In situ 4D (three spatial dimensions plus time) observation of crack formation and subsequent filling of the crack by a load bearing reaction product is crucial for understanding the self-healing behavior. Furthermore, quantification of the spatial and temporal dependence is needed to validate and develop new micromechanical models for crack healing currently under development. While the challenge of in situ observation and (low level) quantification of closing and healing of cracks were already extremely demanding for (polymeric) materials that fail and heal at room temperature23, the experimental challenges become orders of magnitude more complex for (ceramic) materials that operate and heal at high temperatures. Until now, it has not been possible to directly observe the crack filling in high temperature ceramics. Thus it has also not been possible to monitor the crack repair or to establish the integrity of the repair. This is true not only for cracks formed in the pristine material, but also for cracks passing through a previously healed region. 4D X-ray tomographic microscopy using the high flux and brilliance of synchrotron X-rays is now a powerful tool for imaging the spatial and temporal evolution of microstructures from macroscopic to submicroscopic scales within a variety of materials24,25,26,27,28,29,30,31.
(a) Image of the mechanical testing rig incorporating the laser-based heating system mounted at the TOMCAT beamline; the sample stage and wedge setup are also visible. (b) Sample and wedge configuration; the arrow indicates the chevron where the cracks are generated and healed.
Mid-section of the sample: (a) first crack, (b) second crack and healed first crack, (c) healed second crack, (d) third crack and reopening of healed second crack, (d) healed third crack.
where k(T) is the temperature T dependent rate constant and n is the rate exponent, respectively. For alumina forming MAX phases n equals about 3, because in general, the oxidation obeys a cubic growth rate law35. However, for the Ti2AlC MAX phase studied here a value of about 4 was observed15. The growth of oxide in the gap of the crack proceeds as long as the oxide surface is accessible to the external environment. Thus, when the crack gap is sealed off, or when the oxide at either side closes the crack gap, the oxide growth ceases.
The global crack healing kinetics is determined from change in the volume V(t) of the crack as it fills with oxide. This volume change is determined from the 3D datasets by counting the number of voxels that represent the crack gap at a given time, t, hence:
The results presented here demonstrate the usefulness of in situ 4D (time-lapse) tomographic imaging when studying crack repair in a self-healing MAX phase materials. The change in crack face gap is observed with a high spatial resolution and has yielded the local, as well as the average, evolution of crack healing in pristine material and for the healing of a crack re-formed along a previously healed crack. For the first time we are able to resolve the spatial and temporal local crack filling kinetics.
The information presented here is crucial for constructing models predicting local damage and healing under practical operating conditions and also in the interpretation of healing kinetics as a function of initial damage topology and material compositions. This study opens new avenues for development and design of self-healing high temperature ceramics, not only for MAX phase materials when optimizing their composition and microstructure for crack healing, but also for self-healing oxide ceramics with sacrificial particles where their composition, size and distribution are crucial37.
In order to quantify the crack face gap (CFG) a median filter was applied to the original X-ray tomographic reconstructions to reduce the background noise. The cracked region in each tomographic slice was then segmented by thresholding. The gap across the faces (CFG) was measured as a function of position by counting the number of pixels occupied by the crack at each location perpendicular to the crack growth direction. In this way the current gap between the crack faces can be mapped for each crack at any stage in the repair process.
How to cite this article: Sloof, W. G. et al. Repeated crack healing in MAX-phase ceramics revealed by 4D in situ synchrotron X-ray tomographic microscopy. Sci. Rep. 6, 23040; doi: 10.1038/srep23040 (2016).
A scanning acoustic microscope (SAM) is a device which uses focused sound to investigate, measure, or image an object (a process called scanning acoustic tomography). It is commonly used in failure analysis and non-destructive evaluation. It also has applications in biological and medical research. The semiconductor industry has found the SAM useful in detecting voids, cracks, and delaminations within microelectronic packages.
