Engineering Failure Analysis 20 (2012) 88–96

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Engineering Failure Analysis

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Failure analysis of steel wire reinforced thermoplastics composite pipe

Houbu Li a,⇑, Milin Yan a, Dongtao Qi a, Nan Ding a, Xuehua Cai a, Shuhui Zhang a, Qi Li a,Xuemin Zhang b, Juanli Deng b

a CNPC Tubular Goods Research Institute, Xi’an 710065, Chinab School of Materials Science and Engineering, Chang’an University, Xi’an 710064, China

a r t i c l e i n f o

Article history:Received 12 January 2011Received in revised form 14 September2011Accepted 3 November 2011Available online 10 November 2011

Keywords:CracksDegradationEnvironmentally-assisted crackingFailure analysis

1350-6307/$ - see front matter � 2011 Elsevier Ltddoi:10.1016/j.engfailanal.2011.11.002

⇑ Corresponding author. Tel./fax: +86 29 8872 63E-mail addresses: houbuli@gmail.com (H. Li), xu

a b s t r a c t

Incident involving failures of steel wire reinforced thermoplastic composite pipes (SRTP)which transport saline wastewater had caused serious burst after several months of usagefrom July 2009. Investigations were performed to identify the most probable causes of thepipe failures. The study was conducted by reviewing the background of the incident, visualphysical inspection, mechanical property test and thermal analysis. Results revealed thatthe failure of this pipe was mainly caused by a typical environmental stress cracking(ESC) of its basal resin. The basic reason for crazing of the resin was that the oxidationinduction time (OIT) of utilized HDPE 63 (4801EX) in the composite pipes was only4.7 min, which was largely lower than that stated in standard (P20 min). The worse ther-mal oxidation stability of the resin gave a hidden peril of ESC to result in the final failure ofthe pipes.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Steel wire reinforced thermoplastic composite pipe, shortly in SRTP, is reinforced with left and right spinning high-strength steel wires and takes high density polyethylene (HDPE) as the basal body. Steel mash skeleton and both innerand outer HDPE are closely connected by a high-performance sticking resin layer (Fig. 1). This kind of bonding resin is ahigh-powered binding material, belonging to modified material of HDPE, which can be melted together with HDPE underheating conditions [1].

SRTP not only combines the advantages of plastic and steel pipes but also eliminates their disadvantages such as lowerpressure resistance for plastic pipe and antirotless for steel pipe. Thus, it has many advantages, such as high strength, two-side anticorrosion, large circular rigidity, small linear expansion coefficient, lower thermal conductivity, light weight, conve-nient transportation and operation, long life time, lower cost, etc. [1,2]. Due to these qualities, SRTP has frequently been usedin urban water supply, oil and gas field, civil engineering, chemical industry, power transfer pipeline, metallurgical mines,seawater transportation, shipbuilding, and agriculture [1,2].

Till now, SRTP becomes the preferred product to replace traditional pipes like steel pipes in oilfield in China [3]. However,after several years of service, the pipelines fail frequently, which makes the user disturbed and results in reduce of the pro-ductivity. Mostly, methods to resolve these incidents are replacing the failed pipelines directly. Few studies on failure modesof SRTP have been conducted, and little suggestions for failure prevention have been put forward. For reliable operation andefficient design of this engineering composite pipe, it is necessary to analyze the factors which affect the pipe’s serving abil-ity. It will be of great benefit to prevent events which could trigger disastrous incidents, thus can reduce much loses in termsof service life and economics.

. All rights reserved.

89.eminzhang82@gmail.com (X. Zhang).

Inner HDPE

Hot melt adhesive

Outer HDPE

Fig. 1. Structure of SRTP [1].

H. Li et al. / Engineering Failure Analysis 20 (2012) 88–96 89

2. Background of the incident

SRTP was planed to be used as the underground pipeline to transport saline wastewater in chemical plant of China Petro-leum Xinjiang Petrochemical Co., Ltd. The temperature of transported wastewater was about 47 �C. The designed SRTP wasreinforced with U 0.8 mm netted steel wire. HDPE 63 with a trademark of 4801EX was utilized as the basal resin of the com-posite pipes.

The whole construction project had been completed on October 2005, while the SRTP had been started to use on July2009. From September 2009, the pipelines had leaked for seven times. Especially, the pipeline burst for three times duringDecember 2–10th 2009, leading to shut down of the whole system. It can be seen from the on-site investigation that all thecracks located on the right top or bottom of the pipes (Fig. 2a). Moreover, all the cracks extended along the axial direction ofthe pipes, as seen in Fig. 2.

