Bruchmechanische Untersuchungen an niedriglegierten Baustählen mit Hilfe des COD-Konzepts

COD Investigations on Low Alloy Steels Fracture toughness tests were performed at low alloy steels using the COD concept. Especially have been investigated the effect of evaluation methods, determination of stable crack initation and the effect of specimen geometry on the critical crack tip opening displacement: The evaluation methods in use today yield different δ_c-values, therefore a standard procedure method may be desirable. The potential method seems to give the best results for characterizing stable crack growth. A control by unloading and marking the crack extension by additional fatigue loading or other methods might be necessary. An effect of specimen thickness on the critical crack opening displacement was only found at thin specimens, in which no plane strain appeared or only arised in a small region within the center of the specimen. The crack opening displacement at the moment of first crack propagation (stable or instable) proved to be independent of specimen thickness if the amount of plane strain exceeded 50%. For this condition also was found no influence of crack length within the range for a/W = 0,2 to 0,65.     

Influence of notch root radius on ductile rupture fracture toughness evaluation with Charpy-V type specimens

The blunt notch fracture toughness of four types of carbon-manganese steel (ASTM A516 grade 70) has been determined by J-integral tests on Charpy-V type samples with different values of notch root radius, ρ. J-ρ plots, determined using specimens with a notch depth to width ratio, a/w, equal to 0.5, have shown the existence of a limiting ρ value (ρ_eff) below which applied J-intergral values at fracture initiation are constant. These ρ_eff values have been seen to depend only on second-phase particle distribution and not on their volume fraction or on the steel ferritic grain size. The procedure for deriving J-integral values at the onset of stable crack growth from J resistance curves in the case of notches has also been discussed. Experiments with Charpy specimens with a/w = 0.2 do not allow the derivation of meaningful J-ρ plots. In all cases, a ductile fracture criterion based on the constancy of the notch tip strain at rupture initiation has been proved when ρ >ρ_eff.     

Computer simulation of the Charpy V-Notch toughness test

We calculated the impact of a striker with the ASTM Type A Charpy V-Notch specimen. We used an elastic-plastic constitutive law with J₂-flow theory and isotropic hardening to model A-533 Grade B class 1 steel at 100°C. Large strain and rotation are accounted for. We found that the notch stress-strain state was independent of the three-point loading type (round- or flat-tip striker or pressure push) and was most strongly correlated with notch-opening displacement. The dynamic stress-strain state at the time of fracture initiation was obtained by comparing the calculated deformed shape with that obtained in interrupted Charpy-V-notch tests where cracking had started. This time agreed with a computed prediction of fracture initiation obtained using a computer model of material damage. Cracking starts at the notch at about 40% of the maximum load found in an instrumented Charpy test. We have also calculated the distribution and partition of specimen energy. Adiabatic heating and strain rate are discussed.     

Condition for determining the static yield and fracture of a polycarbonate plate specimen with notches

The fracture behavior of notched flat bars of polycarbonate known as a ductile polymer has been studied for a wide range of notch geometries in slow tensile tests at room temperature. The sharply notched specimen fails in a brittle manner, and the bluntly notched specimen fails in a ductile manner. The predominant factors for determining the ductile-brittle transition of fracture mode are the notch root radius and the thickness of a specimen. The experimental results have been discussed in terms of the maximum stress at the root of the notch and the relative stress distribution near the notch root.     

Notch root contraction as a fracture measurement

The value of displacement measurements in fracture toughness testing is discussed; in particular the measurement of notch root contraction. The mechanisms of plastic deformation at the notch tip are described, leading to a discussion of theoretical relationship between notch root contraction and crack opening displacement. Experimental work is described in which notched bend specimens of a low alloy steel were loaded to fracture. Each specimen was simultaneously instrumented with notch contraction, COD, and “clip-in” gauges. The equality between notch root contraction and COD was confirmed and observations are made on the relative value of the different measurements. Further observations on the distribution of contraction around the notch tip were made.     

