Key Engineering Materials Vols. 452-453

Abstract: The tensile fracture behavior of polymer matrix composite materials was investigated with the aid of a nondestructive evaluation (NDE) technique. The materials, E-glass fiber reinforced epoxy matrix composites, which are applicable to bogie materials in railway vehicles to reduce weight, were used for this investigation. In order to explain stress-strain behavior of polymer matrix composite sample, the infrared thermography technique was employed. A high-speed infrared (IR) camera was used for in-situ monitoring of progressive damages of polymer matrix composite samples during tensile testing. In this investigation, the IR thermography technique was used to facilitate a better understanding of damage evolution, fracture mechanism, and failure mode of polymer matrix composite materials during monotonic loadings.

Abstract: Accurate three-dimensional stress-strain constitutive properties are essential for understanding of complex deformation and failure mechanisms for glass-fiber and carbon-fiber reinforced polymer-matrix composites. A large number of different methods and specimen types, which are currently required to generate three-dimensional allowables for structural design, slow down material characterization. Also, some of the material constitutive properties are never measured due to prohibitive cost of the specimens needed. This work shows that simple short-beam shear specimens are well-suited for measurement of 3D constitutive properties for composite systems. In particular, a methodology to measure tensile and compressive material properties, generate shear stress-strain curves and measure the shear strength in a simple short beam shear test will be presented. The methodology is based on the Digital Image Correlation (DIC) full-field deformation measurement. Short-beam and curved-beam tests are accomplished to generate 3D stress-strain response for glass/epoxy and carbon/epoxy tape composite material systems. Accuracy of constitutive properties is also verified using standard methods and data available in the public domain.

Abstract: Practically exact solutions of stress intensity factor for several two-dimensional standard specimens were calculated and shown in numeric tables. The solutions were confirmed to converge until 6 significant figures through a systematical computation of discretization analysis. The convergence analyses were carried out by using a general purpose program based on a body force method.

Abstract: Reliable prediction of fracture conditions is a major concern in the integrity assessment of structural components. This is specifically critical within the transition regime where there is a significant scatter in fracture test data. In recent years local stress based approaches that use a "Weibull distribution function" have been examined to predict probability of cleavage fracture at lower shelf temperature. Furthermore the role of constraint in toughness prediction has been noted. An extensive experimental programme known as "Euro fracture dataset" aimed at characterisation of the "Ductile-to-Brittle" transition (DBT) behaviour of ferritic steels. Recently this data set was used by authors to propose a set of "Global" equations for determination of temperature and thickness dependence of Weibull distribution parameters. In this paper finite element simulations of fracture tests are carried out firstly to verify the experimental findings and secondly to examine and validate the proposed "Global" equations. This objective has been achieved through the comparison between the experimental data, predictions of "Global" curves and the results of performed finite element simulations.

Abstract: Characterisation of failure of components subjected to impact fatigue has received much interest in recent years. Critical stress intensity factor, i.e. fracture toughness, is a characteristic parameter for fracture conditions. Evaluation of this parameter is therefore of primary importance in the study of structures containing cracks. Due to its significance numerous research work have been carried out to provide dynamic stress intensity descriptions under cyclic, impulse and impact loading conditions. These methods are mainly based on numerical analyses and / or experimental techniques led to a range of approximate models. This paper firstly provides a review of fatigue failure due to impact loading and explains the principles of impact mechanics concepts including impact loading, stress wave equation and resulting stress distributions. Then, based on available experimental studies on developing and propagating cracks under impact loading, suggests a simple model leading to an approximate analytical solution for determination of dynamic stress intensity factor, kd under high strain rate loading. Calculated values based on the suggested solution compare well with the experimental data.

