Key Engineering Materials Vols. 488-489

Abstract: In this paper, advanced numerical simulations of curved crack growth in the case of mul­tiple crack systems in combination with the analysis of the plastic limit load by the lower bound theorem of plasticity are presented. In order to take additionally initiated cracks during the crack growth process into account, the numerical simulation algorithm has been extended by using the Smith-Watson-Topper (SWT) parameter in combination with a linear fatigue damage accumula­tion.

Abstract: The carrying capacity decreases continuously as the material property deteriorates under the action of some external factors. It is important to evaluate the damage of concrete, in which the damage parameter plays an important role. An experimentation method is designed in this paper, with which we can simulate the initial damage by mixing initiator into concrete. Using the elastic modulus of axial tension experimentation, the tension initial damage D0 and tension damage Peak value Df have been gained. The Size effect of concrete tension damage parameters is verified. The curve of damage-parameters and size is gained, the trend of the tension damage parameters is obtained.

Abstract: In the present paper, the mechanical model of dynamic propagation interface crack of the compression-shear mixed mode is proposed by using the elastic-viscoplastic constitutive model. Then the governing equations of propagation crack interface at crack tip are given. The numerical analysis is accomplished for the interface crack of compression-shear mixed mode by introducing a displacement potential function and some boundary conditions at interface crack tip. The distributed regularities of stress-strain fields of interface crack tip are discussed with several special parameters. The numerical results show that the viscosity effect is a main factor of interface propagating at crack-tip field, and the interface crack-tip is a viscoplastic field that is governed by viscosity coefficient、Mach number and singularity exponent.

Abstract: Thin-rim gears are often used in aircraft applications in order to reduce weight. The objective of this study is to investigate the effect of rotational speed (centrifugal force) on bending fatigue crack initiation life of thin-rim gear manufactured from case carburized 14NiCrMo13-4 steel. Stresses in gear are determined from two-dimensional finite element model, assuming plane stress conditions. The fact that, in actual thin-rim gear operation, a significant reversed stress occurs at the root of the tooth adjacent to the loaded tooth is considered. Material is assumed to be homogenous, isotropic and linear elastic. Elastic strains are calculated from obtained stresses and corrected using Neuber’s rule to account plasticity effects. The number of load cycles required to initiate bending fatigue crack is predicted using strain-life approach for variety of gear rotational speeds and rim thicknesses. Strain-controlled fatigue properties were approximated from material hardness, while the mean stress as well as residual stress effects are included through Morrow’s mean stress correction. The proposed approach is validated by comparison with available experimental data from literature and used for parametric studies. Predicted numbers of load cycles required to initiate potential bending fatigue crack are presented for the variety of cases studied.

Abstract: In Korea, the KC-100, which is a small scale piston propeller general aviation aircraft, has been developed to establish a domestic certificate infrastructure and system through the BASA(Bilateral Aviation Safety Agreement) program by KAI(Korea Aerospace Industries, Ltd.). This aircraft adopted the whole composite structure concept for an environmental friendly aircraft through low fuel consumption due to structure weight reduction. However the carbon/epoxy composite structure, which is mainly used for this aircraft, is very weak against foreign object damage. Therefore the purpose of the damage tolerance design philosophy is to ensure that the aircraft can operate safely for a period of time with damage present within the airframe. This study is to investigate the residual compressive strength of the carbon/epoxy UD and fabric laminate due to impact damages. Through investigation on compressive strength, design allowable of carbon/epoxy laminate is determined by the experiment to address design criteria of the composite structure.

Abstract: With the development of damage mechanics, many researchers have used it to analyze the constitutive equation of concrete. Since the special environment in the cold marine regions, the offshore structures are common to subject to the comprehensive effects of freeze-thaw action and chloride erosion. This might cause concrete materials degradation and reduce the mechanical performance of concrete seriously. In this paper, based on the analysis and mechanical experiments of concrete materials under the comprehensive effects of freeze-thaw action and chloride ion erosion, the damage evolution equation of concrete elastic modulus along with the freeze-thaw cycles and chloride ion contents was established. The effects of chloride ion were investigated during the process of concrete degradation. According to the damage evolution equation, a new constitutive equation of concrete under freeze-thaw action and chloride erosion was established. And then, by means of the element simulation analysis of concrete beams when subjected to the comprehensive actions, the feasibility and applicability of the equation was examined and discussed. In this equation, both the freeze-thaw action and chloride ion erosion were considered together. It will be more suitable for analyzing the durability of concrete structures in the real cold marine regions. It will also provide some references for concrete constitutive theory.

Abstract: Finite element models with a constitutive material behavior that represents the non-linear response of fiber composites are used to simulate the compressive failure mechanism i.e. kinkband formation. A constitutive material law in framework of micromechanical modeling containing comprehensive constitutive equations for the constituent materials is adopted to model the non-linear behavior of the unidirectional layered materials. This material law is implemented as UMAT user subroutine in ABAQUS/Standard to study kinkband formation. The methodology provides a procedure to investigate the kinkband formation due to fiber misalignment for various complex structures having voids and material discontinuities that occur normally during the manufacture of these unidirectional fiber composites.

Abstract: The fatigue crack propagation behaviour of a new third generation Al-Cu-Li alloy type 2050-T84 developed for aeronautical applications is studied in comparison to a new generation Al-Cu-Mg alloy type 2022-T851. The alloy resistance against crack growth is shown to depend on alloy composition, aging condition and atmosphere environment. The crack path and the growth rate at moderate DK and in the near-threshold domain are discussed in terms of the slip morphology with respect to the microstructure. The different crack propagation regimes, as identified by mean of micro-fractographic observations and EBSD analysis are discussed on the basis of a modelling framework elaborated for conventional metallic alloys.

Abstract: The paper contains the main results of an investigation of an effect of shot peening on residual fatigue life of specimens made of an aircraft V-95 Al-alloy (equivalent to 7075 alloy) containing physically short fatigue cracks. In the first part, an adaptation and verification of direct current potential drop method for detection and measurement of short crack growth is described and discussed. The specific properties of the material and quite large dimensions of flat specimens with side necking with a low stress concentration factor had to be considered when the position of electrodes was specified. The specimen type and dimensions were proposed considering the investigation of shot peening effects. Physically short fatigue cracks of the length from 0.2 mm to more than 3 mm, most of them between 0.8 – 1.5 mm, were prepared under high cycle fatigue loading of the constant nominal stress amplitude ±160 MPa. Specimens with existing short fatigue cracks were shot peened using two different groups of parameters. Crack growth characteristics after shot peening were measured and compared with crack growth in specimens without shot peening. Retardation of crack growth was significant particularly for cracks shorter than 2 mm. Results are also discussed form the viewpoint of fracture mechanics approach. For the specific stress amplitude, evaluated results enable to estimate threshold length of defects, which after the application of shot peening will be reliably arrested.