共查询到20条相似文献,搜索用时 46 毫秒
1.
We propose an approximate semiphenomenological model of the joint process of cold plastic deformation and fracture of metals.
Within the framework of this model, for 10kp, 20kp, 20G2R, and 38KhGNM steels, we show that moving dislocations overcome barriers
through a force process. The formation of nascent microcracks is also realized through a force process, i.e., local stresses
in the “head” of an arrested dislocation pileup attain the levels of theoretical strength. We also suggest a general algorithm
of the application of the proposed model to the prediction of fracture of metals in technological processes of plastic metal
working.
Ufa State Aviation Technical University, Ufa, Russia. Translated from Problemy Prochnosti, No. 1, pp. 76–85, January–February,
1999. 相似文献
2.
The synthesized theory is generalized to take into account the influence of the loading rate on plastic deformation by introducing
the integral of inhomogeneity in the formula for the distance to the planes whose motion causes plastic deformation. We take
into account the displacements of the planes not only in front of the stress vector but also behind it. The integral of inhomogeneity
depends on the history of loading rate, which enables us to describe the processes of hardening, softening, and stabilization
and the Bauschinger effect in plastically deformed materials.
Technical Institute, Pedagogical University of Bydgoszcz. Bydgoszcz, Poland. “L'vivs'ka Politekhnika” State University, L'viv.
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 36, No. 1, pp. 39–42, January–February, 2000. 相似文献
3.
A new etching technique for revealing the plastic deformation zone in an Al–Cu–Mg alloy has been developed. The etching with
the proposed etching agent was conducted on the deformed sample after being heated to 673 K for 3 h. With this etching technique,
the plastic deformation zone was clearly observed even under low magnification. This was due to the change of microstructural
characteristics in the plastic deformation zone after the heating process, in which there is significant precipitation of
Al2Cu and Mg2Si, caused by the high energy arising from the severe deformation. 相似文献
4.
The combined plastic and discrete fracture deformation framework for finite-discrete element methods
Esteban Rougier Antonio Munjiza Zhou Lei Viet Tuan Chau Earl Eugene Knight Abigail Hunter Gowri Srinivasan 《International journal for numerical methods in engineering》2020,121(5):1020-1035
The combined finite-discrete element method (FDEM) was originally developed for fracture and fragmentation of brittle materials, more specifically for cementitious and rock-like materials. In this work, a combination of a discrete crack and plastic deformation has been combined and applied to FDEM simulation of fracture. The deformation is described using a FDEM-specific mechanistic approach with plastic deformation being formulated in material embedded coordinate systems leading to multiplicative decomposition and plastic flow, that is, resolved in stretch space; this is combined with the FDEM fracture and fragmentation criteria. The result and main novelty of the present work is a robust framework for simulation of large strain solid deformation combined with a multiplicative decomposition-based model that simultaneously involves elasticity, plasticity, and fracture. 相似文献
5.
V. V. Pokrovskii P. V. Yasnii P. V. Tokarev B. T. Timofeev 《Strength of Materials》1989,21(11):1446-1453
The influence of preliminary (before crack origin) cyclic plastic deformation by tension-compression (R = min/max = –1 on the brittle fracture resistance of 15Kh2MFA heat resistant steel in two conditions was investigated. The preliminary loading was done at 293 K and the fracture at 123 and 293 K. It was established that in the case of relatively short preliminary deformation the change in static fracture toughness is caused by a change in yield strength as the result of the Bauschinger effect. However, with an increase in cyclic deformation the total irreversibly dispersed specific energy of inelastic deformation and the structural changes in the material are the determining factor. A physically based method is proposed for prediction of the influence of preliminary cyclic plastic deformation on the brittle fracture resistance of cyclically softening steels.Institute of Problems of Strength of the Academy of Sciences of the Ukrainian SSR, Kiev. Leningrad. Translated from Problemy Prochnosti, No. 11, pp. 14–20, November, 1989. 相似文献
6.
The demand on micro-parts is significantly increasing in the last decade due to the trend of product miniaturization. When the part size is scaled down to micro-scale, the billet material consists of only a few grains and the material properties and deformation behaviors are quite different from the conventional ones in macro-scale. The size effect phenomena occur in micro-scale plastic deformation or micro-forming and there are still many unknown phenomena related to size effect, including geometry and grain size effects. It is thus critical to investigate the size effect on deformation behavior, especially for the fracture behavior in micro-scale plastic deformation. In this research, tensile test was conducted with annealed pure copper foils with different thicknesses and grain sizes to study the size effects on fracture behavior. It is found that flow stress, fracture stress and strain, and the number of micro-voids on the fracture surface decrease with the decreasing ratio of specimen size to grain size. Based on the experimental results, dislocation density based models which consider the interactive effect of specimen and grain sizes on fracture stress and strain are developed and their accuracies are further verified and validated with the experimental results obtained from this research and prior arts. 相似文献
7.
