Interfacial shearing stress in pull-out testing of dual-coated lightguide specimens

1993 ◽  
Vol 11 (12) ◽  
pp. 1905-1913 ◽  
Author(s):  
E. Suhir
Keyword(s):  
Shearing Stress ◽  
Interfacial Shearing Stress ◽  
Pull Out

Assembly Bonded at the Ends: Could Thinner and Longer Legs Result in a Lower Thermal Stress in a Thermoelectric Module Design?

2012 ◽  
Vol 79 (6) ◽  
Author(s):  
E. Suhir ◽  
A. Shakouri
Keyword(s):  
Thermal Stress ◽  
Stress Level ◽  
Thermoelectric Module ◽  
Interfacial Strength ◽  
Interfacial Stress ◽  
Electrical Performance ◽  
Shearing Stress ◽  
Interfacial Shearing Stress ◽  
Numerical Example ◽  
Module Design

An analytical (mathematical) thermal stress model has been developed for an electronic assembly comprised of identical components bonded at their end portions and subjected to different temperatures. The model is used to assess the effect of the size (dimension in the x-direction) and compliance of the bonded regions (legs) on the maximum interfacial shearing stress that is supposedly responsible for the mechanical robustness of the assembly. The numerical example is carried out for a simplified two-legged Bismuth-Telluride-Alloy (BTA)-based thermoelectric module (TEM) design. It has been determined that thinner (dimension in the horizontal, x-direction) and longer (dimension in the vertical, y-direction) bonds (legs) could result in a considerable relief in the interfacial stress. In the numerical example carried out for a 10 mm long (dimension in the x-direction) TEM assembly with two peripheral 1 mm thick (dimension in the x-direction) legs, the predicted maximum interfacial shearing stress is only about 40% of the maximum stress in the corresponding homogeneously bonded assembly, when the bond occupies the entire interface between the assembly components. It has been determined also that if thick-and-short legs are employed, the maximum interfacial shearing stress might not be very much different from the stress in a homogeneously bonded assembly, so that there is no need, as far as physical design and robustness of the assembly is concerned, to use a homogeneous bond or a multileg system. The application of such a system might be needed, however, for the satisfactory functional (thermo-electrical) performance of the device. In any event, it is imperative that sufficient bonding strength is assured in the assembly. If very thin legs are considered for lower stresses, the minimum acceptable size (real estate) of the interfaces (in the horizontal plane) should be experimentally determined (say, by shear-off testing) so that this strength is not compromised. On the other hand, owing to a lower stress level in an assembly with thin-and-long legs, assurance of its interfacial strength is less of a challenge than for an assembly with a homogeneous bond or with stiff thick-and-short legs. The obtained results could be used particularly for considering, based on the suggested predictive model, an alternative to the existing TEM designs, which are characterized by multiple big (thick-and-long) legs. In our novel design, fewer small (thin-and-short) legs could be employed, so that the size and thickness of the TEM is reduced for the acceptable stress level.

Lattice-Misfit Stresses in a Circular Bi-Material Gallium-Nitride Assembly

2012 ◽  
Vol 80 (1) ◽  
Author(s):  
E. Suhir
Keyword(s):  
Gallium Nitride ◽  
Cross Sections ◽  
Circumferential Stress ◽  
Lattice Misfit ◽  
Theory Of Elasticity ◽  
Shearing Stress ◽  
Interfacial Shearing Stress ◽  
Stress Theory ◽  
Circular Configuration ◽  
Gan Film

A simple and physically meaningful analytical (“mathematical”) predictive model is developed using two-dimensional (plane-stress) theory-of-elasticity approach (TEA) for the evaluation of the effect of the circular configuration of the substrate (wafer) on the elastic lattice-misfit (mismatch) stresses (LMS) in a semiconductor and particularly in a gallium nitride (GaN) film grown on such a substrate. The addressed stresses include (1) the interfacial shearing stress supposedly responsible for the occurrence and growth of dislocations, for possible delaminations, and for the cohesive strength of the intermediate strain buffering material, if any, as well as (2) normal radial and circumferential (tangential) stresses acting in the film cross-sections and responsible for the short- and long-term strength (fracture toughness) of the film. The TEA results are compared with the formulas obtained using strength-of-materials approach (SMA). This approach considers, instead of the actual circular substrate, an elongated bi-material rectangular strip of unit width and of finite length equal to the wafer diameter. The numerical example is carried out, as an illustration, for a GaN film grown on a silicon carbide (SiC) substrate. It is concluded that the SMA model is acceptable for understanding the physics of the state of stress and for the prediction of the normal stresses in the major midportion of the assembly. The SMA model underestimates, however, the maximum interfacial shearing stress at the assembly periphery and, because of the very nature of the SMA, is unable to address the circumferential stress. The developed TEA model can be used, along with the author's earlier publications and the (traditional and routine) finite-element analyses (FEA), to assess the merits and shortcomings of a particular semiconductor crystal growth (SCG) technology, as far as the level of the expected LMS are concerned, before the actual experimentation and/or fabrication is decided upon and conducted.

Bonding strength of a carbon nanofiber array to its substrate

2006 ◽  
Vol 21 (11) ◽  
pp. 2922-2926 ◽  
Author(s):  
Yi Zhang ◽  
Ephraim Suhir ◽  
Yuan Xu ◽  
Claire Gu
Keyword(s):  
Bonding Strength ◽  
Carbon Nanofiber ◽  
Copper Substrate ◽  
Applied Force ◽  
Stress Model ◽  
Shearing Stress ◽  
Shearing Force ◽  
Interface Failure ◽  
Interfacial Shearing Stress

The bonding strength of a carbon nanofiber array (CNFA) grown on a copper substrate is evaluated based on the measured shearing force-at-failure and the developed analytical stress model that enables one to determine the magnitude and the distribution of the interfacial shearing stress causing the measured (given) shearing force. The experiment is conducted using specially designed test specimens. A table version of the Instron tester is used to measure the applied force and the corresponding displacement in shear. The maximum predicted shear-off stress is about 300 psi (0.211 kgf/m2), and was determined, based on the developed stress model, as a product of the measured 5 kgf/m force at the interface failure and the computed parameter k = 0.0422 m–1 of the interfacial shearing stress.

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