Hiroomi Homma

Pulsed holographic microscopy as a measurement method of dynamic fracture toughness for fast propagating cracks

Abstract A method of pulsed holographic microscopy is applied to take instantaneous microscopic photographs of the neighborhoods of crack tips propagating through PMMA or through AISI 4340 steel specimens at a speed of several hundred meters per second. The cracks are in the opening mode. A fast propagating crack is recorded as a hologram at an instant during its propagation. A microscopic photograph of the crack is taken with a conventional microscope to magnify the reconstructed image from the hologram. From the microscopic photograph, crack opening displacement (COD) is measured along the crack in the vicinity of the crack tip. The COD is of the order often to one hundred microns, and in proportion to the square root of the distance from the crack tip. The dynamic fracture toughness K ID is obtained using the formula for COD in the singular stress field of a fast propagating crack. Simultaneous K ID measurement both through pulsed holographic microscopy and through the caustic method is furthermore carried out with PMMA specimens. The values of K ID obtained through pulsed holographic microscopy are in agreement with those through the caustic method. Microcracks accompanied by a main crack are also photographed with the method of pulsed holographic microscopy.

Crack Tip Plasticity By Classic Dislocation Dynamics

Under very short pulse loads in range from 25 to 100 μs, crack tip plasticity a head of the crack tip in the mode I condition was investigated by discrete dislocation dynamics. The obtained dislocation array parameters such as the number of dislocations, dislocation distribution density, crack tip opening displacement and plastic zone size increase with the magnitude of stress intensity factor, KI and pulse durations. The numerical results were well compared with the experimental ones.

Generation and characterization of longitudinal and transverse cracks in thick weldments of pressure vessel steel

Abstract The detection, characterization and eyaluation of flaws that are crack-like, in thick weldments of pressure vessels, are important, Usually this work is performed through non-destructive evaluation (NDE). Volumetric NDE techniques use the standard references to provide an approximate assessment, but specimens with natural flaws can give more accurate reference data in accordance with actural situations. The present research focuses attention on: (i) a methodology to generate definite-sized and oriented cracks; (ii) the effect of new material used to introduce cracks on the microstructure and mechanical property; and (iii) characterization of cracks. 20 and 125 mm thick weldments of carbon-manganese pressure vessel steel (SA-299/ASTM standard) were provided for the investigation. This paper discusses both the use of an innovative technique to generate cracks in weldments and also the crack characterization by NDE.

Degradation of bending strength and stiffness in glass-fiber-reinforced plastic under repeated raindrop impact

Repeated rain drop impact plausibly brings about a definite damage in airplane surface materials while the airplane flies in rain fall. When the materials are fiber reinforced plastics (FRP), the damage is more serious. However, FRP is a very attractive material to the aircraft, because its specific strength, that is the strength per unit density, is very high as compared with the other materials such as aluminum alloys. This paper investigates the damage mechanism of the glass fiber reinforced plastics subject to repeated rain drop impact and the residual strength after the certain number of the impacts. This experimental result shows that the residual strength decreases significantly after delamination takes place in the FRP.

Numerical analysis of fatigue striations

Abstract A computational model was developed to numerically analyse fatigue striations. The inclined strip yield model with continuous distributions of infinitesimal dislocations was utilized to express the crack tip plasticity in this model. The fatigue crack tip blunting process was approximated by sequential activation of two slip lines under loading, and crack closure during unloading was taken into account. The plastic zone at a growing fatigue crack tip at the maximum load was independent of the crack growth up to ten cycles while the reversed plastic zone decreased in a size to one twentieth of that at the maximum load as the crack grew. The ratio of these plastic zone sizes and also the crack tip opening displacement were quite different from the simple prediction by J.R. Rice for a stationary crack. The computed striation spacings were compared with the observed ones and moderate agreement between them obtained.

Numerical Analysis of Kidney Stone Fragmentation by Short Pulse Impingement: Effect of Geometry

To better understand the behavior of stresses generated inside a kidney stone by direct pulse impingement during extracorporeal shock wave lithotripsy (ESWL), numerical analyses are performed in this work. LS-DYNA, an explicit Finite Element code for non-linear dynamic analysis is employed to investigate the effect of stone geometry to the stress field evolution inside the stone when subjected to short pulse wave. Circular disks with parts removed from the front and the back are used to model the stones that assumed have already had initial fracture. The other variation of spherical geometry such as ellipse is also considered in the numerical calculation.

Viscoelastic Effect on the Fracture Toughness of GFRP: Experimental Approach

The dynamic fracture tests were carried out for a glass fiber reinforced plastic specimen with a crack and dynamic fracture toughness was evaluated by examination of cracking at an initial slit root. Before the crack initiated at the slit root, a whitened damage zone was created surrounding the slit tip. The damage zone consists of micro cracking in the matrix, debonding between a fiber and the matrix, and fracture of the fiber. The comparison of the dynamic fracture toughness and the static fracture toughness value shows that the former is around 12 MPa√m and apparently higher than the later, which is 7 MPa√m. To understand those experimental results and mechanics of the damage zone, a dynamic debonding test was carried out and dynamic bonding strength was estimated as around 70 MPa.

Cleavage Fracture in Steel Specimens Under Short Stress Pulse Loading

Cracks in SM50A, steel for welding structure chilled to -40 and -160 C were loaded by various stress intensity pulses with durations of 20, 40, and 80 μs to generate experimental data of critical stress intensity levels for crack instability. Fracture surfaces were observed by a scanning electron microscope to examine cleavage nucleation origins ahead of the crack tips. The obtained experimental results were discussed from the minimum time criterion and the dislocation dynamics.

The Mechanism of Stable Crack Growth in CFRP

A stress analysis is carried out to clarify a mechanism of stable crack growth in CFRP. Damage zones in which the fibers are debonding from the matrix are extended ahead of the crack tip. The fibers in the damage zone are modeled by the line spring, and the infinitesimal dislocation distribution theory is used for the numerical stress analysis. The model successfully describes the behavior of stable crack growth in CFRP.

Behavior of Thin-Walled Square Tube and Tubular Hat Sections Subjected to Low Velocity Impact Loading

In this paper, the prediction and comparison of the behavior of thin-walled prismatic structures (square tube, top-hat and double-hat sections) in absorbing energy using theoretical and numerical analysis are presented. Equations to predict crushing length and dynamic mean crushing force of top-hat and double-hat sections were applied for material mild steel St37 and the effect of spot weld positions are also figured out. For comparison, an explicit non-linear commercial finite element code LS-DYNA was used to predict the response of the structures subjected to axial crushing. It was found that results of numerical methods and theoretical prediction have good agreement. Assuming that the failure of spot-weld is neglected, mean dynamic crushing force of double-hat section is 90% higher compared to that of square tube.