Enhancing the Performance of the T-Peel Test for Thin and Flexible Adhered Laminates

Symmetrically bonded thin and flexible T-peel specimens, when tested on vertical travel machines, can be subject to significant gravitational loading; with the associated asymmetry and mixed-mode failure during peeling. This can cause erroneously high experimental peel forces to be recorded which leads to uncertainty in estimating interfacial fracture toughness and failure mode. To overcome these issues, a mechanical test fixture has been designed for use with vertical test machines, that supports the unpeeled portion of the test specimen and suppresses parasitic loads due to gravity from affecting the peel test. The mechanism, driven by the test machine cross-head, moves at one-half of the velocity of the cross-head such that the unpeeled portion always lies in the plane of the instantaneous center of motion. Several specimens such as bonded polymeric films, laminates, and commercial tapes were tested with and without the fixture, and the importance of the proposed T-peel procedure has been demonstrated

A New Phenomenon: Sub-Tg, Solid-State, Plasticity-Induced Bonding in Polymers

Polymer self-adhesion due to the interdiffusion of macromolecules has been an active area of research for several decades [70, 43, 62, 42, 72, 73, 41]. Here, we report a new phenomenon of sub-Tg, solid-state, plasticity-induced bonding; where amorphous polymeric films were bonded together in a period of time on the order of a second in the solid-state at ambient temperatures nearly 60 K below their glass transition temperature (Tg) by subjecting them to active plastic deformation. Despite the glassy regime, the bulk plastic deformation triggered the requisite molecular mobility of the polymer chains, causing interpenetration across the interfaces held in contact. Quantitative levels of adhesion and the morphologies of the fractured interfaces validated the sub-Tg, plasticity-induced, molecular mobilization causing bonding. No-bonding outcomes (i) during the compression of films in a near hydrostatic setting (which inhibited plastic flow) and (ii) between an 'elastic' and a 'plastic' film further established the explicit role of plastic deformation in this newly reported sub-Tg solid-state bonding

A mathematical analysis of a one dimensional model for dynamic debonding

In this thesis we develop a mathematical analysis for a dynamic model of peeling test in dimension one. In the first part we give existence and uniqueness results for dynamic evolutions. In the second part we study the quasistatic limit of such evolutions, i.e., the limit as inertia tends to zero. In the model the wave equation $u_{tt}-u_{xx}=0$ is coupled with a Griffith's criterion for the propagation of the debonding front. Our first results provide existence and uniqueness for the solution to this coupled problem under different assumptions on the data. This analysis is extended when we study the initiation of the debonding process. We also give an existence and uniqueness result for solutions to the damped wave equation $u_{tt}-u_{xx}+u_t=0$ in a time-dependent domain whose evolution depends on the given debonding front. We then analyse the quasistatic limit without damping. We find that the limit evolution satisfies a stability condition; however, the activation rule in Griffith's (quasistatic) criterion does not hold in general, thus the limit evolution is not rate-independent. This behaviour is due to the oscillations of the kinetic energy and of the presence of an acceleration term in the limit. The same phenomenon is observed even in the case of a singularly perturbed second order equation \eps^2 \ddot{u}_\eps + V_{x}(t,u_\eps(t))=0, where $V(t,x)$ is a potential. We assume that $u_0(t)$ is one of its equilibrium points such that $V_x(t,u_0(t))=0$ and $V_{xx}(t,u_0(t))>0$. We find that, under suitable initial data, the solutions u_\eps converge uniformly to $u_0$, by imposing mild hypotheses on $V$. However, a counterexample shows that such assumptions cannot be weakened. Thus, inertial effects can not, in general, be captured by a pure quasistatic analysis

Blast mitigation and design of laminated glass facades

Laminated glass is an essential component for any building designed against blast loading. A polymer interlayer is sandwiched between glass layers in order to keep the cracked glass fragments intact. In successful design, the interlayer deforms and absorbs the blast load without tearing. A novel testing method was developed to capture the bending response of laminated glass under high rate uniform loading. Specimens with a dimension of 700 mm x 60 mm were tested. It was found that the failure mechanism of the laminated glass is controlled by the thickness of the interlayer. In addition, a new framing arrangement was tested which showed that the load on the laminated glass can be reduced by increasing the energy absorbing capacities within the frame. By allowing the frame to purposefully deform under loading, the chances of failure of the laminated glass are reduced. Tensile tests were conducted on cracked laminated glass in the temperature range of 0◦C−60◦C. Single-cracked and randomly-cracked specimens were tested. It was found that the optimum temperature for the greatest energy absorption of the cracked laminated varies between 10◦C − 40◦C depending on the interlayer thickness. The single-cracked results were further expanded to calculate a bond fracture toughness for the separation of the interlayer from the glass. A finite element model was developed to simulate the bond separation between the glass and the interlayer at different testing-rates and temperatures. It was found that the bond is testing-rate dependent in the range of 0.01 /s − 200 /s, but temperature independent in the range of 20◦C − 60◦C. The single-degree-of-freedom model is conventionally used for predicting the response of a laminated glass pane subject to blast loading. The single-degree-of- freedom model includes mass and load transformation factors, which vary with the deflected shape of the structure. In this thesis a finite element model was used to derive the deflected shapes and transformation factors for a range of loading conditions, boundary conditions and pane aspect ratios. The time-varying deflected shape was taken into account in this analysis, as this is currently not included in other single-degree-of-freedom models. The transformation factors were found to be insensitive to aspect ratio. In addition, at high loading rates, the deflected shape–time history proved to be important.Open Acces

