Experimental study of sandwich structures as armour against medium-velocity impacts

An experimental impact study has been conducted on sandwich structures to identify and improve armour solutions for aeronautical applications. The objectives are to find the best configurations, i.e. the non-perforated targets with the minimal weight and back deformations. Medium-velocity impacts (120 m/s) have been conducted using a 127 g spherical projectile. The targets are simply supported at the rear of the structure. Two potential choices of front skin have been identified for the sandwich structure: 3 mm thick AA5086-H111 aluminium plates and dry aramid stitched fabrics (between 8 and 18 plies). The dry stitched fabrics appear to be an original solution, which associates a lightweight structure and a good perforation resistance. Moreover, a strong coupling has been found between the front skin and the core. The impact tests indicate that aluminium honeycomb core associated with aluminium skins show mitigated results. However, the combination of dry fabric front skin and aluminium honeycomb show better performances than aluminium sandwiches, with a global weight decrease

Validation of low velocity impact modelling on different stacking sequences of CFRP laminates and influence of fibre failure

This paper presents a validation of low-velocity impact Finite Element (FE) modelling. Based on switching ply location of reference layup [02,452,902,-452]s T700GC/M21 laminated plates from Bouvet et al. (2012) [1], twelve possible layups under a constraint of double-ply, mirror-symmetric, balanced, and quasi- isotropic are allowed. However only seven layups are chosen for the study and one of them reveals the importance of longitudinal fibre compressive failure during impact events. Therefore, the second aspect of this work is the introduction of a fibre compressive failure law associated with fracture damage development. This makes it possible to improve the simulation for all seven different layups. Good correspondence is achieved between simulation and experiment for aspects such as delamination areas/shapes and force–displacement responses. The influence of the addition of fibre compressive failure according to fracture toughness in mode I is discussed

Failure analysis of CFRP laminates subjected to Compression After Impact: FE simulation using discrete interface elements

This paper presents a model for the numerical simulation of impact damage, permanent indentation and compression after impact (CAI) in CFRP laminates. The same model is used for the formation of damage developing during both low-velocity / low-energy impact tests and CAI tests. The different impact and CAI elementary damage types are taken into account, i.e. matrix cracking, fiber failure and interface delamination. Experimental tests and model results are compared, and this comparison is used to highlight the laminate failure scenario during residual compression tests. Finally, the impact energy effect on the residual strength is evaluated and compared to experimental results

Low velocity impact modeling in composite laminates capturing permanent indentation

This paper deals with impact damage and permanent indentation modeling. A numerical model has been elaborated in order to simulate the different impact damage types developing during low velocity/low energy impact. The three current damage types: matrix cracking, fiber failure and delamination, are simulated. Inter-laminar damage, i.e. interface delamination, is conventionally simulated using interface elements based on fracture mechanics. Intra-laminar damage, i.e. matrix cracks, is simulated using interface elements based on failure criterion. Fiber failure is simulated using degradation in the volume elements. The originality of this model is to simulate permanent indentation after impact with a ‘‘plastic-like’’model introduced in the matrix cracking elements. This model type is based on experimental observations showing matrix cracking debris which block crack closure. Lastly, experimental validation is performed, which demonstrates the model’s satisfactory relevance in simulating impact damage. This acceptable match between experiment and modeling confirms the interest of the novel approach proposed in this paper to describe the physics behind permanent indentation

Modelling of impact damage and permanent indentation on laminate composite plate

This paper deals with impact damage and permanent indentation modelling. A model enabling the formation of damages developing during a low velocity / low energy impact test on laminate composite panels has been elaborated. The different impact damages developing during an impact test, i.e. matrix cracking, fibres failure and interfaces delamination, are simulated. The interlaminar damages, i.e. interfaces delamination, are classically simulated thanks to interface finite elements based on the fracture mechanics. The particularity of this model is to account for the intralaminar damages, i.e. matrix cracks, thanks to interface finite elements which respect their discontinue character. These interface elements allow equally to simulate the permanent indentation during the impact unloading. This impact mark modelling is very original in the literature, and should allow to entirely design a composite structure thanks to impact damage tolerance

