Small Compact Tension Research Articles

Measurement and analysis of fatigue crack deformation on the macro- and micro-scale

The paper describes an experiment which performs in-situ loading of a small compact tension specimen in a scanning electron microscope. Images are collected throughout a number of successive loadincg cycles. These are then analysed using digital image correlation (DIC) in order to produce crack flank displacements as a function of load. This data is then compared with a simple elastic approach, and it is concluded that elastic-plastic analysis is required in order to accurately capture the displacements close to the crack tip. A simple approach due to Pommier and Hamam is therefore employed. This gives a better representation of the data, but predicts a variation of crack tip displacement, ?, which is difficult to explain from a physical perspective. The need for a more sophisticated analysis of the data is therefore highlighted.

Effects of temperature on stress corrosion cracking behavior of stainless steel and outer oxide distribution in cracks due to exposure to high-temperature water containing hydrogen peroxide

Cracking growth tests were conducted in high-temperature water containing hydrogen peroxide (H2O2) at 561–423K to evaluate the effects of H2O2 on stress corrosion cracking (SCC) of stainless steel (SS) at temperature lower than the boiling water reactor (BWR) operating temperature. Small compact tension (CT) specimens were prepared from thermally sensitized type 304SS. Despite the observation of only a small portion intergranular SCC (IGSCC) near the side groove of the CT specimen at 561K in high-temperature water containing 100ppb H2O2, the IGSCC area expanded to the central region of the CT specimens at 423 and 453K. Effects of H2O2 on SCC appeared intensely at temperature lower than the BWR operating temperature because of a reduction in the thermal decomposition of H2O2. To estimate the environment in the cracks, outer oxide distribution on the fracture surface and the fatigue pre-crack were examined by laser Raman spectroscopy and thermal equilibrium calculation was performed.

Morphology of stress corrosion cracking due to exposure to high-temperature water containing hydrogen peroxide in stainless steel specimens with different crevice lengths

Crack growth tests were performed in high-temperature water containing hydrogen peroxide (H2O2) to evaluate the relationships between the crevice structure and H2O2 on stress corrosion cracking (SCC) growth morphology of stainless steel (SS). Small compact tension (CT) specimens were prepared from thermally sensitized type 304 SS. 20–300ppb H2O2 was injected into the high-temperature water at 561K. Intergranular SCC (IGSCC) and transgranular SCC were observed near the side grooves and the central region of the original CT specimens, respectively. Chevron notches were removed from the CT specimens after fatigue pre-crack introduction. Owing to pre-crack shortening, the IGSCC area expanded to the central region of the CT specimens and increased with H2O2 concentration. The effects of H2O2 on SCC appeared intensely near the surfaces exposed to high levels of H2O2. Microanalysis and distribution examination of oxide layers were performed and the percentage of H2O2 remaining in the crack was calculated.

Fracture toughness characterization of JLF-1 steel after irradiation in HFIR to 5 dpa

Fracture toughness specimens of the ferritic–martensitic steel JLF-1 were investigated before and after irradiation at two different temperatures in the High Flux Isotope Reactor. Small (12.5 mm in diameter with thickness of 4.6 mm) disk-shaped compact tension specimens were irradiated at average temperatures of ∼250 °C and 377 °C to ∼4 dpa. Small, 3.33 × 3.33 × 25 mm, pre-cracked Charpy specimens were irradiated at ∼300 °C and 500 °C to 5 dpa. Transition fracture toughness was evaluated in terms of the reference temperature T 0 for each irradiation temperature and dose and compared to unirradiated T 0. Current fracture toughness shifts compared with T 0 shifts of F82H and 9Cr2WVTa steels irradiated at similar conditions. The present results show that JLF-1, F82H, and 9Cr–2WVTa steels have very similar resistance to radiation embrittlement after doses of 4–5 dpa in the temperature range from 250 °C to 500 °C.

MicroCT (microtomography) quantification of microstructure related to macroscopic behaviour: Part 1 – Fatigue crack closure measured in situ in AA 2090 compact tension samples

Microcomputed tomography (microCT) quantified crack opening in situ in a small compact tension specimen (AA 2090 T8E41) as a function of 3D position and of applied load. The same specimen was observed twice at two crack lengths. Some crack face areas were dominated by large asperities, while others were relatively flat. Large openings followed the sides of large asperities, with small openings elsewhere. The crack path and pattern of openings are discussed in terms of the variation in mesotexture. Before and after the crack extension, the pattern of open (and closed) voxels did not appear to change, suggesting that contact at a point near the crack tip will continue at the same portion of the fatigue cycle after the tip has receded. Testing parameters and macroscopic closure measurements link the present microCT measurements to studies in the literature.

