Browsing by Author "Merwin, John E."
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Item A characterization of surface roughness(1982) Abdul Rahim, Abdallah Adel; Merwin, John E.; Lutes, Loren D.; Bourland, Hardy M.A study is made on the surface finish of five ground specimens in an attempt to improve on their surface roughness characterization. Surface finish is characterized by a single statistic referred to in practice as the Center Line Average height value (CLA). This CLA height value is a measure of the roughness contribution to surface texture. A fourth order, low pass Butterworth filter is used to separate the contribution of roughness, waviness and error of form that are present in the surface texture. Applying this filtering technique eliminates the need of varying meter cutoff lengths to maximize and minimize those contributions. A statistical analysis provided a choice of ranges of cutoff periods where the CLA height values appear to be more representative of the roughness characteristics of the surface texture. Additional studies using other surfaces are necessary to improve on the single number characterization of surface roughness.Item A comparison of fatigue life improvements obtained by weld reinforcement and toe grinding(1981) Mullen, Christopher Louis; Merwin, John E.; Lutes, Loren D.; Bourland, Hardy M.A study is made of two treatments which help to improve the fatigue life of a welded structure. Experimental data are examined to assess the relative magnitude of the improvements under a bending type of loading. The results of this investigation are discussed in the light of stress distribution information obtained from a finite element analysis of two welds having different weld shapes. Reinforcement is found to achieve reductions in peak stress at the weld toe not only by reducing the angle at which the weld bead meets the plate surface but also by increasing the leg length which reduces the moment developed at the weld toe location. These reductions in stress are accompanied by reductions in strain which, in the presence of repeatedly applied tensile forces, make the weld toe more resistant to the formation and propagation of cracks. Toe grinding is shown to modify the shape and surface finish of the weld toe. These modifications vary significantly with the tool used in the grinding. At low load levels toe grinding with a tool that not only provides a beneficial curvature to the toe but also removes potential crack initiation sites may lead to improvements which are greater than those obtained by reinforcement.Item An experimental parametric study of stochastic fatigue(1989) Brown, Martin Wilson; Merwin, John E.Fatigue damage due to random stress histories is typically estimated by either Rainflow range counting (a time history approach), or by the Rayleigh approximation (a power spectral density approach). These methods are not accurate in all applications. A new time history method of estimating fatigue life for a time history with undefined power spectral density is proposed. This is called the peak moment Rayleigh estimate, and it is based on the standard deviation of the random signal from knowledge of the odd moments of the peaks and valleys of the time history. This study also evaluates a recent spectral method of fatigue life estimation, the Lutes Single Moment estimate, using data from a previous experimental study by Sarkani for loadings with defined power spectral densities. This estimate is similar in form and ease of application to the Rayleigh estimate, but results in more accurate fatigue life predictions for processes with very wide bandwidths.Item Analysis of testing procedures for determining mechanical properties of composite specimens manufactured with zoned pressure molding(1999) Lawton, Kimberly Anne; Merwin, John E.A program of testing various material properties for fiberglass and carbon fiber composites processed using a new proprietary process called Zoned Pressure Molding developed by Stewart Automotive Research (SAR) was developed. Problems associated with the testing of advanced composites are presented along with recommended solutions for these problems. Ultimate tensile strength, Young's modulus, ultimate compressive strength, compressive modulus, ultimate shear strength and shear modulus were determined for a carbon fiber reinforced composite material. Fiber volume fraction was determined for various fiberglass preforms. Finally, ultimate tensile strength and Young's modulus were found for fiberglass reinforced composite coupons made in a test press at SAR.Item Approximation of the plastic behavior of rocks at elevated mean pressures using a bi-linear yield envelope(1980) Gerecci, Gultekin S.; Cheatham, John B.; Wilhoit, James C.; Merwin, John E.In this study, the stress-strain curves of four different rocks are obtained by using true tri-axial test apparatus. First, results using this test apparatus are compared with results using conventional tri-axial test equipment and the dependability is confirmed. Mohr circles, based on the stress-strain curves have been drawn to provide yield envelopes for the rocks tested in this study. The yield envelope is approximated by a bi-linear curve and by extending the wedge indentation theory for this case, the penetration force equation is derived. The calculated values for the penetration force are then compared with the experimental results obtained by previous investigations. The rocks tested are: Danby Marble, Carthage Marble, Ohio Sandstone and Indiana Limestone. These rocks are tested under confining pressures