Session 3 – Composites / Non‐metallic materials Delamination tool – latest developments Authors: A. Arteiro, P.P. Camanho, A.R. Melro (INEGI, University of Porto, Porto, PORTUGAL) Due to the severe consequences of delamination on the load‐carrying capacity of laminated composite structures, robust analysis methods for delamination should be used to predict the damage tolerance of such structures. The analysis methods should be able to accurately predict the location and sequence of failure modes in laminated composite structures. In this work, the location and sequence of failure modes occurring in a satellite sandwich panel, with particular emphasis on delamination onset and propagation, is assessed using two different levels of analysis, according to the best practice guidelines proposed in the DELamination Assessment Tool (DELAT) project, together with the most recent ply failure criteria. These analysis levels include ABAQUS’ VCCT (level 2) and ABAQUS’ cohesive elements (level 3). The results of these analyses are used in the detailed design of a demonstrator test campaign, which will be used in the future to validate the best practice analysis methods proposed in the DELAT project. ************ On the evaluation of delamination resistance in composites Authors: Dr.ir. R.C. Alderliesten (TU Delft) The evaluation of delamination resistance in composites comprises both experimental characterization of delamination resistance, and subsequent development of theories and models to predict the delamination growth in relevant structures. The experimental characterization of quasi‐static delamination growth in composites is commonplace these days. The fracture toughness is related to critical values of the Strain Energy Release Rate (SERR) or to value at onset of growth. Nonetheless, there seems to be a discrepancy between the various formulations of SERR used for similitude in fatigue loading conditions. It will be shown how some formulations may lead to misinterpretation of the data. This misinterpretation specifically plays a role in the development of mixed mode theories, where the superposition of different modes with different stress ratios may lead to physically inconsistent theories. In addition, the characterization of the delamination resistance is in most cases only determined for specific fibre orientations at both sides of the delamination interface. Various tests performed at TU Delft illustrate that the delamination resistance may significantly differ if the growth occurs at an interface between fibres oriented under angles with respect to that direction. This influence of interface topography is not considered in the formulation of the SERR. Related to the discussion on the similitude parameter, this paper will illustrate that the delamination resistance should not be represented as single curves (plotted for example as da/dN against Gmax or G). Neither the mean (or maximum) stress nor the amplitude (or range of) stress are sufficient to describe a load cycle; a combination of both the monotonic and cyclic component are required to define that cycle. Therefore, it will be shown that the delamination resistance constitutes a plane in a three‐dimensional graph, where da/dN is plotted against both the monotonic and cyclic component of the applied similitude parameter. The three‐dimensional representation of the delamination resistance seems to relate to two‐parameter models, proposed in the literature, where both the cyclic and monotonic SERR are used in the equation. However, based upon fractographic analysis of the fracture surfaces, it will be demonstrated that the formulation of the equations based upon the two parameters require further investigations. It will be shown that formulations applying superposition rather than multiplication of the parameters provide better correlation with data obtained from tests at TU Delft, and various data sets from the literature. Physically, this means that it should be considered up to what extent the monotonic and cyclic SERR are inter‐related in the delamination growth characterization, or whether their contributions should be superimposed. ************ Assessment of residual strength of CFRP/al honeycomb sandwich panels subjected to low velocity impact damage Authors: 1
2
2
K, Kalnins , G. Sinnema , A.J. Graham
1
2
( Riga Technical University, ESA/ESTEC) Current study summarise the results obtained in experimental study in order to evaluate the residential strength of CFRP/Al honeycomb panels subjected to a low velocity impact. The aim of research is to establish the best practice for assessment of barely visible damage and its progression up to the failure of structure. Up to now there is a lack of industry standard for evaluation of residual strength of thin wall sandwich structures exposed to damage. This is due a fact that conventional compression after impact methodology which is accepted in aeronautics is intended for four to six times thicker composites. This restriction applies in particularly once impact energy level should be scaled down in order to create permanent/barely visible damage in CFRP skins. For assessment readily available CFRP/Al honeycomb sandwich structure with relatively thin (300 microns) skin was utilised for current investigation. Initially the attempt to introduce the indent in panels was not successful as energy level not causing skin penetration was in range below 2 Joules. Such low energy level readings were compromised by data acquisition sensitivity for available equipment, thus quasi‐static indentation instead of low velocity impact was employed. In total forty two edgewise compressions after impact (CAI) test specimens have been produced and tested up to the failure, while recording the strain distribution during these tests. Both strain gauge readings and optical – digital image correlation strain visualisation by ARAMIS system has been extracted. Moreover non‐destructive inspection by ultrasonic C‐
scan was performed at each stage of damage introduction/evaluation. The level and severity of indent caused delamination in corresponding plies has been identified. Reference sandwich panels and panels with different diameter indent and dent severity as well as location has been tested in edgewise compression. The optical strain measurements acquired during these tests showed sensitivity level appropriate to track the indent propagation during the test and just before the fibre failure or local buckling of damaged region. A conclusion has been drawn for establishing the best testing practice, which afterwards could be implemented in verification of numerical analysis of residual strength of full scale composite sandwich structures including impact caused damage propagation up to failure. ************ Residual stress and micro‐defects caused by curing of composites Authors: Jan‐Patrick Jürgens, Prof. Dr.‐Ing. Vasily Ploshikhin (Airbus Professorship for Integrative Simulation and Engineering of Materials and Processes (ISEMP), University of Bremen) Simulations of the residual stress caused by the curing process of carbon fibre reinforced plastics are carried out on the mesoscopic scale taking into account the real morphologies of composite microstructures, i. e. the statistical distribution of carbon fibres in the polymer matrix. A systematic numerical analysis reveals the major mechanisms for the stress formation and distribution as well as interactions between the residual stress and the possible process induced micro‐defects like micro‐cracks and delamination. A correct description of the boundary conditions is essential in order to make reliable predictions on cracking and fracture. Fig. 1: Microstructure of a representative
volume element of a fibre reinforced
plastic (non-woven fabric 0° / 90°).
The presented approach work demonstrates the potentials of the computational micromechanics for analysis of the thermo‐mechanical behaviour of realistic composites micro‐structures during the manufacturing process. It opens the way for further developments of the multiscale analysis towards application of the nano‐ and micro‐scales simulations for prediction of the macroscopic Fig. 2: Crack formation after the cooling process of a
properties of composites. CFRP layer (2D FEM-Simulation; excessive deformation)
************ The impact of EU Legislation on the Mechanical Performance of Adhesives Authors: Jason Williamson (ESA/ESTEC) The EU chemical industry is undergoing a radical change as it try to comply with stricter EU legislation with respect to the use of chemicals (e.g. REACH, RoHS etc.). In addition these changes are being further pre‐empted by suppliers anticipating changes and rationalising their supply chain. Whilst these changes are of benefit to the health and well being of the EU population they often have unforeseen consequences on the materials. These detailed consequences are often overlooked by the suppliers. ESA's Materials Space Evaluation & Radiation Effects Section has been evaluating the changes and their impact on some common adhesive materials. Often the changes whilst superficial upon first examination have been found to impact on the performance. The presentation will detail three case studies for Araldite AV138M, Scotchweld 2216 and Scotchweld DP 490 where more detailed studies have been done to establish the effects of "minor" product changes.