Non-standard Specimens Research Articles

Numerical study of the applicability of the η-factor method to J-resistance curve determination of steam generator tubes using non-standard specimens

The suitability of the η-factor method for the determination of J-resistance curves of thin walled steam generator tubes was evaluated using elastoplastic finite element analysis. Different candidate non-standard specimens’ geometries were considered. It was shown how high constraint conditions associated with deep cracks geometries and prevailing bending loads favor the η-factor validity while low constraint configurations resulted in a η-factors exhibiting a higher dependence on the applied load. It was also verified that η-factors based on the crack mouth opening displacement (CMOD) showed much less dependency on the loading level than η-factors derived from load line displacement (LLD).

RETRACTED ARTICLE: Investigation of Fracture Behavior of Axially-Cracked Thin-Walled Tubular Components Using Non-standard Specimens and Test Setups

RETRACTED ARTICLE: Investigation of Fracture Behavior of Axially-Cracked Thin-Walled Tubular Components Using Non-standard Specimens and Test Setups

Interlaminar fracture micro-mechanisms in toughened carbon fibre reinforced plastics investigated via synchrotron radiation computed tomography and laminography

Synchrotron Radiation Computed Tomography (SRCT) and Synchrotron Radiation Computed Laminography (SRCL) permit 3D non-destructive evaluation of fracture micro-mechanisms at high spatial resolutions. Two types of particle-toughened Carbon Fibre Reinforced Polymer (CFRP) composites were loaded to allow crack growth in Modes I and II to be isolated and observed in standard and non-standard specimen geometries. Both materials failed in complex and distinct failure modes, showing that interlaminar fracture in these materials involves a process zone rather than a singular crack tip. The work indicates that incorporating particle/resin, fibre/interlayer and neat resin failure is essential within models for material response, since the competition between these mechanisms to provide the energetically favourable crack path influences the macro-scale toughness. The work uniquely combines the strengths of SRCT and SRCL to compare failure micro-mechanisms between two specimen geometries, whilst assessing any edge effects and providing powerful insight into the complex micro-mechanical behaviour of these materials.

Advances in Fracture Toughness Test Methods for Ductile Materials in Low-Constraint Conditions

Fracture toughness is an important material property in the fracture mechanics methods, and often described with the fracture parameters of J-integral and crack-tip opening displacement (CTOD) for ductile cracks. ASTM, BSI and ISO have developed their own standard test methods for measuring initial toughness and resistance curves in terms of J and CTOD using deeply cracked bending specimens. Such specimens are of high crack-tip constraints and give conservative toughness or resistance curves.Actual cracks in pressure vessels and welds are often shallow and dominated by tensile forces, resulting in low crack-tip constraint conditions and rising resistance curves. Thus the standard resistance curves could be overly conservative for a shallow crack in real structures. To obtain more reasonable fracture toughness for ductile cracks in low-constraint conditions, many experimental and analytical methods have been developed in the recent decades. This paper presents a critical technical review of fracture toughness test methods for standard and non-standard specimens, including (1) ASTM, BSI and ISO standard test methods for high-constraint specimens, (2) constraint correction methods for determining a family of constraint-dependent resistance curves, and (3) direct test methods for a low-constraint specimen: single edge-notched tension (SENT) specimen. This includes the basic concepts, basic methods, estimation equations, test procedures, limitations, historical effects, and recent progresses.

Consolidation of weak lime mortars by means of saturated solution of calcium hydroxide or barium hydroxide

Abstract This paper presents research results on the effects of repeated treatments with saturated solutions of calcium hydroxide (lime water) or barium hydroxide (barium water) on consolidating a friable lime mortar. The influence of lime or barium water treatment on various mainly mechanical characteristics of consolidated lime mortar was studied in detail by means of tests on non-standard specimens fabricated from a poor mortar of 1:9 vol. lime-to-sand ratio. The traditional lime water technology and barium hydroxide treatment were further compared with distilled water and lime water with added metakaolin. Lime water treatment of a specific lime mortar was shown to be effective after a sufficiently large number of applications (160 saturations) into a weak lime mortar. No consolidating effect of distilled water on the compressive strength of the tested mortar with a low lime content (1:9) was observed. The mechanical characteristics of the tested mortar were not improved by treatment with lime water with added metakaolin. Barium water treatment significantly increased mainly the tensile strength of the tested lime mortar.

