Effect of Surface Forces on Subcritical Crack Growth in Glass

Data on the growth of cracks tested in aqueous solutions were interpreted in terms of surface force theory. For applied stress intensity factors greater than 0.25 MPa · m½, the position and the slope of the curves and their dependence on pH and ion concentration can be explained in terms of surface force theory, provided these forces are of a magnitude and range that are representative of those involved in the cohensive bonding of solids. Weaker forces, such as structural, double-layer, or dispersion forces, have little effect on crack growth in silica glass for K _I > 0.25 MPa · m½.     

Subcritical Crack Growth in a Phosphate Laser Glass

The rate of subcritical crack growth in a metaphosphate Nd-doped laser glass was measured using the double-cleavage-drilled compression (DCDC) method. The crack velocity is reported as a function of stress intensity at temperatures ranging from 296 to 573 K and in nitrogen with water vapor pressures ranging from 40 Pa (0.3 mmHg) to 4.7 × 10⁴ Pa (355 mmHg). The measured crack velocities follow region I, II, and III behavior similar to that reported for silicate glasses. A chemical and mass-transport-limited reaction rate model explains the behavior of the data except at high temperatures and high water vapor pressures where crack tip blunting is observed. Blunting is characterized by an arrest in the crack growth followed by the inability to reinitiate slow crack growth at higher stresses. A dynamic crack tip blunting mechanism is proposed to explain the deviation from the reaction rate model.     

Measurement of Very Slow Crack Growth in Glass

The rate of very slow crack growth in glass is measured by inducing small, controllable changes in the direction of propagation of Hertzian cone cracks at known times. After completion of a growth sequence, the sample is sectioned to reveal the fracture surface. The stress intensity factor at each stage of crack growth is calculated by using finite-element modeling of the stresses near the crack tip. Data are presented for crack growth velocities as low as 10⁻¹⁴ m/s in soda–lime glass. These data provide strong evidence for the existence of a subcritical limit for crack growth in this material.     

Effect of Corrosion Processes on Subcritical Crack Growth in Glass

Crack-growth studies were conducted on soda-lime-silica, soda borosilicate, and two binary soda-silica glasses immersed in solutions of 1 M Li⁺, 1 M Cs⁺, or deionized water at different pH values. A definite effect of the Li⁺ and Cs⁺ was observed on the V-K₁ curves in all but the soda-lime glass. A plateau in crack velocity in the range 10-⁸ to 10-¹⁰ m/s was measured on the binary soda-silica glasses for K₁<0.35 MPa.m^1/2. These data are analyzed in terms of both the ion exchange and SiO₂ dissolution steps of the corrosion process. A model of crack growth in corrosive conditions is proposed.     

Water Entry into Silica Glass During Slow Crack Growth

Hydrogen concentration depth profiles on surfaces of SiO₂ glass fractured slowly in water and rapidly in oil were determined by nuclear reaction analysis. It was found that water enters SiO₂ glass during slow crack growth in water.     

Subcritical Crack Growth of Macrocracks in Alumina with R-Curve Behavior

Subcritical crack growth of macroscopic cracks in two Al₂O₃ ceramics is investigated with single-edge-notched bending specimens under constant load. The resulting v - K _I-curves are in complete contrast to the behavior of natural cracks. In spite of the monotonic increase of the externally applied stress intensity factor due to crack extension, the crack growth rates first decrease. This behavior is caused by crack shielding due to crack border interaction and can be described by a rising crack growth resistance. Two methods are applied to determine the R -curve under subcritical crack growth conditions.     

Subcritical Crack Growth in Soda–Lime Glass under Mixed-Mode Loading

Subcritical crack growth in glasses and ceramics has been studied extensively under Mode I loading. In this study, subcritical crack growth in soda–lime glass under mixed-mode loading has been determined, using the double cleavage drilled compression (DCDC) specimens with a hole offset from the center line. With this test configuration, cracks are nucleated at the pole of the hole and propagated parallel to the centerline of the specimen under mixed-mode loading. Under mixed-mode loading, subcritical-crack-growth rates are significantly less than those under Mode I loading at the same energy-release rates. Possible mechanisms for this increased resistance to subcritical crack growth under mixed-mode loading are discussed.     

