Marcelo F.S.F. de Moura

Residual Strength after Low Velocity Impact in Carbon-Epoxy Laminates

The aim of present work is to study the influence of low energy impacts on residual strength of carbon-epoxy laminates. Experimental tests were performed on [0,90,0,90]2s and [0,90]8 laminates using a drop weight-testing machine. The influence of the laminate stacking sequence is analysed under 1.5 J, 2 J, 2.5 J and 3 J impact energies, corresponding to a 0.91 ms-1, 1.05 ms-1, 1.18 ms-1 and 1.29 ms-1 of impact velocity, respectively. The impacted plates were inspected by CScan to evaluate the size, shape and position of the delaminations through the thickness of the plate. The same plates were inspected by C-Scan before the impact, to evaluate the eventual presence of defects produced during the manufacturing process. The residual flexural strength showed that the [0,90,0,90]2s laminates have better performance than the [0,90]8 ones. The explanation is related with the lower flexural stiffness of the antisymmetric lay-up relatively to the symmetric one.

Application of the end loaded split and single-leg bending tests to the mixed-mode fracture characterization of wood

Abstract This work describes the application of end loaded split and single-leg bending tests to the mixed-mode fracture characterization of wood. Experimental tests and numerical validation analyses were performed. A new data reduction scheme based on the crack equivalent concept is proposed. The method overcomes the difficulties inherent to these tests, such as crack length monitoring during propagation and influence of clamping conditions. The single-leg bending test is simpler to execute and provided accurate results. The obtained mixed-mode fracture energy is associated with the pure mode values and the obtained trend point to a linear fracture criterion as a candidate to describe the fracture behavior of the Pinus pinaster Ait. wood.

Data reduction scheme for measuring G(IIc) of wood in end-notched flexure (ENF) tests

Abstract Numerical and experimental studies of end-notched flexure (ENF) fracture tests were performed to obtain mode II R curves for maritime pine (Pinus pinaster Ait.) wood in the radial-longitudinal (RL) crack propagation system. Three- (3D) and two-dimensional (2D) finite element analyses were conducted to determine the mode II release rate of the critical strain energy (G IIc). The 3D analysis revealed that a small spurious mode III component did not affect G IIc measurements and that the 2D model was very accurate. A new scheme for data reduction based on the equivalent crack concept is proposed to overcome the difficulties related to accurate crack length measurement during propagation. This method does not require previous experimental tests to obtain the elastic modulus, which varies markedly for different wood specimens. Experimental ENF tests were performed to verify the numerical results. The results demonstrate the accuracy of the data reduction method proposed.

Determination of cohesive laws in wood bonded joints under mode I loading using the DCB test

Abstract The cohesive laws (CLs) have been investigated by means of direct and inverse methods concerning wood bonded joints under pure mode I. The experimental results were obtained by tests with double cantilever beam. The direct method is based on the differentiation of the relation between strain energy release rate and crack opening displacement at the crack tip. An equivalent crack method was used to evaluate the strain energy release rate in the course of the test without monitoring the crack length, which is difficult to observe exactly. The crack opening displacement was determined by postprocessing local displacements measured by digital image correlation. The inverse method requires a previous assumption of the CL shape, and as such, a trilinear law with bilinear softening relationship was selected. The cohesive parameters were identified by an optimization procedure involving a developed genetic algorithm. The idea is to minimize an objective function that quantifies the difference between the experimental and the numerical load-displacement curves resulting from the application of a given law. A validation procedure was performed based on a numerical analysis with finite elements. Both methods in focus provided good agreement with the experimental data. It was observed that CLs adopted by the inverse method are consistent with the ones obtained with the direct method.

Progressive Damage Modelling

The application of bonded joints is increasing due to their several advantages to alternative bonding methods. As a result, more efficient predictive tools are necessary to increase the confidence of designers. In this context, cohesive and continuum damage models acquire special relevancy owing to their capacity to simulate damage onset and growth. Both of these methodologies combine strength of materials with fracture mechanics, thus overcoming the limitations of each method. A cohesive mixed-mode damage model based on interface finite elements and accounting for ductile behaviour of adhesives is presented. The cohesive parameters of the constitutive softening law are determined using an inverse method applied to fracture characterization tests under pure modes, I and II. In this context a new data reduction scheme based on crack equivalent concept is developed and applied to fracture characterization tests. Good agreement between the numerical and experimental results was obtained for strength versus overlap length in single-lap joints. A continuum mixed-mode damage model is also presented using a triangular softening law adequate only for brittle or moderately ductile adhesives. In these models the material properties degradation occurs inside of the solid elements, which is advantageous relatively to cohesive methods mainly when damage propagation onset and path are not known a priori.

