A model for the axisymmetric growth and coalescence of small internal voids in elastoplastic solids is proposed and assessed using void cell computations. Two contributions existing in the literature have been integrated into the enhanced model. The first is the model of Gologanu-Leblond-Devaux, extending the Gurson model to void shape effects. The second is the approach of Thomason for the onset of void coalescence. Each of these has been extended heuristically to account for strain hardening. In addition, a micromechanically-based simple constitutive model for the void coalescence stage is proposed to supplement the criterion for the onset of coalescence. The fully enhanced Gurson model depends on the flow properties of the material and the dimensional ratios of the void-cell representative volume element. Phenomenological parameters such as critical porosities are not employed in the enhanced model. It incorporates the effect of void shape, relative void spacing, strain hardening, and porosity. The effect of the relative void spacing on void coalescence, which has not yet been carefully addressed in the literature. has received special attention. Using cell model computations, accurate predictions through final fracture have been obtained for a wide range of porosity, void spacing, initial void shape, strain hardening, and stress triaxiality. These predictions have been used to assess the enhanced model. (C) 2000 Elsevier Science Ltd. All rights reserved.

An extended model for void growth and coalescence - application to anisotropic ductile fracture

Slow crack growth (SCG) is a time-dependent quasi-brittle type of failure that polyethylene (PE) materials exhibit under low stress levels. The interplay between the crack and its plastic zone dictates complex damage behavior of continuous and discontinuous SCG. Under a continuum mechanics approach, SCG can even remain a commonality in PE mechano-chemical degradation (MCD). Therefore, a deep understanding of SCG is critical since it’s the controlling mechanism for most field failures and consequently the formalism of PE lifetime prediction models. In this work, a comprehensive review of the recent advances on PE SCG modeling is presented. PE SCG and MCD macro and micro mechanisms are explained. The evaluation test methods of SCG are discussed. Owing to PE continuous and discontinuous SCG modes, using the parameters of conventional Fracture Mechanics, e.g., stress intensity factor, J-integral, or crack opening displacement, may be considered only within their applicable range. The crack-layer (CL) theory was found capable in accounting for PE SCG kinetics and thermodynamics including its two fracture modes. The theory framework along with several applications are demonstrated. Hence, the main objective of the manuscript is to provide a thorough review of SCG in PE materials to give a direction to making reliability assessments more predictive. 

Recent advances in slow crack growth modeling of polyethylene materials

In the industries today, metal components are increasingly being produced by powder
 metallurgy (PM) method. The PM method is highly efficient in cost of production and
 materials usage. However, PM components suffer from inhomogeneous density
 variation and are more likely to have internal cracks. These deficiencies usually make
 PM components prone to sudden fracture failure. A parameter that is known to define
 the rate at which cracks grow in a material is fracture toughness. Unfortunately, the
 fracture toughness of most metal powder compacts have not been determined due to
 lack of suitable test technique. This study developed a notching device that has the
 capability to provide uniform notches on the surfaces of powder compacts. The
 effectiveness of the notching device enhanced the determination of mode I fracture
 toughness (KIC) of two metal powder compacts; iron and copper. A method known as
 the modified diametrical compression test technique (MDCTT) was also developed to
 measure the mode II fracture toughness (KIIC) of the powder compacts. Finally, the
 study examined the influence of density on the rate of crack propagation in the
 compacts and developed mathematical relation that predicts fracture toughness from
 the relative density of either the iron or copper powder compacts. Notched samples of
 two types of metal powder; Hoaganas ASC100.29 iron powder and pure copper powder
 were prepared by uniaxial compaction in a rigid die using universal testing machine.
 The relative density of the powder compacts was determined as a fraction of the
 density of the compact to their corresponding solid metal before the diametrical
 compression test was carried out for each sample. The behavior of the cracks around
 the tip of the notch was examined using scanning electron microscope (SEM). A new
 equation was developed to calculate the values of KIIC from the MDCTT. The results of
 KIC for the iron powder compacts showed close agreement with values mentioned in
 the literature. The KIC values for copper powder compacts range from 0.32 to 0.58
 MPa.m0.5 while the KIIC for the iron and copper powder compacts ranged from 0.30 to
 0.57 MPa.m0.5 and 0.28 to 0.59 MPa.m0.5 respectively. The ratio KIIC/KIC for the iron
 and copper powder compacts from this study showed good agreement with the
 predicted values of 0.87 and 1.04 based on the maximum tangential stress (MTS) and
 the minimum strain energy density (SED) criteria respectively. The agreement implies that the developed MDCTT is reliable and can be used to measure the KIIC of other
 metal powder compacts. Furthermore, the results also show that the rate of crack
 extension reduced as the density of the powder compacts increases. A generalized
 mathematical expression that relates fracture toughness and relative density has been
 successfully developed. This relationship will be beneficial for further analysis of crack
 propagation within metal powder compact.

Fracture toughness of iron and copper powder compacts using modified diametrical compression test technique

Fracture toughness characterization of polymers-based composites using essential work of fracture method

The use of high-density polyethylene pipes in gas and water distribution networks is steadily growing worldwide. If the resistance of plain pipes is at present time well established using appropriately designed standards, welding issues continue to be globally approached equally in terms of structure and mechanical properties. Consequently, further practical investigations should be aimed at studying mechanical properties in the weld region which includes the melt zone and its heat-affected zones. This work presents a method based on removing layers in order to assess localized variances in mechanical properties throughout the weld seam in both radial and circumferential directions. An experimental plan based on specific machining operations allowed testing 39 standard specimens representing the weld volume matter in three concentric layers for given pipe dimensions and their counterpart standard unwelded ones. The typical stress–strain behavior of semi-crystalline materials is preserved in welded and unwelded specimens but with different characteristic limits. At the weld inner layers, properties such as elastic modulus, yield, and failure stresses displayed lower values, whereas in welded outer layers, the tendency is inversed. The cold drawing extend remained approximately steady for unwelded and welded cases across the pipe wall. This property is less affected by the presence of the weld as it described a constant material flow which is mostly a function of available material quantity for yielding. The approach developed in this study gives consistent indications on welding quality around the pipe weld and across the thickness. Accordingly, outermost and innermost welded layers may exhibit lower or even bad-quality welds as imperfections can concentrate stresses at the joint interface because of cold weld problems. Such method enabled detecting 23% of failures at the weld seam from outer and inner layers while the middle layer did not reveal any failure at the weld. The causes of this behavior are approached using crystallinity evolution in welded and unwelded pipes.

A method for mechanical property assessment across butt fusion welded polyethylene pipes

The impact fracture behavior of a high-density polyethylene (HDPE) material is investigated experimentally and theoretically Single-edge notched bending (SENB) specimens are tested in experiments with three-point bending and in the Charpy impact tests An energy model is proposed for evaluating the HDPE impact toughness, which provides a description of both brittle and ductile fracture

Impact fracture toughness evaluation for high-density polyethylene materials

Study of void growth and coalescence in HDPE material: a numerical investigation using computational voided cells

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Impact fracture toughness evaluation for high-density polyethylene materials

Study of void growth and coalescence in HDPE material: a numerical investigation using computational voided cells