Initial Damage Threshold Research Articles

Bioinspired hybrid helical structure in lobster Homarus Americanus: Enhancing penetration resistance and protective performance

The high impact and penetration resistance of the cuticle in the lobster Homarus americanus can be attributed to its unique twisted plywood structure. Hybridization of two helical structures was observed in the exoskeleton, with different pitch angles and ply thickness of the exocuticle and endocuticle within the exoskeleton. Given the survival challenges posed by the natural environment, it is worthwhile to study why it evolved into a hybrid helical rather than a uniform helical structure. In light of this, we have proposed an innovative bionic hybrid helical structure design strategy that goes beyond simply reducing the pitch angle. In this paper, we replicated complex biological microstructures by introducing two types of pitch angles and ply thicknesses for regional configurations. Quasi Static Indentation (QSI) full-penetration experiments were conducted. Experimental results revealed that the exocuticle with a large pitch angle and small ply thickness possesses a higher knee load under the penetration test, which provides a higher initial damage threshold. On the other hand, the endocuticle with a small pitch angle and large ply thickness induces higher damage dissipation, which provides superior protective performance. These research findings demonstrate that ply hybridization can be a promising method for microstructure design to improve the penetration resistance of thin laminates and serve as a reference for developing more refined gradient helical structure designs.

Effects of initial damage on self-healing of fly ash-based engineered geopolymer composites (FA-EGC)

The effect of initial damage levels on self-healing properties and the mechanism of fly ash-based engineered geopolymer composites (FA-EGC) was investigated. FA-EGC paste with an ultimate tensile strain exceeding 5.4% was prepared using fly ash, composite alkali activator, and PVA fibers. The FA-EGC specimens were pre-damaged by 1.5%, 2.0%, and 2.5% tensile strains, then self-healed for 7, 14, and 28 days. The self-healing properties of FA-EGC were studied by uniaxial tension, apparent crack characteristic, water absorption, and ultrasonic pulse velocity methods. The self-healing products were characterized by XRD, FTIR, BET, and SEM-EDS. The model of the relationship between initial damage levels and initial internal crack characteristics was built. The results showed that FA-EGC had significant multiple cracking characteristics, strain-hardening behavior, and ultra-high tensile ductility after 28 days of self-healing. After self-healing, the ultimate tensile strength of pre-damage FA-EGC recovered to 97.86% compared with the control specimen, and the ultimate tensile strain recovered to 94.50%. Moreover, the strain of ultra-high tensile ductility FA-EGC was up to 5%. Self-healing products were mainly amorphous aluminosilicate and calcium carbonate phases. The self-healing rate was determined by initial internal crack width according to the model, and FA-EGC had an initial damage threshold (about 2.0% tensile strain) with the lowest mean crack width and the highest self-healing rate.

Meloidogyne Haplanaria: an Emerging Threat to Tomato Production in Florida

The Mi-gene is widely used in different tomato cultivars to resist several Meloidogyne spp. (root-kot nematode; RKN), including M. incognita, M. javanica, and M. arenaria. Tomato cultivars with the Mi-gene are widely used in fields. However, factors such as temperatures, high initial population densities, and gene dosage can interfere with the expression of this gene. In addition, the presence of virulent species of RKN can limit the usefulness of the gene. One of the virulent species is M. haplanaria, which was identified infecting RKN-resistant tomato in Florida in 2015. The objectives of this study were to determine the initial damage threshold of M. haplanaria on tomato under greenhouse conditions and to analyze the impact of temperature and genetic background on virulence in tomato cultivars. The results showed a preliminary damage threshold of three eggs and J2/cm3 of soil. In addition, it was observed that M. haplanaria has a shorter life cycle than the virulent M. enterolobii and can infect, reproduce, and damage homozygous or heterozygous RKN-resistant tomato plants. This research demonstrated that M. haplanaria should be considered highly virulent on RKN-resistant tomato and is an important threat to agriculture in Florida.

