Detect cracking

Automatic setting of threshold of the depth of detected cracks... 

Considering the success of detecting crack tip echoes from defects at the near probe surface, future work will deal with the detection and sizing of defects on the far probe surface. Future work also relates to carrying out defect sizing in anisotropic austenitic stainless steel welds and... 

Detection Capability Can detect HTHA only after cracks have formed. Cannot detect microscopic damage. Can detect and distinguish microfissuring and micro voids due to HTHA from other indications very reliably. Can detect HTHA only after macro size cracks have formed. Cannot detect microscopic damage. Blisters are readily apparent when present. However, HTHA may frequently occur without the formation of surface blisters. Reported to be capable of detecting cracks. Currently not known whether microscopic damage can be detected. 

The preparation of precious metal supported catalysts by the HTAD process is illustrated by the synthesis of a wide range of silver on alumina materials, and Pt-, Pt-Ir, Ir-alumina catalysts. It is interesting to note that the aerosol synthesis of alumina without any metal loading results in a material showing only broad reflections by XRD. When the alumina sample was calcined to 900°C, only reflections for a-alumina were evident. The low temperature required for calcination to the alpha-phase along with TEM results suggest that this material was formed as nano-phase, a-alumina. Furthermore, the use of this material for hexane conversions at 450°C indicated that it has an exceptionally low surface acidity as evidenced by the lack of any detectable cracking or isomerization. 

Most specifications give a set strain and exposure period but it is preferable to examine test pieces at a series of times such that data can be obtained on the relationship between strain and time to appearance of cracks. ISO 1431 requires examination to be carried out with a lens of x7 magnification but, unfortunately, any examination of cracks is to some extent dependent on the eyesight of the operator. In practice, many workers say a crack is only a crack if they can see it with the naked eye. The alternative procedure of measuring relaxation in stress will be discussed later. An optical method of automatically detecting cracks has been described by Zeplichal51 but this is relatively complicated and has not been considered for standardisation. 

Fig. 9.1. Diagram of dead sound test of grinding wheel to detect cracks.

Gc can be measured directly from fracture experiments by detecting crack initiation and measuring the work in initiating the crack. 

A more detailed examination of the intranuclear channel structures and interface textures developed during these reactions demonstrated that no detectable cracking penetrated the reactant in advance of the reactant/product contact [107], This is consistent with the above conclusions. No considerable strain is associated with the interface, so that reaction is not a consequence of water loss at surfaces exposed following crack propagation through stress relief in the vicinity of and associated with the phase transition. 

Diamond Quality Assessment. The physical-chemical characteristics of nitroxyl radicals make it possible to evaluate crystal quality of diamonds, for example, with high sensitivity and reliability. It is possible to detect cracks on the crystal surface, to introduce quantitative criteria for estimation of the quality of diamonds, and to carry out defect differentiation (74). 

At the rule-based level, there are typicaUy changes in the setup to accommodate different customers, different products, or different process conditions. Thus, in Drury and Kleiner (1990), a job aid was produced to help inspectors of roUer bearings reason more effectively from customer specifications to choice of inspection standards. In aircraft-structure inspection, the calibration of an eddy-current meter must be changed by knovm rules to accommodate different thicknesses of fuselage skin or detect cracks of different depths. Errors at this level consist of misapplication of rules. 

Conventional structures, such as a bridge or a civil aircraft, are still commonly passive structures. These structures are designed to withstand the maximum expected loads, even in the presence of small cracks that may occur in service, due to corrosion, impacts with external objects or any other reason. The maximum crack size before catastrophic failure can be predicted, and the detectable crack size (of course, much smaller than the critical size), are defined according to the available non-destructive methods also the crack growth by dynamic loads can be predicted. Therefore, the time between inspections can be defined to avoid any incipient but undetected crack to become critical. This is the so called maintenance on schedule , and it has proven to be a very safe method - presently the percentage of aircraft accidents due to structural failure is very low. But the cost of these inspections is high, because they require sophisticated NDl and many labour hours. The cost of maintenance is about a quarter of the total life-cycle costs of an aircraft, similar to the fuel, crew, or acquisition costs. 

Dye penetration/magnetic particles tests— to detect cracks in welded joints... 

The GPR device is used for rapid and non-destructive determination of pavement layer thickness. It may also be used to detect cracks and determine crack depths, discontinuities within layers in general, presence of moisture, voids within unbound materials, particularly below concrete slabs, and position of reinforcement. 

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