Specimen Size Requirements Research Articles

Heat Flow in Fractured Rocks: Stress and Moisture-Dependent Thermal Contact Resistance

The thermal conductivity of fractured rock masses is an important parameter for the analysis of energy geosystems, yet, its measurement is challenged by specimen size requirements. Fluids within fractures have lower thermal conductivities than rock minerals and heat flow lines constrict through contacting asperities. Together, heat flow constriction and phonon boundary scattering cause an apparent temperature discontinuity across the fracture, typically represented as a thermal contact resistance. We investigate the thermal contact resistance in fractured limestone and its evolution during loading and unloading (σ’=10 kPa to σ’=3000 kPa) for clean and gouge-filled fractures, under both air-dry and water-saturated conditions. The fracture thermal contact resistance decreases during loading because of the increase in the true contact area, gouge and asperity crushing, and fracture filling by produced fines that contribute new conduction pathways. These processes convey high stress sensitivity and loading hysteresis to the fracture thermal contact resistance. Water fills the fracture interstices and forms menisci at mineral contacts that significantly improve heat conduction even in partially saturated rock masses. The rock mass effective thermal conductivity can be estimated by combining the intact rock thermal conductivity with measurements of the thermal contact resistance of a single fracture under field boundary conditions.

Evaluation of Ballistic Limit Velocity Using Instrumented Indentation Test of 7xxx Aluminum Alloys After Friction Stir Welding

Friction stir welding has the advantages of producing less material deformation and of simple joining of relatively thick materials, but the disadvantages of forming relatively large heat-affected zones around weld areas. In addition, in research on reductions in bulletproof performance in heat-affected zones, it is necessary to evaluate the ballistic limit velocity (V50), which measures bulletproof performance, and this requires specimens of a certain minimum size and has only limited application to local areas such as heat-affected zones. Instrumented indentation testing (IIT), a method of measuring material properties by utilizing load-depth curves measured by a small indenter leaving fine marks on the material, has no specimen size requirements; in addition, it has simple test procedures and is nondestructive. Here a theoretical model is proposed for evaluating V50 of aluminum alloys by ductile hole formation using IIT. Heat-affected zones generated after friction stir welding of for 7000-series aluminum alloys were simulated, and the model was validated through comparison with the conventional V50 test. In addition, there is currently no way to directly evaluate V50 for welded areas in 7000-series aluminum, but here IIT was used to assess V50 for each local part of the welded specimen. The conventional V50 test takes 15 min per shot and cannot be reused of specimens, but the test using IIT takes 1 min to complete one test and has benefits in terms of time, cost and safety.

A new method to estimate the plane strain fracture toughness of materials

AbstractAlthough the testing method for fracture toughness KIC has been implemented for decades, the strict specimen size requirements make it difficult to get the accurate KIC for the high‐toughness materials. In this study, different specimen sizes of high‐strength steels were adopted in fracture toughness testing. Through the observations on the fracture surfaces of the KIC specimen, it is shown that the fracture energy can be divided into 2 distinct parts: (1) the energy for flat fracture and (2) the energy for shear fracture. According to the energy criterion, the KIC values can be acquired by small‐size specimens through derivation. The results reveal that the estimated toughness value is consistent with the experimental data. The new method would be widely applied to predict the fracture toughness of metallic materials with small‐size specimens.

An X-Ray CT Validated Laboratory Measurement Method for Air Voids Distribution Over Depth in Asphalt Pavement: A Step Toward Simplified Oxidation Modeling

Recent efforts relating pavement failure to binder hardening have resulted in a pavement oxidation model. A key parameter, the diffusion depth, the ratio of binder volume to accessible air void surface area, is determined via X-ray computed tomography (CT). Toward an alternative determination, in this work volumetric techniques were employed to measure total air voids of core slices over the pavement depth. Because the 0.4-inch-thick slices are insufficient to meet standard method specimen size requirements, precision was evaluated and alternative maximum theoretical specific gravity methods were compared. A vacuum dryer increases efficiency without oxidizing the binder. Air void distributions compare favorably with X-ray CT measurements.

