Measurements of fibre orientation in short-fibre-reinforced thermoplastics

A technique is described for accurately measuring the spatial variation of fibre-orientation-distribution averages in injection-moulded plaques of short-fibre-reinforced carbon-fibre/PEEK composites. It is based on measurements of the ellipticity and orientation of the elliptical images of circular fibres meeting a polished surface of a single section through the plaque when viewed in reflected light. Automatic image analysis of the approximately 10 000 fibre images per section was carried out on the COSMOS facility at the Royal Observatory, Edinburgh.

Details are provided of the estimated uncertainties of these orientation averages in order to establish that the technique has sufficient accuracy and resolution to characterise quantitively the skin-core-skin structures expected for such materials. These and additional data will be used in a subsequent paper to explain, qualitatively and quantitatively, the variation in the fracture toughness of the material as a function of position.     

Micromechanical interpretation of fracture toughness of particulate-filled thermoplastics

The toughness behaviour of particulate-filled thermoplastics is determined by different failure mechanisms in the plastic zone and fracture process zone in front of the macrocrack such as particle-matrix debonding, shear processes or crazing and fracture of matrix fibrils. Theoretical expressions describing the critical strain causing microcrack initiation as well as the critical crack opening and the criticalJ integral value for unstable crack initiation are derived on the basis of a micromechanical analysis. Matrix properties, particle diameter, filler content and phase adhesion are taken into account. Critical particle contents and diameters caused by matrix morphology are discussed. Model calculations are compared with experimental results from acoustic emission analysis and dynamic fracture mechanics tests on PS, PVC and HDPE filled with CaCO₃ or SiO₂ particles.     

Prediction of ductile fracture initiation using micromechanical analysis

In the paper ductile fracture initiation analysis of low-alloyed ferritic steel has been made by application of two micromechanical models: the Rice–Tracey void growth model and the Gurson–Tvergaard–Needleman (GTN) model. The aim of the study was to analyse transferability of micromechanical parameters determined on specimens without initial crack to pre-cracked specimens. A significant part of the research has been carried out through participation in the round robin project organised by the European Structural Integrity Society (ESIS). Tensile tests have been performed on cylindrical smooth specimens and CT specimens. Critical values of micromechanical parameters determined on smooth specimen for both applied models, have been used for prediction of the crack growth initiation in CT specimen. Modelling of the first phase of ductile fracture––void nucleation––has been carried out using quantitative metallographic analysis of non-metallic inclusion content in tested steel. For determination of critical values of model parameters corresponding to ductile fracture initiation a simple procedure has been applied based on a combination of experimental and numerical results. Evaluated J-integral values corresponding to onset of crack growth, J_i, are in good agreement with experimental result and both models have proved to be suitable for determination of the ductile fracture initiation in tested steel. The effect of FE size at a crack tip on J_i-value has been particularly analysed: it has been established that the calculation with FE size corresponding to the mean free path λ between inclusions in steel gives results that are in accordance with the experimental ones.     

On the post-mortem fracture surface morphology of short fiber reinforced thermoplastics

The fracture surface morphology of short fiber reinforced thermoplastics (SFRTs) has often been used to assess qualitatively the degree of fiber–matrix interfacial adhesion. Mechanical properties such as tensile strength, fracture toughness and failure strain, etc. are then correlated with the morphology. Fracture surfaces showing fibers surrounded by a large amount of matrix material is commonly regarded as indication of strong fiber–matrix interfacial adhesion while smooth fibers are characteristic of weak interfacial adhesion. Many experimental results of SFRTs have been so interpreted. However, it is shown in this paper that strictly speaking, such interpretations are generally incorrect. Moreover, the amount of matrix material does not provide a quantitative measure of the adhesion. Correct implication of the morphology of fracture surfaces is clarified. Short glass fiber reinforced polyamide 6,6/polypropylene (PA 6,6/PP) blends toughened by rubber are employed as examples for SFRTs since the PA 6,6/PP blend system by changing PA 6,6 concentration in the matrix blend represents a wide range of matrix materials. It is demonstrated that the fracture surface morphology of such composites is dependent on both fiber–matrix interfacial adhesion strength and matrix shear yield strength. Consequently, tensile failure strain is well correlated with the post-mortem fracture surface morphology of these SFRTs.     

