Composite materials are widely used in demanding applications in aerospace and other industries. In order to understand the complex behaviour of the composite materials and their components, standardised test methods are used. One example is the double cantilever beam (DCB) test in which the test specimen is loaded in an opening, i.e., tensile mode. Failures in composite materials loaded compression are different from those in tension, for example, kink band or buckling-like failures can occur. In this project, several DCBs are designed and a new fixture which allows for compression testing of a DCB is developed for an existing Instron testing machine. The fixture overcomes a known problem of tensile peak causing the failure of the adhesive at the inner surfaces of the DBC by applying additional compressive loads along the outer surfaces of the DBC. The compressive forces can induce the desired kink band formation so that researchers can better study the failure mode. The conceptual development of the new DCBs and the new fixture are presented. Several prototypes of the specimens and the fixture are modelled using the three-dimensional (3D) computer-aided design software Creo Parametric 2.0.  One of the fixtures is selected to further study. The different DCB specimens are studied in order to obtain information about the kink band using 3D finite element analysis with the software programme Abaqus CAE. The selected fixture is analysed to determine if there are any areas of concern. Finally, the behaviour of the compression stress along the DCB using two pairs of forces is studied. Unfortunately, it is determined that the tensile peak experienced by the adhesive cannot be eliminated by the application of two pairs of compressive loads, one at the free end and the other in the vicinity of the tensile peak. Several suggestions are made for future work which might serve to reduce the tensile peak; e.g., the movable force couple is applied as a surface load instead of a point load. For this, the fixture will have to be modified with a new geometry, although the DCB could be the same. This will allow further work to focus on the combined behaviour of the tensile peak and the fixture.

This work describes the development of an improved mode I fracture testing procedure for adhesively-bonded wood, and demonstrates the sensitivity of this approach. The two significant improvements were: 1) the use of the flat double cantilever beam (DCB) geometry, which has been uncommon for wood and 2) the application of an established and powerful data analysis using a corrected compliance method from beam theory. Three studies were conducted using various wood adhesives and DCB specimens were fabricated from yellowpoplar (Liriodendron tulipifera) sapwood. The sensitivity of this methodology showed significant differences in fracture performance as the degree of cure increased for a phenol formaldehyde adhesive, and yielded maximum strain energy release rate (SERR) values of 370 - 560 J/m2. A second study showed performance differences between two polymeric diphenylmethane diisocyanate (pMDI) adhesives and one polyurethane adhesive. Typical maximum SERR values were 160 and 130 J/m2 for the pMDI adhesives and 160 J/m2 for the polyurethane adhesive. A third study investigated the effect of loading rates on a cross-linked polyvinyl acetate adhesive and maximum SERR values of 370 - 560 J/m2 were achieved. Adhesive penetration and cure were determined by image analysis with fluorescence microscopy, and by micro-dielectric analysis, respectively. Since the geometry of the fracture procedure dictates the absence of wood failure, the resulting fractured surfaces were readily analyzable. The surface analysis techniques of laser ionization mass analysis, solid-state nuclear magnetic resonance and field emission scanning electron microscopy were used to investigate the locus of failure for the smooth fractured surfaces. / Master of Science

