bluuebunny/crossref_metadata_2025 huggingface dataset:
Near-Crack-Tip Transverse Strain Effects Estimated with a Large Strain Hollow Cylinder Analogy
Authors: Merkle JG
Published: March 2009

In order to study effects of constraint on fracture toughness, it is reasonable to study the region of large strains close to the crack tip within which the microscopic separations that lead to fracture often take place. The first step in this direction was taken in 1950 by Hill, who postulated that close to a circular notch tip the principal stress directions would be radial and circumferential, so that the plastic slip lines (maximum shear stress trajectories) would be logarithmic spirals. The resulting equation for stress normal to the notch symmetry plane, neglecting strain hardening, was identical to that for the circumferential stress near the bore of an ideally plastic thick-walled hollow cylinder under external radial tension, because the relevant geometries are identical. Hill's hypothesis was extended algebraically by Merkle to include strain hardening with a generalized-plane-strain small-strain hollow cylinder analogy, and numerically in a more general way geometrically for plane strain and large strains by Rice and Johnson. Large strain finite element analyses have shown that a wedge-shaped zone ahead of a blunting crack tip deforms like a hollow cylinder. This paper extends the generalized plane strain hollow cylinder analogy to large strains. The strain equations are derived by analyzing the constant volume deformation of a differential cylindrical element. The circumferential strain is singular at the tip of an initially sharp crack. With the strain distribution determined, the stresses are obtained by integrating the equation of radial equilibrium. An approximation is developed for the first increment of radial stress near the strain singularity. Calculations show that the in-plane stresses are only slightly sensitive to transverse plastic strain.
Developing J-R Curves Without Displacement Measurement Using Normalization
Authors: Lee K , Landes JD
Published: March 2009

The method of normalization is used in a new inverse way to develop J-R curves from test records that contain only load-versus-crack length values with no measure of displacement. These data may represent a special case where measurement of crack length is easier experimentally than the measurement of displacement. The method of normalization uses the principle of load separation to relate the three variables of load, displacement, and crack length. This relationship is expressed by a functional form so that, given any of the two variables, the third can be determined. Previously, the method of normalization has been used to determine crack length when given only load and displacement; however, it can also be applied to determine displacement when given only load and crack length.

In this new way of applying normalization, some problems arise; namely, the calibration points needed to evaluate the functional form of normalized load are not available. A major thrust of this paper is to solve the problem of how to determine these calibration points. To do this, the method is first applied to data for which all of the three variables have been already determined. The displacement is assumed to be missing and the J-R curves are determined from only the load and crack length values; these J-R curves are then compared with the ones generated for the data with all of the variables available. Three methods of determining calibration points were used in the comparison. Of the three, an approach called the power law fit is the best for determining the calibration points. The method is then applied to data for which there are only load-versus-crack length data. The results of this study show that the method of normalization works well for developing J-R curves from load-versus-crack length data.
An Asymptotic Analysis of Static and Dynamic Crack Extension Along a Ductile Bimaterial Interface/Anti-Plane Case
Authors: Yuan H , Schwalbe K-H
Published: March 2009

In the present paper, the asymptotic near-tip stress and velocity fields of a quasi-statically and dynamically growing crack under steady-state conditions along an interface between two ductile materials are presented. The ductile materials are characterized by J2-flow theory with linear plastic hardening. Only the antiplane strain case is considered in the present paper. The linear hardening solutions are assumed to be of variable-separable form with a power-law singularity in the radial distance to the crack tip. Results are given for the singularity and for the distribution of the stress and velocity fields as functions of the hardening parameter and the crack propagation velocity. It is found that an interface between two ductile materials with large plastic hardening only slightly affects the angular distributions of stress and deformation velocity, whereas the singularity in this case is determined by both strain-hardening factors. If a crack is lies along on an interface with a small strain hardening, the singularity is dominantly determined by the lower strain-hardening material and the angular variation of stress and deformation velocity fields in the higher hardening material will be changed drastically. Differences of the elastic shear moduli will mainly influence the deformation velocity distribution, the stress variation will hardly be affected. Increasing the elastic modulus in the small strain-hardening material can slow down the appearance of the plastic reloading. The crack propagation velocity can enlarge the plastic loading zone and diminish the singularity.
An Application Methodology for Ductile Fracture Mechanics
Authors: Landes JD , Zhou Z , Brown KH
Published: March 2009

A ductile fracture methodology is presented here that takes the load-versus-displacement data from a precracked laboratory test specimen, such as a compact specimen, and through a series of analysis steps that will allow direct prediction of the load-versus-displacement behavior of a structural component containing a crack-like defect. This is based on a ductile fracture methodology originally presented by Ernst and Landes that was used to evaluate maximum load in a structure under complex loading. The load versus displacement from the test is analyzed by using the methodology in reverse so that it can be divided into two outputs, calibration functions, and fracture toughness. The calibration functions used here are based upon the load separation principle; the fracture toughness is given in terms of the J-R curve. The calibration functions and fracture toughness are then transferred from those relevant for the test specimen to those for the structure to be analyzed. Having done this, the methodology is used to predict the load-versus-displacement behavior of the structural component.

