Fused deposition modelling (FDM) is a promising additive manufacturing technology and an alternative of conventional processes for the fabrication of fibre reinforced composites due to its ability to build functional parts having complex geometries. Continuous fibre reinforced thermoplastic composites (CFRTPCs) are becoming more significant in industrial applications due to their inherit advantages such as excellent mechanical performance, recycling and potential lightweight structures [1,2]. However, a major concern affecting the efficient use of 3D printed composites is the effect of impact damage on the structural integrity, compared to conventional pre-preg composites. The aim of this study is to evaluate the effect of build orientation, layer thickness and fibre volume content on the impact performance of 3D printed continuous carbon, glass, and Kevlar® fibre reinforced nylon composites, manufactured by FDM technique. Charpy impact tests are carried out to determine impact strength. SEM images of fractured surfaces are examined to assess failure mechanics of the different configurations. It is observed that the effect of layer thickness of nylon samples on the impact performance was different for flat and on-edge samples. Impact strength increases as layer thickness increases in flat samples but, conversely, it decreases in on-edge samples, depicting a more brittle fracture. In addition, the results show that impact strength increases as fibre volume content increases in most cases. Glass fibre reinforced samples exhibits the highest impact strength and carbon fibre reinforced samples the lowest one and similar to nylon performance. Furthermore, on-edge reinforced samples exhibit higher values of impact strength than flat reinforced samples. Finally, the results obtained demonstrate that impact strength exhibited by 3D printed composites are significantly higher than the usual 3D printed thermoplastics and, in some cases, even better than common pre-preg materials.

Impact damage resistance of 3D printed continuous fibre reinforced thermoplastic composites using fused deposition modelling

High performance composite materials, such as Carbon–Fibre Reinforced Plastic (CFRP) composites, are being increasingly used in aerospace industry, such as fuselage primary structures in Boeing 787 or Airbus 350, where high strength and stiffness are required at minimum weight [1] . The design of composite structures frequently includes discontinuities such as cut-outs for access and fastener holes for joining and they become critical regions under thermo-mechanical loading. Understanding of notched specimen behaviour is necessary for the design of complex structures where parts are mostly connected with bolts and rivets [2] . The effect of these discontinuities on the behaviour of composite materials is an important topic because it causes a relatively large reduction in strength compared to the unnotched laminate [3] . In the first part of the current work, the assessment of the damage process taking place in notched (open-hole) specimens under uniaxial tensile loading was studied. Two-dimensional (2D) and three-dimensional (3D) Digital Image Correlation (DIC) techniques were employed to obtain full-field surface strain measurements in carbon–fibre/epoxy M21/T700 composite plates with different stacking sequences in the presence of an open circular hole. Penetrant enhanced X-ray radiographs were taken to identify damage location and extent after loading around the hole. DIC strain fields were compared to numerical predictions. In the second part of the study, DIC techniques were used to characterise damage and performance of adhesively bonded patch repairs in composite panels under tensile loading. This part of work relates to strength/stiffness restoration of damaged composite aircraft that becomes more important as composites are used more extensively in the construction of modern jet airliners. In the current work, external bonded patches have been studied. Adhesively bonded repairs are the most common type of repair carried out with composite materials [1] , [4] . The behaviour of bonded patches under loading was monitored using DIC full-field strain measurements. Location and extent of damage identified by X-ray radiography correlates well with DIC strain results giving confidence to the technique for structural health monitoring of bonded patches.

Damage monitoring and analysis of composite laminates with an open hole and adhesively bonded repairs using digital image correlation

Simulation of delamination growth in multidirectional laminates under mode I and mixed mode I/II loadings using cohesive elements

Analysis of adhesively bonded repairs in composites: Damage detection and prognosis

Impact resistance and damage tolerance are of great significance in the design of composite structures. This study investigated the damage and failure mechanism of thin composite laminates under low-velocity impact and compression-after-impact (CAI) loading conditions. Four levels of impact energy were included in the test matrix. Delamination induced by low-velocity impact was captured using ultrasonic C-scan, and a three-dimensional (3D) digital image correlation (DIC) system was employed to measure full-field displacement during the CAI tests. Infrared thermography was also used to online monitor the thermal field variation of the test specimen during the impact and CAI process. The cross sections of typical tested specimens were inspected using an optical microscope and a scanning electron microscope (SEM). A 3D damage model that considers both interlaminar and intralaminar damage was proposed to study the complex damage and failure mechanism. Excellent correlation was obtained between the experimental results and the numerical results. The experimental results obtained from various tests and the results from the numerical simulation were combined to provide a new and deep insight of damage evolution and failure mechanisms under low-velocity impact and CAI loading conditions.

Damage and failure mechanism of thin composite laminates under low-velocity impact and compression-after-impact loading conditions

https://hal.archives-ouvertes.fr/hal-01997671/document

Assessment of failure criteria and damage evolution methods for composite laminates under low-velocity impact

Tensile behaviour of patch-repaired CFRP laminates

On the determination of delamination toughness by using multidirectional DCB specimens

In this paper, a new fatigue life prediction methodology is proposed by combining stiffness degradation and temperature variation measured by InfraRed Thermographic (IRT) camera. Firstly, the improved thermographic method is used to determine the fatigue limit by using the data of stabilized temperature rising. Following this, a two-parameter model is proposed to characterize the stiffness degradation of CFRP laminates with the increase of cycle numbers. After the calibration parameters and the calculation of the normalized failure threshold stiffness, the whole S - N curve can be obtained in a very short time. The proposed model is applied to both the experimental data of triaxially braided CFRP laminates from literature and those of unidirectional CFRP laminates obtained from our fatigue tests. Results show that predicted S - N curves have a good agreement with traditional tests. The principal interests of this model could be listed as follows: (i) it is a more general criterion applicable to different materials; (ii) it has more physical senses; (iii) it allows the determination of the total S-N curve for composite materials in a short time.

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A new model for fatigue life prediction based on infrared thermography and degradation process for CFRP composite laminates

The architecture of textile reinforcement affects the deformation and failure behavior of the textile reinforced composites. This paper presents an approximate method that incorporates the in-plane periodic meso structure of the textile composite in finite element models. In this approach, the representative unit cell (RUC) of a textile composite is divided into sub-cells. Instead of obtaining a homogeneous equivalent, these sub-cells are idealized and represented with laminates of different layups using shell elements. In this way, an RUC can be constructed with a small number of elements. This method holds the promise of creating a textile composite FE model with an improved accuracy without compromising computational efficiency. The sub-cell FE model was examined by modeling the stress–strain response of a triaxially braided composite and the results were encouraging. The predicted tensile and compressive responses were in good agreement with the experimental results.

Xiaojing Gong

Papers

Assessment of failure criteria and damage evolution methods for composite laminates under low-velocity impact

Tensile behaviour of patch-repaired CFRP laminates

On the determination of delamination toughness by using multidirectional DCB specimens

A new model for fatigue life prediction based on infrared thermography and degradation process for CFRP composite laminates

Strength prediction of a triaxially braided composite