Key Engineering Materials Vols. 488-489

Abstract: In this study the adhesive joint fracture behaviour of a nano-toughened epoxy adhesive was investigated. Two experimental test methods were used; (i) the standard tapered double cantilever beam (TDCB) test to measure the mode I adhesive joint fracture energy, G_IC, as a function of bond gap thickness and (ii) a circumferentially deep notched tensile test to determine the cohesive strength of the adhesive for a range of constraint levels. It was found that the fracture energy of the adhesive followed the well-known bond gap thickness dependency [1]. SEM analysis of the TDCB fracture surfaces revealed significant plastic void growth. Finally, numerical modelling of the experimental tests suggested that most of the fracture energy was dissipated via highly localised plasticity in the fracture process zone ahead of the crack tip.

Abstract: Seam casing pipe used in an oil drilling rig, manufactured by high frequency (HF) contact welding of API J55 steel, is tested. The influence of an initial defect (machined surface crack) is analysed, by performing pressure test of a pipe segment closed at both ends. Besides the damages at the internal surface, casing pipes are exposed to damage at the external surface, which is why such configuration is analysed here. Measurement of strains and crack mouth opening displacement (CMOD) enabled the application of direct method for J integral evaluation. This procedure is based on the path independence of the J integral and can be applied both in laboratory conditions (on specimens) and on structures. However, it requires a demanding experimental - computational procedure, which is accomplished here using the developed routine. Additionally, the behaviour of the pipe under internal pressure, including fracture mechanics parameters determination, is modelled numerically (by finite element method) in software package Abaqus. The pressure is applied as distributed load acting on the inner surface of the three-dimensional model, and axial tension is applied at the end of the pipe to simulate the closed end. J integral values determined numerically and using direct method are used for estimation of the critical pressure corresponding to the crack growth initiation. Additionally, plastic limit load, i.e. pressure which causes yielding of the ligament, is determined. Based on the results, criteria for pipe integrity assessment are discussed

Abstract: The inherent resiliency, hardness and relatively low friction coefficient of the fullerene-like (FL) allotrope of carbon nitride (CN_x) thin solid films give them potential in numerous tribological applications. In this work, we study the substitution of N with P to grow FL-CP_x to achieve better cross- and inter-linking of the graphene planes, improving thus the material’s mechanical and tribological properties. The CN_x and CP_x films have been synthesized by DC magnetron sputtering. HRTEM have shown the CP_x films exhibit a short range ordered structure with FL characteristics for substrate temperature of 300 °C and for a phosphorus content of 10-15 at.%. These films show better mechanical properties in terms of hardness and resiliency compared to those of the FL-CN_x films. The low water adsorption of the films is correlated to the theoretical prediction for low density of dangling bonds in both, CN_x and CP_x. First-principles calculations based on Density Functional Theory (DFT) were performed to provide additional insight on the structure and bonding in CN_x, CP_x, and a-C compounds.

Abstract: The objective of this work is to describe part of the selecting process of a rubber-coated fabric material model. The material is used to construct an air cushion that is a carrying element of the cassette pontoon bridge unit. During operation the air cushion is permanently in contact with a metal component, fresh water and air. Therefore various interactions, such as a contact problem, flow of medium and thermodynamics can occur. The basic material model for numerical simulation was selected based on the uniaxial tensile test. The simple method was used to describe time-dependent material properties for numerical analysis, which allows computation to take a reasonable time. In order to assess the usefulness of the selected material model the impact puncture test was modelled with the same conditions and properties as in the laboratory testing machine called Instron. Moreover, an attempt of simulating the damage process is described. The energy absorbed by the material was registered during the laboratory test which was compared with the results of numerical analysis. An acceptable compatibility of the results is noticed.

Abstract: A 3D elastic-plastic FEM model for prediction of planar fatigue crack growth is presented. The model is based on the concept of local low-cycle fatigue of a small material volume in front of a high cycle crack. A local crack front advance is modelled by the successive release of finite element mesh nodes in the plane of propagation. The release of the nodes is controlled by the value of the Smith-Watson-Topper fatigue damage parameter in the surrounding elements. The effect of the single tensile overload on the fatigue crack growth and on the fatigue crack front shape is modelled.

Abstract: Fracture mechanics is an engineering discipline, which originated from Griffith energy-balance concept. The energy method is the most powerful analytical tool for fracture mechanics. Starting from energy principle in crack propagation, quasi-static governing equations and energy release rate of DCB specimen can be derived by energy principle in this paper. Finally, some correlative problems are discussed.

Abstract: The temperature monitoring on the surface of the railway brake disc was performed using high-speed infrared (IR) camera. The railway brake disc was developed for disc braking of maximum train speed of 180 km/h. The braking tests were conducted with a full scale dynamometer, and a high-speed infrared camera was employed to monitor temperature evolution on the brake disc during braking operation. The high-speed IR camera provides the measurement of temperature change during braking as well as the images of temperature contour on the brake disc surface. In general, the hot spot generation has been considered the main degradation mechanism in railway brake disc. In this investigation, damage evolution due to generation of hot spots on railway brake disc was investigated using the infrared thermography method.

Abstract: An approach to the structure buckling reliability analysis for the underwater super-speed supercavitating vehicle is proposed in this paper. The circular ribbed part-cabin of the vehicle is simplified as the thin cylindrical shell with variable thickness. The critical buckling load coefficient of the thin cylindrical shell is solved by using semi-analytical FEM (finite element method). The structure buckling reliability index is obtained by SFEM (Stochastic Finite Element Method) combining Limit Step Length Iteration Method. A numerical example is given to analyze influences of the number of circular ribs, the coefficient of variation of elastic modulus and the coefficient of variation of the thickness of the thin shell on the structure buckling reliability index.

Abstract: Several hypotheses and concepts exist for the prediction of crack deflection angles from the analysis of the undisturbed stress distribution at the crack front. In this paper, a finite element analysis of small penny shaped secondary cracks with deflection angles ψ₀ = π/4 at a pure mode III loaded crack front is presented. The results are compared with the solutions of the well-known 3D fracture criteria. The fracture mechanical analysis is performed by the highly effective MVCCI-method.

Abstract: In fracture tests involving the Single-Edge-Notched beam loaded in Three or Four-Point-Bending as well as in other specimen shapes, the specimen weight contributes to the overall loading of the system. Unless special methods are put into practice, the contribution of the specimen weight is not compensated, leading to misevaluations of fracture parameters obtained in the experiments. In this work a method taking into account the exact contribution of the specimen self-weight is proposed to evaluate the Resistance-curve of mortar. Cohesive crack modeling accounting for the structure self-weight is used to generate the necessary load-displacement response to perform the validation of the proposed method.