A critical step in the methodology is the transfer of the calibration functions and fracture toughness from those for the specimen to those for the structure. The fracture toughness transfer must be made knowing the effects of three categories on toughness; size, geometry, and thickness constraint. Presently, the data in the literature show conflicting trends for the effect of size and constraint. Therefore, a transfer based on known principles is not possible. The approach used here is to try to get a conservative value of the fracture toughness. Fortunately, for many structural components, the fracture toughness is not the controlling input, and the calibration functions have more influence on such things as maximum load prediction.

A procedure for transferring the test specimen calibration function to the structural calibration functions has been developed. Two important things must be addressed for this transfer. A functional form for the calibration function is needed for the structure. This form can be based on the J-calibration equation and is known for common geometrical shapes. For more complex geometries, this form must be determined.

In this paper, several example structural components are taken to illustrate the methodology. A compact specimen is used for the laboratory specimen geometry and six structural component models are analyzed to illustrate how well the method works and to evaluate the importance of the various steps in the method. From these examples, suggestions are made for further work that could improve the overall method.
Level-3 Crack-Tip Opening Displacement (CTOD) Assessment of Welded Wide Plates in Bending—Effect of Overmatching Weld Metal
Authors: Berge S , Eide OI , Fujikubo M
Published: March 2009

Butt-welded wide plates with semi-elliptical surface cracks along the fusion line were tested in four-point bending. The material was normalized low-carbon microalloyed steel that conformed to offshore specifications. The testing temperature was in the range of -40° to 0°C. The test results were analyzed by the crack-tip opening displacement (CTOD) Level-3 method that compared different assumptions for the reference strain in order to assess the effect of overmatching weld metal. The original Level-3 plane stress model and a proposed plane strain model with a modified definition of effective primary stress were compared. Consistent results were obtained by the latter method. A detailed finite-element method (FEM) model was validated, and results from comprehensive analyses of welded and nonwelded models are presented.
Limit Pressure Analysis of a Cylindrical Vessel with Longitudinal Crack
Authors: Chen X , Albrecht P , Joyce J
Published: March 2009

This paper presents equations for the limit pressure of a cylindrical vessel with a longitudinal crack. The calculations are based on a log-spiral slip-line and plane-strain conditions. The calculated limit pressure is compared with the pressure measured in the Pressurized-Thermal-Shock Experiment No. 2, PTSE-2, performed at the Oak Ridge National Laboratory. It was found that in the PTSE-2A test the short crack extended in a region of thermal tensile stress at a measured pressure lower than the calculated limit pressure. However, in the PTSE-2B test, the long crack extended in a region of decreasing temperature gradient at a pressure about equal to the calculated limit pressure. The vessel collapsed when the pressure exceeded the limit pressure. In comparison, the crack in compact specimens [C(T)] fabricated from the same steel as the vessel extended at the limit load calculated on the basis of the flow stress. The differences in fracture behavior likely resulted from differences in degree of constraint in the C(T) specimens and the vessel.
A Deep Part-Through All-Around Circumferential Crack in a Cylindrical Vessel Subject to Combined Thermal and Pressure Load
Authors: Chen L , Paris PC , Tada H
Published: March 2009

The primary purpose of this work is to develop analytical solutions that could be used to assess the safety of irradiated nuclear reactor pressure vessels against unstable fracture during thermal shock events with repressurization present. This methodology makes use of a constraint straight beam model under combined tension and bending for simplicity. It will be shown that this model can represent the deep part-through circumferential crack in a cylindrical vessel under pressure and thermal load. The thermal load can be treated as an induced equivalent bending moment. Elastic and fully plastic solutions are presented for such deep cracks at the inner surface of the wall. The results show that such a crack would be stable under pressurized thermal shock load.
Study of a Crack-Tip Region Under Small-Scale Yielding Conditions
Authors: Sciammarella CA , Albertazzi A , Mourikes J
Published: March 2009

The computer-assisted moire technique was used to measure displacements and strains in the neighborhood of a crack tip. An aluminum 6061-T6 compact tension specimen was utilized to perform the measurements. The u (parallel to the crack direction) and v displacement fields (perpendicular to the crack direction) were determined, as well as the corresponding strains. Tension and fracture tests were performed to obtain the properties of the material. The stress-strain curve was fitted with a Ramberg-Osgood type of constitutive law. The stresses were computed from the strains using a two-dimensional generalization of the Ramberg-Osgood constitutive equation. The J-integral was computed along several paths. Good agreement was found between the KI computed from the J-integral with the KI obtained from the application of the ASTM Test Method for Plane-Strain Fracture Toughness of Metallic Materials (E 399-83) standard polynomial. The HRR solution was compared to the experimental results. Some important conclusions can be obtained: 1. In small-scale yielding, the HRR field can only be observed very close to the crack tip. 2. The region dominated by the HRR field is only a few crystalline grains in size. 3. The HRR field models the radial stresses very poorly but gives a good estimate of the tangential stresses. 4. The experimental results support the view point that in small-scale yielding, the J-dominance is independent of the validity of the HRR solution.
Analysis of Circumferential Cracks in Circular Cylinders Using the Weight-Function Method
Authors: Mettu SR , Forman RG
Published: March 2009

