EXPERIMENTAL STUDY OF CRACK-INDUCED DYNAMIC DAMAGE GROWTH IN FIBRE COMPOSITES

SUMMARY

The fracture process in composites proceeds by the formation and propagation of a damage zone. The microfracture mechanisms within the damage zone are primarily responsible for energy absorption and dissipation in the macroscopic fracture and, consequently, influence fracture toughness of composites. With this in mind, crack-induced dynamic damage growth has been studied in transparent glass-polyester composites with chopped strand mat and woven fabric as reinforcements. Damage growth has been recorded by photographing the specimens in quick succession using a Cranz-Schardin-type multiple spark camera.

The dynamic damage zone has been observed to propagate, similar to static growth, generally perpendicular to the loading direction; but sometimes the damage zone splits analogous to crack branching in homogenous materials. Higher fibre volume fraction reduces linear damage propagation and increases damage spread in other directions. Damage propagation velocity in the composite appears to be slightly lower than the crack propagation velocity in a polyester resin.     

Light-pulse Propagation through Atomic Caesium Vapour in Conditions of Coherent Interaction with Two-photon Absorbing Medium

Abstract

The propagation effect of a 2πn pulse in atomic caesium vapour in conditions of two-photon resonance has been observed. Comparison of experimental and theoretical results is made.     

Trace Elements in Superalloys

Abstract

In this study, effects of superheated steam on cyclic crack propagation behavior of a heat resistant steel were investigated. Crack propagation experiments were carried out on NF616 (9Cr-0.5Mo-2WVNb) in pressurized superheated steam (600°C/10MPa) under cyclic loading either with or without holding time at constant load. Superheated steam environment has two opposing effects on cyclic crack growth, acceleration and retardation. A modified tarnish rupture (TR) model has been proposed to explain the crack propagation behavior. The crack propagation rate estimated based on the TR-type model well agreed with the experimental data.     

Fatigue dislocation structure and crack initiation in low carbon alloy steel

Abstract

The evolution of dislocation structure in a quenched and tempered low carbon alloy steel with increasing number of fatigue cycles has been investigated using transmission electron microscopy. The changes in dislocation structure can be divided into three stages: the production of the dislocation structure having fatigue features, the formation of microscopic fatigue slip bands, and the initiation and propagation of wide fatigue deformation bands. Microscopic fatigue cracks are formed during the propagation of these bands.

MST/1037     

Effect of liquid gallium on fatigue crack propagation in brass

Abstract

To date there have been few studies of the effects of liquid metals on fatigue crack propagation. In this paper, the effects of liquid gallium on the room temperature fatigue crack propagation characteristics of a leaded brass are examined. In particular, the effects of load ratio and cyclic frequency are studied. Previous models for the effect of liquid metal on fatigue crack propagation are discussed in the context of this work and a new model for fatigue crack propagation in liquid metals is proposed.

MST/1889     

Beam propagation factor of decentred gaussian and cosine—gaussian beams

Abstract

On the basis of the second moment method, the beam propagation factor (M ²?factor) of decentred Gaussian beam has been derived, and analysed physically. Then, the result is extended to novel sinusoidal-Gaussian beams, one type of which is the cosine—Gaussian beam, which can be regarded as a superposition of two decentred Gaussian beams.     

Effect of cyclic loading on interfacial shear stress in SiC–CAS glass ceramic matrix composite

Abstract

Mechanical fatigue has been observed to occur in the Nicalon–CAS continuous fibre reinforced glass ceramic matrix composite under cyclic loading at room temperature, and both microcrack proliferation and propagation are induced. In situ fibre push down tests within a scanning electron microscope have then been used to assess changes in interfacial properties as a result of this mechanical cyclic loading. Both the interfacial shear stress and the interfacial fracture energy decrease when specimens are subjected to mechanical cyclic loading. It is deduced that a decrease in interfacial shear stress is the most likely mechanism driving stable and progressive microcrack propagation.     

