Ductile fracture mechanisms in a structural steel

Microstructural fracture processes in a BS4360 Grade 50D structural steel with lower sulphur content were studied in smooth tensile specimen tests and Charpy-size bend bar tests. Based on the experimental analysis, an experimental void growth relation with the plastic strain and stress triaxiality and multiplying factor on void growth were determined. Experimental results show that the void growth relation can be reasonably used to estimate the constraint in the specimens containing the notch or crack, also they can be used to evaluate the variations of the stress triaxiality in front of the notch and crack tip under general yielding condition. Side-grooves obviously increase the constraint of the CVN specimens. Strain hardening leads to increasing the stress triaxiality, and decelerating the net void growth. This is especially true for the values of stress triaxiality more than about one. Additionally, the effect of the stress triaxiality on the critical void growth corresponding to the onset of ductile tearing was preliminarily investigated. In this work, a large number of smaller specimens were tested to investigate the ductile-brittle transition behaviour of the structural steel. A void growth rate explanation was suggested for evaluating the temperature transition behaviour. The elastic-plastic fracture tough-ness values based on small specimen tests, such as pre-cracked side-grooved bending specimen and short bar tensile specimen, may give large overestimates of the plane strain fracture toughness.

Development and evaluation of a non-destructive adhesion test

The object of this work was to further develop the idea introduced by Muaddi et al (1981) which enables some of the disadvantages of earlier destructive adhesion test methods to be overcome. The test is non-destructive in nature but it does need to be calibrated against a destructive method. Adhesion is determined by measuring the effect of plating on internal friction. This is achieved by determining the damping of vibrations of a resonating specimen before and after plating. The level of adhesion was considered by the above authors to influence the degree of damping. In the major portion of the research work the electrodeposited metal was Watt's nickel, which is ductile in nature and is therefore suitable for peel adhesion testing. The base metals chosen were aluminium alloys S1C and HE9 as it is relatively easy to produce varying levels of adhesion between the substrate and electrodeposited coating by choosing the appropriate process sequence. S1C alloy is the commercially pure aluminium and was used to produce good adhesion. HE9 aluminium alloy is a more difficult to plate alloy and was chosen to produce poorer adhesion. The "Modal Testing" method used for studying vibrations was investigated as a possible means of evaluating adhesion but was not successful and so research was concentrated on the "Q" meter. The method based on the use of a "Q" meter involves the principle of exciting vibrations in a sample, interrupting the driving signal and counting the number of oscillations of the freely decaying vibrations between two known preselected amplitudes of oscillations. It was not possible to reconstruct a working instrument using Muaddi's thesis (1982) as it had either a serious error or the information was incomplete. Hence a modified "Q" meter had to be designed and constructed but it was then difficult to resonate non-magnetic materials, such as aluminium, therefore, a comparison before and after plating could not be made. A new "Q" meter was then developed based on an Impulse Technique. A regulated miniature hammer was used to excite the test piece at the fundamental mode instead of an electronic hammer and test pieces were supported at the two predetermined nodal points using nylon threads. This instrument developed was not very successful at detecting changes due to good and poor pretreatments given before plating, however, it was more sensitive to changes at the surface such as room temperature oxidation. Statistical analysis of test results from untreated aluminium alloys show that the instrument is not always consistent, the variation was even bigger when readings were taken on different days. Although aluminium is said to form protective oxides at room temperature there was evidence that the aluminium surface changes continuously due to film formation, growth and breakdown. Nickel plated and zinc alloy immersion coated samples also showed variation in Q with time. In order to prove that the variations in Q were mainly due to surface oxidation, aluminium samples were lacquered and anodised Such treatments enveloped the active surfaces reacting with the environment and the Q variation with time was almost eliminated especially after hard anodising. This instrument detected major differences between different untreated aluminium substrates.Also Q values decreased progressively as coating thicknesses were increased. This instrument was also able to detect changes in Q due to heat-treatment of aluminium alloys.

