Specialized glassware, including Schlenk line tubes, separatory funnels, vials, desiccators, autoclavable tubing/glassware, micro-fabrication and thin film deposition devices, laser optics, etc., are prone to breakage from hairline cracks and from dropping the item. Wear slip-resistant or cut-resistant gloves when handling glass to prevent cuts, abrasions, and skin puncture.
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High vacuum is often applied to remove the last traces of solvent from a sample. Implosion can occur due to the use of a high vacuum and flaws in glass apparatus. Therefore, researchers should check for cracks in glassware before starting work using a Schlenk line. They should also check for the presence of condensed liquid air (oxygen condensation) in the cryotrap before starting a Schlenk line work because this condition may lead to explosions. If needed, the vacuum trap valve should be opened to dissipate cryotrapped oxygen (light blue color).
The first step in any repair process is assessing the damage to determine whether it's simple or major. Minor damage to a sheet metal structure includes missing or damaged rivets, scratches or small dents, a small crack, and a corroded sheet metal surface. Damage that exceeds the scope of these items is, in most cases, major. For example, deformed rivets often indicate damage to a sheet metal structure, and your inspection should include an area well past the deformed rivets.
If you own a production airplane, call a knowledgeable A&P mechanic, and if you own a homebuilt, you might want to call the kit manufacturer or designer. Damage to the surrounding area might be hidden, and the manufacturer should be able to offer guidance on how to properly make the repair. A damaged control surface is an example. The repair might be simple, but any change to the surface affects its balance, which can lead to flutter in flight. To complete the repair, you'll need to rebalance the surface and perhaps other components or surfaces attached to it.
Scratches happen, and you should repair them to prevent corrosion from taking place. In addition, scratches can lead to cracks. To prevent this, in most cases you can burnish or polish scratches smooth, and the best tool to use is a high-speed grinder with a Cratex abrasive wheel. This special, rubberized wheel is designed for use on sheet metal. Available at most industrial supply houses, it will allow you to easily remove the damaged area without further damage.
Small cracks are a common problem on sheet metal airplanes. Created by vibration, you'll often find them developing on areas like the engine cowling. The common fix is to "stop drill" the crack with a small diameter drill bit. In other words, you drill a small hole at each end of the crack in the hope of stopping its growth. This fix is not a repair. To repair the crack, after you stop drill it, rivet a small sheet metal patch of the same type and thickness metal over the crack to restore the area's strength and to keep vibration from acting on it further (see Figure 1).
If you have a crack in a 0.032-inch aluminum skin, stop drill both ends of the crack. Then cut a small patch out of a piece of 0.032-inch aluminum, making sure the patch provides adequate room for riveting and has no abrupt changes in its shape. Rounded or several-sided patches are preferable over a square patch. AC 43-13 1B is your reference for determining the patch's rivet layout. With the rivet layout complete, you can drill the holes using the proper size bit and hold the patch in place with Cleco fasteners. Once you're satisfied with the fit, remove the Cleco fasteners, apply zinc chromate to the backside of the patch to prevent corrosion from forming, and then rivet it in place.
To progress, we need a means of extracting quantitative information about the local crack-tip environment. Emerging techniques such as digital image correlation allow the crack-tip deformation field to be probed at the surface, but many cracks are either inherently three dimensional or initiate in the interior. This review describes the concept of three-dimensional X-ray microscopy, combining diffraction and imaging modes as in a transmission electron microscope to probe the conditions at the crack tip (figure 1). Instruments capable of this are beginning to emerge, such as ID15 at the European Synchrotron Radiation Facility (ESRF) and JEEP at the Diamond Light Source. This review explores the types of information that can be provided by diffraction (figure 1a) and by CT imaging (figure 1b). For reasons of space, many practical aspects associated with residual stress mapping by diffraction and imaging by computer tomography are not covered in this paper, but excellent introductions to both techniques are available; for example, [12] and [13,14] for diffraction and imaging, respectively.
The recent emergence of high-brilliance, hard X-ray beamlines at third-generation synchrotron sources allows crack-tip strain fields to be mapped deep within test pieces [20,21]. Further, the very high intensity and the absence of significant beam divergence means that very small beams can be used to illuminate the sample. Indeed, sample gauge sizes as small as tens of micrometres in the lateral dimensions are feasible, although the gauge size is often somewhat longer along the beam direction.
