Single Test Specimen Research Articles

Insights from large‐scale shake table testing: Key criticisms and potential solutions

AbstractThis article presents the authors' experience with large‐scale shaking table tests conducted in Japan using the E‐Defense shaking table. The discussion focuses on four criticisms often addressed regarding the utilities of large‐scale shaking table tests. Potential solutions to mitigate such criticisms are discussed based on shaking table tests conducted for a pair of three‐story wooden houses. The first criticism is that the test specimen anchored rigidly to a rigid shaking table is not a reproduction of actual structures supported by soils and foundations. A model ground was developed in a large sandbox, which occupied about 85% of the total specimen weight, supported the house, and the entire soil‐structure system was shaken. Considerable sliding occurred, having lessened the earthquake forces exerted and resultant damage to the superstructure. The second criticism is that a single specimen test, regardless of its size, cannot provide sufficient information for generalizing the behavior and performance. Empirical equations between the maximum story drift and the change in the natural frequency were developed from a series of shaking table tests. Using such empirical equations might promote quick damage assessment of individual houses when suffering from actual earthquakes. The third criticism is the importance of public appeal and eventual support from the general public to secure the budget to operate large‐scale testing facilities. The example test featured two nearly identical specimens placed on the table with different support conditions. The apparent difference in response revealed the effect of support conditions on seismic performance. The fourth criticism is the importance of increasing the number of experimental projects to balance the operation budget. Most of the preparation in the example test was accomplished in an open yard adjacent to the shaking table, and the test specimens were quickly assembled on the table using indoor cranes. The table occupation was four out of 35 weeks of the entire test duration.

Examination of the Dry Density Importances in TDR Use and of the ASTM D6780 Standard Based on a New Calibration Procedure

Abstract The use of time domain reflectometry (TDR) to monitor soil water content at either field or laboratory scales is highly common. Regarding the soil dry density, ρd, effect on the TDR application, three main approaches are found in the literature: (1) Ignore the density issue and use calibration to evaluate the water content regardless of the density. This approach is basic, yet common. (2) Include soil density in the calibration equation and use calibration for the water content as a function of both the TDR output and soil density. In this approach, the soil density should be provided from an independent source. (3) Determined ρd from the TDR waveform. In this case both water content and ρd are set from the TDR analysis. In cases where the ρd is not available, in order to use TDR for water content measurement, a decision has to be made whether to address the density issue although it requires resources, or to neglect it. This article reviews this issue and validates the density effect using an efficient TDR calibration methodology. A calibration scheme that controls ρd of the specimen has been developed and applied. The methodology creates continuous calibration curves over a range of volumetric water content from a single test specimen. Based on the results, it has been demonstrated that the soil density has a major effect on the TDR output and it cannot be ignored. In addition, this article presents a unique examination of ASTM D6780, Standard Test Methods for Water Content and Density of Soil In Situ by Time Domain Reflectometry (TDR). The ASTM dry density estimation was found to provide reliable results; however, the gravimetric water contents estimation gives unsatisfactory results, especially for low densities.

Design of a Cost-Effective and Statistically Validated Test Specification with Selected Machine Elements to Evaluate the Influence of the Manufacturing Process with a Focus on Additive Manufacturing

Due to their versatile advantages, the use of additively manufactured components is growing. In addition, new additive manufacturing processes are constantly being developed, so that a wide range of printing processes are now available for metal. Despite the same starting material, the microstructure and thus also the final mechanical properties differ greatly compared to conventional processes. In most cases, only direction-dependent characteristic values from the uniaxial tension are used to qualify a printing process before it is used. The literature, on the other hand, demonstrates that the results are not transferable to other loading conditions. In this work, several engineering tests were integrated into a single test specimen so that they can be determined on the same specimen. The test specimen can be used to test tooth root strength, bending strength, notched bar impact energy, and thread strength depending on the mounting direction, thus representing industrial loading cases. In this study, test specimens were fabricated by conventional manufacturing (machining), L-PBF (Laser Powder Bed Fusion), and WA-DED (Wire Arc Direct Energy Deposition), and the results were compared using statistical methods. Factors to capture manufacturing influence and buildup direction were statistically validated on 316L. The work shows a benchmark with a typical initial microstructure of rolled and milled material, L-PBF, and WA-DED parts on loads close to the application and thus simplifies an industry-oriented evaluation of a new manufacturing process.