The fracture behaviour of notched specimens of polymethylmethacrylate

The fracture behaviour of notched specimens of polymethylmethacrylate has been examined for a wide range of geometries in Charpy impact tests, and in tensile and slow bend fracture tests. It was found that the failure of the very sharply notched specimens was consistent with linear elastic fracture mechanics and defined a constant fracture toughness K _IC for a constant notch tip radius, whereas the blunt notched specimens failed at a constant critical stress at the root of the notch.     

Determination of critical CTOD using crack flank opening angle method in general yield regime

The crack tip opening displacement (CTOD) evaluation from CMOD values using compact tension (CT) specimen geometry and plastic hinge model (PHM) is well standardized in the literature. However, the model is found to be conservative for the general yielding situation. Moreover, the knowledge of the location of apparent axis of rotation which in turn depends on plastic rotational factor, r_pl, is essential for the conversion from CMOD to CTOD. In the present investigation, the ‘crack flank opening angle’ (CFOA) method has been suggested for the evaluation of plastic CTOD. This approach has facilitated the exact determination of apparent axis of rotation. The effect of the specimen thickness and notch root radius on CTOD has been investigated using the PHM and CFOA method. These results have been compared with finite element analysis results. Results achieved point out that the CTOD obtained from the traditional PHM are underestimated for the general yield situation.     

Dynamic fracture toughness testing of epoxy resin filled with sio2 particles

The fracture behavior of epoxy resin used as one of electrical insulation materials is generally brittle compared with that of metals. Therefore, when epoxy resin is used as a structural material, the effect of impact loading must be taken into consideration in design. In the present study, the dynamic fracture toughness of epoxy resin filled with SiO₂ particles has been evaluated both by the absorbed energy method and by the impact load obtained from the instrumented Charpy type impact test. Therefore, the absorbed energy has been analysed to evaluate the real fracture toughness. Moreover, the influence of inertial loading on the impact load must be also considered; therefore, the dynamic fracture toughness has been evaluated by the formula taking the inertial loading effect into consideration. Thus both fracture toughness values evaluated from absorbed energy and from impact load have been compared; as a result, a good agreement has been ascertained.It is common to perform impact test on specimens with blunt notches since they are easy to be prepared. However, variation of fracture toughness with notch root radius in the brittle material cannot be ignored. Therefore, the influence of notch root radius on the fracture toughness has been examined. As a result, it has been ascertained that the variation of fracture toughness with notch root radius follows the formula presented by Williams.     

Effect of specimen size in determining ductile to brittle transition temperature

Ductile to brittle transition temperature (DBTT) for 9Cr–1Mo steel has been determined from Charpy impact testing for full size and subsized specimens. DBTT was obtained at various percentage of upper shelf energy (USE). Assuming that most of the energy is spent in crack initiation, notch root volumes of subsized specimens (V_NS) were normalised with full size specimen (V_NF), and a power law relationship between DBTT and notch root volume has been established. From finite element method, it is observed that the sum of von Mises stress (σ_eq) and hydrostatic stress (σ_h) reaches ～2400 MPa (fracture stress, σ_f*) as the specimen dimension decreases at a temperature corresponding to 33% USE. This corresponds to ～68 J of full size specimen used in the determination of nil ductility transition temperature.     

Creep properties of a U‐type notch plate in Ni‐based superalloy

In the present investigation, the effect of notch on creep rupture behavior and creep rupture life of a Ni‐based superalloy has been assessed by performing creep tests on smooth and U‐notched plate specimen under 0°C. The finite element analysis coupled with continuum damage mechanics are carried out to understand the stress distribution across the notch throat and the creep damage evolution under multi‐axial stress state. The creep rupture life of U‐notched specimen is much larger than that of plane plate specimen under the same stress condition, indicating that there is a strengthening effect on notch specimen. Creep rupture life increases with increasing the notch radius, the smaller notch radius can induce the creep rupture easier. The effect of notch on the creep damage is also studied. It is found that the location of the maximum creep damage and the maximum equivalent creep strain initiates first at the notch root and gradually moves to the inside as the notch radius increases.     

