Finite Element Simulation of a Crack Growth in the Presence of a Hole in the Vicinity of the Crack Trajectory

The aim of this paper was to present a numerical simulation of a crack growth path and associated stress intensity factors (SIFs) for linear elastic material. The influence of the holes’ position and pre-crack locations in the crack growth direction were investigated. For this purpose, ANSYS Mechanical R19.2 was introduced with the use of a new feature known as Separating Morphing and Adaptive Remeshing Technology (SMART) dependent on the Unstructured Mesh Method (UMM), which can reduce the meshing time from up to several days to a few minutes, eliminating long preprocessing sessions. The presence of a hole near a propagating crack causes a deviation in the crack path. If the hole is close enough to the crack path, the crack may stop at the edge of the hole, resulting in crack arrest. The present study was carried out for two geometries, namely a cracked plate with four holes and a plate with a circular hole, and an edge crack with different pre-crack locations. Under linear elastic fracture mechanics (LEFM), the maximum circumferential stress criterion is applied as a direction criterion. Depending on the position of the hole, the results reveal that the crack propagates in the direction of the hole due to the uneven stresses at the crack tip, which are consequences of the hole’s influence. The results of this modeling are validated in terms of crack growth trajectories and SIFs by several crack growth studies reported in the literature that show trustworthy results.

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

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.

Influence of Embedded Gap and Overlap Fiber Placement Defects on Interlaminar Properties of High Performance Composites

Automated fiber placement (AFP), once limited to aerospace, is gaining acceptance and offers great potential for marine structures. This paper describes the influence of manufacturing defects, gaps, and overlaps, on the out-of-plane properties of carbon/epoxy composites manufactured by AFP. Apparent interlaminar shear strength measured by short beam shear tests was not affected by the presence of defects. However, the defects do affect delamination propagation. Under Mode I (tension) loading a small crack arrest effect is noted, resulting in higher apparent fracture energies, particularly for specimens manufactured using a caul plate. Under Mode II (in-plane shear) loading there is a more significant effect with increased fracture resistance, as stable propagation for specimens with small gaps changes to arrest with unstable propagation for larger gaps.

Experience nature as a basis for building strong composite structures

Introduction. The article is devoted an analytical overview of the methods of applying the Nature solutions for designing structures made of plastics reinforced with fibers, in particular, using rational curved fiber trajectories. The first section provides an overview of different structural models and some approaches to the micromechanics of composites. Materials and methods. Sections 2-7 discuss: analysis of rational elastic-strength properties of wood and composites for crack arrest by weak interface; methods for constructing curved paths of fibers of “flowing holes”; analyzes the applied and promising technologies for manufacturing attachment points, in which holes are formed using curvilinear fiber paths; “nature-inspired” principles of optimal design of pipe composite structures similar in structure to ladder of bamboo stalk; examples of the effective use of fibrous composites in elastic elements such as leaf springs; developing additive technologies for 3D printing of fiber composite parts with fiber laying along calculated trajectories. Results. Each section of the article presents conclusions related to the peculiarities of composites structures calculation and design: calculations show that in order to increase the crack resistance of fibrous composites, it is necessary to significantly increase the shear characteristics of the binder and strive for rational properties created by Nature in wood; as a result of the calculation, it turns out that the maximum stress per fiber at the optimal reinforcement structure becomes about 3–4 times less than with a uniform rectilinear laying; rational reinforcement leads to a significant reduction in local stresses per fiber, elimination of splits and damages of fibers and an increase in the carrying capacity of the assembly; it has been shown that the bamboo rings are arranged to prevent the barrel from splitting from bending compressive stresses and tangential stresses when the barrel is twisted by wind load; analyzed the relationship of equal-strength profiling with Leonardo’s rule for tree crown branching. The works on creation of bio-similar shape and structure of curvilinear reinforcement of specimens for correct determination of unidirectional composites strength at tension along fibres were discussed; analyzed the role of composite technologies in modern mechanical engineering, in particular, in the creation of composite structures in open space. Conclusions. The article is devoted to the analysis of the tasks of fibrous composites macromechanics, therefore, in the opinion of the authors, the three most promising and related areas in macromechanics of composites that require further research are biomechanics of strength, computer modeling of optimal structures and technological mechanics of composites.

Evaluation of Brittle Crack Arrest Toughness for Highly-Irradiated Reactor Pressure Vessel Steels

The crack arrest fracture toughness, KIa, values for highly-irradiated reactor pressure vessel (RPV) steels are estimated according to the linear relationship between crack arrest toughness reference temperature, TKIa, and the temperature corresponding to a fixed arrest load, equal to 4 kN, TFa4kN, obtained by instrumented Charpy impact test. The relationship between TKIa derived from the instrumented Charpy impact test and fracture toughness reference temperature, To, was expressed as an equation proposed in a previous study. The coefficients in the equation could be fine-tuned to obtain a better fitting curve using the present experimental data and previous KIa data. The KIa curve for RPV;A533B class 1 steels irradiated up to 1.3 × 1020 n/cm2 (E > 1 MeV) was compared with a KIR curve defined in JEAC4206-2016. The KIR curve was always lower than the 1%ile curve of KIa for these irradiated RPV steels. This result indicates that the conservatism of the method defined in JEAC4206-2016 to evaluate KIa using the KIR curve is confirmed for highly-irradiated RPV steels.