共查询到20条相似文献,搜索用时 31 毫秒
1.
This paper investigates the effect of load shedding steps on constant-ΔK testing. ASTM fatigue crack growth specimens have been used in constant-ΔK tests for many fatigue investigations. In the tests, load levels need to be gradually stepped down to keep the stress intensity
factor constant. This work points out that care should be taken in choosing the length of the step in order to control ΔK variation. Formulas for ΔK as well as crack growth rate variability are derived. The experimental application of these techniques is also presented. 相似文献
2.
An investigation is presented on the suitability and accuracy of a thermoelastic technique for the analysis of fatigue cracks.
The stress intensity factor ranges ΔK
I and ΔK
II are determined from thermoelastic data recorded from around the tip of a sharp slot in a steel specimen under biaxial load,
in order to assess the accuracy of the technique. ΔK
I and ΔK
II are determined to within 4% and 9% of a theoretical prediction, respectively. The results from a similar test on a fatigue
crack under biaxial load are also presented. These show that thermoelastic stress analysis is a rapid and accurate way of
analyzing mixed-mode fatigue cracks. A discussion is given on the potential of thermoelastic stress analysis of propagating
cracks. 相似文献
3.
A novel impact three-point bend test method for determining dynamic fracture-initiation toughness 总被引:6,自引:0,他引:6
A novel impact three-point bend test method has been developed for determining the dynamic fracture-initiation toughness,K
Id, over the range of loading rates
. The split-Hopkinson pressure-bar technique is used to measure dynamic loads applied to a bend specimen with a fatigue precrack.
The stress-intensity-factor histories for the bend specimen are evaluated by means of a dynamic finite-element technique and
the standard formula (ASTM E 399-83) based on the measured dynamic loads. The time of crack initiation is determined using
a strain gage mounted near a crack tip. The results for 7075-T6 aluminum alloy and Ti−6A1−2Sn−4Zr−6Mo alloy indicate that
the reliableK
Id data can only be obtained by evaluation procedures which take the inertial effects into account. It is shown that the novel
impact bend test method in conjunction with dynamic finite-element analysis provides an effective means of characterizing
the dynamic fracture-toughness parameterK
Id. 相似文献
4.
ZHAO Yong-xiang 《应用数学和力学(英文版)》2005,26(8):1093-1099
IntroductionStudy on the model of fatigue crack growth rate (da/dN) and measurement of theprobabilistic curves of the model are a basis of the fatigue damage tolerate design and theprobabilistic safety assessment. Some benefit efforts have been made in th… 相似文献
5.
Measurement of velocity fields in ball mills 总被引:1,自引:0,他引:1
This paper describes a computer vision technique used in measuring the velocity field related to the motion of balls in a
ball mill. A camera combined with two color flashes facing an experimental ball mill is used to take pairs of pictures by
triggering the flashes in sequence with a time interval Δt. In order to ensure consistency, the pictures are of different colors and in a single frame. The ball positions in each image
are detected by image-processing techniques, and the velocity is then the displacement of each ball between two successive
images divided by Δt. The measurement error is estimated by measuring the displacements from a pair of images when the balls remain stationary. 相似文献
6.
