Fatigue crack growth under non-proportional mixed-mode loading in ferritic-pearlitic steel

Mode II fatigue crack growth tests as well as tests in sequential mode I and then mode II were performed on ferritic‐pearlitic steel. For ΔK_II ranging from 7 to , bifurcation occurs after 12–450 μm of coplanar growth at a decreasing speed. By contrast, hundreds of micrometres of constant speed coplanar growth were obtained under sequential mode I and then mode II loading, for and ΔK_I ranging from 0.25 to 1.0 ΔK_II . The crack growth rate is a simple sum of the contributions of each mode for ΔK_I= 0.25 ΔK_II but above this value a synergetic effect is found. The mechanism of this fast‐propagation mode is discussed in the light of strain range maps ahead of the crack tip obtained by digital SEM image correlation and elastic–plastic finite element calculations. The stability of the crack path according to the maximum growth rate criterion is demonstrated.     

The fracture of wood under impact loading

High speed motion picture photography was used to study the fracture, under impact loading, of wood beams. Photographs were taken at the rate of 500 frames per second, which permitted the crack development during the fracture event to be monitored. Load vs. time data during the test were also recorded. This paper presents a photographic record of the crack patterns which developed when the beams were tested in an instrumented impact machine.     

Dynamic fracture initiation and propagation in 4340 steel under impact loading

Dynamic fracture initiation and propagation in 4340 steel was investigated experimentally using the optical method of reflected caustics combined with high speed photography. A new crack propagation testing configuration consisting of a three point bend specimen loaded in a drop weight tower was used. It was found that prior to crack initiation the stress intensity factor time record calculated using the dynamic tup load and a static formula disagrees with the actual stress intensity factor measured by caustics. During crack propagation, the crack tip velocity and stress intensity factor time records varied smoothly and repeatably allowing for a straightforward interpretation of the data. The experiments show that for the particular heat treatment of 4340 steel used, the dynamic fracture propagation toughness depends on crack tip velocity through a relation that is a material property.

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Résumé On a étudié par voie expérimentale l'amorçage dynamique et la propagation d'une rupture dans de l'acier 4340, en utilisant une méthode optique de caustiques réfléchies, combinée à l'emploi de la photographie à grande vitesse.On a utilisé une nouvelle configuration d'essai de propagation de fissure, consistant en une éprouvette de flexion sur trois points mise en charge dans une installation d'essai Drop Weight. On a trouvé qu'avant amorçage de la tissure l'évolution dans le temps du facteur d'intensité de contraintes, calculée en utilisant l'impact de la charge dynamique et une formule applicable en conditions statiques, n'est pas en accord avec le facteur réel d'intensité de contraintes mesuré par la méthode des caustiques.Au cours de la propagation de la propagation de la fissure, les enregistrements de la vitesse de l'extrémité de la crique et de l'évolution dans le temps du facteur d'intensité de contraintes varient de manière monotone et reproductible, ce qui permet une interprétation immédiate des données d'essais. Ces derniers montrent que, pour le traitement thermique particulier considére pour la nuance 4340, la ténacité vis-à-vis de la propagation d'une rupture dynamique dépend de la vitesse de l'extrémité de la crique, via une relation que est fonction des propriétés du matériau.

     

Pulse current treatment effect on the strength of reinforcing steel and its weld joint under impact loading

We present results of static and impact tension tests of as-received reinforcing steel specimens, specimens with weld joints in their test portion, as well as specimens pretreated by high-density pulse current. As test results demonstrate, an increased strain rate enhances strength, and the pulse current treatment greatly influences the strength and plasticity of a defect-containing weld metal under static and impact tension. Translated from Problemy Prochnosti, No. 3, pp. 89–96, May–June, 2009.     

The effects of ferrrite grain size on fracture of low carbon steel under mixed modes I and II

In the previous article a new methodology is proposed for the study on fracture criterion for the notched or cracked specimens under mixed Modes I and II. In this methodology, any value of K_II/K_I can be applied to the thin-walled hollow cylindrical specimen with notch or crack, the length of which is perpendicular to the specimen axis. Thus the data can be obtained covering all values of K_II/K_I ranging from ∝ to 0, that is, from mode II to mode I.Using this method, the experimental studies were carried out on the effects of ferrite grain diameter upon the fracture of low carbon steel under mixed Modes I and II. The experimental results were compared with various fracture criteria hitherto proposed in literatures. The fracture criterion experimentally obtained is a function of ferrite grain diameter and the value of K_II/K_I, at fracture increases with increase of ferrite grain diameter. On the other hand, the overall direction of the crack growth obeys approximately maximum stress criterion or energy momentum tensor criterion, independent of ferrite grain diameter. Furthermore, this also shows that the fracture criterion does not reveal directly overall direction of crack growth. These characteristics are quite similar in trend to those of the unnotched specimens.     

The effect of thermo-mechanical loading on fracture-related parameters of austenitic steel

Failure analysis shows that 80–85% of emergency boiler shutdowns at power plants result from heating surface damage. When in service, the tubes are exposed to alternating thermo-mechanical loads, which trigger phase change affecting the individual service life. The service life sometimes differs significantly from its estimated value. This research is based on the hypothesis about the role and influence of internal structural stresses on the actual strength and long-term performance of tube products made of steels and alloys. The purpose of this work is to determine the limit state region of internal stresses of the first kind, in which microdamage will not lead to fracture. For that, we did a set of experiment studies to model the accelerated aging processes by thermal cycling and cold cyclic deformation. We chose a tube made of austenitic steel 10Cr13Mn12Si2Ni2Cu2Nb (Di59) as the object of research. The methods used were XRD analysis, microhardness testing, X-ray spectroscopy, and microstructure analysis. Due to redistributing properties, steel has no stable states: all its states are short-term and dependent on external disturbances. In accordance with the suggested hypothesis, the research findings make it possible to forecast the trends and direction of changes in the material properties. This allows us to evaluate the achievement of the limit state by the object and to use relaxations of internal stresses as a sign determining the conditions of crack initiation and propagation. The results are confirmed by the data obtained from the microstructure analysis of fractured tubes of a superheater from a functioning boiler.     

