摘要:

A new approach for analyzing indentation plasticity and for determining indentation stress fields is presented. The analysis permits relations to be established between material properties (notably hardness, yield strength, and elastic modulus) and the dimensions of the indentation and plastic zone. The predictions are demonstrated to correlate with observations performed on a wide range of materials. The indentation stress fields are computed along trajectories pertinent to three dominant indentation crack systems: radial, median, and lateral cracks. The peak load and residual tensile stresses are shown to be consistent with observed trends in indentation fracture.

ISSN:0021-8979    

DOI:10.1063/1.329930

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The problem of fracture initiation within the vicinity of elastic/plastic indentations is examined. The fractures are considered to originate from one of two sources: preexistent flaws (such as surface cracks formed during mechanical finishing) or stress singularities created by shear bands. The initiation of radial cracks from preexistent flaws has been predicted and shown to correlate quite successfully with threshold data obtained using a range of brittle solids. The trends in fracture initiation associated with the existence of a nucleation step (as appropriate for many median and lateral cracks) are also discussed.

ISSN:0021-8979    

DOI:10.1063/1.329932

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

In this paper we present dynamic fatigue of fused silica optical fibers. Time‐to‐failure of the fibers under repeated stress has been investigated theoretically and experimentally, and the results have been compared with the time‐to‐failure under constant stress and increasing stress at a constant rate. As a result, it is found that the time‐to‐failure under repeated stress is related to the maximum stress with the same parameternas those representing the times‐to‐failure by the other tests. The lifetime under repeated stress can be predicted by using two constants measured by the increasing test stress at a constant rate.

ISSN:0021-8979    

DOI:10.1063/1.329933

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

A general definition of dynamic fragmentation can encompass any impulsive process which partitions a body of material into discrete domains. Two examples are fragmentation due to brittle fracture under impact loading and fragmentation due to shear banding in shock‐compression plastic deformation. In application, prediction of fragment size or shear band spacing is frequently either the objective, or else requisite to understanding the process. An approach is presented whereby surface or interface area created in the fragmentation process is governed by an equilibrium balance of the surface or interface energy and a local inertial or kinetic energy. Fragment size can be approximately related to surface or interface area. Relations provided by the analysis compare well with experimental dynamic fracture and shock‐wave shear‐band results.

ISSN:0021-8979    

DOI:10.1063/1.329934

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Upon cooling typical Pb‐alloy Josephson junction devices from 300 to 4.2 K, strain is introduced into the junction electrode films due to the thermal expansion coefficient mismatch between the films and their underlying Si substrates. To reduce the probability of device failure, it is desirable for the strain to be supported elastically by the electrodes. Studies were undertaken by transmission electron microscopy and x‐ray diffraction to correlate film microstructure with the elastic strain that can be supported at 4.2 K by Pb‐alloy films containing Au and/or In. Grain size was confirmed to be one of the key factors that control the level of the elastic strain supported at 4.2 K when the films contained no misfit dislocations. The critical grain size (gc) (the value larger than which strain relaxation occurs upon cooling to 4.2 K) was found to be ∼0.6 &mgr;m for the Pb and Pb‐alloy films prepared at 298 K by nucleating them on an ultrathin Au or oxidized In layer in a vacuum lower than 1×10−7Torr. The Pb‐alloy films prepared at 77 K supported more strain elastically; no strain relaxation was observed when their grain sizes were about a factor of two larger than thegc. When films contained misfit dislocations as observed at Pb/In interfaces in Pb‐In and Pb‐In‐Au alloy films prepared at 298 K, strain relaxation by the dislocation glide was observed even though the average grain sizes were smaller thangc. No evidence was obtained that Pb3Au and AuIn2intermetallic compounds observed in Pb‐Au and Pb‐In‐Au films, respectively, control the level of elastic strain they can support at 4.2 K. However, these compounds were found to retard grain growth of the Pb‐alloy films and to improve their grain size uniformity.

