CASE HISTORY—PEER-REVIEWED

Failure Analysis and Design Optimization of the Steady RestHanger Rod Pipe Assembly

Bharat Joshi • John George • Brian Rose •

Yin Chen

Submitted: 13 August 2013

� ASM International 2013

Abstract This article investigates the failure of the

steady rest hanger rod pipe assembly weld joints of an

automotive exhaust system. Rig testing of the exhaust

system showed the presence of crack at the steady rest

hanger rod and brace weld joints. Metallurgical investiga-

tion was performed in order to determine the root case of

failure and contribution factors. Metallurgical analysis

methods included visual examination, thickness measure-

ments, optical and scanning electron microscopy, chemical

analysis of the material and weld evaluation. A CAE

analysis was performed to simulate the rig test. Finite

element simulation of the system also showed high damage

at the steady rest hanger and brace weld locations. A DOE

study was conducted to identify the design variables that

could impact the dynamic response of the system like the

thickness of the parts, the weld characteristics of joints, etc.

Design changes were proposed; to improve the fatigue life

of steady rest hanger rod pipe assembly based on the results

of DOE-based study. The new design was analyzed using

finite element analysis and compared with the original

design for fatigue life, which showed a considerable

improvement in the durability of the joint.

Keywords Automotive exhaust systems �Steady rest hanger rod � Failure analysis � Fatigue failure �Finite element analysis � Design of experiments (DOE)

Introduction

Rig testing is the most commonly used method employed

by exhaust system manufacturers to validate the durability

of an exhaust system. During the rig test, the exhaust

system is subjected to the various operational loads typi-

cally observed during the life span of an exhaust system

[1, 2]. Rig test establishes the integrity of various joints of

the exhaust system. Once the system passes the rig test, it is

deemed a safe design. Conversely, if it fails the rig test,

remedial measures have to be introduced before the system

is released for production.

This paper looks at failure analysis and design optimi-

zation of a steady test hanger, which failed during a typical

rig test. Failure of a joint could be due to a variety reasons;

like a joint not meeting the weld specification, the welding

processes imparting too much heat to the joint, the joint not

meeting the design or chemical specification, or it could be

due to a fundamental design flaw. The first step of the

remedial process is to establish the root cause of the failure.

Various metallurgical methods like stereoscopic inspection,

metallographic examination, optical emission spectroscopy

analysis, scanning electron microscope analysis, etc., were

carried out to determine the root cause of the failure. CAE

methods could be employed to address the design issues as

they offer significant cost and time benefits [3]. CAE ana-

lysis of the system was carried out to simulate the dynamic

behavior of the exhaust system with the same inputs that is

used to drive rig test [4, 5]. The first step in any numerical

simulation is to validate the numerical model with test

results. The life of the steady rest hanger joint predicted

using numerical simulation was correlated with the life

observed during the rig test. The validated model was used

to conduct a DOE to optimize the design of the failed joint.

Finally, based on the results from DOE study, an updated

B. Joshi (&) � B. Rose

Materials Engineering and Warranty Analysis Laboratory,

Faurecia Emission Control Technologies, Columbus, IN 47201,

USA

e-mail: bharat.joshi@faurecia.com

J. George � Y. Chen

Computer Aided Engineering, Faurecia Emission Control

Technologies, Columbus, IN 47201, USA

123

J Fail. Anal. and Preven.

DOI 10.1007/s11668-013-9781-3

design which would pass the rig test was proposed and

verified with finite element analysis.

The picture of the steady rest hanger considered in this

study is shown in Fig. 1. The steady rest hanger rod pipe

assembly consisted of the hanger rod and the support brace

welded to the pipe.

Root cause Failure Analysis

Visual and Stereoscopic Inspection

Visual examination (Fig. 2) showed the cracked hanger rod

and brace weld joints on the pipe assembly. The hanger rod

to pipe joint showed a presence of repair weld. The weld

was repaired without discontinuing the ongoing rig testing,

after the failure details were marked and noted, to check

the durability performance of the entire exhaust system.