3. Failure description

Fig. 3 shows the visual inspection of failed SRTP (DN315 � 13.5 mm–1.25 MPa) cut from the burst pipeline. Clearly, itscross section has an elliptical shape with the maximal and minimal diameters are 332 mm and 291 mm, respectively. At

Cracks

(b) (a)

Cracks

Fig. 2. On-site incident of SRTP: (a) outer surface and (b) inner surface.

Cracks

Fig. 3. Failed SRTP.

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the minor axis of the ellipse, there are two obvious splitting cracks along the axis of the pipe. The two cracks with a length of184 mm and 110 mm, respectively, have brittle fracture surface and penetrate the whole thickness of the pipe completely.The maximal widths of corresponding cracks on outer surface of SRTP are 2 mm and 1.5 mm, while the widths on the innersurface are 4.5 mm and 3.5 mm, respectively. Thus, the cracks appear ‘‘opening mouth’’ by observing from inner surface ofthe pipe, as shown in attaching figure in Fig. 3. However, the steel wires are not damaged at all, indicating that the failure ofSRTP is mainly caused by fracture of the basal resin.

Furthermore, a smaller ‘‘opening mouth’’ crack with a length of 60 mm (Fig. 4a) is also detected at the minor axis of theellipse on inner surface of the above failed SRTP. This crack has not penetrated the wall thickness completely from the innerto the outer surface. Nevertheless, indentation of press can be obviously found on outer surface of this crack (Fig. 4b). Mean-while, implied crack which corresponds to inner ‘‘opening mouth’’ crack is also observed along the axis of the pipe.

4. Methodology

Studies conducted are comprised of four fundamental aspects. First, the background information for understanding thesequence of events and the operational conditions that might lead to the failures of the pipe should be investigated in detail.Then, probable causes of SRTP failure are systematically analyzed by kinds of measurements, such as flattening propertyanalysis, Vicat softening temperature (VST) test and oxidation induction time (OIT) test. Next, after locating the actual reasonwhich results in the failure of SRTP, different studies are carried out to further testify its facticity. Failure process of SRTP isfinally simulated, followed by bringing forward the measures to failure prevention.

The detailed measurements for failure analysis of SRTP are described as follows. Flatting property of SRTP is determinedby using a SANS CMT 7104 universal testing machine (Shenzhen SANS material testing machine Co., Ltd., China). Surfacemorphology of basal rein cut from SRTP is detected by scanning electron microscope (SEM, JEOL-6700F, Tokyo, Japan), whilethe elemental analysis is conducted by energy dispersive spectroscopy (EDS). To check chemical modification of the resin, X-ray photoelectron spectroscopy (XPS) is obtained by using a PHI5300 X-ray photoelectron spectrometer (PE Corp., USA). Heatdeformation temperature (HDT) and VST Tester (XRD-300DL, Chengde Jinjian Testing Instrument Co., Ltd., China) with sili-cone oil as the heating medium is employed in testing VST of the basal resin. Thermal analysis for resin is carried out on athermal gravimetric analysis–differential scanning calorimetry (TGA–DSC) (Netzsch STA 409C, Germany) in nitrogen gaswith a ramping rate of 10 �C/min.

5. Findings and discussions

5.1. Review of background information

From the aforementioned analysis of cracks in failed pipes (Figs. 3 and 4), it can be predicted that the outer surface of SRTPmay suffer from a persistent press by stiff rocks or gravels, which will result in a tensile stress on inner surface of the pipe.Moreover, the tensile stress will be enhanced by working pressure applied on the pipe. Under such persistent stress, crackingoccurs firstly on inner surface and then extends along the axis direction of the pipe, meanwhile, enlarges down the radial ofthe pipe to outer surface. However, the whole SRTP construction project had been completed on October 2005. Failures of thepipeline were just detected after several months of usage from July 2009. It is indicated that the pipeline had not failed be-fore starting transport the wastewater. It must have other reasons to cause the failure of SRTP.

5.2. Analysis on pipe’s failed section

The damaged resin which centered at the crack in Fig. 4a was cut from SRTP, as shown in Fig. 5. It can be seen from Fig. 5athat fracture surface of the damaged resin is totally smooth and regular, belonging to a typical brittle fracture. Additionally,

Implied crack

Indentation

Implied crack

(b)

Opening-mouth crack

(a)

60mm

Fig. 4. Failed SRTP: (a) crack on inner surface and (b) morphology of outer surface corresponds to inner crack.

Fig. 5. Damaged resin cut from SRTP: (a) smooth and regular fracture and (b) fracture surface.