ISSN:0021-8979    

DOI:10.1063/1.329935

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Previous investigations have shown that the stability of Pb‐alloy Josephson junction devices during repeated thermal cycling between 298 and 4.2 K can be improved by using &egr;‐phase Pb‐Bi films as the counter electrode. The improvement was attributed to the hcp structure that has more resistance to strain relaxation by dislocation glide than fcc materials. In the present paper the strain behavior and microstructure changes of &agr;(fcc) and &egr;(hcp)‐phase Pb‐Bi films, which were prepared by co‐deposition of Pb and Bi onto Si substrates from a single source at 273 K and then cooled to as low as 4.2 K or heated to 350 K, have been studied by using x‐ray diffraction and transmission electron microscopy (TEM). Most of the strain introduced into the films upon cooling to 4.2 K due to the thermal expansion coefficient mismatch between the films and the substrate was found to be supported elastically by &egr;‐phase Pb‐Bi films thinner than 0.8 &mgr;m, and no change in microstructure was observed by TEM upon cooling 0.2‐&mgr;m‐thick films to 100 K. However, significant strain relaxation was observed in &agr;‐phase Pb‐Bi films as thin as 0.2 &mgr;m. The dominant strain relaxation mechanism in the &agr;‐phase film at low temperatures was very similar to that observed in Pb films: Dislocations glided across the grains to relax the strain, and the glide motion was impeded by the grain boundaries, the native oxide on the film surface, the substrate and twins. Upon warming the film to room temperature, a large number of dislocations remained, and did not slip out as in the case of the Pb films. In addition to strong resistance to strain relaxation of the &egr;‐phase films due to the hcp structure, the fiber structure of the films was found to be unfavorable for dislocation glide. Grain growth in the &agr;‐phase films commenced upon heating to 340 K, while in the &egr;‐phase films grain growth was first observed at the higher temperature of 350 K, despite the fact that the &egr;‐phase film has a lower melting temperature. From the present results it is clear that the thermal stability of the &egr;‐phase Pb‐Bi films is superior to that of the &agr;‐phase films.

ISSN:0021-8979    

DOI:10.1063/1.329936

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Changes in the microstructure of &egr;‐phase Pb‐Bi films with various thicknesses that were deposited onto oxidized Si substrates and then repeatedly thermally cycled between 298 and 4.2 K were studied using x‐ray diffraction, transmission, and scanning electron microscopy. The level of strain supported elastically by the films had a strong dependence on the film thickness. For 0.2‐&mgr;m‐thick films, almost all of the strain was supported elastically. For 1.0‐&mgr;m‐thick films most of the strain was relaxed upon cooling to 4.2 K, and grain rotation, hillock formation, and dislocation slip steps were observed after repeated cycling. For 0.4‐&mgr;m‐thick films in which about 0.1% of the strain was relaxed upon cooling to 4.2 K, such changes in the microstructure were observed after ∼200 cycles. Although these changes were significantly suppressed by 0.6‐&mgr;m‐thick SiO layers coated onto the films, the grain rotation and hillock formation were observed after 400 cycles for SiO‐coated, 0.4‐&mgr;m‐thick films similar to those used for the counter electrodes of Pb‐alloy Josephson junctions.

ISSN:0021-8979    

DOI:10.1063/1.329893

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Commercial Manganin gauges were used as in‐material gauges in planar shock experiments. The Hugoniot curve for 2024‐T351 Al was determined in the 0–10 GPa range and was found to coincide with that of 2024‐T4 Al given in the literature and measured by other techniques. The longitudinal sound speed was determined as a function of stress and it was found that Poisson’s ratio decreases while shear modulus increases in this stress range.

ISSN:0021-8979    

DOI:10.1063/1.329895

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The behavior of copper and aluminum slabs under tension, resulting from the detonation of a slab of explosive in contact with them, was studied. A new technique, based on microwave interferometry, was employed for continuous observation of the free‐surface velocity. Retrieved metal discs were scanned for microfractures. Tensile stresses, especially in regions where no observable damage occurred, were studied. They were found to be significantly lower than the negative pressures calculated by the common codes, already at early times of tension. This discrepancy increases in a gradual fashion with increased duration of tension. Observable damage appears only after a considerable reduction in the tensile stresses has already taken place. Elastic behavior of copper was studied by observing successive shock wave reverberations in the metal.

ISSN:0021-8979    

DOI:10.1063/1.329896

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The validity of a deformation model proposed by M. L. Wilkins and M. W. Guinan is tested at a microscopic scale for copper projectiles. In particular metallographic investigations confirm the curved plastic front built up during the first microseconds of impact. At higher velocities the material present in the peripheral region of the impact end is recrystallized by thermal effects caused by strong plastic deformation work. The influence of thermal softening on the position of the plastic front is discussed.

ISSN:0021-8979    

DOI:10.1063/1.329897

出版商:AIP    

年代:1982

数据来源: AIP