9 shows a graphical analysis

of the ALT results and field data on a Weibull plot. These methodologies were valid in pinpointing the weak designs respon-

sible for failures in the field and were supported by two findings in the data. The location and shape also, from the Weibull

plot, the shape parameters of the ALT, (b1), and market data, (b2), were found to be similar.

6.2. Parameter design with ALTs and corrective action plans

The pitting of the evaporator tubing in both the field products and the ALT test specimens occurred in the inlet/outlet of

the evaporator tubing (Fig. 10). The design flaw of the cotton adhesive tape resulting in high corrosive stress areas can be

corrected by extending the length of the contraction tube from 50 mm to 200 mm and replacing the cotton adhesive tape

with a generic transparent tape. The design improvements correspond to the missing key control parameter (KCP) as listed

in Table 3.

20 Min

F (140PPM)

1 hour

..

4, 500 ALTMission Cycles

Fig. 7. Duty cycles of repetitive corrosive load F on the evaporator tubing.

Table 2

Operating cycles of the Kimchi refrigerator.

Item Operating cycles (times)

1 day 10 years

Normal Worst Normal Worst

Kimchi refrigerator 22 98 80,300 357,700

Fig. 8. Failed products in field and ALT.

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The repetitive corrosive force in combination with the high chlorine concentration of the cotton tape and the crevice be-

tween the cotton adhesive tape and the evaporator tubing contained the condensed water as an electrolyte may have beenpitting.

The parameter design criterion of the newly designed samples was more than the target life ofB1 which was 10 years. The

BX life of the sample can be calculated as:

LbB ffi x n h AF

b

r 18

Fig. 9. Field data and results of ALT on Weibull chart.

Fig. 10. Structure of pitting the corrosion tubing in field.

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The confirmed values ofAFand b in Fig. 8 were 100.0 and 6.41, respectively. The recalculated test cycles and sample size

in Eq. (7) were 5300 and 8 EA, respectively. Based on the BX and sample size, two ALTs were performed to obtain the designparameters and their proper levels. In the two ALTs the outlet of the evaporator tubing was pitted in the first test and was not

pitted in the second test.

Table 3

Confirmed key parameters based on the marketplace data and ALTs.

CTQ Parameters Unit

Pitting KNP N1 Corrosive force (or chlorine concentration) N (or PPM)

KCP C1 Length of the contraction tube

C2 Type of the adhesive tape

Fig. 11. A redesigned evaporator tubing.

Fig. 12. Results of ALT plotted in Weibull chart.

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Fig. 11 shows a redesigned evaporator tubing with high corrosive fatigue strength. Based on the modified design param-

eters, corrective measures taken to increase the life cycle of the evaporator tubing system included: (1) extending the length

of the contraction tube (C1) from 50.0 mm to 200.0 mm; (2) replacing the cotton adhesive tape (C2) with the generic trans-

parent tape. With these modified parameters, the Kimchi refrigerator can reserve the food for a longer period without failure.

Fig. 12 and Table 4 show the graphical results of ALT plotted in a Weibull chart and the summary of the results of the ALTs,

respectively. Over the course of the two ALTs the B1 life of the samples increased by over 10.0 years.

7. Conclusions

Robust methods were used to correct the failed cooling function of the evaporator tubing in a Kimchi refrigerator. Knowl-

edge of the failure modes and mechanism to the pitted evaporator tubing in the field were applied. The evaporator tubing

was improved by changing the design parameters through failure analysis and a series of accelerated life testing. The follow-

ing general conclusions were obtained:

(1) Based on the products that failed both in the field and in the controlled ALTs, pitting occurred both in the inlet and the

outlet of the evaporator tubing. The design parameters of the failed evaporator tubing that were problematic were the

cotton adhesive type and the length of the contraction tube.

(2) Based on the failure analysis and two ALTs, the values for the design parameters were determined to meet the lifecycle requirements. The yearly failure rate and B1 life of the redesigned evaporator tubing, based on the results of

ALT, were less than 0.1% and 10 years, respectively.

(3) Inspection of the failed product, load analysis, and two rounds of ALT indicated that the parameter design was greatly

improved by using the new robust design methodologies for the mechanical evaporator tubing system.

References

[1] Taguchi G. Off-line and on-line quality control systems. In: Proceedings of the international conference on quality control. Tokyo, Japan; 1978.

[2] Taguchi G, Shih-Chung T. Introduction to quality engineering: bringing quality engineering upstream. New York: American Society of Mechanical

Engineering; 1992.

[3] Ashley S. Applying Taguchis quality engineering to technology development. Mech Eng 1992.

[4] Wilkins J. Putting Taguchi methods to work to solve design flaws. Qual Prog 2000;33(5):559.

[5] Phadke M. Quality engineering using robust design. Englewood Cliffs (NJ): Prentice Hall; 1989.