H. Li et al. / Engineering Failure Analysis 20 (2012) 88–96 91

multiple cracking characters are observed on its fracture surface. Multiple individual cracks with stretched fibrils are initi-ated, and subsequently coalesce into a unified fracture (Fig. 5b). All the above characteristics of the fracture can be obviouslysummarized to a typical environmental stress cracking (ESC) [4]. ESC in plastics means the failure at about room temperaturecaused by continuously acting external and/or internal stresses in the presence of surface active substances (known as stresscracking agents) [4]. In ESC, the initiation and growth of a crack is caused by the combined action of stress and a corrosiveenvironmental liquid. The action of either a tensile stress or a corrosive liquid alone would not be enough to cause failure.ESC is one of the most common causes of unexpected brittle failure of thermoplastic polymers. A recent study showed that25% of plastic part failures are related to ESC [4].

5.3. Microstructural analysis

As we know, the synergistic effect of stresses and environmental agents results in ESC. Stresses in ESC include tensilestress, residual stress and stress concentration of microscopic defects or points, while environmental agents contain solu-tions, corrosive liquids, chemicals agents, organic fluids, etc. [4].

Combined with analysis of the elliptical cross section (Fig. 3) and surface morphology of the failed SRTP (Fig. 4b), it can bededuced that the pipeline underwent a persistent stress along its radial direction after embedding under the ground. Theinner surface of the installed composite pipe would thus suffer from a continuous tensile stress, which gives an essentialcondition to result in ESC.

Defects such as pores and micro-cracks are observed in the damaged HDPE by using SEM (Fig. 6). These defects can causestress concentration, and in the other hand, can absorb the corrosive agents to initiate crazing of the resin. As aforemen-tioned description, the SRTP pipeline failed one after the other after starting use on July 2009. It is revealed that, underthe combination of transported saline wastewater which acts as the environmental agent and tensile stress, ESC of the basalrein is happened after several months of usage. EDS analysis confirms the absorption effect of HDPE, as seen in Fig. 7. Com-

Fig. 6. SEM of the damaged resin (fracture surface).

Fig. 7. EDS analysis of resin at (a) damaged surface and (b) undamaged surface.

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pared with undamaged resin (Fig. 7b), lots of saline elements such as potassium, calcium, aluminum and iron are detected onthe surface of the damaged resin (Fig. 7a), indicating the existence of absorbed transported medium.

In conclusion, the failure of SRTP is mainly caused by ESC of the basal resin (HDPE). Further analysis will be conducted asfollows to study the actual reason of ESC as well as the failure mechanism of the pipe.

6. Probable cause of SRTP failure

6.1. Flattening property analysis

In order to inspect whether the surface of SRTP craze or not when subject to a radial compress (simulating the stress stateafter embedding under ground), flattening property of the pipe was tested based on standard SY/T 6662-006 [5]. Accordingto the test procedure of standard, the un-failed pipe with a length of 100 mm should be compressed under a parallel plateloading to half of its diameter, followed by observing the specimen whether there are cracks or not. The tested pipe shows anexcellent result that no cracks of the resin and no failures of the pipe are found, even when the pipe is compressed to com-pletely contact of the inner surface (Fig. 8). The results reveal that the deformation resistance of this composite pipe is per-fect, and the cracking stability of the resin is also in good condition when suffering from a compressive stress. Consequently,the pure stress is not the only reason for cracking of the basal resin.

6.2. Vicat softening temperature (VST) test

VST is one of the most important parameters to evaluate thermal resistance and mechanical properties of thermoplasticmaterials when subject to heating. VST is largely helpful for quality control of thermoplastic materials, although it cannotdirectly estimate the ultimate-use temperature of thermoplastics. To investigate thermal resistance of the resin used in SRTP,VST of the material was measured according to ASTM D1525-006 [6]. Applied total mass and temperature rising rate wereset as 50 N and 50 �C/min, respectively. The tested VST of damaged resin is 72.9 �C, a little lower than undamaged one of75.4 �C. Both of them are much higher than the temperature of transported wastewater (47 �C), revealing that working tem-perature will not have any effect on the failure of the resin.

Fig. 8. Flattening property test (compress to completely contact).

H. Li et al. / Engineering Failure Analysis 20 (2012) 88–96 93

6.3. Oxidation induction time (OIT) test

Thermal oxidation resistance of polyolefin material is the intrinsic factor to cause its qualitative change, further to formits brittle cracking. It will directly influence the shelf time and service time of the fabricated pipe. Thus, it is necessary toevaluate thermal oxidation resistance of applied basal resin in SRTP. OIT test is an effective method to realize such analysis[7]. The aim of this measurement is to estimate the thermal stability of polymeric materials under oxidative conditions. It is astandardized test performed in a DSC as described as ISO 11357-6 2008 [8]. During this test, the time between melting andthe onset of decomposition of tested materials in isothermal conditions is measured. The atmosphere is nitrogen up to melt-ing and then oxygen.