Prediction of Elastic Moduli Development of Cement Mortars Using Early Age Measurements

This paper presents an investigation of the time development of dynamic and static elastic moduli of eight mortars made using CEM I cements with contrasting properties. Mixing and the preparation of dynamic testing prism specimens conformed to EN 196-1. A non-standard specimen and bending test setup was devised for measuring the development of static Young's modulus. It was determined that dynamic Young's moduli determined using an ultrasonic pulse velocity test are often but not always greater than those determined using a resonant frequency test. The ratio of static Young's modulus to dynamic Young's modulus for a particular cement mortar increased nearly linearly with increasing static modulus after age 3 days, and this increase can be predicted using the measured 3-day static modulus. Also, methods for predicting the increase of static and dynamic modulus and decrease of Poisson's ratio of mortars, from age 3 to 56 days, using measured 3-day elastic moduli are presented. Predicted 56-day values of static Young's modulus, dynamic modulus, the ratio of these, and Poisson's ratio for the eight different mortars were within 5 % of the measured values. The observed relations between different elastic properties may be useful for verification of prediction models.

Verification of the Fatigue Test Method Applied with the Use of Mini Specimen

In special situations the fatigue properties of the construction material can be determined using non-standard specimens, for example smaller than the normative ones (the so-called mini specimens). The research presented was made for the aluminum alloy based on the high-cycle fatigue testing methodology. The verification was made by breaking down the results with own tests which involved the use of standard specimens and stands as well as with the literature reports.

On fracture resistance parameter from non-standard fracture test specimens of titanium alloy

The current available design rules of submersible structures from ship classification societies are based on the philosophy of strength design. As a supplement, damage tolerant design becomes an important consideration in avoiding the catastrophic failure caused by material defects. Titanium alloys with relatively high ductility are widely used in pressure hull of submersibles nowadays. Much work should be devoted to systematic investigation of the fracture properties evaluation for candidate titanium alloys for submersible structures. Traditional fracture toughness test with normal specimen size may not give reliable fracture resistance value. More applicable non-standard specimens can be designed for engineering purpose. Then the establishment of a reliable fracture resistance parameter from the non-standard fracture test specimen for fracture ductility evaluation is necessary. The present work is to primarily study the potential proper fracture resistance parameters for ductile titanium alloys. Series of non-standard specimens of Ti80 are designed for the test and analysis. Thickness effect is specifically investigated using specimens with similar ligament size. Some fundamental proposals are accordingly brought forward for ductility evaluation of titanium alloys used in submersible structures.)

The Roman mortars used in the construction of the Ponte di Augusto (Narni, Italy) – A comprehensive assessment

The ruins of the Ponte di Augusto bridge, built in 27 BC close to Narni, have been investigated in recent years in order to assess the seismic resistance of the bridge. The inner structure of the existing pier was surveyed by means of four vertical core drillings, which enabled an assessment of its structural integrity, and provided material samples of its cast mortar masonry. The article presents results of tests that aim to understand the particular function of the mortars that were used. Alongside the macroscopic observations, the following instrumental methods were applied in a study of the extracted material: a macroscopic description of the mortar, thin section optical microscopy, binder SEM–EDS analysis on broken mortar samples, determining the basic physical characteristics, thermal analyses (TGA) and (DTG), and bending and compressive strength testing according to a special methodology for testing very small non-standard specimens. The measured mechanical characteristics (strength and stiffness) vary in a rather large range of values, which may indicate that the cast mortars were not set with suitable technological care. Although it cannot be stated unequivocally that the different compositions of the Ponte di Augusto pier mortars were designed intentionally to take into account changes in the load carrying capacity, from the point of view of the statics of the structure, the strongest mortar was used logically and it was probably also intentionally designed. This strongly interdisciplinary study contributes substantially to our understanding of the work of ancient Roman engineers.