Application of the Viscoelastic Continuum Damage Mechanics to Asphalt Mixtures under Indirect Tensile Load

Traditional studies that applied the viscoelastic continuum damage (VECD) model to asphalt materials were mostly accompanied by uniaxial tests whose specimen size requirement provided challenges for mixtures cored from thin asphalt layers. In this paper, an analysis methodology is presented for determining the pseudostrains and damage characteristic curves of asphalt mixtures from both the horizontal (for tension) and vertical (for compression) axes of the indirect tension (IDT) specimen simultaneously. To validate this methodology, laboratory tests were conducted on asphalt specimens in the IDT mode. Complex modulus tests were conducted at various temperatures and frequencies to allow for the construction of a dynamic modulus master curve, which, in turn, was used to obtain the relaxation modulus. In addition, constant-crosshead strength tests were conducted at two temperatures (10 and 20°C) to evaluate the behavior of the asphalt specimens under growing damage. The results of the laboratory study revealed that the damage characteristic curve obtained for compression was located above its counterpart for tension. It also was found that the material showed more favorable damage characteristics at 10°C than at 20°C for both tension and compression and that the damage characteristic curves obeyed the time-temperature superposition principle, which is consistent with the uniaxial test results. These results indicate that the VECD model can be applied to the biaxial IDT test for characterizing tensile and compressive damage characteristic curves simultaneously.

ISRM-Suggested Method for Determining the Mode I Static Fracture Toughness Using Semi-Circular Bend Specimen

The International Society for Rock Mechanics has so far developed two standard methods for the determination of static fracture toughness of rock. They used three different core-based specimens and tests were to be performed on a typical laboratory compression or tension load frame. Another method to determine the mode I fracture toughness of rock using semi-circular bend specimen is herein presented. The specimen is semi-circular in shape and made from typical cores taken from the rock with any relative material directions noted. The specimens are tested in three-point bending using a laboratory compression test instrument. The failure load along with its dimensions is used to determine the fracture toughness. Most sedimentary rocks which are layered in structure may exhibit fracture properties that depend on the orientation and therefore measurements in more than one material direction may be necessary. The fracture toughness measurements are expected to yield a size-independent material property if certain minimum specimen size requirements are satisfied.

Specimen size effects on the compressive strength and Weibull modulus of nuclear graphite of different coke particle size: IG-110 and NBG-18

The effects of specimen size on the compressive strength and Weibull modulus were investigated for nuclear graphite of different coke particle sizes: IG-110 and NBG-18 (average coke particle size for IG-110: 25μm, NBG-18: 300μm). Two types of cylindrical specimens, i.e., where the diameter to length ratio was 1:2 (ASTM C 695-91 type specimen, 1:2 specimen) or 1:1 (1:1 specimen), were prepared for six diameters (3, 4, 5, 10, 15, and 20mm) and tested at room temperature (compressive strain rate: 2.08×10−4s−1). Anisotropy was considered during specimen preparation for NBG-18. The results showed that the effects of specimen size appeared negligible for the compressive strength, but grade-dependent for the Weibull modulus. In view of specimen miniaturization, deviations from the ASTM C 695-91 specimen size requirements require an investigation into the effects of size for the grade of graphite of interest, and the specimen size effects should be considered for Weibull modulus determination.

Analysis of Representative Volume Element for Asphalt Concrete Laboratory Shear Testing

The primary purposes of this paper are to develop a method of quantifying the precision and bias in repeated simple shear test at constant height (RSST-CH) laboratory test results for different-sized specimens and to determine the effects of this precision and bias on predicted rutting performance. The effect of RSST-CH variability was quantified by using a statistical sampling method called bootstrapping. The contribution of test variability to variability in predicted in situ rutting performance was determined by performing Monte Carlo simulations that used a shear-based incremental-recursive rutting analysis model. Results indicated that significant bias exists between the predicted rut depths of different specimen sizes. Increasing the specimen size decreased the test variability. Specimen size requirements for two different mix types are proposed on the basis of the analysis. The effect of test temperature on test results variability was also determined. In addition, analyzing various rutting perform...

Quantification of ductile crack growth in 18G2A steel at different constraint levels

A constraint theory in fracture mechanics is used to analyze the test data of 18G2A steels using single edge-notched bend (SENB) specimens with various crack depth to specimen width ratios ( a/ W). A bending correction factor is included in the two-parameter ( J– A 2) asymptotic solution to improve the theoretical prediction of the stress field for deep cracks under large-scale yielding condition, where J is the J-integral and A 2 is the constraint parameter, which depends on the in-plane geometry of the cracked body ( a/ W). As a result, the valid region for a traditional J-controlled crack growth is extended, and the ASTM specimen size requirements for fracture toughness testing can be relaxed. In addition, it is shown that the functional dependence of J– R curves on A 2 for 18G2A steels is established with test data; and the predicted J– R curves agree very well with the experimental curves. This ensures the transferability of laboratory test data to an actual structure provided the constraint level ( A 2) of the cracked structure is known or determined. This procedure allows an appropriate J– R curve with the same constraint level to be constructed and used in flaw stability analysis of any cracked body.

