INFLUENCE OF LONG-TERM ANNEALING ON RESIDUAL STRESS DISTRIBUTION AND QUASI-BRITTLE FAILURE PROPERTIES OF TALCUM REINFORCED PIPE GRADE POLYPROYPLENE

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1 INFLUENCE OF LONG-TERM ANNEALING ON RESIDUAL STRESS DISTRIBUTION AND QUASI-BRITTLE FAILURE PROPERTIES OF TALCUM REINFORCED PIPE GRADE POLYPROYPLENE Florian J. Arbeiter, Department of Polymer Engineering and Science, Montanuniversitaet Leoben, AUT Gerald Pinter, Department of Polymer Engineering and Science, Montanuniversitaet Leoben, AUT Andreas Frank, Polymer Competence Center Leoben GmbH, AUT Abstract In this study a polypropylene material with talcum reinforcement used for sewer pipes has been subjected to an annealing procedure at 80 C, roughly 60 C above the actual application temperature, in air for a time period of 18 months. As expected, examination of the material showed no significant decrease in mechanical or fracture mechanical properties due to the temperature exposure. However, samples stored at higher temperature showed better resistance against quasi-brittle failure in fatigue tests compared to unconditioned samples. This could mainly be attributed to the decrease of residual stress in the pipe wall. Even though pipes have been annealed for very long times above T g, residual stress could not be totally relaxed within 18 months. Introduction Parts of this work have been taken from the PhD thesis Evaluation of long-term properties of polymeric pipe grade materials using fatigue tests and fracture mechanics [1] of Florian J. Arbeiter and revised to fit the requirements of publication at ANTEC Anaheim Long-term failure of polymer pipes is always a crucial topic within the pipe community. A lot of effort has gone into establishing new test methods to account for this type of failure within the last years. One of these methods is the cyclic cracked round bar (CRB) -test which has been developed to characterize the quasi-brittle failure of polyethylene pipe grades (PE) [2 4] and is now also a standard within ISO [5]. This test makes it possible to rank PE-pipe grade materials with regard to their resistance against slow crack growth (SCG), within hours to a few days, compared to months, or even years older commercial tests take. Due to its excellent performance with PE-pipe grade materials, the test has also been under investigation for other pipe materials, such as polypropylene (PP) [6 8]. Besides intrinsic material properties, there are a lot of different aspects which can influence the performance of a polymer pipe in application. For example point loads, bedding and scratches on the outside can severely influence the lifetime of pipes. Furthermore, residual stress in pipe walls has been discussed, as a possible influence several years ago [9], and also in the recent past [10 13]. Since the level of residual stress can be quite significant, compared to stress due to application conditions it is vital to take these into consideration as well. To decrease the level of residual stress, annealing is a typical solution. However, short time annealing of pipes usual is not sufficient to fully release all residual stress [13], even though negative aspects, such as further shrinkage can be vastly reduced. While residual stress in neat polyethylene and polypropylene pipes has been researched quite thoroughly, research on reinforced pipe materials has only been grazed so far. Therefore, this publication aims to shed light on the residual stress situation in talcum reinforced polypropylene pipes, the long-term annealing of aforementioned stress and its influence on the cyclic CRB performance. Material The material used in this study was a compounded polypropylene block-copolymer with 47wt.% talcum (PPtv). This type of polymer can typically be used as a stiff middle layer in sewer multi-layer pipes. Classical mechanical properties of the material can be seen in Table 1. Table 1. Mechanical properties of the investigated PP-tv at 23 C Mechanical Property Value Unit Young s Modulus (E) 3970 MPa Yield Stress (s ys ) 28 MPa Yield Strain (e ys ) 3 % Strain at break (e br ) 24 % Annealing of the pipes Experimental To examine the influence of annealing on the residual stress in a pipe wall, sample pipes have been stored in an oven room set at 80 C. The pipes had a nominal diameter (DN) of 160 mm and a wall thickness (h) of 19.1 mm and SPE ANTEC Anaheim 2017 / 1989