Determination of the Fatigue Properties of Aluminum Alloy Using Mini Specimen

There are situations where taking normative specimens is impossible due to the dimensions of the objects investigated (e.g. extruded sections) and one of the solutions is to use mini specimens. As for non-standard specimen testing, it is essential to define the effect of size on fatigue strength. The research methodology facilitates the determination of fatigue characteristics (S-N) for EN AW-6063 aluminum alloy. The material is used to manufacture the extruded section in the process of extrusion of the material through the extruding die. The methodology assumes the geometry of the mini specimen and the normative specimen. As for the material strength identification, a static tensile test for the specimens made directly from finished elements and preliminarily strained in cycles was carried out. As a result of the cyclic material reinforcement, an increase in yield strength Re was observed, which, in turn, rejects Re as the upper criterion of the high-cycle fatigue range. The essential fatigue tests were performed based on unilateral cyclic tension (R = 0.1). The effect of size on fatigue strength was defined. Theoretically aluminum alloy non-sensitive to changes in the size of the cross-section showed a different strength in mini and normative specimens.

Investigation into Shear Coefficient for Circle Shear Test Method of Asphalt Mixture

In order to efficiently evaluate the shear properties of asphalt mixture, the circle shear test method is proposed. However, the thickness of upper layer surface and the middle-surface of the asphalt pavement in China are usually less than the height of standard specimen in circle shear test, as well as some laboratory specimens. In order to determine the shear strength for samples at different height, the circle shear test is conducted to the specimens at different height, and the height calibration coefficient for non-standard specimens are proposed. With which the shear strength for samples at different height can be unified and easily determined.

Fracture Toughness of HSLA Coiled Tubing Used in Oil Wells Operations

Coiled Tubings are thin walled steel tubes of 25–89 mm diameter and thousands meters long, used in the oil industry for production and maintenance services. They suffer plastic deformation during unwinding of the reel, passing through a goosneck arch guide and an injector unit. Strain levels are of 2–3%, making the tubing fail by low cycle fatigue in around 100 wrap–unwrap cycles. As coiled tubing material generally behaves in a ductile manner at surface and down well temperatures, the R curve has to be known to make instability analyses. J-R curves were determined to characterize the fracture toughness of nonused coiled tubing, using nonstandard specimens due to difficulties with their small thickness and diameters. Different crack lengths and crack locations were tested to analyze the 2C0/W ratio and the influence of the longitudinal weld. The R curves obtained show crack arc length dependence and are influenced by the position of the longitudinal weld.

A load-separation technique to evaluate crack growth and fracture resistance behaviour of thin-walled axially cracked tubular specimens

Integrity assessment of thin-walled tubular components such as nuclear reactor fuel pins during postulated and beyond-design-basis accident scenarios is an important issue for limiting the release of radioactivity into the surrounding fluid. The fracture resistance behaviour of these tubes cannot be evaluated using standard ASTM techniques. It is because of the inability of these axially cracked specimens to meet the stringent plane strain requirement due to their geometry and the high ductility of the zirconium alloy used for their fabrication. Moreover, the measurement of crack growth during the testing by conventional methods is a cumbersome process and sometimes it is not possible to use them due to the small size of these specimens. Alternative methods such as load-normalization and load-separation techniques are suitable for these types of situations. In this article, the crack growth data during loading of the axially cracked tubular specimens have been evaluated using a load-separation method. Several specimens have been tested in order to evaluate their fracture resistance behaviour. These specimens have different values of initial crack lengths. A modified procedure for the estimation of the material constant ‘m’ has been developed in this study so that data from multiple specimens can be used. This will lead to a more reliable estimation of crack growth during the loading process. The J–R curves, obtained using the current method, have also been compared with those obtained using load-normalization technique. It was observed that this modified method of load separation is a convenient and reliable technique for the evaluation of fracture resistance behaviour of non-standard specimens.