The fatigue crack growth characteristics of
CrlMo steel have been investigated at 861 K over the R-ratio range 0.1–0.7 utilising a dwell time of 10 min. at maximum load. All tests were conducted under load control in a laboratory air environment. It was established that the R-ratio significantly affected the fatigue crack extension behaviour inasmuch that with increasing R-ratio, the critical ΔK level for the onset of creep fatigue interactive growth, ΔKIG, decreased from 20 to 7 MPa√m and the threshold stress intensity, ΔKth, decreased from 9 to about 3 MPa√m. At intermediate ΔK levels, i.e. between ΔKth and ΔKIG, the fatigue crack extension rates, for all R-ratio values, resided on or slightly below the CTOD line, which represents the upper bound for contrnuum controlled fatigue crack growth. Creep fatigue interactive growth was typified by crack extension rates that reside above the CTOD line with a ΔKIG dependence; the attainment of some critical creep condition or crack linkage condition which causes the abrupt change in crack extension behaviour at ΔKIG; and crack extension occurs almost exclusively in an intergranular manner. The R-ratio and ΔKIG followed a linear relation. A literature review concerning the effect of temperature on the threshold fatigue crack growth characteristics of low alloy ferritic steels demonstrated powerful effects of temperature; the magnitude of these effects, however, were dependent upon the testing temperature regime and R-ratio level. The effect of R-ratio on ΔKth was greatest at temperatures >400°C, significant at ambient temperatures and least in the temperature range 90°C to <300°C. The relationship between temperature and ΔKth, at a given R-ratio, exhibited a through and a minimum ΔKth value was observed in the temperature range 200–250°C. The magnitude of the temperature effects on ΔKth decreased with increasing R-ratio. Such effects of temperature and R-ratio on ΔKth was reasonably explained in terms of crack closure effects. Finally, the present elevated temperature fatigue crack growth data exhibited massive crack extension enhancement values when compared to ambient near-threshold fatigue crack growth data for
CrlMo steel. Such large enhancement values were the combined effects of temperature (environment) and frequency. 相似文献
7.
This paper describes the influence of material toughness degradation, through reversed temper embrittlement (RTE) and mean stress on the near threshold fatigue crack growth characteristics of a CrMoV turbine bolting steel at ambient and elevated temperatures. It was established at ambient temperatures that strong effects of R-ratio and material condition (toughness) were observed on near threshold fatigue crack growth characteristics. At elevated temperatures it was shown that for the non-embrittled material that only under low R-ratio conditions did increased temperature increase the level of threshold stress intensity ΔKth, by some 20%. In the case of embrittled material, increasing the temperature increased ΔKth levels by around 30% and decreased near threshold growth rates by an order of magnitude at low to intermediate R-ratio levels.The effects of R-ratio on ΔKth for all material and mechanical testing conditions could be simply expressed by the difference between ΔKth at R = O and a constant B multiplied by R.Quantitative fractographic observations indicated that, generally, the incidence of intergranular failure prevalent in embrittled and non-embrittled steels exhibited a maximum at some specific ΔK level. Also in embrittled steels large effects of environmental assisted crack (EAC) growth were observed at near threshold fatigue crack growth rates. It was suggested that this was the result of the much reduced material cohesive strength which was caused by the presence of both impurity and hydrogen atoms. 相似文献
8.
The accuracy of large-eddy simulation (LES) of a turbulent premixed Bunsen flame is investigated in this paper. To distinguish
between discretization and modeling errors, multiple LES, using different grid sizes h but the same filterwidth Δ, are compared with the direct numerical simulation (DNS). In addition, LES using various values
of Δ but the same ratio Δ/h are compared. The chemistry in the LES and DNS is parametrized with the standard steady premixed flamelet for stochiometric
methane-air combustion. The subgrid terms are closed with an eddy-viscosity or eddy-diffusivity approach, with an exception
of the dominant subgrid term, which is the subgrid part of the chemical source term. The latter subgrid contribution is modeled
by a similarity model based upon 2Δ, which is found to be superior to such a model based upon Δ. Using the 2Δ similarity model
for the subgrid chemistry the LES produces good results, certainly in view of the fact that the LES is completely wrong if
the subgrid chemistry model is omitted. The grid refinements of the LES show that the results for Δ = h do depend on the numerical scheme, much more than for h = Δ/2 and h = Δ/4. Nevertheless, modeling errors and discretization error may partially cancel each other; occasionally the Δ = h results were more accurate than the h ≤ Δ results. Finally, for this flame LES results obtained with the present similarity model are shown to be slightly better
than those obtained with standard β-pdf closure for the subgrid chemistry. 相似文献
9.
