Hydrogen-assisted cracking studies of 4340 steel by using the optical method of caustics

The optical method of caustics is used to measure the effect of hydrogen on the localized deformation within the fracture process zone (PZ). In particular, the role of the PZ in hydrogen-assisted cracking (HAC) of 4340 steel in hydrogen gas at 1 atm is examined. No change in the caustic diameter was detected prior to crack initiation but an “anomalous” enlargement of the caustic diameter as a function of crack growth was observed as a result of crack tunneling. The “center” of the caustic follows the position of the internal crack front, thus providing a simple means to monitor the growth of internal cracks. It is demonstrated that the technique provides a sensitive means for detecting HAC initiation (the smallest crack depth change detected is 35 μm) and insight into the three-dimensional character of the PZ.     

Solid state bonding of superplastic AA 7475

Abstract

Solid state (diffusion) bonds have been developed in AA 7475E sheet material using different regimes of bonding temperature, pressure, and time. The bonds produced have exhibited shear strengths in the range 30–150 MN m^?2 and have been found to be capable of withstanding peel during a slow high-temperature superplastic forming operation. However, the bonds formed under any one set of bonding conditions showed an extremely wide variation in both shear strength and shear fracture mode. The source of this wide variation has not yet been identified.

MST/601     

Tracking nanoparticles in an optical microscope using caustics

An elegant method is proposed and demonstrated for tracking the location and movement of nanoparticles in an optical microscope using the optical phenomenon of caustics. A simple and reversible adjustment to the microscope generates caustics several orders of magnitude larger than the particles. The method offers a simple and relatively inexpensive method for visualizing such phenomena as the formation of self-assembled monolayers and the interaction of nanoparticles with chemically functionalized surfaces.     

Topographic study of superplastically formed AA7475 hemispherical domes

Sliding of grain groups occurs during superplastic stretching of 7475 aluminium alloy with a spherical punch. Such co-operative grain boundary sliding (CGBS) is accompanied by cavity formation in intersection sites of CGBS surfaces, formation of striated bands and migration of sliding grain boundaries. Fibres were observed evolving from the striated bands between grains separated due to CGBS. This fibring process anticipates extreme ductility of the material and could be considered as microsuperplasticity originating from operation of a diffusion-like process or incipient melting.     

Analysis of the optical method of caustics for dynamic crack propagation

In the interpretation of experimental data on dynamic crack propagation in solids obtained by means of the optical method of caustics, it has been customary to neglect the effect of material inertia on the stress distribution in the vicinity of the crack tip. In this paper, the elastodynamic crack tip stress field is used to establish the exact equations of the caustic envelope formed by the reflection of light rays from the surface of a planar solid near the tip of a propagating crack. These equations involve the instantaneous crack tip speed, the material parameters and the instantaneous dynamic stress intensity factor, and they can be used to determine the stress intensity factor for given material parameters and crack tip speed. The influence of inertial effects on stress intensity factor measurements for system parameters typical of experiments with PMMA specimens is considered. It is found that the stress intensity factor values inferred through a dynamic analysis may differ by as much as 30–40% from values based on a quasi-static analysis.     

Grain boundary sliding during superplastic stretching of AA7475 cylindrical cups

Topographic and microstructural examinations were performed at the deformed surface and also in the bulk superplastically stretched 7475 aluminum alloy cups. Sliding of grain groups at shear surfaces spaced at approximately four grain diameters was observed at strain level, ε=0.6. The spacing between shear surfaces of such a cooperative grain boundary sliding (CGBS) decreases when the strain increases; it is approximately three grain diameters at ε=0.85. The pattern of the CGBS surfaces is consistent with that predicted using slip line field theory. Long range correlated migration of sliding grain boundaries, the formation of dispersoid free zones and fibres have also been observed. These processes can be rationalized in terms of CGBS. This revised version was published online in November 2006 with corrections to the Cover Date.     

