Change In Fracture Mechanism Research Articles

Thermomechanical properties of glassy cereal foods

The main objective of this paper is to discuss the relationship between physical state, fracture mechanism, and texture for low moisture cereal-based foods. Experiments were also carried out to get a better understanding of the role of water. At room temperature, extruded bread and white bread (previously) dehydrated, then rehydrated in atmospheres with controlled humidities) exhibited a brittle behavior up to around 9% moisture. At 13.7% moisture, they were ductile. A significant loss in the crispness of extruded bread was observed between 8.5 and 10% moisture. The glass transition temperature (Tg) was measured, using dynamic mechanical thermal analysis (DMTA), for samples with up to 40% moisture. The resultingTg curve showed that the important changes in fracture mechanisms and crispness occurred while the samples were still in the glassy state. The viscoelastic behavior of both extruded and white breads suggested that a secondary relaxation occurred around 10‡C. Another event was observed around 70‡C for low moisture sample, using DMTA. This event was attributed to disruption of low energy interactions.

The influence of hot deformation on the microstructure of Ni-Al and Ni-Al-Fe alloys with small Ti 2B addition

Results of microstructure investigation of Ni-Al and Ni-Al-Fe alloys with addition of 0.5at.% Ti 2B after hot deformation are presented. Ti partially dissolves in the matrix, promoting formation of a discontinuous phase and precipitates of γ′ phase of Ll 2 structure during hot deformation of Ni-Al and Ni-Al-4.3at.%Fe alloys. In all investigated alloys precipitates of different compositions, enriched in Ti, were noticed. A compression test proved that due to the grain boundaries strengthening, the fracture mechanism changes into a transcrystalline one, improving fracture toughness of the hot deformed alloy. Fracture occurs in the deformation induced Ll 0 martensite.

Fracture of solids by radiation pulses as a method of ensuring safety in space

To determine the optimum conditions of fracturing or altering the trajectory of dangerous space objects (large iron or rock space bodies, threatening to collide with the Earth), and also destroy space debris in the space around the Earth, the author presents a mathematical formulation of the problem of calculating the dynamic strength of solids under the effect of high-energy loading pulses. The results of numerical modeling are compared with the experimental data for the formulation of cupola-shaped swollen areas on the rear surface of a metallic plate subjected to laser radiation. The change in the fracture mechanisms (front and rear spallation) with increasing energy of the effect has been detected. Recommendations are given for optimizing the pulsed laser effect on dangerous space objects to ensure the fragmentation or change their orbit.

Toughening isotactic polypropylene and propylene-ethylene block copolymer with styrene-ethylene butylene-styrene triblock copolymer

Dynamic mechanical properties, low-temperature impact behavior, flexural modulus and heat distortion temperature (HDT) of isotactic polypropylene (i-PP) and propylene-ethylene block copolymer (Co-PP) toughened with styrene-ethylene butylene-styrene triblock copolymer (SEBS), at blending ratios of 0–30 phr, were studied and compared. A scanning electron microscopic morphology study of the impact-fractured surfaces demonstrated the changes in fracture mechanisms at various temperatures and SEBS contents. SEBS remarkably improves the impact endurance in the lower-temperature range when blended with Co-PP in comparison with i-PP, due to the increased compatibility in the interface between SEBS particles and the Co-PP matrix.

岩石の破壊機構の変化と寸法効果

岩石の破壊機構の変化と寸法効果

Glass transitions and the fracture behaviour of gluten and starches within the glassy state

Granular particles (crumbs) of gluten, wheat starch, and waxy maize starch were obtained by extrusion in a twin-screw cooker extruder and conditioned to a range of moisture contents (4–18%). The textures of the granules were assessed by a small sensory panel and the sounds produced during crushing (acoustic emission) measured. Solid, essentially amorphous, bars of gluten, wheat starch and waxy maize starch were produced by heat setting under pressure. The glass transition temperatures were measured by differential scanning calorimetry and dynamic mechanical thermal analysis, crystallinity by X-ray diffraction and the fracture behaviour examined using a three-point bend test. Heat-set gluten failed via elastic (brittle) fracture below its glass transition temperature, whereas, as the moisture content was increased, the glassy gelatinised starches underwent a brittle-ductile transition while still in the glassy state. This change in fracture mechanism explains why extruded gluten granules exhibited crispness at higher moisture contents than extruded starch granules, despite gluten having a lower glass transition temperature. The results suggest that the prediction of brittle textures (crisp, crunchy) from a knowledge of the glass transition temperature alone is not possible because bipolymers can exhibit different fracture mechanisms when in the glassy state.