One of the most promising approaches to characterizing the stable tearing process for lower constraint configurations in metallic
materials has been indicated to be the crack tip opening angle (CTOA) criterion. In this paper we examine the effect of measurement
distance selection and the impact of measurement procedures/analysis on the experimental CTOA-Δa resistance curve behavior of a Ti-6Al-4V and an Al5083 alloy. A new, systematic procedure for carrying out the experimental,
surface CTOA measurement process has been used, which has uncovered artifacts of measurement distance selection that have
not been presented prior to this study. By studying the CTOA-Δa behavior using a rigidly defined and consistent method, it is apparent that the measurement distance behind the crack tip
at which angles are measured has a direct impact on the shape of the early transitional behavior of the CTOA-Δa behavior as well as the critical, experimental CTOA values produced. 相似文献
10.
The influence of the shear number on the turbulence evolution in a stably stratified fluid is investigated using direct numerical
simulations on grids with up to 512 × 256 × 256 points. The shear number SK/ε is the ratio of a turbulence time scale K/ε to the shear time scale 1/S. Simulations are performed at two initial values of the Reynolds number Re
Λ= 44.72 and Re
Λ= 89.44. When the shear number is increased from small to moderate values, the nondimensional growth rate γ= (1/SK)dK/dt of the turbulent kinetic energy K increases since the shear forcing and its associated turbulence production is larger. However, a further increase of the
shear number from moderate to large values results in a reduction of the growth rate γ and the turbulent kinetic energy K shows long-time decay for sufficiently large values of the shear number. The inhibition of turbulence growth at large shear
numbers occurs for both initial values of the Reynolds number and can be explained with the predominance of linear effects
over nonlinear effects when the shear number is sufficiently high. It is found that, at the higher initial value of the Reynolds
number, the reduction of the growth rate occurs at a higher value of the shear number. The shear number is found to affect
spectral space dynamics. Turbulent transport coefficients decrease with increasing shear number.
Received 23 June 1998 and accepted 25 February 1999 相似文献
11.
The accuracy of turbulent kinetic energy (TKE) dissipation rate measured by PIV is studied. The critical issue for PIV-based
dissipation measurements is the strong dependency on the spatial resolution, Δx, as reported by Saarenrinne and Piirto (Exp Fluids Suppl:S300–S307, 2000). When the PIV spacing is larger than the Kolmogorov scale, η, the dissipation is underestimated because the small scale
fluctuations are filtered. For the case of Δx smaller than the Kolmogorov scale, the error rapidly increases due to noise. We introduce a correction method to eliminate
the dominant error for the small Δx case. The correction method is validated by using a novel PIV benchmark, random Oseen vortices synthetic image test (ROST),
in which quasi-turbulence is generated by randomly superposing multiple Oseen vortices. The error of the measured dissipation
can be more than 1,000% of the analytical dissipation for the small Δx case, while the dissipation rate is underestimated for the large Δx case. Though the correction method does not correct the underestimate due to the low resolution, the dissipation was accurately
obtained within a few percent of the true value by using the correction method for the optimal resolution of η/10 < Δx < η/2. 相似文献
12.
设计了一种研究裂纹在脆性带板(Strip)中传播行为的实验装置。将矩形无预制裂纹的脆性有机玻璃(PMMA)试件上下边固支,在万能试验机上施加拉伸载荷直到给定幅值,然后在试件侧边中央部位开启一个尖锐裂纹。由于预加的拉伸载荷作用,新开启的裂纹将沿试件中部快速传播,导致整个平板试件断裂。采用印刷在试件表面的导电断裂丝记录裂纹的动态传播历史。设计了一种逻辑电路,可将多达128路电阻丝的断裂信号转换为1路输出信号,使用高速波形记录仪测量该信号,得到裂纹传播距离与时间关系。对不同形状的试件施加不同幅度的预加载荷,完成断裂实验,确定了裂纹的稳定传播速度。通过大量实验,获得了材料中稳定裂纹速度v0与动态裂纹传播能Gc之间的关系。测试结果表明:PMMA的传播能Gc是传播速度v0的递增函数;当裂纹平均的稳定速度超过某一临界值时,出现明显的速度振荡。 相似文献
13.
