Failure of malleable cast iron sprinkler pipe end cap due to freezing of water within

    Failure of malleable cast iron sprinkler pipe end cap due to freezing of water within Tuan Son Nguyen PhD, Kee Bong Yoon PII: DOI: Reference:

S1350-6307(16)30379-X doi:10.1016/j.engfailanal.2016.09.013 EFA 2971

To appear in: Received date: Revised date: Accepted date:

24 May 2016 12 September 2016 30 September 2016

Please cite this article as: Nguyen Tuan Son, Yoon Kee Bong, Failure of malleable cast iron sprinkler pipe end cap due to freezing of water within, (2016), doi:10.1016/j.engfailanal.2016.09.013

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Failure of Malleable Cast Iron Sprinkler Pipe End Cap due to Freezing of Water Within

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Tuan Son Nguyen1, Kee Bong Yoon2*, Jin Ho Choi3, Jung Soo Song3

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PhD Candidate, Professor, and Graduate Students Department of Mechanical Engineering, Chung Ang University 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Korea

* Corresponding Author:

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(Tel) +82-2-820-5328 (Mobile) +82-10-3267-5327 (Fax) +82-2-812-6474 (E-Mail) [email protected]

May, 2016

Manuscript for Submission to Engineering Failure Analysis

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ABSTRACT

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Failures in end caps of sprinkler firewater systems frequently occur due to the freezing of

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the water inside the sprinkler systems on cold winter days. In these failures, a flat top of an

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end cap of a sprinkler system fails and completely separates from the threaded body of the end cap. In this study, metallurgical investigations including fractography, metallography, EDS analysis, and hardness measurement were performed for the failure analysis of the end

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cap. Finite element analysis was employed to identify both the maximum stress location and the maximum stress magnitude resulting from the freezing of and interaction between the ice and end cap when temperature dropped below the freezing point.

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Microstructure observations confirmed the ferritic malleable cast iron of the end cap. The hardness of the end cap was significantly below the hardness range specified for standard

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ferritic malleable cast iron. Small cracks formed during the casting of the end cap and the low

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strength of the end cap caused the final fracture. Failure stress determined by finite element analysis simulating the freezing of the inside water could accurately predict the failure stress

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and failure temperature. High tensile stress and poor manufacturing quality caused the crack to propagate in the cleavage mode and at the final stage in the ductile mode.

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Keywords: Sprinkler; Pipe Burst; Crack; Freezing; Ferritic Malleable Cast Iron

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1. Introduction Water in a piping system freezes when the environmental temperature unexpectedly drops

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down below 0 C. This causes high internal pressure and leads to the bursting of the pipe and

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the components fitted to it. Although the freezing and bursting of pipes appears to be a simple

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and preventable problem, the cost of failure loss and damage is considerable [1,2]. In regions that are not typically characterized by low winter temperatures, the improper insulation of piping systems frequently leads to failures due to freezing. The experimental investigations

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conducted by researchers including Gordon [1], Akyurt [2], and Smith [3] revealed that bursting of pipes was caused by the high pressure of the water confined between the ice plugs.

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The increase in the growth of the ice results in an increase in the compression of confined water since ice occupies more volume than water. The direct pressurization of the pipe material by the ice in the pipe was not considered as the major reason for failure. However,

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this type of failure occurred occasionally. If the pipe is filled with ice and the solid ice is

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cooled further, the volume expansion of the ice could generate additional pressure on the pipe and its fittings, causing cracking and failure. Freezing damage and other types of failure were

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also reported within fire protection systems [4,5]. This study investigated end cap in a pipe fitted to a sprinkler system in an industrial plant that failed during an abnormally cold winter due to the freezing of water in the pipe. The

