Features Testing of Stud Material Under Low Constraint Ductile Tearing

Defect tolerance assessments are carried out to support the demonstration of structural integrity for high integrity components such as nuclear reactor pressure vessels. These assessments often consider surface-breaking defects and assess Stress Intensity Factors (SIFs) at both the surface and deepest points. This can be problematic when there is a high stress at the surface, for example due to the stress concentration at the root of a screw thread. In the past this has led to the development of complex and costly 3D finite element analyses to calculate more accurate SIFs, and still resulting in small apparent limiting defect sizes based on initiation at the surface point. Analysis has been carried out along with supporting materials testing, to demonstrate that the increased SIF at the surface point is offset by a reduction in crack-tip constraint, such that the material exhibits a higher apparent fracture toughness. This enables a more simplistic assessment which reduces the effective SIF at the surface such that only the SIF at the deepest point needs to be considered. This then leads to larger calculated limiting defect sizes. This in turn leads to a more robust demonstration of structural integrity, as the limiting defect sizes are consistent with the capability of non-destructive examination techniques. The high SIF at the surface location, and the concomitant reduction in crack-tip constraint, meant that it was not possible to demonstrate the material response with conventional tests, such as those using shallow-notched bend specimens. Instead it was necessary to develop modified specimens in which semielliptical defects were introduced into a geometry which replicated the notch acuity at the root of a screw thread. These feature tests were used to demonstrate the principle, prior to testing with more conventional specimens to fit more accurately the parameters required to represent the material response in a defect tolerance assessment. Margins in defect tolerance assessments are usually measured against the initiation of tearing, even though the final failure for the material may occur at a higher load following stable crack extension. This work measured and assessed the benefit of reduced crack-tip constraint on both the point of initiation and on the development of the tearing resistance curve. This demonstrated that the effect of constraint was valid with tearing for this material and that there was additional margin available beyond the onset of tearing. The feature test geometry also provided evidence of the tearing behaviour at the surface and deepest points of a surrogate component under representative loading. This paper provides an overview of the range of tests performed and the post-test interpretation performed in order to provide the R6 α and k constraint parameters.

Wide Range Compliance Solutions for Various Fracture Test Specimens Using Crack Mouth Opening Displacement

This work addresses the development of wide range compliance solutions for tensile-loaded and bend specimens based on CMOD. The study covers selected standard and non-standard fracture test specimens, including the compact tension C(T) configuration, the single edge notch tension SE(T) specimen with fixed-grip loading (clamped ends) and the single edge notch bend SE(B) geometry with varying specimen spam over width ratio and loaded under 3-point and 4-point flexural configuration. Very detailed elastic finite element analysis in 2-D setting are conducted on fracture models with varying crack sizes to generate the evolution of load with displacement for those configurations from which the dependence of specimen compliance on crack length, specimen geometry and loading mode is determined. The extensive numerical analyses conducted here provide a larger set of solutions upon which more accurate experimental evaluations of crack size changes in fracture toughness and fatigue crack growth testing can be made.

Performance of All-Polypropylene Composites at LNG Temperatures

In the small scale LNG infrastructure, composite materials are scarcely employed. Potentially, cost effective solutions for LNG applications could be developed thanks to the advantages of composite materials over metals such as weight savings, design flexibility and recyclability. The research presented in this paper focuses on the mechanical performance of fully recyclable, thermoplastic Polypropylene (PP) composite tapes at cryogenic LNG temperatures. Quasi-static tensile tests performed on [±45] laminates made of plain woven plies of PURE® show that at −196°C the behavior is bilinear with the failure strain of 6.5% and failure stress of 37 MPa. Such non-brittle failure behavior of PP is desirable for cryogenic applications. The other results presented in the paper contains [0/90] laminate results and the interlaminar shear strength characteristics at room and cryogenic temperatures.