Circumferential cracks in hollow circular cylinders are often used as idealizations of flaws in weldments caused by lack of penetration. The most common cases occur for butt welds in pipes, nozzles, and cylindrical pressure vessels. The objective of the present work is to develop a methodology to compute accurately values of stress intensity factor for the entire range of radius ratio, Ri/Ro (inner to outer), from 0 to 0.9999 and crack-depth-to-thickness ratio, a/t, from 0 to 1.0 for general loading using the weight-function approach. The p-version of the finite-element method was used to obtain stress intensity factors and crack face displacements for the reference loading of uniform tension. The reference solution was obtained for selected values of the geometrical parameters, Ri/Ro and a/t, covering their whole range. Both internal and external cracks were treated. Piecewise cubic Hermite interpolation techniques were then used to compute the quantities corresponding to intermediate values of the geometrical parameters. The derivatives of crack face displacements needed in the weight function method were obtained numerically from a three-term Williams series fit to the displacements obtained from the finite-element analysis. The results obtained were compared to existing solutions for uniform tension loading and excellent agreement was found. Results for uniform tension loading obtained from the weight function method were compared with those from finite-element analysis and error bounds were established.
An Evaluation of Fracture Mechanics Properties of Various Aerospace Materials
Authors: Henkener JA , Lawrence VB , Forman RG
Published: March 2009

This paper presents results from a series of fracture mechanics tests that used a variety of aerospace materials, including 6063 aluminum in the T5 and T6 tempers, 2014-T651 aluminum welded with a 4043 filler material, Inconel 718, A286 steel, superalloy MP35N, beryllium SR-200E, magnesium AZ-31B-H24 and ZK-60A-T5, beryllium-copper CDA172, and aluminum-bronze CDA630. The fatigue crack growth data for these materials were curve fit using the crack propagation and other related equations that will be incorporated into an updated version of the NASA/FLAGRO computer program. This program will help achieve more reliable fracture mechanics assessments of space systems hardware.
Leak-Before-Break and Fatigue Crack Growth Analysis of All-Steel On-Board Natural Gas Cylinders
Authors: Bhuyan GS
Published: March 2009

Fracture-mechanics methods have been used to provide a basis for assessing the structural integrity of the all-steel (AISI 4130 grade, 31 HRC) compressed natural gas cylinders used as storage fuel tanks on vehicles. A leak-before-break (LBB) failure mode of the cylinders in the event of fatigue crack growth has been established from the cylinder tests as well as from the analysis. Results indicated a safety factor of 1.3 on the materials' fracture toughness for the LBB performance in the worst operating condition. Length of the leaks varied from 2.6 to 3.5 times the cylinder wall thickness for the single-crack-initiated fatigue cracks. Average growth for internal cracks, pressurized with water, was found to be 1.5 times faster than that of external cracks. Flaws shallower than the 30% of wall thickness were not critical for the subsequent five-year service period.
Intergranular Delamination and the Role of Artificial Aging Conditions on the Fracture of an Unrecrystallized Aluminum-Lithium-Zirconium (Al-Li-Zr) Alloy
Authors: McKeighan PC , Hillberry BM , Sanders TH
Published: March 2009

The influence of aging and grain morphology on the fracture behavior of an Al-2.6Li-0.09Zr alloy was investigated by varying the extrusion parameters that affect grain structure; namely, extrusion ratio and product geometry. Fracture specimens were partially fractured and serially sectioned to characterize crack progression into the plastic zone and to assess the interaction between the crack front and the grain structure. Fracture in the higher toughness, underaged condition is characterized by crack tunneling and grain boundary microcracking in a zone surrounding the continuous crack surface. The fracture process for the overaged material is distinguished by large, intergranular delamination cracks perpendicular to the main fracture surface and extending deep into the plastic zone. This delamination cracking phenomenon was investigated using linear, two-dimensional finite-element simulations of the delaminations at the crack tip. The results indicate that when a delamination crack is longer than one half of the section thickness, the stress intensity factor (KI) for the delamination crack is independent of crack length. Furthermore, a section with multiple delamination cracks has a higher load carrying capacity than a section with only one crack, assuming that failure is KI dependent. This delamination formation reduces through-thickness constraint and hastens development of plane-stress conditions. The effect of this delamination cracking on the toughness of aluminum-lithium (Al-Li-X) alloys is in accordance with the functional form of the Bilby, Cottrell, and Swinden model developed for plane-stress fracture and defined by Kc ∞ (B1Eϕf) where B1 is delamination spacing, E is Young's modulus, and ϕf is the flow stress.
Fatigue Crack Growth Behavior of Titanium Aluminide Ti-25Al-25Nb
Authors: Balsone SJ , Maxwell DC , Broderick TF
Published: March 2009

