High Toughness Materials Research Articles

Principles of the German approach to the analysis of flawed structures

The analysis of flawed structures in the linear elastic regime (LEFM) has been treated very extensively, especially in the framework of the 4th German nuclear programme. Investigations with large specimens have demonstrated the applicability of the LEFM for a variety of materials ranging from low to high toughness levels. In the elastic-plastic (EPFM) regime, however, especially the J- integral method has been used for quantifying the safety margin, because with a required value of upper shelf Charpy energy alone this quantification is not possible. A correlation between the crack initiation value J i and Charpy upper shelf energy was evaluated experimentally, justifying the generally upgraded upper shelf energy level adopted in the Code. Furthermore, this correlation can be employed when fracture toughness values are not available as is mostly the case for irradiated material from surveillance programmes. To cope with the problems resulting from repeated transients in a complex component, incipient crack intitiation and propagation under cyclic thermal load have been investigated experimentally and theoretically on a RPV nozzle corner in the HDR plant and on a thick-walled (200 mm) hollow cylinder subjected to pressurized thermal shock (PTS). Since the OCA code covers only the linear elastic range, for this loading case elastic-plastic fracture mechanics calculations have been carried out by means of the Finite Element Method. The first test performed with high toughness material has shown good agreement with the J- integral approach. Additional validation tests are under way to demonstrate the lowest tolerable toughness level to withstand PTS without catastrophic failure. With respect to critical flaw sizes in degraded piping and vessels, the ‘leak before break’ limit curve for different loading conditions has been established and experimentally validated using piping and model vessels of different sizes, crack configurations and toughness levels. The still existing uncertainties in the detection, sizing and interpretation of signals from nondestructive examination are the background of NDE validation programmes for both medium size and full size reactor pressure vessels. Acoustic emission trials as well as extensive ultrasonic (US) examinations will be pursued in cooperation programmes, last but not least in the framework of PISC III.

Crack Propagation in MG-PSZ

ABSTRACTCertain coarse-grained Mg-PSZ's exhibit marked R-curve behavior, e.g., the crack resistance increases from ∼5 to ∼15 MPam1/2 with the first 500 microns of crack extension. These high toughness materials also show unusually-good flaw tolerance.Crack propagation and crack path development in Mg-PSZ were investigated during stable crack growth using a modified DCB specimen geometry. Crack extension was monitored with a traveling microscope equipped with a video recording system. PSZ's with various heat-treatments were tested to determine the effect of microstructure on R-curve behavior.Cracks were found to propagate discontinuously and to arrest. Smaller microcracks would develop around the arrested crack, and link together to form the macrocrack which led to eventual failure.

極低温構造材料の開発と構造設計基準 核融合炉用超電導コイルを中心として

Japan Atomic Energy Research Institute (JAERI) successfully developed high field and large coils in Cluster Test Program and Large Coil Task, respectively. When these coils are designed and constructed, there were few data of ordinary 304, 316 stainless steels and no design standard. Therefore, JAERI studied effect of carbon and nitrogen on strength and toughness of 304, 316 stainless steels to determine LCT coil structural materials and adopted strength at 4K as design stress.From these experience, JAERI concluded that it was indispensable to develop high strength, high fracture toughness and high rigidity materials and to establish design standard in order to realize superconducting coils for fusion reactor. JAERI began to develop new structural steels which had yield strength over 1, 200MPa and fracture toughness over 200MPa √m in collaboration with steel industries in 1982 and has successfully developed 6 stainless steels.AC losses which generated in metallic materials due to pulsed magnetic field is serious problem in superconducting coils. Therefore, non-metallic structural materials which have high strength and rigidity must be developed as soon as possible. In addition, material test standards at 4K have to be established to generate data base which is used in superconducting coil design. On the other hand, design standards of superconducting coil for fusion reactor is not yet established such as ASME code in commercial fission reactor at present. However, this establishment is indispensable to design and construct high field and large superconducting coils for fusion reactor.Development of cryogenic structural materials, evaluation technique of materials at 4K and key items to establish design standard are described here from viewpoint of superconducting coils development for fusion reactor.