Ductility of filled polymers

AbstractThe ductility of a calcium carbonate‐filled amorphous copolyester PETG in a uniaxial tensite test was examined as a fiction other filler volume fraction. A ductile‐to‐quasibrittle transition occurred as the volume fraction of filler increased. This transition was from propragation of a stable neck through the entire gauge length of the specimen to fracture in the neck without propagation. The draw stress (lower yield stress) did not depend on the filler content and was equal to the draw stress of the unfilled polymer. It was therefore possible to use a simply model to predict the dependence of the fracture strain on the filler volume fraction. It was proposed that when the fracture strain decreases to the draw strain of the polymer the fracture mechanism changes and the fracture strain drops sharply. The critical filler content at which the fracture mode changes is determined primarily by the degree of strain‐hardening of the polymer. © 1994 John Wiley & Sons, Inc.

Dynamic grain growth and ductility enhancement at intermediate temperature

Dynamic grain growth has been observed during hot tensile tests of copper at intermediate temperatures from 0.50 to 0.75 T m over the strain rate range of about 3 × 10 −5 3 × 10 −3 s −1. Microstructural examination revealed that at strain rates < 3 × 10 −5 s −1, dynamic recovery dominates and the grain sizes of the specimens after the test are comparable to that of the original material. When the strain rate is increased above 3 × 10 −5 s −1, dynamic grain growth occurs in that the sparsely nucleated grains are able to glow freely during the test, resulting in the formation of exceedingly large grains. As the strain rate is increased further above 3 × 10 −3 s −1, dynamic recrystallization sets in and the final grain size drops to below the original value. When dynamic grain growth occurs, the stress-strain curves become serrated with a few coarse ripples of gradually decreasing peaks. Interrupted tests further revealed that the stress drop during the test is associated with the occurrence of rapid grain growth in the material. Boundary serration is also a common feature under the test conditions when dynamic grain growth is observed. Detailed metallographic and fractographic studies further showed that the occurrence of dynamic grain growth is responsible for the upturn of the hot tensile ductility of copper with increasing temperature, during which the fracture mechanism changes from profuse grain boundary cavitation with low ductility to sparse wedge cracking with considerable ductility.

Electrical and Mechanical Properties of Pb(Zr, Ti)O3 Ceramics Related to Mn Ion Diffusion

The electrical and mechanical properties of Pb(Zr, Ti)O3 (PZT) ceramics with Mn ions injected by diffusion at high temperature were studied. The rise of mechanical quality factor (Qm) and the change of fracture mechanism from intergranular fracture to transgranular fracture was observed with increased diffusion temperature. These changes are related to the diffusion of Mn ions from the grain boundaries into the grains.

Creep of copper in the temperature range 423–623 K

Abstract The creep of copper in the temperature range 423–623 K was investigated and creep curves modelled by combining semi-empirical equations describing deformation and damage. Previous work has indicated that in this temperature range there is a change in the creep behaviour which is usually ascribed to a change in deformation mechanism from power law creep to power law breakdown. Evidence for this change comes mostly from measurements of steady state creep behaviour which does not account for the possibility of creep damage during testing. The purpose of this investigation is to demonstrate that if the creep curve is viewed as a whole, i.e. deformation and damage are considered simultaneously, the transition in behaviour can just as well be described in terms of a single deformation mechanism with a change in the dominant fracture mechanism.

Effect of M7C3→M23C6transformation on fracture behaviour of cast ferritic stainless steels

The fracture behaviour of three 29 wt-%Cr ferritic steels, two containing zirconium and titanium respectively, has been investigated in the as cast condition and after annealing at 660°C for different times up to 2210 h. The fracture energy and the mode of fracture depend on both the morphology and the nature of the eutectic, which consists of carbides and ferrite. In the as cast condition, fracture is predominantly transgranular cleavage and it can be associated with the discontinuous morphology of the M7C3 carbides present in the eutectic as coarse particles surrounded by the eutectic ferrite. After prolonged heating, the ambient fracture energy decreases and the interdendritic mode of fracture is enhanced. This change in fracture mechanism is associated with transformation of the M7C3 to M23 C6 carbides. The M23 C6 carbides, unlike the coarse M7C3 carbides, form a continuous network within the eutectic mixture and constitute an easy path for crack propagation. The zirconium and titanium additions result in a more massive morphology of the carbides in the eutectic mixture and accelerate the M7C3 to M23C6 transformation during the heat treatments, enhancing the interdendritic mode of fracture both in the as cast and in the annealed condition.MST/1734