Experimental study of low-velocity impact behavior and damage characteristics of flat-knitted spacer fabrics reinforced composites

Flat-knitted spacer fabrics are three-dimensional fabrics with good formability and impact resistance, which can be rapidly produced complicated shape preforms. In this work, low-velocity impact behavior and damage characteristics of flat-knitted spacer fabrics reinforced composites were studied and the mechanism of the effects of spacer yarn structure on the impact properties of the composites were explored. Flat-knitted spacer fabrics with three spacer structures were produced using ultrahigh molecular weight polyethylene (UHMWPE) filaments, and composite plates were prepared by vacuum-assisted resin infusion. Low-velocity impact tests were carried out using a drop-weight impact testing machine equipped with a hemispherical impactor. It was revealed that the impact process of the flat-knitted spacer fabrics reinforced composites could be divided into three areas. The effect of initial impact energy on the impact properties of the sample was mainly reflected in the damage speed and initial damage threshold in the first areas. With the increase of spacer yarn span, the damage tolerance, stiffness, and impact resistance of the composite were also increased. Besides, the main damage mechanism of flat-knitted spacer fabric reinforced composites was plastic deformation, matrix fracture, fiber-matrix debonding, and tow splitting.

Improving the crack propagation response of layered and bonded structures through dissipation mechanisms at different length scales

The wide use of layered and bonded parts in structural applications is limited by the brittle crack propagation response of the weak interface zones. A vast body of literature has focused on the improvement of the bond properties, often focusing exclusively on the increase of fracture toughness and without a detailed definition of the design requirements in terms of macroscopic interface properties or structural response. In this work, macroscopic interface properties are defined in terms of Cohesive Zone Models (CZM), and the capital role of the cohesive length in determining the more or less gradual nature of the damage and crack propagation behaviour at the structural level is highlighted through examples on a Double Cantilever Beam (DCB) test. A concept based on a two-mechanisms CZM response with very different cohesive lengths is proposed to ensure a gradual crack propagation behaviour at the structural level, without significantly decreasing the initial damage threshold. A possible strategy to obtain such a macroscale interface response, based on the creation of bridging ligaments, is analysed and the link between the ligament elastic and failure response and the macroscopic interface response are discussed.

The effect of carbon nanofibers on transverse cracking in carbon fiber reinforced polymer: A 3D finite element modeling and simulation

The influence of carbon nanofibers (CNFs) on carbon fiber (CF) reinforced polymer (CFRP) is investigated in this study. A program is developed to generate the finite element model (FEM) for the nanocomposites. The randomness of both CNFs and CF in CFRP is considered. The effects of volume fraction of CNFs and CFs, as well as interfacial strength are analyzed numerically. The improvement in strength and toughness of composites under different loading conditions is investigated. It shows that CNFs lead the redistribution of stress in the matrix, which releases stress concentration between CFs and polymer and increases the initial damage threshold.

A variational approach for a nonlinear one-dimensional damage-elasto-plastic second-gradient continuum model

A one-dimensional displacement second-gradient damage continuum theory has been already presented within the framework of a variational approach. Damage is associated with strain concentration. Therefore, not only non-local effects of displacement second-gradient modelling should be considered in a comprehensive model, but also any plastic effects. The aim of this paper is therefore to extend such a model to plasticity. The action is intended to depend not only with respect to first and second gradient of displacement field and to a scalar damage field, but also to further two internal variables, i.e. the accumulated plastic tension and the accumulated plastic compression. A constitutive prescription on the stiffness is given in terms of the scalar damage parameter in a usual way, i.e. as in many other works, it is prescribed to decrease as far as the damage increases. On the other hand, the microstructural material length (i.e. the square of the constitutive function in front of the squared displacement second-gradient term in the action functional) is prescribed to increase as far as the damage increases, being this last assumption connected to the interpretation that a damage state induces a microstructure in the continuum and that such a microstructure is more important as far as the damage increases. Initial damage threshold and yield stresses are naturally introduced in the action in front of linear terms, respectively, of damage and plastic internal variables. The hardening matrix is also introduced in a natural way as the coefficient matrix in front of the quadratic terms of the two plastic internal variables. At a given value of damage and plastic parameters, the behaviour is referred to second-gradient linear elastic material. However, the damage and plastic evolutions make the model not only nonlinear, but also inelastic. The second principle of thermodynamics is considered by assuming that the scalar damage and plastic parameters do not decrease their values in the process of deformation, and this implies a dissipation for the elastic strain energy. A novel result of this investigation, where displacement second-gradient and plastic effects are combined, is that the distributed and concentrated external double forces do not make work on the displacement gradient but only to its elastic part and this means that the displacement gradient cannot be prescribed, at the border, independently of the plastic internal variables.