Birefringent coatings have been employed to study the effectiveness of an adhesively bonded repair of a center-cracked tension
panel. The repair was one sided, with photoelastic coatings applied to the opposite side. Photoelastic coatings were also
applied over the patch. Analysis methods are presented to permit the stress intensity factor to be determined from the isochromatic
fringe patterns recorded from both continuous andX- andY-edged coatings. The results showed that the one-sided adhesively bonded patch reduced the stress intensity factor; however,
the repair did not markedly change the character of stress distributions. Fringe loops formed near the crack tips for both
the cracked and repaired tension panels. The primary difference was in the size of the loops. The reduction inK
I
due to repair was smaller than anticipated, but even small improvements in ΔK
I
markedly enhance the life of a repaired panel. The Paris power law is used to show the relation between the reduction in
ΔK
I
and the improvement in the crack growth rateda/dN. Fringe patterns from the birefringent coatings applied to the patch provided a means not only to investigate the stresses
in the patch but also to detect the initiation of the local debonding of the adhesive in the neighborhood of the crack. The
birefringent coating on the patch is an approach for producing an optically “smart” repair. 相似文献
14.
Two stress-acoustic coefficients,K
1 andK
2, are required to determine the state of biaxial surface stress from ultrasonic Rayleigh-wave velocity or time of flight measurements in elastic, initially isotropic solids. An experimental technique is described for the precise measurement of these two coefficients in uniaxially stressed test specimens. The technique is applied to aluminum 2024-T351 and 6061-T651 alloys. The influence on measurement results of various parameters such as material anisotropy and temperature is considered. 相似文献
15.
This paper presents an experimental method for measuring the attenuation and the velocity of longitudinal ultrasonic waves
propagating through flat epoxy polymer samples. The study takes place in the first phase of epoxy polymer's polymerization,
where these materials pass slowly from liquid state to the solid state. For this purpose an experimental setup was introduced,
suitable for the accurate evaluation of the acoustic properties Δα andc
ℓ
e
, when the epoxy polymers are in their first phase of polymerization, while they are cured for 24 hours at room temperature
(20°C). The ultrasonic method used is the pulse echo-through transmission technique. From the variation ofc
ℓ
e
and Δα during the first phase of epoxy polymers curing, the three characteristic states: liquid, semi-solid and solid, are
clearly determined. It is also observed that plasticizer reduces the testability and the semi-solid state shows greater attenuation
than either the liquid or the solid state. 相似文献
16.
J. O. Linde 《Applied Scientific Research》1955,4(1):73-86
Summary The change in resistivity due to cold-work has been determined for a number of binary alloy series having copper, silver or
gold as basic metal. The effects observed are very different in the different alloy systems. A change as large as 42% of the
total resistivity is found in one case. Alloys from the systems gold-chromium and gold-iron show a decrease of resistivity
when cold-worked. In addition to the binary series, some alloys containing three or four components have also been investigated,
and on the basis of the results obtained the question of the influence of the different solutes upon the change of resistivity
is discussed. Results are given regarding the recovery of the resistivity changes as a function of time for different annealing
temperatures. The resistivity change Δϱ as a function of temperatureT and time τ is found to obey the law Δϱ=cτ
n
exp (−nE/RT), wheren is a number of the order 0.1–0.4 andE is the activation energy of the process;R is the gas constant, andc a quantity of proportionality. Alloys belonging to the following binary systems have been investigated: Cu-Al, Cu-Si, Ag-Al,
Ag-Mn, Ag-Sn, Au-Al, Au-Cr, Au-Mn, Au-Fe and Au-Sn, and the following multi-component systems: Au-Mn-Cr, Au-Mn-Cr-Fe and Au-Al-Cr. 相似文献
17.
