Won Ho Yang

Mixed mode fatigue crack growth in aluminum plates with composite patches

Fatigue crack growth behavior in Al 6061-T6 thick aluminum plate with composite material patch was studied. A 0, 15, 30, 45 and 60° inclined crack plates repaired with patch were tested. The effect of patch and inclined angle were studied and compared to each other. Also, we investigated the crack propagation direction and debonding behavior during the fatigue crack growth test. In this paper, a study was conducted to get a fatigue life, fatigue crack growth ratio and crack growth direction. Finally, the effectiveness of composite patch on inclined cracked plate was investigated. The results demonstrated that there was a definite variation in fatigue life and fatigue crack growth behavior depending on the inclined crack angle.

Approximate weight function method for elliptical arc through cracks in mechanical joints

Mechanical joints such as bolted, riveted or pinned joints are widely used to join the constituent parts of structural components. Reliable stress intensity factor analysis of arbitrary cracks in mechanical joints is required for the safety evaluation or fracture mechanics design. It has been reported that cracks in mechanical joints usually nucleate as the corner crack and grow as the elliptical arc through crack. The weight function method is a useful technique to calculate the stress intensity factor using the appropriate weight function for a cracked body and the stress field of an uncracked body. In this paper, the weight function method for the two surface points of elliptical arc through cracks in mechanical joints is developed to analyze the mixed-mode stress intensity factors. Unknown coefficients included in the weight function are determined using the reference stress intensity factors obtained from finite element analysis.

Stress Intensity Factors for Elliptical Arc Through Cracks in Mechanical Joints by Virtual Crack Closure Technique

The reliable stress intensity factor analysis is required for fracture mechanics design or safety evaluation of mechanical joints at which cracks often initiate and grow. It has been reported that cracks in mechanical joints usually nucleate as corner cracks at the faying surface of joints and grow as elliptical arc through cracks, In this paper, three dimensional finite element analyses are performed for elliptical arc through cracks in mechanical joints. Thereafter stress intensity factors along elliptical crack front including two surface points are determined by the virtual crack closure technique. Virtual crack closure technique is a method to calculate stress intensity factor using the finite element analysis and can be applied to non-orthogonal mesh. As a result, the effects of clearance on the stress intensity factor are investigated and crack shape are then predicted.

Influence of inclined holes in measurement of residual stress by the hole drilling method

The hole drilling method is widely used in measuring residual stress in surfaces. In this method, the inclination of holes is one of the sources of error. This paper presents a finite element analysis of the influence of inclined holes on the uniaxial residual stress field. The error in stress has been found to increase proportionally to the correct inclined angle of the hole. The correction equations by which one may easily obtain the residual stress, taking account of the inclined angle and direction, have been derived. The error of stress due to the inclined hole has been reduced to around 1% using the correction equations.

Finite Element Analysis of the Stress Intensity Factor and the Residual Stress by Cold Expansion Method in CT Specimen

Cold expansion method retards the crack initiation due to the compressive residual stress developed on a hole surface. Most previous researches have shown only the beneficial distribution of residual stresses in the retardation of the crack initiation at the stress concentration area. Also, there have been only few studies on the relation between crack growth and residual stress around other adjacent holes. A few fastener holes of aircraft structures is a shot distance which is less than 20mm between holes. The purpose of this study is to provide better understanding of the residual stress effect around a hole in a structure as crack growth starts from another hole. By finite element method, this study showed that residual stress in a CT specimen is redistributed by cold expansion process and that tensile stress increases in proportion to the cold expansion ratio in the vicinity of the crack. Stress intensity factor increases as the cold expansion ratio increases.

3-Dimensional Finite Element Analysis of the Residual Stress by Cold Expansion Method and Interference Fit

Cold expansion method and interference fit of fastener hole have been used for over 40 years by the aircraft industry to improve the fatigue life of structures because they induce compressive residual stresses around holes. Especially, interference fits are very widely applied in the industry, because of their simple manufacturing process. There have been only few studies on the difference in the effect between cold expansion method and interference fit. The purpose of this study is to compare the effect between cold expansion method and interference fit according to plate thickness. Furthermore, residual stress distribution according to real clamping force is also investigated.

Analysis of the joint structure of the vehicle body by condensed joint matrix method

It is often necessary that the joint characteristics should be determined in the early stage of the vehicle body design. The researches on identification of joints in a vehicle body have been performed until the recent year. In this study, the joint characteristics of vehicle structure were expressed as the condensed matrix forms from the full joint stiffness matrix. The condensed joint stiffness matrix was applied to typical T-type and Edge-type joints, and the usefulness was confirmed. In addition, it was applied to the real center pillar model and the full vehicle body in order to validate the practical application.

Finite Element Analysis of the Residual Stress by Cold Expansion Method under the Influence of Adjacent Holes

Cold expansion of fastener holes is a mechanical process widely used in the aerospace industry. This treatment leads to an improvement of fatigue behavior due to the developed compressive residual stresses on the hole surface. Despite its importance to aerospace industries, little attention has been devoted to the accurate modeling of the process. This study is devoted to the modeling and simulation of the residual stress resulting from the cold expansion of two adjacent fastener holes. Simultaneous cold expansion of two adjacent holes lead to much higher compressive residual stress than sequential cold expansion.

A Study on Fatigue Characteristics under Residual Stress by Cold Expansion

Cold expansion method is an effective process to retard the crack initiation at a hole and to improve fatigue life by developing the compressive residual stress around the hole. Several researches have been treated for the crack propagating toward a hole around which the residual stress by cold expansion exists. The objective of this paper is to evaluate the fatigue characteristics under the residual stress by cold expansion. Fatigue tests are performed for the plate with a central hole to which cold expansion has been applied, and the effects of the residual stress on the fatigue life are evaluated. Also the fatigue crack growth tests are conducted for the CT specimen in which the residual stress by cold expansion has been generated. The change of fatigue crack growth characteristics by the residual stress is investigated and the effects of cold expansion ratio are evaluated.

A fracture mechanics analysis of a crack in a variable thickness plate having a fillet (thin-to-thick type)

Abstract Crack tip stress intensity factors for the cracks in variable thickness plates having a fillet subjected to tensile (mode I) loadings were obtained by three-dimensional finite element analysis. Stress intensity factors were calculated by the crack opening displacement method proposed by Ingraffea and Manu [Int. J. numer. Meth. Engng15, 1427–1445 (1980)]. The correction factor of the stress intensity factors was introduced in the form of F (λ, β, ω, ρ). And characteristics of the fatigue crack propagation behavior near the thickness interface were examined for 7075-T6 aluminum specimens. A series of thin-to-thick type variable thickness specimens having a fillet (CCT type) was prepared. Fatigue tests were performed under load control in tension-tension.