The fatigue crack growth rate of an orthorhombic + beta titanium aluminide, nominally Ti-25Al-25Nb, was studied as a function of temperature (25 to 750°C), environment (air and vacuum), frequency (0.001 to 1.0 Hz), and superimposed hold times (1 to 1000 s) under computer-controlled constant Kmax testing conditions. In addition, fatigue crack growth rates from the near-threshold region to rates greater than approximately 10-7 m/cycle were determined at room and elevated temperatures. Results show that the fatigue crack growth rate exhibits a combination of cycle- and time-dependent behavior and is sensitive to environment over the entire temperature range. At elevated temperature, crack growth per cycle is found to increase with decreasing frequency in both laboratory air and vacuum, suggesting a contribution from environmentally assisted crack growth. Growth rates in vacuum are as much as an order of magnitude lower than those obtained in air. Further, hold times of increasing duration are found to slightly decrease and then increase the crack growth rate at elevated temperature. At elevated temperatures, crack growth behavior appears to be a complex interaction of environmental degradation at the crack tip, crack-tip blunting due to creep, and cyclic fatigue (resharpening of the crack tip). An attempt was made to correlate the observed fatigue crack growth rates with the mechanism, or mechanisms, of fracture. The crack growth characteristics were compared with those of the alpha-2 titanium aluminide, Ti-24Al-11 Nb, and a conventional high-temperature titanium alloy, Ti-1100.
Fatigue Crack Growth Rate Measurements in Aluminum Alloy Forgings: Effects of Residual Stress and Grain Flow
Authors: Bush RW , Bucci RJ , Magnusen PE , Kuhlman GW
Published: March 2009

An investigation to determine the causes of increased variability in fatigue crack growth (FCG) rate measurement in aerospace aluminum forgings compared to that observed in aluminum flat rolled plate was conducted. Fatigue cracks were grown at constant applied ΔK, consistent with ASTM Test Method for Measurements of Fatigue Crack Growth Rates (E 647-88a), through regions of rapidly changing grain-flow direction within thin 7075-T73 precision forgings and large closed-die 7050-T74 forgings. The local grain flow in each specimen was correlated subsequently with measured crack growth rates so that the contribution of grain-flow variability to the scatter in measured crack growth rates could be determined. These results were found to be comparable to FCG results from 7075-T7351 and 7050-T7451 plate, once residual stress biases were removed by means of closure measurement. Hence, it is concluded that the effects of microstructural features such as grain flow on FCG rate are minimal, and the inherent FCG resistances of aerospace aluminum forgings and plate are comparable. Consequently, the analytical fracture mechanics framework applicable to damage tolerance verification of plate product should extend equally to forging product forms. However, the results indicate that failure to account for residual stress effects in interpretation of forging test results can lead to erroneous conclusions about the material performance.
Development of a Fatigue Crack Growth Rate Specimen Suitable for a Multiple Specimen Test Configeration
Authors: Deshayes FR , Hartt WH
Published: March 2009

A new experimental approach developed by fatigue crack growth rates can be determined for multiple specimens tested in series. This involves a geometry such that the response to cyclic loading in each specimen is mutually independent. Relationships between the stress intensity factor, compliance, and crack length and between direct-current potential drop and crack length have been analytically evaluated for the proposed specimen geometry. An appropriate side-groove depth that avoided crack deviation from the intended plane of extension was determined experimentally, and an analytical expression that accounted for reduced thickness in the crack plane was verified. Calibration curves between crack length and compliance and between crack length and potential drop were experimentally verified as well, and the stress intensity-crack length characteristics for the specimen were confirmed by comparing da/dN-ΔKI curves with those for conventional C(T) specimens. A customized, five specimen frame was designed and fabricated in conjunction with a 98-kN MTS actuator and interfaced with a commercially available control system, the hardware and software of which were modified to accommodate multiple-specimen control and data acquisition. The utility of the proposed specimen and test procedure is discussed within the context of threshold and near-threshold crack growth rate determinations where long test times are normally required for data development, particularly for low cyclic frequency applications such as nuclear and offshore.
Ultrasonic Characterization of Fatigue Crack Closure
Authors: Thompson RB , Buck O , Rehbein DK
Published: March 2009