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annual mean temperature of this region was −1.3 C in January. However, the temperature of the day when the failure occurred was between 3 C and −11 C. The design pressure of the pipe was 1.4 MPa and service pressure was 0.98–1.0 MPa. Visual examination revealed that the failed pipe section was not insulated, as shown in Fig. 1(a); only sections of the pipe near the ventilation system were insulated. The failed end cap is shown in Fig. 1(b), and the separated flat top of the end cap is shown in Fig. 1(c). Since the pipe end cap was not insulated, heat transfer occurred from the water to the surrounding environment in axial direction on the outer surface of the end cap as well as in radial direction on the surface of the pipe cylinder. Ice could initially form at the inner surface of the end cap with the drop in temperature, and it subsequently could grow radially to center and also along the pipe at the same time to fill the entire pipe. This phenomenon was also reported by Akyurt [2]. Hence, direct pressurization due to volume expansion of the water during freezing may have been 3

ACCEPTED MANUSCRIPT applied to the pipe and end cap during freezing of the water. In this study, metallurgical

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inspection and stress analysis were carried out to investigate the reasons for failure.

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2. Metallurgical Investigation

Only the flat top of the end cap was inspected after the failure. Figure 1(c) reveals that the flat top was completely separated from the end cap. Fracture path marked in Fig. 1(b), 1(c)

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indicates that the fracture path was along the corner of the end cap. The flat top was then sectioned into several parts for fractography, microstructural observations, and further

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metallurgical investigations as indicated in Fig. 2.

2.1 Microstructure and composition

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Specimen B shown in Fig. 2 was mounted and prepared for microstructural observation by

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metallographic techniques and etching in 2% nital, the results of the optical microscopy are as shown in Fig. 3. Figure 3 indicates that the microstructure consisted of tempered carbon

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nodules in a ferritic matrix. This microstructure is as anticipated of a typical ferritic malleable cast iron as characterized in the ASM Metals Handbook [6], and it also matches the description of ferritic malleable cast iron as per ASTM-A47 standard [7]. This material was

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designed for use in general engineering including pipe fittings. The chemical composition of the malleable iron generally conforms to the range shown in Table 1 [6]. In the mid-section of specimen B, Scanning Electron Microscopy (SEM) and semi-quantitative Energy Dispersive Spectroscopy (EDS) were conducted, and Fe(92.4–92.6 %), C(5.4–5.6), Si(1.4– 1.5), and Mn(0.6) were detected, thereby verifying normal cast iron composition.

2.2 Hardness Following the microstructural observations, hardness tests were conducted on the mounted specimen. The hardness values were measured in the Rockwell B scale (HRB) at 4 points. The average was 38.8 HRB. This is equivalent to 81.5 HB in Brinell hardness scale as per the conversion equation in ASTM-E140 [8]. According to the ASM Metals Handbook [6], the hardness range for ferritic malleable cast iron is 110–156 HB (approximately 62–82 HRB). 4

ACCEPTED MANUSCRIPT The recommended ASM hardness range and the measured hardness values of the failed end cap are plotted together in Fig. 4. Figure 4 shows that the measured hardness of the end cap is

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significantly below the typical hardness range of the ferritic malleable cast iron.

2.3 Fracture Surface

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Fracture surfaces of specimen A in Fig.2 were observed via SEM. Figure 5(a) shows a low magnification image of the fracture surface through the thickness. At many locations near the inner surface of the end cap, microstructures with non-uniform graphite distribution were

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observed (Fig. 5(b) and Fig. 5(c)). Similar non-uniform regions with micro-cracks and voids are also shown in Fig. 3. These micro-cracks and voids could be introduced by improper

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preparation of the casting die or improper cooling rate of the end cap during casting. Localised cleavage fracture was observed, assumed to be in the ferrite matrix, in the middle

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of the fracture surface (Fig. 5(e), Fig. 5(f), and Fig. 5(j)). However, ductile dimples were observed near the outer surface (Fig. 5(d) and Fig. 5(g)–Fig. 5(i)). This suggested that the

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fracture was initiated at the defects near the inner surface, and it propagated in a cleavage

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mode in the middle section and then in a ductile mode at the final stage. The cleavage fracture could be caused either by the stress triaxiality near the crack tip or by ductile-to-

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brittle transition at low temperatures.

