Stable Ductile Tearing Research Articles

On the Remarkable Fracture Toughness of 90 to 97W-NiFe Alloys Revealing Powerful New Ductile Phase Toughening Mechanisms

Tungsten is generally too brittle to serve a robust structural function. Here, we explore the fracture toughness of 90 to 97%W (by wt.) liquid phase sintered tungsten heavy alloys (WHAs). The room temperature (RT) maximum load fracture toughness (KIc or KJm ≈ 38 to 107 MPa√m) of WHA, containing only 3 to 10 wt.% of a NiFe-based ductile phase (DP), is ≈ 5 to 13 times higher than KIc typical of monolithic W (≈ 8 MPa√m). RT tests for a range of precracked bend bar sizes generally show extensive stable ductile tearing (DT), except in the case of the 97 wt.%W alloy, where elastic fracture occurs in all but the smallest specimen tests. Nevertheless, even in this case the KIc = 38 ± 4 MPa√m, which is still almost 5 times higher than that for monolithic W. Tests with the smallest specimens down to -196°C, to partially emulate irradiation hardening, show decreasing toughness and a transition to elastic fracture at a temperature of -150°C for 90W to -25oC for 97W. However, even at -196°C, the 97W KIc is ≈ 3 times that of monolithic W at RT. In contrast to classical ductile phase toughening by macrocrack bridging, WHA toughening mainly involves new mechanisms associated with arrest, blunting and bridging of numerous dilatationally shielding process zone microcracks.

Influence of Loading Rate on the Fracture Toughness of High Strength Structural Steel

It is known that rates of loading influence the fracture behaviour of most ferritic steels. High loading rates could change a stable ductile tearing behaviour to an unstable brittle fracture by altering the ductile-to-brittle transition curve. This is predicted to be material dependent, with lower strength structural steels showing a larger tensile property loading rate sensitivity compared to high strength structural steels. A programme of mechanical testing was carried out on S690QL and S960QL to determine the influence of loading rate on the fracture behaviour of high strength structural steels with yield strength > 690 MPa and yield-to-tensile ratio above 0.90. The loading rates considered are those anticipated in offshore in-service conditions, with K-rates up to the order of magnitude of 106 MPa√m/s. Results from tensile tests show that the strengths of these grade of steels are relatively unaffected by the effect of loading rate. However, brittle fracture, which is controlled by material strengthening as a result of principal stress in front of the crack, is both loading rate and temperature dependent. Results from tests at quasi-static and elevated loading rates show changes in the fracture behaviour in terms of transition temperature. A shift to a higher ductile-to-brittle transition temperature was observed as the loading rate increases. This was associated with a reduction in the fracture toughness value on the lower transition region. The reference temperature, T0, at a K-rate of 1 MPa √m/s using Master Curve concepts is estimated to be around -116 °C and -108 °C for Charpy-sized pre-cracked and standard (25x25 mm) SENB specimens respectively, under quasi-static conditions for S690QL. The dynamic T0,d is -70.4 °C in the same steel for Charpy-sized pre-cracked specimens at K-rates up to 106 MPa √m/s.

Determination of GTN Damage Parameters for Application to Pipe Ductile Fracture Simulation

A number of works have been reported in the literature up to present on finite element ductile failure analysis. Depending on the model employed for damage simulating, the Gurson-Tvergaard-Needleman model can be broadly used. That model is using a micro-mechanical model for ductile fracture, incorporating void nucleation, growth and coalescence.As many researchers have already published a number of papers using these methods, applicability and validity of these methods have been well discussed in the literature. In this paper, to simulate long, stable ductile tearing in large-scale pipes using finite element analysis by GTN model. The GTN damage model and associate parameters are determined from fracture toughness test.

Static Strength of Cracked Square Hollow Section T Joints under Axial Loads. I: Experimental

A series of static strength tests were performed on full-scale precracked square-to-square hollow section welded T joints. These joints were subjected under brace end axial load until failure using a special purpose designed test rig. An alternating current potential drop technique was employed to monitor the crack propagation during the tests. In addition, three sets of linear variable displacement transducers were used to record the crack mouth opening displacements. The tests were performed at room temperature; therefore, the material is on the upper shelf of the ductile-brittle transition curve where failure occurs in a ductile manner. In all these tests, a stable ductile tearing was observed with a small tearing region. Based on the yield line theory, the plastic collapse load is estimated, and then applied to the failure assessment diagram (FAD) of BS7910. The results show that BS7910 Level 2A FAD provides an adequate procedure for the assessment of cracked square-to-square hollow section T joints.

Microstructural fracture processes accompanying growing cracks in tough rubber-modified polyamides

The process of massive cavitational deformation in an ethylene—propylene—diene grafted maleic-anhydride modified polyamide 66, initiated from cavitating rubber particles and associated with stable crack propagation in compact tension and Izod impact experiments, has been studied in the region ahead of the crack front by scanning electron microscopy. The very tough stable ductile tearing behaviour of the material was manifested by steady-state crack opening angles of 60° and large stress-whitened zones. The process zone ahead of the advancing crack comprised two regions with distinct morphologies: (a) within a radius of 30 μm in an ‘intense’ deformation zone in front of the crack, the cavities are elongated to large draw ratios (≈ 10); and (b) in the remaining ‘weak’ zone cavities were found to be substantially equiaxed. The material with the elongated cavities of the crack front is parted by the propagating crack and immediately sheared by ≈ 60° and left lying parallel to the crack flank surfaces. The very tough behaviour of the material has been analysed by the well known mechanics of fracture of tough structural materials, and the calculated tearing moduli were found to be quite comparable with those of the toughest structural steels.