of 25, 5, 75, 1,, 2, and 3, psi.Item Bending of pipes under tension using a trilinear moment-curvature relationship(1973) LeBlanc, Steven John Hoa; Merwin, John E.An analytical procedure is developed in this thesis for the bending of pipes under tension using a nonlinear moment-curvature diagram. The procedure is used to analyze an offshore pipeline during construction operations. It was concluded that the length of the stinger required to lay offshore pipelines can be reduced by allowing the pipeline to yield during construction operations. The method of analysis used in the thesis was determined to be of limited use due to the sensitivity of the solutions when some of the parameters were allowed to exceed certain values.Item Combined gradient-restoration algorithm for optimal control problems(1983) Basapur, Venkatesh K.; Miele, Angelo; Douglas, Andrew S.; Merwin, John E.The problem of minimizing a functional I subject to differential constraints, nondifferential constraints, and general boundary conditions is considered in this thesis. It consists of finding the state x(t), the control u(t), and the parameter-rr so that the functional I is minimized, while the constraints and the boundary conditions are satisfied to a predetermined accuracy. A combined gradient-restoration algorithm is developed. This is an iterative algorithm characterized by variations Ax(t), Au(t), Air leading toward satisfaction of the optimality conditions, while simultaneously leading toward constraint satisfaction. The variations Ax(t), Au(t), Air are generated by requiring the first variations of the augmented functional J and the constraint error P to be negative. The procedure leads to a linear, two-point boundary-value problem, which is solved via the method of particular solutions. The descent properties of the algorithm are studied, and schemes to determine the optimum stepsize are discussed. In order to improve the convergence characteristics, the inclusion of a restoration phase is studied. In this connection, three versions of the algorithm are studied: the combined gradient-restoration algorithm (CGRA); the combined gradient-restoration algorithm with alternate restoration (CGRA-AR); and the combined gradient-restoration algorithm with complete restoration (CGRA-CR). A comparison of these versions with the sequential gradient-restoration algorithm (SGRA) is also made. Three numerical examples are presented to illustrate the different approaches. Key Words. Numerical methods, optimal control, gradient methods, combined gradient-restoration algorithm, differential constraints, nondifferential constraints, general boundary conditions.Item Elastic behavior and design of arches(1979) Leung, Alen Wah Kwok; Austin, Walter J.; Holt, Edward C.; Merwin, John E.A detailed parametric study of the elastic behavior of symmetric arches with hinged and fixed ends is presented herein. The study is restricted to parabolic arches of moderate and low rise (<.25 span) with prismatic cross-section subjected to a uniform dead load and a uniform live load which extends from the left support over a variable length. Both the dead and live loads are of uniform intensity on the horizontal deck of the arch. The effects of load magnitude, slenderness ratio, rise/span ratio, live/dead load ratio and length of live loading on the internal forces, displacements and reactions are investigated. Approximate equations to predict stresses and deflections are derived. Special study has been made of the occurrence of first yielding. The live load length, load magnitude and location on the arch axis corresponding to first yielding have been found for a wide range of parameters. Approximate procedures are presented for estimation of the load at first yielding. The analytical study was made with the use of a computer program. The information presented herein is expected to be useful for the design of arch bridges.Item Empirical and analytical correlation of three-dimensional optimum structural frames(1977) Arguello Carazo, Xavier; Merwin, John E.The present investigation deals with the correlation between optimally designed space frames and experimental results obtained from testing small-scale modeled three-dimensional frameworks loaded beyond the elastic limit. The optimum design procedure used was developed by M. Khalifa. A justification for using the linear optimization procedure as opposed to the non-linear one is presented. Efficient techniques for performing tests in small-scale modeled three-dimensional frames loaded up to failure are developed. Two small-scale modeled space frames are fabricated and tested; one subjected to vertical loads and the other to vertical and lateral loads. A comparison between the optimally designed space frames and the models tested is made. The results and conclusions are presented and discussed.Item Experimental system identification of model frames using frequency-domain and time-domain methods(1994) Smyth, Andrew Willem; Merwin, John E.Two different metal test frames are excited using the impact-hammer method. The force-input and specific acceleration-output time-histories were recorded, and through frequency-domain, and time-domain structural system identification methods, the systems were identified. Artifacts of the test method and equipment, in the input-output data are highlighted, and their effect upon the identification is diminished through digital signal processing techniques. With some explainable differences, the correspondence between the systems identified in the frequency-domain, the systems identified in the