- Fast production control- Standards : IPC A610, Mil-Std883, J-Std-035, Esa, etc- Parts sorting- Inspection of solder pads, flip-chip, underfill, die-attach- Sealing joints- Brazed and welded joints- Qualification and fast selection of glues, adhesive, comparative analyses of aging, etc- Inclusions, heterogeneities, porosities, cracks in material
To better understand the formation and evolution of hierarchical crack networks in shales, observations of microscopic damage, and crack growth were conducted using an in situ tensile apparatus inside a scanning electron microscope. An arched specimen with an artificial notch incised into the curved edge was shown to afford effective observation of the damage and crack growth process that occurs during the brittle fracturing of shale. Because this arched specimen design can induce a squeezing effect, reducing the tensile stress concentration at the crack tip, and preventing the brittle shale from unstable fracturing to some extent. Both induced and natural pores and cracks were observed at different scales around the main crack path or on fractured surfaces. Observations indicate that the crack initiation zone develops around the crack tip where tensile stresses are concentrated and micro/nanoscale cracks nucleate. Crack advancement generally occurs by the continuous generation and coalescence of damage zones having intermittent en echelon microscopic cracks located ahead of the crack tips. Mineral anisotropy and pressure build-up around crack tips causes crack kinking, deflection, and branching. Crack growth is often accompanied by the cessation or closure of former branch cracks due to elastic recovery and induced compressive stress. The branching and interactions of cracks form a three-dimensional hierarchical network that includes induced branch cracks having similar paths, as well as natural structures such as nanopores, bedding planes, and microscopic cracks.
Fine scratches, pores, and pullouts can be distinguished better in DF than in BF. Irregularities like pores or cracks reflect the light into the lens while all the well-polished areas are dark. This technique permits easy differentiation of pores and inclusions, very fine crack propagation, and evaluation of polish quality.
N2 - A novel approach has been designed to observe stress corrosion cracking (SCC) as it occurs in-situ, in real time. State-of-the-art contact mode high-speed atomic force microscopy (HS-AFM) has been utilised to measure in-situ SCC propagation with nanometre resolution on AISI Type 304 stainless steel in an aggressive salt solution. SCC is an important failure mode in many metal systems but has a complicated mechanism that makes failure difficult to predict. Prior to the in-situ experiments, the contributions of microstructure, environment and stress to SCC were independently studied using HS-AFM. Uplift of grain boundaries before cracking was observed, indicating a subsurface contribution to the cracking mechanism. Focussed ion beam milling revealed a network of intergranularcracks below the surface lined with a thin oxide, indicating that the SCC process is dominated by local stress at oxide-weakened boundaries. Subsequent analysis by atom probe tomography of a crack tip showed a thin Cr-rich oxide at the surface of the open crack. This study shows how in-situ HS-AFM observations in combination with complementary techniques can give new insight into the mechanisms of SCC.
AB - A novel approach has been designed to observe stress corrosion cracking (SCC) as it occurs in-situ, in real time. State-of-the-art contact mode high-speed atomic force microscopy (HS-AFM) has been utilised to measure in-situ SCC propagation with nanometre resolution on AISI Type 304 stainless steel in an aggressive salt solution. SCC is an important failure mode in many metal systems but has a complicated mechanism that makes failure difficult to predict. Prior to the in-situ experiments, the contributions of microstructure, environment and stress to SCC were independently studied using HS-AFM. Uplift of grain boundaries before cracking was observed, indicating a subsurface contribution to the cracking mechanism. Focussed ion beam milling revealed a network of intergranularcracks below the surface lined with a thin oxide, indicating that the SCC process is dominated by local stress at oxide-weakened boundaries. Subsequent analysis by atom probe tomography of a crack tip showed a thin Cr-rich oxide at the surface of the open crack. This study shows how in-situ HS-AFM observations in combination with complementary techniques can give new insight into the mechanisms of SCC.
Using scanning acoustic microscopy, optical microscopy and scanning electron microscopy, in conjunction with fractography of fractured surfaces, the crack formation and growth kinetics of subsurface fatigue cracks and surface breaking fatigue cracks near rivets have been characterized in detail in this research. The scanning acoustic microscope was used to quantitatively investigate subsurface fatigue cracks (even when they were very small) at and near countersunk rivets in riveted lap joint specimens that are similar to the riveted lap joints found in the fuselages of many aircraft. 153554b96e