Lithosphere strength inferred from fracture strength of rocks at high confining pressures and temperatures

A model is proposed for the strength of the crust or lithosphere, in which the peak strength is in the brittle regime, but not on the brittle-ductile transition. The model is based on experimental evidence which suggests that the fracture mechanism changes from ordinary brittle fracture to a high-pressure type when the compressive strength equals the frictional strength. The model assumes that a high-pressure type of fracture is dominant in the earth's lower crust, except where very high pore pressures exist. It takes account of the scale effect on the strength of rocks. The strength of the high-pressure regime is estimated as a function of depth, or pressure and temperature, using experimental results derived at high confining pressures and temperatures. In the upper part of the brittle zone, where the frictional strength is lower than the compressive strength, Byerlee's friction law is used. In the ductile regime, the steady-state creep law is used. Two cases of lithospheric strength, dry and wet (hydrostatic water pressure), respectively, are evaluated for thrust, strike-slip and normal faulting mechanisms. With the exception of a “wet” lithosphere and a normal faulting mechanism, the strength between the frictional and creep regimes is approximately constant with depth, although the peak strength is in this part.

Change of Fracture Mechanisms of Austempered Ductile Iron in High-Cycle Fatigue.

We carried out high-cycle internal pressure fatigue tests using a large number of thin-wall cylinders of austempered ductile iron (ADI). As a result, we observed a distinctive feature often seen in high-strength steel, that the SN diagram inclined again after about 107 cycles, in spite of once revealing a horizontal part at 106- 107 cycles, and that an obvious fatigue limit could not been observed. Corresponding to this characteristic SN diagram, Weibull plots of the fatigue data were dramatically changed between high-and low-pressure levels. The thorough SEM observation of the fracture surfaces clarified that the fracture origins changed from casting defects to relatively small graphite particles between high-and low-pressure levels. This means that different fracture mechanisms exist between high-and low-pressure levels. These two mechanisms competed in the high-cycle region and brought about the distinctive fatigue behavior of ADI.

Energy‐consuming micromechanisms in the fracture of glassy polymers. I. Chain scission in polystyrene

AbstractThe number of chain scissions per unit area that occur during the fracture of partially annealed latex films from Mn ≃ 180,000 g/mol polystyrene particles of about 275 Å radius were measured and correlated to annealing times. A curve with four regimes was found. At short annealing times the curve is nearly flat, in what is called the chain pull‐out regime. In the second regime, the number of chains broken per unit area increases with a 0.8 power of annealing time as entanglement of the diffusing polymer chains increases in neighboring host particles. This is in good agreement with Wool's theory which predicts a 0.75 power dependence. Then, after reaching a peak, the number of scissions decreases in the third regime, indicating a change in fracture mechanism. The number of chain scissions increases again in the fourth regime, as final healing of the film interface takes place. Fracture surface analysis reveals a rough surface for short annealing times and a smooth surface for longer annealing times. The number of polymer chain scissions per unit area of fracture surface showed no dependence on initial molecular weights for t ≫ τr where t and τr are annealing and relaxation times, respectively. The number of chain bridges crossing a unit area of interface was suggested as the basic molecular property. © 1992 John Wiley &amp; Sons, Inc.

Effect of mineral fillers in low concentration on the mechanical properties of polymeric materials. Part 1: Static and fatigue fracture of polypropylene, qualitative aspects

Injection moulded polypropylene samples containing low concentrations of talc, mica or wollastonite fillers were studied by differential scanning calorimetry, scanning electron microscopy (SEM), and static and dynamic (fatigue) flexural testing. All the fillers under study displayed a nucleating effect on crystallization. This effect is probably responsible for a decrease of the macromolecular orientation, especially in the ‘shear zone’. The most noticeable change in static flexural properties was an increase (50%) of the ultimate stress, presumably due to a change of fracture mechanism induced by the presence of mineral particles. The mineral particles also induced a decrease of the flexural fatigue lifetime. This effect can be attributed directly to the initiating effect of mineral particles on cracking, but also indirectly to a change of skin-core structure as a result of filler nucleating effects during processing. Three crack propagation modes were observed by SEM: matrix/filler debonding, partial debonding with crack reinitiation at the particle edge, and linear propagation with particle rupture. However, the mode of crack propagation seemed to have little influence on the fatigue lifetime, which was essentially controlled by the number of particles per unit volume.