Study of fatigue fracture characteristics for duplex stainless steel STHE welded joint specimen

A shell and tube heat exchanger (STHE) welded joint is considered for fatigue fracture mechanism. The joint is made up of a protruding tube fabricated to a tubesheet by means of welding. A super duplex stainless steel (DSS) material is used for the tube and tubesheet. Fatigue test was newly conducted using the single tube test specimen. The initial cracking plane was moved from the minimum sectional-area plane that was perpendicular to the loading direction, and the crack growth was almost planar. Stress analysis is carried out using finite element method (FEM) and the stress distributions are carefully studied by comparing the simple perforated plate. In addition, three-dimensional crack propagation (CP) simulation is performed employing extended finite element method (X-FEM). Fracture phenomenon is verified using the experimental and numerical crack front geometries. The fracture mechanics parameter is evaluated and verified with empirical formulae. CP behaviors are carefully discussed based on the fracture mechanics parameter. The CP life is presented for three loading amplitudes. The FAT class of the STHE welded joint is studied based on the available design standards such as BS, IIW and JSSC. Finally, the fatigue fracture characteristics of the STHE welded joint specimen are shown through the experimental and numerical methods.

Application of TDR in compacted clay soils: Issues of dry density, water content range, and calibration

The paper deals with three issues relevant to the application of TDR methods for monitoring water content in compacted clay soils: ( i) the dependence on dry density, ( ii) the ability to monitor changes in water content in clay soil approaching saturation, and ( iii) difficulties in performing efficient and reliable calibrations. A calibration scheme which inherently controls dry density of the specimen has been developed and applied. The methodology creates continuous calibration curves over a range of volumetric water content from a single test specimen. The new approach has been applied in monitoring the advance of a wetting front through a compacted swelling clay in a laboratory environment.

Study of dual grain microstructure heat treatment in steel specimen representing gas turbine rotor disc

Gas turbine discs which support the rotor blades operate at relatively high speeds and high rim temperatures and are therefore subjected high creep damage at the rim area and to very high low-cycle fatigue damage at the bore. Any heat treatment which results in homogenous grain structure and one small band of grain size will limit the life of the disc either by creep at the rim or by the fatigue at the bore. If the heat treatment parameters are optimized to increase the grain size at the rim area closer to the ASTM 6.5 and to finer grain size at the bore area closer to ASTM 11.5, the heat treated disc is likely to have much superior operational life as compared to that of heat treated which results in homogenous grain structure and size throughout the disc. There is therefore the need for exploring possibilities of obtaining variable grain structure during heat treatment operation.This study is aimed at evaluating two possible heat treatment processes that are likely to result in distinctly different grain size in a single test specimen such as a circular rod. These test rods represent rotating gas turbine discs and the objective of achieving differential grain size is to optimize and balance the creep life at one half portion of the test rod (representing the disc rim or Outer Diameter) and fatigue life at another half portion of the test rod(representing the bore or Inner Diameter).

A constraint correction method based on use of a single test specimen

Commonly, fracture toughness tests on deeply cracked specimens are used to assess defects in large-scale components. The paper presents a method for selection of test specimen type, size and crack length in order to obtain fracture toughness estimates relevant to defects in cracked pipes. The method uses available closed-form T-stress, stress intensity factor and limit load solutions to determine the required specimen dimensions. The paper reports elastic–plastic finite element analyses for single edge notched bend (SENB) and single edge notched tension (SENT) specimens and cracked pipes which demonstrate good agreement of the matching approach, although care is needed in selecting the appropriate limit load solution for SENT geometries.

Modified DSM framework for limit states prediction of simply-supported Z-section purlin with sheeting

A framework to predict the limit states of a single span unbridged Z-section purlin restrained by sheeting under uplifting wind load is presented. The flexural–torsional buckling of the member takes into account the diaphragm effects and the effect of load eccentricity on the warping-torsional behaviour of the purlin. The limit state loading for flexural–torsional buckling when under bi-moments and biaxial bending moments is then calculated. The variation of the signature curve of the Z-section member is also studied with the finite strip method. The interaction of different buckling behavior and limit states loadings is studied with the Direct Strength Method resulting in a reduction of load factor on the applied moment. The proposed model is then calibrated with a database of 45 single span specimen tests with good results. The proposed framework has been demonstrated feasible and accurate for limit states prediction of Z-section single span purlin with sheeting.

