Papers by Keyword: Delamination

Abstract: Carbon Fibre Reinforced Thermoplastics (CFRTP) are expected to be used in various fields for the point of their superior mechanical properties. CFRP laminates with continuous fibres tend to be damaged by microcracks in the layer and interlaminar delamination. Especially, it is necessary to evaluate the mode II delamination growth property, which is correlated with compression after impact (CAI) strength. It is reported that CF/Epoxy laminates with a thicker interlaminar resin layer show higher toughness. By applying an extra thick interlaminar resin layer to CFRTP in which thermoplastic resin with relatively higher fracture toughness is used for the matrix, CFRTP with higher interlaminar fracture toughness can be developed. In this study, the mode II delamination growth property of CFRTP laminates under static loading was evaluated for the specimens with various layer thicknesses of polyamide (PA) resin in the middle layer of the laminates. Their moldability and damage propagation properties were evaluated by three-point bending tests and end notched flexure (ENF) tests. CF/PA laminated composites with a thicker PA layer showed superior mode II delamination growth property under static loading since they had more ductile fracture due to a thicker PA layer.

Abstract: A delamination of thermal barrier coatings (TBC) applied to turbine blades in gas turbine could be caused by a high-velocity impingement of various foreign objects. It is important to accurately predict the size of interfacial crack for safety operation of gas turbine. In this study, in order to establish a practical equation for prediction of the length of interfacial crack, a high velocity impingement test and a finite element analysis (FEA) based on a cohesive model were conducted. As the result, the length of interfacial crack is linearly increased with the impact velocity. In addition, it was confirmed that it was accurately estimated by the FEA. The equation for prediction of the length of interfacial crack was formulated based on these results and the energy conservation before and after impingement. Finally, the applicability of the equation was demonstrated in a wide range of impact velocity through a comparison with the experimental results.

Abstract: Composite materials damage behaviour is, nowadays, extensively investigated in the frame of aerospace research programmes. Among the several failure mechanisms which can affect composites, delamination can be considered as the most critical one, especially when combined to compressive loading conditions. In this context, nanofillers can represent an effective way to increase the composites fracture toughness with a consequent reduction of the delamination onset and evolution. Hence, in this paper, the toughening effect of the nanofillers on the delamination growth in composite material panels, subject to compressive load, has been numerically studied. A validated robust numerical procedure for the prediction of the delamination growth in composite materials panel, named SMXB and based on the VCCT-Fail release approach, has been used to perform numerical analyses by considering two different types of nanofillers. Reference material, without nanofillers insertion, has been used as benchmark in order to assess the capability of nanofillers to enhance the fracture toughness in composite laminates.

Abstract: Delamination is a well-known issue in polymeric composite materials. Interlaminar Shear Stress (ILSS) plays an important role in delamination initiation and thus its value needs to be monitored when designing a composite component. In this paper a Short Beam Shear experimental test of a carbon/epoxy specimen reproducing a Formula 1 component laminate is represented through a Finite Element analysis of two different models, one featuring 2D shell elements and the other 3D elements. ILSS results from both models are compared to experimental data in order to assess whether a 2D shell model can be used instead of its 3D counterpart, in favour of design versatility.

Abstract: This paper describes the physics of the interaction of the eddy current probe and the delamination in multidirectional carbon fiber reinforced polymer (CFDR). Currently, there is an opinion that eddy current signals obtained due to delamination in multidirectional CFRP are caused by the redistribution of the vertical eddy current arising from the excitation of eddy currents in the sample by a rectangular coil whose surface is orthogonal to the surface of the sample. The analysis performed by us shows that the possibility of creation and deformation of the vertical eddy current is very questionable, and that the observed eddy current signals associated with the delamination are most likely caused by carbon fiber plies misalignment towards the sample surface. In this paper was compare eddy-current signals from the delamination in CFRP in samples where the existence of vertical eddy current is possible in theory and in samples where this is not possible. The obtained signals had similar measurements, and this indirectly indicates the doubtfulness of the hypothesis of the delamination detection in CFRP samples due to vertical eddy current.