Effect of Postweld Heat Treatment Conditions on Microstructure of 9Cr-1Mo-V Steel Welds for Pressure Vessel

We investigated the effects of post-weld heat treatment conditions on the microstructure of the multi-pass submerged arc weld metal of 9Cr-1Mo-V steel used in pressure vessels. The microstructural properties were analyzed under three conditions (as-weld, Larson-Miller parameter (LMP) = 21.38 × 103, and LMP = 21.99 × 103). The precipitation behavior was observed using scanning electron microscopy, and the difference in precipitation behavior in the “as-welded” and “reheated” regions of the prepared multi-pass weld metal was clarified at the different LMP values. The precipitate was analyzed using scanning transmission electron microscopy. An oxide and two types of precipitates were identified, and a dislocation network pinned by MX-type carbides was visualized under the low-LMP condition. The effects of LMP on the effective grain size and dislocation amount were also evaluated using electron back-scattering diffraction. All microstructural change along the LMP had a positive effect on the toughness of weld metal.

Relating ASME Flaw Evaluation to ASME Task Group on Carbon Fiber Repairs

As most nuclear power stations in the US have surpassed their initial 40 years of operability, the industry is now challenged with maintaining safe operations and extending the operating life of structures, systems and components. The US Nuclear Regulatory Commission (NRC), Nuclear Energy Institute (NEI), and Electric Power Research Institute (EPRI) have identified safety related buried piping systems as particularly susceptible to degradation. These systems are required to maintain the structural factors of the ASME Construction Codes under pressure and piping loads, which includes seismic wave passage. This paper focuses on evaluation approaches for metallic buried piping that can be used to demonstrate that localized thinning meets the requirements of the Construction Code. The paper then addresses a non-metallic repair option using carbon fiber reinforced polymers (CFRP) as the new pressure boundary.

Improved Elastic Compliance Equation and its Inverse Solution for Compact Tension Specimens

ASTM E1820 is a well-developed fracture test standard and has been used worldwide for fracture toughness testing on ductile materials in terms of the J-integral or J-R curve. This standard recommends the elastic unloading compliance technique for measuring crack length in a single specimen test, and an accurate elastic compliance equation is needed to estimate physical crack length. Compact tension (CT) specimen is one of the most often used standard specimens with crack length ratios of 0.45≤a/W≤0.70 prescribed in E1820 for J-R curve testing. The stress intensity factor K of CT specimens used in E1820 was developed by Srawley (IJF, 1976) and has been commonly accepted as the most accurate solution. The compliance equation of CT specimens was developed by Saxena and Hudak (IJF, 1978) and has been used in ASTM E1820 for decades. However, recent results showed that the load-line displacement (LLD) compliance equation is not consistent with that determined from its K solution, and the maximum error of LLD compliance can be larger than 7% at a/W = 0.32 and ∼ 5% at a/W = 0.45 (E1820 standard crack size). The FEA results confirmed that the K solution in E1820 is indeed very accurate, but its compliance equation is less accurate. Thus, an improved compliance equation with high accuracy is developed from the accurate K solution using the numerical integration technique and shooting method.

Evaluation of Fatigue Crack Propagation Behavior Crossing Interface in Cladded Plates

In nuclear power plants, inner surface of reactor pressure vessel, which is made of low-alloy steel, is cladded by austenitic stainless steel to improve corrosion resistance of vessel. In an evaluation of fatigue crack propagation behavior at J-welding portions of bottom mounted instrumentation in reactor pressure vessel, crack propagation crossing dissimilar materials interface between base metal of low-alloy steel and stainless steel cladding is assumed. Fatigue crack growth rate of the cladding is predicted to be captured by that of the base metal because thickness of the cladding is very thin compared that of the base metal. In this study, fatigue crack propagation analyses of cladded plates under cyclic loading were performed using XFEM, which can model crack independently of finite elements, as a preliminary study for crack propagation analyses considering dissimilar materials interface.

