Thin Tubular Specimens Research Articles

An experimental and numerical study of the fracture behaviour of tubular specimens in a pin-loading-tension set-up

Evaluation of a valid crack tip loading parameter using standard ASTM practice may not be feasible in many situations, e.g. in thin tubular specimens with diametrically opposite axial through-wall cracks (loaded in opening mode) that are obtained from zircaloy fuel pins of nuclear reactors. Recently, a pin-loading-tension (PLT) experimental set-up has evolved for this purpose. However, the required geometry functions (which should be obtained from a three-dimensional (3D) finite-element (FE) analysis procedure) for evaluation of the stress intensity factor (SIF) for such a set-up is not available in the literature. In this work, a detailed 3D FE analysis of the PLT set-up has been carried out where both the pin and the mandrel have been modelled. Analysis has been carried out for the full loading path (i.e. for applied displacements beyond the limit load of the specimen) in order to demonstrate the effectiveness of the model and the results have been compared with those of the experiment. In addition, the SIFs (and geometry functions) of the PLT set-up (with different initial crack lengths) have been evaluated from the FE results using extrapolation techniques and were compared with the results from two different analytical techniques.

Prediction of fatigue lifetime under multiaxial cyclic loading using finite element analysis

Life prediction for GH4169 superalloy thin tubular and notched specimens were investigated under proportional and nonproportional loading with elastic–plastic finite element analysis (FEA). A strain-controlled tension–torsion loading was carried out by applying the axial and circular displacements on one end of the specimen in the cylindrical coordinate system. Uniaxial cyclic stress–strain data at high temperature were used to describe the multi-linear kinematic hardening of the material. The comparison between FEA and experimental results for thin tubular specimen showed that the built model of FE is reliable. A fatigue damage parameter was proposed to predict the fatigue crack initiation life for notched specimen. The results showed that a good agreement was achieved with experimental data.

Creep-fatigue life prediction under fully-reversed multiaxial loading at high temperatures

A multiaxial fatigue damage parameter based on the critical plane approach was proposed to calculate the pure fatigue damage under uniaxial/multiaxial loading at constant high temperatures. For the fully-reversed low-cycle fatigue loading under low frequency at high temperature, one-half of the maximum equivalent stress response value at cyclic stabilization is used as the creep stress to evaluate the multiaxial creep damage. The linear damage accumulation rule is used to predict the multiaxial creep-fatigue life at high temperature. The creep-fatigue experimental data of thin tubular specimens with GH4169 superalloy and 2.25Cr–1Mo steel were used to verify the proposed creep-fatigue life prediction model. The results showed that the proposed creep-fatigue damage calculation model can be used under either uniaxial or multiaxial nonproportional loading at high temperature. The proposed model is used to predict multiaxial creep-fatigue life, and a good agreement is demonstrated with experimental data.

Multiaxial fatigue behavior of Ni-based superalloy GH4169 at 650 °C

The fatigue behavior of a Ni-based superalloy GH4169 was investigated using thin tubular specimens under multiaxial loading at 650 °C. Fatigue testing included multiaxial constant amplitude loading and variable amplitude block loading. The test results showed that the characteristics of cyclic hardening or softening depended on the strain loading paths, the sequence of the loading paths, and the loading parameters. Additional hardening due to non-proportional cyclic loading was still present at 650 °C, which caused the fatigue life to decrease. The sequence of the loading paths has an obvious influence on fatigue life, as shown by the increase in loading strain amplitude. Creep effect was observed in the multiaxial fatigue process at high temperature, and an increase in the number of cycles gradually strengthened the creep effect. Fractography showed intergranular fracture phenomena, which reflects the fatigue–creep behavior under non-proportional cyclic loading at high temperature.

Mechanical behaviour of SiC reinforced aluminium thin walled tube under combined axial and torsional loading

Metal matrix composite materials are designed and engineered to exhibit properties that are most desirable for a given application. The properties of the composite will depend upon the particular constituents and volume fraction used, but most MMCs show improvement in stiffness, wear and fracture resistance over the matrix material. The yield and ultimate tensile strength, and the elastic modulus of the material, increased with heat treatment and volume fraction of silicon carbide particles at the expense of ductility. This paper examines the mechanical behaviour of an AlSiC metal matrix composite with 17.8% volume fraction of 3μm SiC particles in a 2124 aluminium matrix. Extruded and heat-treated (T4) thin tubular specimens were used in the experiments. The specimens were subjected to non-proportional tension and torsion loadings. In the first set of experiments, specimens were subjected to an initial angle of twist in the elastic range followed by axial load, maintaining initial angle of twist. In the second set of experiments, specimens were subjected to an initial axial displacement in the elastic range followed by torsion load, maintaining initial axial displacement.

