Relationships between LRI process parameters and impact and post-impact behaviour of stitched and unstitched NCF laminates

The general context of the development of out-of-autoclave processes in the aeronautics industry raises the question of the possible links between these new processes and impact behaviour. In this study, a Taguchi table was used in a design of experiment approach to establish possible links. The study focused on the liquid resin infusion process applied to laminates made with stitched or unstitched quadri-axial carbon Non-Crimp Fabric(NCF). On the basis of previous studies and an analysis of the literature, five process parameters were selected (stitching, curing temperature, preform position, number of highly porous media, vacuum level). The impact energy was set at 35 J in order to obtain enough residual dent depth. The parameters analysed during and after impact were: maximum displacement of the impactor, energy absorbed, permanent indentation depth, and delaminated surface. Then, compression after impact tests were performed and the corresponding average stress was measured. The interactions found by statistical analysis show a very high sensitivity to stitching, which was, of course, expected. A very significant influence of curing temperature and a significant influence of preform position were also found on the permanent indentation depth and a physical explanation is provided. Globally, it was demonstrated that the resin infusion process itself did not influence the impact behaviour

Core crush criterion to determine the strength of sandwich composite structures subjected to compression after impact

In this study a core crush criterion is proposed to determine the residual strength of impacted sandwich structures. The core of the sandwich is made of a Nomex Honeycomb core and the faces are laminated and remain thin. The mechanism of failure of this kind of structure under post-impact compressive loading is due to interaction between three mechanical behaviors: geometrical nonlinearity due to the skin’s neutral line off-set in the dent area, nonlinear response of the core and damages to the skins. For the type of sandwich analysed in this study, initially the core crushes at the apex of the damage. Using a finite element discrete modelling of the core previously proposed by the authors, the load corresponding to the crushing of the first cell can be computed and it gives the value of the residual strength for our criterion. Some geometric and material hypotheses are assumed in the damaged area mainly based on nondestructive inspection (NDI). The criterion is then applied to tests modelled by Lacy and Hwang [Lacy TE, Hwang Y. Numerical modelling of impact-damaged sandwich composites subjected to compression after impact loading. Compos Struct 2003;61:115–128]. It is shown that the criterion allows a good prediction of the tests except in the case of very small dents. Several sensitivity studies on the assumptions were made and it is shown that using this approach, the criterion is robust

Discrete ply model of circular pull-through test of fasteners in laminates

In aeronautical structures, assemblies with thin laminates are becoming increasingly usual, especially for fuselage design. In these structures, out-of-plane loads can appear in bolted joints and can lead to progressive punching of the fastener’s head in the laminate resulting, in some cases, in a failure mode called pull-through [1]. This complex phenomenon, which occurs in assemblies, was studied firstly by using a simplified ‘‘circular’’ pull-through test method. Qualitative micrographic examinations showed damage very similar to that observed in impacted specimens. The research presented here extends the Discrete Ply Model Method (DPM) developed by Bouvet et al. [2] to this case. The finite elements model is based on a particular mesh taking ply orientations into account. Cohesive elements are placed at the interfaces between solid elements to represent matrix cracks and delamination, thus allowing the natural coupling between these two damage modes to be represented. The model shows good correlation with test results, in terms of load/displacement curve, and correct prediction of the damage map until failure, including the splitting phenomenon

Contribution à l'étude des structures sandwichs dissymétriques

Les structures sandwichs dissymétriques sont un cas particulier de la famille des structures sandwichs. Elles présentent une peau dite travaillante qui est chargée en membrane. Celle-ci est stabilisée par une âme en nid d'abeille Nomex et une contre-peau dite peau stabilisatrice. Cette configuration génère un comportement non linéaire géométrique. Plusieurs thèories analytiques des poutres et plaques sandwichs dissymétriques ont été élaborées basées sur le minimum de l'énergie potentielle ou le principe des puissances virtuelles associées à une méthode de discrétisation de Ritz. Ces théories intégrent de plus la modélisation a priori ou a posteriori de la compression de l'âme. Parallélement un montage d'essai sous sollicitations combinées compression/cisaillement a été développé et des essais multiaxiaux ont été réalisés sur éprouvettes neuves et impactées. Une comparaison théorie/expérience est aussi réalisée

Modelling of low-energy/low-velocity impact on Nomex honeycomb sandwich structures with metallic skins

