Influence of crack depth and strength matching on deformation and plastic zone for welded joint specimen

By using the 2-dimensional elasto-plastic finite element method, the effects of crack depth and strength matching properties of welds on the deformation and plastic zone have been studied for three point bend specimens. The results indicate that, in the loading process of a welded joint specimen, the influence of crack depth and strength matching properties on the deformation parameters such as loading point displacement, crack mouth opening displacement, plastic rotational factor, and development of plastic zone is evident. The boundary between shallow crack and deep crack specimens is also influenced by the strength matching of welded joints. Moreover, the crack depth and strength matching of the welded joints have an important effect on the fracture-resistant capability of the specimens. The fracture-resistant capabilities of shallow crack specimens and overmatched joint specimens are better than those of deep crack specimens and undermatched joint specimens, respectively.     

On the size of plastic zone ahead of crack tip

A method for the crack tip analysis of a tensile loaded crack (mode I) due to yielding of the material is developed. The stress/strain distribution within the plastic zone, as well as size of the plastic zone are presented. The development is based on the energy interpretation of the strain hardening exponent, and an analogy between mode III and mode I for the case of small scale yielding. Predictions of the proposed method are compared with the experimental results, and a fairly good agreement is observed. A number of proposed methods to estimate the plastic zone size for ductile materials are also discussed.     

Analysis of crack growth rate based on rotation at the crack tip plastic zone

The crack growth rate of a line crack is obtained from a linear elastic analysis of work required in the formation of a crack tip plastic zone. Equations of crack growth rate are derived from rigid body rotation at the root of Dugdale's plastic zone. The proposed relations are used to study the fracture behaviour of materials in tension tests and the three point bending tests. This revised version was published online in August 2006 with corrections to the Cover Date.     

A note on the plastic zone size ahead of the propagating fatigue crack

Crack length and plastic zone size measurements were made on a limited number of mild steel tensile specimens of ASTMGS No.3.5 and No. 7.0 grain size for various stress amplitudes and various mean stresses. The data are in agreement with the predictions of Bilby, Cottrell and Swinden concerning the static yielding, and indicate that maximum shearing stress rather than shear stress amplitude should be used for best correlation.

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Zusammenfassung An einer beschränkten Anzahl Zugprüflingen aus weichem Stahl der Korngrösse ASTMGS N 3,5 und No 7,0 wurden Risslänge und Grösse der plastischen Zone an der Risspitze bei verschiedenen Spannungsamplituden und verschiedenen mittleren Spannungen gemessen.Die erhaltenen Daten stimmen mit den von Bilby, Cottrell und Swinden für den Fall des statischen Versagens vorhergesagten Daten überein und zeigen an, dass im Hinblick auf eine gute Uebereinstimmung mit der Theorie eher die maximale Schubspannung als die Schubspannungsamplitude verwendet werden soll.

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Résumé Sur un nombre limité d'éprouvettes de traction d'un acier doux, dont la grosseur du grain ASTM était de 3,5 et de 7,0, on a mesuré la longueur de la fissure de fatigue, et l'étendue de la zone de déformation plastique précédant celle-ci, en fonction de diverses amplitudes de la tension alternée, et de divers niveaux de la tension moyenne.Les résultats sont en accord avec les prédictions de Bilby, Cottrell et Swinden concernant l'élongation statique et montrent que pour obtenir une corrélation optimale il convient de considérer la contrainte maximale de cisaillement plutôt que l'amplitude de la tension appliquée.

     

Influence of deformation anisotropy on the size of the plastic zone at the tip of an opening mode crack

The form and dimensions of the plastic zone at the tip of an opening mode crack in a plate made of a material with deformation anisotropy were investigated within the limits of the elastic solution. The anisotropy was caused by strengthening during plastic deformation until formation of cracks by loading in a straight trajectory located in the plane of the plate. It was shown that in the case of anisotropy caused by loading in a trajectory which is oriented on a normal to the crack edges the size of the plastic zone decreases and its boundaries are rotated in the direction opposite to the crack growth. Loading in a trajectory in the direction of crack growth leads to broadening of the plastic zone in the transverse direction.Translated from Problemy Prochnosti, No. 1, pp. 73–76, January, 1990.     

Influence of specimen size and microstructure on the fracture toughness of a martensitic stainless steel

The fracture toughness of alloy HT-9,² a martensitic stainless steel under consideration for fast reactor and fusion reactor applications, was determined from circular compact tension specimens using the multi-specimen R-curve method. Specimens with thicknesses of 11.94, 7.62 and 2.54mm and widths of 23.88 and 11.94 mm were tested to investigate the effects of specimen size on fracture toughness. The test results obtained from all specimens are in good agreement and thickness requirements for a valid J_1c test are satisfied. The experiment indicates that small specimens of HT-9 may be used for post-irradiation fracture toughness testing.Fractographic examination of the fracture surfaces reveals that fracture in HT-9 is significantly influenced by delta ferrite stringers present in the material. The fracture surface examination and crack opening displacement measurements for specimens tested at various temperatures are consistent with the temperature dependence of the J_1c results.     

