Photoelastic investigation on the crack-arrest capability of a hole

Dynamic photoelasticity employing a 16-spark gap Cranz-Schardin camera system was used to determine certain conditions leading to fracture arrest by a circular hole ahead of a propagating crack. Photoelastic models of 3/8×10×10-in. Homalite-100 plates with a 1/2-in. edge crack were loaded in a fixed-grip configuration and crack arrest was made possible by central holes of 1/2, 1/4, and 0.15-in. diameters. In one test of a uniformly loaded plate with a central hole of 0.15-in. diameter, the propagating crack continued through this hole. Changes in dynamic-stress-intensity factors, as the crack tip approaches the hole, as well as changes in the dynamic-stress-concentration factors at the far side of the hole were studied, and these results were compared with the corresponding static results determined by finite-element analysis. This comparison shows that the static analysis can be used to qualitatively assess the arrest capability of the hole using the maximum static-stress concept or the proposed concept of strain energy released as the crack penetrates the hole.     

Experimentally determined stress-intensity factors for single-edge-crack round bars loaded in bending

Dimensionless stress-intensity factors were determined for single-edge-crack solid and hollow round bars loaded in bending. These factors were calculated from experimental compliance (inverse slope of load-displacement curve) measurements made on round bars loaded in three-point bending. The compliance specimens had span to diameter ratios of 6.67 and 3.33, and measurements were made over a range of dimensionless crack lengths from 0.002 to 0.70. The tests were made using 3-in. (76-mm) and 6-in. (152-mm) solid and hollow round bars notched on one side; the hollow bars had an inner to outer diameter ratio of 0.33. A comparison was made with data in the literature for rectangular bars; for ana/D of 0.0001, the dimensionless stress-intensity factor for a solid round bar is 1.3 vs. 2.0 for a rectangular bar.     

Strain energy density approach to stable crack extension under net section yielding of aircraft fuselage

In this paper, the incremental theory of plasticity is used in conjunction with the strain energy density criterion to determine the stress field in 4-in. wide test specimens containing 3 holes. These specimens, made from 0.04-in. thick sheets of 2024-T3 aluminum, also contained small collinear cracks emanating from the holes. The initial crack sizes varied from 0.15 to 0.26 in. Residual strength tests conducted with these specimens revealed that stable tearing occurred before failure. Analyses were performed to predict the stable crack extension and failure by plastic collapsed. Because of the complexities involved with nonlinear stress analysis combined with subcritical crack extension, the finite element method was used with the grid pattern adjusted for each increment of stable tearing. Reasonable correlation between the experimental data and predicted results was achieved.     

Dynamic behavior of concrete

Concrete and cement-paste specimens, representing a model of the actual structural material and of its adhesive component, respectively, were subjected to static and dynamic tests. Static tests on virgin specimens were carried out in order to evaluate the strength, elasticity and Poisson's ratio of the materials. The dynamic experiments were conducted in order to ascertain the response of the specimens to the propagation of one-dimensional pulses. Transient loading was accomplished by the central longitudinal impact of a 1/2-in.-diam steel sphere on a ballistically suspended 3/4-in.-diam Hopkinson bar of the material at an initial velocity of about 3260 or 1650 ips. The shocked specimens were also subsequently examined to determine whether changes in static material properties had occurred as a result of passage of the waves. Both static and dynamic tests yielded consistent results for a number of specimens cast and cured in identical fashion. Comparison of the properties of the virgin and the shocked specimens indicated little, if any, shock damage. While some minor grain damage was observed in microscopic examination of thin sections taken from some of the shocked specimens, other sections did not indicate any visible cracking of the grains. The wave-propagation process appeared to occur without dispersion and relatively little attenuation, indicating that the material could be represented on a macroscopic scale as an “elastic” substance with a small structural-damping coefficient. The obvious inhomogeneities of the concrete affected the gage response whenever a gage was mounted directly over a piece of aggregate. The dynamic response of the materials has been compared with the response of several types of rocks.     

Stress-intensity factor for plain concrete in bending—Prenotched versus precracked beams

A series of experiments conducted to compare the behavior of beams with notches to those with natural cracks has recently been completed. A total of 42 beams with notches formed by casting teflon strips into the concrete were tested to failure. A companion series of 42 beams were statically precracked following the procedure described in Ref. 5. The ranges of crack depth to beam depth varied from 0.3 to 0.7 (nominally). Two strengths of concrete were used and the 3-in.×4-in.×15-in. (span) beams were tested on three-point or four-point bending. Comparisons are made on the basis of computed stress intensity using a finite-element program developed by the writers. Results of this study show the following: (1) in all cases the naturally cracked beams yielded higher failure loads and stress-intensity values than notched beams with the same crack length; (2) the averageK ₁ values for precracked beams were approximately 38 percent, 77 percent and 96 percent greater than for notched beams for crack-depth ratios of 0.3, 0.5 and 0.7 respectively.     