2.4 Secondary Cracks The inner surface of specimen D (Fig. 2) was examined using SEM. Several secondary cracks were found near the main crack in the direction parallel to the main crack. As shown in Fig. 6, the shape of the secondary cracks was jagged and not linear. A single major crack with a straight linear crack growth pattern should have formed if this crack was formed owing to high stress caused by the internal pressure during freezing. However, many jagged secondary cracks were observed as stated before. These cracks could be formed during manufacturing owing to an improper casting process. Figure 7 shows the EDS analysis results of the metal surface and the interior of the secondary crack. The chemical composition of the metal surface was almost similar to the measured chemical composition of specimen A (Fig. 2) (detailed in Section 2.1). In addition, 5

ACCEPTED MANUSCRIPT oxygen was detected. However, Zn was detected inside the secondary crack. Because the material used in paint coating was composed of Zn, it could be inferred that the crack was

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formed before the painting process, thereby confirming that the cracks were manufacturing

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defects.

Figure 8(a) shows the location of a secondary crack. The specimen was cut across this

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secondary crack to obtain the smaller specimens C and D. Specimen D was used for observing the cross sections of the crack and the material beneath the inner surface of the end cap as shown in Fig. 8(b)–Fig. 8(g). The typical microstructure of ferritic malleable cast iron

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can be clearly observed in Fig. 8(b). The tempered carbon nodules were uniformly distributed in the middle of the thickness section but not in the region near the inner surface. At higher

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magnifications, Fig. 8(c) shows that the depth of the secondary crack measured from the inner surface was almost 1 mm, and it grew via neighbouring graphite nodules. Furthermore, inter dendritic micro cracks formed up to the depths of 0.2 mm below the inner surface as

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shown in Fig. 8(d). EDS analysis was conducted at a location inside the secondary crack as

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shown in Figs. 8(e) and 8(f). Figure 8(g) indicates the presence of zinc and oxygen in addition to C, Si, Mn, and Fe. The presence of these elements (zinc and oxygen) inside the

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crack was unexpected since the chemical composition of ferritic malleable iron does not include these two elements. Hence, the cracks are thought likely to have formed during the

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casting process.

3. Failure Prediction The following four scenarios for freezing in the ice–water system were identified i) supercooling of water, ii) nucleation and dendritic ice formation, iii) growth of concentric solid ice rings, and iv) final cooling of solid ice [2]. During stages (i) and (ii), there will be no pressure variation in the pipe. However, if the water was confined between two ice plugs and if the volume of the ice plugs increased, then the compressed water could cause the burst failure of the pipe as shown in Fig. 9(a). Conversely, if all the water is converted to ice, then the volume expansion of the ice could cause the failure of the pipe or pipe fittings as shown in Fig. 9(b).

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3.1 Stress analysis The scenario indicated in Fig. 9(a) is not plausible as the failure occurred at the end cap and

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ice formed from inner surface of end cap and pipe. Thus, pressurization by ice volume

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expansion and interaction between the ice and pipe fitting were simulated. Finite element analysis was performed to evaluate the impact of ice expansion and the effect of minimum

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freezing temperature. The axisymmetric model including the pipe, cap, and ice was generated using ABAQUS. A total of 7600 four-node thermally coupled axisymmetric quadrilateralbilinear displacement and temperature elements (CAX4RT) were used for this model. The

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dimensions of the pipe and threaded end cap were determined based on the measured value of the failed end cap and relevant ASME codes [9,10]. The model details are shown in Fig.