The characterization of fatigue crack closure is an important objective because of its influence on fatigue crack propagation, particularly under conditions of variable amplitude loading. This paper describes a nontraditional technique for characterizing closure, in which ultrasonic scattering measurements are used to obtain estimates of the number density and size of asperities bridging the crack faces, with subsequent estimates of the crack-tip shielding being based on those geometrical parameters. The paper first reviews the experimental configuration and the basic elasto-dynamic theory underlying the technique. It then presents recent results obtained in studies of the influence of block overloads and load shedding on the growth of fatigue cracks in aluminum alloys. In both cases, the change in the closure state after the overload can be seen unambiguously even in the raw data. Moreover, data analysis suggests that it may be possible to predict when the crack will reinitiate based on more subtle changes in the ultrasonically inferred closure state. In the case of load shedding, a massive closure region is observed, whose characteristics appear consistent with the notion that threshold phenomena can be explained in terms of crack closure.
Translaminate Fracture of Notched Graphite/Epoxy Laminates
Authors: Harris CE , Morris DH
Published: March 2009

The strength of three notched graphite/epoxy laminates has been shown experimentally to be a function of laminate thickness. The notched strength of the [0/±45/90]ns and [0/90]ns laminates decreased toward asymptotic values with increasing laminate thickness. Conversely, the notched strength of the [0/±45]ns laminate increased toward an asymptotic value with increasing laminate thickness. For all three laminate types, the notched strength decreased with increasing notch size, regardless of thickness. The fracture of the thick laminates were essentially uniform and self-similar with the notch. The “universal” value of the general toughness parameter, Qc/εtuf = 1.5√mm (0.30√in.), developed by Poe successfully predicted the notched strength of the thick laminates. The strength of specimens with surface cuts was predicted using a linear elastic, homogeneous, isotropic fracture mechanics solution.
Static Fatigue in Dilatant-Zone-Toughened Ceramics
Authors: Duan K , Cotterell B , Mai Y-W
Published: March 2009

An approximate model is presented for the effect of crack-tip shielding due to dilatant zone toughening on subcritical crack growth in ceramics. This model relies on estimating the transformed zone from the applied stress intensity factor. Slow crack growth can be predicted with this model in ceramics toughened by the stress-triggered martensitic transformation in zirconia-bearing ceramics, if the slow' crack growth data for the same ceramic with the transformation either already completed by thermal aging or suppressed by stabilizers is known.
Fracture Energy Dissipation Mechanism of Concrete
Authors: Guo Z , Yon J-H , Hawkins NM , Kobayashi AS
Published: March 2009

Moiré interferometry was used to determine the crack opening displacements (COD) along a stably or dynamically propagating crack in three-point-bend concrete specimens. For stable crack growth studies, the measured COD together with applied loading was input to a finite element model of the specimen to determine directly the crack closure stress (CCS) versus COD relationship along the fracture process zone (FPZ) that trailed the crack tip. For dynamic crack growth studies, the COD data together with three strain gage data taken along the crack path were used in an inverse analysis to extract the CCS versus COD relationships using a dynamic finite element model of the fracturing specimen. Variations in energy release and dissipation rates with crack extension were determined and the FPZ was identified as the major energy sink in concrete fracture.
Rapid Crack Propagation in Polyethylene Pipes: The Role of Charpy and Dynamic Fracture Testing
Authors: Leevers PS , Yayla P , Wheel MA
Published: March 2009

Pressurized pipelines of extruded ductile polyethylenes can, under severe conditions, fail by brittle rapid crack propagation (RCP). Critical pressures, below which RCP always arrests, have been measured for grades of medium-density (MDPE) and modified high-density polyethylene (HDPE) pipe, using a small-scale test. For 180-to 250-mm-diameter pipe of 11-to 24-mm wall thickness, HDPE performs much better than MDPE; but both materials show temperature, crack velocity, and thickness-promoted ductile-brittle transitions, beyond which their behaviors converge. Instrumented Charpy impact tests reflect the transitions, but only qualitatively. High-speed double-torsion tests for dynamic crack resistance across a spectrum of crack velocities yield only plane-strain data, even at 0°C and for specimens only 6 mm thick. They therefore do not help to locate the thickness-dependent fracture transition, but more efficiently characterize the worst case of a highly constrained crack in a thick pipe.
Intrinsic Confined and Unconfined Load-Deformation Properties of Geotextiles
Authors: Ballegeer JP , Wu JTH
Published: March 2009

This paper presents intrinsic load-deformation properties of different geotextiles under confined and unconfined conditions. The confined load-deformation properties were determined by a test method (the intrinsic confined test) proposed by Wu (1991) for design and specification of geotextile-reinforced soil structures. The intrinsic confined test has three distinct characteristics: (1) it is an “element” test, thus the load-deformation properties determined from the test are the intrinsic properties of the geotextile; (2) the test measures the confined stiffness and strength of geotextiles without inducing soil-geotextile interface adhesion, thereby simulates the predominant operational condition in geotextile-reinforced soil structures; and (3) the stiffness and strength obtained from the test are conservative values if soil-geotextile interface slippage does occur. A detailed procedure for the intrinsic test method is described. The described method uses only a thin rubber membrane, without soil, for confinement.Four nonwoven geotextiles and one woven geotextile were tested.Discussions of the test results are presented.
Index and Performance Tests for Geocells in Different Applications
Authors: Cancelli A , Rimoldi P , Montanelli F
Published: March 2009

