Warm Prestressing Research Articles

Effect of fracture mode on warm prestressing

Warm prestressing (wps) is the phenomenon by which the fracture resistance of a material containing a crack is apparently enhanced if the material is (warm) prestressed at a temperature above that at which it is eventually fractured. The wps effect is manifested by materials, such as ferritic steels, which exhibit a fracture toughness transition temperature. Above this temperature a cracked specimen may be loaded, without fracturing, to a crack-tip stress intensity level, K(wps), which significantly exceeds the fracture toughness of the material below the transition temperature, K(ic). If this load is held constant and the specimen is cooled below the transition temperature, K(wps) is unaltered but the specimen does not fracture, even though K(wps) > K(ic). Additional load is required to fracture the specimen. If the crack-tip stress intensity factor calculated from the fracture load at the lower temperature is K(f), then K(f)-K(ic) is the apparent increase in fracture toughness produced by the wps. Tests of this type, which involve loading, cooling under load and then fracture at the lower temperature, are termed LCF tests. A similar wps effect is observed if, following the warm prestress, the specimen is unloaded, cooled to the lower temperature and then fractured. The apparentmore » stress intensity level at fracture, K(f), is again found to be in excess of K(ic) in the absence of prestressing. Tests which involve an unloading step between wps and low temperature fracture are known as LUCF tests. There is a clear wps effect in steels in which the fracture mode is either mixed intergranular/cleavage or wholly intergranular. The magnitude of the effect appears to be similar to that when purely cleavage fracture is involved, though, in the case of LUCF tests, this needs to be confirmed. It would appear that the beneficial effects of wps can be relied upon in circumstances where metallurgical changes alter the fracture mode from cleavage to intergranular fracture.« less

Effects of clad yielding and warm prestress on fracture-margin assessment of pressurized water reactor pressure vessels

Potential benefits from the inclusion of clad-yielding and warm-prestress (WPS) effects toward decreasing the conditional probability of vessel failure (P(F/E)) of a pressurized water reactor vessel are summarized. Potential reduction in P(F/E) is quantified based on comparison with (P(F|E) calculated within a linear-elastic fracture mechanics (LEFM) framework that does not take into account either clad-yielding or WPS effects. Three postulated pressurized-thermal shock (PTS) transients that simulate conditions of no repressurization, early and late repressurization to full operating pressure are considered. Preliminary analysis results for the transient without repressurization indicate that inclusion of clad-yielding effects can significantly reduce P(F|E) relative to LEFM-based values. Numerical results also indicate that clad-yielding can similarly reduce P(F|E) for the transients with repressurization, and that the reduced P(F|E) can be conservatively estimated based on results for the case of LEFM analysis of the transient without repressurization. The decrease in P(F|E) is predicted to be greater for the transient involving early vs. late repressurization.

THE INFLUENCE OF PLASTIC PRESTRAINING ON BRITTLE FRACTURE RESISTANCE OF METALLIC MATERIALS WITH CRACKS

Abstract The purpose of the present work was to study the influence of different regimes of overloading of pressure vessel steels in different states which correspond to the steel properties at the beginning of a reactor operation and at different degrees of embrittlement (simulated by heat treatment). The experiments were performed on 25, 50 and 150 mm thick specimens with short and long cracks of various shape in the temperature range from 293 to 623 K corresponding to the service temperature range of those steels. The following factors were investigated contribution of different effects (residual stresses, strain hardening, crack tip blunting) into the enhancement of the brittle fracture resistance of steels after warm prestressing, stability of the positive warm prestressing effect during subsequent exposure of the steels to different service loading conditions; size effect on optimal regimes of thermo-mechanical prestressing and on the brittle fracture resistance characteristics of the steels studied after warm-prestressing. An approach is proposed to predict the increase in the brittle fracture resistance of steels with cracks after warm prestressing.

Effect of the yielding scale at warm prestressing on the behaviour of crack initiation

The effect of large-scale yielding at warm prestressing (WPS) is investigated, and it is found that the small-scale yielding solutions can completely represent the actual toughness obtained after WPS in a wide range of WPS level. Both Je and Jw can be regarded as the parameter to explain the WPS phenomenon.

