Subsurface cracks

This work presents two procedures of quantitative evaluation of the material discontinuities, using the eddy current method. One of the procedures concerns the long surface or subsurface crack-type discontinuities in a flat conductive body. The second procedure allows a quantitative evaluation of short discontinuities, such as voids, inclusions etc. 

Figure 6-2. Magnetic particle testing is used to detect discontinuities such as fine surface or subsurface cracks.

Cracks in the HAZ are usually sited either at the weld toe, the weld root or in an underbead position. These positions are shown schematically for fillet welds and butt welds in Fig. 1.1. In fillet welds, HAZ cracks are usually oriented along the weld length, but in butt welds subsurface cracks can be transverse to the weld. Hydrogen cracks examined in sections of a weld under the microscope may be intergranular, transgranular or a mixture with respect to the transformed microstructures in which they lie. 

CF cracks are always initiated at the surface, unless there are near-surface defects that act as stress concentration sites and facilitate subsurface crack initiation. Crack initiation takes place independently of fatigue limit in air as it can be decreased or eliminated through the increase of dissolution rates at anodic sites. Localized corrosion such as pitting favors fatigue crack initiation through stress concentration and a local acidic environment. The two main mechanisms of CF are anodic slip dissolution and HE (80). 

Surface fatigue wear occurs during repeated sliding or rolling over a track. Surface or subsurface crack formation leads to breakup of the surface. 

This equation corresponds to Archard s law with = l/ crit- The wear coefficient in this case represents the inverse of the critical number of asperity contacts that lead to fracture by fatigue. At each passage of an asperity the subsurface crack grows by an increment until it reaches a critical size at which fracture occurs. The quantity Merit can be associated to the number of cycles leading to fracture in a common fatigue test. Many parameters affect the value of Mj in a wear test and it is not possible to calculate it from first principles. For the model structure shown in Figure 10.19, the critical number of cycles can be brought in relation with the number of inclusions or precipitates that act as crack initiation sites [10]. 

The four wear mechanisms described in this section all lead to Archard s law, but the physical interpretation of the wear coefficient differs. In the adhesive wear model the wear coefficient expresses the probability that an adhesive junction leads to formation of a wear particle. In the abrasive wear model the wear coefficient depends only on the geometry of the abrasive. The wear coefficient in delamination wear characterizes the critical number of cycles leading to fatigue fracture of microscopic subsurface cracks. Finally, the wear coefficient in oxidative wear includes the growth constant and the critical thickness of a surface oxide film. 

FIG. 2—Particle erosion of brittle material (A) subsurface crack formed by Impacting particle (B) the result of multiple Impacts causing fracture linkage and chipping out of material. 

Ultrasonic Uses high-frequency sound waves to locate defects. Very sensitive can detect very fine surface and subsurface cracks. Equipment is portable Personnel must be trained to interpret equipment response. Not effective on rough surfaces or welds with backing rings. 

Flow patterns in the surface around indents and their significance are dealt with in Chapter 3. The development of surface and subsurface crack patterns is the subject of Chapter 5. Changes of shape with time and temperature—that is, the relaxation and creep of indentations—are covered in Chapter 4. All the indentation methods can be adapted to make studies at high and low temperatures and with various degrees of atmosphere control with the result that in a quantitative sense indentation has massively... 

Fatigue Wear Associated with cyclic stress variations and therefore, the lifetime of the material is dependent on the number of cycles. Cyclic deformation of the contacting surfaces leads to the initiation and propagation of microcracks (Rowe, 1980). Subsurface crack initiation generally occurs in the region of maximum shear stress which wilt depend upon the geometry of the materials... 

Claude Bathias Paul C. Paris, 2010. Subsurface crack initiation and propagation mechanisms in gigacycle fatigue [J]. Acta Material, (7) 6047. 

A Atrens, W Hoffelner, TW Duering, J Allison, Subsurface crack initiation in high cycle fatigue in Ti-6A1-4V and a typical martensitic stainless steel, Scripta Metallurgica, 1983,17, 601. 

The development of new technologies and the improvement of existing ones for suspension checks and detection of subsurface cracks in wheels and rails are essential. 

Earthquake Focal Mechanisms Earthquake focal mechanisms describe the nature of deformation at the earthquake source. Most VT earthquakes are double-couple, indicative of shear motion on a fault plane. The orientation of focal mechanism describes the stress field at the source. Rotations of the focal mechanism with time from, for example, strike-slip to reverse, have been reported before some eruptions and interpreted in terms of magma pressurization changing the orientations of the principal stresses in the vicinity of the magma body (Roman and Cashman 2006). Non-double-couple earthquakes indicate a more complex source, including tensile failure and events associated with finite volume changes, such as explosions or collapses. In Iceland, for example, such sources have been associated with tensile failure during the opening of subsurface cracks. 

Todo S, Tomita N, Kitakura T, Yamano Y. Effect of sliding locus on subsurface crack formation in ultra-high-molecular-weight polyethylene knee component. Biomed Mater Eng 1999 9(1) 13—20. 

Scanning in Total Elbow 34.5 Assessing Subsurface Cracking... 

As shown in Chapter 22, UHMWPE components can undergo oxidative degradation both on the shelf and in vivo. One of the consequences of sevCTe UHMWPE oxidation is subsurface cracking that ultimately leads to delamination [24]. 

FIGURE 34.11 Examples of subsurface cracking in total hip arthroplasty, total elbow arthroplasty, and total knee replacement. 

This damage mode is prevalent in historic total knee replacements however, it also occurs in contemporary UHMWPE components that have been sterilized in an inert environment. Currier et al. have hypothesized that severe oxidation can lead to delamination of the rim of highly crosslinked UHMWPE acetabular liners [25]. Therefore, it is important to be able to investigate the subsurface cracking and delamination that can occur in UHMWPE. 

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