China
Africa
Explore chapters and articles related to this topic
Testing and Reliability Characterization Methods for Flexible Hybrid Electronics
Published in Katsuyuki Sakuma, Krzysztof Iniewski, Flexible, Wearable, and Stretchable Electronics, 2020
Pradeep Lall, Jinesh Narangaparambil, Kartik Goyal
Stress relaxation test was performed to understand the substrate behavior when subjected to stretching. Stress relaxation is a material property to study the decrease in stress over a period of time. In this study, instead of stress, we will be looking at the decrease in resistance over a finite period of time. The sample was stretched to 8% and left for 14 h. Figure 13.52 (top) shows the change in resistance for the whole period of time. We can see an initial spike in first few minutes, and over a period time, it stabilizes to 100% change, which can be regarded as a plastic deformation. To analyze the initial spike, a zoomed-in graph is plotted in Figure 13.52 (bottom). When the sample is stretched to 8% level, the highest change in resistance is around 250%, which plateaus out eventually. This decrease may be attributed to the change in properties of the stretchable substrate when it is in a stretch state. Polyurethane undergoes multiple changes while it is stretched such as microphase separation, change in the polymer bonds, which may be the reason for the decrease in the resistance after a period of time. It can also be argued that this decrease is the stretch recoverable property of the PE874 conductive silver paste used on the substrate.
Structure and Properties of Polymer Matrix
Published in Noureddine Ramdani, Polymer and Ceramic Composite Materials, 2019
In addition, the modulus of polymers also increases as the cross-link density of polymers rises. Crystalline and filled polymers resemble polymers with low cross-link density at temperatures below the melting temperature, Tm. On the other hand, the crystallinity of the polymer improved the strength because in the crystalline phase the intermolecular bonding is more abundant. Thus, the deformation of a polymer resulted at higher strength provoking oriented chains. The mechanical properties of the polymer are significantly dependent on the temperature. Generally, by increasing the temperature, the elastic modulus and tensile strength are reduced, while the ductility is enhanced. In addition, at a fixed temperature, when the polymer is strained to a given value, then the stress needed to keep this strain decreases with time. This phenomenon is known as stress relaxation and occurs because the molecules of polymers relax with time, and to maintain the level of strain, somewhat smaller values of stress are required.
Thermomechanical Analysis and Design
Published in Michael Pecht, Handbook of Electronic Package Design, 2018
Stress relaxation phenomena have a critical adverse influence on the fatigue damage of the component during each strain cycle. This is qualitatively illustrated in Figures 8.13 and 8.14. Figure 8.13 shows the stress-strain plot of a material exhibiting no stress relaxation when subjected to a cyclic strain Δϵ. One school of thought postulates that the area within the cyclic strain hysteresis loop is a qualitative measure of the fatigue damage the material has suffered. This damage criterion is based on the contention that fatigue damage can be viewed as a progressive exhaustion of ductility and can be characterized by the inelastic work done during each cycle. Figure 8.14 shows the same figure for a material that exhibits stress relaxation. The large increase in the hysteresis area indicates earlier ductility exhaustion and shorter fatigue life.
The role of geometry on the viscoelastic materials properties: tensile and stress relaxation of the yarn
Published in The Journal of The Textile Institute, 2019
Nasrin Hashemi, Azita Asayesh, Ali Asghar Asghariyan Jeddi, Mohammad Amani Tehrani
Textile materials are subjected to various types of forces during use. Tensile, compressive, flexural, torsional, and even their complex combinations are some typical examples of these. All these forces lead to deformation which is time-dependent. Also during mechanical processing, yarns are subjected to different levels of strains for varying durations of time, while operating on loom and knitting machines (Subrata & Anindya, 2006). In materials science, stress relaxation is the observed decrease in stress in response to the same amount of strain generated in the structure (Meyers & Chawla, 1999). Stress relaxation in yarns affects the processing and end-use performance of the final product (Gao, Meng, Sun, & Sun, 2011). Zou (2012) showed that the stress relaxation behavior of yarn is influenced by the yarn type, strain level, tensile rate, and yarn count. He observed that when the tensile strain is lower and the tensile rate is higher, the vortex spun yarn displays a more obvious stress relaxation.
Viscoelastic modeling of extrusion damage in elastomer seals
Published in Soft Materials, 2019
R. J. Windslow, J. J. C. Busfield
Elastomers are viscoelastic in nature; hence, their deformation in response to the application of a load or a pressure changes with the time of loading. Viscoelastic relaxation in elastomers is known to manifest as two related but different phenomena, creep, and stress relaxation (7). Whilst the physics behind the two are similar, as both have physical and chemical contributions, creep is the gradual deformation of an elastomer under a fixed loading whereas, stress relaxation is the gradual drop off in force as an elastomer is held at a fixed displacement. Elastomer seals are installed into fixed volume glands, so their displacement is generally held constant. This would suggest that any relaxation should manifest itself as stress relaxation in most cases. Nevertheless, seals do exhibit creep at times, particularly during extrusion problems. When sealing against high pressures, creep will cause the seal to gradually flow into the extrusion gap over time, even if the initial deformation did not lead to extrusion. Therefore, by not accounting for the elastomer’s viscoelasticity, previous researchers have probably overestimated the fluid pressure required to cause extrusion, and extrusion fracture.
Effects of loading rate, applied shear strain, and magnetic field on stress relaxation behavior of anisotropic magnetorheological elastomer
Published in Mechanics of Advanced Materials and Structures, 2022
Tran Huu Nam, Iva Petríková, Bohdana Marvalová
In a stress relaxation test, a constant strain ε = ε0 is applied and the stress is measured. The relaxation modulus is estimated using the formulation G(t) = σ(t)/ε0. Viscoelastic analysis using fractional derivative models requires an analytic expression for the relaxation modulus. Therefore, in the viscoelastic analysis using the four-parameter fractional derivative Zener model, the stress relaxation modulus is obtainable by applying the Laplace transform to Eq. (3). The relaxation modulus of the four-parameter fractional derivative Zener model is expressed as follows: