W. R. Tyson

Ring Hoop Tension Test (RHTT): A Test for Transverse Tensile Properties of Tubular Materials

Tube hydroforming has been receiving increasing attention, particularly for making automotive components. In this process, tubular metal pats are formed within a die cavity by forcing pressurized fluid inside so that the part expands into its final shape in a die. Process control requires an improved knowledge of mechanical properties in the transverse direction of tubular products. Several laboratory tests have been used to evaluate tube properties. One test method that has been used for zirconium alloy cladding tubes in the nuclear industry is the ring test, in which an expansion force is applied from inside the ring by separating two die inserts. In the present work, this test method is adapted to determine the hoop stress-strain curve of tubular materials. The new protocol allows measurement of the tensile properties without altering those properties in specimen preparation, which occurs using the conventional flattened strip tensile test. This paper presents the new test and data analysis procedure, along with typical data for a commercial steel hydroforming tube.

Crack Size Evaluation Using Unloading Compliance in Single-Specimen Single-Edge-Notched Tension Fracture Toughness Testing

Crack-mouth-opening displacement (CMOD) unloading compliances of single-edge-notched tension (SE(T)) specimens, both pin-loaded and clamped, have been evaluated by finite element analysis (FEA), including evaluation of the effect of rotation during testing on compliance. For pin-loaded specimens, FEA results are compared with compliance correction equations based on a rigid-body rotation model recommended by Joyce and Link. The results agree reasonably well when the load line is far from the midsection of the ligament. However, the discrepancy increases when the load line approaches the midsection of the ligament where the bending load becomes small or negligible compared with the tension component and the rigid-rotation model breaks down. For clamped SE(T) specimens, factors affecting specimen rotation, such as the ratio H/W of span between load points H to width of the specimen W, normalized crack size, a/W, and material strain hardening coefficient, N, were investigated. This work was performed for specific application to surface circumferential cracks in pipes, for which the best constraint matching has been found to occur for clamped specimens with H/W = 10. For this geometry, an equation is proposed to correct compliance for rotation and to estimate crack size from rotation-corrected CMOD compliance. Crack sizes evaluated using this equation are compared with measurements on broken specimens. Excellent agreement has been achieved for initial and final crack sizes of both plain-sided and side-grooved SE(T) specimens.

Fracture Toughness Evaluation of High Strength Steel Pipe

Stress fields and constraint parameters (Q and A2 ) of circumferentially-cracked high strength pipe in displacement-controlled tension are compared with those of small-scale single-edge notched samples tested in tension (SE(T)) and bending (SE(B)). The factors affecting transferability of fracture toughness (J-resistance) data from small-scale laboratory tests to cracked high strength pipe are discussed. The crack-tip stress field is of similar form for a circumferential crack in a pipe and a SE(T) test specimen, while for a SE(B) specimen there is a significant gradient in the crack-tip stress field. Hence, the fracture toughness can be characterized by only two parameters (J and Q or J and A2 ) for tension-loaded pipe and SE(T) tests, but for SE(B) tests one more parameter is needed to describe the bending term. It is concluded that the constraint in a SE(T) test with ratio of span between load points to width H/W = 10 provides a reasonable match to that for a circumferential crack in a pipe subjected to tensile loading.© 2008 ASME

Evaluation of Fracture Toughness of X100 Pipe Steel Using SE(B) and Clamped SE(T) Single Specimens

