R. G. Hoagland

Bio: R. G. Hoagland is an academic researcher from Battelle Memorial Institute. The author has contributed to research in topics: Fracture mechanics & Crack closure. The author has an hindex of 3, co-authored 4 publications receiving 540 citations.

Influence of Microstructure on Fracture Propagation in Rock

Abstract: Influence of Microstructure on Fracture Propagation in Rock This paper describes the results of research to correlate the fracture resistance with microstructural features of Salem limestone and Berea sandstone. Tests were conducted on wedge-loaded double-cantilever-beam specimens containing machined slots as crack starters. The fracture resistance of Salem limestone measured in terms ofR, the energy dissipated per unit area of projected surface, was found to increase in the initial stage of crack extension but finally reached a constant value which was strongly dependent on crack orientation with respect to the bedding plane. For this limestoneR ranges from about 50 joules/m2 to 230 joules/m2 (1 joule/m2 = 104 erg/cm2). The sandstone which is a more compliant rock exhibited similar fracture behavior while theR values ranged from 465 joules/m2 to 1580 joules/m2. In addition, tests in liquid nitrogen which were aimed at eliminating plastic deformation in the rocks during fracture showed little difference inR for the limestone but a substantial reduction inR for the sandstone which, in the latter case, may be caused by differential thermal expansion between the quartz grains and the calcite cement. Acoustic emissions were detected in both rocks at very early stages in the tests indicating the occurrence of microcracking near the initial slot tip at low loads. The mode of fracture and the fracture path in both materials were identified by fractography over a broad range of magnifications. The evidence gained from this work strongly points to the existence of an extensive array of microcracks produced in a region surrounding the main crack tip and which advances with it. The energy dissipated in fracturing of rock is associated with the creation of the large amount of surface area contained in this microcrack array. This picture provides a self-consistent explanation for the puzzling discrepancy between the measured tensile strength of rocks and the strengths predicted from measuredR values.

Local Yielding Attending Fatigue Crack Growth.

Abstract: Fatigue crack growth rate measurements were performed at 100°C on an Fe-3Si steel in three thickness conditions and at different ΔK-levels. The test pieces were subsequently sectioned and etched to reveal the plastic deformation attending crack growth both on the surface and in the interior. Unlike preceding studies, the Fe-3Si steel displayed classical cyclic crack growth: well-defined fatigue striations with a spacing close to the per-cycle growth rate, and essentially the same growth rates that have been reported for low and medium strength steels. A highly strained region, approximately one-fifth the size of the monotonie plastic zone, is identified as the cyclic plastic zone. On this basis three regions with distinct cyclic strain histories that precede the crack are identified: a microstrain region wherein the material receives ∼103 to 104 strain cycles in the range 0 < Δe P ≲ 10-3; a cyclic plastic zone corresponding to ∼200 cycles in the range 10-3 < Δ∈ P ≲ 10-1, and a COD-affected zone that receives ∼10 strain cycles in the range 10-1 ≲ Δ∈ P ≲ 1. It is suggested that the damage associated with the instabilities in the fatigue substructure to overstrain contribute to the growth mechanism.

Mechanisms of fast fracture and arrest in steels

Abstract: Studies of the unstable propagation and arrest of brittle fractures were conducted on four steels: plain carbon steel, 3 pct Si steel, A-517, and 4340. Unstable fractures were initiated in double-cantilever-beam test specimens by forcing a wedge between the two beams under compression. These fractures propagate at essentially constant wedge opening displacement and can be made to arrest within the confines of the specimen. The strain energy stored in the specimen at the onset of propagation was varied systematically by changing the root radius of the starting slot. The experiments show that Ka, the stress intensity at arrest, is not a materials constant but depends on the strain energy stored in the specimen. Values of άrcR, the average energy dissipation rate during propagation, calculated for the four steels, are in the range23- GIc ≲ άcrR ≲ G{Ic}. Detailed metallographic examinations show that brittle fractures appear highly segmented on interior sections, but that the individual segments are interconnected. This morphology is attributed to isolated, difficult-to-cleave regions, comparable in size to the grains, which are bypassed and remain unbroken at relatively large distances behind the crack front. Etching studies conducted on a silicon steel reveal that the plastic deformation attending crack propagation is largely confined to the plastic stretching of the ligaments behind the crack front. Increases in the size, number, and toughness of the ligaments with temperature coincide with the brittle-to-ductile transition. An analytical model consisting of an elastic crack with a regular array of tractions representing the ligaments supports the view that the ligaments are the principal source of brittle crack propagation resistance in the steels.

