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Omni first-ply-failure (FPF) envelopes are an elegant yet conservative approach to assess composite laminate failure on a global level. Omni envelopes can be found increasingly in recent publications. However, the development process of those envelopes shows a lack of clarity. At some point the illustration switches from a laminate-strain basis (ɛx,ɛy,γxy) to the particular case of laminate principal-strain (ɛI,ɛII) basis. The latter is elegant, as the principal-strain space can be easily plotted in 2D. This article presents two procedures to directly determine omni FPF envelopes and it clarifies the transfer to principal strains.While the Tsai–Wu criterion is used in almost all available publications, the present article uses Cuntze’s failure mode concept (FMC). The article provides a simple example case, which demonstrates the application of omni envelopes in context of FEA based CFRP design.

Abstract: T-bolt joints are widely used in the wind turbine industry. This paper describes the constitution and function of the different components of the joint, showing the different parameters that affect its performance, and presents a parametric analysis, useful for joint design and optimization, relating laminate thickness, bolt and cylinder diameters to pitch distance, and pretension of the bolt with external load. In order to provide information on the influence of the T-bolt geometry on the strength of the laminate in the net tension and bearing zones, tests for two different glass fiber – epoxy laminates, under different pitch/diameter ratios have been performed, forcing either bearing or tension failure modes. Stress and strain concentrations in the laminate have been estimated trough a three-dimensional finite element model that takes into account pretension effects and contact nonlinearities. How experimental results correlate with simple and finite element model analysis is discussed. Stress Point Criteria may be applied for joint tension failure with a characteristic distance of d 0 =0.9mm. [Copyright &y& Elsevier]

In part A of the exercise, the failure criterion was presented and some failure envelopes were calculated on the basis of some materials properties that were supplied by the organizers. Here, in part B, some experimental results, also supplied by the organizers, are analyzed and compared with the theoretical predictions. In some cases, there is a large deviation between theory and practice. It is shown that the huge scatter in the experimental results and the arbitrary selection of some tests and material properties might be the reason for that. It is also suggested that, in some cases, a different mode of failure that is not covered by this criterion caused the failure. [Copyright &y& Elsevier]

The successive failure of different laminates subjected to a variety of loading conditions has been treated by a layer-by-layer failure analysis. Three sources of non-linearity are considered, namely material non-linearity due to microdamage, matrix cracking, and changes in fibre angle with increasing strains. In general there is good agreement between predictions and experimental results. Some disagreement is due to imperfections in certain tests, therefore, these tests should be repeated. Three categories of laminate configuration/loading condition can be distinguished: (I) laminates with 3 or more fibre directions with arbitrary loading conditions; (II) balanced angle ply laminates with stress ratios in accordance with netting analysis; (III) laminates with 2 fibre directions and loadings which are not in accordance with netting analysis. The analysis of category (I) is straight forward. Category (II) is sensitive to the stiffness degradation after the onset of matrix cracking. Category (III) fails at low stresses and large strains of the laminate due to a rapid deterioration. An intensive discussion is necessary to define a failure limit for category (III).For the detection of the different modes (A, B, C) of interfibre fracture (IFF), refined action plane related IFF-criteria developed by Puck on the basis of Mohr''s and Hashin''s considerations on brittle fracture are used. They provide much more information than has been reported from the experiments. Their unique ability to predict the inclination of the fibre parallel fracture plane is the key for assessing of the risk of delamination and local buckling due to a wedge effect which occurs when oblique fracture planes are exposed to high transverse compression. [Copyright &y& Elsevier]

The paper concerns the progressive failure analysis of laminates with the in-plane shear nonlinearity accounted for. The nonlinear shear response of the layer is described by the constitutive relation treating the stresses as a function of strains. Thus it can be easily incorporated into the displacement-based FEM codes. The brittle failure mechanisms of the fibers and the matrix of the layer are recognized with the use of the Hashin criterion. The model is implemented into the non-commercial FEM program for the analysis of multifold shells which is based on the 6-parameter nonlinear shell theory of the Cosserat type. [ABSTRACT FROM AUTHOR]