2 a length of 1 m each. The duration of the annealing at 80 C was 6, 12 and 18 months. Additionally, a pipe sample has been investigated as received without any additional treatment for reference. Samples for both residual stress analysis and CRB testing have been taken directly from these pipes as well. Residual stress analysis There are several methods available to analyze residual stress in pipe walls. For example, there is the common slitting method, which has also been used in this work, hole-drilling methods, or more complex but informative methods, as developed in [9, 14]. The slitting method is a rather simple method, where segments of a pipe ring are cut out and the deformation of the residual ring is measured as a function of time. If the time dependent modulus of the same material is known, it is possible to calculate residual stress in the pipe wall based on basic mechanics. This is commonly done, by assuming a linear distribution of residual stress in the pipe wall. In this case, since the pipe walls are rather thick, compared to the diameter, a correction function for thickwalled pipes according to Williams [9] has been used. The rings were chosen with a length of 20 mm to avoid interactions between residual stress in hoop and axial direction. Since previous research [9, 14] has shown, that the values from linear distribution tend to overestimate tensile, and underestimates compression stresses, it is suggested to examine the non-linear behavior of residual stress in the pipe wall, if more exact values are needed. Poduška et. al. [10, 14] have developed a set of easy to use formulae for PE and PP pipes, which has shown to be quite independent of pipe dimension and wall thickness to diameter ratio, and can be used to better describe the non-linear behavior of residual stress in the pipe wall. Handy enough, it can also be applied to data from ringslitting tests done in the past. The formula for the resulting stress is given in (1) and has an exponential shape. The constants of c 1 and c 2 have to be calculated for each pipe and are mainly dependent on the pipe geometry, the deformation and modulus of a pipe ring segment at the same time. Cyclic CRB tests 3.2x s res ( x) = c1 + c2e (1) The cyclic CRB test, which is a fracture mechanics based fatigue test on a circumferentially notched cylindrical specimen, can be used to characterize the failure behavior of pipe grade materials in a quasi-brittle failure mode. Testing parameters have been chosen similar to ISO [5]. Only the applied testing frequency has been decreased to 5 instead of 10 Hz to account for the higher dampening of PP. The values of applied initial stress intensity factors K I,ini, were also adapted to better fit the requirements of polypropylene. K I,ini, describes the stress situation in front of a crack tip (2), accounting for actual crack length (a) and its influence on the geometry of the specimen (Y). Tests were performed at 23 C and a load ratio of R=0.1 (s min /s max ). For the correction term Y the solution of Benthem and Koiter [15] has been used to describe the change in K I,ini as a function of changes in crack length a. Tests were performed on a servo-hydraulic testing machine from MTS (MTS, Eden Prairie, USA). K σ Stabilizer concentration ay I, ini = I, ini (2) Since the main goal of this work is to characterize the annealing of polymeric materials under heat storage for long periods of time, possible material degradation has to be monitored in order to correctly interpret results. Therefore stabilizer concentration of materials has been observed using oxidation induction time (OIT) measurements and high pressure liquid chromatography (HPLC). Specimens of 15±2 mg were tested according to the Austrian pipe standard ONORM EN 728 [16]. A type DSC1 (Mettler Toledo, Columbus, OH, USA) was used for these measurements. Samples were heated up to 210 C under nitrogen atmosphere (50 ml/min) and afterwards exposed to oxygen (50 ml/min) to induce oxidation. The time until the onset of degradation was determined. The HPLC was also used to detect stabilizer concentration in the polymer as a function of ageing time. Additives were extracted from specimens at temperatures <40 C to prevent the samples from thermal influence. A device of the type 1260 Infinity LC System (Agilent, Santa Clara, CA, USA) was used to analyze the type and amount of stabilizers. Residual stress analysis Results and Discussion The results of the residual stress analysis are shown in Figure 1. In this figure, the distribution of the residual stress in the pipe wall is shown from the inside pipe wall (position 0) to the outside pipe wall (position 1). As expected, tensile stress can be found on the inside of the pipe wall and compression stress on the outside pipe wall. SPE ANTEC Anaheim 2017 / 1990

3 For the linear evaluation, resulting residual stress (s res ) on the pipe wall surfaces are close to 4.3 MPa in tension and compression for untreated samples. Using the formulae from [10, 14], the non-linear distribution of residual stress in the pipe wall was also calculated. This asymmetric distribution shows a clear shift of stresses towards compression, decreasing the residual tensile stress to around 2.5 MPa and increasing the compression value to > 6 MPa. To validate the influence of the annealing, the same procedure was performed on the conditioned samples. It can be seen, that annealing at 80 C clearly reduces the level of residual stress in the pipe wall, as expected. Values were reduced to around ±2.1 MPa for the linear, and 1.3 and -3.8 MPa for the non-linear approach. The respective values for c 1 and c 2 of the exponential formula are shown in Table 2. level for a mayor part of the service life of a talcum reinforced PP pipe. Since stress levels around 2.5 MPa are quite high compared to the actual long-term stress levels in non-pressure pipes they should