Nonlinear Analysis of Thin Fracture Specimens Using Solid, Isoparametric Finite Elements

This report examines the performance of various "solid" finite elements for the analysis of thin shell structures often encountered in nonlinear fracture mechanics studies. Such models require solid elements in the crack front region to capture strong through-thickness effects; modeling of the entire test specimen-structural element with solid elements then proves convenient. Unfortunately, the standard 8-node "brick" element with full integration exhibits strong shear-locking under bending deformations and thus overly stiff behavior. Three alternative elements are examined here: the 8-node element with single-point integration, the 8-node element with enhanced (incompatible) modes and the 20-node (quadratic) element. Element performance is assessed through analyses of a thin M(T) fracture specimen loaded in remote tension. This specimen generates strong compressive (T- )stresses parallel to the crack growth direction which leads to out-of-plane bending in the crack front region (triggered by a small normal force). The displacements obtained with a refined mesh of thin shell elements provide the reference solution for evaluation of the solid element performance. The analyses include large-displacement effects, but linear material response for simplicity, and are performed with Abaqus 5.6 and Warp3D. The results show clearly that both the 8-node element with enhanced modes and the 20-node element with conventional reduced integration provide solutions of accuracy comparable to the thin shell element. Mixed 8 and 20-node element meshes for ductile fracture analyses with transition elements to maintain displacement compatibility are demonstrated to provide an accurate and efficient modeling strategy.NASA-AMES Research CenterNASA-Langley Research CenterContract Nos. NCC 2-5126 and NAG 2-112

Evapotranspiration monitoring methods within an irrigated mixed vegetation environment

The Colorado River system is one of the most heavily used river systems in the world and as such, accurate water accounting methods are vital. The U.S. Bureau of Reclamation (Reclamation) is charged with accounting for the Colorado River\u27s water use. One tool Reclamation uses to accomplish this is the Lower Colorado River Accounting System (LCRAS). This system uses a combination of remote sensing (RS) and a crop coefficient method to calculate agricultural and phreatophyte evapotranspiration (ET), a crucial component to any water budget. In this study, ET was measured within an irrigated mixed vegetation field (sapling cottonwood and willow, alfalfa, and noxious weeds), within Cibola, AZ, using a Bowen-Ratio (BR) flux tower, from May 18, 2006 through January 9, 2007. In the same field, ET estimates were calculated using LCRAS methodology and three RS Vegetation Index (VI) techniques were tested using various regression analyses. In this study, a published regression technique for estimating ET from VI data was tested and a local regression equation was developed using data collected in the study field. Cost effectiveness analyses were completed assessing the use of all methods to estimate phreatophytic ET along the lower Colorado River; The accuracy of all ET estimates was determined by comparison with BR flux tower ET measurements. LCRAS ET estimates ranged from a root mean error (RME) of 1.1 -- 2.3 mm per day, while RS ET estimates ranged from a RME of 0.5 to 2.5 mm per day. This study found that: RS VI methods for estimating ET within complex phreatophyte communities had the potential to be more accurate than LCRAS ET estimates; ET estimates based on local data outperformed estimates based on regional data; and that the tested RS techniques were not sensitive to different VI but were sensitive to sensor resolution and local empirical calibration data. This research demonstrates that estimating ET using VI techniques shows promise within mixed vegetation environments, but the accuracy of such estimates is improved by the availability of local ET measurements

Structural and metabolic studies on normal and pathological bone

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Bone is refractory to most conventional biochemical Procedures. However because it is now possible to cut sections (e. g. lopm) of fresh, undemineralized adult bone, this tissue can be analyzed by suitably modified methods of quantitative cytochemistry. A new substrate for assaying hydroxyacyl dehydrogenase activity demonstrated that bone cells may use fatty acids as a major source of energy: detailed analysis of the activities of key enzymes indicated that the paradox of ‘aerobic glycolysis’ of bone could be explained by fatty acid oxidation satisfying the requirements of the Krebs' cycle and directing the conversion of pyruvate to lactate The influence of glucose 6-phosphate dehydrogenase (G6PD) activity in aerobic glycolysis has been considered. The inverse relationships between this activity and that of Na-K-ATPase led to the development of a new method for the latter, based on a new concept in cytochemistry ('hidden-capture' procedure). A major feature of fracture-healing is increased periosteal G6PD activity. The association with the vitamin K cycle has been investigated by feeding rats with dicoumarol which not only inhibited bone-formation but also G6PD activity. The stimulation of this activity in fracture-healing has been linked with ornithine decarboxylase (ODC) activity, for which a new method has been developed. Rats deficient in pyridoxal phosphate (cofactor for ODC) had decreased G6PD responses and also appeared to become osteoporotic. Studies on osteoporotic fractures in the human showed the presence of relatively large apatite crystals close to the fracture-site, and disorganized glycosaminoglycans (demonstrated by the new method of ‘induced birefringence’)