Optical microscopy was used to determine the crack

location. The macroscopic images (Fig. 3) show the crack

initiation at the toe of the weld in heat affected zone and

propagation into the base metal. Fracture surface exami-

nation showed presence of multiple thumbnails (Fig. 4)

indicative of fatigue failure that initiated from multiple

sites [6].

Fig. 1 Steady rest hanger rod pipe assembly

Fig. 2 Visual observation showed the cracked hanger rod and brace

weld joints on the pipe assembly

Fig. 3 The macroscopic images show the crack initiation at the toe of

the weld in heat affected zone and propagation into the base metal

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Thickness Measurement at the Crack Boundary

Thickness measurements were conducted near the failure

locations to check the condition of base metal material near

the failure location for substantial thinning. Thickness

measurements (Table 1) met the print pipe thickness

requirement of 1.5 ?/� 0.08 mm. The thickness profiles

(Fig. 5) for hanger rod and brace weld joints showed

minimum and maximum values of 1.46 and 1.52 mm,

respectively, within the specification requirement.

Optical Emission Spectroscopy (OES) Analyses

The chemical composition analysis was performed on the

pipe sample in accordance with test method of ASTM E

1086-08 by means of Spectro analytical instruments optical

emission spectrometer [7]. The chemical composition met

the chemical requirements of 439 stainless steel per ASTM

A240/A240M-09a specification [8]. Table 2 shows chem-

ical compositions of the sample.

Metallographic Examination

Grain structure analysis was performed on the pipe at the

crack and away from the crack locations after etching with

Viella’s reagent. No major difference in grain size (Fig. 6)

was observed at both the locations indicating nearly uni-

form heat input exposure at both the locations. This

evidence indicates that failure at the toe of the weld is not

caused by high heat input application during welding

process.

Fig. 4 Fracture surface examination showed presence of multiple

thumbnails indicative of fatigue failure that initiated from multiple

sites

Fig. 5 Thickness profiles

measured at the crack boundary

locations

Table 1 Thickness measurements results at the crack boundary met

the print requirements

Locations

Thickness measurement at the crack boundary locations,

mm

Hanger rod joints Brace weld joints

Left Right Left Right

1 1.51 1.49 1.49 1.49

2 1.52 1.47 1.50 1.49

3 1.49 1.46 1.51 1.51

4 1.48 1.48 1.51 1.50

5 1.50 1.51 1.50 1.50

Average 1.50 1.48 1.50 1.50

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Weld Evaluation

Weld evaluation was conducted at the hanger rod to pipe

and brace to pipe joint locations to inspect the weld quality.

The evaluation results met the print requirement weld

specification (Fig. 7) for all the locations. Figures 8 and 9

demonstrate the weld evaluation measurements for hanger

rod and brace weld joint respectively. Table 3 shows the

weld evaluation results.

Fig. 6 No major difference in grain size for both the locations

indicate no major high heat input exposure at the crack location

during welding process

Fig. 7 The weld specification per the print requirement

Fig. 8 Weld evaluation results (a) left hanger rod and (b) right

hanger rod weld joints. The red colored circle indicates failure

location (Color figure online)

Table 2 Chemical composition of stainless steel pipe

Elements

Pipe (439

stainless)

Chemical requirements

per ASTM A240/A240M-09a

UNS#43035 (439 stainless steel)

Carbon 0.02 0.07 max

Manganese 0.23 1.00 max

Silicon 0.35 1.00 max

Sulfur 0.001 0.03 max

Phosphorus 0.025 0.04 max

Chromium 17.45 17.0-19.0

Nickel 0.24 0.50 max

Nitrogen 0.012 N/A

Titanium 0.39 0.20?4(C?N) min 1.10 max

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Scanning Electron Microscope Analysis

The fracture surface analysis was conducted using JEOL

JSM-5900LV scanning electron microscope. Examination

of the fracture surface under scanning electron microscope

exhibited surface ripples known as striations. The presence

of striations (Fig. 10) indicates fatigue as the mode of

failure. The most accepted mechanism for the formation of

striations on the fatigue fracture surface is the successive

blunting and re-sharpening of the crack tip [9].