Investigation of failure behavior of two different types of Zircaloy clad tubes used as nuclear reactor fuel pins

Nuclear reactor fuel pins act as barriers to the release of radioactive fission products to the coolant flowing around these thin-walled tubes and hence they prevent the leakage of radioactivity to the surroundings of reactor core. These tubes are of small thickness in order to have less resistance in the path of heat flow from the fuel to the coolant. Investigation of failure behavior of these fuel clad tubes is of utmost importance to the designers and plant operators in order to ensure the maximum residence time of the fuel bundles inside the reactor core as well as to ensure minimal activity during operation and refueling activities. Various types of zirconium based alloys are used to manufacture these pins. The focus is to obtain better strength, ductility, corrosion resistance, oxidation resistance, and minimal creep including those due to irradiation-assisted damage and deformation processes. Two number of such types of alloys, namely, re-crystallization annealed Zircaloy-2 and stress-relief annealed Zircaloy-4, have been investigated in this work for their fracture behavior. As standard fracture mechanics specimens cannot be machined from these thin-walled tubes, non-standard specimens with axial cracks have been used in this work. Load normalization technique has been used to evaluate crack growth during loading of these specimens. It was observed that the re-crystallization annealed Zircaloy-2 specimens have higher initiation fracture toughness as well as higher resistance to crack growth compared to the other type of specimens. In order to understand the micro-structural aspects of the fracture resistance behavior of these materials, further investigation incorporating optical and transmission electron microscopy have also been carried out. It was concluded that the higher fracture resistance behavior of the re-crystallization annealed Zircaloy-2 specimens can be attributed to the presence of finer grain and sub-grain micro-structure, very low dislocation density and other defects in the material.

Arcan-Richard Specimens: Is there a Pure Shear Mode?

Traditionally, applications of fracture mechanics have been mainly focused on cracks growing under the opening or mode I mechanism (three point bend specimen, middle tension specimen, crack tension specimen, etc.). When investigating instability under mixed mode loading conditions, three different alternatives can be envisaged consisting of: a) machining a hole in standard specimens, b) creating a crack oriented under a given angle, and c) using non-standard Arcan-Richard specimens. In this work, finite element calculations are performed to analyze the initial values of the fracture parameters in Arcan-Richard specimens. First, the influence of the normal stress mode to the shear stress mode ratio is analyzed, then the effect of the constraint level is discussed, and finally, the initial propagation angle of the daughter crack is derived.

Fatigue Crack Propagation in SAW Seam Welds of API 5L X42 Steel Pipe in the Radial Short Direction

Abstract Fatigue crack propagation (FCP) rates in submerged arc welding (SAW) seam welds of 1524 steel (API 5L X42) pipe were measured by using an arc-shaped bend specimen with the same radius of curvature of the body of the pipe so the material did not have to be cold worked to get a flat shape nor extensively machined. The test direction was girth radial and the stress intensity factor (K) function was calibrated for this type of nonstandard specimen. The FCP tests were carried out in air at room temperature, testing the three zones: The base metal, the deposited metal, and the heat affected zone (HAZ). A fractographic analysis was done to analyze the role of the microstructure in the FCP in the three zones. It was found that the zone of greater resistance to FCP was the base metal, whereas the deposited metal showed the least resistance to crack propagation. FCP in the deposited metal and the HAZ behaved according to the Paris law, unlike the base metal, which showed a high data dispersion. The behavior in the base metal was attributed to the propagation of the crack in the transverse direction of the preferential alignment of the microstructure, while the deposited metal and the HAZ had a more homogeneous microstructure.