When tested at 200 �C, OIT of the damaged resin of SRTP is only 0.6 min, while the one of undamaged material is just4.7 min. Both of them are much lower than the stated value of standard (P20 min). The results indicate that the utilized4801EX HDPE 63 has a much worse thermal oxidation stability. It will be easily degraded at a little higher temperaturedue to the lower thermal oxidation resistance. Such oxidative degradation of polymeric material makes its physical andmechanical properties change a lot, which may lead to an absolute failure of the material.

6.4. TGA–DSC analysis

The pyrolysis behaviors of undamaged resin and damaged resin are investigated by TGA–DSC (Fig. 9). The weight lossbehaviors of these two resins are alike, both exhibiting a tremendous weight loss from about 464 �C to 495 �C, as indicatedin TGA curves in Fig. 9. However, the overall weight losses correspond to undamaged and damaged resin are 96.1 wt% and89.0 wt%, respectively. The difference of weight loss between these two resins is mainly caused by the absorbed salts whichstill reside in the final pyrolysized products of damaged resin.

The first endothermic peak for undamaged resin of SRTP is centered at about 135 �C, seen the DSC curve in Fig. 9a. Thispeak attributes to melting peak of the resin, revealing that the resin starts to melt at this temperature [9]. Then, small mol-ecules due to breakage of side chain of the polymeric resin will slowly evaporate out, contributing to a slowly exothermalreaction (Fig. 9a). Subsequently, breakage of the main chain of the polymer results in an obvious endothermic peak at around483 �C (Fig. 9a). Hence, the highest weight loss at this stage results from pyrolysis of the resin. After completely decomposeof the organic groups, thermal change of the resin becomes much smooth.

The DSC curve of damaged resin is alike with undamaged one, except a visible exothermic peak at about 360 �C (arrow inFig. 9b). The peak greatly results from a large decomposition of low molecular weight species which may contain two as-pects: absorbed salts (few percentage) and oxides owing to thermal oxidation of the resin. At higher temperature, these spe-cies will much more easily evaporate out, forming an obvious exothermic peak. On the other hand, appearance of this peakindicates that the resin at fracture surface of SRTP has already degraded because of the thermal oxidation.

6.5. XPS analysis

The chemical bonding structures of resin surface are further characterized by XPS (Fig. 10). Like the EDS analysis, ele-ments detected on undamaged resin surface by XPS are mainly carbon, oxygen and silicon (Fig. 10a). However, new elementssuch as nitrogen, sodium and sulfur are also found on the damaged resin surface (Fig. 10b). Such new elements are in thebinding state of sodium salt, ammonium salt and sulfate, respectively, and no complex structures are formed. It is believedthat the corresponding salts with these new elements are physical absorption of the damaged resin rather than chemicallycorroded reaction.

It can be seen from high-resolution spectrum for carbon signal of undamaged resin that the C1s signal is deconvolutedinto two peaks (Fig. 11a). The peak at around 284.6 eV corresponds to C–C or C–H in (CH2CH2)n structure. According to Beam-son and Briggs [10], the C–O functional group should shift the C1s signal of involved carbon atoms to approximately

Fig. 9. TGA–DSC analysis of (a) undamaged resina and (b) damaged resin.

Fig. 10. XPS analysis of (a) undamaged resin and (b) damaged resin.

Fig. 11. C(1s) XPS signal for (a) undamaged resin and (b) damaged resin.

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286.5 eV. Therefore, the weak peak at about 286.4 eV assigns to carbons in C–O functional group, revealing that HDPE hasalready oxidized a little during its fabrication and usage stages.

The C1s XPS signal for damaged resin is also described into two obvious peaks at 284.6 eV and 286.4 eV, which attributeto C–C or C–H and C–O functional groups, respectively (Fig. 11b). By comparing these two figures (Fig. 11a and b), it is clearlyshown that the intensity of C–O signal for damaged resin is much higher than that of undamaged one, while the intensity ofC–C or C–H peak decreases a lot, further indicating an oxidation reaction of the resin has occurred. This result is entirely con-sistent with thermal analysis of the resin by OIT test and TGA–DSC.

7. Simulation study

From all above analysis, the failure process of SRTP can be simulated as follows.