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10(a) and the model after meshing is shown in Fig. 10(b). The threaded assembly between the pipe and cap was securely fixed; therefore, the threaded

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contact was adequately modeled by a partition. The water was still in the liquid phase when the temperature decreased to the freezing point. In this condition, the stress in the pipe and

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fitting components was caused by the water pressure which was approximately 1 MPa. The

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situation became worse when the water underwent a phase change to ice and the ice temperature decreased further. At this point, the decrease in temperature resulted in further expansion of the ice, thereby resulting in higher internal pipe stress. The simulation was

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conducted by assuming that ice was fully formed inside. The reported friction coefficient between ice and steel was in range of 0.02–0.05 [11,12]; in this study, a value of 0.03 was adopted. Table 2 summarizes the mechanical and physical properties for ice, steel, and ferritic malleable cast iron as given by previous researchers [7,10,13,14]. Ice differs from most materials as it expands when temperature decreases. This is characterized as negative thermal expansion. A negative value of the thermal expansion coefficient was used in the simulations. The applied natural convective heat transfer coefficient on the outer surface of the pipe and end cap was assumed to be 20 (W/m 2. C). The corner radius of the cap was assumed to be 2 mm. Various environmental temperatures from −5 C to −20 C and various corner radii of the end cap from 1 mm to 4 mm were also simulated in order to investigate their effects on the resultant maximum stress.

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ACCEPTED MANUSCRIPT The resultant maximum pressure on the surfaces of the ice, pipe, and end cap during the steady state was approximately 21.9 MPa. The results from the previous study by Akyurt [2]

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reported that the maximum pressure during freezing reached 27.0 MPa. The result in this

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study was comparable to the reported pressure value, thereby supporting the accuracy of the current simulation. The distribution of stress in the pipe shown in Fig. 11 confirmed that the

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maximum stress occurred at the corner of the end cap. Maximum stress variations of 16 simulation cases are graphically illustrated in Fig. 12. A significant influence of the environmental temperature on the resultant maximum stress was observed. However, there

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was a limited influence of the radius of the end cap. Since environmental temperature determined the degree of thermal expansion, smaller decreases in temperature resulted in

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larger expansions in ice. This in turn resulted in the higher end cap corner stresses where the

3.2 Strength Estimation

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failure occurred.

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The tensile strength of normal ferritic malleable cast iron is 345 MPa within the hardness

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range of 110 HB–156 HB in the Brinell hardness scale (approximately 62 HRB–82 HRB in Rockwell hardness B-scale) [6]. However, the measured hardness of the failed end cap was only 38.8 HRB, which was far below the normal range. This implied that the actual tensile

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strength of the end cap was also low. The relationship between the hardness and tensile strength for non-austenitic steel is shown in Fig. 13 [15]. The solid symbols represent the conversion data available in ASTM-A370, while the open symbols indicate the regression in connecting the end of the available data and the origin where both hardness and strength are zero. This strength-hardness conversion relationship is used for a wide range of nonaustenitic steels. Within the non-austenitic steel hardness range [6], the strength of the ferritic malleable cast iron is located near the lower bound of the standard data. This indicated that the strength of the normal ferritic malleable cast iron was relatively low. The material of the end cap was of an even lower strength given that its measured hardness was considerably below the recommended range. In order to estimate the tensile strength of the end cap, a straight line originating from the middle of the hardness range and at 345 MPa was drawn parallel to the linear regression line as shown in Fig. 13. The corresponding tensile strength 8

ACCEPTED MANUSCRIPT was estimated as 135 MPa from the intersection of this straight line and the measured hardness of 38.8 HRB. The failure of the end cap occurs when the stress value obtained from

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the finite element calculations equals the material strength of 135 MPa.

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4. Discussion

Overpressure bursting is the most commonly suspected cause for the failure of pipes and fittings when water is confined between the ice plugs (Fig. 9(a)). However, in the current case,

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the ice plug began forming from the inner surface of end cap side and grew in the opposite direction while the failure occurred at the end cap. Hence, the burst pressure of the confined water did not cause the failure. The pipe end cap contracted when temperature decreased, but

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the ice expanded as shown in Fig. 9(b). This phenomenon resulted in high contact pressure. Finite element analysis results indicated a contact pressure of approximately 21.9 MPa.