A universal expression for the propagation rate of the normal phase over a high-temperature superconducting film with a transport current

The transport-current-initiated propagation of the normal phase over a high-temperature superconducting film located on a thermally stabilized substrate has been studied theoretically. A universal expression for the propagation rate of the normal phase is obtained that takes into account the influence of the substrate on the evolution of thermal instability in the film and can be applied for an arbitrary dependence of the critical current on temperature. The resulting expression is shown to describe the experimental data satisfactorily. Pis’ma Zh. Tekh. Fiz. 24, 22–26 (January 26, 1998)     

Numerical Solution of the Fourth-moment Equation for a Point Source

Abstract

This paper presents an accurate numerical solution of the fourth-moment equation for a point source. This moment describes the intensity fluctuations arising from wave propagation in random media. The problem has remained intractible due to the nature of the solution and its rapid evolution with range. An adaptive grid method has been developed to deal with these difficulties, and its application to this equation provides a reliable and efficient way of obtaining solutions for a wide range of scattering strengths.     

Multi-Gaussian Ultrasonic Beam Modeling for Multiple Curved Interfaces—an ABCD Matrix Approach

ABSTRACT

A multi-Gaussian beam model uses a superposition of Gaussian beams to simulate the waves radiated from an ultrasonic transducer. We show that propagation and reflection/transmission laws for Gaussian beams in fluids and elastic solids can be written in the form of A , B , C , D matrices that are analogous to the A, B, C, D scalars used in Gaussian optics. This representation leads to simple expressions for a Gaussian beam even after that beam has been transmitted or reflected at multiple curved interfaces and produces a highly modular multi-Gaussian beam model that is also computationally very efficient. Some examples of the use of this model for both planar and curved interfaces are given.     

Thermal cracking of multicomponent white cast iron

Abstract

The effect of the volume fraction of eutectic carbides on the thermal fatigue resistance of multicomponent white cast iron has been investigated. Thermal fatigue tests were carried out for 100 and 500 cycles. Nucleation of thermal fatigue cracks took place mostly at the specimen surface, induced by mechanical and metallurgical stress raisers. The crack nucleated in the matrix as well as at the carbide/matrix interface or at the carbide itself. The surface crack density increased slightly for increasing volume fraction of eutectic carbides from 9 to 14%, approximately. Crack propagation took place mostly at the carbide/matrix interface or through the carbide. The propagation rate was affected by the carbide distribution: the higher was the 'carbide continuity/mean free path between carbides' ratio, the higher was the propagation rate. The propagation rate decreased with increasing test time, regardless of the volume fraction of eutectic carbides.     

Development of new material and heat treatment for tools ensuring high performance in hot seamless tube rolling

Abstract

A new technology has been developed to improve the life of tools used for high temperature steel rolling in seamless tube manufacturing. It is a joint technology combining a new chemical composition and a special heat treatment. The new steel tool material has high Cr, C, and Ni contents compared with those of the conventional tools in order to increase the volume fraction of carbide which increases wear resistance. Through the new heat treatment, the macroscopic distribution pattern of eutectic carbide changes from meshlike to granular form increasing resistance against crack propagation. The validity of the results in the laboratory has been verified on the production line.     

Effect of boron on hot ductility oflow carbon low alloyed steel

Abstract

The influence of boron on the hot ductility of C-Mn-Al-Cr steel has been investigated. At <980°C M(CB)₃ precipitated out and about half of the boron content was in solution in austenite at >900°C. It was found that solute boron atoms segregate to austenite grain boundaries and occupy the vacancies induced by deformation. This prevents the formation and propagation of microcracks at boundaries and results in improved hot ductility and a reduced dynamic recrystallisation temperature.     