Phase contrast stereometry: fatigue crack mapping in three dimensions

The work reported below describes development of a novel method, termed X-ray phase contrast stereometry, for reconstructing the three-dimensional (3D) positions of features within a specimen. The approach takes measured positions of a given feature in phase microradiographs recorded at different viewing angles and refines that feature's position numerically. The approach is designed to address limitations in conventional laboratory or synchrotron X-ray absorption micro computed tomography (microCT), particularly those inherent in studying plate-like samples. The phase stereometric reconstruction technique was applied to a small compact tension sample of aluminium alloy AA 2090 (20.3 mm wide from notch tip to back face and 2.7 mm thick) containing a fatigue crack and was used to map the 3D crack surface non-invasively. Up to ten viewing angles were used for each of 2269 points mapped on the approximately 2.7 mm × 6.0 mm surface. The 3D stereometry-derived crack surface agreed with the surface map from absorption microCT of the same sample. The phase technique was superior in terms of its crack sensitivity; the phase images allowed the crack to be followed to its tip, even with zero applied load while absorption microCT could only detect about 5 mm of the crack 6 mm long when loaded to 40 kgf. The origin of the contrast used for stereometric mapping was confirmed to be from the peaks on the crack face; phase microradiographs from a second (intact) sample showed a pattern of features identical with those revealed on the surface when the sample was split open. Crack opening as a function of in-situ load was measured for the same points on the crack surface using both phase stereometry and absorption microCT; the openings were in good agreement.

Effect of High Temperature Air-Exposure on Room Temperature Fatigue Crack Growth of Ni-Base Single Crystal Superalloy CMSX-10

Room temperature fatigue crack growth behaviour of un-exposed(as-received) and high temperature air-exposed(at 1100°C for 24 to 1000 h) CMSX-10 was investigated. A small compact tension specimen with [001] loading direction and [110] fatigue crack growth direction was used. The fatigue crack growth results were analyzed using a stress intensity parameter K . It is shown that the air-exposure has no deteriorative effect on the crack growth behaviour, even though the microstructure has changed by the exposure(coarsening of γ' ). The crystallographic facet fracture of (1-11) and (-111) planes appeared and the intersection of the planes forms sets of lines(ridge and valley) parallel to [110] direction. The fatigue crack tip front is schematically shown.

Effects of high-temperature hydrogen exposure on room temperature tensile properties and fatigue crack growth behaviour of TiAl base alloy

Tensile, notched tensile, and fatigue crack growth tests of hydrogen-exposed (in 0.1 MPa 99.9995% H 2 at 800 °C for 6 and 24 h) TiAl base alloy were carried out in ambient air. The results are compared with those of the material before exposure. By the H 2 exposure, the tensile ductility and tensile strength were reduced significantly, and greater reduction in the strength of the notched specimen was observed. The H 2-exposed specimen fractured in the elastic deformation range, while the non-exposed specimen deformed plastically before fracture. A small compact tension specimen (CT) was used to correlate the fatigue crack growth rate d a/d N and stress intensity factor range Δ K. In the near-threshold region of the fatigue crack growth diagram, the H 2-exposure had no effect on d a/d N. With increasing Δ K, d a/d N of the H 2-exposed material increased to about ten times higher than before exposure. The acceleration is due to the hydrogen embrittlement (reduction in ductility).

Constitutive behavior and fracture toughness properties of the F82H ferritic/martensitic steel

A detailed investigation of the constitutive behavior of the International Energy Agency (IEA) program heat of 8 Cr unirradiated F82H ferritic–martensitic steel has been undertaken in the temperature range of 80–723 K. The overall tensile flow stress is decomposed into temperature-dependent and athermal yield stress contributions plus a mildly temperature-dependent strain-hardening component. The fitting forms are based on a phenomenological dislocation mechanics model. This formulation provides a more accurate and physically based representation of the flow stress as a function of the key variables of test temperature, strain and stain rate compared to simple power law treatments. Fracture toughness measurements from small compact tension specimens are also reported and analyzed in terms of a critical stress–critical area local fracture model.

Effects of Initial Crack Length and Specimen Thickness on Fracture Toughness of Compact Bone.

This paper deals with the determination of fracture mechanics parameters, such as critical stress intensity factor, Kc, and critical strain energy release rate, Gc, of compact bone. Haversian bone samples taken from fresh bovine femurs were machined into small compact tension (CT) specimens with various initial crack lengths and thicknesses, and then tested under quasi-static mode I loading condition at room temperature. Factor Kc was determined following the ASTM standard E399-90, and the compliance method was used for the determination of Gc. The values of Kc and Gc were larger in the specimens with transversely oriented initial cracks than in those with longitudinally oriented initial cracks. Both Kc and Gc were dependent on initial crack length as well as specimen thickness, and these fracture characteristics could be interpreted by a hypothesis based on the observation of damage accumulation, or microcracking ahead of the main crack front.