time-domain, and analytical models of the test structures was quite good. Structural system identification, particularly through discrete time-domain methods (due to the digital nature of future implementations), enables the calibration of synthesized finite element models using prototype data, is an excellent means of non-destructive damage evaluation, and is required for active structural control.Item Fatigue analysis for non-normal stochastic stress(1982) Hu, Sau-Lon James; Lutes, Loren D.; Merwin, John E.; Pharr, George M.The basic goal of this study is to determine the relationship between the fatigue damage predicted for Gaussian and nongaussian processes when both are analyzed by the rainflow counting method. Due to a lack of references on nongaussian process simulation, some initial effort is devoted to finding a simple way of generating nongaussian processes. In this study, in addition to mean and variance (the usual two parameters of a normal distribution), kurtosis is chosen as a third parameter to indicate the degree of non-normality. A theoretical prediction of the fatigue damage due to a nongaussian process is also obtained for the special situation of a narrow-band process with the exponent in the S-N curve limited to integer values. The effect of non-normality on the empirical rainflow results (for any bandwidth process) is shown to be approximately the same as the effect predicted theoretically for the corresponding narrow-band process. A practical example associated with the fatigue life design of an offshore platform is given. It incorporates non-normality considerations into the design procedure. It is concluded that the effect of non-normality should not be neglected. It is also noted that the influence of nonnormality is affected not only by the kurtosis of the stress process, but also by the slope of the S-N curve (which is a material property).Item Fatigue tests of axially loaded K-joints(1980) Nezames, Theodore Hercules; Merwin, John E.; Bourland, Hardy M.; Krahl, Nat W.This thesis deals with the procedures, equipment and results of testing large scale, tubular, axially loaded, simple K-joints to failure in fatigue. Each K-joint had a 2" diameter chord, a 18" diameter 3° branch and a 12 3/4" 9° branch. Measures were taken to ensure axial forces with little bending in both branches. This was accomplished using a specially designed load frame with capability for full adjustment of alignment. The design is presented in an appendix. Sophisticated load control was used to apply a variable force of constant peak amplitude in which the mean was zero. Various methods of crack initiation detection are discussed ranging from acoustic emissions to visual inspection. Hot spot strains were measured using rosette type strain gages with locations determined from a finite element program. Data such as strains, load versus displacement loops, and crack lengths were taken frequently throughout the test. Substantial amounts of accumulated data, which include strains and load displacement loops, are placed in the appendicies. Test results are presented in graphical form and tables using information calculated from the raw data. Complete crack progression data were kept throughout each test and presented on drawings of projections of the weld toe onto a plane, and in graphs of crack length versus cycles for the major cracks.Item Higher order equivalent linearization in random vibration(1979) Hseih, Jeder; Lutes, Loren D.; Merwin, John E.; Veletsos, Anestis S.This study considers third-order linear systems and a two mode linear system as candidates for equivalence to the bilinear hysteretic system subjected to a Gaussian white noise excitation. In each linear system some parameters are chosen to match stationary mean squared levels of both displacement and velocity response to those of the bilinear hysteretic system (using empirical values for the nonlinear system). The particular types of equivalence being sought here involve the power spectral density (psd) of stationary response and transient buildup of mean squared response. The values predicted by the linear models are compared with empirical data for the nonlinear system. The bilinear hysteretic systems considered have the slope ratio between post-yielding and pre-yielding stiffnesses of a - 1/2 (moderately nonlinear system) and a = 1/21 (nearly elastoplastic system). Two ways of choosing the parameters of the third-order linear system are considered. One method involves choosing two damping factors for equivalence and the other involves choosing one damping factor and one spring stiffness for equivalence. The two mode linear system is a model with two uncorrelated modes whose undamped natural frequencies correspond to the initial stiffness and the reduced stiffness of the bilinear hysteretic system, and with two damping factors chosen for equivalence. Based on the comparisons of both psd and transient response, it is concluded that the third-order system is the better equivalent linearization overall (arbitary a), although the two mode linear system is somewhat better in the particular case of a nearly elasto-plastic system.Item Natural and induced anistropy in rocks under plastic conditions(1981) Allen, Michael Bruce; Cheatham, John B.; Bowen, Ray M.; Merwin, John E.The object of this study was to investigate the effect of anistropy on wedge indentation tests. These tests are used to qualitatively investigate the deviation forces encountered when drilling through an anisotropic medium. The preliminary investigation of anisotropic rocks lead to the