Abstract: Corrosion modifies the steel-concrete interface in reinforced concrete structures. Moreover in structures subjected to cyclic load, a simultaneous mechanical deterioration due to the load is present. Both phenomena can lead to an evolution of cracks width during the service life. In order to evaluate the crack openings increase, an experimental campaign on reinforced concrete ties subjected to simultaneous loading and corrosion have been realized. Transversal crack opening, initially due to loading and longitudinal corrosion cracks evolution is monitored. Results highlight the differences in terms of corroded and uncorroded specimens, static and cycling test and also different loading amplitude. Finally it can be observed as the growing of the damage is significantly different when a cycling action, combined with a chemical attack is present.

Abstract: The low-cycle fatigue properties of cast nearly lamellar TiAl alloys with diverse Nb content at room temperature and 750°C were compared. Monotonic tensile curves, cyclic stress-strain curves (CSSC) and combined fatigue life curves were obtained at both temperatures. After cyclic straining the surface relief and the fracture surfaces were observed using scanning electron microscopy. The increase in Nb content significantly improves tensile properties and CSSCs at both temperatures. The combined fatigue life curves especially at 750 °C are shifted to higher fatigue lives. The effect o Nb content on the tensile and cyclic strength is discussed in relation to the thickness of the lamellae. Persistent slip markings formed along interlamellar interfaces were predominant locations for fatigue cracks. At both temperatures and materials showed similar crack initiation and propagation behavior. The cracks initiated at surface or in sub-surface region creating smooth flat areas corresponding to the persistent slip bands.

Abstract: The wedge splitting (WS) test is now a promising method to perform stable fracture mechanics tests on concrete-like quasi brittle materials. Fracture parameters, such as fracture toughness and critical crack opening displacement and et.al, are however not easy to determined since formulae available from stress intensity factor manual are restricted to standard specimen geometry. The paper attempts to compute expressions for commonly used fracture parameters for a general wedge splitting specimen. By means of finite element analysis program, test simulation was performed on non-standard wedge splitting specimen with different depth and initiation crack length, and thereafter expressions were proposed for stress intensity factor at the pre-cast tip and crack mouth opening displacement on the load line. Based on the work above, size effect on the unstable fracture toughness and crack extension were investigated, and the consistency of fracture toughness data for various specimen depth as well as initiation crack length is demonstrated. The crack extension is little sensitive to the initiation crack length, it increases with the depth of specimen, which can be explained by the boundary influence of the specimen.

Abstract: A crack in the tooth root is probably the least desirable problem in gear unit operation; it often leads to failure. Signals produced by a gear with a crack in the tooth root, produced through real operating conditions, and signals caused by a faultless gear are used for the analysis. By monitoring vibrations it is possible to detect the presence of a crack. A fatigue crack in the tooth root brings about significant changes in tooth stiffness. Other faults are usually linked with modifications of other dynamic parameters. Time Frequency Analysis tools, e.g. Wavelets Analyses, are used to analyse a non-stationary signal. The wavelet transform is chosen for the analysis. The wavelet function similar to the dynamic reaction of the crack in the tooth root is selected. By means of the methods and the analysis presented in this paper, the reliability of determining modifications in signal vibrations is improved.

Abstract: The influence of the cooling condition after solution treatment on the high temperature fatigue resistance of 23Cr-26Ni heat resistant steel was investigated. Two different cooling conditions were applied to the steel after solution treatment at 1230oC for 3 hours. One specimen was water quenched immediately after the solution treatment. The other one was furnace cooled at a rate of 0.5oC/min down to 750oC after the solution treatment. Then, both specimens were aged at 750oC for 5 hours. The low cycle fatigue (LCF) test was conducted to investigate the influence of high temperature on the LCF behaviors of the heat-resistant 23Cr26Ni alloy. Under two different heat treatment conditions, the LCF test was performed at total strain amplitudes ranging from ±0.4~0.9% at room temperature (RT) and 600°C. During the test, initial cyclic hardening occurred at both experimental temperatures. This phenomenon was attributed to the increase in the dislocation density due to cyclic deformation, which resulted in the interaction between the newly created dislocations and precipitates. Cyclic softening was observed in the later stages of the LCF test at RT. The formation of precipitates and increase in the dislocation density were observed using TEM. Also, the XRD and EDS techniques were used to verify the type and composition of the precipitates.