Creep and Stress Relaxation Behavior of Homogeneous and Reinforced Compliant Mechanisms and Segments

2018 ◽  
Author(s):  
Joshua Crews ◽  
Lokeswarappa R. Dharani ◽  
Ashok Midha
Keyword(s):  
Stress Relaxation ◽  
Compliant Mechanisms ◽  
Test Results ◽  
Stress Relaxation Behavior ◽  
Relaxation Resistance ◽  
New Class ◽  
Engineering Plastics ◽  
Physical Tests ◽  
Metallic Reinforcement ◽  
Creep And Stress Relaxation

Two critical disadvantages of compliant mechanisms constructed of engineering plastics are poor creep and stress relaxation resistance. Metallic reinforcement is investigated as a method to improve the creep and stress relaxation behaviors of compliant mechanisms and compliant segments. The stress relaxation and creep behaviors of homogeneous compliant segments are compared to those of metallic reinforced compliant segments. Special specimens and fixtures were designed for conducting physical tests. Test results show that metallic reinforced compliant segments significantly outperform homogeneous compliant segments with respect to both creep and stress relaxation. This vein of research is undertaken using metallic reinforcement (inserts) toward the development of a new class of compliant mechanisms with significantly greater performance, particularly insofar as the problems of fatigue and creep are concerned.

Reduction of Stress in Plastic Compliant Mechanisms by Introducing Metallic Reinforcement

2017 ◽  
Author(s):  
Joshua Crews ◽  
Ashok Midha ◽  
Lokeswarappa R. Dharani
Keyword(s):  
Stress Relaxation ◽  
Compliant Mechanisms ◽  
Experimental Testing ◽  
Testing Device ◽  
Bending Stress ◽  
New Class ◽  
Metallic Reinforcement ◽  
Creep And Stress Relaxation

A method is provided and validated for redesigning compliant segments to improve their fatigue, creep, and stress relaxation performance. The method reduces the bending stress in the polymer portion of the compliant segment without the need for overall mechanism redesign, by introducing metallic reinforcement and by matching the force-deflection response of the redesigned segment to that of the baseline segment. An example redesign case study is presented and validated with experimental testing using a unique deflection testing device designed for fixed-free compliant mechanisms. This vein of research is undertaken using metallic reinforcement (inserts) toward the development of a new class of compliant mechanisms with significantly greater performance, particularly insofar as the problems of fatigue and creep are concerned.

Fatigue and Failure Behavior of Homogeneous and Reinforced Compliant Mechanism Segments

2018 ◽  
Author(s):  
Joshua Crews ◽  
Lokeswarappa R. Dharani ◽  
Ashok Midha
Keyword(s):  
Compliant Mechanisms ◽  
Fatigue Testing ◽  
Compliant Mechanism ◽  
Qualitative Assessment ◽  
Failure Behavior ◽  
Test Results ◽  
New Class ◽  
Metallic Reinforcement ◽  
Baseline Test ◽  
Comprehensive Study

This paper presents a comprehensive study of the fatigue and failure behavior of both homogeneous and metallic-reinforced compliant segments. Baseline test results are presented for a homogeneous, fixed-free compliant segment constructed of thermoset urethane. The advantages of both polymeric and metallic materials for compliant mechanism construction are leveraged by designing and testing compliant test specimens containing a polymer casing and a metallic reinforcing element. Results obtained from fatigue testing of fixed-free compliant segments in a cyclic loading configuration show that the metallic-reinforced compliant specimens offer superior fatigue performance when compared to the homogeneous baseline specimens. Fractography, both macroscopic and microscopic, is used for a qualitative assessment of the failure behavior. This vein of research is undertaken using metallic reinforcement (inserts) toward the development of a new class of compliant mechanisms with significantly greater performance, particularly insofar as the problems of fatigue and creep are concerned.

Comparative Study of Creep and Stress Relaxation Behavior for 7055 Aluminum Alloy

2011 ◽  
Vol 314-316 ◽  
pp. 772-777 ◽  
Author(s):  
Li Hua Zhan ◽  
Yan Guang Li ◽  
Ming Hui Huang ◽  
Jian Guo Lin
Keyword(s):  
Aluminum Alloy ◽  
Creep Rate ◽  
Stress Relaxation ◽  
Relaxation Rate ◽  
Constitutive Equations ◽  
Relaxation Behavior ◽  
Experimental Results ◽  
Stress Relaxation Behavior ◽  
Creep And Stress Relaxation ◽  
7055 Aluminum Alloy

In order to study the similarities and dissimilarities between creep and stress relaxation behavior of age formed aluminum alloys, both creep ageing and stress relaxation ageing experiments have been conducted with plate shaped 7055 aluminum alloy specimens on the 100 KN tensile testing machine performed at 120 °C for 20 h, under different stress levels from 190.0 to 357.8 MPa. The experimental results show that similar variation trends for creep and stress relaxation behavior were observed. Both creep and stress relaxation curves can be divided into two stages. During the first stage, higher creep rate and stress relaxation rate occur, which increase with stress levels but decrease with ageing time. While during the second stage, both the creep rate and the stress relaxation rate reach its lowest value and keep constant. A set of unified creep ageing constitutive equations has been developed and calibrated from creep experimental data, which can be used to predict the creep strain under age forming conditions perfectly. But the experimental results from stress relaxation ageing tests cannot be predicted with the established creep ageing constitutive equations, which shows that there is not a one-to-one correspondence between creep and stress relaxation, creep deformation is the most important but not the only reason for stress relaxation under age forming condition.

scholarly journals Fully-compliant statically-balanced mechanisms without prestressing assembly: concepts and case studies

2011 ◽  
Vol 2 (2) ◽  
pp. 169-174 ◽  
Author(s):  
G. Chen ◽  
S. Zhang
Keyword(s):  
Stress Relaxation ◽  
Force Feedback ◽  
Compliant Mechanisms ◽  
Compliant Mechanism ◽  
High Fidelity ◽  
Constant Force ◽  
Force Transmission ◽  
Actuation Force ◽  
New Concepts ◽  
Creep And Stress Relaxation

Abstract. The purpose of this paper is to present new concepts for designing fully-compliant statically-balanced mechanisms without prestressing assembly. A statically-balanced compliant mechanism can ideally provide zero stiffness and energy free motion like a traditional rigid-body mechanism. These characteristics are important in design of compliant mechanisms where low actuation force, accurate force transmission or high-fidelity force feedback are primary concerns. Typically, static balancing of compliant mechanisms has been achieved by means of prestressing assembly. However, this can often lead to creep and stress relaxation arising in the flexible members. In this paper two concepts are presented which eliminate the need for prestressing assembly of compliant mechanisms: (1) a weight compensator which employs a constant-force compliant mechanism, (2) a near-zero-stiffness mechanism which combines two multistable mechanisms. In addition to the advantages provided by statically-balanced compliant mechanisms, two other notable features of these statically-balanced mechanisms are their ability to be monolithically fabricated and to return to their as-fabricated position without any disassembly when not in use.

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