Wear Behaviour of SAE 4340 Steel in Comparison with Single Test Specimen

This study addresses the progressive damage of a surface/specimen/component caused by another substance in relative motion. This could cause the change in geometry, dimensions of the part which loses the practical functionality. So, an attempt was made to study the wear behavior of SAE 4340 steel using a pin-on-disc wear test machine at different loading, test speed, and time. Two cases viz. (1) single steel specimen, (2) multiple specimens were considered for the wear test and the obtained wear (μm), and frictional force (N) was compared.

Towards a better understanding of the mechanical behavior of a fixed bed of eucalyptus charcoal in a blast furnace using a specific compression test

The properties of wood charcoal layers have an effect on the performance of small-scale blast furnaces. In order to characterize the mechanical behavior of a fixed bed of eucalyptus charcoal, a specific uniaxial compression test was designed and used with charcoal layers of different characteristics. This layer test has the potential to be standardized, and it made it possible to consider the bulk properties of randomly layout charcoal pieces, which was better adapted than single specimen tests in the fiber direction. A total of eight charcoal layers were prepared with two carbonization temperatures (500 °C and 900 °C), two granularities (10 mm and 20 mm), and two different testing temperatures (20 °C and 300 °C). Characteristic parameters of the compression tests were then determined as the particle size distribution, the mechanical energy, and the mean power. The charcoal produced at 900 °C and with a granularity of 20 mm was more resistant to breakage than the others were, and a high quantity of large particles remained after the tests. Significant correlations existed between the carbonization temperature, granularity, and mechanical power of the compression test. The mechanical power was the main parameter that determined the resistance to breakage of a charcoal bed in compression.

Impact of neutron irradiation on the strength and ductility of pure and ZrC reinforced tungsten grades

The strength and ductility of two tungsten products, developed for application in nuclear fusion environment, are studied before and after neutron irradiation using uniaxial tensile tests. The first product is a commercially pure tungsten, produced by AT&M company according to ITER specification, and the second one is reinforced with zirconium carbide (W–ZrC) particles. The addition of ZrC particles leads to a reduction of the ductile to brittle transition temperature (DBTT) in non-irradiated conditions down to 50–100 °C without loss of strength and of other attractive properties of tungsten.The neutron irradiation was performed in the range 625–700 °C up to 1.125 dpa. The tests were performed to screen the shift of the DBTT as well as to characterize the evolution of the strength and ductility at the irradiation temperature. In addition, a series of interrupted tensile tests were performed in order to determine the variation of the yield strength as a function of temperature using an original single specimen test method.The neutron irradiation causes the reduction of the total elongation of both tungsten products. The DBTT range, which was evaluated from the tensile test results, of W–ZrC lies in the 300–500 °C range (while it is ∼100 °C in non-irradiated state). The DBTT range of pure tungsten is between 400 and 575 °C i.e. higher than that of W–ZrC. The irradiation hardening, measured at ∼600 °C, which is close to the irradiation temperature, leads to an increase of the proof stress by a factor of two in both studied grades. Despite that the irradiation induced hardening, both products retain a total elongation of about 10% prior to fracture. W–ZrC exhibits a similar total elongation at 500 °C, thus maintaining a significant ductility resource, while pure W becomes brittle at 500 °C and below.

Effect of Temperature Variation and Pre-Sustained Loading on the Bond between Basalt FRP Sheets and Concrete.

The coupled effects of temperature variation and pre-sustained loading on the bond between basalt fiber reinforced polymer (BFRP) sheets and a concrete substrate were studied. Single lap-shear test specimens were exposed to temperatures of 15, 30, 40, 50, and 60 °C for 3 h with pre-sustained loading at 35% of the ultimate load capacity (Fu). Compared with the case of 15 °C, the interfacial fracture energy of the specimens at 30 and 40 °C increased by 46% and 11%, respectively, whereas those reduced by 73% and 77% at 50 and 60 °C, respectively. The coupled effects of temperature and pre-sustained loading on the effective bond length are insignificant for the specimens at both 15 and 30 °C and the effective bond length increased to 300 mm when the temperature exceeded 40 °C. The failure crack still occurred in the concrete substrate at the temperatures of 15 and 30 °C, and changed to the debonding of the adhesive layer from the concrete substrate at the temperature above 30 °C.