The use of geocells in base reinforcement of structural fill allows confinement of the soil from lateral spreading, therefore preventing shear failure and increasing the soil bearing capacity.Geocell are typically loaded by the live loads on the surface and the overall structure is tensioned while the underlying foundation soil settles. Geocells are regularly used for soil confinement on steep slopes to prevent severe erosion. The geocells are sometimes installed even over slippery geomembranes to contain and reinforce the cover soil.The geocells in these applications should withstand high tensile stresses both in the strands and the junctions without breaking and sliding down the slope. A need for more accurate and specific geocells testing is needed by the design engineers when designing such a critical applications. The testing procedures appropriate for geocells will be investigated, performed and analyzed in this paper. The description of the above tests and some preliminary results are presented and discussed. A final proposal for test standards concerning geocells will be presented for consideration by the existing geosynthetics testing committees.
Poll-out Testing Of Geogrids in Cohesive Soils
Authors: Farrag KA , Griffin P
Published: March 2009

The growing interest in utilizing on-site cohesive soils in reinforced-soil structures raises the need for development of testing procedures to evaluate their interaction properties (i.e. pull-out resistance and shear stress-strain characteristics). In determining the pull-out resistance of geosynthetics in clay, several factors can influence the measured properties. These factors are generally related to the testing equipment, the associated boundary effects, testing procedure, pull-out rate, geosynthetics characteristics, soil properties (e.g. soil density, grain size distribution and moisture content), compaction procedure and confining pressure.

This paper presents a part of the pull-out testing program conducted in the Geosynthetic Engineering Research Laboratory at the Louisiana Transportation Research Center to evaluate the effect of related physical parameters on the pull-out resistance of geogrids. Tests are conducted on HDPE geogrids in two pull-out boxes. The large pull-out box has dimensions of 1.5 m (60 in.) length, 0.9 m (36 in.) width, and 0.76 m (30 in.) height. The small box is 1.22 m (48 in.) long, 0.6 m (24 in.) wide, and 0.45 m (18 in.) high. A compaction procedure is developed in order to control soil density and moisture content. An instrumentation array is implemented to monitor the pull-out load, pull-out rate, normal pressure, and the displacement distribution along the geogrid specimens.

The results of the pull-out tests in both boxes provide an evaluation of the effect of various parameters on the pull-out mechanism and suggest a standardized procedure for control of box boundary effects and other influencing parameters on the pull-out resistance of geogrids.
The Influence of Test Parameters and Procedures on the Tensile Modulus of Stiff Geogrids
Authors: Austin DN , Wu KJ , White DF
Published: March 2009

Geogrid tensile strength at low strains is difficult to determine due to the non-linear, viscoelastic behavior of thermoplastic polymers. Their response to applied tensile loads is profoundly different from that of elastic materials including elastic structural metals. The stress-strain properties of plastics are not constant, varying greatly and nonlinearly with the test parameters. Therefore, if testing conditions are not controlled and accounted for, they can be inaccurate predictors of tensile performance. The measurement of tensile modulus is affected by such factors as temperature, sample length, gauge length, extension rate, and tension and strain measuring equipment.

It is not easy to define the effect of the numerous variables involved with the tensile testing of thermoplastic materials. However, the general effect of a few of the more important and controversial variables was investigated in order to provide some appreciation of the sources of variability. This paper presents the results and observations from an inter-laboratory study that determined tensile modulus of stiff polypropylene geogrids. Various test parameters and procedures affecting the measured results and their potential relevance to field performance were studied. Conclusions and guidelines are provided, and recommendations are made for modifications to current test methods when tensile modulus at low strains is of particular interest.
High Strength Polyester Geotextile Testing and Material Property Evaluation
Authors: Paulson JN
Published: March 2009

High strength woven polyester geotextiles were tested for manufacturing quality control purposes, and for project approvals.This testing included both single end breaks, and full wide width strength testing. Wide strip tensile strengths were determined using ASTM D4595, “Tensile Properties of Geotextiles by the Wide Width Strip Method”.

Results show consistency in single end breaks, with significant variability when full wide strip tensile specimens were tested. Results were variable both in ultimate strength determinations and at 5 and 10% Several laboratories also tested this material, with their results presented. A summary of the results is discussed followed by recommendations for modification to ASTM D4595 to limit results variability.
Material Parameters Used in Design of Oeosynthetic Reinforced Soil Structures
Authors: Berg RR , Collin JG
Published: March 2009

Limit equilibrium design of reinforced soil structures requires input of several material parameters. Properties required for design and status of standardized test methods and procedures by ASTM for definition of these properties are summarized. Results of a limited parametric study investigating effect of small variances (i.e., 10%) in material property values on design of reinforced soil structures are presented. Small variations in both soil friction angle and direct shear coefficient between soil and geosynthetic reinforcement have an economically significant impact on the design of reinforced walls and slopes, for the cases examined.
Geosynthetic Installation Damage Under Two Different Backfill Conditions
Authors: Koerner GR , Koerner RM , Elias V
Published: March 2009