SIZE REQUIREMENTS OF COMPACT SANDWICH SPECIMEN FOR TESTING OF BONE

It is well understood that bone quality deteriorates due to aging, disease, etc., and may be affected by factors at different length scales due to its hierarchical microstructure. Fracture toughness is one of the properties that assess bone quality. The compact sandwich (CS) specimen gives a better choice of bone sample size, and therefore suits a wide variety of fracture toughness testing needs and constraints. Reliable and statistically valid overall CS specimen size requirements are established in this paper; these serve as guidelines for choosing the CS specimen size. Finite element analysis (FEA) is used for simulating fracture toughness tests. Experimental fracture toughness tests are carried out to verify the FEA results. The experimental results are verified qualitatively by performing scanning electron microscopy (SEM) on the fractured specimen surfaces.

Evaluation of the ISO J initiation procedure using the EURO fracture toughness data set

The multiple specimen J 0.2/BL initiation fracture toughness test procedure from the ISO standard, ISO 12135:2002, is evaluated using the EURO fracture toughness data set. This standard is also compared with the ASTM standard, ASTM E 1820, multiple specimen J Ic procedure. The EURO round robin data set was generated to evaluate the transition fracture toughness methods for steels. However, many of the tests resulted in ductile fracture behavior giving final J versus ductile crack extension points. This is the information that is measured in a multiple specimen J initiation fracture toughness test. The data set has more than 300 individual points of J versus crack extension with four different specimen sizes. It may be the largest data set of that type produced for one material. Therefore, its use to determine J initiation values can provide an important evaluation of the standard procedures. The results showed that a J 0.2/BL value could be determined from the ISO standard for three of the four specimen sizes, the smallest size did not meet the specimen size requirement on J. The construction line slopes in this method are very steep compared with the ASTM construction line slopes. This resulted in low J initiation values, about a factor of two lower than the one from the ASTM method. Of the various criteria imposed to determine a valid J 0.2/BL value, the one limiting the maximum J value was the most questionable. It had an effect of eliminating small specimen data that was identical to acceptable large specimen data.

A Two-Parameter Method for Determining the Fracture Toughness of Materials from Subsized Specimens

The fracture toughness of brittle materials can be determined easily using the Linear Elastic Fracture Mechanics (LEFM) approach, and the only demand for change is the desire to decrease specimen size requirements. Specimen size effect on the normally measured KQ and PMAX/PQ values was revisited in order to establish the suitability of a relationship between these parameters. A linear dependence was confirmed from experimental data of materials as different as isotactic polypropylene, AISI 4140 steel, a titanium alloy, and an aluminum alloy. The load ratio was interpreted as a second parameter, and thorough this dependence, KQ values measured on subsized specimens were extrapolated to linearity conditions PMAX/PQ = 1, determining a value identified as K1.0. When some simple conditions were met, this K1.0 value matched the toughness of evaluated materials. For the analyzed materials, this toughness parameter could be obtained from subsized specimens with important size reduction compared with that needed for KIC valid value determination. Moreover it permitted a simple characterization of material as difficult to characterize as isotactic polypropylene, which initially motivated this study.

Effects of Temperature and Loading Rate on Fracture Toughness

It is known that fracture toughness value is affected by test temperature, specimen thickness and loading rate. In the present study, specimen size and test temperature are varied widely with the obtained data then being analyzed using rate parameter. Additionally, the fracture toughness values are obtained using round bar-type specimen with a circular notch. This result is compared with the result of the CT specimens, and the advantage of using the round bar-type specimen with a circular notch to modify specimen size requirement is discussed. Sample material used is HT780 high tensile strength steel. The test specimens were 1T, 2T and 4T-CT that are described in ASTM E399. Notched round bar-type specimen with a diameter of 15mm and notch root radius of 0.25mm is also used. The test temperature is varied from a low temperature to room temperature, and loading rate is varied about the 1T-CT specimen and the notched round bar-type specimen between static and 1000mm/sec. The test temperature and the loading rate dependency of the fracture toughness values were arranged by the rate parameter. The fracture toughness value has decreased with the decrease in test temperature and with the increase in specimen thickness and loading rate. The fracture toughness value obtained from the notched round bar-type specimen indicated a value close to 2T-CT specimen. It is shown that valid fracture toughness value can be obtained with a small test specimen by the notched round bar-type specimen. The test temperature and the loading rate dependency of the fracture toughness values can be successfully arranged by the rate parameter that can express both temperature and strain rate dependencies. Feasibility of using round bar-type specimen to obtain valid fracture toughness values with less specimen mass was demonstrated.