Composite laminates with linearly varying fiber angles under static and dynamic loads

In this paper, natural frequencies, large deflections and failure safety factors of variable stiffness composite laminate (VSCL) plates with curvilinear fibers are studied. In each ply of these rectangular VSCLs, the fiber-orientation angle changes linearly with respect to the horizontal coordinate. To carry out the static and dynamic analysis of the laminates, a new p-version finite element, which follows third-order shear deformation theory, is employed. Large deflections are considered, hence, the analysis is in the non-linear regime. To predict the failure onset, Tsai-Wu criterion is used. Taking into account manufacturing restrictions regarding the fiber curvatures, natural frequencies, deflections, and failure safety factors of some VSCL plates are determined. These results are compared with natural frequencies, deflections, and failure safety factors of constant stiffness composite laminate (CSCL) plates.

The Effect of the Impactor Diameter and Boundary Conditions on Low Velocity Impact Composites Behaviour

The aim of present work is to study the influence of the impactor diameter and boundary conditions on low velocity impact on carbon-fibre-reinforced epoxy laminates. Experimental tests were performed on [04,904]s laminates, using a drop weight-testing machine. Circular plates were tested under low velocity impacts for two diameters of the hemispherical impactor, 12.7 mm and 20 mm, and considering similar impact energies, 2.6 J for the first impactor and 3 J for the second one. Rectangular and square plates were analysed under low velocity impacts with different boundary conditions. The impacted plates were inspected by X-radiography. Numerical simulations were also performed considering interface finite elements compatible with three-dimensional solid elements including a cohesive mixed-mode damage model, which allows to model delamination between layers. The impact tests showed that both the impactor’s diameter and boundary conditions have influence on the delaminated area. Good agreement between experimental and numerical analysis for shape, orientation and size of damage was obtained.

Strength Prediction and Experimental Validation of Adhesive Joints Including Polyethylene, Carbon-Epoxy and Aluminium Adherends

Polyolefins are especially difficult to bond due to their non-polar, non-porous and chemically inert surfaces. Acrylic adhesives used in industry are particularly suited to bond these materials, including many grades of polypropylene (PP) and polyethylene (PE), without special surface preparation. In this work, the tensile strength of single-lap PE and mixed joints bonded with an acrylic adhesive was investigated. The mixed joints included PE with aluminium (AL) or carbon fibre reinforced plastic (CFRP) substrates. The PE substrates were only cleaned with isopropanol, which assured cohesive failures. For the PE CFRP joints, three different surfaces preparations were employed for the CFRP substrates: cleaning with acetone, abrasion with 100 grit sand paper and peel-ply finishing. In the PE AL joints, the AL bonding surfaces were prepared by the following methods: cleaning with acetone, abrasion with 180 and 320 grit sand papers, grit blasting and chemical etching with chromic acid. After abrasion of the CFRP and AL substrates, the surfaces were always cleaned with acetone. The tensile strengths were compared with numerical results from ABAQUS® and a mixed mode (I+II) cohesive damage model. A good agreement was found between the experimental and numerical results, except for the PE AL joints, since the AL surface treatments were not found to be effective.

Mode II Fracture Characterization of Pinus Pinaster Wood

This paper describes experimental and numerical studies on the application of the End Notched Flexure (ENF) and End Loaded Split (ELS) tests to mode II wood fracture characterization. In this context, ENF and ELS specimens were used to determine GIIc of a clear Pinus pinaster wood in the RL system, which is the most relevant for structural design. In mode II fracture tests the crack faces are in contact, thus hindering a rigorous visualization of the crack tip. This makes classic methodologies based on crack length measurement during experimental tests inadequate, since they induce significant errors on the mode II fracture properties. To overcome this experimental problem a Compliance Based Beam Method (CBBM) is used. This new data reduction scheme does not require crack length monitoring and includes the effect of the Fracture Process Zone (FPZ) ahead of crack tip. Furthermore, the clamped cross-section rotation of the ELS specimen is also taken into account. In the present work a numerical analysis considering a cohesive damage model was performed with a cohesive damage model in order to validate the application of the CBBM to the experimental results. The results confirmed the adequacy of the CBBM and the applicability of the ENF and ELS tests for mode II wood fracture characterization.

The Effect of the Carbon Epoxy Plate’s Size under Low Velocity Impact

The aim of present work is to study the influence of the plate’s size on low velocity impact on carbon-fibre-reinforced epoxy laminates. Experimental tests were performed on [04,904]s laminates, using a drop weight-testing machine. Circular, square and rectangular plates were tested under low velocity impacts using a hemispherical impactor with 20 mm diameter and 3 J impact energies. The impacted plates were inspected by X-radiography. Numerical simulations were also performed considering interface finite elements compatible with three-dimensional solid elements, which allows to model delamination onset and growth between layers. The results showed that the plate’s size has influence on the delaminated area. Good agreement between experimental and numerical analysis for shape, orientation and size of the delaminations was obtained.