Six different geotextiles and one geogrid were placed on an angular, poorly graded gravel, backfilled with the same type of gravel,compacted in a standardized manner, and then exhumed within the same day. A visual damage assessment indicated that the exhumed geotextiles had 60 to 108 holes per square meter and the geogrids had approximately 4 damaged ribs per square meter. A large number of strength tests were performed on the exhumed samples and compared to the original (as-received) strengths to obtain an average percent retained strength. The results for the geotextiles were from 30% to 58% strength retained while the geogrid resulted in a 71% strength retained.

At a second site the entire study was repeated with new materials but now the soil was a poorly graded sand both beneath and above the geosynthetics. The same types of geotextiles and geogrids as with the first site were used in these tests. Here the results were very different, with no holes or damaged ribs, and average geotextile strengths retained from 72% to 96%. The geogrid had its full strength retained.

The data results in quantifiable values for installation damage partial factors-of-safety (the inverse of the strength retained), as well as guidance on the type of backfill soil to use with polymeric reinforcement. Clearly, angular gravel can be damaging to both geotextiles and geogrids and should only be used with considerable caution or with the addition of protection materials.
A Review of the Degradation of Geosynthetic Reinforcing Materials and Various Polymer Stabilization Methods
Authors: Hsuan YG , Koerner RM , Lord AE
Published: March 2009

The paper describes the two major degradation mechanisms of geosynthetics: oxidation of polyolefins (polyethylene and polypropylene), and hydrolysis of polyester (polyethylene terephthalate). These are the polymers which comprise the vast majority of geotextiles and geogrids used for soil reinforcement. Details of the degradation mechanisms of each material and their consequences are described based on the presently available literature. The influence of material structure, in particular the orientation, and ambient environmental conditions on the degradation are also discussed. The mechanisms and methods of stabilization which are currently used to minimize degradation are also presented. Finally, the paper concentrates on various possible test methods which can be used for monitoring the degradation of geosynthetic materials.
Creep Buckling of Asymmetric Laminated Beam Columns
Authors: Vinogradov AM
Published: March 2009

This paper examines the creep buckling response of asymmetric composite beam columns under general loading conditions. The material properties of the laminates are defined by the constitutive equations of the linear viscoelastic theory. The problem is formulated in terms of convolution integral operators and is solved by means of the quasi-elastic method. As a result of the analysis, a safe load limit for composite beam columns made of limited creep viscoelastic materials is determined. The creep buckling characteristics of composite beam columns are compared with the time-dependent behavior of respective homogeneous structures. It is observed that, by tailoring the composition and properties of asymmetric composite laminates, their long-term performance can be improved considerably.
Design, Analysis, and Hydrotesting of a Composite-Aluminum Cylinder Joint for Pressure-Hull Applications
Authors: Andersen SM , Gillespie JW , Lamontia MA , Olson BD , Sloan JG , Newan KE , Kodokian GKA
Published: March 2009

The design, analysis, and hydrotesting of a composite-aluminum cylinder joint subjected to external hydrostatic pressure is presented. This study addresses the feasibility of utilizing an adhesive bond to join composite and metallic pressure-hull components. A preliminary design methodology is outlined and applied to the joint. The design provides for axial load transfer through shear in the adhesive and bearing in the butt end of the cylinder. A thickened area of aluminum in the vicinity of the joint serves as a king frame to enhance stability of the structure. Detailed finite element analysis is performed to provide accurate failure predictions. Dimensional details of the joint are designed to obtain the desired ratio of shear/bearing load transfer and to minimize bending of the shell over the joint to delay interlaminar shear failure. An in situ thermoplastic filament-wound cylinder is manufactured and adhesively bonded to an aluminum cylinder. The instrumented structure is then tested to failure through application of external hydrostatic pressure. Experimental results including collapse pressure, failure mode, and strain field are compared with predictions to verify finite element model accuracy and validate the design methodology. The results of this study demonstrate both accurate predictive capability and use of an effective design methodology for composite pressure-hull joints.
The Effects of Constituent Properties on the Compression Strength of Advanced Composites
Authors: Crasto AS , Kim RY
Published: March 2009

A miniature sandwich specimen was employed to determine the influence of matrix modulus and fiber-matrix interfacial bond strength on composite compression strength. This specimen generates significantly higher compression strengths for unidirectional carbon/ epoxy composites compared to conventional coupons and also exhibits a smaller scatter of the data. The matrix modulus of AS4/3501-6 and AS4/APC-2 composites was varied by testing specimens over a range of temperatures. In both composites, compression strength increases with matrix shear modulus up to a critical value of the shear modulus, above which it remains constant. At low matrix shear moduli, failure initiates via fiber microbuckling, while at high shear moduli, failure is dominated by the fiber compression strength. The influence of the fiber-matrix bond strength was investigated with epoxy composites reinforced with two different carbon fibers having treated and untreated surfaces. Composite compression strength varied directly with the 90° tensile strength, which was adopted as a measure of relative interfacial bond strength.
Experiments to Reveal the Role of Matrix Properties and Composite Microstructure in Longitudinal Compression Strength
Authors: Yurgartis SW , Sternstein SS
Published: March 2009