Effect of grain size on grain boundary strengthening of copper bicrystals under cyclic loading

Abstract

To clarify the strengthening effect of grain boundaries (GB), cyclic deformation behaviour of really grown [4¯79] ∥ [1¯25] copper bicrystals with different widths (4, 6, and 8 mm, denoted RB-4, RB-6, and RB-8) of com-ponent crystals and a combined copper bicrystal (denoted CB-6), obtained by sticking component single crystals G1 [4¯79] and G2 [1¯25] together, was investigated. The results showed that the cyclic saturation stresses increased in the order of bicrystals of CB-6 < RB-8 < RB-6 < RB-4. It is indicated that the GB effect caused different degrees of strengthening, which increased with the decreasing width of the RB bicrystals. By surface observation, it was found that only the primary slip system was activated in the combined bicrystal during cyclic deformation. However, an additional slip system appeared near the GB within the crystal G2 [1¯25] in the RB bicrystals (except in the primary slip system), and formed a GB affected zone (GBAZ). The width of the GBAZ was about 400 and 600 μm at plastic strain amplitudes of 0·1% and 0·2% respectively. Meanwhile, using an electron channelling contrast (ECC) technique in the SEM, the dislocation patterns near the GB and within the component crystals were observed. It was found that a two phase structure of persistent slip bands (PSBs) and matrix (or veins) can form in these bicrystals, similar to that in copper single crystals. But these PSBs cannot transfer through the GB during cyclic deformation. Based on the results above, the effect of grain size on GB strengthening of copper bicrystals was discussed.     

An investigation into the effect of grain size on fracture behaviour of a steel using crack path analysis

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of prior-austenite grain size and fracture mode on the fracture toughness of 12% Cr steel

The prior-austenite grain size of a 12% Cr steel has been varied by altering the austenitising temperature. The effect of these variations on the K _Ic fracture toughness has been studied for the as-quenched condition and after subsequently tempering at temperatures up to 550°C. The fact that different fracture modes occurred within this range of heat-treatment has allowed the effect of grain size variations on fracture toughness to be compared for specific fracture morphologies.

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Résumé En modifiant la température d'austénitisation d'un acier à 12% Cr, on a fait varier son grain austénitique primaire et l'on a étudié l'effet de ces variations sur la ténacité à la rupture K _Ic dans l'état brut de trempe et après des revenus allant jusqu'à 550°C. Pour ces divers traitements thermiques, on a constaté des modes de rupture différents, ce qui a permis d'évaluer les effets de variations de la taille du grain sur la ténacité à la rupture, pour des morphologies de rupture bien spécifiques.

     

The effect of temperatures on hybrid composite laminates under impact loading

The present experimental investigation deals with the impact responses of hybrid composites (carbon–glass fiber/epoxy) under various temperatures. A number of samples were subjected to increasing impact energy at the temperature range of ?20 to 60 °C until complete perforation of samples. With this use of increasing impact energies, it was possible to examine the impact response and failure mechanisms of hybrid composites until perforation of sample. An Energy Profiling Diagram (EPD) was used to obtain the penetration and perforation thresholds of hybrid composites. Besides, the temperature effects on impact characteristics such as load, contact time and permanent deflection were also presented in figures. Test results showed that temperature variations affect the impact characteristics of hybrid composites and they get their maximum values at ?20 °C or 60 °C.     

The investigation of aggregate grain size effect on fracture toughness of ordinary concrete structures

The material under investigation was ordinary conerete of B 20 class, different in the sizes of coarse aggregate and water-cement ratio of the same value w/c=0.55. It was found during the tests that there were two kinds of sample cracking mechanisms. Some samples cracked at the moment of the initiation of slit (when P _Q=P _max); in others the decohesion force P _max exceeded P _Q, which was identified as a minor deflection or relative extremum of the curve. The frequency of the second type of cracking increased with the growing size of coarse aggregate grains. Together with the growth of the grain size, the angle of curve inclination also became smaller. This phenomenon can be explained by the fact that, in the process of decohesion, the aggregate grains are relocated and the system of contact between them changes, so the K _Hc depends on coarse aggregate.     

The shear fracture of concrete under impact loading using end confined beams

An investigation of the shear fracture of plain and steel fibre reinforced concrete (SFRC) under impact loading was carried out on both unconfined and confined beams. It was found that confinement played a significant role on the behaviour and properties of both plain and SFRC beams.A shear mode of failure could be obtained with increasing end confinement stresses. End confined concrete beams were stronger and tougher than unconfined ones, as indicated by the increase in both peak load and fracture energy. However, they were found to be less stress rate sensitive than unconfined concrete beams. This was due to the change in the failure mode from flexure to shear with sufficient end confinement. Using end confinement appears to be a promising method for the study of shear fracture in concrete.     

Effect of the ferrite grain size on the deformation and fracture characteristics of a structural steel

Strength of Materials -