Simulations of delamination in CFRP laminates: effect of microstructural randomness

Due to their high specific strength and stiffness, fibre-reinforced composite materials are being increasingly used in structural applications where a high level of performance is important (e.g. aerospace, automotive, offshore structures, etc.). Performance in service of these composites is affected by multi-mechanism damage evolution under loading and environmental conditions. For instance, carbon fibre-reinforced laminates demonstrate a wide spectrum of failure mechanisms such as matrix cracking and delamination. These damage mechanisms can result in significant deterioration of the residual stiffness and load-bearing capacity of composite components and should be thoroughly investigated. The delamination failure mechanism is studied in this paper for a double cantilever beam (DCB) loaded in Mode I. Several sensitivity studies are performed to analyse the effects of mesh density and of parameters of the cohesive law on the character of damage propagation in laminates. The microstructural randomness of laminates that is responsible for non-uniform distributions of stresses in them even under uniform loading conditions is accounted for in the model. The random properties are introduced with the use of Weibull’s two-parameter probability density function. Several statistical realisations are carried out which show that the effect of microstructure could significantly affect the macroscopic response emphasizing the need to account for microstructural randomness for accurate predictions of load-carrying capacity of laminate composite structures

Modelling the effect of moisture on the depth sensing indentation response of a stereolithography polymer

Stereolithography (SL) resins are highly hygroscopic and their mechanical properties are significantly affected by the level of moisture in the environment. In addition, the load response of these materials is highly time dependent, hence, an appropriate rate dependent constitutive model is required to characterise their mechanical behaviour. In this work, the time dependent mechanical behaviour of an SL resin is investigated under varying humidity conditions using depth sensing indentation (DSI) tests. In the experimental study, a DSI system fitted with a humidity control unit was used to explore the influence of moisture on the mechanical properties of a SL resin. Samples were tested with 33.5%, 53.8%, 75.3%, 84.5% relative humidity (RH) inside the chamber while the temperature was kept constant at 22.5 C. It was seen that hardness and modulus decreased with increasing absorbed moisture in the resin. Material parameters obtained through bulk tests were used to develop a coupled stress-diffusion finite element model incorporating rate dependent material behaviour. It is proposed that this model can be used in predicting the effect of the environment on the performance of SL manufactured components

Drilling in cortical bone: a finite element model and experimental investigations

Bone drilling is an essential part of many orthopaedic surgery procedures, including those for internal fixation and for attaching prosthetics. Estimation and control of bone drilling forces are critical to prevent drill-bit breakthrough, excessive heat generation, and mechanical damage to the bone. An experimental and computational study of drilling in cortical bone has been conducted. A 3D finite element (FE) model for prediction of thrust forces experienced during bone drilling has been developed. The model incorporates the dynamic characteristics involved in the process along with geometrical considerations. An elastic-plastic material model is used to predict the behaviour of cortical bone during drilling. The average critical thrust forces and torques obtained using FE analysis are found to be in good agreement with the experimental results

Development of delamination in cross-ply laminates: effect of microstructure

Various aspects of the effect of microstructural randomness exhibited by carbon fibre-reinforced cross-ply laminates on the delamination damage mechanism is investigated in this paper. In the first part, the matrix cracks with different spacings measured in experiments are simulated using finite elements in order to obtain the levels of degradation and effective properties for a composite beam loaded in bending. The results show significant levels of degradation of obtained effective properties depicting the importance of accounting for the inherent stochasticity in these laminates. In the second part of the paper, initiation of delamination at an interface between 0° and 90° layers due to stress concentrations at tips matrix cracks is simulated for a beam under tension. Stochastic cohesive zone elements with fracture parameters presented as random fields are used to model this interface in a composite. Different values of the axial stress are obtained for initiation of damage for a number of realisations based on this approach. The results emphasize the need to take into consideration the microstructural randomness in fibre-reinforced laminates for adequate predictions of damage and load carrying capacities

Finite element modeling and experimentation of bone drilling forces

Bone drilling is an essential part of many orthopaedic surgery procedures, including those for internal fixation and for attaching prosthetics. Estimation and control of bone drilling forces are critical to prevent drill breakthrough, excessive heat generation, and mechanical damage to the bone. This paper presents a 3D finite element (FE) model for prediction of thrust forces experienced during bone drilling. The model incorporates the dynamic characteristics involved in the process along with the accurate geometrical considerations. The average critical thrust forces and torques obtained using FE analysis, for set of machining parameters are found to be in good agreement with the experimental results

Investigation of the effect of relative humidity on polymers by depth sensing indentation

Stereolithography (SL) resins absorb varying amounts of moisture dependent on the relative humidities, which can significantly affect the mechanical properties. In this work, the influence of relative humidity (RH) on the mechanical behaviour of an SL resin is investigated using depth sensing indentation (DSI). The samples were conditioned by two methods. In the first method, samples were pre-conditioned at 33.5, 53.8, 75.3 and 84.5% RH using saturated salt solutions. These preconditioned samples were tested at 33.5% RH, using a humidity control unit (HCU) to control RH in the DSI system. In the second method, samples were conditioned and tested at 33.5, 53.8, 75.3 and 84.5% RH by regulating humidity in the DSI system using the HCU. Temperature was kept constant at 22.5 C for the conditioning and DSI testing. It was seen that hardness and modulus decreased with increasing RH and conditioning time but recovered significantly when tested after drying. This study demonstrates that RH needs to be taken into account during the DSI testing of polymers