A Single-Specimen Determination of JIc by the Ultrasonic Method and Analysis of Its Statistical Distribution Characteristics

Abstract This paper reports the summary of JIc test procedures based on an ultrasonic method proposed earlier. The single-specimen JIc measurements for low- and intermediate-strength, high-toughness materials show excellent agreement with those determined by multispeciment test procedures. The JIc values of the weldment were also determined from a single specimen. In addition, a statistical analysis of the JIc distribution was done.

Investigations into the fatigue crack initiation and propagation behaviour in austenitic stainless steel X5 CrNi 18 9 (1.4301)

AbstractControlled load fatigue‐crack growth rate tests were conducted using three point bend type specimens. The austenitic stainless steel X5 CrNi 18 9 (material number 1.4301, similar to AISI 304) was tested. For this high toughness material, the crack initiation and crack propagation is discussed in terms of linear‐elastic analysis even for the highest plasticity regime. The curve for the onset of stable crack growth during fatigue starting from a notch is discussed as well as the stable crack growth up to large plasticity. From the standpoint of load controlled fatigue it is indicated that the time between crack initiation and crack length at the „end of life”︁ is large.

潜函工事用ボルトの疲労強度

In construction works, bolts used for connecting structural components are often subjected to severe conditions. These bolts are not always made of high toughness materials. Therefore, collapses of the components are sometimes caused by failure of the bolts.In this study, to evaluate the strength of these bolts, tensile and fatigue tests were carried out on two kinds of bolts employed to build up caisson shafts. One of them was a group of the bolts which had been actually used in caisson works and the other was the unused one. Then by assuming a simplified model for a caisson shaft, stress in the bolt needed to connect blocks of the shaft was estimated by calculation and the results were compared with the experimental data.A large amount of scatter was found in both tensile and fatigue strengths for the used bolts, as compared with those of the unused ones. It was also found that the maximum stress obtained by calculation reached almost to the yield strength of the tested bolts and the calculated stress amplitude was greater than their endurance limit.From these results, the followings are recommended to prevent failure of the bolts for caisson construction: (1) to employ the bolts with higher strength than those which are generally accepted at present and (2) not to use the bolts which have been used once.

Modified manjoine WOL specimen for [formula omitted] and [formula omitted] tests

A procedure is presented by which a modified WOL specimen can be designed to withstand the load required to achieve a δ i , or J Ic test for a low yield strength, high toughness material, using information from the test of a standard WOL specimen that has suffered plastic collapse of its loading arms. The K- calibration for the family of geometries used is obtained from published data fitted to a 2-dimensional half-power series, and the load point and guage point compliances are obtained through a connected beam model that is calibrated from existing compliance data to imitate a WOL specimen. Prom these calibrations the J- integral tensile component correction factor is evaluated and found to be applicable for this type of specimen.

An assessment of acoustic emission for nuclear pressure vessel monitoring

Recent research has greatly improved our understanding of the basic mechanisms of deformation and fracture that generate detectable acoustic emission signals in structural steels. A critical review of the application of acoustic emission (AE) to the fabrication, proof testing and in-service monitoring of nuclear pressure vessels is presented in the light of this improved understanding. The detectability of deformation and fracture processes in pressure vessel steels is discussed, and recommendations made for improving source location accuracy and the development of quantitative source assessment techniques. Published data suggest that AE can make an important contribution to weld fabrication monitoring, and to the detection of defects in lower toughness materials during vessel proof testing. In high toughness materials, however, the signals generated during ductile crack growth may frequently be too weak for reliable detection. The feasibility of AE for continuous monitoring has not yet been adequately demonstrated because of high background noise levels and uncertainty about AE signal strengths from the defect growth processes that occur in service. In-service leak detection by AE shows considerable promise. It is recommended that further tests are carried out with realistic defects, and under realistic conditions of loading (including thermal shock and fatigue) and of environment.

Structure and property relationships in microlaminate Ni-Cu and Fe-Cu condensates

Microlaminate metal matrix composites offer considerable potential as high strength high toughness materials with isotropic properties in the plane of the sheet. In this paper we deal with the preparation of Fe-Cu and Ni-Cu microlaminate composites by electron beam evaporation from two sources and alternating deposition onto a rotating substrate. The thickness of the laminae was varied by changing the evaporation rates of the metals and the speed of rotation of the substrate. The deposits were characterized by optical microscopy, scanning electron microscopy and electron microprobe analysis to study the thicknesses of the laminae and interdiffusion of elements between the layers. X-ray diffraction was used to study the composition of the laminates. The microhardness and tensile strength at room and high temperatures of the laminates were studied and correlated with the thicknesses of the laminae and the microstructure. The results showed very marked increases in strength and hardness as the size of the laminae decreased below about 2 μm. Superplastic behavior was observed at a certain thickness of the laminae, strain rate and temperature.