Investigation on material mixing during FSW of AA7475 to AISI304

AA7xxx and AISI304 stainless steel (SS) are employed in promising applications. Al alloy-to-SS dissimilar joining is difficult and challenging. Major challenge in the joining of these alloys is the difficulty in mixing of these materials which possess exotic and widely distant properties. AA7475-T761 is a high strength aluminum alloy which is used in key aircraft components. Maiden AA7475-T761 and AISI304 dissimilar joints were fabricated using friction stir welding. Welding was performed with tool having pin diameter of 4 mm and offset of 1.25 mm on Aluminum side. Tool rotational speed, traverse speed and shoulder diameter were varied in the range of 450–560 rpm, 50–63 mm/min, and 12–14 mm, respectively. Mechanical tests showed that joint formed with 14 mm diameter, 560 rpm and 50 mm/min gave the best joint efficiency of 71% of Al-alloy at 7.31% elongation. The materials mixing issues during processing were analyzed with SEM mircrostructure and fractography. Metallography also revealed that offset is critical to the success of joint as it controlled effective mixing of SS and Al in 15 and 85 vol.%, respectively. SS fragments from thermo-mechanically affected zone of SS were found to be partially sheared forming layer of thickness equal to tool traverse/revolution ratio.     

Mechanical behaviour of AA 7475 friction stir welds with the kissing bond defect

Friction stir welding (FSW) is restricted to non-safety–critical aerospace components because there is no reliable method for detecting kissing bonds (KB), which may have a significant effect on fatigue life. The effects of KB defects on the tensile and fatigue properties of 7475-T7351 friction stir welds were quantitatively evaluated with respect to a reference weld without any flaws and a base material. Various KB defects were investigated with the aim of determining the defect size that does not have a significant influence on the fatigue life of joined 6.35-mm-thick plates. A critical value for the KB geometry appears to be a depth of 0.3 mm considering the influence on fatigue life for the investigated configurations. This paper also presents results from micromorphological analyses of the fatigue crack initiation from the KB and from the analysis of the weld cross-section microstructure.     

Application of the shadow optical method to fatigue and crack closure studies

In this paper it will be shown that crack closure and related crack shielding mechanisms can be studied successfully by applying the shadow optical method, also known as the method of caustics. It is shown that at least one mechanism - plasticity induced crack closure - can be identified and determined from measurements of the transverse diameter of the caustic in the crack tip region. For given fatigue conditions at which crack closure occurs, several experiments have been performed to investigate the effectiveness of the crack opening in a typical fatigue cycle.
Two different methods of closure determination have been used. A comparison is made between one of the most currently used methods of back face strain (BFS) compliance measurement and the shadow optical method (SOM). The underlying features of these two experimental techniques provide the key to finding the extent of the load in the fatigue cycles over which the crack is actually open. This redefines the value of K_min and the meaning of ΔK. The driving force, ΔK_eff is shown to be reduced in or near the fatigue threshold of the alloy, because crack closure is most effective in this part of fatigue loading. The introduction of SOM to fatigue crack closure provides a suitable alternative for finding the effective part of the stress intensity range between the minimum and maximum loads. Conclusions drawn from this work were that in addition to determining that part of the fatigue cycle over which the crack is actually open, the shadow optical method allowed an accurate interpretation of the entire fatigue cycle between K_max and K_min     

Recent application of caustics on experimental dynamic fracture studies

Combining the caustic method with high‐speed photography is an efficient optical measurement technique to study the dynamic fracture behaviours of homogenous and isotropic material. In the last decade, the main emphasis is extended to study dynamic fracture of anisotropic material and dynamic propagation of multi‐cracks and interface cracks in practical engineering materials. In this paper, the recent advances and applications about the dynamic caustic method in China are reviewed, such as impact response and dynamic fracture of composite materials (fibre composites, functionally gradient materials and nanometre composites), and dynamic interaction and propagation of multi‐cracks and interface cracks. Particularly, some new numerical methods were developed to solve the complicated caustic equations by introducing both the maximum characteristic size and the relevant angles in caustic patterns. Also, some important experimental results in fracture mechanics are described, and the potential research prospects about dynamic caustics are included as well.     