J-resistance testing using a single-specimen unloading compliance technique has been performed on single-edge-notched tension (SE(T)) specimens of X100 pipe steel base material at room temperature and at −20°C, using a procedure developed at CANMET. J-resistance testing using single-edge-notched bend (SE(B)) specimens according to ASTM E1820 was also conducted for comparison. The specimens included two nominal through-thickness pre-crack aspect ratios (a/W = 0.25 and 0.5). The results show that shallow-cracked (a/W∼0.25) bend and tension specimens produce higher resistance curves than deeply-cracked (a/W∼0.5) specimens; ductile propagation was observed at both temperatures. Resistance curves are slightly higher at −20°C than at room temperature for both bending and tension, especially for shallow-cracked specimens. Crack length predicted from unloading compliance of crack mouth opening displacement for the SE(T) specimens was validated by optical measurement of initial crack length (ao ) and final crack extension (Δa>1.0 mm) after heat-tinting, as per ASTM E1820. Predicted crack growths show acceptable agreement with measured values in all cases. The effect of side-groove depth on the resistance curve and straightness of the crack front was briefly investigated. For both bending and tension, resistance curves for 10% (total) side-grooved specimens were close to those from plain-sided specimens when other testing conditions, such as precrack and testing temperature, were the same, whereas 20% (total) side-grooved specimens showed lower toughness. It was occasionally observed that the crack grew faster at the side for 20% side-grooved bend and tension specimens, resulting in a crack front of concave curvature. For 10% side-grooved specimens a rather straight crack front or slightly faster crack growth in the middle of the specimen (convex curvature) was observed.Copyright

Low-Constraint Toughness Testing: Results of a Round Robin on a Draft SE(T) Test Procedure

Assessment of the effect of girth weld flaws on pipeline integrity requires knowledge of a number of factors: pipe geometry, applied loads, flaw size, and pipe mechanical properties. Of the latter, strength and toughness are the primary factors. Toughness has conventionally been measured using specimens tested in bending to maximize constraint. While this gives a conservative estimate of toughness, it would be better to use a test that would reveal the toughness in constraint conditions typical of girth weld flaws: namely, relatively shallow flaws loaded in tension. Consequently, there is a trend to evaluate toughness using pre-cracked single-edge-cracked tension (i.e. SE(T)) specimens, and one procedure has already been standardized. However, this procedure requires the use of multiple specimens to generate a resistance curve. With the objective of devising a more economical test, a single-specimen procedure has been developed at CANMET. The viability of this procedure has been assessed by means of a round robin involving test and research laboratories from around the world. In this presentation, the results of the round robin will be presented and discussed.Copyright

Effect of Side Grooves on Compliance, J-Integral and Constraint of a Clamped SE(T) Specimen

The effect of side grooves on crack mouth opening displacement (CMOD) compliance, distribution of J-integral and crack-tip constraint parameters Q and A2 along the thickness of a clamped single-edge-notched tension (SE(T)) specimen were studied by finite element analysis (FEA). Focus was on the effect of depth of side grooves on J-integral and constraint parameters Q and A2 for shallow and deep cracks. The 3-D results were compared with those of SE(T) specimens in plane strain. The results show that the effective thickness equation used in ASTM E 1820 to evaluate compliance of side-grooved SE(B) and C(T) specimens can be used for clamped SE(T) specimens with reasonable accuracy. The results also suggest that the depth of the side grooves affects the distribution of the J-integral: the highest J-integral is at the center of the thickness for a SE(T) specimen with side grooves equal to or less than 10% of total thickness, and near the root of the side grooves for side grooves greater than 10% for a deeply-cracked specimen when the applied load P≥PY . The FEA results also show that the depth of side grooves affects the distribution of the constraint parameters: the crack-tip constraint is highest at the center of the thickness for a specimen with 0% side grooves (plain-sided), and near the root of the side grooves for side grooves equal to or greater than 10%. It was also found from FEA that the crack-tip constraint of a SE(T) specimen with 20% side grooves with shallow (a/W = 0.2) or deep (a/W = 0.5) crack is higher than that of a SE(T) specimen with the same crack depth in plane strain. As a result, the J-resistance of a SE(T) specimen with 20% side grooves may be lower than that of the same specimen in plane strain.Copyright

J-Resistance Results From Multi-Specimen and Single-Specimen Surface Notched SEN(T) Geometry