Effect of notch root radius on tensile behaviour of 316L(N) stainless steel

Abstract

Type 316L(N) stainless steel (SS) is used as the major structural material for high temperature components of sodium cooled fast reactors. The influence of notch root radius on the tensile behaviour of 316L(N) SS under multi-axial stress state was investigated. Double U-notches with five different kinds of notch geometry were incorporated symmetrically into the tensile testing specimens by changing the notch root radius while keeping the gross diameter, net diameter and notch depth as the same for all the notches. The notch root radius was varied as 0·25, 0·5, 1·25, 2·5 and 5 mm. Tensile tests were carried out on the notched specimens at room temperature (298 K) and at 923 K at a constant strain rate of 3×10^?3 s^?1. The tensile strength and yield strength of notched specimen of 316L(N) SS increased with decrease in notch radius at both the temperatures and the notch severity was less pronounced at high temperature. The fractured notch surface was analysed using scanning electron microscope and unfractured notch was sliced along the axis and observed under optical microscope. Finite element analysis was performed on the models of notched specimens with various notch root radii. These results showed that Von Mises equivalent stress which was derived from triaxial stresses decreased with decrease in notch radius. The shift of location of peak values of maximum principal stress and hydrostatic stress towards the axis of the specimen, leading to formation of cracks, occurred at a lower nominal stress when the notch radius was increased.     

Fracture Toughness Test with a Sharp Notch Introduced by Focussed Ion Beam

The validity of fracture toughness data obtained from tests with V-notched bending bars is affected by the notch root radius and the presence of R-curve behavior. A macroscopic test specimen has been developed that contains a notch introduced by focused ion beam machining. This produces a notch root radius of less than 0.1 μm, so that notch effects can be ignored for most ceramics. Also, due to the very small notch depths the influence of a rising R-curve should be very close to that of natural cracks. First tests, carried out on a Ce-doped zirconia ceramic resulted in a toughness of K _Ic ≈ 5.9 MPa√m.     

Assessment of the period to fatigue macrocrack initiation near stress concentrators by means of strain parameters

A new approach to the experimental assessment of the local strain at a stress concentrator has been presented. It is based on a procedure of notch opening displacement measurements at certain points in the vicinity of a notch related to the effective notch radius ρ _eff = ρ + d * , where ρ is the notch radius and d * is a material constant. Different stress concentrators in structural elements were modelled for a wide variation of notch radii ( ρ = 0.1–6.5 mm) and different geometries of specimens. Hence, a basic relationship, which directly relates the local strain range Δ ε* to the period of fatigue macrocrack initiation N _i has been established. Thus, by applying the value of Δ ε* , assessed from a direct measurement at the notch root, it is possible to determine the period N _i to initiate a fatigue macrocrack of length a _i = d * for some structural components of complicated geometry.     

高速列车制动盘SiCp／A356颗粒增强铝基复合材料的热疲劳性能研究

通过SiCp／A356颗粒增强复合材料切口试样在20℃-300℃循环下的热疲劳试验,获得热疲劳裂纹形成寿命与试样切口半径及厚度等几何尺寸的关系。采用热弹塑性有限元法模拟热疲劳试验中试样切口根部的应力．应变响应,进而揭示出残余应力形成机制。结合热疲劳试验的裂纹形成寿命与有限元模拟的应力．应变响应,建立起考虑平均应力影响的...     

Numerical prediction of notch bluntness effect on fracture resistance of SM490A carbon steel

The present work investigates the notch radius effect on fracture resistance using the finite element (FE) damage analysis based on the multiaxial fracture strain model. The damage model was determined from experimental data of notched bar tensile and fracture toughness test data using a sharp‐cracked compact tension specimen. Then, the FE damage analysis was applied to simulate fracture resistance tests of SM490A carbon steel specimens with different notch radii. Comparison of simulated results with experimental data showed good agreement. Further simulation was then performed to see effects of the specimen size, thickness, and side groove on J‐R curves for different notch radii. It was found that effects of the specimen size and thickness became more pronounced for the larger notch radius. Furthermore, it was found that without side groove, tearing modulus for notched specimens was similar to that for cracked specimens, regardless of the notch radius.     