[6] Byrne D, Taguchi S. The Taguchi approach to parameter design. Qual Prog 1987;20(12):1926.

[7] Box G, Jones S. Designing products that are robust to the environment. Total Qual Manage 1987;3:26582.

[8] Vining G, Myers R. Combining Taguchi and response surface philosophies: a dual response approach. J Qual Technol 1990;22:3845.

Table 4

Results of ALT.

First ALT Second ALT

Initial design First design iteration

In 5300 cycles, Corrosion

of evaporator pipe is

less than 1

1130 cycles: 1/18 pitting 5300 cycles: 8/8 OK

1160 cycles: 2/18 pitting

1680 cycles: 4/18 pitting

1680 cycles: 11/18 OK

Evaporator pipe structure

Material and spec. Length of the contraction tube, C1: 50.0 mm? 200.0 mm

Adhesive tape type, C2: cotton type ? generic transparent

tape

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[9] Woo S, Pecht M. Failure analysis and redesign of a helix upper dispenser. Eng Fail Anal 2008;15(4):64253.

[10] Woo S, ONeal D, Pecht M. Improving the reliability of a water dispenser lever in a refrigerator subjected to repetitive stresses. Eng Fail Anal

2009;16(5):1597606.

[11] Woo S, ONeal D, Pecht M. Design of a hinge kit system in a Kimchi refrigerator receiving repetitive stresses. Eng Fail Anal 2009;16(5):165565.

[12] Woo S, Ryu D, Pecht M. Design evaluation of a French refrigerator drawer system subjected to repeated food storage loads. Eng Fail Anal

2009;16(7):222434.

[13] Woo S, Pecht M, ONeal D. Reliability design and case study of a refrigerator compressor subjected to repetitive loads. Int J Refrig 2009;32(3):47886.

[14] McPherson J. Accelerated testing, packaging, electronic materials handbook. ASM International 1989;1:88794.

[15] Ryu D, Chang S. Novel concept for reliability technology. Microelectron Reliab 2005;45(3):61122.

Dr. Woo has a BS and MS in Mechanical Engineering, and he has obtained PhD in Mechanical Engineering from Texas A&M. He

major in energy system such as HVAC and its heat transfer, optimal design and control of refrigerator, reliability design of

thermal components, and failure Analysis of thermal components in marketplace using the Non-destructive such as SEM &

XRAY. In 1992.03 1997 he worked in Agency for Defense Development, Chinhae, South Korea, where he has researcher in

charge of Development of Naval weapon System. Now he is working as a Senior Reliability Engineer in Side By Side Refrigerator

Division, Digital Appliance, SAMSUNG Electronics, and focus on enhancing the life of refrigerator as using the accelerating life

testing. He also has experience about Side-by-Side Refrigerator Design for Best Buy, Lowes, Cabinet-depth Refrigerator Design

for General Electrics.

Dr. Dennis L. ONeal has a BS in Nuclear Engineering, Texas A&M University, MS in Mechanical Engineering, Oklahoma State

University, and PhD in Mechanical Engineering, Purdue University. Currently he is a Department Head of Mechanical Engi-

neering, Texas A&M University Professional Engineer, an ASHRAE Fellow. He has become the Holdredge/Paul Professor,

Department of Mechanical Engineering in 2002.

He has an interested in the areas of Heating, ventilating, and air conditioning; frost formation on heat exchangers; heat pump

system defrost performance and dynamics; ventilation air heat pumps; and aerosol mixing in ventilation systems. He has been

leading an energy system laboratory in Texas A&M and has written a several of heat-transfer related papers in the ASHRAE

Transactions.

Dr. Michael Pecht has a BS in Acoustics, an MS in Electrical Engineering and an MS and PhD in Engineering Mechanics from the

University of Wisconsin at Madison. He is a Professional Engineer, an IEEE Fellow and an ASME Fellow. He has received the 3M

Research Award for electronics packaging, the IEEE Undergraduate Teaching Award, and the IMAPS William D. AshmanMemorial Achievement Award for his contributions in electronics reliability analysis. He has written eighteen books on elec-

tronic products development, use and supply chain management. He served as chief editor of the IEEE Transactions on Reli-

ability for eight years and on the advisory board of IEEE Spectrum. He is chief editor for Microelectronics Reliability and an

associate editor for the IEEE Transactions on Components and Packaging Technology. He is the founder of CALCE (Center for

Advanced Life Cycle Engineering) and the Electronic Products and Systems Consortium at the University of Maryland. He is also

a Chair Professor. He has been leading a research team in the area of prognostics for the past ten years, and has now formed a

new Electronics Prognostics and Health Management Consortium at the University of Maryland. He has consulted for over 50

major international electronics companies, providing expertise in strategic planning, design, test, prognostics, IP and risk

assessment of electronic products and systems.

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