be considered when talking about service life, similar as in [10 12] for PE pipes. Cyclic CRB tests The results of the cyclic CRB tests on talcum reinforced PP at 23 C and a loading ratio of R=0.1 are shown in Figure 2. Results are displayed as a function of the initial applied K I value (corrected to actual crack length according to ISO after testing). It can be seen, that the samples with the full symbols, which are evaluated according to ISO show a difference between untreated and treated samples. The K I,ini values range between 0.85 and 0.80 MPam 0.5 and cycles to failure between 1,500,00 and 2,000,000 cycles. Compared to untreated samples, values of the annealed samples are shifted towards higher applicable K I,ini values or higher cycle numbers till fracture at similar K I,ini values (0.91 to 0.85 MPam 0.5 at 800,000 to 4,000,000 cycles). Figure 1. Residual stress distribution in a pipe wall: from 0 (inner pipe wall surface) to 1 (outer pipe wall surface); calculated according to linear and non-linear theory [10, 14], Data recomputed from [17] Table 2. Residual stress distribution coefficients c 1 and c 2 Sample C 1 C 2 As received Annealed for 6 months Annealed for 12 months Annealed for 18 months Interestingly, it seems that further time at elevated temperature does not significantly decrease residual stress in the pipe wall further. Even after close to 13,000 hours of annealing at 80 C, residual stress remains constant at around 1.3 and -3.6 MPa in the pipe wall. Also the coefficients (c 1 and c 2 ) of the stress distribution (shown in Table 2) do not change significantly after 3 months of annealing. Similar to previous work for PE pipe grade materials [13, 18], this indicates that residual stress will stay at this Figure 2. Fracture curve of cyclic CRB-test on talcum reinforced PP, depending on annealing time at 80 C To account for measured residual stress in the samples, results from Figure 1 have been added in the calculation of stress intensity factor and can also be seen in Figure 2 as K I,ini (s nom + s res ). Since all cyclic CRB tests were performed in tensile-tensile fatigue (R=0.1) the tensile residual stress level at the position of the crack tip (indicated in Figure 1 with orange lines) has been used. By adding the residual stress into the calculation, K I,ini values are shifted to higher levels (0.91 to 0.97 MPam 0.5 ). However, the difference in results between treated and untreated samples almost completely vanishes. The scatter of data in the measurements was found to be significantly higher, compared to unreinforced materials, which is not surprising, given the additional influence of the included minerals. As shown in Figure 3, minerals can be found on the fracture surfaces in the range SPE ANTEC Anaheim 2017 / 1991

4 of >10-50 µm. Agglomeration of reinforcing particles can also be an issue, considering that the material consists of 47wt.% talcum. Inclusions or even agglomerates of this size can definitely influence stress distribution and fracture behavior if positioned in close proximity to the initial circumferential notch, or a growing crack. Stabilizer concentration As previously mentioned, to ascertain that measured results are only depending on annealing processes and are not influenced by ageing, stabilizer concentration has been monitored. In Figure 4 (top) results of OIT and HPLC measurements can be seen as a function of annealing time. While processing stabilizers are already depleted after 3 months of annealing, two other stabilizer packages are still working even after 18 months of exposure. Even though a higher depletion of stabilizers can be seen at the surface of the pipe segments, the material is still sufficiently protected after 18 months of testing. This affirms the assumption that changes in CRB results can mainly be attributed to the annealing of residual stress. After 18 months of annealing at 80 C material degradation does not yet play a vital role. Figure 4 (bottom) also shows a correlation between normalized OIT and HPLC values. Both methods were found to correlate quite nicely (R2 = 0.998) as indicated in literature [19, 20] for polyolefins. 50 µm 10 µm 20 µm Figure 3. Fracture surface of the quasi-brittle failure area in talcum reinforced PP with ruptured fibrils ( as received at KI,ini = 0.83 MPam0.5 Figure 4. Stabilizer concentration OIT as a function of annealing time at 80 C in air (top) and correlation of OIT and results from HPLC (bottom) Conclusions Residual stress levels in highly talcum reinforced polypropylene block-copolymer pipe grade material were examined for untreated and annealed samples. While untreated samples showed residual stress levels around 2.5 MPa on the inside of the pipe (exp. evaluation acc. to [10]), annealing at 80 C lead to a decrease of roughly 50% to around 1.3 MPa. Interestingly, long-term annealing of up to 18 months at 80 C lead to no further notable decrease of residual stress. To characterize the long-term failure behavior of the examined material, cyclic CRB tests were performed. The CRB testing yielded fracture curves around 0.85 MPam0.5 to 0.80 MPam0.5 at 1,500,000 to 2,000,000 cycles