Probabilistic optimization of a continuum mechanics model to predict differential stress-induced damage in claystone

Phenomenological modeling of anisotropic damage in rock raises many fundamental thermodynamic and mechanical issues. In this paper, the maximum likelihood method is used to analyze the performance of the Differential Stress Induced Damage (DSID) model recently formulated by Xu and Arson [1]. The stress/strain relationship is a nonlinear function of parameters including unknown constants (i.e., damage constitutive parameters) and known variables (e.g., elastic parameters and controlled stress state). Logarithmic transformation, normalization and forward deletion are employed, in order to find the optimum number of constitutive parameters, as a trade off between accuracy and simplicity. For Eastern France claystone subject to deviatoric stress loading (e.g., triaxial and proportional compression loading), it is found that (1) only one damage parameter (a2) is needed in the expression of the free energy to predict stress/strain curves; (2) a2 controls the deviation of the current principal directions of stress to the principal directions of damage; (3) model parameters involved in the damage criterion can be related to a2. As a result, a2 is the only parameter needed to model differential-stress induced damage in Eastern France claystone. It is also shown that within the set of assumptions made in this study, the DSID model is not sensitive to the initial damage threshold C0, except for C0>106Pa, a range of values in which only one constitutive parameter becomes insufficient to predict the stress/strain curves of damaged claystone. Coupling probabilistic calibration and optimization methods to numerical codes promises to allow adapting the complexity of anisotropic damage models to different rocks and stress paths.

Inductively Coupled Plasma Modified Silica Material Technology Research

The using inductively coupled plasma etching techniques, the surface of fused silica was etched by CF4 as etching reaction gases. The results show that the fused silica surface scratches buried defects can be effectively removed with no replication effect, with the etching process proceeds, fused silica at 355nm laser light R: 1 under irradiated, the initial damage threshold of component surface is worth to an effective improved, damage resistance level of element surface more balanced, low damage threshold is worth to significantly improve.

Experimental comparison of damage performance induced by nanosecond 1 laser between K9 and fused silica optics

The initial damage threshold, the damage growth threshold and the damage growth laws of K9 glass and fused silica glass are tested respectively in the same experimental condition with 1 nanosecond laser. The comparison of the laser induced damage performance between the two glass materials is performed. The results show that the initial damage thresholds of the two glass materials are the same, the damage area growths both obey the exponential increase rule and their damage depth growths obey the linear increase rule. However, there is apparent difference in the rule of damage growth. For example, the damage growth threshold is lower and the damage growth coefficient is higher for K9 glass. This can be explained by the difference in the material mechanical strength between the two glass materials. The present research is very important for choosing transparent optical materials used in high power laser.

Influence of laser passivation on fused silica damage after CO2 laser mitigation

A 10.6 m CO2 big size laser beam is used to passivate the ablation deposit on surface of damage site which is repaired with single laser pulse. When the irradiated region is placed on front surface, modulation by 355 nm UV ns laser pulse is inhibited effectively; when the irradiated region is situated on rear surface, the initial damage threshold exceeds the substrate initial damage threshold. Second repair makes the thermal stress distribution enlarged obviously and the thermal stress magnitude weakened. The damage growth examination shows that once thermal stress is released, the initial phase damage growth exhibits an exponential tendency and as shoot number increases the damage size approaches to a constant value.