The characteristics of plastic flow and a physically-based model for 3003 Al–Mn alloy upon a wide range of strain rates and temperatures

The uniaxial compressive responses of 3003 Al–Mn alloy upon strain rates ranging from 0.001/s to about 10⁴/s with initial temperatures from 77 K to 800 K were investigated. Instron servohydraulic testing machine and enhanced split Hopkinson bar facilities have been employed in such uniaxial compressive loading tests. The maximum true strain up to 80% has been achieved. The following observations have been obtained from the experimental results: 1) 3003 Al–Mn alloy presents remarkable ductility and plasticity at low temperatures and high strain rates; 2) its plastic flow stress strongly depends on the applied temperatures and strain rates; 3) the temperature history during deformation strongly affects the microstructure evolution within the material. Finally, paralleled with the systematic experimental investigations, a physically-based model was developed based on the mechanism of dislocation kinetics. The model predictions are compared with the experimental results, and a good agreement has been observed.     

Thermomechanical behavior of casting sands: Experiments and elastoplastic modeling

The thermomechanical behavior of casting sands is discussed from an experimental and a theoretical point of view. Uniaxial compression tests at temperatures ranging from 20°C to 950°C and at different values of strain rate (ϵ = 10⁻² s⁻¹, ϵ = 10⁻³ s⁻¹ and ϵ = 10⁻⁴ s⁻¹) have been performed. They show that casting sands exhibit no strain rate effect in the temperature range 20–600°C, and that an elastoplastic model is well suited to describe the experimental results. Three thermoelastoplastic models, derived from Cam Clay and Hujeux models have been developed. These new models take into account the cohesion of the material. The physical parameters needed for these models have been obtained in the temperature range 20–300°C by using triaxial tests, uniaxial compression tests, isotropic compression tests and die pressing tests. An original triaxial apparatus has been built allowing a temperature of 800°C and a pressure of 4 MPa to be reached. In the temperature at which the parameters have been obtained (20–300°C), two additional triaxial compression tests at different confining pressures are used to check the validity of the thermoelastoplastic models used. The best quantitative results are obtained with the revised modified Cam Clay model.     

Temperatures and stresses in ablating hemispheres

Hypersonic wind-tunnel tests of thirteen 7 3/4-in.-diam Plexiglas and nylon hemispheres were designed and conducted at the Polytechnic Institute of Brooklyn utilizing a shrouded-model technique to simulate Mach 15 to 20 flight. The models were instrumented with thermocouples ans strain gages from which continous time histories of temperature and strain were recorded. In addition, ablation measurements were made after themodels were tested. The experimental results are compared with theoretical calculations of strain, temperature and ablation. Using a point-symmetric heat-balance method to determine the ablation and temperature histories, a thermal stress analysis is conducted in which variations in the modulus of elasticity with temperature are approximated by a concentric-layer technique. The effects of axisymmetric pressure distributions, as well as the effects of the boundary condition imposed by the model support, are considered in an axisymmetric alyer analysis. A comparison between theroy and experiment indicates good qualitative results and good-to-poor quantitative results. The details of the analyses, and the comparison between the theoretical predictions and experiment are discussed.     

爆炸冲击波作用下颅脑损伤机理的数值模拟研究

爆炸引起的颅脑损伤已经成为现代战场单兵的主要致伤形式,而相关的致伤机理尚未完全阐明。本文中,针对头部在爆炸冲击波作用下的动态响应及相关致伤机理进行了数值模拟研究。首先,基于颅脑的核磁共振切片建立了人体头部三维数值模型,该模型真实地反映了颅脑的生理特征与细节构造;利用该模型对人体头部碰撞实验进行数值模拟,模拟结果与实验结果吻合良好,验证了头部模型的有效性。在此基础上,基于欧拉-拉格朗日耦合(Euler-Lagrangian coupling method,CEL)方法发展了爆炸冲击波-头部流固耦合模型,对头部受到爆炸冲击波正面冲击工况进行了数值模拟,分别从流场压力分布、脑组织压力、颅骨变形与加速度等方面对头部动态响应过程进行了分析。爆炸冲击波峰值压力在流固耦合作用下增大为入射波的3.5倍,致使受到直接冲击处的颅骨与脑组织发生高频振动,相应的振动频率高达8 kHz,这与碰撞载荷下的脑组织动态响应是完全不同的。同时,该处颅骨的局部弯曲变形会沿着颅骨进行“传播”,影响着整个颅骨的变化构型,从而决定了脑组织压力与损伤的演化过程。     

