Residual stresses and warp generated by one-sided quench of an epoxy-resin beam

This paper discusses the warp and accompanying residual stress in a rectangular epoxy beam produced by water cooling its lower surface. First, the theoretical values of this warp and residual stress are obtained by the linear-viscoelastic theory. The specimen is then subjected to quenching. The variations in the warp are observed. After quenching, the residual stress is measured by a layer-removal method. The experimental and theoretical results are then compared and discussed.     

A simplified optical method for measuring residual stress by rapid cooling in thermosetting resin strip

This paper presents a simplified optical method for measuring the residual stresses by rapid cooling in thermosetting resin strips. First, the fundamental equations for calculating the residual stress from the residual birefringence were obtained by the linear photoviscoelastic theory. The specimens were then subjected to rapid cooling. After rapid cooling, the residual stress was measured by two methods, the simplified optical method mentioned above and the well-known layer-removal method. The effectiveness of the simplified optical method was discussed by comparing results of the two methods.     

Effect of a Shock Pulse on a Floating Ice Sheet

The vibrations of a viscoelastic plate lying on an elastic liquid base subjected to pulse loading have been studied theoretically and experimentally. The effect of the variable depth of the reservoir, plate thickness, and strain relaxation time on the value of the plate vibration amplitude and the length and curvature of the flexural gravity wave profile are analyzed. Good agreement of theoretical and experimental results is obtained.     

复合材料层合板的低速冲击损伤及其剩余压缩强度研究

本文采用理论和实验方法研究了复合材料层合板的低速冲地及其剩余压缩强度。文中利用有限元方法和能量转换原理计算了层合板受到低速冲击的受载最危险状态，以及此时的应力分布；并用Ｔｓａｉ－Ｗｕ张量准则判断损伤情况，对产生损伤的单元进行相应的刚度折减，且作重复计算直至不产生新的损伤为止；最后，对受冲击的层合板还进行剩余压缩强度计算。在实验中，采用激光全息无损检测法测量了层合板的冲击损伤，并对受冲击的层合板进行     

Viscoelastic analysis of residual stress in quenched thermosetting resin beams

The residual stress generated by the molding process of thermosetting resins exerts serious influences upon their mechanical properties. This residual stress is generally classified by two groups: one produced by shrinkage in the curing reaction of monomers, the other produced by the nonuniformity of the temperature distribution in the cooling process. This paper is concerned with the theoretical and experimental analysis of the generation of residual stress of the latter type, using examples of rectangular beams of thermosetting resins quenched on both the upper and lower surfaces. First, a viscoelastic model is applied to make a qualitative prediction of the residual stress in quenched beams. Second, using linear-viscoelastic theory, fundamental equations are derived for the residual stress in a viscoelastic rectangular beam, where an unsteady and nonuniform temperature distribution is assumed in the direction of depth. The theoretical values of the residual stress in rectangular beams are calculated under various quenching conditions for two resins having different viscoelastic characteristics, i.e., epoxy and unsaturated polyester. The theoretical residual-stress distributions agree fairly well with the residual stress measured experimentally at every quenching condition for both resins. The qualitative prediction that the residual stress in quenched beams is compressive in the vicinity of the upper and lower surfaces and is tensile in the inner parts is confirmed. The relaxation modulus of epoxy resin changes more greatly with time and temperature than that of unsaturated polyester resin. The theoretical and experimental analysis shows that the residual stress for the former resin is larger than that for the latter. Therefore, it is concluded that the generation of residual stress is more significant where the relaxation modulus of resin changes greatly with time and temperature.     

A membrane model for the response of thin plates to ballistic impact

The axisymmetric response of an infinite plate to an impacting projectile is determined analytically on the hypothesis that, for large deformations, a ductile plate behaves to a good approximation like a membrane under uniform tension. The lowest projectile velocity that results in perforation (the ballistic limit), and the residual velocity after perforation, then are determined on the basis of a critical-strain failure criterion. A figure of merit that depends only on the material properties of the target and characterizes the resistance of the material to impact appears naturally in the analysis. Variations in the ballistic limit with target thickness and projectile dimensions can be determined when this figure of merit is known. The theoretical ballistic limit and residual velocity for a steel cylinder impacting a titanium plate are found to agree with available measured values. Further support for the membrane model and an estimate of its range of validity are obtained by comparing the maximum displacement of an impulsively-loaded, circular membrane with experimental data for circular plates.     