In a collaborative program, J-R results were compared from multi-specimen and single-specimen procedures for toughness testing with single-edge notched tensile loaded SEN(T) specimens. Grade 483 MPa (X70) grade pipeline steel was used to prepare surface notched axially loaded specimens. Test procedures followed the multi-specimen method and a single-specimen recommended practice recently developed at the CANMET Materials Technology Laboratory (recently name-changed to CanmetMATERIALS) in a program jointly funded by PERD, PRCI, and DOT. The multi-specimen method adopted side grooves with the objective of comparison with results from the single–specimen method. The specimen geometry was B × B for both test procedures. The target ao/W was about 0.5.The multi-specimen testing was performed at BMT Fleet Technology. The single-specimen testing was performed at the CANMET laboratory, formerly located in Ottawa.The paper describes experimental and analysis details, and compares results from the two techniques, using J expressions developed at CANMET. The results showed similar J-resistance curves at small crack extensions for both techniques, and higher J-resistance values at larger crack extensions for the multi-specimen method. The results are discussed in terms of initial crack length and the analysis methods adopted for the two techniques. Validity criteria according to ASTM E1820 were applied and the results are presented.Copyright

Numerical Simulation of Ductile Crack Growth in Pipeline Steels

This paper presents numerical studies on stable crack extension of high toughness gas pipeline steels (X80) using the 2D and 3D computational cell approach. The Gurson-Tvergaard dilatant plasticity model for voided materials is used to describe the degradation of material stress capacity. Fixed-size, computational cell elements defined over a thin layer at the crack plane provide an explicit length scale for the continuum damage process. Outside this layer, the material is modeled as undamaged by void growth. The key micro-mechanics parameters are D, the thickness of the computational cell layer, and ƒ0 , the initial cell porosity. Calibration of these parameters is conducted using analysis of ductile tearing from testing of Charpy-sized bending specimens. The resulting computational model enables the study of effects on crack growth of specimen size, geometry and loading mode. Computational and experimental studies are described for shallow and deep DWTT (drop weight tear test) specimens under quasi-static loading conditions.Copyright

Effective Modulus for Crack Size Measurement With SE(T) Specimens using Unloading Compliance

Low-constraint toughness tests have been increasingly used in pipeline flaw assessment. A number of protocols have been published using a test employing a clamped single-edge tension (SE(T)) specimen with the distance between the clamps H equal to ten times the specimen width W, and specimen width W equal to thickness B. For this geometry, the constraint over the bulk of the specimen varies between plane stress and plane strain. This affects the elastic compliance, commonly used to estimate crack size. It was the intent of the present work to assess the effects of constraint on the compliance and to identify the best combination of compliance equation and modulus to estimate crack size from CMOD compliance where CMOD is the crack mouth opening displacement. To do this, values of compliance from finite element analyses were provided by the authors of this paper from three separate laboratories. 2D plane strain, 3D plane-sided, and 3D side-grooved specimens were analyzed. The data were used to assess available compliance equations and to propose a new one for the specific geometry of interest in this work.

Flow Behaviour and Ductile Fracture Toughness of a High Toughness Steel

This paper reports results of tests on flow and ductile fracture of a very high toughness steel with Charpy V-notch absorbed energy (CVN energy) at room temperature of 471 J. The microstructure of the steel is bainite/ferrite and its strength is equivalent to X80 grade. The flow stress was determined using tensile tests at temperatures between 150°C and −147°C and strain rates of 0.00075, 0.02 and 1 s−1 , and was fitted to a proposed constitutive equation. Charpy tests were carried out at an initial impact velocity of 5.1 ms−1 using drop-weight machines (maximum capacity of 842 J and 4029 J). The samples were not broken during the test, i.e. they passed through the anvils after significant bending deformation with only limited crack growth. Most of the absorbed energy was due to deformation. There was little effect of excess energy on absorbed energy up to 80% of machine capacity (i.e. the validity limit of ASTM E 23). As an alternative to the CVN energy, the crack tip opening angle (CTOA) measured using the drop-weight tear test (DWTT) has been proposed as a material parameter to characterize crack propagation resistance. Preliminary work on evaluating CTOA using the two-specimen CTOA test method is presented. The initiation energy is eliminated by using statically precracked test specimens. Account is taken of the geometry change of the specimens (e.g. thickening under the hammer) on the rotation factor and of the effect of strain rate on flow stress.Copyright