An improvement to the single specimen test method for fracture characterization of elastomer materials using DENT specimen

Fracture behavior of elastomer materials is investigated based upon single specimen and multiple specimen test methods using the practically recommended double edge notched tension (DENT) specimen. Combining analytical, experimental, and numerical analysis, the critical J integral is determined based upon η factor concept. The main challenge with this approach is how to extract the η (or η*) factor. For DENT specimen, previous studies have proposed that η* factor, unlike η, could be considered approximately constant, and even estimated from its homologous LEFM-based formula (ηel). In this paper, it is shown that the values of coefficients η and η* may significantly depend on the material type and the constitutive model and even on the load level. Moreover, η* values deviate significantly from ηel predictions. Consequently, the claim of a constant coefficient η* for DENT specimen is not correct, though numerical estimations of this variable factor by using the appropriate constitutive parameters could still be provided.

Optimized Analysis Method for Evaluating the Shear Strength Parameters of Rock Joint Surfaces

The results obtained from the mechanical test of rock samples inevitably suffer dispersion owing to discrepancies between test specimens. In view of these deficiencies, the present study proposes a method based on the empirical equation of shear strength developed by Barton to determine the shear strength parameters of joint surfaces using a single test specimen. This approach is then applied to optimize the analysis of multiple specimens. An analysis of experimental results verifies that the shear strength parameters of joint surfaces obtained by the proposed method can more accurately reflect the shear mechanics of multiple specimens than conventional multiple sample analyses; meanwhile, the results are reasonable and reliable. More importantly, the optimized method ensures the shear strength parameters are no longer affected by the sequence of specimens employed during shear test. The optimized analysis method eliminates the effect of differences between specimens and the influence of subjective factors on test results and therefore provides more realistic evaluations of shear strength parameters.

Bond stress and behavior of interface between untreated aluminum alloy surface and concrete

This paper explores the viability of using aluminum alloy (AA) plates as an externally bonded reinforcement (EBR) material that could overcome some of the shortcomings of the prevailing steel and FRP materials and provides new advantages. A total of 32 prisms with different concrete strengths have been prepared as single shear test specimens with AA plates bonded to the prisms at different lengths using epoxy adhesive. Single shear tests were conducted on the prism specimens until failure. Load-extension and load-strain values were recorded. Bond stress and bond slip values were calculated. Test results have shown that plain untreated AA surface is susceptible to premature debonding; however treating the AA surface, by roughening it, increases its bond strength and ultimate load capacity considerably. Generally, the ultimate load and maximum bond stress increased with the increase of the bonded length and concrete strength, on the average, for the AA roughened surface compared to the AA plain untreated surface, by up to 457%, 327%, respectively. It is also observed that the modes of failure were influenced by the changes in the bonded length, concrete strength and AA surface treatment.

Об уровнях вибрационных воздействий при испытаниях бортовой радиоэлектронной аппаратуры космических аппаратов

The tightening of avionics lifetime and reliability requirements necessitates a profound and efficient experimental method. At the same time, the not infrequent absence of even two equipment samples for conducting tests (considering the variation of properties of materials and technological processes) makes the problem of confirming hardware strength characteristics difficult to solve. The present paper describes a methodology for vibration tests of spacecraft onboard electronics that allows us to reliably confirm the strength of a random sample of the given equipment based on the testing of a single test specimen. Safety margins for different stages of equipment vibration testing are determined. These margins are necessary to ensure the given probability of design defect detection. The proposed methodology is compared with current foreign regulatory documents dedicated to testing spacecraft onboard electronics. The proposed method allows a flexible approach to the selection of vibration test levels since it takes into account the design features of the equipment affecting strength spread from sample to sample during the determination of the scope of required tests.