Electrodeposition on magnesium alloy diecasting

Magnesium alloy diecasting AZ91CC, AZ61CC', AZ91HC and AZ71HC were electroplated using different pretreatment sequences which incorporated conventional zincate immersion processes. Satisfactory peel adhesion in excess of 7. 7 KNm -1 was achieved on AZ61CC using a sequence which was designated Canning. The comparatively low adhesion achieved on the AZ91HC was due to its poor surface quality as cast. Growth of deposits was monitored using a strip-and-analysis technique and the morphology of the various deposits were studied using scanning electron microscopy. Different pretreatment sequences resulted in different surface responses for the alloys but all alloys behaved in a similar manner in a particular sequence with regard to potential time-curves and the rate of zinc deposition. The role of fluoride in both the second stage solution and zinc immersion stages of the Canning pretreatment sequence was studied using techniques listed above and Auger electron spectroscopy. Complete coverage of the magnesium alloy surface with immersion zinc was achieved when fluoride was absent from the zincating solution. However, a zero adhesion value was indicated in both thermal cycling and peel tests. The presence of fluoride in the immersion zinc solution suppressed the rate of zinc deposition and affected the time taken to reach equilibrium during potential-time determinations. A mechanism is suggested to explain the significance of fluoride additions to the processing solutions. pH and composition of the zincating solution had a significant effect on the time taken to produce the step observed in the potential/time curves and hence equilibrium potential. Immersion zinc deposition occurred rapidly at first but then changed to a lower uniform rate at a point corresponding approximately to the step in the potential/time curve. Although the minimun levels of adhesion, using the Canning sequence, varied from 7.72 KNm-1 for alloy AZ61CC to 1.54 KNm-1 for alloy AZ91HC, all the alloys revealed ductile failure characteristics in the surface layer of the substrate after peel testing. Plated magnesium alloys exhibited good corrosion resistance when appropriately pretreated and overplated with adequate nickel chromium coatings. The immersion zinc layer was not preferentially attacked when pits penetrated to the coating/substrate interface. Hemispherical pits formed and attack on the substrate was severe. Of the pretreatment sequences investigated, the Canning one was the most premising with respect to peel adhesion and corrosion behaviour.

Fracture toughness of a series of high chromium abrasion resistant alloys

The fracture properties of a series of alloys containing 15% chromium and 0.8 to 3.4% carbon are investigated using strain fracture toughness testing techniques. The object of the work is to apply a quantitative method of measuring toughness to abrasion resistant materials, which have previously been assessed on an empirical basis; and to examine the relationship between microstructure and K10 in an attempt to improve the toughness of inherently brittle materials. A review of the relevant literature includes discussion of the background to the alloy series under investigation, a survey of the development of fracture mechanics and the emergence of K10 as a toughness parameter. Metallurgical variables such as composition, heat treatment, grain size, and hot working are ???? to relate microstructure to toughness, and fractographic evidence is used to substantiate the findings. The results are applied to a model correlating ductile fracture with plastic strain instability, and the nucleation of voids. Strain induced martensite formation in austenitic structures is analysed in terms of the plastic energy dissipation mechanisms operating at the crack tip. Emphasis is placed on the lower carbon alloys in the series, and a composition put forward to optimise wear resistance and toughness. The properties of established competitive materials are compared to the proposed alloy on a toughness and cost basis.

Isothermal fatigue of an aluminide-coated single-crystal superalloy:Part I

The isothermal fatigue behavior of a high-activity aluminide-coated single-crystal superalloy was studied in air at test temperatures of 600 °C, 800 °C, and 1000 °C. Tests were performed using cylindrical specimens under strain control at ∼0.25 Hz; total strain ranges from 0.5 to 1.6 pet were investigated. At 600 °C, crack initiation occurred at brittle coating cracks, which led to a significant reduction in fatigue life compared to the uncoated alloy. Fatigue cracks grew from the brittle coating cracks initially in a stage II manner with a subsequent transition to crystallographic stage I fatigue. At 800 °C and 1000 °C, the coating failed quickly by a fatigue process due to the drastic reduction in strength above 750 °C, the ductile-brittle transition temperature. These cracks were arrested or slowed by oxidation at the coating-substrate interface and only led to a detriment in life relative to the uncoated material for total strain ranges of 1.2 pet and above 800 °C. The presence of the coating was beneficial at 800 °C for total strain rangesless than 1.2 pet. No effect of the coating was observed at 1000 °C. Crack growth in the substrate at 800 °C was similar to 600 °C; at 1000 °C, greater plasticity and oxidationrwere observed and cracks grew exclusively in a stage II manner.

