Stress Wave Radiation From a Crack Tip During Dynamic Initiation

1992 ◽  
Vol 59 (2) ◽  
pp. 356-365 ◽  
Author(s):  
V. Prakash ◽  
L. B. Freund ◽  
R. J. Clifton
Keyword(s):  
Fracture Mechanics ◽  
Plane Strain ◽  
Dynamic Fracture ◽  
Rear Surface ◽  
Crack Tip ◽  
Interparticle Spacing ◽  
Stress Fields ◽  
Wave Radiation ◽  
Normal Velocity ◽  
Square Root

Plate impact experiments are conducted to study the dynamic fracture processes which occur on submicrosecond time scales. These experiments involve the plane strain loading of a plane crack by a square tensile pulse with a duration of approximately one microsecond. The crack-tip loading rates achieved are K1 ˜ 108MPams−1, which are approximately two orders of magnitude higher than those obtained in other dynamic fracture configurations. Motion of the rear surface caused by waves diffracted from the stationary crack and by waves emitted by the running crack is monitored at four points ahead of the crack tip using a laser interferometer system. The measured normal velocity of the rear surface of the specimen agrees very well with the scattered fields computed using an assumed elastic viscoplastic model, except for the appearance of a sharp spike with a duration of less than 80 nanoseconds. This spike, which is not predicted by the inverse square root singular stress fields of linear elastic fracture mechanics, is understood to be related to the onset of crack growth and coincides with the abrupt and unstable ductile growth of a microstructural void to coalescence with the main crack. The crack initiation process is modeled as the sudden formation of a very small hole at the crack tip. This admits the possibility of dynamic crack-tip stress fields with crack-tip singularities stronger (˜r−3/2) than the inverse square root singular fields of fracture mechanics. The elastodynamic radiation resulting from the formation of a traction free hole at the crack tip is applied first to the case of antiplane shear deformation and then to the corresponding plane strain problem. The radiated fields predicted by the strongly singular solutions are found to be in good agreement with the spikes observed in the experiments. The radius of the hole, which appears as a parameter in the solution for the radiated field, agrees reasonably well with the interparticle spacing.

scholarly journals A Numerical Analysis of Selected Elastic-Plastic Fracture Parameters for DEN(T) Plates under Plane Strain Conditions

2017 ◽  
Vol 22 (1) ◽  
pp. 49-80 ◽  
Author(s):  
M. Graba
Keyword(s):  
Numerical Analysis ◽  
Fracture Mechanics ◽  
Plane Strain ◽  
Plastic Material ◽  
Crack Tip ◽  
Limit Load ◽  
Material Model ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Elastic Plastic

Abstract This paper provides a numerical analysis of selected parameters of fracture mechanics for double-edge notched specimens in tension, DEN(T), under plane strain conditions. The analysis was performed using the elastic-plastic material model. The study involved determining the stress distribution near the crack tip for both small and large deformations. The limit load solution was verified. The J-integral, the crack tip opening displacement, and the load line displacement were determined using the numerical method to propose the new hybrid solutions for calculating these parameters. The investigations also aimed to identify the influence of the plate geometry and the material characteristics on the parameters under consideration. This paper is a continuation of the author’s previous studies and simulations in the field of elastic-plastic fracture mechanics [4, 6, 16, 17, 31].

Influence of Porosity on Plane Strain Tensile Crack-Tip Stress Fields in Elastic-Plastic Materials: Part II

1993 ◽  
Vol 60 (4) ◽  
pp. 883-889 ◽  
Author(s):  
Y. Miao ◽  
W. J. Drugan
Keyword(s):  
Plane Strain ◽  
Plastic Material ◽  
Normal Component ◽  
Symmetry Plane ◽  
Crack Tip ◽  
Stress Fields ◽  
Elastic Response ◽  
Tensile Crack ◽  
Rapid Variation ◽  
Porosity Level