Liquid metal embrittlement of Zr-2.5% Nb alloy

Liquid metal embrittlement (LME) of zirconium alloys in mercury is entirely transgranular. Grains not oriented forpseudo-cleavage on the basal plane fail by slip on the prism planes to form the characteristic ‘fluting’. Because of this, fracture in Hg of Zr-2.5 Nb small compact tension specimens (cut from five different CANDU pressure tubes) showed large differences in the macroscopic fracture topography that could be related to differences in the local continuity of the basal planes in the structure, even though the average textures were very similar. These differences could be related to differences in the fracture toughness of the tubes. Although LME in zirconium alloys has been thought to be generally transgranular because of previous experience in Hg, Cs/Cd mixtures, a Hg/In/TI eutectic, and Li, tests in Ga showed no transgranular cracking, but small amounts of intergranular cracking were observed that appeared to be limited by poor wetting of Zr by Ga.

Fatigue crack propagation in residual stress fields of welded plates

This paper presents some recent results of a research programme on the evaluation of the fracture mechanics properties, principally the crack propagation rate, of plasma welded aluminium joints, 2219-T851. Crack propagation tests were carried out on large centre cracked tension (CCT) specimens and on small compact tension (CT) specimens. The main difference between the two typs of specimens concerned the residual stresses, which were not present in the CT specimens, due to their reduced dimensions. The residual stresses were measured in cracked and uncracked plates, by means of strain gauges. The results of the crack propagation tests carried out on the two types of specimens were fully comparable in the case of a crack which propagates along the weld bead, in the heat-affected zone, as the residual stress effect was quite low, while the crack propagation rate measured in the CT specimens was lower than that in the CCT specimens in the case of a crack which propagates perpendicularly to the weld bead. Residual stress measurements allowed the evaluation of the residual stress effects on the crack propagation rate and close agreement was found between the results of the two set of tests.

High ResolutionR-Curve Characterization of the Fracture Toughness of Thin Sheet Aluminum Alloys

AbstractThe plane-strain initiation fracture toughness and plane-stress stable crack growth resistance were determined with a single small compact tension (C(T)) specimen for each of three precipitation hardened aluminum alloy sheets (AA2024-T3, AA2519-T87 (+Mg+Ag), and AA2650-T6). Crack length was monitored precisely with direct current potential difference (DCPD) measurements, and specimen plasticity was accounted for with the J-integral. The DCPD technique resolves a small amount of crack-tip process-zone damage (≈20 µm) that constitutes crack initiation under plane-strain constraint. Two measures of initiation toughness are calculated: the elastic-plastic fracture toughness detected by DCPD (JICi, KJICi) and the toughness based on ASTM Standard E 813 () and the toughness based on ASTM Standard E 813 (JIC, KJIC). High resolution of fracture initiation is necessary to obtain a lower bound initiation toughness,). High resolution of fracture initiation is necessary to obtain a lower bound initiation toughness, KJICi, because plane-strain constraint is present ahead of the fatigue precrack but is rapidly lost with crack extension in thin sheet. KJIC overestimates toughness due to constraint loss coupled with the offset blunting line definition of fracture initiation. The J-integral/DCPD method provides a reproducible measure of the plane-stress linear-elastic resistance curve (KJ − Δa) that compares reasonably to R-curves determined for large middle-cracked tension specimens. The small specimen method is effective for studies pertaining to alloy development, environmental effects, and fracture mechanisms.

The studies on fracture toughness of nuclear pressure vessel steels by using a small round compact tension specimen and a comparison with a Charpy-size specimen

This paper presents a single-specimen method of evaluating the fracture toughness of nuclear pressure vessel steels by using a small round compact tension (RCT) specimen with deep side-grooves. The method has been used to estimate the fracture toughness parameters of the nuclear pressure vessel steels A508CL3-A, A508CL3-B and A508CL3-A′ produced by two plants. A comparison of the measured fracture toughness results has been done among the small RCT specimen, the Charpy-size specimen and the large specimen, which GB2038 and ASTM E813 require. The research results indicate that the fracture toughness values obtained from the RCT specimens with the double-size deep side-grooves have little difference when compared with the valid fracture toughness results. Finally, this paper discusses the possibility of using the RCT specimens in nuclear pressure vessel steel embrittlement surveillance.

