A two-strain-gage technique for determining mode I stress-intensity factor

A finite element analysis and a experimental test were performed to show that the terms with r^−1/2 and ^1/2 in the eigenfunction expansion of the strains can describe the crack tip strain distribution with sufficient accuracy. A set of two linear equations can be obtained to determine the stress intensity factor K_I using only two strain-gages. Errors within 5% can be achieved provided that the two strain-gages are placed at the appropriate locations. The technique can be developed to treat crack bodies with irregular geometry and complex loading.     

Measurement of mode I stress-intensity factors by scattered-light photoelasticity

The feasibility of using the scattered-light technique to determine Mode I stress-intensity factors is demonstrated for the case of an edge-cracked beam subjected to pure bending. Photoelastic-fringe data were utilized to obtain an expression for the fringe gradient in the singular region surrounding the crack tip. Mode I stress-intensity factors were then determined by relating the fringe gradient to the local stresses in the singular region and extrapolating these results to the crack tip. Experimental and analytical results showed good agreement and the technique is suggested for application to three-dimensional fracture-mechanics problems.     

Scattered-light determination of mode I stress-intensity factors by a fringe gradient independent technique

Integration of the scattered-light stress-optic law and established bending and singular crack-tip relationships yielded new experimental equations for calibration and for the determination of mode I stress-intensity factors which are independent of a stress-fringe gradient. Scattered-light stressoptic coefficients determined from four-point bending tests and an integrated scattered-light bending equation show good agreement with values based on stress-fringe gradients computed with polynomials. Excellent agreement was also shown between mode I stress-intensity factors predicted by the integrated stress-optic equation and analytical solutions available in the literature. Favorable comparisons were also made with predictions based on a polynomial-finite-difference method of determining a stress-fringe gradient. Analyses were limited to flaw geometries and locations where there was minimal rotation of the refraction tensor.     

Use of photoelasticity to determine orthotropicK I stress-intensity factor

A new experimental method of obtaining orthotropic stress-intensity factor,K _I , is presented. The orthotropic photoelasticity and orthotropic linear-elastic fracture-mechanics laws are combined. The combined set of equations is used along with half-fringe photoelasticity to determineK _I in a compact-tension specimen made of a transparent unidirectional fiberglass-epoxy material. The results are compared with finite-element-method solutions.     

Experimental determination of the dynamic stress-intensity factor using caustics and photoelasticity

The methods of photoelasticity and caustics were used in conjunction with high-speed photography to determine the dynamic stress field near a moving crack. The photographs were analyzed to extract information on crack speed and the dynamic stress-intensity factor. The stress-intensity-factor histories obtained from both techniques were compared to determine the reliability of the two techniques.     

Dependence of crack acceleration on the dynamic stress-intensity factor in polymers

The caustics method in combination with high-speed photography was employed to study velocity effect on the dynamic-stress-intensity factor of fast cracks in polymethyl methacrylate and in Araldite D. The specimen geometry was so determined that both the accelerating and decelerating crack propagation occurred noticeably in one fracture event. Instantaneous crack velocity as well as its acceleration were expressed as a function of the crack length by using polynomials of the ninth order which were given on the basis of the least-square method. The results show that the dynamic-stress-intensity factor depends not only on the crack velocity but also on crack acceleration, and that the accelerating crack has a smaller value stress-intensity factor than the decelerating crack at the same velocity.     

Three-dimensional speckle interferometric investigation of the stress-intensity factor along a crack front

The variation in Mode I stress-intensity factor throughout the thickness of an ASTM standard compact tension specimen was determined using scattered-light speckle interferometry. Two very thin sheets of coincident coherent light traveling in opposite directions were passed through a Plexiglas specimen normal to the crack faces. A double-exposed photograph of the scattered-light speckle pattern was taken while the specimen was subjected to a small load increment. From this double-exposed photograph, the change in the crack-opening displacement could be determined. From the information about the crack-opening displacement in the region of the crack tip, the stress-intensity factor was calculated for various interior planes and on the surface of the specimen. For the compact tension specimen tested, the stress-intensity factor did not vary throughout the specimen's thickness. The method of scattered-light speckle interferometry proved to be very powerful in solving this complex three-dimensional problem.     

Determination of stress-intensity factors of fillet-welded T-joints by computer-assisted photoelasticity

Fillet-welded T-joints with crack-like discontinuities are analyzed using the photoelasticity method. Stress-intensity factors of opening modes are determined for undercut, overlap and center-line crack of fillet joints with respect to the various loading conditions and the weld dimensions. The photoelastic analysis uses a 20-point overdeterministic approach. The accuracy of fringe measurement is obtained by iteration of the measured results on a pattern-recognition software.Paper was presented at 1983 SESA Spring Meeting held in Cleveland, OH on May 15–20, 1983.     