An estimation of the plastic zone size for a bimaterial interfacial crack

This note deals with the size of the plastic zone ahead of an interfacial crack between two dissimilar isotropic elastic materials. Dugdale's concept of finite stress at the tip of the crack is used in the study. The plastic zone size is determined for plane stress problems under the von Mises yield condition.     

Ⅰ型裂纹尖端塑性区和无位错区及其对裂纹扩展的影响

用裂纹与位错的相互作用模拟了刃型位错从Ⅰ型裂纹尖端沿多个滑移面的发射,得到了裂纹尖端周围塑性区和无位错区的形状和大小.结果表明,与宏观断裂力学算出的塑性区形状相比,本文给出的塑性区向裂纹前方倾斜;无位错区的形状与塑性区的相似.以应变能密度因子理论为判据,当存在明显的无位错区时,塑性区使裂纹扩展的潜力下降,但扩展方向不变;而当塑性区充分发展、无位错区的作用减小或消失后,裂纹扩展的方向可能发生变化.     

Process zone analysis for the single-edge notched specimen: Part I. Process zone size and crack profile

A systematic analysis of the process zone is presented. Relations among the process zone size, load and crack opening displacement (COD) are derived using the weight function method and a power function closing pressure distribution in the process zone. These relations are specialized and used to study the behaviour of the single-edge notched (SEN) specimen loaded in tension and in bending. This study shows that the method presented here may be successfully used to analyze the fracture of finite-size specimens.     

Influence of the cyclic plastic zone size on the propagation of the fatigue crack in case of 12NC6 steel

Under cyclic loading, the plasticized zone becomes complicated; it contains in particular a second plasticized zone, resulting from the local compression which occurs at the time of the closing of the crack to each cycle. The two plastic zones, monotonous (r_m) and cyclic (r_c), are proportional to (K_max/R_e)² et (ΔK/R_e)², respectively. The objective of this work is to study the evolution of the fatigue crack grown rate (FCGR) and the influence of the plastic zone size (r_c), which represents the seat of the residual stresses, on this evolution in the case of 12NC6 steel. Generally, the plastic zone size increases with the crack advance. The FCGR can be correlated with the energy absorptive in these plastic zones.     

Effect of specimen size on fatigue crack growth rate in AISI 4340 steel

An investigation has been carried out to study the influence of specimen size parameters (thickness, with and aspect ratio) on fatigue crack growth rate. Compact tension specimens with a TL orientation, prepared from aircraft quality AISI 4340 steel and heat treated to a yield strength level of 1000 MPa, were used. All testing was done at a constant δK level. The investigation demonstrates that specimen thickness and width have no significant influence on fatigue crack growth rate for AISI 4340 steel. On the other hand, fatigue crack growth rate was found to increase marginally at high aspect ratios (a/W0.55). Paris constants C and m were also evaluated.     

A new method of plastic zone size determined based on maximum crack opening displacement

This paper presents a new method to determine the crack-tip plastic zone size (R_y) in the center cracked plate in tension. The maximum crack opening displacement (MCOD) is used to estimate R_y in this method. Based on the series of calculation results by finite element analysis, the explicit expression for the crack-tip plastic zone size versus MCOD is fitted by least square method. The expression enables to eliminate the influences of the yield stress of material, crack geometry, plate sizes and transverse stress. Therefore, the presented method of R_y determined by MCOD is suitable to any center crack finite plate of any material under uniaxial or biaxial tension.     

Process zone analysis for the single-edge notched specimen: Part II. Process zone growth and crack propagation

Process zone growth and crack propagation in the single-edge notched (SEN) specimen are studied using the relations among applied load, notional crack and process zone lengths, and crack opening displacement derived in the first part of this work [1]. Process zone growth is simulated by increasing the notional crack length while keeping the traction-free crack length constant. A model for crack propagation based on either critical crack tip opening displacement (CTOD) or critical process zone length, as criteria for traction-free crack extension is proposed. The influence of closing pressure distribution, initial traction-free crack length, and crack extension criterion on the behavior of load vs. CMOD curves is discussed. The present model can be used to model load-deformation behavior from initial loading through softening to failure of nonlinear materials, as is verified by comparing the theoretical and experimentally determined load vs. crack mouth opening (CMOD) curves for concrete beams.