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Akyurt [2] experimentally measured the maximum pressure between the ice plugs during

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freezing and reported 27.5 MPa internal pressure. The current finite element analysis result was quite similar to experimental result obtained by Akyurt. Figure 11 shows that the stress is

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highly concentrated along the corner of the end cap. The location of maximum stress is same as the fracture location.

The change in the maximum stress occurring at the corner of the end cap when the

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environmental temperature was decreased from -5 C to −20 C is shown in Fig. 12. The maximum stress increased from 79 MPa at −5 C to 300 MPa at −20 C when the corner radius of the end cap was approximately 2 mm. Thus, failure should have occurred at approximately from −8 to −9 C since the strength of the end cap was 135 MPa. This prediction was very reasonable because the recorded lowest environmental temperature had been −11 C during the night when failure occurred. Figure 12 shows the maximum stress variation according to different corner radii of the end cap. The maximum stress decreased from 169 MPa to 134 MPa when the radius increased from 2 mm to 4 mm at −11 C. McDonald estimated the cooling and freezing time of water in cylindrical pipes exposed to temperature range from -25 C to -5 C [16]. At −11C, the time required for cooling from 20C and freezing was predicted as 170 minutes. This means the freezing can occur overnight. 9

ACCEPTED MANUSCRIPT However, if the pipe was insulated the estimated time was increased to 7,000 minutes. Therefore, insulation of the pipe end cap can be effective for delaying freezing and prevent

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this type of failure.

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The crack shown in Fig. 8(c) grew preferentially between graphite nodules. Similar crack growing pattern was also reported for graphite in a study conducted by Hutter where the

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ferritic malleable cast iron was subjected to tensile stress [17]. However, the mechanism explained by Cocco [18], which is graphite particles debonded from the ferritic matrix and “onion-like” mechanism was not observed. Conversely, graphite particles themselves had

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broken in a brittle manner along the crack path, and the ferrite matrix also underwent a cleavage failure as shown in Figs. 5(e), 5(f), and 5(j). Ductile cracking appeared only at the

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last stage of the crack growth as shown in Figs. 5(g), 5(h), and 5(i) when the remaining crosssection of the material was reduced. Metallurgical evaluation indicated severe damage in the material beneath the inner surface of the end cap. Inter dendrite cracks had formed tightly up

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to a depth of approximately 0.2 mm from the inner surface. This indicated the poor quality of

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the manufacturing process. Low manufacturing quality and high tensile stress combined with a low temperature were believed to cause the failure of the end cap. Since failure at an end of

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pipe due to freezing is also probable as reported in this study, as well as the burst failure in the middle due to confined water between two ice plugs, the end cap should be designed to

freezing.

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have enough strength to resist internal pressure caused by volume expansion of water

5. Conclusions The failure of an end cap made of ferritic malleable cast iron in a sprinkler firewater piping system occurred due to the freezing of the water inside the sprinkler pipe. Metallurgical investigations and stress analysis were carried out for failure analysis. The conclusions of this study are as follow:  Metallurgical investigation showed that the end cap was made from ferritic malleable cast iron. The measured hardness of the end cap was far below the specified standard range. The tensile strength corresponding to the hardness value was estimated as 135 MPa. 10

ACCEPTED MANUSCRIPT  When temperature decreased below the freezing point, the pressurization from ice expansion and the contraction of the pipe fittings generated maximum stress along

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the corner of the end cap. The internal pressure was approximately 21.9 MPa. The

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maximum stress varied from 79 MPa to 229 MPa when the temperature decreased from −5 C to −15 C. Failure was predicted when the temperature was −9 C. The

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actual minimum environmental temperature was approximately −11 C. This supported the accuracy of the simulation conducted in this study.  High tensile stress at the corner of the end cap and low strength caused the crack to

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propagate in the cleavage mode in the mid-thickness section and in the ductile mode

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at the last stage of failure.

