Won J. Park

An Optimal Design of Simple Symmetric Laminates Under the First Ply Failure Criterion

Abstract : Optimal design of various types of symmetric laminates for T300/5208 graphite epoxy composites was investigated under the first ply failure criteria. The symmetric laminates considered include continuous laminate and angle ply laminate. The optimal design angles 0 were obtained and presented in graphic form as functions of the loading conditions (N1, N2, N6). The results presented here are directly useful for designers for making a choice of composites for optimal performance. (Author)

Non-Uniform Percentage Brokerage Commissions and Real Estate Market Performance

The effect of non-uniform commissions on the market duration of residential properties is ambiguous. While the brokers search effort is positively related to the size of the percentage commission, so is the sellers reservation price. Each of these relationships imply a time-on-market effect in the opposite direction of the other. A powerful statistical technique, survival regression, is employed to determine which relationship dominates. Because the probability that a property will sell at any given point in time is inversely related to the size of the percentage commission, we conclude that the negative search effects associated with low commission rates are more than offset by the positive reservation price effects. Copyright American Real Estate and Urban Economics Association.

More goodness-of-fit tests for the power-law process

The power-law process is often used as a model for reliability growth of complex systems or for reliability of repairable systems. There are many results on estimation and hypothesis testing concerning parameters of the power-law process. Goodness-of-fit tests for the power-law process were presented in Park & Kim (1992) using Kolmogorov-Smirnov, Cramer-von Mises, and Anderson-Darling statistics. This paper considers the same problem using three statistics, Kuiper, Watson and weighted Watson. Tables of critical values for the three statistics are presented and the results of a power study are given under the alternative hypothesis that failure data come from a nonhomogeneous Poisson process with log-linear intensity function. The power study shows that the tests have acceptable power for various parameter values and the Cramer-von Mises Statistics, in Park and Kim (1992), has the highest power among the six statistics. An example from the Cox air conditioning repair data is presented. >

Statistical analysis of a power-law model for repair data

The power-law process is an alternative model to the homogeneous Poisson process for analyzing repair data. When many nominally-identical systems are in service, the repair data for all systems can be used to assess the aptness of the power-law model. To test the hypothesis of a homogeneous Poisson process versus the power-law process, the maximum likelihood estimates of the power-law process parameters are used. A table of the critical values for the estimates is given, along with an example of their use.

Symmetric Three-Directional Optimal Laminate Design:

N RECENT YEARS CONSIDERABLE ATTENTION HAS BEEN FOCUSED ON THE Idesign of composite laminates because the weight savings may be obtained by efficient design procedures, and the objective of this paper is to describe such a procedure. Park [1,2] developed a method based on first ply failure criterion or Von Mises failure criterion for simple symmetric laminates. Wurzel [5] considered a design method of symmetric bidirectional composites, namely cross-ply and angle-ply laminates. Tsai and Massard [3] presented principle stress design method and design method by laminate ranking. In this paper we utilize a first ply failure criterion as objective function in optimal design of laminates and ply orientation angles and ply thickness are considered as the design variables. We consider designing three-directional symmetric laminates under various in-plane single loading conditions. Our design

Lognormal distribution - model for fatigue life and residual strength of composite materials

The differential equation of Yang for residual strength degradation in composite materials subject to fatigue loading is considered for obtaining the lognormal distribution as a model for the fatigue life of a composite material. Using this model, an ad-hoc method for estimating the physical parameters of the composite system is suggested. An example illustrates the use of our method in analyzing data. Since Yangs differential equation is for composite materials, our objective was to derive a model for composite materials. However, the methodology can be extended to other metals and materials.

Statistical analysis of censored motion sickness latency data using the two-parameter Weibull distribution

The suitability of the two-parameter Weibull distribution for describing highly censored cat motion sickness latency data was evaluated by estimating the parameters with the maximum likelihood method and testing for goodness of fit with the Kolmogorov-Smirnov statistic. A procedure for determining confidence levels and testing for significance of the difference between Weibull parameters is described. Computer programs for these procedures may be obtained from an archival source.

An Optimal Design of Simple Symmetric Laminates Under Von Mises Failure Criterion

N THE PROBLEMS OF OPTIMAL LAMINATE DESIGN, IT IS WELL UNDERSTOOD th4t the most important design variables are ply orientation angles, ply thicknesses and volume fractions of fibers. Park [1] has considered an optimal laminate design problem, in which an optimal ply orientation angle was computed for various multiple loadings (N,, N2, N6) under the first ply failure criterion. This paper considers a similar problem extending the failure criterion to the Von Mises failure criterion. Various simple symmetric laminates of the composite material T300/5208, such as

Percentiles of Pooled Estimates of Weibull Parameters

In fatigue testing of metals or composite materials under different levels of stress, the underlying distributions are often assumed to be Weibull with a common shape parameter. The problems of estimation and hypothesis tests of the common shape parameter and scale parameters in the Weibull distributions are considered in this paper. The results are: 1) the maximum likelihood equation for estimating the shape parameter, 2) a s-confidence interval and hypothesis test for the shape parameter, 3) s-bias factors and variances of the pooled estimator of shape parameter, and 4) s-confidence interval and hypothesis test for the scale parameters. Examples are given for composite material applications. Results are based on Monte Carlo simulation methods.