About the impact behavior of woven-ply carbon fiber-reinforced thermoplastic- and thermosetting-composites: A comparative study

This study is aimed at comparing the response of TS-based (epoxy) and TP-based (PPS or PEEK) laminates subjected to low velocity impacts. C-scan inspections showed that impact led to diamond-shaped damage resulting from different failure mechanisms: fiber breakages in warp and weft directions, more or less inter-laminar and intra-ply damage, and extensive delamination in C/PEEK and C/epoxy laminates. The permanent indentation can be ascribed to specific mechanisms which mainly depend on many factors including the ultimate out-of-plane shear strength, and the interlaminar fracture toughness in modes I–II–III. In TP-based laminates, the matrix plasticization seems to play an important role in matrix-rich areas by locally promoting permanent deformations. Fiber-bridging also prevents the plies from opening in mode I, and slows down the propagation of interlaminar and intralaminar cracks in modes II–III. Both mechanisms seem to reduce the extension of damages, in particular, the subsequent delamination for a given impact energy. In epoxy-based laminates, the debris of broken fibers and matrix get stuck in the cracks and the adjacent layers, and create a sort of blocking system that prevents the cracks and delamination from closing after impact

Permanent indentation characterization for low-velocity impact modelling using three-point bending test

This paper deals with the origin of permanent indentation in composite laminates subjected to low-velocity impact. The three-point bending test is used to exhibit a non-closure of matrix crack which is assumed as a cause of permanent indentation. According to the observation at microscopic level, this non-closure of crack is produced by the blocking of debris inside matrix cracking and the formation of cusps where mixed-mode delamination occurs. A simple physicallybased law of permanent indentation, ‘‘pseudo-plasticity’’, is proposed. This law is qualitatively tested by three-point bending finite element model and is lastly applied in low-velocity impact finite element model in order to predict the permanent indentation. A comparison between low-velocity impact experiments and simulations is presented

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

Reversible Rail Shear Apparatus Applied to the Study of Woven Laminate Shear Behavior

The multitude of in-plane shear tests existing in the literature seems to demonstrate the complexity of developing a test adapted to all experimental works. In a general framework of investigation of translaminar cracks in thin laminates, a test able to reproduce a pure in-plane shear loading was required. The laminate studied is notably employed as helicopter blade skin, and cyclic torsion induced by aerodynamic load involves cyclic in-plane shear. This particular application established some specifications for the test needed to carry out this study. To comply with them, an original technological solution has been developed from a three-rail shear test apparatus. This paper describes the resulting “reversible rail shear test” solution and its application to the study of in-plane shear behavior of a thin glass-epoxy laminate. The results concern plain and notched coupons under quasi-static loading, and crack growth tests under cyclic loading

Experimental and numerical study of AA5086-H111 aluminum plates subjected to impact

An experimental and numerical study of medium-velocity impact (within the range of 120 m/s) has been conducted on thin AA5086-H111 aluminum square plates. Targets with different thicknesses (between 2.5 and 4 mm), stratifications and aluminum alloys have been normally impacted by projectiles with 30 mm diameter and 127 g weight. Experimental results show that a compromise is to be found between the alloy strength and ductility, taking into account the impact velocity and energy. Ductile aluminum like AA5086-H111 grade subjected to medium-velocity impacts, showed the best perforation resistance. A finite element analysis was carried out using the ABAQUS finite element code. Slightly modified versions of the JohnsoneCook models of flow stress and fracture strain were applied. A good correlation between experimental and numerical results was found. The effect of strain rate appears to be predominant in the rupture initiation for the aluminum under consideration. Stratification seems to be advantageous compared to monolithic solutions. However, there are limitations to this tendency