Three-dimensional numerical and experimental study of fracture saturation in panel paintings

Panel paintings—complex multi-layer structures consisting of wood support and a paint layer composed of a preparatory layer of gesso, paints, and varnishes—are among the category of cultural objects most vulnerable to relative humidity fluctuations and frequently found in museum collections. The current environmental specifications in museums have been derived using the criterion of crack initiation in an undamaged, usually new gesso layer laid on wood. In reality, historical paintings exhibit complex crack patterns called craquelures. The present paper analyses the structural response of a paint layer with a virtual network of rectangular cracks under environmental loadings using a three-dimensional model of a panel painting. Two modes of loading are considered—one induced by one-dimensional moisture response of wood support, termed the tangential loading, and the other isotropic induced by drying shrinkage of the gesso layer. The superposition of the two modes is also analysed. The modelling showed that minimum distances between cracks parallel to the wood grain depended on the gesso stiffness under the tangential loading. Despite a nonzero Poisson’s ratio, gesso cracks perpendicular to the wood grain could not be generated by the moisture response of the wood support. The isotropic drying shrinkage of gesso produced cracks that were almost evenly spaced in both directions. The modelling results were cross-checked with crack patterns obtained on a mock-up of a panel painting exposed to several extreme environmental variations in an environmental chamber.

EPFM Analyses of a Complex Crack in Dissimilar Metal Welded Pipes

Accurate estimation of elastic-plastic fracture mechanics (EPFM) parameters for a crack in nuclear pipes was considered as an important factor for leak-before-break (LBB) design and evaluation. Yet few EPFM studies have been made to predict the crack opening displacement (COD) and J-integral of dissimilar metal welded pipes, which consist of two-layered materials (TLMs), due to the difficulty of complicate analysis encompassing both through-wall crack and internal surface crack in radial and circumferential directions. In this study, a series of finite element (FE) analyses to determine the typical EPFM parameters were carried out considering idealized complex-cracked pipes with TLMs. The analyses were elaborated through applying three loading conditions of axial tension, bending moment and internal pressure. Both J-integral and COD values were calculated by assuming two kinds of equivalent materials based on weighted average concepts as well as two different materials. The proposed equivalent schemes can be utilized in not only improvement to existing solutions but also more accurate detailed LBB assessment of complex cracked nuclear piping with TLMs.

A Modified Phase-Field Fracture Model for Simulation of Mixed Mode Brittle Fractures and Compressive Cracks in Porous Rock

In this work, we propose a modified phase-field model for simulating the evolution of mixed mode fractures and compressive driven fractures in porous artificial rocks. For the purpose of validation, the behaviour of artificial rock samples, with either a single or double saw cuts, under uniaxial plane strain compression has been numerically simulated. The simulated results are compared to experimental data, both qualitatively and quantitatively. It is shown that the proposed model is able to capture the commonly observed propagation pattern of wing cracks emergence followed by secondary cracks driven by compressive stresses. Additionally, the typical types of complex crack patterns observed in experimental tests are successfully reproduced, as well as the critical loads.

Experimental Study on Compression Failure of Composite Laminates with Prefabricated Surface Cracks

A new compression test fixture was designed in the present work to study the damage tolerance of composite laminates with surface cracks or notches. The compression failure behaviors of CCF300/5228A quasi-isotropic composite laminates with prefabricated surface cracks were studied experimentally. Through the size design of the test fixture and specimens and an application of a simple test method, the complex crack growth process was captured. The experimental results showed that the compression failure modes were mainly affected by crack angles and depths, and there were two typical failure modes, which were local intra- and inter-laminar damage propagating from the crack tips and delamination growth induced from the crack leading edge. This study verified the validity of the test fixture and test method, and revealed the compression failure mechanisms of composite laminates with surface cracks.

A Unified Abaqus Implementation of the Phase Field Fracture Method Using Only a User Material Subroutine

We present a simple and robust implementation of the phase field fracture method in Abaqus. Unlike previous works, only a user material (UMAT) subroutine is used. This is achieved by exploiting the analogy between the phase field balance equation and heat transfer, which avoids the need for a user element mesh and enables taking advantage of Abaqus’ in-built features. A unified theoretical framework and its implementation are presented, suitable for any arbitrary choice of crack density function and fracture driving force. Specifically, the framework is exemplified with the so-called AT1, AT2 and phase field-cohesive zone models (PF-CZM). Both staggered and monolithic solution schemes are handled. We demonstrate the potential and robustness of this new implementation by addressing several paradigmatic 2D and 3D boundary value problems. The numerical examples show how the current implementation can be used to reproduce numerical and experimental results from the literature, and efficiently capture advanced features such as complex crack trajectories, crack nucleation from arbitrary sites and contact problems. The code developed is made freely available.

Quantification of the Complex Crack Geometry Effect On Fracture Resistance Using Strain Based FE Damage Analysis

This paper examines the effect of complex crack geometry on the J-resistance curves obtained by strain-based ductile tearing simulation of complex cracked tension (CC(T)) specimens. The damage model is determined by analyzing the results of a smooth bar tensile test and a C(T) specimen toughness test on an SA508 Gr.1a low-alloy steel at 316 ?. The validity of the damage model and simulation method is checked by comparing the fracture test data for two CC(T) specimen tests. To investigate the effect of the complex crack geometry on the crack growth profiles and J-resistance curves, two geometric parameters (namely, the through-wall crack length and the surface crack depth) are systematically varied. It is found that the J-resistance curves for the CC(T) specimens with various through-wall crack lengths and surface crack depths are consistently lower than the corresponding 1T C(T) J-resistance curves. The effect of the through-wall crack length upon the J-resistance curve is found to be less significant than that of the surface crack depth. Moreover, the J-resistance curve decreases continuously with increasing surface crack depth.

Variable-order fracture mechanics and its application to dynamic fracture

This study presents the formulation, the numerical solution, and the validation of a theoretical framework based on the concept of variable-order mechanics and capable of modeling dynamic fracture in brittle and quasi-brittle solids. More specifically, the reformulation of the elastodynamic problem via variable and fractional-order operators enables a unique and extremely powerful approach to model nucleation and propagation of cracks in solids under dynamic loading. The resulting dynamic fracture formulation is fully evolutionary, hence enabling the analysis of complex crack patterns without requiring any a priori assumption on the damage location and the growth path, and without using any algorithm to numerically track the evolving crack surface. The evolutionary nature of the variable-order formalism also prevents the need for additional partial differential equations to predict the evolution of the damage field, hence suggesting a conspicuous reduction in complexity and computational cost. Remarkably, the variable-order formulation is naturally capable of capturing extremely detailed features characteristic of dynamic crack propagation such as crack surface roughening as well as single and multiple branching. The accuracy and robustness of the proposed variable-order formulation are validated by comparing the results of direct numerical simulations with experimental data of typical benchmark problems available in the literature.

Crack morphologies in drying suspension drops

Drops of aqueous suspensions of nanoparticles placed on a substrate form a solid deposit as they dry. The initial concentration of particles governs both the shape of the deposit and the complex crack morphology.