Bend stress relaxation

The bend stress relaxation (BSR) technique involves forcing a fiber into a loop of radius (Rq)- After thermal exposure, a creep induced radius of curvature (Rq) is observed in the fiber at room temperature. The salient parameter of the BSR test is the BSR ratio, m = 1 -Rt/R. The higher the value of m, the more creep resistant the material. For a complete description of the BSR technique, see DiCarlo and Dutta, 1995. 

FIGURE 3-14 One-hour bend stress relaxation ratio of polymer-derived SiC fibers and other polycrystalline SiC and AI2O3 fibers. Source Reprinted from Composites Science and Technology, Volume number 51(2), J.A. DiCarlo, Creep limitations of current polycrystalline ceramic fibers, Pp. 213-222. Copyright 1994, with permission from Elsevier Science. 

FIGURE 3-21 One-hour bend stress relaxation ratio for directionally solidified eutectic YAG/alumina fibers grown by edge-defined film growth (EFG), for polycrystalline alumina-based fibers, and for c-axis sapphire fibers. Source Morscher et al., 1995. 

Morscher, G.N., S. C. Farmer, and A. Sayir. 1995. Bend stress relaxation of AI2O3-YAG eutectic fibers. Ceramic Engineering and Science... 

G.N. Morscher, H-M. Yun, and J.C. Goldsby, Bend Stress relaxation and Tensile Primary Creep of a Polycrystalline a-SiC Fiber, Plastic Deformation of Ceramics, eds. R. Bradt, C. Brooks, and J. Routbort (New York Plenum Publishing, 1995), pp. 467-478. 

The creep resistance of a fiber can also be studied by analyzing the bend stress relaxation (BSR) test parameter m, which is defined as m = 1 - RJR. In this equation, Ro is the initial radius imposed on the fiber. R is the permanent radius taken by the fiber after a one hour relaxation treatment at a temperature T. When m approaches 1, the fiber does not creep at temperature T, and its ability to exhibit creep increases as the value of m decreases. For Fiber FP and PRD-166 fibers, a value m = 0.5 is achieved after a relaxation test is performed at 1000 and 1100°C, respectively. In contrast, the test temperature has to be raised to about 1500 C to observe the same m value for the best SiC based fibers known to date [77] [79]. 

A simple bend stress relaxation test can be used to compare the creep resistance of individual filaments [43]. A bending stress is applied with a graphite jig to the fiber which acquires a radius of curvature, R . The assembly is submitted to a heat treatment. After cooling, the graphite jig pieces are separated revealing a fiber with a creep induced curvature of radius R (with R > R ). The stress relaxation is quantified by a parameter m defined as m = 1 - (Ro/R). For given test conditions, creep resistant fibers are characterized by m-values close to 1 (i.e., R Ro) whereas creeping fibers display low m values (R Ro). 

Stress relaxation performance is usually determined in bending, where the initially imposed stress is 50 to 100% of the yield strength at the tension side of the stressed samples. Test details are presented in Reference 23. Data is presented as the percentage of the initial stress that remains as a function of time at exposure temperature. 

Stress relaxation between manufacture and testing. Between manufacturing and testing, relaxation of the residual stress will occur as a result of the viscoelastic properties of the matrix, which controls the behaviour of the 90° layer. Measurements to determine the relaxation of the ply-to-ply residual stress of the carbon-polyetherimide lay-up, with the highest level of residual stresses of the laminates tested ([904c/04c]s), have been presented previously [5]. It shows that the level of residual stress follows a power law with time. For the purpose of this study, it was chosen to perform the bending experiments 240 hours after fabrication, as the level of residual stress remains more or less constant. The level of residual stress as calculated in (1) is then altered by a reduction factor,/,. The relaxed residual stress. 

The axial stress at the initiation of the first visible transverse crack is now considered and results are given in Table 4. These average values include for each lay-up the relaxed residual stress, the bending stress and the total stress at the occurrence of the first transverse crack, as well as the relative standard deviation. 

Grzywinski and Woodford [118.119] report on the use of stress relaxation tests for the determination of design data for polycarbonates, in trying to determine long-term tensile and creep behavior from a 24 hour relaxation test, w hilc Tsou et al. [120] examine the stress relaxation of several types of plastic film in bending and tension. 

Dilation Membrane Shear Deformation Stress Relaxation and Strain Hardening New Constitutive Relations for the Red Cell Membrane Bending Elasticity... 

An asymmetric temperature gradient gives rise to a more complex situation as the plate will bend to relax some of the stresses. 

The T of an ethylene-propylene copol er rubber is —50 C. When the EPR is blended wito PA6.6, the fracture resistance of the blend in slow bending tests on notched bars iBrst shows a significant increase at -50°C. In this test, the time taken to reach peak load is Is. Toughening occurs when the rubber particles have time to stress-relax and to initiate plastic deformation in the PA6.6 matrix. Calculate the minimum tenq)eratuie required to adiieve toughening in the Qiar] impact test Use the i F equation,... 

Fundamental Properties of Polymers, Metals and Ceramics (e.g., strength in compression, tension and bending elasticity/plasticity failure mechanisms phase diagrams transition temperatures surface roughness hydrophobicity) Mechanical Properties of Biological Tissues (e.g., elastic viscoelastic, hysteresis, creep, stress relaxation)... 

Abstract. The viscosity is a physical parameter which controls not only the melting and fining of melts, but also the stress relaxation and the nucleation and crystallization phenomena. Here the basis of viscous flow is presented and discussed. Rheological models and some measurement methods fiber extension, beam bending and indentation are described. 

By using dynamic mechanical analysis (DMA), tensile, compression, bending and torsion tests can be carried out under static or dynamic conditions. Elastic moduli can also be obtained by DMA. With DMA, measurements of constant stress or constant strain can also be made. Thus thermal expansion coefficients, stress relaxation and creep can be investigated by DMA. Here we only give an example of the measurement of the thermal expansion coefficient. In Figure 4.96, the thermal expansion curves of Fe-Co-Si-B amorphous alloy are plotted. Curves 1 and 2 are the measured results for a relaxed glass and as-quenched glass. 

Mechanical Properties. The measurement of mechanical properties is concerned with load-deformation or stress-strain relationships. Forces may be applied as tension, shear, torsion, and compression and bending. Stress is the force divided by the cross-sectional area of the sample. Strain is the change in a physical dimension of the sample divided by the original dimension. The ratio of the stress to strain is referred to the modulus. Stress maybe applied continuously or periodically at varying rates for dilferent tests. The characteristic stress-strain curve, stress relaxation, or impact behavior is very important in determining the applications and limitations of a polsrmer. 

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