The Art of Embedding Tissue for Frozen Section. Part II: Frozen Block Cryoembedding

This article describes an original technique referred to as frozen block cryoembedding that is used for precision embedding of tissues for frozen section. Two devices are described that aid in this technique. The technique makes possible precise, on-edge embedding, even in the presence of minute samples, thin membranous or flimsy tissues, perforated and torn specimens, rubbery/curling tissues, and/or thin tubular specimens.

Effects of Torsional Buckling on the Cleavage Failure of Low-Alloy Steel Tension Pipe Specimens

The local approach criterion of fracture mechanics, initially developed by Beremin for brittle cleavage fracture, is applied here to A508 class 3 low-alloy ferritic steel. This criterion, based on the maximum principal stress and Weibull statistics, has previously been verified in the case of uniaxial tests. In this study, it is extended to multiaxial loading tests, that can lead to more significant levels of plastic strain, and thus permit a study of the effect of plastic strain on cleavage fracture. Uniaxial tests on axisymmetric notched tensile bars (AE2-6) were used to determine Beremin’s model parameters m and σu. The cleavage fracture behavior, described by these parameters, was then verified by multiaxial tension-torsion tests carried out on thin tubular specimens. Numerical simulations of the tension-torsion tests, by the finite element method, were also performed, taking into account the nonlinear geometrical effects and the specimen plastic buckling. The buckling critical loads were calculated and used to ascertain whether fracture was associated with the instability phenomenon. Beremin’s model is shown to correctly describe experimental data which are not affected by buckling.

疲労過程におけるＣＦＲＰの内部損傷の検討

Development of internal damage evolution in plates and thin tubular specimens of CFRP laminates under static and dynamic loadings are discussed by means of Acoustic Emission measurements and micrographical observations. The mechanical behavior of three kinds of specimens, i.e. undamaged laminate plates [+45°4/-45°4]s, damaged plates [+45°4/-45°4]s subjected to drop-weight impact and undamaged tubular specimens [±45°]4, under quasi-static and fatigue loadings is observed first. Then the mechanism of the resulting inelastic behavior and the change in the mechanical properties are discussed in relation to the evolution of internal damage. Finally the distribution and the evolution of matrix cracks and delamination in the sliced section of the specimens are measured quantitatively in several stages of fatigue process. The dependence of damage distribution on the loading condition is elucidated. Namely, in the case of the stress ratio R=-0.25, the growth of damage zone involving the main crack is localized, and the main crack forms large delamination. On the other hand, for the stress ratio R=0, small cracks are distributed sparsely, but the main crack is not observed until the final stage of the fatigue process.

２軸組合せ負荷におけるＣＦＲＰの損傷面に関する検討

Internal damage evolution of thin tubular specimens of CFRP was studied by means of AE (acoustic emission) measurement. The AE signals were detected first in the tension tests of CFRP tubular specimens of 0°, 90° and ±45° fiber orientation, and three damage modes induced by the tests, i.e., matrix crack, delamination and fiber breakage were identified by the frequency analysis of AE signals. Then, tension-torsion tests of specimens of ±45° were performed, and the behavior of damage development in every damage stage was analyzed from the frequency analysis of AE signals. AE count rate was also measured in order to detect the onset and the development of material damage. The change in the elastic properties were related to the AE count rate. Then, by performing damage tests under biaxial loading, the initial and subsequent damage surfaces were identified by use of AE count rate.