Fast fracture of rubber-toughened thermoplastics used for the shells of motorcycle helmets

The effects of outdoor ageing and of accelerated UV exposure on the grade of acrylonitrile butadiene styrene copolymer (ABS) used by UK manufacturers of motorcycle helmets were compared. Although the surface layer becomes embrittled the helmets still survive an impact test after conditioning at -20° C because the cracks are arrested before they penetrate the full thickness of the shell. A 4 mm thick rubber-toughened polycarbonate, used in helmets from continental suppliers, shows a transition from tough to brittle crack propagation behaviour at -15° C. For 4 mm thick ABS this transition had not occurred at the lowest test temperature of -20° C, but if 6 mm sheet is tested the transition is at -15° C. Temperature increases in the 40 sec delay between conditioning a helmet at -20° C and impact testing are shown to be significant.     

Tewksbury Lecture: Putting fracture to work

It is emphasized that the science and crafts associated with the parting of solids are of considerable industrial, cultural and historical interest. Some principles of the modern theory of fracture which may be relevant to the controlled separation of a solid into pieces are reviewed. The use of path independent integrals in the analysis of indentation fracture is discussed, and some of the subtleties involved in treating the motion, deviation and forking of cracks and the energy balance in crushing and shattering are considered. The paper concludes with a brief account of some recent work on the theory of flint knapping and of the influence of the environment on the fracture process.     

A micromechanical model for the prediction of the temperature fracture behaviour dependence in metallic alloys

In the present paper, a micro-mechanical model based on energetic considerations is developed to simulate the effect of environmental temperature on the fracture toughness of metallic alloys. By considering a reference elementary volume (REV) with the same composition of the real material, the stress-strain field inside such a volume and the corresponding strain energy due to a temperature variation is determined. The energy balance to determine the material fracture toughness is generalised in order to take into account the temperature effects. The proposed micro-mechanical model is governed by few parameters which can be simply estimated, and allows us to determine the fracture toughness for any temperature below the room temperature. Such a model is applied to three metallic alloys which show a ductile-brittle transition temperature: ASTM A471, Carbon Steel D6ac, Steel S275 J2. From the comparison of theoretical results with experimental data, it can be concluded that the model seems to be able to correctly predict the fracture toughness at low temperatures.     

A micromechanical constitutive model for dynamic damage and fracture of ductile materials

This paper proposes a detailed theoretical analysis of the development of dynamic damage in plate impact experiments for the case of high-purity tantalum. Our micro-mechanical model of damage is based on physical mechanisms (void nucleation and growth). The model is aimed to be general enough to be applied to a variety of ductile materials subjected to high tensile pressure loading. In this respect, the work of Czarnota et al. (J Mech Phys Solids 56:1624–1650, 2008) has been extended by introducing the concept of nucleation law and by entering a nonlinear formulation of the elastic response based on the Mie-Grüneisen equation of state. This later aspect allows us to consider high impact velocities. All model parameters are directly assessed by experimental measurements to the exception of the nucleation law which is characterized by the way of an inverse identification method using three free-surface velocity profiles (at low, intermediate and high impact velocities). It is shown that the nucleation law can be consistently determined in the range of operating pressures. The nucleation law being identified, the development of internal damage happens to be a natural outcome of the modelling. The model is applied to predict damage development and free-surface velocity profiles for various test conditions. The variety and the quality of results support the physical basis (in particular micro-inertia effects) upon which the proposed model of dynamic damage is based.     

Contribution of deformation-induced martensite to fracture appearance of austenitic stainless steel

Fracture surface commonly carries the evidence of high-energy (ductile/tough) and low-energy (brittle) regions involved in fracture history, the macroscopic appearance of a fracture surface has often been utilised to qualitatively evaluate toughness of materials. For metastable austenitic stainless steels (AISI 304LN), the degree of martensitic transformation affects the fracture appearance and thus depends critically on the strain rate. The two dimensional ductile tearing ridge pattern quantified from many tensile fractographs are observed to predict the nature of disparity in deformation and fracture responses with systematic variation in strain rate of the steel under ambient atmosphere. The spatial distribution of deformation-induced martensite under tension at various stress/strains and strain rates strongly influences void nucleation, growth, coalescence and hence, keeps the impression on the ductile tearing ridge morphologies and dimple geometries on the fracture surface, where the initial inclusion content was constant.     