Abstract: The main results of an experimental program concerning masonry pillars reinforced by CFRP strips (also provided by spike anchors) subjected to single lap shear tests are described in this paper. The experimental results, also compared with a previous experimental program, allowed to analyze the increment of bearing capacity produced by spike anchors to CFRP sheets having different bonding length.

Abstract: Delamination or interlaminar fracture often occurs in composite laminate due to several factors such as high interlaminar stress, stress concentration, impact stress as well as imperfections in manufacturing processes. In this study, finite element (FE) simulation of mode I delamination in double cantilever beam (DCB) specimen of carbon fiber/epoxy laminate HTA/6376C is investigated using cohesive zone model (CZM). 3D geometry of DCB specimen is developed in ANSYS Mechanical software and 8-node interface elements with bi-linear formulation are employed to connect the upper and lower parts of DCB. Effect of variation of number of elements on the laminate critical force is particularly examined. The mesh variation includes coarse, fine, and finest mesh. Simulation results show that the finest mesh needs to be employed to produce an accurate assessment of laminate critical force, which is compared with the one obtained from exact solution. This study hence addresses suitable number of elements as a reference to be used for 3D simulation of delamination progress in the composite laminate, which is less explored in existing studies of delamination of composites so far.

Abstract: The fighter planes of the fifth generation, as compared to the fourth, has increased the composite materials use since high mechanical properties enhance the operational performance. However, composite materials present some negative characteristics such as damage progression, leading to delamination or matrix failure. This phenomenon often results in an unknown behaviour. This paper deals with the delamination phenomenon after casual low energy impacts. On different specimens, we performed different investigations through dynamic characterization, followed by modal parameters elaboration. After completing the first phase an opportune simulation model by FEM was developed to compare the obtained results whose values were in agreement with the real case.

Abstract: In this paper a novel Cohesive Zone Model (CZM) is derived within the framework of continuum thermodynamics to describe cracking and delamination behaviour of coatings at high-temperatures. The separation variable in the Traction-Separation-Law (TSL) is decomposed into elastic and inelastic part. For evolution of inelastic separation, a power-law in combination with a damage evolution law is used to consider the tertiary stage of inelastic separation of the interface, additionally. Thereby, damage evolution is related to the corresponding thermodynamic driving force and the inelastic opening rate. For reasons of simplicity the resulting thermo-mechanical problem only considers heat conduction through the interface. Due to the fact that standard Newton-Raphson procedure gets unstable (e.g. snap-back) when softening occurs which is the case by using a CZM, this model is enhanced with the damage gradient, similar to approaches in phase field modelling. Further on, this extension is done to investigate if it is possible to overcome the size dependence of CZMs. Finally, the model is reduced to pure Mode I opening and an example for a Double Cantilever Beam (DCB) is analysed by the finite difference method.

Abstract: Along with high tensile strength, high carbon steel wires must possess sufficient torsional ductility is to avoid longitudinal splitting along the wire axis, known as delamination. Often, wires ductile in the bright (asdrawn) surface condition exhibit delamination failure after undergoing a post-drawing surface treatment such as hot-dip galvanizing. The objectives of this study were to examine the influence of postdrawing heat treatment time and temperature on drawn wire mechanical properties, and to identify possible treatment conditions that suppress delamination. Stelmor cooling or lead patenting prior to drawing were used to develop a pearlitic microstructure. Salt pot heat treatments simulated postdrawing heat treatments; experimental heat treatments were conducted between 325 °C and 475 °C for 20 s or at 450 °C for immersion times between 1 s and 20 s. Tension and torsion tests quantified the changes in mechanical properties due to aging. Lead patented wires experienced greater tensile strength and torsional ductility changes with aging time and temperature when compared to the Stelmor cooled wires. This increased sensitivity was attributed to greater dislocation recovery in the patented wire after drawing.