Dynamic crack propagation and arrest in structural steels

Abstract: This is the second of two Ship Structure Committee reports describing a three-year investigation of the crack propagation and arrest characteristics of ship-hull steels. The earlier report (SSC-242), which dealt principally with development of experimental and analytical techniques, is briefly discussed. Results are then presented for the following steels: ASTM-A517F (high strength low alloy), 9% Ni (for cryogenic service), ABS-C and ABS-E (two plates, one of which is high strength and designated EH).

Perspective on the Development of High‐Toughness Ceramics

Abstract: The science governing the strength and fracture of structural ceramics has developed from a mostly empirical topic in 1965 into a mature discipline that now sets the standards in the field of mechanical behavior. The intent of this review is to provide a perspective regarding this evolution, followed by succinct descriptions of current understanding. The rapid developments in the field are considered to have commenced upon the first concerted attempt to apply fracture mechanics concepts to ceramics, beginning in the middle 1960s. This allowed a distinction between the separate contributions to strength from the flaws in the material and from the microstructure, as manifest in the fracture toughness. Another contribution that accelerated the learning process was the development of indentation techniques, which allowed trends in the damage resistance of new ceramics to be assessed on a routine basis. However, the most important development, which originated at about the same time, was the discovery of toughened zirconia alloys. The ensuing research on these alloys established two vital precedents.

Subcritical crack growth in geological materials

Abstract: A review is presented of the experimental data on subcritical crack growth in geological materials. The main parameters describing subcritical crack growth are the critical stress intensity factor Kc, the subcritical crack growth limit Ko, and the stress intensity factor-crack velocity (K-v) relationship between Ko and Kc. The K-v data are presented in terms of an equation in which the crack velocity depends on stress intensity factor raised to a power n because this is common practice in experimental studies. These data are presented as tables and in synoptic diagrams. For silicates the value of n increases as the environment becomes depleted in hyroxyl species and with increase in the microstructural complexity of the solid. Values of n as low as 9.5 have been found for tensile cracking of quartz in basic environments and as high as 170 for tensile cracking of basalt in moist air. Insufficient experimental data are available to predict subcritical crack growth behavior at depth in the earth's crust without major extrapolations of the data base. Schematic outlines are presented, therefore, of the probable influence on subcritical crack growth of some key parameters in the crustal environment. These include stress intensity factor, temperature, pressure, activity of corrosive environmental agent, microstructure, and residual strains. In addition, a discussion is presented of the likely magnitude of the subcritical crack growth limit. For stress corrosion tensile crack growth of quartz a limit of approximately 0.2 of the critical stress intensity factor is inferred from theoretical calculations. Further problems discussed with regard to the extrapolation of experimental data to crustal conditions include the choice of a suitable equation to describe crack growth and the magnitude of parameters in these equations. A brief discussion of the double torsion testing method is presented in order to aid the interpretation of experimental results because it is almost the sole method used to study subcritical cracking in rocks.

Elastic Solids with Many Cracks and Related Problems

Abstract: Publisher Summary This chapter discusses some basic problems in mechanics of elastic solids containing multiple cracks. A number of mathematical aspects that frequently constitute fields of their own (like various numerical techniques) are discussed very briefly in the chapter. The focus is on physically important effects produced by crack interactions and to present results in the simplest form possible. The problems considered in this chapter can be divided into two groups: (1) The impact of interactions on individual cracks, particularly on the stress intensity factors (SIFs), and (2) the effective elastic properties of solids with many cracks. Problems of the first group are, generally, relevant for the fracture-related considerations; solutions are sensitive to the positions of individual cracks. Problems of the second group deal with the volume average quantities; they are relatively insensitive to the information on individual cracks. The chapter discusses, in this connection, whether correlations exist between these two groups of quantities; in particular, whether microcracking can be reliably monitored by measuring changes in the effective elastic moduli.