The paper presents the results of an experimental investigation of the influence of warm prestressing (WPS) on fracture toughness
characteristics of large-size specimens. The WPS has been found to be an efficient method for enhancing brittle fracture resistance
of large-size bodies from the investigated materials and can be recommended for practical realization in nuclear reactors
and other critical structures whose brittle fracture is impermissible both in the process of normal operation and in emergency
situations. The optimum temperature-loading regime of the WPS is defined by both the properties of a given material and its
thickness which governs the intensity of plastic deformation in the process of WPS. Based on the established mechanisms of
the WPS effect, a physicomechanical model has been developed for the prediction of fracture toughness for pressure-vessel
heat-resistant steels after WPS taking into account the influence of the stress state at the crack tip. The model makes it
possible to predict fracture toughness for large-size bodies subjected to WPS with the given temperature and loading regimes
from the results of testing small laboratory specimens. The most optimum regimes of the WPS can also be determined using this
model and even those for several materials making up a structural component and subjected to the WPS.
Translated from Problemy Prochnosti, No. 3, pp. 39–54, May–June, 1997. 相似文献
8.
S. F. Gnyusov V. P. Rotshtein S. D. Polevin S. A. Kitsanov 《Technical Physics Letters》2010,36(9):801-803
Features of the plastic deformation and dynamic spall fracture of Hadfield steel under conditions of shock wave loading at
a straining rate of ∼106 s−1 have been studied. The shock load (∼30 GPa, ∼0.2 μs) was produced by pulses of a SINUS-7 electron accelerator, which generated
relativistic electron bunches with an electron energy of up to 1.35 MeV, a duration of 45 ns, and a peak power on the target
of 3.4 × 1010 W/cm2. It is established that the spalling proceeds via mixed viscous-brittle intergranular fracture, unlike the cases of quasi-static
tensile and impact loading, where viscous transgranular fracture is typical. It is shown that the intergranular character
of the spall fracture is caused by the localization of plastic deformation at grain boundaries containing precipitated carbide
inclusions. 相似文献
9.
A model has been developed for predicting fracture toughness characteristics on the lower shelf of the fracture toughness
temperature dependence considering the influence of specimen dimensions. The model is based on the physicomechanical model
of brittle fracture governed by microcleavage critical stress which in its turn depends on plastic prestraining. This model
is also based on the assumption that the change in the body dimensions influences fracture toughness as much as the change
in the stress state caused by it, which is evaluated by the introduced parameter χ, and affects plastic deformation preceding
brittle fracture by cleavage. According to the given model, in some cases an increase in specimen thickness can lead only
to a shift of the brittle-to-ductile transition temperature, and in other cases to both a shift of the brittle-to-ductile
transition temperature and a decrease in the critical SIF value at which this transition occurs. The paper also presents further
elaboration of the probabilistic approach to the prediction of size effect for the cases of brittle fracture based on Weibull's
three-parametric distribution. A distinctive feature of this approach is the determination of one of the parameters of the
cumulative distribution function, namely, the parameter of location Kc
min
, directly in the experiment and not by conventional statistical methods. This reduces appreciably the requirements for the
size of the sample and simplifies the body of mathematics. In this case, the parameter of location Kc
min
corresponds to the critical SIF value during the first unstable jump of a fatigue crack K
fc
(1)
which, according to a large number of experimental data, is minimal and the most invariant among all other critical SIF values
obtained experimentally.
Translated from Problemy Prochnosti, No. 2, pp. 21–31, March–April, 1997. 相似文献
10.
N. I. Bobyr’ A. P. Grabovskii A. P. Khalimon A. V. Timoshenko A. N. Maslo 《Strength of Materials》2007,39(3):237-245
The paper presents a procedure whereby the damage accumulation kinetics in structural materials, such as steel 45, stainless
steel 12Kh18N10T, aluminum alloy D16T, and titanium alloy VT22, under elastoplastic deformation is studied based on variation
parameters of elastic modulus and resistivity. For complex stress conditions, a continuum model for damage accumulation is
proposed which relates the damage parameter to the intensity of accumulated plastic strains. The data calculated by the proposed
continuum model are compared to the experimental findings of the investigation of the damage accumulation kinetics for some
structural metals.