Of course, the method can also be applied using synchrotron X-rays, but the increased penetration into the surface can complicate the depth analysis using a reflection geometry [33]. This approach can, however, easily be adapted to exploit the penetration of hard X-rays, by using a transmission arrangement (using a similar arrangement to that used for stress mapping in figure 3a), so that the different depths beneath the fracture surface (crack plane) can be probed non-destructively. Consequently, this technique need not be confined only to post-mortem fractography analysis but could be applied in situ during fatigue crack growth.
Many natural materials are extremely anisotropic, giving rise to extensive crack deflection, for example wood, seashells, bone [76], bovine enamel/dentin [77] and elephant tusk [78]. For example, for bone (see 4c(i)) three-dimensional imaging of cracks growing transverse to the length of the bone shows marked crack deflections and (out-of-plane) twists as they interact with the underlying Haversian structure [79,80]. This is an important source of toughening for cracks grown in this orientation.
Combined use of diffraction contrast tomography (DCT) and CT data to identify crack-bridging grain boundary structure for intergrannular corrosion in a stainless steel wire. (a) Integranular cracking is evident from the segmented attenuation contrast image (top) but the relationship to the underlying grain structure is only evident from the magnified DCT image (bottom). (b) Two-dimensional section of the grain boundaries identified by DCT compared with the path identified by CT. The low-angle boundaries Σ1 (orange), Σ3 (red), Σ9 (blue) and other boundaries
(a) Fracture toughness resistance curve data for the transverse and longitudinal orientations in hydrated human cortical bone [89] and tomographs showing the crack growth path for cracks growing (b) transverse to the axis of the bone [80] and (c) along the length of the bone [89]. In (b) crack deflection/twisting dominates and in (c) crack bridging dominates the toughening. (Online version in colour.)
Increase in crack-tip driving force with crack length compared with that nominally applied (red squares) for a 35vol% SiC fibre/Ti alloy composite as inferred from the crack-tip stress field (orange circles) using the approach of 5a, from the stresses in the bridging fibres (green diamonds) as described in 5b and the local crack-opening displacements (purple triangles) as described in 5d. (After [49].) (Online version in colour.)
In materials science, fatigue is the initiation and propagation of cracks in a material due to cyclic loading. Once a fatigue crack has initiated, it grows a small amount with each loading cycle, typically producing striations on some parts of the fracture surface. The crack will continue to grow until it reaches a critical size, which occurs when the stress intensity factor of the crack exceeds the fracture toughness of the material, producing rapid propagation and typically complete fracture of the structure.
To aid in predicting the fatigue life of a component, fatigue tests are carried out using coupons to measure the rate of crack growth by applying constant amplitude cyclic loading and averaging the measured growth of a crack over thousands of cycles. However, there are also a number of special cases that need to be considered where the rate of crack growth is significantly different compared to that obtained from constant amplitude testing. Such as the reduced rate of growth that occurs for small loads near the threshold or after the application of an overload; and the increased rate of crack growth associated with short cracks or after the application of an underload.[2]
If the loads are above a certain threshold, microscopic cracks will begin to initiate at stress concentrations such as holes, persistent slip bands (PSBs), composite interfaces or grain boundaries in metals.[3] The stress values that cause fatigue damage are typically much less than the yield strength of the material.
Historically, fatigue has been separated into regions of high cycle fatigue that require more than 104 cycles to failure where stress is low and primarily elastic and low cycle fatigue where there is significant plasticity. Experiments have shown that low cycle fatigue is also crack growth.[4]
Fatigue failures, both for high and low cycles, all follow the same basic steps: crack initiation, crack growth stages I and II, and finally ultimate failure. To begin the process, cracks must nucleate within a material. This process can occur either at stress risers in metallic samples or at areas with a high void density in polymer samples. These cracks propagate slowly at first during stage I crack growth along crystallographic planes, where shear stresses are highest. Once the cracks reach a critical size they propagate quickly during stage II crack growth in a direction perpendicular to the applied force. These cracks can eventually lead to the ultimate failure of the material, often in a brittle catastrophic fashion. 2ff7e9595c
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