study of an elastic - linear hardening limestone, Cordova Cream Limestone, also known as Austin Chalk. A yield condition and hardening rule, which could accurately account for the strain induced anisotropy, were developed. After carefully weighing the advantages and disadvantages of using this rock in wedge indentation tests, it was decided not to proceed further with this material. Instead, Pierre Shale was used, because preliminary tests indicated a high degree of anisotropy. Conventional triaxial tests were performed, in order to establish a yield condition. This yield condition was used in conjunction with four different theories to predict the vertical and horizontal forces encountered in wedge indentation tests. The four theories consisted of an adaptation of R. McLamore's preferred chip analysis, a plane strain slip-line field solution assuming no lip and a perfectly rough wedge, and two limit analysis solutions using the plane strain yield condition; one assumed a perfectly rough wedge while the other assumed the wedge to be frictionless. The slip-line field solution and the limit analysis solutions successfully bounded the vertical force data. The slip-line and limit analysis solutions for the horizontal force very nearly coincided. Although the scatter in the horizontal force data prevented quantitative confirmation of any of the above theories, the general trends, direction of forces and symmetry, were supported. The scatter in the data also did not convincingly confirm or contradict McLamore's theory, although previous observations and experiments do contradict this theory while supporting the plasticity solutions. Additional tests with various wedge and rock orientations were also performed to investigate the effects on the problem. The conclusions that can be made from this study are: (1) That the strength characteristics of Cordova Cream Limestone can be modeled successfully with Ziegler's modification of Prager's hardening rule in the principal stress space. (2) That plasticity theory can successfully predict the forces on a wedge penetrating anisotropic Pierre Shale. The consequences of this study are that the forces on the wedges and therefore on a drill bit are in the up-dip direction when the bedding plane is inclined less than 45°, while the forces are in the down-dip direction for inclinations greater than 45°.Item Nonlinear torsion problems approached by theory of perturbation(1967) Lee, James Der-Yi; Merwin, John E.The work presented herein is to describe how the theory of perturbation can be applied to nonlinear torsion problems. The torsion of circular, rectangular, triangular, and elliptic cross sections has been investigated. The applicability of this approach is shown by a comparison with exact solutions obtained for a circular section.Item Nonnormality in the seismic response of primary-secondary systems(1990) Chen, Chen-Kang David; Merwin, John E.Response nonnormality is investigated for a yielding primary structure and a linear secondary system (P-S) subjected to a normally distributed ground acceleration. The nonlinearity considered is bilinear hysteretic (BLH) yielding in the primary structure. The coefficient of excess (COE), which is a normalized fourth cumulant function, is used as a measure of the nonnormality in the current study. An initial effort focuses on the nonnormality of primary absolute acceleration, since this is the base excitation of a light secondary system. Analytical and numerical results for a nonlinear but nonhysteretic substitute structure are shown to be in good agreement with those from simulation for both mean squared levels and COE of response. It is shown that the acceleration of the primary system can be significantly nonnormal in some situations. Linear substitute methods are used for analytically evaluating the nonnormality of secondary response. The basic concept is to use a linear model with nonnormal excitation to replace the nonlinear primary element with normal excitation, with the goal of matching the trispectrum for the acceleration of these two systems. The trispectrum is the frequency decomposition of the fourth cumulant function. Periodogram analysis (a special FFT technique for obtaining polyspectra) is developed for evaluating the trispectral function of BLH primary acceleration. A two filters model (with a more narrowband fourth cumulant filter) gives good approximations for the COE values of secondary response in most cases including both cascade and noncascade analysis. The probability of failure of secondary response affected by nonnormality due to nonlinearity in the primary is investigated. A nonnormality correction factor (NCF) which is equal to the ratio of the expected life for a Gaussian process to the expected life for the non-Gaussian process is used as an index of the nonnormality effect. Analytical approaches based on knowledge of the first four response cumulants are developed to approximate the NCF values. It is shown that the NCF for first-passage failure generally is more significant than for fatigue failure based on the cases in this study, and both failure modes can be significantly affected by the nonnormality in some situations.Item Prediction of fatigue life in relation to surface finish parameters(1989) Abdulrahim, Abdallah Adel; Merwin, John E.One can safely claim that not one single surface roughness parameter, in terms of completely characterizing a surface, can be exhaustive. This study is both an experimental and theoretical study of characterizing surface finish in relation to fatigue; and the role of this