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Acknowledgments

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This work was supported by the Power Generation and Electricity Delivery Core Program of KETEP funded by the Korean Government, MOTIE (No. 20141010101850). This research

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was supported by the Chung-Ang University Graduate Research Scholarship in 2016 (Jin Ho Choi). The corresponding author wishes to express sincere thanks to the company that

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provided the failed component and the relevant data for analysis.

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References [1]

J.R. Gordon, An investigation into freezing and bursting water in residential

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construction, Research report No. 96-1, Building Research Council. School of

M. Akyurt, G. Zaki, B. Habeebullah, Freezing phenomena in ice–water systems,

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[2]

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Architecture, University of Illinois at Urbana-Champaign; 1996.

Energy Convers. Manag. 43 (2002) 1773–1789. [3]

K.M. Smith, M.P. Van Bree, J.F. Grzetic, Analysis and testing of freezing phenomena

Massachusetts, USA; (2008) 1–6. [4]

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in piping systems, Proceedings of IMECE 2008, October 31–November 6, Boston,

J. Pfaendtner, Material failures in fire protection systems, Proceedings of SUPDET

[5]

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2014, March 4–7 Univ. of Central Florida; 2014.

P.M. Bravo, M. Preciado, J.M. Alegre, Failure analysis of galvanized iron pipeline

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accessories of a fire protection system, Eng. Fail. Anal. 16 (2009) 669–674. ASM International, ASM Metals Handbook Vol.15 – Casting; 1992.

[7]

ASTM International, Standard Specification for Malleable Iron Castings, ASTM A47;

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[6]

[8]

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1999.

ASTM International, Standard Hardness Conversion Table for Metals Relationship Among Brinell Hardness, Vickers Hardness, Rockwell Hardness, Superficial hardness,

[9]

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Knoop Hardness and Scleroscope Hardness, ASTM E140; 2002. ASME International, Malleable Iron Threaded Fittings, ASME B16.3; 1998.

[10] ASME International, Stainless Steel Pipe, ASME B36.19M; 2004. [11] L. Makkonen, M. Tikanmäki, Modeling the friction of ice, Cold Reg. Sci. Technol. 102 (2014) 84–93. [12] H. Liang, J.M. Martin, T.L. Mogne, Experimental investigation of friction on lowtemperature ice, Acta Mater. 51 (2003) 2639–2646. [13] V.F. Petrenko, R.W. Whitworth, Physics of ice, Oxford University Press,1999. [14] J.J. Petrovic, Review Mechanical properties of ice and snow, J. Mater. Sci. 38 (2003) 1–6. [15] ASTM International, Standard Test Methods and Definitions for Mechanical Testing of Steel Products, ASTM A370; 2003. 12

ACCEPTED MANUSCRIPT [16] A. McDonald, B. Bschaden, E. Sullivan, R. Marsden, Mathematical simulation of the freezing time of water in small diameter pipes, Appl. Therm. Eng. 73 (2014) 140-151

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[17] G. Hütter, L. Zybell, M. Kuna, Micromechanisms of fracture in nodular cast iron:

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From experimental findings towards modeling strategies - A review, Eng. Fract. Mech. 144 (2015) 118–141.

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[18] V. Di Cocco, F. Iacoviello, M. Cavallini, Damaging micromechanisms characterization

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of a ferritic ductile cast iron, Eng. Fract. Mech. 77 (2010) 2016–2023.

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List of Tables

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Table 1 Chemical composition of malleable cast iron (weight %) [6]

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Table 2 Material and physical properties used in the finite element analysis in this study

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List of Figures

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Fig. 1 Frozen and failed section of the sprinkler pipe: (a) uninsulated pipe, (b) failed end cap, (c) separated flat top end cap.