Mechanical behaviour of poly(methyl methacrylate)

A series of tensile and three-point bending studies was conducted at various temperatures and loading rates using a commercial poly(methyl methacrylate) (PMMA). Tensile properties and fracture toughness data were obtained for the various conditions. In general, both tensile strength and fracture toughness increase with increasing loading rate and decreasing temperatur E. However, when the temperature reaches the glass transition region, the relationships between fracture toughness, loading rate, and temperature become very complex. This behaviour is due to the simultaneous interaction of viscoelasticity and localized plastic deformation. In the glass transition region, the fracture mechanism changes from a brittle to a ductile mode of failure. A failure envelope constructed from tensile tests suggests that the maximum elongation that the glassy PMMA can withstand without failure is about 130%. The calculated apparent activation energies suggest that the failure process of thermoplastic polymers (at least PMMA) follows a viscoelastic process, either glass orβ transition. The former is the case if crack initiation is required.

Relationship between evolution of mechanical properties of various cast duplex stainless steels and metallurgical and aging parameters: outline of current EDF programmes

The long term aging behaviour of a large number of Mo bearing and Mo free heats of cast duplex stainless steels has been studied between 300 and 450°C in terms of hardness and impact Charpy toughness. The results demonstrate the importance of the ferrite, Cr, and Mo contents in controlling aging response. Owing to the complexity of microstructural evolution and related changes in fracture mechanisms, the extrapolation of higher (i.e. 450 and sometimes 400°C) towards lower (i.e. service) temperature is not straightforward. Aging kinetics cannot be accounted for with a single activation energy Arrhenius plot; instead, a range of energies increasing with decreasing temperature must be considered. In the lower temperature range, this energy is of the order of 250 kJ mol−1, in agreement with the metallic element controlled mechanisms of microstructural evolution.MST/1186

Creep rupture behavior in particle-strengthened Ni-NiO eutectic alloys.

The relationship between ductility at creep rupture and the change in the creep fracture mechanism has been discussed. The Ni-NiO eutectic alloys have considerably greater elongation at failure than pure Ni. A creep rupture strain has been divided into three groups of strains; i. e., strain for primary creep regions, for steady-state creep regions and for tertiary creep regions. Both stress and temperature dependence of the amount of these three strains have been investigated. The strain corresponding to the tertiary creep region increases with increasing stress. Consequently, the rupture strain for both materials increases. The rupture strain for both materials linearly increases with the strain for the steady-state creep region. It is suggested that greater elongation in Ni-NiO alloys may be attributed to the high ductilitiy in the nickel matrix which is produced by the addition of nickel oxide. The change in the fracture mechanism from intergranular creep fracture to transgranular creep fracture results in the increase of strain corresponding to the tertiary creep regime.

Tensile fracture and fractographic analysis of 1045 spheroidized steel under hydrostatic pressure

Void formation in tensile test under hydrostatic pressure is characterized through quantitative metallography, and the fracture mechanism under pressure is analyzed by fractography. Transition of the fracture surface from the cup-and-cone under atmospheric pressure to a slant structure under high pressure is explained on the basis of the void development leading to fracture and the concomitant change in fracture mechanism. The concept of “shear blocks” is introduced to illustrate the features observed on the fracture surface of specimens tested under high pressure. It is postulated that shear blocks evolve to connect the central crack regions with the shear crack initiated on neck surface due to the severe necking deformation under applied pressure.

A study of tensile properties of ferritic compacted graphite cast irons at intermediate temperatures

Results of an investigation of the influence of carbon and silicon content on tensile properties of ferritic compacted graphite cast irons from room temperature to 500° C are presented. The tensile flow stress plateau of ferritic compacted graphite cast irons occurs owing to dynamic strain ageing from 200 to 400° C with various carbon and silicon contents. Because the ferritic compacted graphite cast irons have irregularly shaped graphite particles, their fracture mechanism is attributed to a mixed fracture mode. With increasing silicon content, the fracture mechanism changes from dimple pattern to transgranular cleavage at room temperature. However, when the test temperature is above the raised transition temperature the specimens fail in ductile manner.