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Chromium (Cr) films are widely used as interlayers to promote the adhesion of copper or gold to substrates. However, the Cr interlayer usually fractures at lower strains than the ductile metal films. In this paper, the cracking and buckling behavior of Cr films on polyethylene terephthalate (PET) substrates were studied in situ under tensile loading with the Atomic Force Microscope (AFM) and optical microscope imaging. Cr films with three nominal thicknesses of 15, 70 and 140 nm were studied. The depth and width of the cracks, as well as the height and width of the buckles, were measured from AFM images acquired at incremental loading steps. The buckle shapes at different strain levels were carefully examined using AFM line profile. It was found that at large strain levels the measured buckle shapes usually deviated from the elastic buckling mode shapes. Further in situ AFM imaging of the buckles at a smaller scan area revealed that in some cases the buckles were cracked at the apex. These in situ nanoscale measurements provided experimental observations and data for further model development and more accurate measurement of the interfacial fracture energy at the Cr-PET interface.

An experimental investigation was conducted on the initiation and growth of wrinkling due to nonuniform tension using the Yoshida buckling test. The initiation of wrinkling was detected by strain gages mounted on both surfaces of the samples in the loading and transverse directions. The bifurcation of aluminum auto body sheets appeared to be smooth and much less abrupt than that observed in a steel sheet. A special fixture was designed to, perhaps for the first time, continuously measure the in situ growth of the buckle heights so that the rates of buckle growth were monitored as functions of strain and stress in the loading direction. In contrast to what is commonly believed, it was found that the buckle height is not predominantly determined by the material yield strength, and lower averager value does not increase the rate of buckle growth. Crystallographic texture components and pole figures of the test materials were also measured, and the relationship of plastic anisotropy with wrinkling behavior was investigated by experiments with specimens aligned in the rolling direction, the transverse direction and 45-deg to the rolling direction of the sheet materials.

This paper describes tests that were conducted on high-quality circular cylindrical shells loaded in axial compression. During the loading sequence the shells were continuously examined by an optical technique employing reflected grid lines. Because of the high sensitivity of the optical system, axisymmetric-ripple-type prebuckling deformations were detected even though the shells collapsed into the conventional dimple-type buckle pattern. Although this axisymmetric-type deformation is predicted by theoretical considerations only limited evidence of its existence has been previously reported.

Experiments on the axial compression buckling of high-quality epoxy cylindrical shells with imposed dimpletype defects are described. Additionally, a technique for the manufacture of high-quality epoxy conical shells which buckle at loads approaching the classical critical load is presented. For both types of shells, prebuckling deformations have been monitored optically. The sizes of defects determined from the optical examination when applied in the space-frame approach to shell buckling have led to predicted knock-down factors which are remarkably consistent with measured knock-down factors (i.e., the ratio of actual collapse to classical critical load).

Two methods for determining the location of and load level to produce instability of compressed cylindrical shells are presented. The first relates the variation in the wall normal stiffness as a function of applied compressive force to the critical load. It uses the distribution of stiffness over the surface of the shell as a guide to buckle location. The second method associates the local dynamic mass with instability behavior. The test data presented show that either method will give excellent prediction capability from low-load-level data for shells of orthodox form. Neither method appears to apply to spirally stiffened shells. This is thought to be due to the fact that there is a substantial difference between the buckle pattern under axial compression and the imperfection shape induced by the normal displacement which is used to ascertain the wall stiffness and the dynamic mass.

Fifteen near-perfect electroformed nickel cylindrical shells of highR/t ratios have been buckled by axial compression using a special loading-rig system. Buckling-stress values averaged 85 percent, and ranged from 80 to 96 percent, of the classical theoretical bucklingstress value. The loading rig described in this report makes use of a simple procedure for accurately aligning the thin shell with the load axis and, also, for maintaining the circularity of the shell at its end by accommodating for slightly different diameters, with the use of tapered circular end caps which provide exact fits in each case. The rig also provides a setup in which shells may be free to twist, tip and translate, or be restrained from these motions. Comparison between restrained and unrestrained tests reveoled no noticeable change in the buckling load. However, with the restraining mechanism, slightly higher postbuckling loads, repeated buckling loads and an increased number of circumferential waves resulted.

This paper presents the results of a series of experiments on the progressive plastic buckling of cylindrical shells under axial compressive load. It shows that, for shell bodies with anR/t less than 100, the normal axisymmetric ring buckling will develop into nonsymmetric patterns. We demonstrate that there exists also a class of shells within this thickness-radius range for which nonsymmetric plastic buckling always occurs without the prior formation of a ring. It appears from the limited number of tests made that, for a particularR, R/t, material and rate of loading, there is a critical value ofL, above which there is a high probability of the buckle pattern developing in a nonsymmetric fashion. It seems probable, too, that there are bands ofR/t for a particularL/R, R, material and rate of loading for which the buckle number will be constant. The experiments tend to indicate that the postbuckling efficiency of the shell decreases with increasing buckle number. The nonsymmetric patterns demonstrated appear to be inextensional deformations. They are very similar in character to the Yoshimura pattern which occurs as the limiting case for thin shells in axial compression and, under impact loading. Load-displacement histories are presented for some of the various modes of failure demonstrated.