Finite element modeling and experimentation of bone drilling forces

Bone drilling is an essential part of many orthopaedic surgery procedures, including those for internal fixation and for attaching prosthetics. Estimation and control of bone drilling forces are critical to prevent drill breakthrough, excessive heat generation, and mechanical damage to the bone. This paper presents a 3D finite element (FE) model for prediction of thrust forces experienced during bone drilling. The model incorporates the dynamic characteristics involved in the process along with the accurate geometrical considerations. The average critical thrust forces and torques obtained using FE analysis, for set of machining parameters are found to be in good agreement with the experimental results

Damage assessment in CFRP laminates exposed to impact fatigue loading

Demand for advanced engineering composites in the aerospace industry is increasing continuously. Lately, carbon fibre reinforced polymers (CFRPs) became one of the most important structural materials in the industry due to a combination of characteristics such as: excellent stiffness, high strength-to-weight ratio, and ease of manufacture according to application. In service, aerospace composite components and structures are exposed to various transient loads, some of which can propagate in them as cyclic impacts. A typical example is an effect of the wind gusts during flight. This type of loading is known as impact fatigue (IF); it is a repetition of low-energy impacts. Such loads can cause various types of damage in composites: fibre breaking, transverse matrix cracking, de-bonding between fibres and matrix and delamination resulting in reduction of residual stiffness and loss of functionality. Furthermore, this damage is often sub-surface, which reinforces the need for more regular inspection. The effects of IF are of major importance due its detrimental effect on the structural integrity of components that can be generated after relatively few impacts at low force levels compared to those in a standard fatigue regime. This study utilises an innovative testing system with the capability of subjecting specimens to a series of repetitive impacts. The primary subject of this paper is to assess the damaging effect of IF on the behaviour of drilled CFRP specimens, exposed to such loading. A detailed damage analysis is implemented utilising an X-ray micro computed tomography system. The main findings suggested that at early stages of life damage is governed by o degree splits along the length of the specimens resulting in a 20% reduction of stiffness. The final failure damage scenario indicated that transverse crasks in the 90 degree plies are the main reason for complete delamination which can be translated to a 50% stiffness reduction

Three-dimensional analysis of the effect of material randomness on the damage behaviour of CFRP laminates with stochastic cohesive-zone elements

Laminated carbon fibre-reinforced polymer (CFRP) composites are already well established in structural applications where high specific strength and stiffness are required. Damage in these laminates is usually localised and may involve numerous mechanisms, such as matrix cracking, laminate delamination, fibre de-bonding or fibre breakage. Microstructures in CFRPs are non-uniform and irregular, resulting in an element of randomness in the localised damage. This may in turn affect the global properties and failure parameters of components made of CFRPs. This raises the question of whether the inherent stochasticity of localised damage is of significance in terms of the global properties and design methods for such materials. This paper presents a numerical modelling based analysis of the effect of material randomness on delamination damage in CFRP materials by the implementation of a stochastic cohesive-zone model (CZM) within the framework of the finite-element (FE) method. The initiation and propagation of delamination in a unidirectional CFRP double-cantilever beam (DCB) specimen loaded under mode-I was analyzed, accounting for the inherent microstructural stochasticity exhibited by such laminates via the stochastic CZM. Various statistical realizations for a half-scatter of 50 % of fracture energy were performed, with a probability distribution based on Weibull's two-parameter probability density function. The damaged area and the crack lengths in laminates were analyzed, and the results showed higher values of those parameters for random realizations compared to the uniform case for the same levels of applied displacement. This indicates that deterministic analysis of composites using average properties may be non-conservative and a method based on probability may be more appropriate. © 2013 Springer Science+Business Media Dordrecht

Damage in adhesively bonded CFRP joints : sinusoidal and impact-fatigue

The main aim of this paper is to investigate the behaviour of adhesively bonded CFRP joints subjected to cyclic low-velocity impacts and to compare this with fracture in specimens tested in standard fatigue (i.e. non-impacting, constant amplitude, sinusoidal fatigue). It is seen that the accumulated energy associated with damage in impact-fatigue is significantly lower than that associated with similar damage in standard fatigue and that the mechanisms of failure are very different for the two loading regimes. For both types of loading, fracture initiates in the adhesive layer and then propagates into the 0º ply of the composite adjacent to the adhesive layer. However, the fracture surfaces after impact-fatigue are generally less uniform and exhibit more signs of high rate/brittle fracture than seen in the fracture surfaces after standard fatigue testing. Various parameters are proposed to characterise damage in standard and impact-fatigue and it is shown that crack velocity, accumulated absorbed energy and normalised maximum force are all useful parameters for characterising damage evolution