The relationship between tensile strength and flexure strength in fiber-reinforced composites

Tensile data on unidirectional composites generated from a flexure test usually yield a higher strength than observed from a standard tensile coupon. According to a statistical-strength theory based on a Weibull distribution, the presence of a stress gradient in the flexure-test results in an apparent increase in tensile strength as compared to the tensile test under uniform stress. In the present paper, this concept is explored by utilizing data from unidirectional graphite-epoxy composites to compare with theoretical results generated from a two-parameter Weibull distribution. A larger variation in tensile strength is observed from tensile-coupon data than from flexure data. Such differences are not in accordance with strength theories based on a uniform flaw distribution and raise questions concerning variability of the test methods, as well as sources of material variability.     

Anisotropic strength of composites

The present investigation is concerned with the development of a theory of strength of anisotropic composite materials and the establishment of sound experimental procedures for the confirmation of the predicted results. A general theory is stated whereby the strength of laminated as well as unidirectional composite materials subjected to any state of combined stress can be predicted once the basic strength characteristics of a unidirectional layer have been determined. The transversely isotropic layer is treated in detail and, based on the understanding of the mechanical behavior of laminated composites, the procedure outlined for determining the strength of laminated systems. An experimental program was conducted, using glass-filament-reinforced resin test spceimens, and data obtained confirmed the results predicted for the strength of unidirectional composites. Based on the results of the analysis presented, composites may then be designed to incorporate an optimum utilization of the inherent strength characteristics of the constitutent materials.     

Prestress for maximum strength

The problem of designing prestress to maximize elastic capacity is treated analytically. Our formulation models structures comprised of a one-material elastic continuum, subject to a single deterministic load configuration.The equations needed to predict the optimal prestress design are derived. They are shown to comprise necessary and sufficient conditions for global maximum strength in the unconstrained design problem. The theory is demonstrated on the design of prestress for a thick-walled cylinder.     

基于静强度分布参数的复合材料剩余强度及寿命预测

为探讨剩余强度及疲劳寿命与初始静强度分布参数之间的关系,构建基于初始静强度分布参数的剩余强度和疲劳寿命计算模型。在模型构建过程中未涉及复合材料结构的层数、铺层厚度和铺层方向,适应性强。模型参数可通过静力试验和剩余强度试验获得,试验成本相对疲劳试验较低。剩余强度和疲劳寿命与初始静强度变异系数及其分布参数有关,当初始静强度服从威布尔分布时,疲劳寿命亦服从威布尔分布,可为开展疲劳可靠性提供借鉴和参考。算例表明,基于建立的模型,剩余强度计算结果最大误差为-1.58%,疲劳寿命与试验数据吻合较好。     

Fracture strength for a high strength steel bridge cable wire with a surface crack

In this paper, the fracture strength of a cracked suspension bridge wire is determined based on linear elastic fracture mechanics (LEFM). The wire is 5 mm in diameter, with an original ultimate strength of 1725 MPa and ultimate elongation that ranges between 5.5% and 6%. The average value of for the wire fracture toughness, K_C, was recently evaluated by the author. The state of practice is to use the ultimate strength of the cracked wire as obtained from tensile tests. This approach may overestimate the strength of the wire due to possible delamination and crack tip plasticity. A case study for a group of in situ wire breaks retrieved from a suspension bridge cable is presented. The failure analysis is performed based on both the fracture toughness criterion and the net section theory. The fracture toughness criterion produced more realistic results for the fracture strength of the wire. The decline in the fracture toughness and the corresponding reduction in the fracture strength of cracked degraded wire are predicted making use of the strain energy density criterion.     

Dynamic tensile strength of water

This paper reports the results of an experimental study of the tensile strength of water under dynamic loading conditions in the absence of a free surface.     

Local strength of thin-walled structures

We suggest efficient methods for studying stress concentrations in thin-walled structural members. The advantages of these methods are determined by their essential distinguishing characteristics as compared with well-known methods. After the separation of variables, the solution of linear ordinary differential equations of shell strain mechanics is analytically determined in the form of a matrix Taylor series. The property of the solution to satisfy arbitrary initial conditions allows one simply to reduce the boundary conditions to the initial conditions prior to calculating. Another method for structures with arbitrary parameters of the shells permits one to transfer the boundary conditions by a multiplicative method to the stress concentration site and to use them as the initial conditions in studying the stress concentration. We present the results of investigations of the stress concentration in transport-launch containers of aircraft by various methods.     

Submicrosecond strength of ultrafine-grained materials

We present the results of measuring the strength properties of metals and alloys with face-centered cubic lattice (copper, aluminum), body-centered cubic structure (Armco iron, tantalum), hexagonal close-packed structure (titanium and titanium alloy BT6) in the original coarse-grained and submicrocrystalline state under shock-wave loading. The grain dimension of the materials under study was changed by intensive plastic deformation. The influence of the grain dimensions on the dynamic yield stress does not always agree with the data of low-rate test even in sign, which is interpreted in the framework of general laws of the strain rate influence on the metal and alloy flow stress. As the grain dimension decreases, there is an increase in the compression rate in the plastic shock wave, a small increase in the fracture strength (spall strength), and an increase in the spall fracture rate.     