Calculated and experimental estimation of preliminary loading effect at elevated temperatures on fracture toughness of pressure vessel materials

Based on the analysis of different factors' effect on the structural material cracking resistance after preliminary warm prestressing (WPS), a conclusion has been drawn that the existing calculation models used for WPS effect prediction are of limited value; this conclusion applies to the Chell model, which is very convenient for practical use, in particular. The testing results on cracking resistance are presented for the structural 15X2MFA steel (of different strengths) and 15X2HFMA steels, as well as a welding 10XMFT joint after WPS. Comparison of the experimental and calculated data showed the possibility of a practical application of the simplified Chell solution of the WPS effect on the structural steels cracking resistance in the wide range of loading up to the subcritical crack growth.

Verification of Warm Prestressing Effect Under a Pressurized Thermal Shock (PTS) Event

Abstract Fracture tests for the verification of WPS (warm prestressing) effect were carried out by using large flat specimens and big compact specimens with low toughness. In the case of monotonical KI increasing during cooling, the specimen broke within the scatter band of KIC. On the other hand, when KI was decreasing during cooling, the specimens did not break even if KI values were beyond the scatter band of KIC. That is, WPS effect was confirmed even for the low toughness steel like reactor pressure vessel wall under neutron irradiation. Also, KI values at fracture can be predicted by Chell’s theory. By applying WPS effect and the predictive equations for irradiation embrittlement for Japanese PWR reactor steels to the PTS integrity analysis, much more temperature margin can be expected.

A promising method for enhancing resistance of pressure vessels to brittle fracture

The purpose of the present work was to study the influence of different regimes of overloading of pressure vessel steels in different states which correspond to the steel properties at the beginning of the reactor operation and at different degrees of embrittlement (simulated by thermal treatment). The experiments were performed on 25-, 50-, 150-mm-thick specimens with short and long cracks of various shape in the temperature range 293–623 K corresponding to the service temperature range of those steels. The following factors were investigated: contribution of different mechanisms (residual stresses, strain hardening, crack tip blunting) into the enhancement of the brittle fracture resistance of steels after warm prestressing; stability of the warm prestressing positive effect during subsequent exposure of those steels to different service loading conditions; size effect on optimal regimes of thermomechanical prestressing and on the brittle fracture resistance characteristics of the steels studied after warm prestressing. An approach is proposed for the prediction of the increase in the brittle fracture resistance of steels with cracks after warm prestressing.

The effect of metallurgical factors on crack resistance of pressure-vessel materials

The paper deals with the investigation of static and cyclic crack growth resistance of pressure-vessel materials obtained through different melting, welding, and heat treatment techniques, and subject to different regimes of warm prestressing. It has been established that heat treatment simulating the radiation effect reduces appreciably the static and cyclic crack growth resistance characteristics. In addition, crack resistance of the investigated materials is affected by a particular welding technique, as well as the content of impurity elements (P, S) and non-ferrous metals (Cu, Su, As, Bi, Co). In general weld metal crack growth resistance is lower than that of base metal and basically determines the lifetime and limiting load carrying capacity of pressure vessels. It has been found that warm prestressing can increase substantially the base and weld metals resistance to brittle fracture. Optimum regimes of warm prestressing have been obtained. The investigation was performed on compact and three-point bend specimens of thickness from 25 to 150 mm having a long through and a short semi-elliptical cracks at the temperature range from 293 to 573 K.

Warm Precracking at 295 K and Its Effects on the 4-K Toughness of Austenitic Steels

Abstract Experiments show that some austenitic steels at extreme cryogenic temperatures are toughened by warm prestress. We demonstrate this for Fe-17Cr-3Ni-13Mn-0.33N steel specimens that were warm precracked at 295 K and then fractured in liquid helium at 4 K where failure occurs by slip-plane cracking. On the other hand, Fe-13Cr-5Ni-22Mn-0.21N steel with higher toughness and a ductile fracture mechanism at 4 K was not affected by similar warm precracking. Austenitic and ferritic steel behaviors are compared, and precracking procedures for 4-K fracture tests are discussed.

Toughening analysis for warm prestressing

Warm prestressing of a cracked structure can result in an apparent elevation of its fracture toughness at lower temperature. In this paper, Chell's theory is re-examined, and it is found that the J e-integral defined by Chell should be reformulated. As a new control parameter the intergal J ̄ w is proposed to explain the warm prestressing phenomenon. By comparison with published warm prestressing data, the J ̄ w-integral is more available.

