Fracture control for composite structures

A fracture control technique for composite structures is presented which takes advantage of the unique capability of composite materials to be tailored in stiffness and fracture toughness. Crack arrestment is achieved through the use of integral ‘buffer’ strips in the primary load-carrying laminate. Experimental uniaxial tension data obtained from damaged laminates containing such buffer strips indicate residual strength capacity in excess of the limit design stress for the selected laminate.     

Fracture parameters for sintered steels

Measurements of the plane strain fracture toughness K _Ic of sintered steels have frequently been invalid because the requirement that P _max/P _Q<1.1 (where P _max = maximum load and P _Q=load used to calculate K _Ic) has not been met. We show that the reason for the criterion not being met is that sintered steels have a considerable crack growth resistance K _R. Values obtained in the past for K _Ic probably have been over-estimates of the initiation value of the crack growth resistance K _i and under-estimates of the maximum crack growth resistance K . The important point is that the assessment of the toughness of sintered steels by a single parameter is not appropriate. Test methods to determine the crack growth resistance of sintered steels are discussed. Crack growth, which is difficult to detect by visual observation, can be determined by compliance techniques. Because of the porous nature of sintered steel, fatigue cracks are unnecessary at the tip of the notch and indeed are undesirable as they can easily cause errors in toughness measurements through inadvertent overloading. The thickness requirement for plane strain measurements can also be relaxed.     

Fracture criterion for inelastic solids

The criterion of inelastic crack growth is found on the basis of the principle of the minimum energy dissipation. According to it the variation of dissipation plus the elastic energy rate per unit of crack growth has to reach the critical value. The path-independent integral of the 2nd type about crack-tip zone is given. Some examples are considered.     

A parameter for power law creep

Experimental investigations on the creep rupture behaviour of type 304, 304£ and 316 stainless steels, a nickel-base superalloy SuperNi 600 and a turbine disc alloy (equivalent to MAR M 200) have been carried out. Stainless steels 304£ and 316 have been tested with and without weldment, and materials 304 SS and SuperNi 600 have been tested with and without corrosive coatings. MAR M 200 was tested in air. A parametric method is suggested for obtaining the master curves for these alloys under test conditions. The applicability of the parameter to 25-20 stainless steel and a modified nuclear grade 316 type stainless steel has been verified.     

A parameter gauge for light fields

Translated from Izmeritel'naya Tekhnika, No. 3, pp. 32–33, March, 1992.     

Fast Bayesian approach for parameter estimation

This paper presents two techniques, i.e. the proper orthogonal decomposition (POD) and the stochastic collocation method (SCM), for constructing surrogate models to accelerate the Bayesian inference approach for parameter estimation problems associated with partial differential equations. POD is a model reduction technique that derives reduced‐order models using an optimal problem‐adapted basis to effect significant reduction of the problem size and hence computational cost. SCM is an uncertainty propagation technique that approximates the parameterized solution and reduces further forward solves to function evaluations. The utility of the techniques is assessed on the non‐linear inverse problem of probabilistically calibrating scalar Robin coefficients from boundary measurements arising in the quenching process and non‐destructive evaluation. A hierarchical Bayesian model that handles flexibly the regularization parameter and the noise level is employed, and the posterior state space is explored by the Markov chain Monte Carlo. The numerical results indicate that significant computational gains can be realized without sacrificing the accuracy. Copyright © 2008 John Wiley & Sons, Ltd.     

Size parameter for thermally emitting particles

For thermal emission from particles the conventional size parameter X = 2pia/lambda does not distinguish between small and large particles. We show that the opacity parameter Omega = 4pika/lambda = 2kX is a more accurate means of demarcating the two emission regimes. Omega is approximately equal to the particle's mean optical depth, and it can be derived from both scattering theory and geometrical optics.     

Optimized independent components for parameter regression

In this paper, a modified ICR algorithm is proposed for quality prediction purpose. The disadvantage of original Independent Component Regression (ICR) is that the extracted Independent Components (ICs) are not informative for quality prediction and interpretation. In the proposed method, to enhance the causal relationship between the extracted ICs and quality variables, a dual-objective optimization which combines the cost function w^TX^TYv in Partial Least Squares (PLS) and the approximations of negentropy in Independent Component Analysis (ICA) is constructed in the first step for feature extraction. It simultaneously considers both the quality-correlation and the independence, and then the ICR-MLR (Multiple Linear Regression) method is used to obtain the regression coefficients. The proposed method is applied to the quality prediction in continuous annealing process and Tennessee Eastman process. Applications indicate that the proposed approach effectively captures the relations in the process variables and use of proposed method instead of original PLS and ICR improves the regression matching and prediction ability.     

Improved time-base for waveform parameter estimation

An improved gated oscillator time-base and associated auto-calibration algorithm for use in a high-accuracy sampling waveform acquisition system are described. The time-base architecture consists of a stable 100 MHz gated oscillator, 24-bit counter chain, and a clock period interpolator. The nominal, uncorrected linearity of the time-base is approximately ±30 ps. By using an iterative, sine-fit based algorithm, the linearity has been improved to <5 ps. Details of the performance and major sources of error of the time-base and correction algorithm in an equivalent time sampling system are also discussed     

Edge Fracture Prediction Using Uncoupled Ductile Fracture Models for DP780 Sheet

This paper is a contribution to the prediction of edge fracture behavior using uncoupled ductile fracture models. A fully integrated simulation framework for the edge fracture prediction is proposed with the shear-induced pre-damage considered. User-defined material subroutines are coded with uncoupled ductile fracture models (Lou-Huh, Oh, Brozzo) incorporated, which are calibrated using the fracture strains of various loading paths. A series of 3D numerical simulations are performed and compared with the results of hole-expansion tests. The effects of pre-damage field and fracture models are analyzed and discussed.