The Tip Field of Mode III Crack in Damage Material

The research of crack-tip field in damage materials is a very complex problem. The core question is how to give the damage constitutive equation and determine the crack tip singularity order of magnitude of the physical quantities. Taking into account the damage process is a process which the energy (strain) accumulates, and the initial damage threshold value existence, on the Lemaitre’s research foundation, the brittle material damage constitutive equation that has the initial damage quantity and the power order relationship with strain is given. Under the assumption that stress (strain) is finite, the construction of III mode crack tip damage fields is studied and the solution of crack-tip stress (strain) asymptotic field is given. Finally, the relationship of the initial crack tip damage zone and full damage zone Scale and material parameters are discusses. These works provide theoretical basis for the material engineering design and the appraisal structure reliability.

Effect of thermal stresses on fused silica surface on the laser induced damage

To study the effect of the thermal stresses on the optics, experiment is performed to measure the damage threshold of the two fused silica samples with one annealed to eliminate thermal stresses while the other not. The measurement includes the effects of the thermal stresses on the initial damage threshold of the fused silica, the damage growth threshold, and the damage growth laws. The results show that the damage threshold decreases as the thermal stresses increase, and the damage induced by the laser is accelerated by the thermal stresses. For the same laser intensity, the damage growth factor is higher for the area with higher thermal stresses, but the damage growth still obeys the exponential increase rule and no obvious effect of the thermal stresses is found on the damage growth threshold. The study here will determine to some extent whether the technology of the CO2 laser pretreatment can be applied to the large-aperture optics or not.

Stochastic approaches for damage evolution in standard and non-standard continua

Damage evolution in quasi-brittle materials is a complex process in which heterogeneity plays an important role. This heterogeneity may imply that the exact failure mode can be highly dependent upon the precise spatial distribution of initial imperfections. To model this inhomogeneity stochastic distributions of material properties must be used in numerical simulations. However, the use of a stochastic approach does not resolve the issue of the change of character of the governing differential equations during progressive damage. To avoid such a change of character higher order terms, either in space or in time, must be added to the standard continuum description (regularization techniques). A simulation technique that describes the failure process properly must incorporate both a regularization technique and a stochastic description of the disordered continuum. This statement will be substantiated here by presenting finite element analyses of direct tension tests with a standard local damage model and with a nonlocal damage model. The randomness in the damage process will be introduced by considering the initial damage threshold of the continuum damage model as a random field, characterized by a relevant distribution and autocorrelation coefficient function. The response statistics calculated by the Monte-Carlo technique will be presented for two different levels of finite element discretization. The nonlocal and random field formulations both rely on the introduction of a length parameter: the internal length scale in case of the nonlocal continuum and the correlation length for the random field. The effect of the relative variation of the correlation length and the internal length scale will also be discussed.

PREDICTION OF THE FAILURE AND SIZE EFFECT IN CONCRETE VIA A BI‐SCALE DAMAGE APPROACH

Simple but also accurate models are needed to predict the failure response of concrete structures. Simplicity involves modelling assumptions while accuracy involves objectivity of both the experimentally identified model parameters and the numerica results. For concrete‐like heterogeneous and brittle materials, the modelling assumptions idealizing the material as a homogeneous continuum with classical linear or non‐linear behaviour, leads to some problems at the identification stage, namely the size effect phenomena. A continuum damage model, representing the non‐linear behaviour due to microcracking, is proposed here for predictive computations of structural responses. A Weibull based theory is used to determine, in a statistical sense, the value of the initial damage threshold. The essential influence of material heterogeneity on the damage evolution, is accounted for with a bi‐scale approach which is based on the idea of the non‐local continuum with local strain. It has already established that the non‐local approaches yield realistic failure predictions and the numerical results are convergent for subsequent mesh refinements. The applications presented here show the ability of the approach to predict the failure response of concrete structures without being obscured by size effect problems.