Elastic–plastic contact force history and response characteristics of circular plate subjected to impact by a projectile

A new elastic–plastic impact–contact model is proposed in this paper. By adopting the principle of minimum acceleration for elastic–plastic continue at finite deformation, and with the aid of finite difference method, the proposed model is applied in the problem of dynamic response of a clamped thin circular plate subjected to a projectile impact centrally. The impact force history and response characteristics of the target plate is studied in detail. The theoretical predictions of the impact force and plate deflection are in good agreements with those of LDA experimental data. Linear expressions of the maximum impact force/transverse deflection versus impact velocity are given on the basis of the theoretical results. The project supported by the National Natural Science Foundation of China (10532020).     

Transient thermal fracture of cracked plates

The effects of a transient thermal load on a cracked plate are studied experimentally using photothermoelasticity. The three crack configurations of an edge crack, an interior vertical crack and an interior crack inclined at 45 deg are analyzed. In each case, the initially heated plates are subjected to cooling along the edge, while the faces of the plate are either completely insulated, or noninsulated, or in a third case, they are covered with heated transparent Plexiglas plates. It is shown that among the three cracks, the largest transient maximum stress-intensity factor occurs for the edge crack. The inclined crack is subjected to a mixed-mode loading. Among the three cooling conditions, the most severe is the insulated faces case while the noninsulated is the least severe. The relative effect of the cooling conditions on the stress-intensity factors for the three crack types is different enough that the results with one cooling condition would not represent those of another one. A comparison of the experimental transient stress-intensity factors for the vertical crack cases to the finite-element results shows good agreement.     

An investigation of the use of strain gages to measure welding-induced residual stresses

The measurement of weld-induced residual stress is important in structures that are subjected to cyclic loading during their service life. Depending on their magnitude, stresses can influence the rate of crack growth under cyclic loading and hence affect the life of the structure. Because the level of residual stress may change during service, measurement of these changes is necessary for accurate life prediction of the structures. The measurement of welding-induced residual stress using strain gages poses significant problems, the most important being the potential damage to the gages by high temperatures generated in the welding process. This laboratory study was undertaken to assess the suitability and signal stability of commercially available resistive strain gages for the measurement of postweld residual stresses in a submarine hull structure. Adhesively bonded and weldable-type strain gages were attached to the surface of a 35 mm thick steel plate, which was then subjected to thermal cycles similar to those encountered during welding construction of a submarine pressure hull. This paper describes the strain gage application procedure, changes in the strain gage output at end of each experimental stage and the history of changes in the residual stress.     

Thermal shock resistance behavior of a functionally graded ceramic: Effects of finite cooling rate

This work presents a semi-analytical model to explore the effects of cooling rate on the thermal shock resistance behavior of a functionally graded ceramic (FGC) plate with a periodic array of edge cracks. The FGC is assumed to be a thermally heterogeneous material with constant elastic modulus and Poisson's ratio. The cooling rate applied at the FGC surface is modeled using a linear ramp function. An integral equation method and a closed form asymptotic temperature solution are employed to compute the thermal stress intensity factor (TSIF). The thermal shock residual strength and critical thermal shock of the FGC plate are obtained using the SIF criterion. Thermal shock simulations for an Al₂O₃/Si₃N₄ FGC indicate that a finite cooling rate leads to a significantly higher critical thermal shock than that under the sudden cooling condition. The residual strength, however, is relatively insensitive to the cooling rate.     

An experimental investigation of residual stress in quenched flat plates

The residual stresses induced in a quenched flat plate are measured experimentally. A flat plate approximating the theoretical infinite flat plate, after being heated to a uniform temperature, is quenched uniformly from both sides in order to cause a residualstress distribution. The residual stresses are obtained by successively grinding the plate on one side and measuring the resulting strains on the opposite side. Experiments are performed on plates experiencing no phase transformation—316 stainless steel. Comparison of results is made with the predicted stress distributions from the analysis of Landau and Weiner, which is derived from the time-temperature history of the plate during its quench.     

On the limits of added-mass theory in separated flows and with varying initial conditions

It remains unclear to what extent inviscid added-mass theory accounts for the forces exerted on an accelerating body subjected to separated flow. In this study, reactant forces and velocity-field data are systematically acquired using experimental measurements and simulations of an accelerating circular flat plate. Cases accelerated from rest are compared to cases accelerated from a steady flow state. When the added-mass forces predicted by potential theory and the resistance forces associated with the instantaneous plate velocity are accounted for, the remaining (residual) forces comprise approximately 20% of the peak force, even at high accelerations. In addition, the computed residual forces during accelerations both from rest and steady-state cases yield good collapse with respect to one another, indicating that the total forces are not a strong function of the initial state of the wake. These results suggests that inviscid added-mass theory is inadequate to predict the full reactant force even in the ‘ideal’ condition of impulsive motion from rest.     