Effects of shot peening on bending fatigue strength of spring steel SS 2090

Abstract

The influence of prior surface condition and of a shot peening treatment on the bending fatigue strength of a standard Si–Cr spring steel (SS 2090) has been investigated. This steel was initially hardened and tempered to a hardness of 52–54 HRC. After shot peening, compressive residual stresses had been introduced into a surface layer of depth ~0·3 mm, with the maximum value of ~1000 MN m^?2 being found close to the surface. The effect of this treatment was to increase the fatigue limit by ~40% to 890 MN m^?2. Coincident with this increase was a change in the site of fatigue initiation from a surface to a subsurface location beneath the compressive residual stress layer. The initiating inclusions, which were 20–40 μm in size, were analysed and found to be Al₂O₃. At stress amplitudes greater than the fatigue limit, initiation was invariably found to occur at the surface and was not always due to inclusions. Inclusion initiated failure has been modelled using the size and spatial distribution of inclusions in the test bars in addition to the variation of applied and residual stresses through the section. A crack propagation criterion based on linear elastic fracture mechanics is used, assuming that propagation is controlled by stress intensity threshold value. It is assumed that small cracks exist at oxide inclusions from the beginning of the fatigue life and that failure is associated with the propagation of one of these cracks.

MST/1392     

A Remark on Intramolecular Transfer of an Injected Electron in C60 and C70

Abstract

A simple analysis on the intramolecular transfer of an injected electron into C₆₀ and C₇₀ has been performed based on the concept of orbital interaction. This analysis contains the partitioning of each fullerene into three parts and the examination of the frontier molecular orbital interaction for propagation of an injected electron into the whole Cgo or C70 molecule. This electron transfer process is shown to be “one-way” from energetical point of view.     

Perturbation theory for the refractive index mismatch between the inclusion and the surrounding tissues

Abstract

Tissue refractive index is one optical contrast mechanism with diagnostic potential, it is very important to investigate the effect of the refractive index mismatch on light propagation through diffusive regions. Here, we present a new analytical solution of perturbation theory for the refractive index mismatch between the small spherical inclusion and the surrounding tissues. The solution has been used to implement fitting procedures in order to obtain the optical properties of a heterogeneous sphere in semi-infinite medium from measurements of diffuse reflectance. Finally, perturbation theory has been validated by comparisons with the results of Monte Carlo simulation. The new perturbation theory would provide a basis for allowing early disease diagnosis and automatic screening.     

Structure and mechanical properties of Mg–Zn–misch metal alloys solidified at different cooling rates

Abstract

The influence of cooling rate on the mechanical properties and structure evolution of Mg–Zn–misch metal alloys has been investigated. The sequence of structural changes has been correlated with stress—strain curves determined from high temperature uniaxial compression tests. Two regions of work hardening have been identified on the stress—strain curves, described by the Ludwigson equation σ=K ₁ ?ⁿ¹±Δ. Microstructural examination at various levels of deformation has been carried out on selected specimens. In the first region of deformation, the appearance of traces of slip lines was observed inside the grains. In the second region, a network of eutectic precipitates located around the grains had undergone fragmentation, enabling the propagation of deformation by transcrystalline slip across two or more grains. Based on results of the present investigation, optimal properties of the alloys for application at elevated temperatures have been established.     

Three-dimensional measurement of short fatigue cracks using scanning acoustic microscopy

Abstract

The three-dimensional crack growth of short fatigue cracks in Al–Li 8090 alloy has been examined using time resolved acoustic microscopy. Two sets of specimens were machined having different orientations of the elongated grains. Depending on the orientation of the pancake shaped grains with respect to the propagation direction of the cracks different growth rates were obtained. Furthermore, the influence of the microstructure (e.g. grain boundaries) on the crack path in the two sets of specimens could be determined.

MST/1838     

Insitu transmission electron microscopy investigation of crack propagation in single crystal Ni3Al

Abstract

The initiation and propagation of a crack has been investigated in a 110110 oriented Ni3Al alloy single crystal by in situ TEM and using tension deformation at room temperature. The result has shown that the macropropagating direction of the crack was parallel to the tensile axis and the crack followed a zigzag path. Trace analysis indicated that the slip on 111 and 111 were activated during the crack propagating process. Calculations show that for this orientation of Ni3Al crystal, the stress concentration arising from the dislocation pile-up decided the choice of secondary slip systems and the macrodirection of the crack.