Analysis of the essential work of fracture method as applied to UHMWPE

The validity of the basic assumptions behind the method of essential work of fracture (EWF), as applied to ultra-high molecular weight polyethylene (UHMWPE), is evaluated using finite element modelling. To define a suitable model of constitutive behaviour, the mechanical properties of UHMWPE have been measured in both uniaxial tension and compression over a range of strain rates. The observed strain rate dependence of stress, including the observed differences in strain rate sensitivity between tension and compression, is interpreted in terms of a single Eyring process. The constitutive theory is constructed comprising an Eyring process and hyperelastic networks, the latter having responses symmetric with respect to tension and compression. This theory is implemented within a finite element scheme, and used to model fracture measurements made on the same material using double-edge notch tensile specimens. Calculations of the non-essential work and of the extent of the plastic zones are thus made possible. It is concluded that the specific non-essential work is essentially constant, but that the shape factor β, assumed constant in the conventional analysis, varies significantly with ligament length. The implication of this finding on the derived EWF value is evaluated and found to be slight.     

Acoustic emission interpretation of ductile fracture processes

Detailed analyses of acoustic emission from several material classes have established that the predominant source in tensile testing is particle decohesion and fracture. Acoustic emission is a smoothly varying parameter with plastic strain in 4340 steel where small carbides predominate; however, in an IN 718 superalloy, it is both bimodal and exhibits a burst phenomena where very large carbides and nitrides as well as a medium size laves () phase contribute. From microscopic and acoustic emission observations, it is found that the fracture process is sulfide decohesion followed by void sheet instability associated with carbides in 4340 steel. In IN 718, large carbide or nitride fractures are followed by void sheets associated with laves phase. Identification of the major particle nucleation sites have allowed an initial interpretation of ductile hole growth models. Application of McClintock hole growth and a Hahn and Rosenfield void sheet instability criterion to the ductile fracture process has provided a good correlation to tensile ductilities, plane-strain crack tip ductilities, and plane-strain fracture toughness, K _IC.

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Résumé Une analyse détaillée de l'émission acoustique en provenance de diverses classes de matériaux permet d'établir que la source prédominente d'émission au cours de l'essai de traction est la décohésion des particules et la rupture.L'émission acoustique se présente comme un paramètre variant de manière continue avec la déformation plastique dans le cas de l'acier 4340 où prédominent de petits carbures. Toutefois, dans un superalliage IN 718 elle se présente de manière bi-modale et fait état d'un phénomène de brusque variation lorsque de très gros carbures et nitrures ainsi que des composants de phase delta de dimension moyenne sont présents.A partir d'observations microscopiques et d'observations d'émissions acoustiques, on trouve que le processus de rupture dans l'acier 4340 consiste en une décohésion des sulfures suivie d'une instabilité lacunaire associée au carbure. Dans le cas de l'alliage IN 718, la rupture des gros carbures ou des gros nitrures est suivie de la formation de lacunes en bandes associées avec la phasedelta.L'identification des zones de nucléation principale a permis une interprétation première des modèles de croissance ductile d'une cavité. L'application du modèle de McClintock et d'un critère d'instabilité des lacunes en bande dû à Hahn et Rosenfield, pour décrire le processus de rupture ductile, a fourni une bonne corrélation avec les ductilités à la traction, les ductilités dans des conditions d'état plan de déformation à l'extrémité d'une fissure et de la ténacité à la rupture en état plan de déformation K _IC.

     

The work necessary to form a ductile fracture surface

The relief visible on a ductile fracture surface is the result of severe plastic deformation. The strain energy per unit area necessary to form this relief is estimated. The result is used to compute $\text{K}{}_{\mathrm{Ic}}$ values which are fairly close to those determined experimentally.     

Modified essential work of fracture model for polymer fracture

The well-known essential work of fracture model, describing the linear relationship between fracture energy and un-cracked ligament length, is further extended to allow for any non-linearity in the relationship as this can potentially lead to a large error in determination of the essential work of fracture. Two different polymers, described in the literature, were chosen to show how the essential work of fracture of a polymer can still be determined in cases when a non-linearity exists in the relationship between fracture energy and ligament length. The new model specifies the condition under which the linear relationship exists, and the condition under which the non-linearity needs to be considered.