The adhesion strength of thin films is critical to the durability of micro and nanofabricated devices. However, current testing methods are imprecise and do not produce quantitative results necessary for design specifications. The most common testing methods involve the manual application and removal of unspecified tape. This overcome many of the challenges of connecting to thin films to test their adhesion properties but different tapes, variation in manual application, and poorly controlled removal of tape can result in wide variation in resultant forces. Furthermore, the most common tests result in a qualitative ranking of film survival, not a measurement with scientific units. This paper presents a study into application and peeling parameters that can cause variation in the peeling force generated by tapes. The results of this study were then used to design a test methodology that would control the key parameters and produced repeatable quantitative measurements. Testing using the resulting method showed significant improvement over more standard methods, producing measured results with reduced variation. The new method was tested on peeling a layer of paint from a PTFE backing and was found to be sensitive enough to register variation in force due to differing peeling mechanisms within a single test.

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Recent research has revealed that Nickel Titanium (NiTi) shape memory alloys can produce residual stresses after undergoing constrained recovery and returning to their low temperature, martensitic state while still constrained. The nature and underlying mechanisms that cause this post constrained recovery residual stress (PCRRS) are not well understood. This paper presents experimental research and results seeking to further understand the PCRRS. Experiments were performed on multiple formulations of NiTi subjected to: 1) Cyclic loading and training before producing PCRRS, 2) Repeated thermomechanical loading with large strains followed by a thermal cycle to create and re-generated the PCRRS, and 3) Creation of the PCRRS followed by repeated cycles of small, 0.5% strains. Experiments found that the training in 1) did not significantly alter the ability to produce PCRRS or its magnitude. Straining samples from the PCRRS state could reduce the residual stress state to zero stress, but the PCRRS could be recreated by repeating thermal actuation with the only significant variation being a reduction in magnitude for the first to second cycle. Multiple small strain cycles applied from the PCRRS state caused an incremental reduction in residual stress. The full PCRRS could be re-created by repeating the initial thermomechanical cycle. The values of the residual stress varied across the first 3 sets of cycles, but from the third set onward the response stabilized. These results indicate that the primary mechanisms for generating a PCRRS are stable and recoverable with only minor and diminishing variations due to training or repeated regeneration of the PCRRS. Grain boundary stabilization and similar mechanisms may be responsible for the minor variation between the first few regenerations of the PCRRS. The incremental reduction in the residual stress after exposure to small 0.5% strains must be due to a recoverable process like partial and accumulating detwinning of the NiTi with each load cycle. Further work is underway to perform microstructural analysis of samples in the various states to further the theorized material states. The ability to generate and control PCRRS has the potential to find new application and advance capabilities in fields like self-healing and fatigue resistant materials by generating stresses without the continuous application of heat energy. New forms of actuation could also be developed based on the potential energy stored in a structure through PCRRS.

Experimental investigations have found that nickel titanium (NiTi) shape memory alloys (SMAs) exhibit a phenomenon wherein the material can be recoverably deformed in its low temperature martensitic state, thermally actuated to its austenite state while constrained from returning to its parent geometry, and upon ceasing actuation and return to the low temperature martensite state it will continue to generate post constrained recovery residual stresses (PCRRS). The ability to generate a PCRRS creates a new way for SMAs to be used but the stability of the PCRRS, response to subsequent loading, and underlying microstructural mechanisms that generate the PCRRS are poorly understood. This paper presents an experimental investigation into the thermomechanical response of NiTi beginning from a PCRRS state. Multiple formulations of NiTi were exposed to training and sequences of cyclic deformation and thermal actuation in this study. It was found that repeated application of 0.5% strain reduced the residual stress with each application and thermal actuation would restore the original PCRRS. Training and mechanically working the material stabilized the thermomechanical response and did not significantly degrade the magnitude of the PCRRS. Understanding of PCRRS supports a new way for materials to be used as actuators, storing potential energy in material structures that may be beneficial to self-healing material, fatigue crack prevention and other applications.

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