HIGH TEMPERATURE SHORT FATIGUE CRACK BEHAVIOUR IN A STAINLESS STEEL

Abstract— Continuous low cycle fatigue (LCF) tests with‐and without‐hold time in push‐pull and torsion loading modes and sequential LCF tests in push‐pull mode were carried out at 650°C in air on thin tubular specimens of 316 stainless steel; the sequential tests involving pure fatigue (PF) and creep‐fatigue (CF) loadings. The growth of short fatigue cracks was studied by taking several replicas from the specimen surface which were subsequently observed under a scanning electron microscope. An analysis was done with respect to both crack density and the orientation of microcracks and macrocrack(s) which led to failure.Crack density was higher on the surface of a CF tested specimen than that of a PF tested specimen. Mainly short cracks oriented at 45° to the specimen axis were observed on a torsion fatigue tested specimen surface. For push‐pull specimens the microcracks propagated perpendicular to the specimen axis to form macrocracks that propagated in the same direction. On the other hand, for torsion specimens the microcracks which initially propagated at 45° to the specimen axis linked to form macrocracks oriented parallel and perpendicular to the specimen axis. However, the macrocrack responsible for the final fracture was always oriented parallel to the specimen axis.Cumulative damage was dependent on the type of loading (PF or CF) in the first part of sequential tests. In particular microcracks initiated during an initial damage phase observed under sequential LCF tests in PF were found to be healed by oxide formation during the hold times applied in the subsequent CF loading and this produced a total damage summation significantly larger than one.

ＡＥ法による炭素繊維強化薄肉円管材の損傷メカニズムの解明

The internal damage evolution of thin tubular specimens of CFRP was studied by means of AE (acoustic emission) measurement. The AE signals were detected first in the quasi static tests on the CFRP tubular specimens of 0°, 90° and ±45° fiber orientations, and three damage modes induced in the tests, i.e., matrix crack, delamination and fiber breakage were identified by the frequency analysis of AE signals. Then, the fatigue tests on the specimens of ±45° were performed under the particular stress ratios R=0 and R=-0.25 by detecting the AE signals for the whole process of fatigue. By use of the results of the quasi static tests, the behavior of damage development in every stage of fatigue was analyzed from the frequency analysis of AE signals. The AE signal related with matrix crack was observed in the middle of the fatigue process, and the AE signal due to fiber breakage was observed in the later stage of the fatigue. The AE signal due to delamination was mainly observed in the later period of the fatigue under stress ratio R=-0.25. The damage state in some stages of fatigue was also observed by a scanning acoustic microscope by interrupting the fatigue test, and the validity of the identification of damage modes by the frequency analysis was confirmed by comparing the results of AE analysis with those of the internal observation. These results agreed also with the results of other measurement of the macroscopic variation of elastic modulus, as well as with those of damage observation on the sliced section of the specimens.

弾塑性−損傷材料のモデル化に対する実験的検討 多軸組み合せ負荷における損傷面の検討

Applicability and fundamental aspects of the irreversible thermodynamics theory of elastic-plastic-damaging materials are discussed by performing a series of multiaxial damage tests. The changes of Young's modulus, Poisson's ratio and shear modulus under tension and torsion are first observed for thin tubular specimens of spheroidized graphite cast iron. The Acoustic Emission (AE) count rate is also measured in order to detect the onset and the development of material damage. The changes in the elastic properties are related to the AE count rate. Then, by performing damage tests under proportional and nonproportional loading, the initial and subsequent damage surfaces are identified by use of AE count rate. The conditions of loading, unloading and neutral loading for the damage surface are established by the experiments. The influence of the hydrostatic stress on the initial damage surface is found larger than that of subsequent surfaces.

Validation of a statistical criterion for intergranular brittle fracture of a low alloy steel through uniaxial and biaxial (tension-torsion) tests

Criteria initially developed by Beremin (1-5) for brittle cleavage fracture are applied to intergranular brittle fracture of a MnNiMo steel submitted to a temper embrittlement heat treatment. These models which are based on the maximum principal stress as a damage loading parameter and on the Weibull statistics are firstly reviewed and discussed. In particular the effect of plastic strain and of temperature on the intergranular fracture stress is emphasized. Then the results of fracture tests on smooth tensile specimens and on notched bars are used to identify the parameters of the models. Finally these models are applied to multiaxial tension-torsion tests carried out on thin tubular specimens. In these tests the observation of the orientation of the fracture surfaces clearly shows the importance of the maximum principal stress. These tests underline also the importance of one of the basic assumptions of the models which is the necessity of plastic deformation to initiate brittle fracture. In particular the existence in the r-tr plane of two domains for fracture, one controlled by the maximum principal stress, the other by plastic yielding is discussed. The results of further tests in which the material was submitted to a predeformation at room temperature before being tested at -196°C are also presented and used to discuss the effect of plastic deformation on intergranular fracture. In all cases it is shown that the Beremin criteria account very well for the scatter in the results and for the size effect observed in the experiments.

