Structure-Property Relationships of Polyamide 12 Grades Exposed to Rapid Crack Extension

Abstract

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.

On the slow crack growth process and associated structure–property relationships in polyamide 12 grades

“…In contrast, an impact modification surely leads to a very fine fibrillation and a significantly increasing plasticity at a microscopic level of the materials S IM and S IM_PGM compared to S Base and S PGM , respectively (see Figure 12). It can, therefore, be assumed that rubber particles have enough time during quasi‐static fatigue to plastically deform and cavitate, enhancing the resistance against crack initiation and SCG 33 . Different theories were proposed to explain the toughening mechanisms due to impact modification: the rubber is stretched during the fracture process and absorbs a large amount of energy 34 within the plastic zone; rubber particles are crack terminators that create a large number of stress concentration points, promoting multiple crazing and subsequent propagation of smaller cracks, which require more energy than a single main crack due to the formation of numerous new surfaces 35 ; a rubbery phase results in cavitation mechanisms that dissipate hydrostatic strain energy and increase shear flow of the matrix 36 …”

“…It can, therefore, be assumed that rubber particles have enough time during quasi-static fatigue to plastically deform and cavitate, enhancing the resistance against crack initiation and SCG. 33 Different theories were proposed to explain the toughening mechanisms due to impact modification:…”

An Experimental Study of the Mechanical Properties of Partially Rehabilitated Cable Tunnels

“…Therefore, the exposed area at the crown of the pipe becomes the only area for rehabilitation, which is shown as the green area in Figure 2b, and the partial lining method is bound to become a major rehabilitation method in these structures; thus, great importance is attached to the studying of partial lining materials and dimensional parameters for a better effect. It can be learnt from previous studies [25][26][27] that for a pipe ring structure, the dangerous sections are located at the crown, invert, and springlines of the pipe under surface load, where the cracks appear and propagate first [28,29]. As shown in Figure 2b, the exposed area contains the dangerous section at the crown of the pipe, and it can be rehabilitated with the partial lining method, which suggests a potential structural enhancement by rehabilitating with this method [30,31].…”

“…It can be learnt from previous studies [ 25 , 26 , 27 ] that for a pipe ring structure, the dangerous sections are located at the crown, invert, and springlines of the pipe under surface load, where the cracks appear and propagate first [ 28 , 29 ]. As shown in Figure 2 b, the exposed area contains the dangerous section at the crown of the pipe, and it can be rehabilitated with the partial lining method, which suggests a potential structural enhancement by rehabilitating with this method [ 30 , 31 ].…”