Fracture behaviour of an aluminide coating on a single crystal nickel base superalloy

Measurement of the coating fracture strain of an aluminide coating on a single crystal nickel base superalloy has been performed both in three-point bending and using variable wall thickness testpieces. As-aged specimens, 28 to 33 μm in thickness, were tested at room temperature, 600, 700 and 750 °C; specimens pre-exposed for 140 h at 850 and 1100 °C in air and vacuum were tested at room temperature. Fracture strains varied from 0.52 to 0.70% for as-aged specimens tested at temperatures up to 700 °C and specimens exposed at 850 °C and tested at room temperature. The crack path for these conditions was intergranular or transgranular in the main coating, along carbide-matrix interfaces in the coating transition zone, and at an angle of 30-45° to the original crack path in the substrate. The as-aged coating tested at 750 °C was ductile; a ductile-brittle transition occurs between 700 and 750 °C for the strain rate used (1 × 10-5 s-1). Following 1100 °C pre-exposure, specimens were ductile at room temperature with fractures strains of several percent. In this condition the crack morphology changed to one of subsurface nucleation in β grains and at β-γ′ interfaces. © 1993.

Anisotropic characterization of crack growth in the tertiary flow of asphalt mixtures in compression

Asphalt mixtures exhibit primary, secondary, and tertiary stages in sequence during a rutting deterioration. Many field asphalt pavements are still in service even when the asphalt layer is in the tertiary stage, and rehabilitation is not performed until a significant amount of rutting accompanied by numerous macrocracks is observed. The objective of this study was to provide a mechanistic method to model the anisotropic cracking of the asphalt mixtures in compression during the tertiary stage of rutting. Laboratory tests including nondestructive and destructive tests were performed to obtain the viscoelastic and viscofracture properties of the asphalt mixtures. Each of the measured axial and radial total strains in the destructive tests were decomposed into elastic, plastic, viscoelastic, viscoplastic, and viscofracture strains using the pseudostrain method in an extended elastic-viscoelastic correspondence principle. The viscofracture strains are caused by the crack growth, which is primarily signaled by the increase of phase angle in the tertiary flow. The viscofracture properties are characterized using the anisotropic damage densities (i.e., the ratio of the lost area caused by cracks to the original total area in orthogonal directions). Using the decomposed axial and radial viscofracture strains, the axial and radial damage densities were determined by using a dissipated pseudostrain energy balance principle and a geometric analysis of the cracks, respectively. Anisotropic pseudo J-integral Paris' laws in terms of damage densities were used to characterize the evolution of the cracks in compression. The material constants in the Paris' law are determined and found to be highly correlated. These tests, analysis, and modeling were performed on different asphalt mixtures with two binders, two air void contents, and three aging periods. Consistent results were obtained; for instance, a stiffer asphalt mixture is demonstrated to have a higher modulus, a lower phase angle, a greater flow number, and a larger n1 value (exponent of Paris' law). The calculation of the orientation of cracks demonstrates that the asphalt mixture with 4% air voids has a brittle fracture and a splitting crack mode, whereas the asphalt mixture with 7% air voids tends to have a ductile fracture and a diagonal sliding crack mode. Cracks of the asphalt mixtures in compression are inclined to propagate along the direction of the external compressive load. © 2014 American Society of Civil Engineers.

Simple room temperature method for polymer optical fibre cleaving

In this paper, we report on a new method to cleave polymer optical fibre. The most common way to cut a polymer optical fibre is chopping it with a razor blade; however, in this approach both the fibre and the blade must be preheated in order to turn the material ductile, and thus, prevent crazing. In this paper, we make use of the temperature-time equivalence in polymers to replace the use of heating by an increase of the cleaving time and use a sawing motion to reduce fibre end face damage. In this way, the polymer fibre can be cleaved at room temperature in seconds with the resulting end face being of similar quality to those produced by more complex and expensive heated systems.