__________
Translated from Problemy Prochnosti, No. 3, pp. 23–34, May–June, 2007. 相似文献
11.
An X-ray powder profile analysis in vanadium pentoxide powder milled in a high energy vibrational ball-mill for different
lengths of time (0–250 h), is presented. The strain and size induced broadening of the Bragg reflection for two different
crystallographic directions ([001] and [100]) was determined by Warren-Averbach analysis using a pattern-decomposition method
assuming a Pseudo-Voigt function. The deformation process caused a decrease in the crystallite size and a saturation of crystallite
size of ∼ 10 nm was reached after severe milling. The initial stages of milling indicated a propensity of size-broadening
due to fracture of the powder particles caused by repeated ball-to-powder impact whereas with increasing milling time microstrain
broadening was predominant. WA analysis indicated significant plastic strain along with spatial confinement of the internal
strain fields in the crystallite interfaces. Significant strain anisotropy was noticed in the different crystallographic directions.
A near-isotropy in the crystallite size value was noticed for materials milled for 200 h and beyond. The column-length distribution
function obtained from the size Fourier coefficients progressively narrowed down with the milling time. 相似文献
12.
We study the processes of quasistatic deformation and fracture of brittle materials under the action of rapidly varying temperature
fields. As a fracture criterion, we use the condition of attainment of the critical levels of stresses. The analyses of the
stressed state and crack growth are performed under the assumptions that the corresponding elements of the stress field are
equal to zero on the newly formed free surfaces and that the conditions of the fracture criterion are satisfied at the ends
of the crack. It is shown that the process of crack propagation is unstable for the major part of modes of thermomechanical
loading: as soon as the critical stresses are attained at a certain point of the body, the crack instantaneously propagates
to a critical size corresponding to a new stable state. It is shown that the mechanical overloading of a specimen can substantially
weaken the effect of instability of development of the fracture zone. Examples of fracture of elastic brittle bodies are presented.
We also perform the numerical analyses of the processes of initiation and propagation of cracks with regard for the plasticity
of the material near its heated surface.
__________
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 6, pp. 55–60, November–December, 2006. 相似文献
13.
The plastic deformation and fracture of aluminium hydroxide filled polypropylene has been investigated. A transition between
two mechanisms with an increase of the filler volume fraction has been observed. Below a critical filler volume content φcr
≈ 20 vol% (designated region 1) adhesive failure processes and polymer deformation in the neighbourhoods of different particles
occur in an uncorrelated manner. Above this critical value (designated region 2) exfoliation along the surface of the initial
portion of inclusions causes the formation of craze-like deformation zones transverse to the direction of the loading. The
concentration of craze-like zones is essentially determined by the filler content and the level of interphase interaction
which in turn depends on the particle size. In region 1 deformation occurs in a macro heterogeneous way with the formation
and growth of a neck. The elongation to break decreases with an increase in the mean diameter of the filler phase. At φ>φcr
composites, filled with small particles, fail in quasi brittle manner with the formation of a short and narrow neck. In contrast
to the case for a small filler concentration, an increase of the inclusion size leads to an increase in the ultimate elongation
and a tendency to macro homogeneous yielding. An explanation of the observed behaviour is proposed based on a change in adhesive
failure conditions with filler content and size.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献
14.
Specimens of a fully lamellar TiAl alloy and a duplex TiAl alloy were tensile tested using load-controlled procedure. The
microcracks were measured for each specimen as it was subjected to various preloading–unloading processes. Loading–unloading–reloading
processes of in-situ tensile tests were carried out in a scanning electron microscope (SEM). Effects of microcrack damage
on the deformation and fracture behavior were evaluated. The following results of microcrack-damage on deformation and fracture
behavior of TiAl alloy were found: (1) The apparent plastic elongation resulted mainly from plastic strain. The elongation
caused by microcracks is negligible. (2) No appreciable effects of microcrack damage on the apparent elastic modulus could
be found. (3) Microcracks damage produced at higher preloading reduced the fracture stress, however, that produced at lower
preloading gave diminished effects. 相似文献
15.