Current micromechanical models of composite longitudinal compression strength are contradictory about the role of matrix strength and matrix modulus. A resolution among these models has been difficult because past experiments had difficulty controlling other potentially critical variables, such as fiber misalignment, fiber distribution, and so on, while changing the intended experimental variables. These difficulties were overcome in the present work by measuring thermoplastic composite compression strength as a function of temperature. Matrix properties are therefore changed while microstructural variables stay constant. Rate of change of matrix modulus and matrix strength with temperature were different, thereby making a resolution between the influence of these properties possible. Polycarbonate matrix composites with XAS and AS-4, sized and unsized, carbon fibers were tested. Both bending and direct compression (IITRI) test geometries were employed. Failure was by microbuckling. Composite longitudinal compression strength was found to depend proportionally on the matrix shear modulus. Matrix strength did not directly correlate to compression strength. The range of fiber/matrix interfacial strengths tested also did not influence the compression strength, suggesting that above an interfacial strength threshold this is not a critical variable in composite compression strength. Finally, using a recently developed technique for measuring small-angle fiber misalignment distributions, the data suggested that small random fiber misalignments on the order of 1 to 2 degrees do have a significant affect on compression strength.
The Role of Fiber, Matrix, and Interphase in the Compressive Static and Fatigue Behavior of Polymeric Matrix Composite Laminates
Authors: Swain RE , Elmore JS , Lesko JJ , Reifsnider KL
Published: March 2009

In the continuing effort to understand the processes involved during the compressive failure of statically and dynamically loaded composite structures, this work is concerned with the role the fiber, matrix, and interphase play in determining a composite's compressive performance. Fourteen material systems representing permutations of four carbon fiber systems, three matrix systems, percentages of fiber surface treatment, and three sizing conditions have been laminated and cut into coupons. The bond strength arising from the particular combination of fiber, surface treatment, sizing, and matrix is quantified by measuring the transverse flexural strength of each material system.

Center-holed, cross-plied specimens of each of the 14 materials were tested in quasi-static compression to determine their structural strength. An attempt was then made to correlate differences in compressive strength to differences in the properties of the three phases. A fixed percentage of each strength value was then utilized to establish the tension-compression fatigue stress level employed for each of the 14 material systems. Damage analysis revealed the mechanisms responsible for the wide variation in fatigue life witnessed. The interphase is seen to play a second-order role in determining the compressive strength of these particular notched cross-plied laminates. On the other hand, the interphase is shown to be a key variable in dictating these laminates' compression-dominated fatigue behavior.
A Global and Local Investigation of Compressive Strength to Determine the Influence of the Fiber/Matrix Interphase
Authors: Lesko JJ , Elmore JS , Case SW , Swain RE , Reifsnider KL , Dillard DA
Published: March 2009

The investigation presented examines the compressive strengths, Xc, of thermoplastic-toughened epoxy composites with fiber surface chemistry altered by various levels of surface treatments and two distinctly different sizings. The thermoplastic, polyvinylpyrrolidone (PVP) size improved the IITRI Xc by 51% over an unreacted bisphenol-A epoxy size for a 100% surface-treated fiber. It is important to point out that effects were produced with the sizings making up less than 1% (by weight) of the composite. Thus, appropriate selection of sizings may present an effective and economical means for tailoring composite material performance. It is suggested that properties other than interfacial shear strength may be important in the description of composite compression strength, e.g., interphase modulus and fracture resistance.
An Extrapolation Method for the Evaluation of Compression Strength of Laminated Composites
Authors: Tan SC , Knight M
Published: March 2009

An extrapolation method is presented for the evaluation of compression strength of unidirectional composites. We recommend that the thickness of the laminate tested range from 12 to 16 plies and that the gage length equal 5.08 mm. Three or more different tapered angles should be used for the end tabs. Each specimen geometry must have at least three replicates. Strain gages (or other instrumentations) must be placed on both sides of a specimen to ascertain that the strength value obtained is not due to premature failure. The strengths are correlated with the end tab tapered angles by a second-order polynomial function. The compressive strength can be extrapolated from this curve by setting the tapered angle of the end tabs equal to zero. At this state, the stress concentration vanishes theoretically. Our limited experimental results show that the compressive strengths of the Gr/Ep AS4/3501-6 and S-Gl/934 unidirectional laminates are -1885 and -1665 MPa, respectively. These values are significantly higher than those obtained using the Celanese or the classical IITRI specimens. Fiber failure was the major failure mode obtained from the specimens proposed here.