Experimental Evaluation of the Size Effect on the Ductile and Brittle Fracture Toughness of a Steel Pressure Vessel

Experiments of fracture toughness with non-standard SENB specimens of five different thicknesses were performed to investigate the size effect on the ductile and brittle fracture for different temperatures. From the experimental results it is found that size effects both brittle and ductile fracture with the same trend but for different mechanical reasons. The ductile fracture toughness increases firstly with increased plastic deformation zone size and plastic fracture strain under general yielding conditions, and then drops down due to the plastic deformation zone size not changing much which is less than the residual ligament width and the increase of the proportion of the high stress triaxiality zone to the whole specimen. The fracture toughness of the lower shelf increases with increasing thickness of the plastic deformation zone size under small scale yielding conditions, and then drops down due to the increase of the high out-of-plane constraint.

Quantitative Estimation of the Growth of Environmentally Assisted Cracks at Flaws in Light Water Reactor Components

Since environmentally assisted cracking (EAC) is an important degradation mechanism affecting the structural materials of nuclear power plants, numerous EAC experiments have been performed in the past three decades using standard specimens in simulated high temperature water environments to evaluate the various core materials used in light water reactors (LWRs). However, the environment, the condition of the material, and the mechanical properties near flaws in LWR components are not absolutely equivalent to those near the crack tip in standard specimens; thus, more research needs to be done before EAC growth in an actual LWR component can be accurately estimated using existing experimental EAC data. By combining the film slip-dissolution/oxidation model with the elastic-plastic finite element method and existing experimental EAC data, we have derived a method by which an estimation of EAC growth at flaws in actual LWR components can be made. In this paper we propose and discuss the use of this method. The results show that this new method basically concurs with the Fracture Research Institute (FRI) model in evaluating EAC growth across a semi-elliptic crack front under a simple tensile load and is also in approximate agreement with the experimental results obtained in evaluating EAC growth along a semi-elliptic crack front under complex loading conditions. The approach is expected to form a bridge between predicting EAC growth rate in core materials and evaluating EAC growth in key structural components in LWRs, and it is also expected that it can be used as a pre-analytical tool for EAC experiments using nonstandard specimens.

Intralaminar toughness characterisation of unbalanced hybrid plain weave laminates

A numerical and experimental investigation on the mode-I intralaminar toughness of a hybrid plain weave composite laminate manufactured using resin infusion under flexible tooling (RIFT) process is presented in this paper. The pre-cracked geometries consisted of overheight compact tension (OCT), double edge notch (DEN) and centrally cracked four-point-bending (4PBT) test specimens. The position as well as the strain field ahead of the crack tip during the loading stage was determined using a digital speckle photogrammetry system. The limitation on the applicability of the standard data reduction schemes for the determination of intralaminar toughness of composite materials is presented and discussed. A methodology based on the numerical evaluation of the strain energy release rate using the J-integral method is proposed to derive new geometric correction functions for the determination of the stress intensity factor for composites. The method accounts for material anisotropy and finite specimen dimension effects regardless of the geometry. The approach has been validated for alternative non-standard specimen geometries. A comparison between different methods currently available for computing the intralaminar fracture toughness in composite laminates is presented and a good agreement between numerical and experimental results using the proposed methodology was obtained.

J–Resistance curve testing of HY80 steel using SE(B) specimens and normalization method

The normalization method is adopted for standard and nonstandard specimens in this paper to develop J– R curves for HY80 steel directly from load versus load-line displacement records without use of automatic crack length measurement. The standard specimens usually contain high crack-tip constraints, while the nonstandard specimens involve low crack-tip constraints. To obtain J– R curves with different constraints, a series of single edge notched bend (SE(B)) specimens with different crack lengths for an HY80 steel are tested in accordance with ASTM standard E1820. The normalization method is then used for determining crack extension and J– R curves for these SE(B) specimens. To validate the normalization method, the J– R curves determined using the normalization method are compared with those obtained by the elastic unloading compliance method for the SE(B) specimens. The comparison shows that good agreements exist between the two methods, and the normalization method is a viable tool to be used to determine J– R curves of the HY80 steel for the standard as well as nonstandard SE(B) specimens. In the J-integral calculations, the resistance curve test method, the basic test method and the modified basic test method specified in ASTM E1820 are evaluated. The results indicate that the modified basic method can be equivalent to the resistance curve method.