Experiments on the stability of spherical and paraboloidal shells

This paper presents the experimental part of a study of the elastic stability of clamped and hinged spherical and paraboloidal shell caps subjected to external uniform pressure loading. Ninety-six poly-vinyl-chloride (PVC) shells of 15-in. base diameter and formed by thermovacuum process, were tested and their critical loads recorded. The effects of geometry, clamped and hinged boundary conditions, height/span and radius of curvature/thickness ratios on the critical pressure were investigated. The results are compared, where possible, with experimental data obtained by previous investigators.     

近地小天体对地撞击成坑模型研究进展

近地小天体对地撞击成坑是行星研究的前沿问题之一。本文中介绍了陨石坑成坑过程与类型、实验室模拟成坑现象和陨石坑成坑模型律，分析了近地小天体对地撞击成坑机理和点源模型的不足，指出了近地小天体对地撞击成坑未来研究的发展趋势。     

Hydrodynamic drag of the dolphin

An experimental study of the hydrodynamic drag of the dolphin is reported. The results are compared with calculations for the steady-state motion of a similar rigid model, and previously reported data on the hydrodynamic drag of the dolphin are refined. The drag turns out to be much less than the calculated drag, even when the initial laminar region is taken into account. Several conclusions can be drawn from the results: 1) The experimental results show quite reliably that hydrodynamically the dolphin is in fact a unique phenomenon. 2) The phenomenon is explained in terms of Gray's paradox for the dolphin. 3) The mechanism by which the dolphin's drag is reduced is apparently controllable. 4) Flow around the dolphin can be assumed to be laminar undetached flow.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 2, pp. 160–164, March–April, 1971.     

Buckling of spherical sandwich shells

An experimental investigation was carried out to determine the critical buckling loads of several shallow spherical sandwich shells. A cold-forming process simultaneously using pressure and vacuum was employed to manufacture the nearly perfect spherical facing layers from 5052 aluminum-alloy sheets of 0.006 and of 0.008-in. thicknesses. Eight shallow spherical-shell specimens of 20-in. base diameter and of 20 and 30-in. radii with 1/8 and 1/4-in. thickness of “Flexcore” have been tested in a 300-psi autoclave specifically designed for these experiments. The pressure on shells was developed by the differential pressure between the inner and the outer chambers separated by the shell being tested. When the inner chamber was maintained at atmospheric pressure and gas pressure was applied in the outer chamber, the testing procedure was termed “soft.” Alternatively, the inner chamber would be filled with fluid with the outer chamber remaining filled with gas. By initially pressurizing both chambers equally, a load on the shell could be developed by the differential pressure due to controlled bleeding of the fluid inside the inner chamber, while the gas in the outer chamber was maintained at the initial pressure. This is an accurate volume-control experiment and this testing procedure was termed “hard.” In the latter case, it was possible to monitor the displacements of the shell for each load increment with a nest of clip gages of an unique design. It was found that there is no substantial difference in the buckling loads between the hard and “soft” systems. All shells buckled in the plastic range. A reasonably good correlation is obtained with a linear theory using the double modulus for the sandwich segments.     

Strain measurements in tubes during rapid transient heating

A measuring apparatus using reluctance-type displacement transducers was successfully used for measuring radial thermal strains in 1-in.-diam × 8-in.-long thin-walled tubes of molybdenum, tantalum and 304 stainless steel. Wall-temperature rates of approximately 300° F/sec were accomplished by rapid heating with a plasma jet and strains at temperatures up to 2500° F were recorded. Excellent agreement between experimental results and a theoretical solution based on temperature profiles was found for temperatures to 2000° F.     

Center deflection of minute glass diaphragms under uniform pressure

A new experimental technique has been applied to measure the center deflection of minute glass diaphragms under uniform pressure. The technique employs a microscope with a monochromatic-light source, an interferometer, and a U-tube mercury vacuum system. The diaphragms (0.0005-in. thick) are from 20 to 30 times thinner than the plates tested by Way⁴ and McPherson⁵. In addition, the diaphragms are glued at the edges as opposed to clamped, and they are glass as opposed to metal. Pressure-deflection data are compared with accepted deflection solutions for three ideal edge conditions to show that the solutions are applicable for predicting the center deflection provided the proper edge condition is assumed.     