Development of a Novel Test Method to Characterize Material Properties in Corrosive Environments for Subsea HPHT Design

Abstract High pressure high temperature (HPHT) design is a significant new challenge facing the subsea sector, particularly in the Gulf of Mexico. API 17TR8 provides HPHT Design Guidelines, specifically for subsea applications. Fatigue endurance (i.e., S–N) and fracture mechanics design are both permitted, depending on the criticality of the component. Both design approaches require material properties generated in corrosive environments, such as seawater with cathodic protection and/or sour production fluids. In particular, it is necessary to understand sensitivity to cyclic loading frequency (for both design approaches), crack growth rates (CGR) (for fracture mechanics approach) as well as fracture toughness performance. For many subsea components, the primary source of fatigue loading is associated with the start-up and subsequent shutdown operation of the well, with long hold periods in-between, during which static crack growth (CG) could occur. These are the two damage modes of most interest when performing a fracture mechanics based analysis. This paper presents the preliminary results of a novel single specimen test method that was developed to provide fatigue crack growth rate (FCGR) and fracture toughness data in corrosive environments, in a timeframe that is compatible with subsea HPHT development projects. Test data generated on alloy 625+ in seawater with cathodic protection are presented along with a description of how the test method was developed. A crack tip strain rate based formulation was applied to the data to rationalize the effect of frequency, stress intensity factor range (ΔK), and maximum stress intensity factor (Kmax).

The effect of residual stresses on the formation of transverse cracks in adhesive joints for wind turbine blades

Transverse cracks in adhesive bonded joints evolve typically due to a combination of mechanical- and residual stresses. In this paper, a new approach that allows the residual stress to be determined on a single test specimen in several different ways is presented. The residual stress measurements were used in combination with a bi-material model to predict the stress at first transverse crack in the adhesive layer of a sandwich specimen (laminate/adhesive/laminate).The model prediction was consistent with measurements from quasi-static tensile tests of the sandwich specimens. These experimental results showed that higher post curing temperature and lower test temperature had a negative effect on the formation of transverse cracks in the adhesive layer i.e. residual stresses were higher and transverse cracks initiated at lower applied mechanical loading. The effect of increased laminate thickness was found to be small under both static and cyclic loading of the sandwich specimens. Furthermore, the cyclic loaded tests confirmed that the design of the sandwich specimen was damage tolerant since multiple cracking of the adhesive developed in a stable manner.

The effects of initial crack length on fracture characterization of rubbers using the J-Integral approach

The effects of initial crack length on the mode I fracture properties of crosslinked elastomers were investigated. A J-integral approach was implemented using single-specimen test procedures. Quasi-static fracture tests were carried out on filled and unfilled acrylonitrile-butadiene rubbers with various pre-cut (initial) crack lengths, a0, at a fixed rate of extension, using single edge notch rectangular bar specimens. Data from these experiments were analyzed and crack resistance curves (R curves) were obtained. A detailed analysis of the single-specimen method was carried out to establish correlation with the fracture properties derived from the more involved multi-specimen method. The weak dependence of geometry factor on rubber composition and displacement (in the neighborhood of crack initiation displacement) was verified. The crack initiation resistance and tearing modulus of the materials were calculated based on the R curve for each initial crack length. Fracture properties of lightly cracked specimens (a0/W<0.4, where W is the specimen width), including crack initiation resistance, strain at crack initiation, tearing modulus and maximum load and extension before fracture, were found to show significant variations with a0. These variations are characterized and discussed for the filled and unfilled rubber materials.

Potential implications of step loading in impression creep testing

The small sample impression creep test method has recently been of interest, as it can give a good indication of expected creep rates in uniaxial creep testing with minimal use of material. The compressively loaded test has also been shown to provide accurate results under multi-step loading conditions for a low alloy steel (½Cr½Mo¼V) to further extract value from a single test specimen. The Electric Power Research Institute (EPRI) has conducted step tests (step temperature and step loading changes) on another low alloy steel (Grade 22), as well as a tempered martensitic 9 Cr steel (Grade 91). Results have shown that there may be potential problematic areas when conducting step-up and step-down steps in these materials. Additional posttest evaluations have shown that material effects, such as strain hardening and strain softening, may add additional complexities when comparing strain rates of multi-stepped loaded stain rates. Hardness testing on posttest impression creep specimens have confirmed strain softening of tempered martensitic Grade 91 and no observed effect for an ex-service Grade 22 alloy. These findings have shown that careful considerations must be made before using creep rates obtained from multi-stepped loaded tests in situ of single loaded tests.