Simulating drop formation at an aperture by means of a Multi-Component Pseudo-Potential Lattice Boltzmann model

The growth and release of a pendent liquid droplet with the complex motion of the phase interface as a result of a liquid flow from an aperture has been studied by means of a Multi-Component Pseudo-Potential Lattice Boltzmann method. In this method, automatic component separation is attained by means of the Shan and Chen (1993) interaction strength G. We demonstrate that droplet formation can satisfactorily be described by combining three elements: using two components α and β (of the same density and viscosity), the G-driven separation of α and β, and gravity working on just α such that it becomes heavy and behaves as a liquid, while the gravity-free β mimics a gas. We present several time sequences of the growth and release of a pendent liquid droplet. Although the simulations were just 2–D, the dynamics of the necking, the tear shape of the droplet, and the motion of the apex after pinch-off all qualitatively agree with literature. The results are interpreted in terms of non–dimensional Bond, Ohnesorge and Archimedes numbers. We find convincing agreement between the relationships derived by dimensional analysis and the numerical simulations.     

The determination of mode I stress-intensity factors by holographic interferometry

The use of linear elastic fracture mechanics generally depends upon the availability of suitable analytical or numerical solutions for the relevant crack-tip stress-intensity factor,K. Convenient experimental verification of such solutions is a valuable aid to their correct application and can provide a practical substitute in real design situations of great complexity. A convenient, new experimental technique for estimating the Mode I stress-intensity factor using holographic interferometry and test pieces cut from thin sheets of commercially available polymethylmethacrylate is described and demonstrated. The test pieces can readily be prepared to model any desired Mode I geometry and boundary conditions. In addition, a prior self-calibration procedure can be employed to enhance both convenience and accuracy. Real-time interference-fringe data from the crack-tip region are easily reduced and plotted to yield a straight line whose slope provides a one-parameter evaluation of the effect of geometry on the stress-intensity factor. This information, together with the crack length and applied stress, completely definesK.     

基于角速率积分法的光纤陀螺标度因数不对称度测量方法

标度因数不对称度是评价光纤陀螺的一项重要指标,对其进行精确测量在高精度导航应用中具有重要意义。传统的测试方法受转台速率控制精度限制,很难精确测量小于1×10-6的不对称度。首次提出基于角速率积分的标度因数不对称度测量方法。该方法给定转台正反方向转动的角度,由固定在转台上的被测光纤陀螺进行角速率测量,并对输出值积分,从而得到标度因数不对称度。该方法基于转台位置控制,避免了转速不稳定及正反向转速不对称等因素造成的影响。还对可能引起测量误差的因素进行了分析。最后采用角速率积分法测得高精度光纤陀螺标度因数不对称度小于1×10-6。     

Measurement of three-dimensional Mode-I stress intensity factor using multiple embedded grid moire technique

Multiple embedded grid moire and strain gage techniques are used to calculate the variation in Mode-I stress intensity factor throughout the thickness of ASTM E-399 standard compact tension specimens of Merlon polycarbonate. The specimen grids near the crack tip on the surface and in the interior were recorded for the unloaded condition and for various loaded states using high resolution photographic techniques. Optical processing produced moire patterns from which the change in the crack opening displacement could be determined. From the information about the crack opening displacement in the region of the crack tip, the Mode-I stress intensity factor was calculated for various interior planes and on the surface of the specimen. The stress intensity factor was found to be higher in the midplane than on the surface, and it causes crack initiation to start at the midplane.     

Rapid proton diffusion in microfluidic devices by means of micro-LIF technique

The micro-LIF (laser-induced fluorescence) technique was applied to the measurement of pH distributions in a chemically reacting flow in a microfluidic device. Two liquid streams were combined at the junction of a Y-shaped microchannel (100-m width and 33-m depth), and allowed to diffuse into each other and react. The results for non-reacting fluids (hydrochloric acid and water) show good agreement with theoretical values calculated using conventional diffusion. When a reaction occurred (hydrochloric acid and sodium hydroxide), a large difference between the measurement results and the theoretical values was observed, indicating rapid proton diffusion compared with the theoretically calculated values.     

Determination of stress-intensity factors by the method of caustics in anisotropic materials

This paper studied the applicability of the method of caustics to anisotropic materials under Mode I and mixed-mode static-loading conditions and introduced the procedure to obtain stress-intensity factors (SIF) in anisotropic materials by the method of caustics.The mapping equations for initial and caustic curves in anisotropic materials were introduced and their computer graphical images were compared to the experimental ones to check the validity of the mapping equations proposed in this paper. The agreement between them was found to be satisfactory.Two kinds of equations to determine SIF in anisotropic materials by the method of caustics are proposed in this paper. Corroborative experiments carried out by using the orthotropic materials under various loading conditions are presented. In the case of Mode I loading condition, the SIF's obtained by this paper's methods were found to be close to the results by another method, i.e., boundary-element method (BEM). And in the case of mixed-loading condition, the SIF's by this paper and BEM show little differences, (2.2–24.4 percent) with respect to the slanted angle of crack.     

Investigation of shock/elastic obstacles interactions by means of a coupling technique

Arrays of obstacles have proved to be an efficient way of attenuating shock waves generated by large scale explosions. The present study intends to take into account fluid–structure interactions that may occur when elastic obstacles are used. A tractable coupling tool based on a partitioned procedure is exposed, validated on the supersonic flutter of a panel and applied to a configuration composed of square section cylinders. Several numerical difficulties related to staggering are emphasized and workarounds discussed. A methodical procedure involving one and two-way coupled simulations highlights the influence of material properties as well as the acceleration of waves in the fluid when an initial motion is prescribed to the obstacles. Finally, it is shown that uncoupled simulations may be relevant to investigate shock mitigation in some given cases.