Submitted by Lucienne Costa (lucienneferreira@ufcg.edu.br) on 2018-04-10T19:51:47Z No. of bitstreams: 1 THIAGO TAKUMI TAN – DISSERTAÇÃO (PPGEM) 2017.pdf: 9018003 bytes, checksum: 45bd9458239c0c274dabe5c7aee2f165 (MD5) / Made available in DSpace on 2018-04-10T19:51:47Z (GMT). No. of bitstreams: 1 THIAGO TAKUMI TAN – DISSERTAÇÃO (PPGEM) 2017.pdf: 9018003 bytes, checksum: 45bd9458239c0c274dabe5c7aee2f165 (MD5) Previous issue date: 2017-03-13 / O objetivo desta dissertação foi desenvolver dispositivos que possam ser utilizados para ensaios de fratura interlaminar modo-I e modo-II em materiais dissimilares, para aplicação de cargas dinâmicas. Para o desenvolvimento dos dispositivos foi utilizada uma adaptação da metodologia de projeto apresentada por Maribondo (2000), que consiste em projeto informacional, projeto conceitual, projeto preliminar, projeto detalhado, fabricação e testes. A escolha dos ensaios que podem ser utilizados para obter tais modos de fratura foi realizada com base no estudo de trabalhos acadêmicos, que apontou o ensaio double cantilever beam (DCB), como sendo o mais utilizado para estudo de fratura interlaminar modo-I, e também foi observado o crescimento do uso do ensaio end-loaded split (ELS), para estudo de fratura interlaminar modo-II. Os dispositivos desenvolvidos foram testados em uma máquina de impacto por queda de peso, onde foram utilizados corpos de prova de junta adesiva de alumínio/epóxi. Realizaram-se testes modo-I e modo-II bem-sucedidos, analisando o comportamento dos dispositivos e dos corpos de prova durante o ensaio e após o mesmo. / The main objective of this dissertation for the devices that can be used for mode-I and mode-II interlaminar fracture test in dissimilar materials. For the development of devices for design methodology that projects in informational design, conceptual design, preliminary design, detailed design, fabrication and testing. The choice of the tests that can be used to obtain such fracture modes was made based on the study of academic papers, which pointed to the double beam cantilever (DCB) test, as being the most used for the study of interlaminar mode-I, and end-loaded split (ELS) for the study of mode-II interlaminar fracture. The developed devices were tested in a weight drop impact machine, where aluminum / epoxy adhesive joints were used. Testing and analysis of the behavior of the devices and test bodies were carried out during and after the test.

No description available.

The primary direction of this thesis was towards exploring qualitative and quantitative characteristics necessary for refining and understanding the flat wood double cantilever beam (DCB) as a valid means for testing Mode I fracture energy in wood adhesive bonds. Southern yellow pine (SYP) adherends were used with epoxy and phenol formaldehyde (PF) impregnated films, providing two systems with different characteristics for investigation. An adhesive penetration analysis was performed for both the epoxy and PF bonds. The PF penetration into the SYP was shown to be relatively shallow. The epoxy penetration was shown to be deeper. Epoxy-SYP DCBs were quasi-statically tested with varying widths (10 mm, 15 mm, and 20 mm), showing decreases in scatter of critical and arrest strain energy release rates, GIc and GIa, with increases in specimen width. Quasi-static fracture testing was also performed on PF SYP-DCBs, showing much higher critical and arrest fracture energy values than the epoxy-SYP DCBs, indicating that deep adhesive penetration is not necessarily a requisite for higher Mode I fracture energy values. Grain distribution influences were computationally investigated because of the stiffness difference between latewood and earlywood growth and the grain angle along the length of the beams. The grain angle and the stiffness difference between latewood and earlywood growth caused the effective stiffness, (ExxI)eff, to vary along the length of the beam. The effective stiffness variation caused variations in the beam's ability to receive and store strain energy, complicating and confounding determination of experimental results. Cyclic loading tests were performed on PF-SYP DCB's. The cycle frequency was 3Hz, with a valley to peak load ratio of R = 0.5. Specimen softening was observed with cycling, with re-stiffening occurring with crack growth. Contrary to expectations, specimen compliance occasionally decreased with small crack extensions. A toughening mechanism was frequently observed, whereby subsequent crack lengths required more cycles to failure than the previous crack length. Monotonically extending the crack length far from the fatigued region created a fresh crack that did not show the toughened behavior. But toughening did resume with subsequent crack lengths. / Master of Science