The feasibility of using a previously developed crack-kinking criterion to predict crack arrest at a tear strap in a pressurized
fuselage was studied with instrumented axial rupture tests of 21 models of an idealized fuselage. A rapidly propagating axial
crack, which was initiated from a precrack, kinked immediately upon extension and propagated diagonally until it turned circumferentially
and propagated along the tear straps. An elastodynamic finite element analysis of the rupturing model fuselage yielded the
mixed-mode stress intensity factors,K
I
andK
II
, and the remote stress component, σ
OX
. This numerical procedure was also used to predict the crack trajectories in full-scale fuselage rupture tests. All numerical
results agreed well with their measured counterparts regardless of size. 相似文献
18.
In this paper, a new experimental, computercontrolled, stress-analysis technique (SPATE) based on the measurement of infrared
emissions from the surface of a cyclically loaded body, has been used to study the stress distributions in rectangular steel
plates and circular rings under cyclic in-plane loading. A typical test procedure for quantitative stress measurement is described.
The effects of surface obliquity and the thickness of the paint coating applied to the specimen surface on the received signal
are described and discussed. Results are given for three series of test specimens and compared quantitatively with relevant
theoretical solutions. Attention is drawn to the changes in the received signal as the applied load range increases beyond
the elastic limit of the material. A new application of the technique to the determination of the stress-intensity factor
in acracked body is also illustrated.
Paper was presented at the 1985 SEM Spring Conference on Experimental Mechanics held in Las Vegas, NV on June 9–14 相似文献
19.
Edwin A. Cowen Russell D. Dudley Qian Liao Evan A. Variano Philip L.-F. Liu 《Experiments in fluids》2010,49(1):77-88
The design, calibration, and testing of a borescopic quantitative imaging profiler (BQuIP) system, suitable for the insitu
measurement of two components of the instantaneous velocity in high sediment concentration flows, are presented. Unlike planar
quantitative imaging techniques, BQuIP has a concentration-dependent field of view, requiring detailed calibration. BQuIP
is demonstrated in unidirectional sheet flow in an open channel flume with a narrow-graded sand with median diameter 0.25 mm.
Acoustic velocity measurements are made in the suspension region above the BQuIP measured region yielding a continuous measurement
of velocity and turbulent stress from the immobile bed to just below the free surface. The temporal history at a point reveals
the sheet flow sediment velocities to be highly intermittent, and the spectra reveal a broad range of temporal scales close
to −5/3 in slope for the streamwise velocity component. At its core BQuIP is a quantitative imaging technique giving it significant
flexibility in terms of both the spatial and temporal analysis parameters (e.g., interrogation subwindow size and Δt, the time between images in a pair to be analyzed), allowing it to have tremendous dynamic range in terms of the velocities
that can be measured. 相似文献
20.
In this paper, the ethylene/oxygen/nitrogen premixed flame instabilities induced by incident and reflected shock wave were
investigated numerically. The effects of grid resolutions and chemical mechanisms on the flame bubble deformation process
are evaluated. In the computational frame, the 2D multi-component Navier–Stokes equations with second-order flux-difference
splitting scheme were used; the stiff chemical source term was integrated using an implicit ordinary differential equations
(ODEs) solver. The two ethylene/oxygen/nitrogen chemical mechanisms, namely 3-step reduced mechanism and 35-step elementary
skeletal mechanism, were used to examine the reliability of chemistry. On the other hand, the different grid sizes, Δx × Δy = 0.25 × 0.5mm and Δx × Δy = 0.15 × 0.2mm, were implemented to examine the accuracy of the grid resolution. The computational results were qualitatively
validated with experimental results of Thomas et al. (Combust Theory Model 5:573–594, 2001). Two chemical mechanisms and two
grid resolutions used in present study can qualitatively reproduce the ethylene spherical flame instability process generated
by an incident shock wave of Mach number 1.7. For the case of interaction between the flame and reflected shock waves, the
35-steps mechanism qualitatively predicts the physical process and is somewhat independent on the grid resolutions, while
the 3-steps mechanism fails to reproduce the instability of ethylene flame for the two selected grid resolutions. It is concluded
that the detailed chemical mechanism, which includes the chain elementary reactions of fuel combustion, describes the flame
instability induced by shock wave, in spite of the fact that the flame thickness (reaction zone) is represented by 1–2 grids
only.
相似文献