(1) Fabrication of SRTP – hidden peril of the failure

4801EX HDPE 63 was selected as the basal body during fabrication of SRTP on August 2005. This kind of resin has a muchlower OIT, thus possessing a bad thermal oxidation stability, thereby leading to a worse ESC resistance. Hence, the lowerthermal oxidation stability of the resin gives a hidden peril of the failure for the composite pipe (Fig. 12a).

(1) Construction process – stress formed

When the whole construction project had been completed on October 2005, the embedded SRTP had been suffering froma compressive stress along its radial direction from top to bottom by a layer of earth containing stiff rocks or gravels. Tensilestress was thus caused on inner surface of the pipe. The cross section of SRTP became an elliptical shape under this persistent

(a) (b) (c) (d) (e)

Wastewater

Craze Leaking

Stress

SRTP

Fig. 12. Simulation of failure process of SRTP.

H. Li et al. / Engineering Failure Analysis 20 (2012) 88–96 95

pressure (Fig. 12b). However, the pipeline had not failed before usage because the wastewater which was an essential envi-ronmental medium to cause ESC had not been transported in SRTP at this stage.

(1) Fluid absorption – physical erosion

SRTP started to use on July 2009. The transported medium was saline wastewater with a temperature of 47 �C (Fig. 12c).The fluid physically diffused or absorbed into the resin firstly rather than directly leaded to a chemical degradation of theresin. Then, swelling, salvation or dissolution of the resin would be occurred. Such physical erosion could damage the inter-molecular force of the polymer and would accelerate the break of its main chain, further resulting in the property change ofthe resin. Meanwhile, pores and micro-cracks in resin provided additional positions to facilitate absorption of the fluid.

(1) Degradation – chemical erosion

Chemical erosion of resin is usually a combination of oxidation, hydrolysis, substitution and crosslinking. With the differ-ences of material compositions and environmental conditions, one of above reactions might be performed as a leader. In thisstudy, thermal oxidation degradation would be the main point for chemical erosion of the resin due to its worse thermaloxidation resistance.

(1) Craze initiation – failure starts

The abovementioned physical and chemical reactions made the resin degrade gradually and became much more brittle.As a result, plenty of cracks would be initiated under the cooperation of external stress and intrinsic defects of the resin(Fig. 12c).

(1) Crack spreading – failure deepening

With the deepening of erosion, the cracks extended longer along the axis of the pipe and enlarged deeper at the radialdirection of the pipe under persistent stress (Fig. 12d). Moreover, spread of the cracks promoted further erosion of the resinand speeded the failure of SRTP.

(1) Leaking – final failure

The resin completely fractured under the physical and chemical erosions as well as the added stresses, leaded to a finalfailure of SRTP. The cracks exhibited a typical ‘‘opening mouth’’ shape from inner surface to outer (Fig. 12e).

8. Conclusions

(1) Based on the usage history of SRTP and analysis of fracture morphologies of its basal resin, it is deduced that the failureof this pipe belongs to a typical ESC of HDPE.

(2) The intrinsic reason for crazing of the resin is that the utilized basal body (4801EX HDPE 63) in SRTP has a much lowerOIT (4.7 min), which is largely lower than that stated in standard (P20 min), thus possessing a much worse thermaloxidation stability.

9. Measures to failure prevention

Measurements for preventing the failure of SRTP can be provided as follows based on the analysis of this incident.

(1) Installment of the pipe should be strictly conducted according to notices of the construction practice. Special attentionshould be carefully paid to the backfill operations of pipelines to avoid pressing of stiff rocks, gravels or frozen earth.

96 H. Li et al. / Engineering Failure Analysis 20 (2012) 88–96

(2) Modify the properties of HDPE 63 by controlling its molecular weight and distribution or by optimizing its structure.Carbon black, antioxidants, fillers, etc., can be added into the resin to improve its resistance to ESC. Alternatively,higher qualitative PE such as PE80, PE100 can be replaced as the basal body of SRTP.

(3) Optimize the fabrication processes of SRTP such as extrusion temperature, traction speed, cooling rate, etc., to preventthe existence of pores and micro-cracks in the basal resin.

References

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2006;25:83–90.[8] ISO 11357-6 2002 Plastics-differential scanning calorimetry (DSC) – part 6 determination of oxidation induction time.[9] Greco A, Maffezzoli A. Correction of melting peaks of different PE grades accounting for heat transfer in DSC samples. Polym Test 2008;27:61–74.

[10] Beamson G, Briggs D. High resolution XPS of organic polymers. Chichester: John Wiley & Sons; 1992.