Andreas Frank

The influence of micro‐structural changes within polyamide 12 (PA12) grades on their resistance against crack initiation and subsequent Slow Crack Growth (SCG) was examined. Possible micro‐deformation mechanisms and processes guiding slow crack extensions in PA12 are discussed. Special focus was put on the formation of plastic zones as predecessors to microscopic cracks prior to crack initiation via crack freezing analysis. Simultaneously, variations in the pure SCG resistance after a recorded craze‐crack transition were determined via cyclic Cracked Round Bar (CRB) tests. In that context, a homologous series of PA12 grades of increasing molecular weight (M_W), as well as a systematically developed series of PA12 grades incorporating a pigment and/or an impact modifier, were selected for this study. Results show a good correlation between the crack initiation resistance and the disentanglement resistance of PA12 chains as well as the size of the plastic zone. In that context, an increasing average M_W, leads to increasing disentanglement resistances due to hydrogen bond effects. An impact modification promotes the development of notably larger plastic zones. Contrarily, colored grades exhibit a reduced plastic zone size, rendering a lower amount of dissipated energy before physical crack initiation. Pure SCG resistance follows a similar trend and is improved by high average M_W as well as the use of impact modifier. Crack growth acceleration, however, is assumed to occur alongside the relatively weak interfaces between polymer matrix and pigments.

Thermoplastic materials have established a reputation for long-term reliability in low-pressure gas and water distribution pipe systems. However, occasional Slow Crack Growth (SCG) and Rapid Crack Propagation (RCP) failures still occur. SCG may initiate only a small leak, but it has the potential to trigger RCP, which is much rarer but more catastrophic and destructive. RCP can create a long, straight or meandering axial crack path at speeds of up to hundreds of meters per second. It is driven by internal (residual) and external (pressure) loads and resisted by molecular and morphological characteristics of the polymer. The safe installation and operation of a pipe throughout its service lifetime therefore requires knowledge of its resistance to RCP, particularly when using new materials. In this context, the RCP resistance of five different polyamide (PA) 12 grades was investigated using the ISO 13477 Small-Scale Steady State (S4) test. Since these grades differed not only in molecular weight but also in their use of additives (impact modifiers and pigments), structure-property relationships could be deduced from S4 test results. A new method is proposed for correlating these results more efficiently to evaluate each grade using the crack arrest lengths from individual S4 test specimens.

Lifetime of plastic pipes can be estimated by integration of a power law describing the crack kinetics. However, this procedure requires an FEM (finite element method) calculation of the possible crack propagation in the pipe to obtain stress intensity factor dependency on the crack length. It is very important for the simulation to consider every possible load that is acting on the pipe. This contribution deals with FEM modelling of a pipe that is loaded by internal pressure, residual stresses and soil loads. Comparison of the factors and pipe lifetime estimation is carried out.

Polyethylene pressure pipes are designed for service lives of at least 50 years. While so far lifetimes have been assessed by pipe pressure tests, due to improved slow crack growth (SCG) resistance and increased testing times such tests are not suitable anymore for modern pipe grades. The paper presents an alternative methodology for lifetime prediction, combining practical advantages of the cyclic cracked round bar (CRB) test and linear elastic fracture mechanics. A major advantage of this procedure is that material characterization is done at ambient temperatures. The key challenge in this context is to measure SCG rates in CRB specimens. After determination of crack kinetics at different dynamic loading ratios, material laws for SCG at static loading are extrapolated. Based on the stress intensity factor concept and realistic considerations of initial defect size and changing crack front geometry, pipe lifetimes of four different PE pipe grades are predicted. The results show that for all materials the required minimum lifetime of 50 years will be reached and under practical assumptions even 100 years will be exceeded.

Molecular structure and reinforcement heavily influence the crack growth resistance of polypropylene materials. Aim of this study is to investigate the fatigue behavior of different unreinforced and reinforced tough polypropylene materials used for piping applications. Due to high resistance against crack growth, these materials cannot be tested in the application relevant quasi‐brittle failure mode within feasible amounts of time. In this work, the new cyclic cracked round bar test, developed for tough polyethylene materials, has been examined as a possible method to characterize this important type of failure mode in homo‐, random‐, and reinforced polypropylene. Even though molecular mass distribution, which is often used to explain differences in crack growth resistance of polymers, was similar for unreinforced materials, fatigue lifetimes differed greatly. The mismatch of molecular mass and fatigue lifetime was mainly attributed to the different buildup and morphology of the base polymer. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43948.

Polymer pipe lifetime is conventionally estimated by internal pressure test. However, important for service are also other types of loading as: additional bending, different type of external loading from sand embedding, or effect of the residual stresses. One of the dangerous type of loading is so‐called “point load” caused, for example, by stones in the soil. The present article is focused on the numerical prediction of this effect on the pipe lifetime. For the lifetime estimation of a polymer pipe under point loading, an approach based on combination of numerical calculation and experimental measurements was used. A special numerical algorithm, which optimizes modeled crack front shape according to the real one during simulation of crack propagation, was proposed. Finally, an analytical relationship for the estimation of fracture parameters of propagating crack across the pipe wall crucial for the pipe lifetime estimation is presented in the paper. A significant reduction of the residual lifetime was found for the cracked pipe loaded by additional point load (consideration of indentation effects of stones in the soil) in comparison with pipe loaded by internal pressure only. The approach presented contributes to the safer service of polyolefin pipes. POLYM. ENG. SCI., 56:79–86, 2016. © 2015 Society of Plastics Engineers