finish in fatigue initiation. A total of forty-two samples, comprising three different groups that exhibited different surface finish properties, were fatigued using constant amplitude loading. The S-N curves for these groups were established. A correlation between surface finish parameters and fatigue initiation was established. The parameters of importance were expected to be the deepest scratch and the rms valley curvature (sharpness) of the measured surfaces. The product of both parameters showed a reasonable degree of correlation with fatigue initiation at different stress levels. This product became a basis of establishing the functional relationships between surface finish parameters and fatigue failure. A number of specimens were prepared having a single scratch that varied in depth from 0.004 inches to 0.02 inches. The scratches had a fixed radius of curvature of 0.003 inches. The specimens were fatigued and the resulting S-N curves were grouped into one spectrum. The single scratch spectrum utilizing the deepest scratch and sharpness was used as a basis to predict fatigue failure. First-passage criteria were used to predict the deepest scratch in longer sampling lengths. The product of the predicted deepest scratches and curvature were used with the single scratch spectra to predict fatigue failure. The predicted life was within the 95% confidence bands of the S-N curves generated from fatiguing the specimens.Item Stochastic dynamics of yielding two-story frames(1980) Brinkman, Carl Rhys; Lutes, Loren D.; Holt, Edward C.; Merwin, John E.An investigation is performed to determine the response of yielding two-story frames subjected to a white noise ground acceleration. Column stiffnesses of the structural model exhibit a bilinear hysteretic behavior. Empirical data for general yielding systems is obtained by a numerical integration procedure which measures mean square displacement and velocity values at each floor level of the structural model. Results are compatible with known data for linear and nonlinear single degree of freedom systems. Attempts at analytical solutions are limited to two-story frames in which only the first floor yields. The first theoretical solution attempts to predict the behavior of a nonlinear system by modifying stiffness and damping characteristics and studying the resulting system as a linear model. It is found that interstory mean square response is underpredicted with this procedure. The second analytic approach defines an equivalent linear system by matching its response parameters with those obtained from numerical results and calculating frequencies and mode shapes of that equivalent system. Results from this interpretive approach indicate that for systems for which the second slope of the bilinear stiffness relation is a small percentage of the initial slope, a linear system which exhibits the same mean square response levels as the original yielding system does not exist.Item Stochastic stress history simulation for fatigue analysis(1983) Zimmerman, James J.; Lutes, Loren D.; Merwin, John E.; Bourland, Hardy M.; Pharr, George M.The basic goal of this study is to find an alternate, more efficient method of simulating stochastic stress histories for fatigue analysis. Stress histories are generated from power spectral densities made up of either one or two rectangular blocks. The currently popular simulation technique produces a normal signal by summing sine waves with random phase angles. The fatigue damage predicted from stress histories simulated by this method is used as the basis of comparison for three other techniques. Two techniques which simulate correlated, Rayleigh distributed peaks and valleys are investigated. Another technique produces correlated peaks which have S. O. Rice's peak distribution. The rainflow method of cycle counting is used to determine the stress ranges from all the stress histories and Miner's rule is used to predict fatigue damage. It is concluded that fatigue damage from processes having single block power spectral densities can be efficiently and accurately predicted from a sequence of correlated peaks which have the peak distribution. This technique is three to four times faster than the currently popular technique. Simulation techniques which generate Rayleigh distributed peaks are found to be overly conservative in their prediction of fatigue damage. None of the three techniques investigated can be used for simulation of processes with two block power spectral densities. It is also concluded that a stress process cannot be sufficiently characterized solely by the spectral width parameter for fatigue damage predictions.Item Stress distribution in the human buttocks(1980) Scott, Allison Cooper; Wilhoit, James C.; Walker, William F.; Merwin, John E.One of the foremost problems in rehabilitation today is the prevention of bedsores or decubitus ulcers in spinal cord-injured patients. For those patients who must spend great lengths of time in a wheelchair, the wheelchair cushion is of vital importance in avoiding the high forces on the skin that could cause tissue breakdown. The finite element method was used to analyze a two-dimensional model of the human buttocks. The normal and shear stresses in the flesh were determined for several body types supported by a variety of cushion materials. Results permitted a comparison of external pressure measurements made experimentally with calculated internal stresses. Thus, analysis supplemented clinical observations in evaluating the ability of cushion materials to minimize stresses in the flesh.