Specimen Size Requirements for Two-parameter Fracture Toughness Testing

Using a single parameter fracture mechanics theory, a minimum specimen size requirement of min(a, b, B) >200J/σ0 in tension and min(a, b, B) >25J/σ0 in bending, where B is the thickness, b the remaining ligament and a is the crack length of the specimen, were derived [Shih and German (1981), International Journal of fracture 17, 27–43] which have provided the basis for modern fracture toughness testing procedures. Two-parameter fracture toughness testing including the constraint, on the other hand, is desirable since it offers a solution to the transferability issue. A size requirement for a valid two-parameter fracture toughness testing based on the J-A2 three-term solution was determined as min(a, b, B) > 11J/σ0 [Chao and Zhu (1998), International Journal of fracture 89, 285–307] in which the limiting case is bend specimens under large scale yielding (LSY). Recent work by Chao et al. (2004, International Journal of fracture, 27, 283–302) has shown that the J-A2 dominance at a crack tip can be significantly enhanced for bending specimens under LSY if a modified J-A2 solution is adopted. This current paper further studies the size of the J-A2 dominant zone using the modified J-A2 solution for deep bend specimens with hardening from low to high and loading from SSY to LSY using finite element analysis. Based on the results, a rather relaxed specimen size requirement min(a, b, B) >6J/σ0 is developed and recommended for a valid two-parameter fracture toughness testing using the J-A2 fracture criterion. Validity of the size requirement is demonstrated by using the experimental J-R curves from non-standard bending specimens for A285 steel.

Determination of constraint limits for cleavage initiated toughness data

The proper constraint limits for cleavage initiated toughness data within the ductile-to-brittle transition regime have been studied extensively using both numerical analysis and analysis of experimental data. Historically, the experimentally based constraint limits have supported less conservative limits. This study conducts analysis of existing and new experimental data developed using data sets targeted to exhibit constraint loss toughness enhancement. Constraint herein is quantified in terms of the scaled specimen deformation level, more commonly known as M. It is expected that data with low M values will exhibit the greatest affect of constraint loss. Large data sets are therefore developed and extracted from the literature that include data with a large range of M levels and at least the required number of uncensored results with M > 30 for valid T 0 measurement as per ASTM E1921-02. Differences in the calculated T 0 values using censoring limits of 5–500 are then determined. The onset of T lim differences due to constraint loss is examined by simply increasing the censoring limit, M lim, utilized in determining the indexing temperature, T lim, and evaluating differences between T 0 and the T lim values obtained using higher constraint limits. Bias resulting from this censoring procedure is examined using a Monte-Carlo analysis and shown to be small. Measurement of a high constraint T 0 in bend specimens is shown to require M lim > 200. As M lim increases from 30 to 200 in bend specimens, the corresponding T lim can increase by approximately 15 °C. Further increases in M lim do not result in substantial increases in T lim. This evolution buttresses previous numerical findings by Dodds and co-workers [Specimen size requirements for fracture toughness testing in the ductile-to-brittle transition regime, J Test Eval 1991;191:123–34; Size and deformation limits to maintain constraint in K Ic and J c testing of bend specimens. In: Kirk M, Ad Bakker, editors. Constraint effects in fracture theory and applications: second volume. ASTM STP 1244, 1995; Numerical investigation of 3-D constraint effects on brittle fracture in SE(B) and C(T) specimens, Int J Fract 1995;74:131–61] and provides a strong justification for changes to ASTM E1921-02 if a conservative, geometry insensitive, and transferable reference temperature, T 0, is to be determined using this standard. Possible short-term changes to ASTM E1921-02 could include raising M lim and requiring an upward shift of SE(B) T 0 values. A more desirable solution is to adjust individual K Jc toughness values before evaluation of T 0 to eliminate specimen geometry bias.

