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

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

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

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

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

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

Impacts on foam stabilised composite structures: experimental and numerical study

A dropweight tester is used to make low velocity tests on specific sandwich type structures. Sandwich are made of glass-epoxy skin and polyurethane foam core. The skins can be straight or little curved, and impact direction is the global skin direction. The aim of these tests is to study the initiation of rupture in such structures :local buckling of skin and foam core rupture. Experimental results are given. They show the evolution of buckling critical stress in the skin when impact velocity increases. The rupture mode in curved skin specimen is also studied : rupture is no more provoked by buckling. A numerical analysis is proposed to model the behaviour of the structure and the rupture initiation. Finally, a method is developed, in order to predict the propagation of skin debonding during impact : an element layer under the skin is damaged with a specific law to simulate debonding

Experimental investigation on mean crushing stress characterization of carbon–epoxy plies under compressive crushing mode

A challenge in numerical simulation of crashworthiness study is to be able to predict the crush damage modes, their evolution during crushing and the energy absorption in any composite structure from elementary material characterisation data. Therefore, it is important to know the behavior of one ply subjected to crushing load, and especially to determine the mean crushing stress that could be used for simulation. For that purpose, quasi-static crushing tests are performed for different configurations of two CFRP materials, UD and fabrics to determine the mean crushing stress of plies alone and inside a laminate. This study shows there is a linear relationship between crushing load and the contact surface of the plies being crushed on a metallic base which enables actually to define a mean crushing stress for a ply. The method to calculate this value is presented in this paper, based on image analysis of specific crushing tests. Experimental results show that for the UD material, the mean crushing stresses in 0° and 90° plies are very close. The value for a balanced fabric is also similar, approximately 270 MPa

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

Buckling of foam stabilised composite structures

An analytical modelling of the symmetrical wrinkling is proposed : from original assumptions on displacements within the core, and from an energy minimisation method, it is possible to predict critical loads and buckling modes better than traditional models do, and to distinguish the influence of each structure component. Compression tests were carried out on sandwich structures to validate the model. Little curved structures were also tested to estimate the influence of skin curvature on rupture and buckling mode. A finite elements analysis has been achieved in parallel : a fine modelling allows to find results close to experimental ones