The method of caustics in environmental cracking

A simple and powerful crack growth criterion under combined environmental conditions and applied loads is formulated for mixed-mode situations based on the method of caustics. It is established that the axis of symmetry of the caustic represents the crack growth direction, while the crack growth condition is expressed in terms of the diameter of the caustic. Caustics can easily be obtained by illuminating the specimen in the vicinity of the crack tip by a laser light beam and recording the optical pattern on a viewing screen at some distance from the specimen. An elegant procedure for the experimental study of environmental cracking is thus developed.     

Effect of stress state on cavitation and hole growth in superplastic AA 7475 aluminium alloy

Abstract

A study has been made of the growth of cavities and of artificial holes in AA 7475 alloy sheet material during both uniaxial and equibiaxial tensile straining, with the object of clarifying the effect of stress state on cavitation during superplastic flow. The growth rate of cavities with strain was observed to be lower for uniaxial tension than for equibiaxial tension. An analysis of artificial hole growth data supports these observations, and is consistent with the view that continuous cavity nucleation and cavity coalescence lead to an increase in the apparent cavity growth rate during superplastic flow.

MST/1149     

Determination of stress intensity factors and J-integrals using the method of caustics

Applications of the optical shadow method of reflective caustics to the measurement of the stress intensity factor and J-integral in various specimens are investigated. The necessary experimental requirements to help in determining an accurate stress intensity factor and J-integral are described. The ratios of r₀ (radius of initial curve)/r_p, (plastic zone size) and r₀/t (thickness of specimen) are found to be very important experimental parameters with which to obtain meaningful stress and/or strain intensities surrounding crack tips. The appropriate ranges to determine accurate values of stress intensity factor and J-integral for polycarbonate (compact tension) and aluminum (c-shaped tension) specimens are presented.     

Comparison of dynamic initiation toughness measured by strain gauges and the shadow optical method

The deformation and fracture behaviour in dynamically loaded Charpy specimens of BS11 rail steel were studied by two different measuring techniques. The transient records obtained from straingauges were compared with results obtained simultaneously by the shadowoptical method. Good agreement was obtained when measuring and comparing the stress intensity factor K in the initial stages of deformation behaviour prior to crack initiation. During fast fracture, the shadow optical method indicated the influence of dynamic effects on crack propagation.     

Further possibilities of application of the method of caustics

International Journal of Fracture -     

Metallography of environment sensitive fracture

Environment sensitive cracks initiate at structurally related sites—grain, twin or interphase boundaries, slip steps or persistent slip bands. That is so even if cracks are initiated from nonstructually dependent pits and whether the mechanism of cracking is essentially dissolution controlled or due to the localized embrittlement of the metal. The subsequent growth of such cracks often, but not invariably, shows the same structural dependence as the initiation stage. The metallography of intergranular, transgranular, and mixed-mode cracking is considered in the context of various metal/environment combinations, together with its implications for some aspects of the mechansims of environment sensitive cracking. Finally, attention is drawn to the importance of the coalescence of environment sensitive cracks, manifest at the microscopic as well as the macroscopic level, in relation to crack growth kinetics and the prediction of the remaining safe life of components.     

The exact form of caustics in mixed-mode fracture: A comparison with approximate solutions

Summary The form of caustics created by stress singularities in elastic problems was up-to-now derived from the Sneddon expressions for the components of stresses at the point of singularity, which are based on the first and singular term of the series expansion of the Muskhelishvili complex stress function. In this paper the closed from expression for the Muskhelishvili complex stress function (z) was used to define the exact form of the caustic. Moreover, the forms of the caustics were constructed for several terms, besides the first one, in the Taylor expansion of (z).The approximate forms with the singular term of (z) and several terms of the Taylor expansion of (z) were compared with the form derived from the exact solution. The discrepancies between exact and approximate solutions were evaluated for the case of a slant crack in an infinite plate under in-plane biaxial loading where theK _I andK _II -mode stress intensity factors were compared as derived from the various solutions. It was concluded that, although the method of caustics yields superior results than any other experimental method, it is possible to improve these results by using either the exact solution for the particular problem, or higher order approximations.With 11 Figures