Fracture in aluminum alloys for aerospace application

Complex structural aerospace components are high-speed machined out of (thick) hard alloy plate. A majority of the material is removed from an integrated unit, providing considerable weight savings. However, the demand for increased performance and reduced weight has lead to more complex loading scenarios – and the development of localized stress states that drive complex microstructure-related mechanisms of failure. Fault-tolerant design is (in turn) complicated by interplay of kinetics of plasticity and deformation incompatibility at the mesoscale. The presentation will begin with a review of experimental research conducted on hard aluminum alloys targeted for use in aerospace structures (a story that follows from the efforts of many graduate students and colleagues). Topics to be detailed include (i) fracture and fatigue response, studied at both the macroscale (Digital Image Correlation) and mesoscale (Electron Back-Scatter Diffraction); (ii) characterization of kinetics through cyclic testing; and (iii) in situ study of lattice strain on a grain-by-grain basis, using high-energy X-ray diffraction microscopy (HEDM) at a synchrotron source. The application of modeling techniques to interpretation of data will be given. In particular, the stress state at the mesoscale will be related to crack turning behavior. Model validation follows from agreement with HEDM results. Simulations of fracture highlight driving forces for crack growth, plasticity developed at grain boundaries and the effect of T-stress. The combined experimental and modeling program provides guidance in fault-tolerant design

Recent Development in the Weibull Stress Model for Prediction of Cleavage Fracture in Ferritic Steels

This paper reviews recent developments in the Weibull stress model for prediction of cleavage fracture in ferritic steels. The procedure to calibrate the Weibull stress parameters builds upon the toughness scaling model between two crack configurations having different constraint levels and eliminates the recently discovered non-uniqueness that arises in calibrations using only fracture toughness data obtained under small scale yielding (SSY) conditions. The introduction of a non-zero threshold value for Weibull stress in the expression for cumulative failure probability is consistent with the experimental observations that there exists a minimum toughness value for cleavage fracture in ferritic steels, and brings numerical predictions of the scatter in fracture toughness data into better agreement with experiments. The calibrated model predicts accurately the toughness distributions for a variety of crack configurations including surface crack specimens subject to different combinations of bending tension

Theoretical and Software Considerations for General Dynamic Analysis Using Multilevel Substructured Models

An approach is presented for the dynamic analysis of complex structure sy~t'=!!!S using the finite element method and multilevel substructured models. The fixedinterface method is selected for substructure reduction because of its efficiency, accurac and adaptability to restart and reanalysis. This method is extended to reduction of substructures which are themselves composed of reduced substructures. Emphasis is placed on the implementation and performance of the method in a general purpose software system. Solution algorithms consistent with the chosen data structures are presented in detail. This study demonstrates that successful finite element software requires the use of software executives to supplement the algorithmic language. As modeling and analysis techniques become more complex, proportionally more implementation effort is spent on data and computer resource management. Executive systems are essential tools for these tasks. The complexity of the implementation of restart and reanalysis porcedures also illustrate the need for executive systems to support the non computational aspects of the software. The example problems show that significant computational efficiencies can be achieved through proper use of substructuring and reduction techniques without sacrificing solution accuracy. The unique restart and reanalysis capabilities developed in this study and the flexible procedures for multilevel substructured modeling allow analysts to achieve economical yet accurate analyses of complex structural systems

An investigation into the hedonic price analysis of the structural characteristics of residential property in the West Rand

A vast amount of literature on hedonic price modelling has been formulated on overseas property markets. Very little currently exists in South Africa and this poses a risk for sellers and estate agents of a residential property when listing it on the open market, as this could result in an extended list period, reducing the original asking price. This paper seeks to examine Gauteng’s West Rand residential property market and formulate a multi-variate regression model to best predict property prices, determined by a property’s structural characteristic. The research tracks residential sales from 1996 to 2009, a thirteen-year sample period from which a composite property index, to account for inflation and real house price growth, has been formalised. Correlation and regression analysis was used to interpret the data at the relevant significance level. In order to account for locational attributes present in property values, the data set was divided into locational quadrants and run as dummy variables. A further regression was run on a screened data set to create an ordinary least squares equation that could be used to show the relationship between property values and structural characteristics. The results indicated a good fit with an R2 of 69.5%. This regression was then applied practically to predict property prices for houses that have transacted in the West Rand property market, and plotted along a value/price graph using the 45-degree true value frontier line. The relevant results were then interpreted, and recommendations given

Finite Element Analysis of Concrete Fracture Specimens

The effects of the descending branch of the tensile stress-strain curve, fracture energy, grid refinement, and load-step size on the response of finite element models of notched concrete beams are studied. The width of the process zone and constraint of crack angles are investigated. Nonlinearity is 1 imited to cracking of the concrete. A limiting tensile stress criterion governs crack initiation. Concrete is represented as linear elastic prior to cracking. Cracks are modeled using a smeared representaion. The post-cracking behavior is controlled by the shape of the descending branch, fracture energy, crack angle, and element size. Unloading occurs at a slope equal to the i nitia 1 modulus of the material. load deflection curves and cracking patterns are used to evaluate the beam's response. Comparisons of the process zone size are made. All analyses are performed on a 200 x 200 x GOO mm concrete beam, with an initial notch length of 80 mm. The fracture energy, tensile strength, and shape of the descending branch interact to determine the stiffness and general behavior of the specimen. The width of the process zone has a negligible influence on the beam's response. The importance of proper crack orientation is demonstrated. The model is demonstrated to be objective with respect to grid refinement and load-step size