Corrosion fatigue testing of eight different steels in an Icelandic geothermal environment : Thorbjornsson, I. Mater. Des. (1995) 16 (2), 97–102

This paper presents some recent results of a research programme on the evaluation of the fracture mechanics properties, principally the crack propagation rate, of plasma welded aluminium joints, 2219-T851. Crack propagation tests were carried out on large centre cracked tension (CCT) specimens and on small compact tension (CT) specimens. The main difference between the two typs of specimens concerned the residual stresses, which were not present in the CT specimens, due to their reduced dimensions. The residual stresses were measured in cracked and uncracked plates, by means of strain gauges.The results of the crack propagation tests carried out on the two types of specimens were fully comparable in the case of a crack which propagates along the weld bead, in the heat-affected zone, as the residual stress effect was quite low, while the crack propagation rate measured in the CT specimens was lower than that in the CCT specimens in the case of a crack which propagates perpendicularly to the weld bead. Residual stress measurements allowed the evaluation of the residual stress effects on the crack propagation rate and close agreement was found between the results of the two set of tests.

Micromechanical toughness test applied to NiAl

A microscratch technique for determining the fracture toughness of brittle intermetallics is presented. Advantages include very small sample sizes and multiple tests on a single, flat polished surface. The fracture toughnesses of single-crystal NiAl in {001}〈100〉, {001}〈110〉, {110}〈110〉, and {110}〈100〉 crack orientations were evaluated using this technique. The results of the scratch testing are compared with measurements on small compact tension samples. Also, transmission electron microscopy is used to help explain the dislocation character of the resulting plastic zones in samples tested. Lastly, a model for determining the dislocation velocity during nanoindentation is presented to help understand the ductile-brittle transition temperature of semibrittle materials.

An experimental and numerical investigation of ductile crack growth characteristics in surface cracked specimens under combined loading

Fracture mechanics tests have been performed in the upper shelf region of a steel on large surface cracked plate (SCT) specimens and on small compact tension (CT) specimens. Some of the SCT specimens were subjected to combined tension and bending in such a way that the loading was strongly non-proportional. Crack growth characteristics were compared between the specimens in order to assess possible influences of geometry and nonproportional loading. The differences observed could unambiguously be reduced to and correlated with differences in constraint. Constraint parameters were evaluated by detailed three-dimensional finite element computations and quantities for growing cracks were interpreted on the basis of deformation theory considerations, in analogy with the commonly used JR-philosophy. It was noted that the initiation of ductile crack growth along a three-dimensional crack front appears to be independent of the degree of local constraint. However, the increase in toughness for a growing crack was markedly affected by the degree of local constraint. Some estimates of the constraints effects regarding stability considerations were also made.

Small specimen compact tension testing of ceramics

The small size compact tensile specimen is useful for measurement of the fracture toughness of ceramics. The tabular data available for the compliance factor are accurately fitted by a polynominal expression. The elimination of residual stress is important for accurate measurement of the fracture toughness. [on SciFinder (R)]

Does the Process Zone Control Crack Growth?

Fracture toughness tests were carried out by using small compact tension specimens of Westerly granite. Relation between the load-line displacement and the initial crack-tip separation changed from linear to nonlinear and then to linear again with increasing loading. The onset of the main crack growth was found to occur in a loading level at which the relation was nonlinear. This result disclosed that the onset of main crack growth in granite occurred without experience of the complete area under the tension-softening curve. The critical value of the initial crack-tip separation for onset of the main crack growth was obtained independently of the initial notch length. Also monitoring of the growth of the fracture process zone by ultrasonic technique showed that its length related with the crack-tip separation independently of the initial notch length. By combining the results just mentioned, it was concluded that the fracture process zone controlled the onset of the main crack growth.

Analyses of circumferential and axial flaws of a pressure vessel under PTS loads and failure estimate on the basis of irradiated RCT test specimens

Initiation and slow propagation of cracks in a pressure vessel loaded by severe PTS-transients were studied. J values were calculated using linear-elastic and elastic-plastic material properties. The calculated J values were compared with the J–R curves of the irradiated Round compact tension (RCT)-test specimens at upper shelf temperature. Cladding and weld seam residual stresses were taken into account in the calculations. For short three-dimensional surface cracks, the influence of cladding depends upon the shape of the crack. In the case of a deep three-dimensional crack, very high J values exist in the cladding and at the cladding/base metal interface. For shallow surface cracks, the influence of cladding is not so significant at this interface. The effect of cladding and its residual stresses was also studied in the laboratory using small cladded compact tension (CT) specimens. The first specimens were made of carbon steel and cladded in two layers. Finite element calculations were performed in order to study the J-integral distribution in these CT specimens. Exact fatigue crack profile and cladding residual stresses were taken into account in these calculations. The shape of the calculated J-integral distribution resembled the observed profile of the crack propagation front.