Finite Element Simulation

Finite element analysis could be used to simulate the full

system durability rig test. The detailed procedure used for

full system durability simulation is described in references

[4] and [5]. The exhaust system that failed during the rig test

was analyzed utilizing transient modal dynamic method,

with the same inputs that was used for driving the rig test.

Numerical simulation also showed high damage at the

steady rest hanger joint. During the rig test, the part lasted

for 1.38 times PG (proving ground) life and numerical

simulation predicted that the part would live for 1.45 PG

lives. It is noted here that the target for the system was to

pass 2.0 times PG life during the rig test. Baseline results

demonstrate that finite element simulation captures the

failure mode of the exhaust system accurately. The location

of the highest damage from numerical simulation and the

failure location from rig test is shown in Fig. 11. The val-

idated finite element model was used for running the DOE

(design of experiment), described in the section followed.

Design of Experiments (DOE) to Identify the Critical

Parameters and There Effects on Steady Rest Hanger

Rod Life

A design of experiment (DOE) study was conducted uti-

lizing CAE analysis to study the effect of various design

variables on the life of the steady rest hanger joint. The

parameters that affect the life of the joint are the brace weld

Fig. 9 Weld evaluation results (a) left brace weld joint and (b) right

brace weld joint. The red colored circles indicate failure location

(Color figure online)

Table 3 Weld evaluation results met the minimum requirement per

the weld specification

Hanger rod weld

joint Left

Minimum

requirements Right

Minimum

requirement

Pipe thickness, T 1.53 N/A 1.52 N/A

Diameter of hanger rod, D 14.35 N/A 14.35 N/A

Joint gap, G 0.28 B1.50 0.28 B1.50

Leg length, L1 4.85 3.59 4.97 3.59

Leg length, L2 6.61 3.59 4.15 3.59

Throat 4.38 2.87 3.86 2.87

Depth of fusion, P1 0.41 C0.2 mm 0.52 C0.2 mm

Depth of fusion, P2 0.56 C0.2 mm 0.54 C0.2 mm

Weld wetting angle, Ø 165 C100� 175 C100�Weld wetting angle, Ø 142 C100� 176 C100�

Brace weld joint Left

Minimum

requirements Right

Minimum

requirement

Pipe thickness, T 1.54 N/A 1.54 N/A

Diameter of hanger rod, D 14.20 N/A 14.20 N/A

Joint gap, G 0.37 B1.50 0.37 B1.50

Leg length, L1 7.20 3.55 5.30 3.55

Leg length, L2 7.11 3.55 5.35 3.55

Throat 5.55 2.84 3.62 2.84

Depth of fusion, P1 0.75 C0.2 mm 0.66 C0.2 mm

Depth of fusion, P2 0.58 C0.2 mm 0.58 C0.2 mm

Weld wetting angle, Ø 173 C100� 166 C100�Weld wetting angle, Ø 134 C100� 160 C100�

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length, the pipe thickness and the hanger weld length. The

DOE was setup with each factor at 3 levels. The levels of

each factor analyzed in the DOE is as shown below

Thickness of the pipe: 1.02/1.42/1.82 mm.

Weld length of hanger: 23/27/31 mm.

Weld length of brace: 21/24.5/28 mm.

Fig. 10 Fatigue striations indicating fatigue as the mode of failure

Fig. 11 Location of highest damage from finite element analysis and

location of crack from rig test

Fig. 12 Variables considered

for performing DOE on current

design

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The picture of the DOE setup is shown in Fig. 12. The

output studied in the DOE was the relative PG life of the

steady rest hanger joint. The relative life of the joint with

thickness = 1.02 mm, weld length of the hanger =

23 mm, and weld length of the brace = 21 mm was set to

1.0. The relative life of all the other iterations was cal-

culated with respect to this baseline. Results from the

DOE study are shown in Table 4. The data from the DOE

study was analyzed using MINITAB, widely used statis-

tical analysis software. The output from MINITAB is

shown in Figs. 13 and 14. Figure 13 is the main effect

plot, which indicates that the pipe thickness and the weld

length of the hanger are the most important design vari-

ables. It also shows that weld length of the brace does not

have any significant influence on the life of the joint.