Finite element analysis of the piezocone test in cohesive soils using an elastoplastic–viscoplastic model and updated Lagrangian formulation

In this research, the finite element analysis of piezocone penetration has been conducted using the elastoplastic–viscoplastic bounding surface model in the updated Lagrangian reference frame. A finite element formulation has been performed considering the viscoplastic contribution of the model and the theory of mixtures has been incorporated to explain the behavior of the soil. The formulated model has been implemented into a finite element program, EPVPCS-S (elastoplastic–viscoplastic coupled system-soil), to analyze the mechanism of piezocone penetration. The results of the finite element analysis have been compared and investigated with the experimental results from the piezocone penetration and dissipation tests conducted using LSU/CALCHAS (Louisiana State University Calibration Chamber System).     

Experimental investigation of the stability of monocoque domes subjected to external pressure

This paper describes the fabrication techniques, testing procedures and experimental results for eight 16-in.-diam rigid-vinyl plastic monocoque domes subjected to external-pressure loads. The domes tested were spherical, elliptical and torispherical, and all had fixed-edge support. They were fabricated in such a manner that wall-thickness tolerances and residual stresses could be minimized. In the test setup, a backup or support block was used to prevent complete collapse or yielding. The support allowed a complete buckle pattern to form, and the resulting pattern was always characteristic of a particular dome shape.These tests on domes withr/t values ranging from 240 to 600 produced remarkably consistent and high critical buckling coefficients (C) which exceeded 80 percent of the classical value (0.60). The results further indicateC to be independent ofr/t as well as the half-opening angle () of the dome shell over the range of values investigated.Paper was presented at 1965 SESA Spring Meeting held in Denver, Colo., on May 5–7.     

On the validation of the methods related to cyclic behavior of metallic structures

This work includes two ‘classical’ experimental and numerical parts. In the first part, an experimental device has been carried out and two tests have been performed on a structure subjected to a biaxial prescribed force (tension and torsion) and cyclic thermal loadings. The effect of short term mechanical overloads in tension is also analyzed with the objective to check the validity of an approach proposed previously in order to take into account that effect. The numerical part has two goals : evaluate the ability of Chaboche's viscoplastic model to describe the overloads effect and compare the assessment of some simplified methods to our experimental results. We have considered two categories of methods. The first concerns the methods based on the elastic analysis (3 S_m and efficiency rules), the second category is related to some more recent methods based on an elastic analysis combined with an elastoplastic analysis limited to the first cycle (Gatt's and Taleb's methods). The following results have been obtained:

• •The forecasts using the approach proposed previously in order to describe the primary overloads effect is confirmed by our tests.
• •The Chaboche's model has a tendency to overestimate the evolution of the strain increments induced by the successive overloads.
• •The predictions given by some methods based on elastic analysis (the efficiency rule in particular) do not always seem conservative. On the other hand, Gatt's and particularly Taleb's methods give some conservative forecasts for the studied cases.

     

Design of an Impact Loading Machine Based on a Flywheel Device: Application to the Fatigue Resistance of the High Rate Pre-straining Sensitivity of Aluminium Alloys

This paper presents a device that has been designed for tensile loading at medium impact rates (up to 10³ s^–1) and for performing either interrupted or failure tests. This machine allows us to apply prescribed pre-straining to the specimen, and then apply subsequent loading histories such as impact fatigue. Two specimen loading systems are considered, which make it possible to carry out tests with various ranges of force and various durations of time. A multi-CCD camera system is triggered by a chosen threshold from the force signal. The system is dedicated to the displacement measurement and gives both qualitative and quantitative information about the stretching mechanism leading to fracture. To illustrate the performance of the device, experimental results concerning impact tensile tests at a strain rate of about 300 s^–1 are presented, as well as consecutive impact-fatigue tests on two aluminium alloys.     

DNS of heat transfer in turbulent and transitional channel flow obstructed by rectangular prisms

Direct numerical simulation (DNS) of heat transfer in a channel flow obstructed by rectangular prisms has been performed for Re_τ = 80–20, where Re_τ is based on the friction velocity, the channel half width and the kinematic viscosity. The molecular Prandtl number is set to be 0.71. The flow remains unsteady down to Re_τ = 40 owing to the disturbance induced by the prism. For Re_τ = 30 and 20, the flow results in a steady laminar flow. In the vicinity of the prism, the three-dimensional complex vortices are generated and heat transfer is enhanced. The Reynolds number effect on the time-averaged vortex structure and the local Nusselt number are investigated. The mechanism of the heat transfer enhancement is discussed. In addition, the mean flow parameters such as the friction factor and the Nusselt number are examined in comparison with existing DNS and experimental data.