Corrosion Fatigue Crack Growth in 7050 Aluminum Alloy Extrusions

As part of a continuing program to provide data to aid in the prediction of materials' behavior, tests were conducted to compare the effects of various corrosive environments on the fatigue crack propagation rates of several 7075-T76511 and 7050-T76511 extruded shapes. The ASTM E399 compact specimen was used, and test variables included stress intensity (A/0 and grain direction, with the stress range ratio and test frequency being held constant (R = 0.1,6 Hz). Other tests included static mechanical properties, constant amplitude fatigue, fracture toughness, stress corrosion resistance, and exfoliation corrosion resistance. The tests showed that the fatigue strengths of the two alloys were similar except that, in the low-load, high-cycle range, the 7050-T76511 provided a slight advantage. Fracture toughness (KIc) ranged from 17 to 50 ksiVm. and was found to vary significantly with specimen orientation. Fatigue crack growth rates (DA/DN) in the various test environment increased in the following order: dry air, moist air, sump water, salt water. The effect of test environment was more prominent than specimen orientation at K however, higher-toughness material did tolerate more stable crack growth before failure and, therefore, would provide improved tolerance to crack damage, i.e., longer life, in a service environment.

Fracture mechanics applied to engineering problems-strain energy density fracture criterion

Traditional design criteria make no attempt to account for the failure mode which is characteristic of a flawed, frangible structure. A design rationale is outlined for materials which contain flaws caused, for example, by metallurgical inclusions, fabrication and erection overloads, and fatigue cracking. Linear elastic fracture mechanics can be employed successfully for high strength-low toughness materials design. Laboratory test results for critical values of K c , the stress intensity factor, are limited to specimens where the loads are applied symmetrically with respect to the crack plane. The evaluation of a structure due to the influence of a crack will in general involve a mixed or combined load situation where the crack will grow in a curved fashion. This necessitates the application of energy-density factor S, which has a geometrically controlled critical direction. θ and can also serve as a measure of the toughness of the material.

Constitutive Modeling Of Functionally GradedMaterial And Its Application To Fracture In FGM

An analytical methodology is shown for studying a crack in a functionally graded material subjected to an intense thermal shock load. A theoretical elastoplastic material modeling of the functionally graded material is presented. Also independently a computational procedure of an elastoplastic constitutive law is introduced with the use of a micromecnanics analysis and a hierarchical neural network algorithm. The material is composed of ZrO and 77-6A/-4V, where the plastic flow is considered to occur in the titanium alloy phase. To detect the crack-tip fracture severity in the highly inhomogeneous media, r*integral parameter is employed for the thermal shock problem of the cracked material. Introduction In high temperature technology, multi-material bodies comprised of ceramics and high toughness materials such as metals have been thought. The discrepancies in the thermal expansion rates and other material properties between the materials causes a strength problems at the interface. Functionally graded material(FGM) is, hence, devised in the manner that the material composition is continuously varied with location in order to remove the strain discontinuity. By properly grading the composition of the constituents, the thermal stresses in the FGM may be minimized. FGM is, thus, inherently inhomogeneous, and in addition thermally inhomogeneous such that the material properties change over the wide range of temperature to that the material is designated to be subjected. The FGM is still on the materials research and development stage. To design the material, the prompt establishment of the analytical methodology is urged. For the analysis of such a material behavior and the structure, rapidly developing computational mechanics is best suited for its complexity. In the computational analysis of the FGM, the local macroscopic constitutive law for the averaged medium, where the matrix and/or the inclusion phases undergo thermo-elastoplasticity, must be known. When plastic flow occurs, if the volume fraction of the inclusion phase is high, interactions between inclusions may give a significant Transactions on Engineering Sciences vol 13, © 1996 WIT Press, www.witpress.com, ISSN 1743-3533