Fatigue and Creep-Fatigue Damage of Austenitic Stainless Steels under Multiaxial Loading

The objectives of the present study are to observe and model physical damage induced by cyclic multiaxial (tension-torsion) loading of 316L stainless steel both at room temperature and at elevated temperature (600 °C). Four types of experiments were carried out on thin tubular specimens: (a) continuous pure fatigue (PF) tests; (b) PF sequential tests with different sequences of push-pull and torsional loading; (c) creep-fatigue (CF) tests with superimposed hold time at maximum tensile strain; and (d) sequential tests involving sequences of PF and CF loadings. Optical microscopy and scanning electron microscopy (SEM) were used to study quantitatively the damage, in particular, to determine the orientation of cracks and to measure the kinetics of crack nucleation and crack growth. It is shown that in pure fatigue at 600 °C, the classical crack initiation stage I is bypassed due to a strong interaction between cyclic plasticity, oxidation, and cracking. Intense slip bands act as diffusional short circuits, leading to the formation of external (Fe2O3) and internal ((FeCr)3O4) oxide scales. The orientation of the microcracks during initiation and propagation stages, which is strongly affected by oxidation effects, explains qualitatively the significant deviations observed in the sequential tests from the Miner linear damage cumulative rule. It is also shown that creep-fatigue damage, which involves intergranular damage, is a complex process rather than a simple superposition of fatigue and creep damage. A stochastic model based on a Monte-Carlo simulation is developed. This model, which accounts very well for the situations in which crack initiation and crack propagation are coplanar, includes damage equations based on quantitative metallographical observations. Damage is modeled as the continuous nucleation of a population of growing cracks which eventually coalesce to lead to final fracture. It is shown that this simulation is able to reproduce with a good accuracy the fatigue lives measured under multiaxial continuous and sequential tests.

Study on Strength and Nonlinear Stress-Strain Response of Plain Woven Glass Fiber Laminates under Biaxial Loading

Strength and stress-strain properties are investigated for plain woven glass fiber laminates using thin tubular specimens under biaxial static and cyclic loadings. Some composites show large distortion and fiber reorientation close to failure under biaxial load ing. For such composites the usage of the 2nd Piola-Kirchhoff stress instead of the nominal stress is proposed for strength evaluation. The experimental result reveals that the Tsai-Wu criterion fits the biaxial strength expressed by the 2nd Piola-Kirchhoff stress much better than the strength expressed by the nominal stress. Strong interaction between axial and shear deformations is found beyond the knee/yield points while, within the elastic range no interaction is found between the axial and shear stiffness. Fiber misalignment with the loading axis affects the observed stress-strain behavior under cyclic loading. More than two S-S curves are distinguishable when the fiber misalignment is higher than 2°. Such stress-strain relations are successfully simulated using an incremental method in conjunc tion with updating fiber orientation. Good agreement between the calculated result and ex perimental one confirms that the proposed model which considers fiber misalignment can explain the observed stress-strain response under biaxial fatigue loading.

Thermal fatigue behavior and dislocation substructures of 316-type austenitic stainless steels

Abstract Results of a study on cyclic behavior and dislocation structures of type AISI 316L austenitic stainless steel arising from cyclic thermal stresses are reported. The test method consists in an ohmic heating device to allow a thin tubular test specimen to serve as its own heater, converting any longitudinal thermal deformation of the specimen into elastic or inelastic deformation. The effects of thermal stress cycling on the cyclic hardening and/or softening behavior and the accompanying microstructural changes in the specimen were evaluated. A cyclic hardening is observed for all temperature changes. This hardening is followed by a continous softening that occupies the major part of total fatigue life. Only for the temperature amplitude of 473–823 K an extended saturation plateau was observed. The dislocation structure formed after failure depends on the temperature amplitude. These structures will evolve from a veins and walls structure for the lowest temperature amplitude to a completely equiaxed cell structure for the highest temperature amplitude.