Figure 14 is the interaction plot, which indicates that

there is no significant interaction between the design

variables.

Implemented Design Change

Results from the DOE study showed that the pipe thickness

and weld length of the hanger are the most important

variables that affect the durability of the steady rest hanger

joint. Based on the DOE results, the weld length of the

hanger was increased to 36 mm. It was determined by the

design team that the adding an additional brace as shown in

Fig. 15 was a more cost effective solution as compared to

increasing the thickness of the pipe. The final updated

design is shown in Fig. 15. CAE analysis of the updated

design showed that the joint would live for 75.6 times PG

life, which represents a 52-fold improvement in PG life as

Table 4 DOE matrix and results

Material

thickness, mm

Weld length,

mm—brace

Weld length,

mm— Hanger rod

Relative

life

1.02 28 31 1.569

1.42 28 31 3.650

1.82 28 31 6.759

1.02 21 23 1.000

1.42 21 23 1.934

1.82 21 23 3.730

1.02 24.5 31 1.456

1.42 24.5 31 3.420

1.82 24.5 31 6.431

1.02 21 31 1.354

1.42 21 31 3.175

1.82 21 31 6.044

1.02 21 27 1.193

1.42 21 27 2.471

1.82 21 27 4.763

1.02 24.5 27 1.310

1.42 24.5 27 2.690

1.82 24.5 27 5.099

1.02 24.5 23 1.109

1.42 24.5 23 2.131

1.82 24.5 23 4.036

1.02 28 27 1.405

1.42 28 27 2.894

1.82 28 27 5.398

1.02 28 23 1.216

1.42 28 23 2.320

1.82 28 23 4.310

Fig. 13 Material thickness is

the most important variable and

weld length of brace is the least

important variable

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compared to the baseline. The results from the CAE

damage analysis is shown in Fig. 16.

Discussion and Conclusion

Failure analysis and design optimization of a steady rest

hanger rod pipe assembly of an automotive exhaust system,

which failed during a rig test, is presented in this paper.

Various types of metallurgical analysis of the failed joint

were conducted to determine the cause of the failure. Weld

evaluation demonstrated that the joint met all the weld

specifications. Optical emission spectroscopy showed that

the components met the chemical composition require-

ments as per ASTM standards. Metallographic examination

of the welded joints did not reveal any concerns regarding

the welding process. The fracture surface analysis of the

failed joint showed fatigue striations, indicating fatigue as

the primary mode of failure. Finally, based on the results

from metallurgical investigation, it was concluded that the

root cause of the failure was a design issue. CAE analysis

of the exhaust system also showed high damage at the

steady rest hanger joint. A DOE study based on numerical

simulation was conducted to optimize the design of the

steady rest hanger. Based on the results from the DOE

study, the design of the steady rest hanger was updated in

order to improve the life of the joint. Finite element ana-

lysis of the updated design showed that the updated design

would improve the life of the joint by 52 times as compared

to the failed joint, which was acceptable to the design team.

Fig. 14 No significant interaction between the variables

Fig. 15 Final updated design

Fig. 16 Plot showing PG life of the updated design

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Highlights

• A failure analysis was performed on a steady rest

hanger rod assembly

• The steady rest hanger rod assembly failed during rig

testing before the expected life duration of 2.0 lives

• Failure mechanism was fatigue

• A design of experiment (DOE) was conducted for problem

solving and design optimization recommendations

References

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evaluation of exhaust system components, SAE 2007-01-0467

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dynamic performance of an automotive exhaust system, 2013 SAE

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