Microstructural fracture processes accompanying growing cracks in tough rubber-modified polyamides

The process of massive cavitational deformation in an ethylene—propylene—diene grafted maleic-anhydride modified polyamide 66, initiated from cavitating rubber particles and associated with stable crack propagation in compact tension and Izod impact experiments, has been studied in the region ahead of the crack front by scanning electron microscopy. The very tough stable ductile tearing behaviour of the material was manifested by steady-state crack opening angles of 60° and large stress-whitened zones. The process zone ahead of the advancing crack comprised two regions with distinct morphologies: (a) within a radius of 30 μm in an ‘intense’ deformation zone in front of the crack, the cavities are elongated to large draw ratios (≈ 10); and (b) in the remaining ‘weak’ zone cavities were found to be substantially equiaxed. The material with the elongated cavities of the crack front is parted by the propagating crack and immediately sheared by ≈ 60° and left lying parallel to the crack flank surfaces. The very tough behaviour of the material has been analysed by the well known mechanics of fracture of tough structural materials, and the calculated tearing moduli were found to be quite comparable with those of the toughest structural steels.

Fracture control in the transition-temperature region of structural steels

Abstract This paper describes the applicability of current research on the crack-tip opening displacement (CTOD) methodology to engineering approaches to fracture control in steel structures in the transition-temperature region. review of the four (not three) broad areas of fracture response that structural steels exhibit throughout their transition-temperature region is presented and the standard fracture toughness test methods applicable to each region are described. These four regions are the lower-shelf (linear-elastic K Ic type behavior), lower-transition (elastic-plastic behavior with no stable crack growth prior to brittle fracture), upper-transition (stable ductile tearing followed by brittle fracture),and upper-shelf (full fibrous ductile tearing). Correlations and relations between Charpy V-Notch impact energy results, K ID , K Ic , J-integral and CTOD test results are described. The importance of loading rate in understanding fracture control is demonstrated using the Standard Oil Development (SOD) wide plate test results of the early 1950s. Finally, actual CTOD test values, in combination with nonlinear finite element analyses, are used to predict the burst pressures of notched pressure vessels. The burst pressures computed agree to within 7% of experimental burst pressures in all five vessels analyzed.

The Effect of Temperature, Specimen Size, and Geometry on the Fracture Toughness of a 3 Pct NiCrMoV Low Pressure Turbine Disc Steel

The variation of fracture toughness with temperature and specimen size of a 3 pct NiCrMoV LP disc steel has been investigated over the temperature range -100 to +100 °C using compact tension and single-edge-notched bend geometries. A number of large ‘half-disc’ three point bend specimens were also tested. Toughness increased up to a transition temperature coinciding with the onset of stable ductile tearing prior to instability. Below this temperature fracture could be described by established linear elastic or post yield fracture analyses. Above this temperature failure was by plastic collapse. The transition temperature decreased with decreasing specimen size, and at similar thicknesses was lower for the bend geometry than for the compact tension so that it was not possible to predict the fracture behavior of the full size service component from small scale tests in the transition region. A further complicating feature was the extreme scatter of some duplicate test results below the transition temperature. The implications for toughness testing in the transition region are discussed. The data obtained in this work have been combined with published data for similar steels to derive an equation which describes the variation of fracture toughness with temperature for steels of this type.

The application of elastic-plastic fracture mechanics parameters in fracture safe design

The report summarizes some of the methods which are currently used for assessing the fracture toughness of materials under elastic and elastic-plastic conditions. The main parameters which are considered are (1) plane strain fracture toughness ( K Ic), (2) equivalent energy ( K Icd), (3) contour integral ( J) and (4) crack opening displacement (COD). Gross strain crack tolerance and stress concentration methods are also discussed. It is concluded that of these parameters, the contour integral and the crack opening displacement have most potential for future development. These two parameters are shown to be equivalent, however, at the present stage of development the COD concept has several advantages over the J concept. Firstly, the COD concept is able to take into account, secondary stresses, such as welding residual stresses. Because these stresses are in equilibrium, they do not appear in energy measurements to evaluate J. Secondly, the COD value is a physical measure of the crack tip conditions which includes the effect of stress state and thickness. It is, therefore, possible to measure and calculate COD levels for cracks in real structures. It is not possible to evaluate J for real structures since J methods are appropriate only to in-plane problems. This also means that partial wall (thumbnail) flaws are better characterized by the COD concept. The COD concept has been developed to a stage where it is possible to estimate the significance of flaws in welded structures provided the toughness of the material and the acting stresses or strains are known. This development is described and the method used to analyze tests on model pressure vessels with 6″ thick walls. A comparison is made with other methods, and it is concluded that although the COD analysis gives conservative estimates of the flaw size to cause failure, further work is necessary to be able to predict vessel burst conditions when failure is preceded by extensive plasticity and stable ductile tearing. A simple nomogram to determine COD levels to ensure leak before break conditions is also developed.