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Fig. 2 Metallurgical investigation and specimen cutting plan for the failed end cap. Fig. 3 Micrograph showing the microstructure of the failed end cap material which was consistent with ferritic malleable cast iron. (Specimen B)

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Fig. 4 Measured hardness value of the failed end cap and a comparison with the hardness range of the typical ferritic malleable iron [6].

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Fig. 5 Fracture surfaces of the failed end cap.

Fig. 6 Secondary cracks found on the inner surface of the end cap.

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Fig. 7 EDS analysis results of the metal surface and the inside of the secondary crack.

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Fig. 8 Secondary crack (a–d) path observed on the cross sectional area between C and D shown in Fig(a) and EDS analysis of the interior of the secondary crack (e–g).

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Fig. 9 The two types of potential pipe failure mechanisms due to freezing.

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Fig. 10 Finite element method for the pipe and end cap: (a) dimension and boundary condition and (b) meshed finite element model. Fig. 11 Von Mises stress near the end cap corner when Rcap = 2 mm and Temp = −10 C. Fig. 12 Variation of maximum stress at the corner of the end cap with respect to the environmental temperature and corner radius. Fig. 13 Hardness conversion for non-austenitic steels and strength estimation of the failed end cap.

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ACCEPTED MANUSCRIPT Table 1 Chemical composition of malleable cast iron (weight %) [6]. C

Si

Mn

S

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Bal.

2.16 – 2.9

0.90 – 1.90

0.15 – 1.25

0.02 – 0.20

0.02 – 0.15

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Fe

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End cap

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170

Ice

Elastic Modulus

Poisson’s Ratio

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(GPa) 0.3

0.3

7200

917

60

2.2

500

500

2000

10−5

10−5

−5.5 × 10−5

Conductivity

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50

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(W/m. C ) Specific Heat

Thermal Expansion

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( / C)

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(J/kg. C)

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0.3

Density (kg/m3)

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Properties

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Fig. 1 Frozen and failed section of the sprinkler pipe: (a) uninsulated pipe, (b) failed end cap,

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and (c) separated flat top of the end cap.

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Fig. 2 Metallurgical investigation and specimen cutting plan for the failed end cap.

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Fig. 3 Optical micrograph showing the microstructure of the failed end cap material which

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was consistent with ferritic malleable cast iron. (Specimen B)

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Fig. 4 Measured hardness value of the failed end cap and a comparison with the hardness

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range of the typical ferritic malleable iron [6].

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Fig. 5 Fracture surfaces of the failed end cap.

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Fig. 6 Secondary cracks found on the inner surface of the end cap.

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Fig. 7 EDS analysis results of the metal surface and the inside of the secondary crack.

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Fig. 8 Secondary crack (a–d) path observed on the cross sectional area between C and D shown in

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Fig(a) and EDS analysis of the interior of the secondary crack (e–g).

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Fig. 9 Two types of probable pipe failure mechanisms due to freezing.

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Fig. 10 Finite element method for the pipe and end cap: (a) Dimension and boundary condition and (b) meshed finite element model.

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Fig. 11 Mises stress near the end cap corner when Rcap = 2 mm and Temp = −10 C.

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Fig. 12 Variation of maximum stress at the corner of the end cap with respect to environmental temperature and corner radius.

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end cap.

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Fig. 13 Hardness conversion for non-austenitic steels and strength estimation of the failed

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ACCEPTED MANUSCRIPT

Highlights

 Sprinkler pipe end cap was failed due to freezing of inside water.

T

 The maximum internal pressure due to freezing was predicted as 21.9 MPa by finite

IP

element analysis.

SC R

 Failure analysis using FEA and metallurgical investigation well predicted the failure temperature.

 Poor manufacturing quality of ferritic malleable cast iron caused cracking and failure.

NU

 Pipe insulation or proper strength of the end cap is recommended for failure

AC

CE P

TE

D

MA

prevention.

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