The interfacial fracture performance of dicyandiamide cured epoxy/steel adhesive systems was thoroughly investigated. Fracture mechanics based testing was utilized to study several variables which were believed to influence the epoxy/steel interphase region, specifically the elasomeric toughener concentration, the dicyandiamide concentration, and the cure temperature. Bulk mechanical measurements were conducted to provide background information for comparison with the fracture data, and surface analyses were carried out on the neat adhesives and failed fracture specimens to provide insight into the locus and causes of failure. The addition of toughener drastically impacted the morphological, bulk mechanical, and adhesive properties in these latent cure systems. Modulus values decreased and bulk fracture toughness values increased with increasing toughener content. Static double cantilever beam (DCB), fatigue DCB, and notched coating adhesion (NCA) interfacial fracture performances all increased. X-ray photoelectron spectroscopy (XPS) and tunneling electron microscopy (TEM) analyses of the failed specimens revealed that chemical changes were more prominent at the epoxy/steel interphase than in the bulk of the materials. Morphological variations were also apparent with toughener level variations, but for a single formulation no differences between the bulk and intephase morphologies were seen. Evaluations were conducted on a series of elastomer modified model epoxy formulations cured with varying amounts of dicyandiamide. The modulus and bulk fracture toughness values were shown to be independent of dicyandiamide concentration, whereas the adhesive performance was greatly influenced. For increases in the concentration of dicyandiamide, single lap shear (SLS) failure strength values increased while quasi-static DCB and NCA test performances decreased. Fatigue DCB results showed improved adhesive performance at both high and low levels of dicyandiamide content. The results of the failure surface evaluations suggest that dicyandiamide variations produce significant chemical changes only in the epoxy/steel interphase region, and not in the bulk. Analyses were conducted on all of the above systems using two additional cure temperatures. The purpose of this work was to alter the dicyandiamide solubility, and possibly the dicy/epoxy reaction mechanisms, and to determine what influence these changes had on the interfacial fracture performance. In general it was found that performance increased as the cure temperature was increased. / Ph. D.

A research group at University of Skövde has developed an interphase element for implementation in the commercial FE-software Abaqus. The element is using the Tvergaard & Hutchinson cohesive law and is implemented in Abaqus Explicit version 6.7 using the VUEL subroutine. This bachelor degree project is referring to evaluate the interphase element and also highlight problems with the element. The behavior of the interphase element is evaluated in mode I using Double Cantilever Beam (DCB)-specimens and in mode II using End Notch Flexure (ENF)-specimens. The results from the simulations are compared and validated to an analytical solution. FE-simulations performed with the interphase element show very good agreement with theory when using DCB- or ENF-specimens. The only exception is when an ENF-specimen has distorted elements. When using explicit finite element software the critical time step is of great importance for the results of the analyses. If a too long time step is used, the simulation will fail to complete or complete with errors. A feasible equation for predicting the critical time step for the interphase element has been developed by the research group and the reliability of this equation is evaluated. The result from simulations shows an excellent agreement with the equation when the interphase element governs the critical time step. However when the adherends governs the critical time step the equation gives a time step that is too large. A modification of this equation is suggested.

Finite element analysis using Cohesive Zone Method (CZM) is a commonly used method to investigate delamination in laminated composites. In this study, two plane strain, zero-thickness six-node quadratic (6-NQ) and four-node linear (4-NL) interface elements are developed to implement CZM. Two main approaches for CZM formulation are categorized as Unified Mode Approach (UMA) and Separated Mode Approach (SMA), and implemented into 6-NQ interface elements to model a double cantilever beam (DCB) test of a unidirectional laminated composite. The results of the approaches are nearly identical. However, it is theoretically shown that SMA spawns non-symmetric tangent stiffness matrices, which may lower convergence and/or overall performance, for mixed-mode loading cases. Next, a UMA constitutive relationship is rederived. The artificial modifications for improving convergence rates such as lowering penalty stiffness, weakening interfacial strength and using 6-NQ instead of 4-NL interface elements are investigated by using the derived UMA and the DCB test model. The modifications in interfacial strength and penalty stiffness indicate that the convergence may be improved by lowering either parameter. However, over-softening is found to occur if lowering is performed excessively. The morphological differences between the meshes of the models using 6-NQ and 4-NL interface elements are shown. As a consequence, it is highlighted that the impact to convergence performance and overall performance might be in opposite. Additionally, benefits of selecting CZM over other methods are discussed, in particular by theoretical comparisons with the popular Virtual Crack Closure Technique. Finally, the numerical solution scheme and the Arc-Length Method are discussed.