散射光弹性技术在化学钢化玻璃表面应力测量中的应用

在智能手机、平板电脑等电子产品领域,化学钢化超薄玻璃的需求不断增加,而玻璃是易碎材料,要提高或保持强度和抗破碎能力,必须正确认识应力分布。如何有效的测量化学钢化玻璃的表面压应力(CS)和压应力层厚度(DOL),是当下一个迫切需要解决的问题。散射光弹性法是利用通过玻璃内部应力的双折射来改变极化激光束的延迟,并且散射光的强度随着激光束的延迟的变化而改变,最后通过偏振光光路上因激光束的延迟而出现的光程差和偏振特性来计算表面压应力和压应力层厚度。对散射光弹性法测量化学钢化玻璃的表面应力的理论依据、测量技术等进行了介绍。     

Effects of Electric Fields on Slow Crack Growth in Glass

Electrical de fields applied through electrodes on either side of a slow-running crack in soda-lime-silica glass resulted in a deviation of the crack plane and in delayed deceleration, arrest, and healing. In some cases crack closure occurred while still under load. Resumption of propagation resumed gradually upon removal of the field. It is suggested that the effects involve displacement of the dilated negatively charged zone at the crack tip, while healing is effected by reformation of bonds between the crack surfaces.     

利用热膨胀仪研究硼硅酸盐玻璃的结构弛豫行为

针对目前差示扫描量热仪、红外光谱和Raman光谱等在分析不同玻璃性质弛豫行为上的局限性,提出了建立在玻璃热膨胀性能测试基础上的分析方法。在Rekhson提出的方案基础上,结合Moynihan法,采用Tool-Narayanaswamy-Moynihan唯象模型对玻璃容积在转变区内的弛豫行为进行了研究。测试了两种具有不同热历史的硼硅酸盐玻璃的热膨胀曲线,得到了相同结构弛豫参数,验证了这种方法的可靠性。结果表明:采用改进Rekhson法得到的硼硅酸盐玻璃弛豫函数在参考温度为830K时的比例系数x为0.89,活化能H为200kJ/mol,弛豫时间τr为200s,扩展指数β为0.55;对比Moynihan热容法,该方法虽在精确性上稍差,但在工业领域应用中具有非常大的潜力。     

Selective Stress Relaxation on Chemically Strengthened Glass Sheets for Conventional Wheel Cutting

We introduce, for the first time to the best of our knowledge, the use of variable frequency microwave (VFM)‐assisted second ion exchange to selectively modify the compressive stress CS in chemically strengthened glass sheets, which allows for separation of these glass sheets into smaller pieces using a conventional cutting tool. The CS in paste‐coated zones, in the regions where it is desired for separation of the glass sheets, was successfully manipulated via VFM‐assisted second ion exchange, substituting the larger alkali ions in glass by smaller ions of the paste. Its effect on chemically strengthened glass was characterized using a strain viewer, an optical microscope, and an electron probe microanalyzer. We also report on the scribability and breakability across the stress relaxation zones. The results reveal a CS loss of only 3%, from 677 to 656 MPa, in the paste‐uncoated zones, and complete CS relaxation, down to 0 MPa, in the paste‐coated regions. The areas of relaxed CS enable the separation of the strengthened glass sheets using conventional wheel cutting.     

An Overview of the Strengthening of Glass Fibers by Surface Stress Relaxation

Pristine glass fiber is well known to become mechanically weaker when heat-treated in the presence of water vapor. However, recently, the same fiber was found to become stronger if heat-treated while held under a subcritical tensile stress at a temperature below the glass transition temperature. The added strength was attributed to the formation of a surface compressive layer on the glass created by a surface stress relaxation process that occurred while being held under the tensile stress in air. Silica glass fibers with strengths estimated to be ~7–8 GPa were produced, exceeding the ~5.5 GPa strength of fresh optical fiber commonly reported at room temperature in air. Similar degrees of strengthening, a 20–30% improvement, have been observed previously for E-glass and is reported here for the first time for soda-lime silicate glasses. This process is a new glass strenthening method that may be applied to all oxide glasses and is not subject to the same constraints as currently available methods.     

Evaluation of Crack Growth in Glass by Using Stress-Wave Fractography

Subcritical crack growth in glass shows three characteristic regions, depending on crack velocity. Among three regions, in region III, where water does not affect crack-growth behavior, the slope of the crack-growth curve can be correlated with the intrinsic nature of glass fracture. In this study, to measure the crack velocity in region III, a periodic stress wave was applied to produce fracture-surface markings in the double-cleavage-drilled compression specimen. The crack-growth data obtained were compared with results obtained via direct observation of the crack front. As a result, this method, by using stress-wave fractography, was found to be effective to obtain the crack-growth curve in region III.     