A two-parameter approach to assessing notch fracture behaviour in clay/epoxy nanocomposites

Fracture toughness and other mechanical properties of epoxy are known to be affected by the addition of nanoclays. Fracture toughness has been shown by many researchers to depend on the nanocomposite structure with well-dispersed and distributed nanoparticles resulting in improvements in this property by up to 50%. Notch fracture toughness depends on the acuity of the notch as well as on the structure of the nanocomposite. In the present work, a two-parameter fracture criterion based on a critical notch stress intensity factor, K_ρ,c, and effective T-stress, T_ef, was used to study the effect of notch severity and nanoclay addition on the fracture toughness of the epoxy matrix. The results show that the average value of K_ρ,c for neat epoxy increased with increasing notch radius while the absolute value of T_ef decreased with notch radius. The addition of nanoclay to pristine epoxy reduced the average value of K_ρ,c and increased the absolute value of T_ef. The critical notch radius was found to be around 1.0 mm and the notch sensitivity was higher for neat epoxy. SEM analysis of the fractured surfaces revealed that the lower K_ρ,c for nanocomposites in both mode I and mixed mode fractures was due to early crack initiation at clay clusters or voids at the notch root.     

Effect of notch root radius on fracture toughness of Ti–18Al–8Nb alloy

Abstract

The effect of notch root radius on the mode I fracture toughness of Ti–18Al–8Nb alloy in beta solution treated and water quenched condition was investigated. The apparent fracture toughness K _IA was found to be independent of the notch root radius below a critical notch root radius ρ ₀ and subsequently increase linearly with the square root of notch root radius ρ^1/2 beyond ρ ₀. The critical notch root radius in this alloy was found to be ～50 μm. The results were explained on the basis of strain controlled fracture model.     

Notch fracture toughness of high-strength Al alloys

In this study, the notch fracture toughness (NFT) of high-strength Al alloys was examined by a non-standardized procedure. The NFT is defined as the critical notch stress-intensity factor (NSIF) K_ρ,c, which is determined by using several methods of analysis and computing. A set of specimens with different notch root radii made from overaged 7xxx alloy forging was selected. The influence of the notch radius on the fracture toughness of the material was considered. It was found that the notch radius strongly affects the fracture behavior of forged 7xxx alloy in overaged condition. The notch fracture toughness was higher than the fracture toughness of a cracked specimen and increased linearly with notch radius. The critical notch radius was related to the spacing of intermetallic (IM) particles which promote an intergranular or transgranular fracture mechanism according to their size. It appeared that ductile transgranular fracture generated by the formation of dimples around dispersoids and matrix precipitates was predominant which indicates that intense strains are limited to a much smaller zone than the coarse IM particles spacing. This double mechanism is also operate for crack propagation of ductile fatigue. The nature and morphology of IM particles exert significant effects on the rate of fatigue crack growth and fracture toughness properties.     

Fatigue strength of steel rollers with failure occurring at the weld root based on the local strain energy values: modelling and fatigue assessment

Weldments geometry with failures occurring at the weld toe or at the weld root cannot, by its nature, be precisely defined. Parameters such as bead shape and toe or root radius vary from joint to joint even in well-controlled manufacturing operations. The worst case configuration can be achieved by modelling as a sharp, zero radius, notch both the toe and the weld root. The intensity of asymptotic stress distributions obeying Williams’ solution is quantified by means of the Notch Stress Intensity Factors (NSIFs). For steel welded joints with failures originated from the weld roots, where the lack of penetration zone is treated as a crack-like notch, units for NSIFs are the same as conventional SIFs used in LEFM. The different dimensionality of NSIFs for different notch opening angles does not allow a direct comparison of failures occurring at the weld toe or at the weld root. In order to overcome the problem related to the variability of the V-notch opening angle, a simple scalar quantity, i.e. the value of the strain energy density (SED) averaged in the structural volume surrounding the notch tip, has been introduced. This energy is given in closed form on the basis of the relevant NSIFs for modes I, II and III. The radius R_c of the averaging zone is carefully identified with reference to conventional arc welding processes being equal to 0.28 mm for welded joints made of steel.The local-energy based criterion is applied here to steel welded rollers produced by Rulmeca and subjected to prevailing mode I (with failures at the weld root). The aim of the paper is firstly to describe the employed methodology for the fatigue assessment and secondly to show the first synthesis of fatigue data by means of local SED for a specific geometry.