A correlation for laminar mixed convection from vertical plates using transient experiments

The applicability of the transient cooling experimental technique is studied for developing a correlation for Nusselt number with other pertinent parameters for laminar aiding mixed convection flows over an isothermal vertical plate. A heated aluminum plate, modeled as a lumped system, is allowed to cool in the mixed convection environment during which the transient response of the plate is recorded using a data acquisition system. The experimentally known transient response of the plate is then compared with the numerically computed transient response to estimate the square of the residual. The minimization of this residual using Levenberg–Marquardt iterative procedure retrieves the values of relevant parameters in the correlation, whose form is assumed a priori. The experiments were conducted for a temperature range of 410–305 K and a corresponding Richardson number range 20–0.04. The retrieved values of the parameters compare well with those available in literature.     

爆炸荷载下矩形板的塑性动力响应

本文采用Jones-Sawczuk控制方程,导出了脉冲荷载下矩形板最大残余挠度的简单理论计算公式,并应用该公式和动态屈服条件,计算了固支方板在爆炸荷载下的最大残余挠度值,与试验结果比较,取得了令人满意的结果。     

A method for the evaluation of design limits for structural materials in a cyclic state of creep

The purpose of the present paper is to demonstrate how the minimum theorems proposed in an accompanying paper (Ponter and Boulbibane, 2002) can be utilised in the prediction of the deformation and life assessment of structures subjected to cyclic mechanical and thermal loadings. The developed method, which is based upon bounding theorems and an associate programming method, the Linear Matching method, takes into account the changes in residual stress field occurring within a cycle. Although the solution provided a bound on the inelastic work, it also appears that generally the displacements predicted by this solution are smaller than those that would be predicted by the rapid cycle solution. By way of illustration a simple non-linear viscous model is adopted and a number of solutions are presented involving a Bree plate problem subjected to cyclic histories of load and temperature. An elastic follow-up factor is identified as a key design parameter for high temperature dwell periods.     

LES Investigation of the Flow Through an Effusion-Cooled Aeronautical Combustor Model

The present study is devoted to the analysis of the behaviour of the flow through an effusion-cooled aeronautical combustor model. High-fidelity calculations are performed on an experimental model of a combustion chamber multi-perforated wall and compared to experimental measurements. The effect of combustion instability on the effusion-cooling system is investigated by studying the interaction of an acoustic wave with the jets-in-crossflow issued from the cooling plate. It is shown that the mass-flow rate through the plate can be drastically reduced by the acoustic wave, which demonstrates the destructive effect that such instability may have on the cooling of an aeronautical combustion chamber.     

爆炸法消除焊接接头残余应力的数值模拟

利用非线性动力有限元法对爆炸处理消除焊接接头残余应力的全过程进行了数值模拟。首先，采用温度场与位移场的间接耦合方法计算了钢板对接焊的焊后冷却及残余应力的生成过程，求得焊接接头处由高温冷却到室温由于变形受到阻碍而产生的不均匀的残余塑性变形和应力。然后，在焊缝区引入移动的爆炸载荷，计算了爆炸波作用下该钢板焊接接头附近应力的变化。计算结果表明，爆炸处理可引起板内应力的重新分布，从而有效地释放超过塑性极限的残余应力。利用炸药爆炸消除大型焊接结构残余应力是一种经济有效的方法，本文的数值模拟为研究炸药爆炸消除焊接结构残余应力的机理提供了有力的工具。     

Formation of the residual stress field under local thermal actions

The one-dimensional process of material deformation due to local heating and subsequent cooling is analyzed in the framework of the classical theory of elastoplastic deformations. The problem of formation of residual stresses in a thin plate made of an elastoplastic material under a given thermal action is solved. The graphs of fields of residual stresses and displacements are constructed.     

Photoelastic analysis of a thick plate with an elliptical hole subjected to simple out-of-plane bending

A three-dimensional photoelastic analysis using the stress freezing and slicing techniques was employed to study the stress distribution and the stress-concentration factors around an elliptical hole in a plate of finite thickness. The plate was subjected to simple out-of-plane bending. A special bending device was designed to produce uniform bending moment at the two opposite free edges of the plate. Six plates with various elliptical holes were studied. The stress variation across the plate thickness at the periphery of the elliptical hole was also investigated. The experimental results were correlated with the existing theoretical solutions.     

Impact response of a circular membrane

The major objective of this study is to investigate the transverse deformation of a finite-circular membrane after normal impact by a blunt projectile. A separation-of-variables technique is used to solve the governing two-dimensional-membrane equation. From observations obtained from a high-speed film of the impact, it is shown that the predicted displacements are in close agreement with experimentation. The theoretical formulation is also supperted by comparison with the experimental results obtained by Beynet and Plunkett, who studied the analogous problem of a thin plate subjected to ballistic impact.