This paper continues the investigation of Drugan and Miao (1992). There we studied analytically the influence of a uniform porosity distribution on the stress field near a plane strain tensile crack tip in ductile (elastic-ideally plastic) material, assuming that material very near the tip is at yield at all angles about the tip. Our solutions exhibited completely continuous stress fields for porosity f ≤ 0.02979, but for higher porosities they involved radial surfaces of radial normal stress jumps. Here we investigate whether, for this higher range of porosity, relaxing our assumption of yield at all angles about the tip will facilitate solutions exhibiting fully continuous stress fields. The answer to this is shown to be yes, with a single near-tip sector assembly providing such solutions for this entire higher porosity range. On either side of the crack symmetry plane, this solution configuration consists of a leading plastic sector possessing radial stress characteristics (“generalized centered fan ”), followed by a plastic sector of constant Cartesian components of stress, followed finally by a sector of purely elastic material adjacent to the crack flank. The angular extents of these sectors vary substantially with porosity level. In regions of purely elastic response, we have accounted for the influence of porosity on the overall, or effective, elastic moduli. Among the interesting features of these new solutions are a significantly enlarged generalized centered fan sector as compared to that of the fully plastic Part I solutions for the same f values, and for f values just slightly above the 0.02979 level, a narrow elastic sector exists in which stresses vary so rapidly with angle that they appear to be nearly discontinuous. This rapid variation spreads out as the elastic sector enlarges with increasing f, and, in contrast to the fully plastic solutions, the radial normal component of stress becomes negative near the crack flank.

Analysis of the Influence of Microstructure Heterogeneity and Mechanical Properties of Welded Joint Constituents on Fracture Toughness for Plane Strain, KIc

2011 ◽  
Vol 488-489 ◽  
pp. 617-620
Author(s):  
Ivica Čamagić ◽  
Nemanja Vasić ◽  
Zijah Burzić ◽  
Aleksandar Sedmak
Keyword(s):  
Mechanical Properties ◽  
Fracture Toughness ◽  
Fracture Mechanics ◽  
Plane Strain ◽  
Welded Joint ◽  
Crack Tip ◽  
Critical Areas ◽  
Theoretical Assumptions ◽  
Geometrical Form

The problem of fracture toughness, KIc, determination at a crack tip localized in a welded joint is placed in principle because fracture mechanics assumes homogenous material, not only around the crack tip but on a distance from it, in order to maintain valid theoretical assumptions and importance of the fracture toughness as the property measured by some of the fracture mechanics methods. Welded joint, as an integral part of a structure, represents inhomogeneity by microstructure and mechanical properties, often by geometrical form, and by the stress field as well, which are affected by various factors as well as residual stresses after welding [1]. However, these general difficulties did not disenable experimental determination of the fracture toughness under plane strain, KIc, in certain critical areas of a welded joint, or welded joint as a whole, but rather there are difficulties in interpreting the meaning of the measured values. Specimens for the fracture mechanics parameters determination are specimens with cracks, and cracks appears in welded joints as the most critical defects.

Applicability of Precracked Charpy Specimens for Determining Fatigue Crack Growth and J-R Properties: Experiments and Assessment of K and J Dominance

2014 ◽  
Author(s):  
Gustavo Henrique B. Donato ◽  
Rodrygo Figueiredo Moço ◽  
Tatiane Rossi Merlo
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Fracture Mechanics ◽  
Crack Growth ◽  
Driving Forces ◽  
Crack Tip ◽  
Structural Steels ◽  
Stress Fields ◽  
Small Scale ◽  
Main Challenge

Structural integrity assessments regarding Fatigue Crack Growth (FCG) and fracture phenomena are based on fracture mechanics theoretical background and rely upon the notion that a single parameter (usually K or J, respectively for linear elastic and elastic-plastic fracture mechanics) characterizes the crack-tip stress fields and controls local damage. However, the validity of K/J as crack-tip driving forces representative of local stress fields is only achieved if SSY (Small Scale Yielding) conditions prevail. It means that plasticity ahead of the crack must be small. Current standards (e.g.: ASTM E399, E1820, E647, ISO 12135) impose severe geometrical restrictions for the specimens (minimum thicknesses and crack depths) looking for plane strain (high constraint) conditions and therefore K and J-dominance. The main challenge is that thicknesses and/or planar dimensions of current real structures made of high toughness structural steels are in several cases not enough for the extraction of “valid” C(T), SE(B) or SE(T) specimens. In this context, subsized specimens are of great interest. As an example, Charpy geometries have been investigated during the last decades. This work is concerned about testing high structural steels and investigates the applicability of fatigue-precracked Charpy specimens for determining FCG (da/dN vs. ΔK) and J-R curves. The main issues are: i) verify the feasibility of the experiments in a servohydraulic machine in terms of scatter, control and repeatability; ii) quantify the validity limits of K and J for such reduced geometries. Samples had notches machined by EDM and were precracked reaching a/W=0.25 and a/W=0.45. FCG and J-R tests were successfully conducted with repeatability and refined 3D non-linear FE models were developed to provide compliance solutions and verify K and J dominance. Consequently, mechanical properties from subsized samples could be obtained and compared to data obtained from standardized C(T) specimens made of the same steel. The applicability of precracked Charpy geometry could be investigated, motivating further investigations in the field.

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