Intermittency and roughening in the failure of brittle heterogeneous materials

Stress enhancement in the vicinity of brittle cracks makes the macro-scale failure properties extremely sensitive to the micro-scale material disorder. Therefore: (i) Fracturing systems often display a jerky dynamics, so-called crackling noise, with seemingly random sudden energy release spanning over a broad range of scales, reminiscent of earthquakes; (ii) Fracture surfaces exhibit roughness at scales much larger than that of material micro-structure. Here, I provide a critical review of experiments and simulations performed in this context, highlighting the existence of universal scaling features, independent of both the material and the loading conditions, reminiscent of critical phenomena. I finally discuss recent stochastic descriptions of crack growth in brittle disordered media that seem to capture qualitatively - and sometimes quantitatively - these scaling features.Comment: 38 pages, invited review for J. Phys. D cluster issue on "Fracture: from the Atomic to the Geophysics Scale

Improvement of cutting tool life for the primary transformation of wood

Le contrôle de l’usure des couteaux de découpe est un grand défi dans l’industrie de la première transformation du bois. En contrôlant le niveau d’usure, les coûts de maintenance sont diminués et une meilleure qualité des produits est obtenue. Les mécanismes d’usure des outils de coupe dans un environnement industriel de transformation du bois sont particulièrement complexes. Dans le présent projet, une caractérisation des principaux mécanismes d’usure a été faite afin de comprendre les conditions de travail en première transformation du bois. Fort de ces résultats, une analyse du choix des matériaux ainsi que différentes conditions de traitement thermique ont été faites afin d‘optimiser la durée de vie des outils de coupe. Trois différents aciers à outil ont ainsi été choisis. Les traitements thermiques ont été effectués selon neuf conditions différentes par acier. Les essais de dureté, d’impact et d’usure par abrasion avec sable (DSRW) selon la norme ASTM G65 ont été effectués. La dernière partie de projet a été axée sur la caractérisation des performances en usure de revêtements céramiques obtenus par la méthode de dépôt physique par phase vapeur (PVD) appropriés pour les conditions d’utilisation retrouvées dans la première transformation du bois. Les essais ont été réalisés sur les échantillons préalablement traités thermiquement selon les conditions optimales déterminées. Six revêtements différents ont été choisis pour faire face aux conditions pour lesquelles la résistance à l’abrasion et aux impacts périodiques sont importantes. Un appareil d’essai de fatigue par impact a été fabriqué pour évaluer la résistance des revêtements aux chocs périodiques sur la surface. De plus, des essais de micro-dureté et de l’usure par abrasion avec sable ont été faits. Afin d’évaluer la condition de la surface après chaque essai, l’analyse d’image par microscopie électronique à balayage (MEB) a été faite. Deux mécanismes dominants qui étaient observés sont la micro abrasion et l’écaillage de l’arête. La micro abrasion sur les surfaces de coupe et de dégagement est différente où il y a un grand écart en grosseur des traces d’abrasion. La résistance optimale pour l’usure par abrasion et impact a été obtenue avec l’acier AISI H13 trempé à 1040°C suivi par un double revenu à 580°C. Le revêtement d’AlCrTiN a été le meilleur pour améliorer la résistance de l’acier AISI A8 à l’usure par abrasion et fatigue d’impact. Pour les aciers AISI S1 et AISI H13, le revêtement de TiAlCrN a fourni la résistance optimale en terme de résistance à l’abrasion qui est la première cause d’usure des outils de coupe utilisés dans le procédé d’équarrissage-fragmentation.Controlling cutting knife wear is a big challenge in industry of primary transformation of wood. By controlling the knife wear, maintenance cost will be reduced and higher quality of products will be achieved. Wear conditions in the wood transformation industry is very intricate. In this project, wear study was performed in order to understand the work condition in primary transformation of wood. Microscopic analysis was conducted to investigate wear mechanisms on the edge of knives. Wear properties on both rake face and clearance face were studied. Material selection and heat treatment practices were done in the second part of the project in order to optimize the tool life. Three steels from different tool steel categories were selected. Heat treatment tests were carried in nine different conditions per steel. Hardness, impact and dry-sand-rubber-abrasion-wear according to ASTM G65 standard (DSRW) tests were applied for evaluating the samples. Last part of this project was about evaluating of industrial coatings by physical vapour deposition (PVD) method for the working condition in primary transformation of wood. Tests were done on optimized samples from heat treatment study. Six different coatings were selected in order to perform high resistance to conditions where both abrasion and periodic impact are present. Impact Fatigue test machine was fabricated in order to evaluate the resistance of coatings to working conditions of periodic shock on the surface. Micro-hardness and DSRW test were done as well. In order to evaluate the surface change after each experiment, SEM imaging was done. The two dominant mechanisms that were observed are micro abrasion and edge chipping. The micro-abrasion patterns found on rake and clearance surfaces were dissimilar and there was a significant difference in size and shape of abrasion marks. The optimum resistance to abrasion wear and impact was achieved for AISI H13 tool steel quenched from 1040°C followed by double-tempering at 580°C. AlCrTiN coating was the best coating to improve the resistance of AISI A8 tool steel to both abrasive and impact fatigue wear. For both AISI S1 and AISI H13 steel, TiAlCrN coating has performed optimum resistance to abrasive working condition which is major cause of tool wear in chipper-cantering