Fracture toughness of unidirectional graphite fibre reinforced/epoxy composite in mode ii (forward shear), using a thin tubular specimen

The fracture toughness of graphite fibre reinforced unidirectional composite in mode II has been determined using a thin tubular specimen, by the method developed by Giare and Campbell, accepted for publication in Engng Fracture Mech. The crack growth resistance at instability and the corresponding initial strain energy release rates were independent of the initial crack in the range of the crack length investigated. G IIC obtained by thin tubular torsion specimen is 42% greater as compared to end-cracked beam method. The end-cracked beam method underestimated the G IIC as reported by J. W. Gillespie Jr, L. A. Carlson and R. B. Pipes [ Compos. Sci. Technol. 27, 177–197 (1986)]. They presented a finite element analysis of the End-Notched Flexure test specimen for mode II interlaminar fracture testing of composite materials. They predicted, using finite elements, that data reduction schemes based upon beam theory underestimate G IIC by approximately 20–40% for typical graphite fibre composite test specimen. It is very encouraging that our results follow very close to their prediction. It is concluded that the thin tubular torsion specimen method is accurate as compared to the end cracked beam method. G IIC of graphite fibre reinforced epoxy composite is found to be half of the glass fibre reinforced/epoxy composites, for practically the same V t of fibres.

A method to determine the fracture toughness of unidirectional fibre reinforced composites in mode II (forward shear), using a thin tubular specimen

A method has been developed, using thin tubular specimens, to determine the fracture toughness of unidirectional fibre reinforced composites in Mode II. The tubular specimens were loaded under torsion and hence produced pure shear at the crack tips located on the circumference of the tube. The cracks were made parallel to the transverse axis and in the mid-length of the tube. Calibration factors for Mode II were obtained. The stress-intensity factors at instability, K IIR(INS) were obtained by experiments on thin tubular specimens through a compliance matching procedure. The crack growth resistance at instability and the corresponding initial strain energy release rates were independent of the initial crack in the range of crack length investigated. The stress-intensity factor obtained by the thin torsion tube method is slightly higher than the stress-intensity factor at instability, K IIR(INS) obtained by the method developed by Giare for end cracked beams [ Engng. Fracture Mech. 20, 11–21 (1984)]. This method may be applied to a different geometrical shapes and hence may be useful in determining the fracture toughness of any closed geometrical sections.

Immediate and delayed thermal expansion of hardened cement paste

Experiments are reported in which the thermal deformations were measured of thin tubular specimens of hcp conditioned to various relative humidities between 0 and 100%. Attention was concentrated on the first cycle of heating and cooling and on a second heating; the level of temperature was also an experimental variable. It was found that the thermal movement could be considered as the sum of two deformations, the immediate (taking place during the temperature change) and the delayed (taking place after temperature equilibrium was reached). The immediate deformation was approximately the same for all three temperature changes, was approximately linear with temperature, and exhibited a maximum value at an intermediate conditioning humidity towards 100%. The delayed deformation had a smaller reversible component, and a larger irreversible component which was associated only with the first heating. With only a few exceptions all the measurements of delayed deformation were of the same sense as the preceding immediate deformation. Measurements were made of two distinct types of evaporable water. There were no significant changes in the quantities of either as a result of temperature cycling.

Plastic Flow of Mild Steel Under Proportional and Non-Proportional Straining at a Controlled Rate

In the present investigation, thin tubular specimens of annealed mild steel (En8) were tested under combined straining, using a newly developed tension-torsion, closed-loop, servo-controlled, electro-hydraulic testing machine. Two types of straining program were used, proportional straining at constant strain rate and nonproportional straining in which the specimen was loaded to yield in pure torsion and then extended with the twist held constant. The resulting load and torque components were continuously recorded during each test, as functions of time. A remarkable feature of the second type of test is that it was possible to obtain almost the whole positive quadrant of the initial yield locus of this material from a single test, since the load-torque trajectory followed closely the yield locus. Both proportional and nonproportional straining paths gave results in good agreement with the von Mises yield condition. The work included two theoretical analyses of the present problem, one using Perzyna’s constitutive laws for rate-sensitive but nonworkhardening material and the other using a rate-independent theory. Comparison with the experimental results of the nonproportional straining program indicates that the time history of the neutral loading path was reasonably well matched by the prediction of both the rate-dependent and rate-independent theories. Some preliminary experimental investigations of the hardening behavior in the proportional paths are also reported.