脆性材料的亚临界裂纹扩展和双向应力的影响

分析了陶瓷,玻璃等脆性材料的断裂机理和表征.指出了脆性材料的失效过程的三种不同的裂纹发展模式.实验研究了玻璃在静载下的亚临界裂纹扩展特征及其受双向应力的影响.从而为脆性材料的可靠性和寿命评价提供理论基础和手段.     

Subcritical Crack Growth in Soda-Lime Glass under Combined Modes I and III Loading

Fracture and subcritical crack-growth characteristics under combined Modes I and III loading were studied using the modified compact tension (CT) specimens of soda-lime glass. The combined mode load was applied to the specimen in the direction β with respect to the initial crack. By superposition of Mode III, the advancing crack begins to rotate at an angle Ψ to the initial crack plane, which nearly maximizes the Mode I stress intensity factor K _I(Ψ), and the crack continues to propagate in the same direction. In this case, unlike combined Modes I and II, the crack breaks into multiple partial fronts, and ligamentary bridging forms fracture lances when these segmented cracks are held together. The crack velocity d a /d t was plotted versus the maximum Mode I stress intensity factor K _I(Ψ) for combined Modes I and III loading. The d a /d t values are initially high, and the crack growth tends to be discontinuous compared with the result for pure Mode I. The subcritical crack growth seems to occur when the K _I value for the initial crack reaches a certain value. The d a /d t - K _I(Ψ) curves for combined Modes I and III lie roughly on the same curve as that for pure Mode I as the crack growth increases.     

Stress Relaxation Modulus of a Commercial Glass

The stress relaxation modulus in compression of a container glass was investigated over a wide range of strain, time, modulus, and temperature. The glass is a linear viscoelastic liquid up to 2% strain, and the modulus is a smooth function of time, with no pseudorubbery plateau apparent down to a modulus of 10° dynes/cm². The data cover 4 decades in time and a range of almost 100°C above the glass transition, T ₀ =536°C. Within experimental error, changes in temperature simply shift the modulus-vs-time curve along the time axis without altering its shape. This behavior implies that the same mechanism controls both the bulk and shear spectra. The shift factors fit the WLF equation well with values for the parameters c₁ and c₂ of 16.7±0.2 and 345±5°C, respectively. Data from the literature for silicate glasses agree with these parameters.     

Numerical Modeling of Viscoelastic Stress Relaxation During Glass Lens Forming Process

With the recent advances in numerical simulation capabilities and computing technology, finite element method (FEM) can be applied to predict the performance of a precision aspherical lens molding process. In this paper, various stages of the lens molding process have been modeled using a commercial FEM code MSC MARC. Stress relaxation effect during the forming stage has been incorporated into the numerical model by using a generalized Maxwell model. Successful comparison of the predicted results has been made with the experimental data. The various aspects of the simulation that would enable a more realistic modeling of the process have been identified for future research.     

Comparison of Slow Crack Growth Behavior in Alumina and SiC-Whisker-Reinforced Alumina

Results of dynamic fatigue experiments in water at room temperature on an indented Al₂O₃/25 wt% SiC whisker composite material have shown that this composite has a high resistance to slow crack growth. Aging tests in water revealed that the residual stress due to the indentation does not play an important role, and interrupted fatigue tests showed that the crack starts to grow at very low stress intensities, but the velocity is very low. Detailed fractography indicated that the slow crack growth path is intergranular in the whisker composite. The slow crack growth behavior in the whisker composite is discussed in association with the indentation residual stress, the change of the crack shape during the bending test. These results are compared with a bio-grade Al₂O₃ with lower resistance to slow crack growth, and important differences are pointed out.     

Subcritical Crack Growth in Soda-Lime Glass in Combined Mode I and Mode II Loading

Subcritical crack growth under mixed-mode loading was studied in soda-lime glass. Pure mode I, combined mode I and mode II, and pure mode II loadings were achieved in precracked disk specimens by loading in diametral compression at selected angles with respect to the symmetric radial crack. Crack growth was monitored by measuring the resistance changes in a microcircuit grid consisting of parallel, electrically conducting grid lines deposited on the surface of the disk specimens by photolithography. Subcritical crack growth rates in pure mode I, pure mode II, and combined mode I and mode II loading could be described by an exponential relationship between crack growth rate and an effective crack driving force derived from a mode I-mode II fracture toughness envelope. The effective crack driving force was based on an empirical representation of the non-coplanar strain energy release rate. Stress intensities for kinked cracks were assessed using the method of caustics and an initial decrease and a subsequent increase in the subcritical crack growth rates of kinked cracks were shown to correlate with the variations of the mode I and the mode II stress intensities.