In the aircraft industry, manufacturers have to decide quickly whether an impacted sandwich needs repairing or not. Certain computation tools exist at present but they are very time-consuming and they also fail to perfectly model the physical phenomena involved in an impact. In a previous publication, the authors demonstrated the possibility of representing the NomexTM honeycomb core by a grid of nonlinear springs and have pointed out both the structural behaviour of the honeycomb and the influence of core-skin boundary conditions. This discrete approach accurately predicts the static indentation on honeycomb core alone and the indentation on sandwich structure with metal skins supported on rigid flat support. In this study, the domain of validity of this approach is investigated. It is found that the approach is not valid for sharp projectiles on thin skins. In any case, the spring elements used to model the honeycomb cannot take into account the transverse shear that occurs in the core during the bending of a sandwich. To overcome this strong limitation, a multi-level approach is proposed in the present article. In this approach, the sandwich structure is modelled by Mindlin plate elements and the computed static contact law is implemented in a nonlinear spring located between the impactor and the structure. Thus, it is possible to predict the dynamic structural response in the case of low-velocity/low-energy impact on metal-skinned sandwich structures. A good correlation with dynamic experimental tests is achieved

Effects of process parameters on the mechanical properties and morphology of stitched and non-stitched carbon/epoxy liquid resin-infused NCF laminate, out of autoclave and out of oven

International audienceThe effects of resin infusion process parameters on the mechanical properties of stitched or non-stitched composite laminates out of autoclave were studied using the design of experiment method. This method was chosen due to the complexity of the problem. The preforms used were laminates of multi-axial quasi-isotropic non-crimp fabric (NCF), either stitched or non-stitched. A literature review identified nine parameters as the key design-of-experiment factors: sewing; the number of NCFs; the number of high-porous media; the interaction between the number of NCFs and the number of high-porous media; the mould temperature, injection temperature and cure temperature; the position of the preform; and, finally, the vacuum level. The mechanical properties studied and the morphological analysis carried out concerned the resistance in tension, compression and shear, the glass transition temperature, the thickness of the finished laminate, and the fibre volume fraction and porosity. The study revealed the best suited manufacturing conditions

Discrete ply modelling of open hole tensile tests

The Discrete Ply Modelling (DPM) method, previously applied with success to out-of-plane loading such as impact or pull-through, is used to model open hole tensile tests. According to the literature, this kind of test is relevant to assess the efficiency of a modelling strategy. Four different stacking sequences are tested and the failure scenario and patterns are well predicted. The main advantages of DPM are the very small number of parameters required and the robustness of the models. The main drawback is the computation cost

Scaling effect in notched composites: The Discrete Ply Model approach

Numerical and experimental investigations were carried out on the size effect in notched carbon/epoxy laminates. This paper presents a computational study of scaled open-hole tensile tests using the Discrete Ply Modeling (DPM) method, which has already proven efficient on both in-plane and out-of-plane loading cases, such as pull through, low velocity impact and compression after impact. The specificities of this finite element model are its discrete nature, the small number of parameters required and its robustness. Three different stacking sequences of thin plies coupled with three sizes of coupons having the same length to width ratio were tested. The results show that the model reflects the reduction in strength when the size of the specimen increases. The influence of different parameters such as mesh size, presence of discrete matrix cracks and fiber fracture toughness that should be used for clustered plies, are discussed. Comparisons with experiments demonstrate that tensile strengths, and failure scenarios and patterns are predicted with acceptable accuracy

Experimental and numerical analysis of Carbon Fiber Reinforced Polymer notched coupons under tensile loading

The behavior of composite laminates subject to notch based stress concentrations is difficult to apprehend, especially the mechanisms of damage progression leading to total failure. Numerical and experimental investigations were carried out on three different stacking sequences of notched, thin ply carbon/epoxy laminates. This paper presents a computational study of notched tensile tests (U-notch) using the Discrete Ply Modeling (DPM) method, which has already proved efficient on both in-plane and out-of-plane loading cases, such as pull through, low velocity impact and compression after impact. The specificities of this finite element model are its discrete nature (interface elements to model delamination and matrix cracks), the small number of parameters required, and its robustness. This work follows on from the study of open-hole tensile tests (same three layups) by the same authors [1] and analyzes the influence of layup and notch shape. Comparisons with experiments (using infrared technology) demonstrate that tensile strengths, and failure scenarios and patterns are predicted with acceptable accuracy

Experimental and numerical analysis of the shear nonlinear behaviour of Nomex honeycomb core: Application to insert sizing

This work is a contribution to the understanding of the nonlinear shear behaviour caused by cell postbuckling in Nomex honeycomb cores. First, an experimental benchmark study was made of different designs for the shear testing of honeycomb cores. Then, several test specimens were fabricated and tested, a 3D DIC system being used to measure and record the displacements. An Artificial Neural Network (ANN) was also used to identify the onset of bucking and collapse of the cells. The influence of the overall boundary conditions of shear tests on the buckling of the cells is presented both experimentally and numerically. The reversibility and test procedure results suggest that it may be possible to allow the shear strength to be increased by up to 35% under certain conditions