Fracture Mechanics at Elevated Loading Rates in the Ductile to Brittle Transition Region

The rate-dependent reference temperature T0,x characterizes the fracture toughness of ferritic steels at the onset of cleavage. Fracture toughness values KJc,d were determined according to the Annex A1 of ASTM E1921 [1] which refers to the high rate annexes A14 and A17 of ASTM E1820 [2]. Results of extensive dynamic fracture toughness experiments at various loading rates, temperatures, specimen types and sizes revealed shortcomings in the transferability of the shape of the Master Curve under quasi-static conditions to elevated loading rates. In particular, the quasi-static Master Curve predicts lower fracture toughness values towards higher temperatures than experimentally observed under dynamic loading causing a steeper Master Curve shape. Fractographic examinations proved the relevance of local crack arrest under dynamic loading conditions, which is consistent with the view of the parallelism of dynamic fracture probability and probability of arrest.

Mechanical Properties of Vanadium Microalloyed High-Strength ASTM A694 Forgings

Recent upstream oil and gas industry experience has raised attention to substandard properties with high strength carbon steel forgings manufactured to the requirements of ASTM A694 and MSS-SP-44. As part of an internal investigation into quality of commodity pipeline flanges, three flanges certified as ASTM A694 grade F60 to F70, were purchased off-the-shelf from three different manufacturers for microstructural and mechanical property investigation. All three flanges were supplied with material test certificates indicating acceptable material properties. Tensile and Charpy impact specimens were extracted from various locations and orientations in each flange. All three flanges failed to meet yield strength requirements for the specified grade. The impact energy and shear area values were well below those reported on the material test certificates. The discrepancy between the sacrificial testing results and the material test certificates is attributed to the use of separately forged test blocks for quality testing instead of integral prolongations or a sacrificial production part, which is permitted by ASTM A694 and MSS-SP-44. Further investigation was made into the chemical composition and heat treating practices. The chemical composition can be characterized as high strength, low alloy steel (HSLA) by virtue of 0.05–0.08 wt. pct. vanadium added to a carbon-manganese steel with CE(IIW) ranging from 0.43 to 0.45. Advanced microscopy showed that the morphology of the vanadium precipitates was inadequate as a strengthener and deleterious to Charpy impact properties for the size of the flanges and the heat treatment practices applied.

Simplified Assessment of Studs Allowing for Low Constraint Conditions

Defect tolerance assessments are carried out to support the demonstration of structural integrity for high integrity components such as nuclear reactor pressure vessels. These assessments often consider surface-breaking defects and assess Stress Intensity Factors (SIFs) at both the surface and deepest points. This can be problematic when there is a high stress at the surface, for example due to the stress concentration at the root of a screw thread. In the past this has led to the development of complex and costly 3D finite element analyses to calculate more accurate SIFs, and still resulting in small apparent limiting defect sizes based on initiation at the surface point. Analysis has been carried out along with supporting materials testing, to demonstrate that the increased SIF at the surface point is offset by a reduction in crack-tip constraint, such that the material exhibits a higher apparent fracture toughness. This enables a more simplistic assessment which reduces the effective SIF at the surface such that only the SIF at the deepest point needs to be considered for many defects. This then leads to larger calculated limiting defect sizes. This in turn leads to a more robust demonstration of structural integrity, as the limiting defect sizes are consistent with the capability of non-destructive examination techniques. An overview of the supporting materials testing is provided in an accompanying paper. The accompanying paper details how it was not possible to demonstrate the required material response with conventional tests, such as those using shallow-notched bend specimens. Instead it was necessary to develop modified specimens in which semi-elliptical defects were introduced into a geometry which replicated the notch acuity at the root of a screw thread. These tests were used to quantify the stud materials sensitivity to constraint. Conventional three-point bend tests were also seen to confirm these values. A series of R6 constraint modified assessments have been considered to understand the benefit from including a loss of constraint, particularly when assessing the surface breaking SIF. This has necessitated a series of complex finite element analyses to define the elastic SIF as well as the elastic constraint parameter, T-Stress, T. Further verification analyses have also been performed to determine the equivalent elastic-plastic J and Q parameters. These have been used to provide guidance on how best to assess surface breaking defects within studs. This has shown that the increased perceived toughness at the surface location means that under the majority of conditions, the assessment can simply be based upon the SIF at the depth location using high constraint fracture toughness. This paper provides an overview of the process undertaken to provide simplified guidance on assessing defects within studs that allows benefit from constraint loss.