The significance of prior overload on fracture resistance: A critical review

The review presented in this paper is limited to prior overload effects in pressure vessels. The incidence of pressure vessel failure is examined, and the effects of prior overloading on residual stresses and fracture toughness are reviewed. It is found that the effectiveness of the mechanical stress relief of residual stresses cannot be guaranteed as the level of relief is dependent on local geometry. Prior overload effects on fracture toughness are divided into proof testing aspects and warm prestress effects. In this context the effects of prestraining, ageing and cyclic loading are reviewed. In warm prestressing it is assumed that the existence of flaws is postulated, and some benefit may be obtained by operating at temperatures lower than the proof test temperature. The empirical evidence of the benefit of warm prestressing is reviewed, and examined in relation to the effects of loading and temperature cycling, and material and geometry effects. The models for predicting warm prestress effects are also reviewed. The models are found to be restrictive in their application, and the effects of warm prestressing cannot be adequately quantified without recourse to experimental work. There is little quantifiable evidence of benefits of proof testing on subsequent behaviour of large-scale pressure vessels. Progress in the last 20 years is considered, and although recommendations have been made in the past, there is little evidence that codes have implemented the more important elements. Finally, recommendations for future work are presented.

Parameters affecting sag resistance in spring steels

Recent trends toward reducing the weight of automobile suspension springs have led to the development of a number of microalloyed steels and a variety of processing treatments which have claimed to improve the sag resistance of springs while increasing their operating stresses. More often than not, however, the subtle effects of varying levels of hardness and prior austenite grain size, as well as small but significant differences in critical elements such as Si, are over-looked in comparing these new steels with the conventional grades. Hysteresis loops measured in tension (related to the Bauschinger effect) were used to determine the relaxation behavior of a number of microalloyed and standard grade spring steels. The effects of hardness level, austenitizing temperature, prior austenite grain size, and warm prestressing on the Bauschinger hysteresis loops were also established. Silicon was found to be the most important factor influencing the size of the hysteresis loops; the greater the Si content of the steel (up to 2.2 pct), the larger the loops at a given strength level and the greater the expected relaxation resistance of the spring. The standard AISI 9261 steel containing 2.2 pct Si showed the same Bauschinger loops as the microalloyed grades containing Nb and V at the same hardness of 50 HRC.

Performance of low-upper-shelf material under pressurized-thermal-shock loading

The second pressurized-thermal-shock experiment (PTSE-2) of the Heavy-Section Steel Technology Program was conceived to investigate fracture behavior of steel with low ductile-tearing resistance. The experiment was performed in the pressurized-thermal-shock test facility at the Oak Ridge National Laboratory. PTSE-2 was designed primarily to reveal the interaction of ductile and brittle modes of fracture and secondarily to investigates the effects of warm prestressing, A test vessel was prepared by inserting a cracklike flaw of well-defined geometry on the outside surface of the vessel. The flaw was 1 m long by ≈ 15 mm deep. The instrumented vessel was placed in the test facility in which it was initially heated to a uniform temperature and was then concurrently cooled on the outside and pressurized on the inside. These actions produced an evolution of temperature, toughness, and stress gradients relative to the prepared flaw that was appropriate to the planned objectives. The experiment was conducted in twoseparate transients, each one starting with the vessel nearly isothermal. The first transient induced a warm-prestressed state, during which K I , first exceeded K Ic . This was followed by repressurization until a cleavage fracture propagated and arrested. The final transient was designed to produce and investigate a cleavage crack propagation followed by unstable tearing. During this transient, the fracture events occurred as had been planned.

Warm prestressing and fractures in structural steel

Warm prestressing, by which structural steel can endure stress without low-temperature brittle fracture after it has previously been exposed to the same stress at a higher temperature, contributes to the effectiveness of preheating in increasing the weldability of steels. It has also been invoked as a safety feature for steel pressure vessels in nuclear reactors. It might be invalidated however by the process of spontaneous delayed brittle fracture. Delayed yielding could cause this, but the effect is likely to be unimportant in practice.

Use of load path dependent material fracture toughness values (WPS) in the safety analysis of RPV stade

The effect of warm prestressing (WPS) has been investigated for the specific material and loading conditions of the central circumferential weld of the reactor pressure vessel of KKS under emergency core cooling. Warm prestressing results in a significant rise of effective fracture toughness. The magnitude of the WPS-effect as a function of warm prestressing, path of unloading, amount of cooling (e.g. change of yield stress) can be predicted on the basis of a theoretical model of Chell et al.. Crack initiation can be excluded for emergency core cooling (ECC)-loading and for material conditions beyond end of life for the central circumferential weld of the pressure vessel of KKS.