Tensorial strength analysis of paperboard

Tensorial-type failure criteria with linear and quadratic terms are used to calculate the strength of paperboard under plane stress. Theoretical predictions and experimental data are correlated in all four quadrants of biaxial normal stress with various levels of shear. Several methods are examined for determining the interaction coefficientF ₁₂. Comparisons are made with optimum values obtained from least-squares analyses. The best analytical-experimental agreement at all levels of shear is obtained approximately by using coefficientF ₁₂ equal to zero. The sensitivity ofF ₁₂ to errors in experimental input data is also studied. Reliable correlation with experiment, as well as operational simplicity, make these criteria attractive for predicting the strength of paperboard.     

单轴荷载下饱水岩石静态和动态抗压强度的细观力学分析

由于单轴荷载下饱水岩石的动态力学特性与静态力学特性存在很大差异,从宏观上进行力学分析存在局限性。根据岩石受压全应力应变曲线的细观机制,分析了静态及动态单轴荷载条件下孔隙水影响饱水岩石裂纹扩展的情况。在静态单轴压缩条件下,初始裂隙受压使自由水产生孔隙水压力,自由水对翼裂纹有向外挤压的应力,促进裂纹扩展。在动态单轴压缩条件下,自由水会产生粘结力,抑制裂纹扩展。根据翼裂纹受压扩展原理,推导出饱水单轴条件下动态抗压强度、静态抗压强度的计算公式,在相同断裂韧度下,饱水岩石静态抗压强度风干岩石静态抗压强度饱水岩石动态抗压强度。对自然风干和饱水砂岩进行单轴静态、动态压缩实验,结果与理论模型的结果相符。     

Diffusion locking effect on long-term strength

The diffusion locking effect on the long-term strength of metals is studied in the case of aggressive media. In order to model this effect, the diffusion equation with a variable diffusion coefficient is solved approximately. It is shown that, when determining the long-term strength of metals in aggressive media, it is necessary to take into account the diffusion locking effect.     

Optimal spherical cupola of uniform strength

Summary This paper concerns the optimal design of a spherical cupola of given base radius that consists of a material without tensile strength. The combined action of the weights of the cupola proper and a cover of uniform thickness is considered, and the designs for minimum total weight (minimum vertical load on foundation) and minimum structural weight (minimum material consumption) are discussed.

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übersicht Die Arbeit betrifft den optimalen Entwurf einer sph?rischen Kuppel von gegebenem Basisradius, die aus einem Material ohne Zugfestigkeit besteht. Die kombinierte Belastung durch das Eigengewicht der Kuppel und das Gewicht einer Abdeckung konstanter Dicke wird behandelt, und Entwürfe für minimales Gesamtgewicht (minimale Vertikalbelastung des Fundaments) und für minimales Eigengewicht (minimaler Materialverbrauch) werden besprochen.

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This note is cordially dedicated to Prof. Dr. Dr. h. c. Hans Ziegler on the occasion of the seventieth anniversary of his birth. This paper is the last publication by Prof. Dr.-Ing. Dr. h. c. mult. William Prager whose sudden death, on 16 March 1980, is a tragic loss to the world of science.     

海洋平台TT型管节点的极限强度分析

利用非线性有限元方法分析了轴向力作用下多平面TT节点的极限强度。在数值分析中,采用三维20结点固体单元模拟管道结构和焊缝形状,将结构有限元网格划分为不同区域,每个区域的网格独立产生,通过合并形成整个结构的有限元网格。通过控制位移增量法得到了加载过程中载荷和位移之间的关系曲线。使用ABAQUS软件分析了TT节点在支管端部承受轴向载荷的变形及与外部载荷之间的关系,得到了不同参数影响下的TT节点极限强度。     

A new three-dimensional Hoek-Brown strength criterion

The Hoek-Brown(HB) strength criterion has been widely applied to the estimation of strength of intact rock and rock mass,while evolving ever since.However,negligence of the effect of the intermediate principal stress still remains in the criterion’s latest version.At the same time,several three-dimensional(3D) HB strength,which can takes into account the influence of the intermediate principal stress,have already been proposed,among which the 3D HB criterion proposed by Zhang and Zhu seems to be the most reasonable one.However,the Zhang 3D HB criterion may have problems with some stress path close to triaxial extension state because of the non-convexity characteristic of its failure surface.In this paper,a new 3D HB strength criterion is presented based on a generalized form of the HB criterion,which also considers the effect of the intermediate principal stress and inherits all the merits of the original version of the HB criterion.In addition,this new criterion can remedy to some extent the shortcomings observed in the Zhang 3D HB criterion.Polyaxial tests for five different rocks from published literatures are used for evaluating this new criterion and comparing it with the Zhang 3D HB criterion.The results show that this new criterion may over-predict or underpredict the polyaxial strength of rocks but the errors are relatively small,and similar results are also found for the Zhang 3D HB criterion,which one is better depends on the type of the rock under estimation.     

Optimal strength of a compound column

A column of fixed length and variable cross section consists of two homogeneous and isotropic components. The components are joined along their side surfaces and have different Young's moduli, but the same Poisson's ratio. One of the components encloses the other that has the smaller Young's modulus. For different values of the ratio of the moduli, the shape of the column, which has the largest critical buckling load under axial thrust, is determined, assuming that the volumes of the components are prescribed. The problem is solved for the case of pinned ends.It appears that the solution of the most general problem, in which each of the areas of the component cross sections may be varied, is a combination of the solutions of some more elementary problems. Therefore, two types of problems are discussed: the compound bar with an inner component of fixed cross section and the general compound bar.The method of solution may be extended to other boundary conditions.