Linear elastic analysis of the loading rate dependency of the fracture properties of structural adhesives in the mixed mode I+II plane

Recently, we have shown that in-service repair of stiction failed MEMS devices is possible with structural vibrations. In order to further understand this phenomenon and better predict, theoretically, the onset of repair we have constructed an apparatus to determine the Mode I, II, and III interfacial adhesion energies of MEMS devices failed on a substrate. Though our method is general, we are specifically focused on devices created using the SUMMiT V process. An apparatus has been constructed that has 8 degrees-of-freedom between the MEMS device, the surface on which the device is failed, and a scanning interferometric microscope. Deflection profiles of stiction failed MEMS (micro-cantilevered beams 1500 microns long, 30 microns wide, and 2.6 microns thick) have their deflection profiles measured with nanometer resolution by the scanning interferometric microscope. Then non-linear elastic models are used in order to determine the interfacial adhesion energy between the failed micro-cantilevers and the surface. In this work we report the interfacial energies from Mode I and Mixed Mode I and II type failures. We also show further experimental results of repair of stiction failed MEMS and corresponding modeling results that use data from the Mode I and II experiments.

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Investigations on critical anisotropic triaxiality at the crack tip under mixed-mode (I + II) fracture

Proceedings of the Institution of Mechanical Engineers Part C Journal of Mechanical Engineering Science ◽

10.1177/0954406219827337 ◽

2019 ◽

Vol 233 (13) ◽

pp. 4688-4706

Author(s):

Ruchin Kacker ◽

Shailendra Singh Bhadauria

Keyword(s):

Mixed Mode ◽

Crack Tip ◽

Polar Angle ◽

Critical Values ◽

Small Scale ◽

Mode I ◽

Linear Elastic ◽

Degree Of Anisotropy ◽

Scale Yielding ◽

Yield Conditions

In this analytical study, anisotropic triaxiality, at the yield loci of the crack tip under mixed-mode (I + II) loading, has been modeled. According to the well-known M-criterion the crack of any orientation from the loading axis, initiates at a polar angle at which criticality of triaxiality occurs. Thus, the critical values of triaxiality have been obtained by statistically evaluating the proposed model for various crack inclinations. The combination of six anisotropic constants classifies the five different yield conditions, which are useful in various alloys. Hill’s generalized anisotropic yield equation generates these yield conditions. The assumption of limited plastic deformation at the crack tip has been the basis of analysis. In particular, the principle of linear elastic fracture mechanics (considering small-scale yielding) holds well. Plastic zone shapes at the crack tip for various crack inclinations are plotted to supplement the results obtained from critical values of triaxiality. For both, plane stress and plane strain conditions, the analysis reveals the regions of the degree of anisotropy for particular Lankford’s coefficient and vice versa where the crack initiation angles show variations.

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