Mark T. Kirk

A framework to correlate a/W ratio effects on elastic-plastic fracture toughness (J c )

Single edge-notched bend (SENB) specimens containing shallow cracks (a/W < 0.2) are commonly employed for fracture testing of ferritic materials in the lower-transition region where extensive plasticity (but no significant ductile crack growth) precedes unstable fracture. Critical J-values Jc) for shallow crack specimens are significantly larger (factor of 2–3) than the Jc)-values for corresponding deep crack specimens at identical temperatures. The increase of fracture toughness arises from the loss of constraint that occurs when the gross plastic zones of bending impinge on the otherwise autonomous crack-tip plastic zones. Consequently, SENB specimens with small and large a/W ratios loaded to the same J-value have markedly different crack-tip stresses under large-scale plasticity. Detailed, plane-strain finite-element analyses and a local stress-based criterion for cleavage fracture are combined to establish specimen size requirements (deformation limits) for testing in the transition region which assure a single parameter characterization of the crack-tip stress field. Moreover, these analyses provide a framework to correlate Jc)-values with a/W ratio once the deformation limits are exceeded. The correlation procedure is shown to remove the geometry dependence of fracture toughness values for an A36 steel in the transition region across a/W ratios and to reduce the scatter of toughness values for nominally identical specimens.

The influence of weld strength mismatch on crack-tip constraint in single edge notch bend specimens

Previous work by Dodds and Anderson provides a framework to quantify finite size and crack depth effects on cleavage fracture toughness when failure occurs at deformation levels where J no longer uniquely describes the state of stresses and strains in the vicinity of the crack tip. Size effects on cleavage fracture are quantified by defining a value termed JSSY: the J to which an infinite body must be loaded to achieve the same likelihood of cleavage fracture as in a finite body. In weld metal fracture toughness testing, mismatch between weld metal and baseplate strength can alter deformation patterns, which complicate size and crack depth effects on cleavage fracture toughness. This study demonstrates that there is virtually no effect of ±20 percent mismatch on JSSYif the distance from the crack tip to the weld/plate interface (Lmin) exceeds 5 mm. At higher levels of overmatch (50 to 100%), it is no longer possible to parameterize the departure of JSSYfor a weldment from that for a homogeneous SE(B) based on Lmin alone. Weld geometry significantly influences the accuracy with which JSSYfor a welded SE(B) can be approximated by JSSYfor a homogeneous specimen at these extreme overmatch levels.

Wide Range CTOD Estimation Formulae for SE(B) Specimens

Fracture testing of single edge notch bend, SE(B), specimens containing shallow cracks is often undertaken to match constraint conditions between specimen and structure, thereby obtaining a more accurate assessment of structural integrity However, standardized methods for measuring fracture toughness in terms of the crack-tip opening displacement (CTOD) do not currently provide formulae which relate experimentally measurable quantities to CTOD for shallow cracks. A CTOD estimation equation, based on results from plane-strain finite element analyses, is proposed to fill this gap. This equation applies to specimens having a wide range of crack depths (0.05 ≤ a/W ≤ 0.70) and Ramberg-Osgood work hardening exponents (4 ≤ n s 50). The proposed equation provides CTOD estimates which are up to five times more accurate than those obtained using the procedures recommended by ASTM E1290. This improved accuracy is achieved by estimating the plastic part of CTOD from a work-like quantity rather than as a scalar multiple of CMOD p l (as is done in E1290). Accuracy improvements of the new equation over the E1290 equation are especially notable for materials having a moderate to high work hardening capacity because the linear proportionality that E1290 assumes between CTOD p l and CMOD p l breaks down in the presence of work hardening. A procedure for estimating the value of the work hardening exponent (n) from the results of a simple tension test is presented and validated. Use of this procedure permits routine application of the proposed CTOD estimation equation in the characterization of fracture toughness. The only material property data that is needed to apply this equation in addition to that currently required by E1290 is the ultimate tensile strength.

Incorporation of residual stress effects into fracture assessments via the finite element method

This paper presents a finite element methodology for the incorporation of residual stress effects into fracture assessments of pressure vessel and piping components. The residual stress is determined through improved welding simulations. Following the welding simulation, interpolation is used to transfer the computed residual stresses onto fine meshes for the evaluation of J integrals. The finite element fracture assessment methodology is used as a baseline to evaluate several approximations to the residual stress field and appropriate analytic solutions for a 15.875 mm (5/8 in.) thick pipe with R/t=10, 25, and 50. The remote yield level approximation is found to be overconservative while the best approximation is the bending moment fit to the residual stress distribution.

Fatigue design and analysis for welded tubular joints: Results from the API Offshore Tubular Joint Research Center

The fatigue performance of tubular connections can limit the life of offshore constructions. Fatigue research for the American Petroleum Institute (API) Offshore Tubular Joints Research Centre focused in its first three projects on fatigue design. An examination of design rules in API RP2A and equivalent standards as well as the basic data has defined several areas where API RP2A can be updated to include new design curves and explicit means of determining hot spot stresses. In addition, several suggestions have been made that would introduce new provisions, such as inclusion of other weld improvements besides grinding and a different method of including the effects of cathodic protection in seawater.

Approximate techniques of J estimation applicable to part-through surface cracks

Abstract This investigation concerns the accuracy with which three different J estimation schemes can predict the variation of applied Jwith applied strain for semi-elliptical surface cracks. The three estimation schemes considered were a weight function technique proposed by Bhandari, a modification of the EPRI estimation scheme proposed by Ainsworth, and Turners Engineering-J design curve. Each of these techniques is simple enough to employ in design, the most arduous calculation required being that needed to determine the linear elastic stress intensity factor. Estimation scheme accuracy was assessed by comparing J predictions to finite-element results for three small semi-elliptical surface cracks in a moderately hardening steel loaded in either pure tension or pure bending. The results obtained indicate that, for applied strains up to three times the yield strain, both the weight function and the modified EPRI schemes under estimate applied J by between 23 and 83% depending on the applied strain level. Conversely, Turners Engineering-ssJ design curve provides accurate or conservative (i.e. over) estimates of applied J in both tension and bending provided total crack size is less than 3% of the total cross sectional area and maximum crack depth is less than 25% of the plate thickness. Application to larger and deeper cracks loaded in tension is not recommended as the design curve does not conservatively account for net section yielding in these situations. The design curve can still be applied in bending to cracks of size up to 7.4% of the total cross sectional area. However, the degree of conservatism inherent in this application may be considered excessive in certain situations.