for untreated samples. Annealing at 80 C led to an improved material behavior and curves around 0.91 to 0.85 MPam0.5 and 800,000 to 4,000,000 cycles, respectively. A strong indication, that this improvement can mainly be attributed to the annealing of residual stress could be found by superimposing applied nominal stress and residual stress levels which lead to almost identical fracture curves for annealed and untreated samples. Monitoring of stabilizer concentration showed a significant decrease over the course of 18 months at 80 C with both OIT and HPLC. However, at the end of the study stabilizers were still present and actively protecting the material from degradation. Therefore, influence of material degradation can be excluded. Overall this study shows, that residual stress levels can be quite significant compared to actual long-term application stress. Annealing at moderate temperature can be used to decrease the level of residual stress. However, even after 18 months of annealing at 80 C which is by far longer than any commercially used treatment, residual stress could not be deleted completely. Since found levels SPE ANTEC Anaheim 2017 / 1992

5 of residual stress (around 2.5 MPa or 1.3 MPa for annealed pipes) can be seen as significant compared to long-term application loads in non-pressure pipes, they should be taken into consideration when estimating the long-term performance of pipes made from talcum reinforced PP. References 1. Arbeiter, F.J.: Evaluation of long-term properties of polymeric pipe grade materials using fatigue tests and fracture mechanics. Dissertation, Montanuniversitaet Leoben (2015) 2. Frank, A., Lang, R.W., Pinter, G.: Accelerated Investigation of creep crack growth in polyethylene pipe grade materials by the use of fatigue tests on cracked round bar specimens. Proceedings Annual Technical Conference - ANTEC, Society of Plastics Engineers, Milwaukee, Wisconsin, USA, (2008) 3. Kratochvilla, T.R., Frank, A., Pinter, G.: Determination of slow crack growth behaviour of polyethylene pressure pipes with cracked round bar test. Polymer Testing 40, (2014). doi: /j.polymertesting Frank, A., Pinter, G.: Evaluation of the applicability of the cracked round bar test as standardized PEpipe ranking tool. Polymer Testing 33, (2014) 5. ISO 18489: Polyethylene (PE) materials for piping systems Determination of resistance to slow crack growth under cyclic loading Cracked Round Bar test method(18489) (2015) 6. Arbeiter, F.J., Pinter, G., Frank, A.: Characterisation of quasi-brittle fatigue crack growth in pipe grade polypropylene block copolymer. Polymer Testing, (2014). doi: /j.polymertesting Arbeiter, F.J., Schrittesser, B., Frank, A., Berer, M., Pinter, G.: Cyclic tests on cracked round bars as a quick tool to assess the long term behaviour of thermoplastics and elastomers. Polymer Testing 45, (2015). doi: /j.polymertesting Arbeiter, F.J., Frank, A., Pinter, G.: Influence of molecular structure and reinforcement on fatigue behavior of tough polypropylene materials. J. Appl. Polym. Sci. (2016). doi: /app Williams, J.G., Hodgkinson, J.M., Gray, A.: The determination of residual stresses in plastic pipe and their role in fracture. Polym. Eng. Sci. 21(13), (1981). doi: /pen Poduška, J., Hutař, P., Kučera, J., Frank, A., Sadílek, J., Pinter, G., Náhlík, L.: Residual stress in polyethylene pipes. Polymer Testing 54, (2016). doi: /j.polymertesting Hutař, P., Ševčík, M., Frank, A., Náhlík, L., Kučera, J., Pinter, G.: The effect of residual stress on polymer pipe lifetime. Engineering Fracture Mechanics 108, (2013). doi: /j.engfracmech Hutař, P., Ševčík, M., Náhlík, L., Frank, A., Kučera, J., Pinter, G.: Numerical Lifetime Prediction of Polymer Pipes Taking into Account Residual Stress. KEM , (2013). doi: / 13. Kim, J.-S., Yoo, J.-H., Oh, Y.-J.: A study on residual stress mitigation of the HDPE pipe for various annealing conditions. J Mech Sci Technol 29(3), (2015). doi: /s Poduška, J., Kučera, J., Hutař, P., Ševčík, M., Křivánek, J., Sadílek, J., Náhlík, L.: Residual stress distribution in extruded polypropylene pipes. Polymer Testing 40, (2014). doi: /j.polymertesting Benthem, J., Koiter, W. (eds.): Method of Analysis and Solutions of Crack Problems, 3rd edn. (1973) 16. ÖNORM: Kunststoff-Rohrleitungs- und Schutzrohrsysteme - Rohre und Formstücke aus Polyolefinen - Bestimmung der Oxidations- Induktionszeit(ÖNORM EN 728 ( )) (1997) 17. Schwab, M.: Residual stress analysis and damage behaviour of reinforced polypropylene pipe materials. Master's Thesis, Montanuniversitaet Leoben (2015) 18. Frank, A., Pinter, G., Lang, R.W.: Prediction of the remaining lifetime of polyethylene pipes after up to 30 years in use. Polymer Testing 28(7), (2009). doi: /j.polymertesting Marshall, D.I., George, E.J., Turnipseed, J.M., Glenn, J.L.: Measurement of oxidation stability of polyolefins by thermal analysis. Polym. Eng. Sci. 13(6), (1973). doi: /pen Foster, G.N.: Analytical methods applied to the testing of oxidation inhibition. In: Pospisil, J., Klemchuk, P. (eds.) Oxidation inhibition in organic materials, 2nd edn., pp CRC Press Inc., Boca Raton, Florida (1990) SPE ANTEC Anaheim 2017 / 1993