<p>A research group at University of Skövde has developed an interphase element for implementation in the commercial FE-software Abaqus. The element is using the Tvergaard & Hutchinson cohesive law and is implemented in Abaqus Explicit version 6.7 using the VUEL subroutine. This bachelor degree project is referring to evaluate the interphase element and also highlight problems with the element.</p><p>The behavior of the interphase element is evaluated in mode I using Double Cantilever Beam (DCB)-specimens and in mode II using End Notch Flexure (ENF)-specimens. The results from the simulations are compared and validated to an analytical solution.</p><p>FE-simulations performed with the interphase element show very good agreement with theory when using DCB- or ENF-specimens. The only exception is when an ENF-specimen has distorted elements.</p><p>When using explicit finite element software the critical time step is of great importance for the results of the analyses. If a too long time step is used, the simulation will fail to complete or complete with errors. A feasible equation for predicting the critical time step for the interphase element has been developed by the research group and the reliability of this equation is evaluated.</p><p>The result from simulations shows an excellent agreement with the equation when the interphase element governs the critical time step. However when the adherends governs the critical time step the equation gives a time step that is too large. A modification of this equation is suggested.</p>

This study focuses on factors that affect the structural integrity of wood-based composites. Wood-based composites exposed to fire may decompose due to the elevated temperatures, resulting in a degradation in performance. Thermal modelling can only predict the structural integrity of construction materials in fire if it is given accurate inputs. Consequently, methods for the characterization of the thermal, physical, and mechanical behaviors of wood and wood-based composites are selected, designed, and benchmarked. The relevant thermal and physical responses characterized includes porosity, permeability and thermal diffusivity. Common construction materials (white pine board, medium density fiberboard and spruce 24) are characterized from room temperature to complete decomposition. The characterization techniques and processes are based on existing literature and relevant ASTM standards. To reduce the number of experiments required for future material characterization, estimates based upon the degree of decomposition and the measured values for the virgin and charred materials are used. For porosity and thermal diffusivity, these models allow values at intermediate temperatures to be estimated with measurements at room temperature and complete decomposition and thermogravimetric analysis (TGA). We find that permeability depends heavily on the microstructure of materials and should be measured independently at the conditions of interest. An additional important aspect of the performance of wood-based composites is the fracture behavior of wood/adhesive systems. Adhesive bonding enables many engineered wood products such as furniture and structural wood joints and the adhesive fracture toughness often determines the durability. The conventional characterization method for wood/adhesive fracture resistance relies on samples with machined grain angles designed to funnel cracks to the adhesive interface. This method of sample preparation is difficult and time-consuming for certain wood species. In this work, a practical and efficient method is developed to characterize adhesive fracture energy of adhesively bonded veneer systems. In the method, auxiliary aluminum adherends are bonded to the veneers in an effort to drive the crack to the wood/adhesive interface. The method is applied to rotary-peeled veneers and saw-cut veneers produced from three species of wood bonded with three commonly used adhesives. The new tests method yields a high interfacial failure rate and successfully identifies differences in the performance of the three adhesives. SPG (one species of the rotary-peeled veneers) demonstrates a rising R-curve behavior (an increase in the fracture toughness with crack length) when bonded on the loose side. This increase in fracture toughness is observed to be a result of adhesive-substrate interaction, which is a developing process zone behind the crack tip consisting of bridged wood ligaments. / Doctor of Philosophy / Construction materials exposed to elevated temperatures from fires may reach temperatures where the material decomposes from the original material to a char. Protected and unprotected structural timber products exposed to fires may exhibit this behavior resulting in a degradation of performance. Understanding the thermal and physical responses of these materials is crucial in evaluating the materials behavior in fire. Additionally, many wood-based products (such as furniture) rely on adhesive bonds. Consequently, their usefulness is determined by the performance of those bonds. In this work, methods are developed to measure key properties impacting the behavior of wood-based systems at elevated temperatures, such as that experienced in fires and when they are subjected to forces attempting to debond one wood material from another. These techniques are demonstrated on common building materials (white pine board, medium density fiberboard and spruce 24) and wood veneers from three different species bonded with three different adhesives. Mathematical models are developed to expand the use of the data beyond the specific conditions for which it is measured.