Sören Östlund

Rethinking Engineering Education

© Springer International Publishing Switzerland 2007, 2014. This book describes an approach to engineering education that integrates a comprehensive set of personal, interpersonal, and professional engineering skills with engineering disciplinary knowledge in order to prepare innovative and entrepreneurial engineers. The education of engineers is set in the context of engineering practice, that is, Conceiving, Designing, Implementing, and Operating (CDIO) through the entire lifecycle of engineering processes, products, and systems. The book is both a description of the development and implementation of the CDIO model and a guide to engineering programs worldwide that seek to improve the education of young engineers.

First order analysis of stiffness reduction due to matrix cracking

In this paper the reduction in effective elastic constants of composite lami nates due to matrix cracking is considered. A general easily applicable theory valid for two- and three-dimensional analysis of thin as well as thick laminates is presented. The theory is based on the change of elastic energy in a laminate at the appearance of a matrix crack in one layer. This information combined with a dilute approximation , i.e., different matrix cracks are considered not to interact with each other, is utilized to estimate the re duction of elastic constants for a certain crack density. The theory is asymptotically exact for nk « 1, where nk is the number of cracks per unit thickness of the cracked ply and a correct value of the slope of the stiffness reduction crack density curve is obtained at nk = 0.

Orthotropic elastic–plastic material model for paper materials

Paper and paperboard generally exhibit anisotropic and non-linear mechanical material behaviour. In this work, the development of an orthotropic elastic plastic constitutive model, suitable for modelling of the material behaviour of paper is presented. The anisotropic material behaviour is introduced into the model by orthotropic elasticity and an isotropic plasticity equivalent transformation tensor. A parabolic stress-strain relation is adopted to describe the hardening of the material. The experimental and numerical procedures for evaluation of the required material parameters for the model are described. Uniaxial tensile testing in three different inplane material directions provides the calibration of the material parameters under plane stress conditions. The numerical implementation of the material model is presented and the model is shown to perform well in agreement with experimentally observed mechanical behaviour of paper.

FEM-remeshing technique applied to crack growth problems

The remeshing option of the commercial FE-code ABAQUS is used to study crack propagation in elastic-plastic materials. Propagation is accomplished with a combination of remeshing and nodal relaxation. The results are compared with analyses using nodal release only. Substantial crack growth is achieved with less computer time and higher accuracy than in conventional FE analysis.

Prediction Of Thermoelastic Properties Of Composite Laminates With Matrix Cracks.

Abstract A theory is presented for the determination of changes in the elastic compliance tensor and the thermal expansion coefficients due to matrix cracking. Effects of residual stresses are taken into account. The theory is valid for two- or three-dimensional laminates of arbitrary lay-ups and is asymptotically exact for dilute micro-crack densities. For large micro-crack densities an alternative model is described which is asymptotically exact for large crack densities. Combined, the two models cover to a good approximation the whole range of micro-crack densities. The theory is applied to a micro-cracking cross-ply GFRP laminate for which the stress/strain curve is calculated. Data are also presented for changes in thermomechanical properties of angle ply laminates as functions of the lay-up angle.

Asymptotic crack tip fields for dynamic fracture of linear strain-hardening solids

Asymptotic crack tip fields, for dynamic crack propagation in an elastic-plastic material, have been calculated. The material is characterized by J2flow theory with linear-strain hardening. The possibility of plastic reloading on the crack flank is taken into account. Numerical results for the strength of the crack tip singularity, the angular positions of elastic unloading and possible plastic reloading regions, and the angular variation of the stress and velocity fields, are presented as functions of the crack tip speed and the ratio between tangent modulus and elastic modulus. Calculations have been performed for crack tip speeds below a certain limit velocity which depends on the tangent modulus and the loading conditions. The different loading modes which have been studied are modes I and II (plane strain and plane stress) and mode III (antiplane strain).

Rethinking engineering education - The CDIO approach, 2nd ed

© Springer International Publishing Switzerland 2007, 2014. This book describes an approach to engineering education that integrates a comprehensive set of personal, interpersonal, and professional engineering skills with engineering disciplinary knowledge in order to prepare innovative and entrepreneurial engineers. The education of engineers is set in the context of engineering practice, that is, Conceiving, Designing, Implementing, and Operating (CDIO) through the entire lifecycle of engineering processes, products, and systems. The book is both a description of the development and implementation of the CDIO model and a guide to engineering programs worldwide that seek to improve the education of young engineers.

Experimental investigation of damage at folding of coated papers

To achieve a better understanding of the folding properties of coated papers pertinent to the mechanical behaviour, a microscopic investigation was performed. The influence on the damage levels in ...

Numerical Verification of a Procedure for Calculation of Elastic Constants in Microcracking Composite Laminates

In two recent papers Gudmundson and Östlund have presented a simple to use theory for calculation of reduced thermoelastic constants in composite laminates with matrix cracks. The theory is valid for two- and three-dimensional laminates of arbitrary layups and is asymptotically exact for dilute and infinite microcrark densities. Further more, the theory is based solely on ply property data and matrix crack densities. No ex perimentally determined or unknown theoretical parameters are required. The theory pre dicts changes in all coefficients of the elastic stiffness and thermal expansion coefficient tensors. In previous papers the accuracy of the present theory has been investigated for cross ply laminates by comparisons to experimental results from the literature and two-dimensional finite element calculations. However, in order to become a powerful tool in the analysis of cracked composite laminates the theory must be verified at intermediate crack densities also for angle ply laminates. Unfortunately no useful experimental results for such lami nates have been found in the literature and numerical methods must be considered for the verification. In the present paper the approximate theory is compared with three- dimensional finite element calculations of cracked thick [⊖, - ⊖ ] M glass fiber reinforced epoxy laminates for a wide range of different matrix crack densities. The finite element calculations can be considered as exact for all crack densities within the present formula tion, and errors are due only to the numerical algorithm. It is found that the dilute theory in combination with the theory for infinite crack densities to a surprisingly good accuracy cover all ranges of crack densities.

Fatigue Propagation of Fibre-Bridged Cracks in Unidirectional Polymer-Matrix Composites

In order to improve the fatigue resistance of polymer-matrix composites by materials design, or to conceive micromechanics based models for life predictions, the underlying micromechanisms must be understood. Experimental investigations have revealed fibre-bridged cracking as a toughening micromechanism that retards further fatigue crack growth in a unidirectional 0° carbon-fibre-reinforced epoxy. The bridging fibres exert a closing traction on the crack surfaces, thereby reducing the driving force for crack growth. In this study, the growth of bridged cracks has been quantified by a surface replication technique. The da/dN–ΔK curve defined in terms of nominal stress-intensity factors shows a crack retarding behaviour. The crack growth curve can be replotted in terms of the effective stress-intensity factor where the contribution of the cohesive crack surface forces from the bridging fibres are taken into account. This curve falls somewhat closer to that of the neat matrix material, but the difference is still considerable, and it shows a decelerating propagation. Therefore, there must be other active toughening mechanisms besides fibre bridging, that slow the crack propagation down, and account for the fatigue resistant behaviour of the tested material. Ways by microstructural design to promote the fatigue resistant mechanisms are discussed.