Proportional limit stress

Fig. 6.5 Cyclic stress-strain behavior observed during the room temperature fatigue of unidirectional SiQ/LAS-II at a maximum stress below the 105 fatigue limit (loading frequency = 10 Hz, crmin/crmax = 0.1). The cyclic stress-strain curves show very limited, if any, hysteresis. Note that the strength and strain capability is retained after cyclic loading below the proportional limit stress. After Prewo.42...

A coincidence between the proportional limit, fatigue limit, o-fi, has also been observed for other composite systems fatigued at loading frequencies of 10 Hz or lower. For example, as shown in Fig. 6.6, under tension-tension fatigue at 1000°C, 0° SCS-6 SiQ/HPSN43 composites exhibit a fatigue limit (5 x 106 cycles) at the monotonic proportional limit stress of 200 MPa. 

There are also many composites where the 106 cycle fatigue limit is above the proportional limit stress. Examples include Nicalon SiCf/172344 (Fig. 6.7), Nicalon SiQ/CAS45 (Fig. 6.8), and 2-D Nicalon SiQ/SiC46 (run-out at 2.5 x 105 at 1 Hz) (Fig. 6.9). 2-D T-300 Q/SiC, which does not possess a proportional limit, has a fatigue limit (106 cycles, 10 Hz) slightly above 300 MPa, which is close to 75% of the ultimate strength of this composite... 

High tensile Proportional Limit Stress (PLS) after CMC processing (allows high CMC design stress and high environmental resistance)... 

For the N22 CMC system, remaining open porosity in the CVI SiC matrix was filled by room-temperature infiltration of SiC particulate or slurry casting, followed by the melt-infiltration (MI) of silicon metal near 1400°C. This yielded a final composite with 2% closed porosity within the fiber tows and 0% porosity between the tows. The final composite system (often referred to as a slurry-cast MI composite) typically displayed a thermal conductivity about double that of a full CVI SiC composite system in which the CVI matrix process was carried to completion. Also the composite did not require an oxidation-protective over-coating to seal open porosity. Decreasing the porosity of the hybrid matrix also increased the N22 CMC elastic modulus, which in turn contributed to a high proportional limit stress. However, since the filler contained some low-modulus silicon, the modulus increase was not as great as if the filler were completely dense SiC. 

Some of the important fracture parameters that are determined from the stress-strain curves are also illustrated in Figure 6, and include the initial modulus, proportional limit stress, ultimate strength and strain to failure. It is often very difficult to determine unambiguously the stress at which the first matrix crack occurs, so the proportional limit stress, i.e. the stress at which the strain deviates by 0.005% from linear loading, is more commonly used to characterize this important stress level. All of the in-plane fracture data reported here was measured at initial strain rates (prior to matrix cracking) between 3 x 10 and 10 " s unless otherwise noted. 

C are shown in Figure 8. Fracture properties were found to be constant with exposure to between 200 and 1000 hours, at which time gradual reductions in the modulus and ultimate strength were observed. Both proportional limit stress and strain to failure values remained relatively constant out to the 4000 hour limit of the test. 

Interface coating Fiber lay-up Fiber volume.% Proportional limit stress, % MPa Proportional limit strain. % Elastic modulus, GPa Ultimate tensile strength, MPa Ultimate tensile strain,%... 

Elastic modulus values were calculated from a mathematical least squares fit on that linear portion of the tensile stress-strain curves that produced a correlation factor of 0.99. Proportional limit stress and strain values were measured at the upper limit of the elastic modulus defined section of the stress-strain curves. 

The effects of600°C oxidation on the proportional stress and strain values are not clear in this study. For both the 5X and 8X test sets, the proportional limit stress and strain values in the as-prepared condition are within the expected range - 80-100 MPa stress and 0.1% strain. The 8X stress value is higher than the 5X value. However with 600°C oxidation, the stress and strain values shift higher and lower with time. The 5X values for stress and strain vary considerably with no clear trend. This may be a measurement artifact, based on the use of the least squares fit mathematics to define the linear range. 

A typical stress-strain curve for a polymer membrane is shown in Fig. 6.6, from which the ultimate tensile strength, tensile strength at break. Young modulus, elongation at break, proportional limit stress and proportional limit strain can be extracted. The area under the curve is proportional to the overall energy needed to break the material. 

Temperature C Micro Cracking Stress Average St.Deviation , Pa Proportional Limit Stress Average St. Deviation Saturation Stress Average St, Deviation MPa ... 

Creep (failed) 1 Hz (failed) — Proportional Limit Stress A 30 Hz (foiled) — Micro-cracking stress Saturation Stress —... 

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