Comparative Analysis of Using Small Cylindrical Specimens for Compressive Strength of Portland Cement Concrete

Cast compressive strength tests performed on Portland cement concrete varies from one standard to another and from one country to another. In many situations, it may be necessary to compare compressive strength values tested using different specimen sizes specially in areas were no specific specimen size requirement is adapted. It is considered that the restraining effect of the platens of the testing machines extends over the entire height of the cube but leaves a part of a test cylinder unaffected. For that reason, many codes prefer to use cylinder of 300 ×150 mm rather than cubes, but this leaves the problem of preparation and handling of relatively large sample (300×150 mm), which creates a difficulty during construction. The proposed cylindrical specimen of 150x75 mm considered in this research will enable a practical and easy handling, at the same time it keeps the effect of machine plates at minimum. For the purpose of this research, specimens were prepared and tested having 100 mm and 150 mm for cubes, and 150x75 mm and 300x150 mm for cylinders. Ten mixes were used in preparation of specimen above having variable water-Cement ratio varying from 0.30 to 0.83. A compressive strength results shows that a relationship exists between the different specimens sizes especially the proposed 150x75 mm cylinder and the standard 300x150 mm cylinder. Based on the outcome of this research it is recommended to use the proposed 150x75 mm cylinder specimen and the correlation developed between the proposed specimen size and the standard cylindrical specimen.

Welded plate and T-stub tests and implications on structural behavior of moment frame connections

A series of tests on simple-welded plate specimens (SWPS) and T-stub tension specimens simulating some of the joint details in moment frame connections were conducted in this investigation. The effects of weld strength mismatch and weld metal toughness on structural behavior of these specimens were considered under both static and dynamic loading conditions. Finite element analyses were performed by taking into account typical weld residual stress distributions and weld metal strength mismatch conditions to facilitate the interpretation of the test results. The major findings are as follows: (a) Sufficient specimen size requirements are essential in simulating both load transfer and constraint conditions that are relevant to moment frame connections, (b) Weld residual stresses can significantly elevate stress triaxiality in addition to structural constraint effects, both of which can significantly reduce the plastic deformation capacity in moment frame connections, (c) Based on the test results, dynamic loading within a loading rate of 0.02 in/in/sec, as used in this study, premature brittle fractures were not seen, although a significant elevation of the yield strength can be clearly observed. However, brittle fracture features can be clearly identified in T-stub specimens in which severe constraint effects (stress triaxiality) are considered as the primary cause, (d) Based on both the test and FEA results, T-stub specimens provide a reasonable representation of the joint conditions in moment frame connections in simulating both complex load transfer mode and constraint conditions.

A Proposal for Specimen Size Requirements in Toughness Qualification with the Weibull Stress Criterion

Abstract The specimen size requirements in toughness qualification are discussed on the basis of a statistical local fracture criterion approach. The Weibull stress criterion for cleavage fracture is applied to the constraint loss field at the crack tip described with the toughness scaling model (TSM). The proposed new model demonstrates that the TSM is a particular case for a material with a particular combination of the Weibull shape parameter, m, and the strain hardening exponent, n. Based on this model, alternative size requirements for ASTM standards are proposed.

LEFM size requirements for the fracture testing of sea ice

The stress-separation curve obtained for the in-situ response of first-year sea ice is used to obtain specimen size requirements for sea ice fracture tests. Issues of notch sensitivity and minimum size requirements for the applicability of linear elastic fracture mechanics are addressed. The role of time dependent deformations in studying sea ice fracture is examined.

Specimen Size Requirements for Cleavage Fracture Toughness based on the Weibull Stress Criterion

The specimen size requirements in toughness qualification are discussed on the basis of statistical local fracture criterion approach. The Weibull stress criterion for cleavage fracture is applied to the constraint loss field at the crack tip described with the toughness scaling model (TSM). Proposed new model (WSSM) demonstrates that the TSM is a particular case fora material with a particular combination of the Weibull shape parameter, m and the strain hardening exponent, n. The correlation between TSM and WSSM were also confirmed with 2 dimensional and 3 dimensional finite element analysis. The 3 dimensional analysis with Jp-δ for the stress intensity suggested the less constraint loss effect rather than 2 dimensional analysis in spite of the strong dependency of the constraint loss effect on Weibull shape parameter, m. The 3 dimensional analysis also suggested that the thickness effect on toughness mainly resulted from the statistical volume effect.The values of the hardening exponent, n and Weibull shape parameter, m, that are the dominant factors for the specimen size requirement suggested from WSSM, were investigated from the experimental results for the practical constructive steels. These results demonstrated that the specimen size requirements prescribed in ASTM could be too conservative.