     

A Lattice Model for Viscoelastic Fracture

A plane, periodic, square-cell lattice is considered,consisting of point particles connected by mass-less viscoelastic bonds.Homogeneous and inhomogeneous problems for steady-state semi-infinitecrack propagation in an unbounded lattice and lattice strip are studied.Expressions for the local-to-global energy-release-rate ratios, stressesand strains of the breaking bonds as well as the crack openingdisplacement are derived. Comparative results are obtained forhomogeneous viscoelastic materials, elastic lattices and homogeneouselastic materials. The influences of viscosity, the discrete structure,cell size, strip width and crack speed on the wave/viscous resistancesto crack propagation are revealed. Some asymptotic results related to animportant asymptotic case of large viscosity (on a scale relative to thelattice cell) are shown. Along with dynamic crack propagation, a theoryfor a slow crack in a viscoelastic lattice is derived.     

Fracture load predictions for adhesive joints

An engineering approach to fracture load predictions for adhesive joints is presented. The approach is based on the premise that the in-situ strength of the bondline can be characterized by the fracture envelope (critical energy release rate as a function of the mode of loading), for a specific adhesive system. By using the J integral for large deformations together with large-deformation beam theory, a simple closed-form expression is obtained for the energy release rate per unit area extension when a crack propagates in the bondline of a generalized adhesive joint (adhesive sandwich). This technique, together with a published method for mode partitioning, enables fracture load prediction by comparing the calculated fracture parameters with the critical ones from the fracture envelope. The approach is shown to predict fracture loads accurately for a variety of joints including the cracked lap shear (CLS), the single lap shear (SLS) and the double strap (DS) joint.     

某型弹用尾翼片裂纹分析

目的分析弹用尾翼片淬火、回火后表面裂纹产生的原因。方法通过对存在裂纹的尾翼片进行金相检测、化学成分分析、硬度检测、断口检测、力学性能检测,对尾翼片产生的原因进行了分析和讨论。结果确定了尾翼片裂纹的性质和产生原因。结论尾翼片裂纹为淬火裂纹;尾翼片裂纹产生的主要原因是由于原材料钢板剪尾翼片条料过程中,在条料的表面产生了冷变形开裂缺陷,出现了较深的线状缺口,导致淬火热处理时产生了严重的应力集中,诱发了裂纹的萌生,同时,原材料钢板中存在的魏氏组织和硫化物夹杂等材质缺陷,也对促进了淬火裂纹的萌生。     

Fracture diagrams for cracked stiffened panels

A general equation is derived for the prediction of structural failure of two-dimen-sional cracked components in which the geometrical features of the component affect the stress intensity factor of the crack. The general equation is used to construct a new fracture diagram for a uniformly stressed sheet containing a crack which is constrained by two stiffening elements fastened to the sheet. By the use of an example it is shown that the shape of the fracture diagram, and hence the fracture behavior of the panel, depends on the spacing of the stiffening elements, rivet locations and the relative stiffness of the cracked sheet and the stiffeners. It is shown that there are stiffened panel diagrams which indicate that failure can occur at fracture toughness levels significantly higher than would be apparent from an unstiffened panel of similar material.     

Fracture energy maps for fibre composites

Analytical expressions are presented which predict the debonding and pull-out lengths observed in brittle-fibre composites. The characteristic lengths are combined with models of four toughening mechanisms to calculate the work of fracture of a composite. The results are presented as maps showing not only contours of toughness but also the dominant toughening micromechanism. The toughness is largely determined by six material parameters, and each map demonstrates the combined effect of changing two of these simultaneously. Maps are presented for glass fibres in epoxy and carbon fibres in epoxy. Their use is demonstrated by showing the effects of hygrothermal aging on the toughness of the composites.Nomenclature A value ofP just less than one - B value ofP just greater than zero - d fibre diameter - E _f tensile modulus of fibre - g(l) general probability distribution ofl - G _II critical strain energy release rate per Mode 2 fibre-matrix bond failure - G ₂ G _II divided by geometry factor - G _m shear modulus of matrix - l _d total debonded length of fibre - l _n maximum pull-out length - ¯l _p mean pull-out length - ¯l average value of a distributed length,l - m Weibull modulus - P cumulative probability of failure - S cumulative probability of survival - W _e elastic work (per fibre) - W _i surface energy of fibre-matrix interface (per fibre) - W _p pull-out work (per fibre) - W _pdf post-debond friction work (per fibre) - x general distance from debond crack - x _c value ofx which only a fractionB of pull-out lengths can exceed - geometry factor from integration of stress field around fibre - constant, but exact value depends on author - surface energy of the composite - _m surface energy of the matrix - _f failure strain of fibre - _m failure strain of matrix - parameter dependent only on the Weibull modulus - ₁, ₂ stresses at which fractions,A, B of fibres have broken - _d debond stress - _f average fibre strength - ₀ characteristic strength of fibre - ₀ shear strength of fibre-matrix adhesive bond - _f frictional shear stress