Numerical investigation of dynamic brittle fracture via gradient damage models

International audienceBackground: Gradient damage models can be acknowledged as unified framework of dynamic brittle fracture. As a phase-field approach to fracture, they are gaining popularity over the last few years in the computational mechanics community. This paper concentrates on a better understanding of these models. We will highlight their properties during the initiation and propagation phases of defect evolution.Methods: The variational ingredients of the dynamic gradient damage model are recalled. Temporal discretization based on the Newmark-β scheme is performed. Several energy release rates in gradient damage models are introduced to bridge the link from damage to fracture.Results and discussion: An antiplane tearing numerical experiment is considered. It is found that the phase-field crack tip is governed by the asymptotic Griffith's law. In absence of unstable crack propagation, the dynamic gradient damage model converges to the quasi-static one. The defect evolution is in quantitative accordance with the linear elastic fracture mechanics predictions.Conclusion: These numerical experiments provide a justification of the dynamic gradient damage model along with its current implementation, when it is used as a phase-field model for complex real-world dynamic fracture problems

Fracture processes in cortical bone: effect of microstructure

Understanding of bone fracture can improve medical and surgical procedures. Therefore, investigation of the effect of bone’s microstructure and properties as well as loading conditions on crack initiation and propagation is of great importance. In this paper, several modelling approaches are used to study fracture of cortical bone tissue at various length scales and different types of loading. Two major problems are tackled: crack propagation under impact loading and bone cutting in surgical procedures. In the former case, a micro-scale finite-element (FE) fracture model was suggested, accounting for bone’s microstructure and using X-FEM for crack-propagation analysis [1, 2]. The cortical bone tissue was modelled as four-component heterogeneous materials. The morphology of a transverse-radial cross section captured with optical microscopy was used to generate FE models; extensive experimental studies provided necessary mechanical input data [3]. The problem of bone cutting was treated within the framework of tool-bone interaction analysis [4, 5]. A two-domain approach was used, with a process zone simulated using a smooth-particle hydrodynamics method. This zone was embedded in a continuum domain with macroscopic anisotropic properties obtained in experiments. This study is supported by analysis of damage induced by interaction between the cutting tool and the bone tissue using wedge-indentation tests and considering also the anisotropic behaviour of the bone

Study of the mechanical and metallurgical properties of AMDRY 9954 HVOF coated Ti-6A1-4V alloy

Ti-6A1-4V alloy is commonly used in gas turbines due to its excellent tensile and fatigue strength, corrosion resistance, and high toughness to mass ratio. In the present study, the metallurgical and mechanical properties of High Velocity Oxygen Fuel (HVOF) thermally sprayed AMDRY 9954 (Cobai.Ni32Cr21Al8Yo.5) superalloy powder on Ti-6A1-4V alloy are examined. The mechanical tests include three point bending, tensile, fatigue, indentation, and microhardness tests. The mechanical tests are applied on Ti-6A1-4V specimens (a) asreceived, (b) as-received heat treated, (c) coated then heat treated and (d) coated without heat treatment. Three-point bending tests are carried out to investigate the coating-base material interface properties and the influence of heat treatment on the interface properties. Tensile tests are performed to evaluate the adhesion of the thermally sprayed coating to Ti- 6A1-4V alloy. The fatigue tests are conducted to study the fatigue resistance behavior of the coated substrate martial under fluctuating load. Finite element method (FEM) is introduced to simulate the bending and tensile testing situations and predict the stress distribution in the workpieces. In addition, the microhardness and the indentation tests are carried out to measure the hardness and estimate the plane fracture toughness of the coating, respectively. The metallurgical characterization and surface morphology prior and after mechanical testing are investigated using SEM, optical microscopy, EDS, and XRD. It is found that heat treatment modifies the elastic modulus of the coating; in addition, tensile and fatigue performance of the specimens subjected to the heat treatment is low