Results of an experimental investigation of the regularities of plastic deformation of titanium alloys in a plane stress state
are analyzed. Tests were performed by loading thin-walled tubular specimens by an axial force and internal pressure under
conditions of a proportional increase in the loads. The alloys are found to be transversely isotropic materials whose isotropic
surface coincides with the cylindrical surface of the specimen. The process of plastic deformation of the alloys under simple
loading is shown to be described well by equations of a previously proposed deformation theory of the plasticity of transversely
isotropic media.
Translated from Problemy Prochnosti, No. 5, pp. 27–35, September–October, 1999. 相似文献
16.
Srinivasan Swaminathan M. Ravi Shankar Balkrishna C. Rao W. Dale Compton Srinivasan Chandrasekar Alexander H. King Kevin P. Trumble 《Journal of Materials Science》2007,42(5):1529-1541
Large plastic strains between 1 and 15 can be imposed in chips formed by plane-strain (2-D) machining of metals and alloys.
This approach has been used to examine microstructure changes induced by large strain deformation in model systems—copper
and its alloys, precipitation-hardenable aluminum alloys, high-strength materials such as titanium, Inconel 718 and 52100
steel, and an amorphous alloy. It is shown that materials with average grain sizes in the range of 60 nm–1 μm can be created
by varying the parameters of machining, which in turn affects the deformation processes. Furthermore, a switch-over from an
elongated subgrain microstructure to an equi-axed nanocrystalline microstructure, with a preponderance of large-angle grain
boundaries, has been demonstrated at the higher levels of strain in several of these materials. This switch-over can be readily
controlled by varying the deformation conditions. Dynamic recrystallization has been demonstrated in select material systems
under particular conditions of strain and temperature. This study may be seen as providing an important bridge between furthering
the understanding of microstructural refinement by large strain deformation and the practical utilization of nanostructured
materials in structural and mechanical applications. Conventional plane-strain machining has been shown to be a viable SPD
method for examining the underlying processes of very large strain deformation. 相似文献
17.
Laws governing the buildup of fatigue fracture under cyclic loading of aluminum alloy polycrystals under conditions of multicycle
fatigue have been studied by analyzing the spacetime dynamics of mesoscopic structures. It has been shown that under the periodic
action of an external mechanical field, deformation domains form in the material at the mesoscopic level. The evolution of
these dynamic mesoscopic domain substructures determines the kinetics of the fatigue fracture of the polycrystals.
Pis’ma Zh. Tekh. Fiz. 23, 51–57 (December 26, 1997) 相似文献
18.
The deformation field in notched metal sheets stretched under tension was analysed experimentally. The results of strain distribution were explained by using the result of the near-tip deformation field of non-linear elastic material, combined with a simple model of the plastic state under a mixed plane stress and plane strain condition. Next, the relationship among fracture mechanics parameters, i.e. the notch-tip opening displacement, the notch-tip contraction and was established based on the rigid plastic strip model. Finally, the effect of the specimen thickness on the toughness value at crack initiation and instability was discussed by improving Bluhm's idea that the total fracture resistance was the sum of the fracture work for slant and flat fractures. 相似文献
19.
《材料与设计》2015
In meso/micro-scaled plastic deformation, material deformation and ductile fracture are quite different from those in macro-scale. The roughness of the free surfaces of workpiece increases with deformation and the decrease of grain number in the sample thickness direction, leading to the nonuniformity of specimen thickness. The so-called size effect and free surface roughening may in turn affect the deformation behavior, ductility and fracture morphology of the samples. To explore the coupled effect of workpiece geometry and grain size on material flow behavior in meso/micro-scaled plastic deformation, uniaxial tensile test of pure copper sheets with different thicknesses and comparable microstructure was performed. The experimental results reveal that the material flow stress, fracture stress and strain, and the number of microvoids on fracture surface are getting smaller with the decreasing ratio of specimen thickness to grain size. In addition, the modified Swift’s equation and the corrected uniform strain are closer to the experimental ones considering the thickness nonuniform coefficient induced by the free surface roughening. Furthermore, the observation of fracture morphologies confirms that the local deformation caused by the free surface roughening leads to strain localization and a decreased fracture strain when there are only a few grains involved in plastic deformation. 相似文献
20.
The effect of the grain size as a basic structural parameter on plastic strain macrolocalization has been studied for polycrystalline
aluminum. The mathematical form of the above dependence has been verified. The limiting cases have been defined both for small-and
coarse-grain ranges. The effect of sample dimension on the macrolocalization period has been considered.
__________
Translated from Problemy Prochnosti, No. 1, pp. 52–55, January–February, 2008. 相似文献