THE EFFECTS OF SUB‐CRITICAL CRACK GROWTH ON THE FRACTURE BEHAVIOUR OF CRACKED FERRITIC STEELS AFTER WARM PRESTRESSING

AbstractThe effects of sub‐critical crack extension on the fracture properties of ferritic steels after they have been subjected to warm prestressing are investigated. The crack growth is assumed to occur at the temperature of the final loading, after the warm prestressing and the subsequent unloading. Predictions of the fracture behaviour are made using continuum mechanics and a fracture criterion based on a modified J‐integral. The results of these calculations are consistent with those of a micromechanistic model of cleavage fracture from a sharp crack. Both the theories predict that the beneficial elevation in fracture load produced by warm prestressing is maintained after sub‐critical crack growth provided the latter is not greater than the compressive yielded zone formed on unloading.

A method for LEFM analysis of RPV during SBLOCA

A somewhat simplified two-dimensional linear elastic fracture mechanics (LEFM) model of the beltline of a reactor pressure vessel is presented. The fracture mechanics model used tends to be conservative in the sense that it ignores possible beneficial effects of warm prestressing and cladding. For LEFM studies that require a large number of analyses on the same geometry but with different loads and material toughnesses, the superposition principle is an accurate and simple method to determine the stress intensity factor K 1 , provided that K 1 due to a unit load (called K∗ in this paper), acting on an arbitrary point on the crack surface is known. The details of the superposition principle and the procedure used for determining K∗ have been presented. Once these K∗ are determined for a specific geometry, then the determination of K 1 for the same geometry can be made accurately and in a manner that permits parametric studies involving thousands of individual analyses. It is believed that the error in the K 1 values so determined is less than 3·5%. An example of the use of the simplified model for a parametric analysis is also presented.

The effect of warm prestressing on proof tested pressure vessels

The beneficial effects of warm prestressing of cracked pressure vessels using a proof test are investigated using experimental and theoretical evidence. An approach based on the reloading capacity is proposed which is dependent on only two parameters: the extent of prior unloading and the fracture toughness. A simple lower bound failure criterion is given which avoids most of the uncertainties in input data which are usually associated with integrity assessments. The approach is validated using available experimental failure data.

Results and conclusions from the first pressurized-thermal-shock experiment

The first pressurized-thermal-shock test of a 148 mm thick steel pressure vessel with a 1 m long flaw was performed to investigate fracture behavior of a vessel under conditions relevant to a flawed nuclear reactor pressure vessel during an overcooling accident. The objectives were to observe crack arrest and stability on the ductile upper shelf and the effects of warm prestressing on crack initiation. Three coordinated pressure and thermal transients were imposed on the vessel, which was preheated to ∼290°C. Two episodes of crack propagation and arrest occurred. The thermal transients were induced by coolant at −29 to 15°C. Pressure transients were as high as 94.4 MPa. The experimental objectives were attained. The inhibiting effects of warm prestressing were definitely demonstrated. Crack propagation was nearly pure cleavage, and arrest at ∼30 K above the onset of the Charpy upper-shelf was experienced in a positive K 1 gradient and with K 1 = 300 MPa√ m. Fracture-mechanics analysis of brittle fracture based on small-specimen toughness measurements was reasonably accurate. Flaw evaluation by procedures of the ASME Boiler and Pressure Vessel Code conservatively predicted vessel failure, which did not occur. No ductile tearing occurred after each crack arrest, although some stable tearing had been predicted on the basis of tearing resistance data.

Fracture assessment of a PWR pressure vessel

The Second Report of the Marshall Study Group [1] reviewed the design of a pressure vessel to identify those regions which were most sensitive to the expected loading conditions. The design and loading conditions, including the pressure test, were also reviewed. A stress analysis data base of sufficient detail was established to permit a full fracture analysis to be carried out. After reviewing currently available methods for fracture assessment the CEGB R6 method was adopted. Selected normal and upset, emergency and fault transients were considered. Initiation defect sizes for normal and upset transients were generally large. In the beltline and nozzle shell course, for example, the minimum initiating height for an extended line defect was approximately 70 mm. For emergency and fault conditions, providing upper shelf material properties apply, the height to initiate semi-elliptical defects exceeds 25 mm and cracks substantially larger than this will extend by only small amounts in a ductile manner and remain stable. If during the large LOCA upper shelf material properties cannot be maintained semi-elliptic defects 6 to 10 mm high may propagate transversely parallel to the inside surface of the vessel. This may be limited by warm prestressing.