Failure under constant load

Subjecting a specimen to a constant load evidently can have one of three results  

Generally the third result is technologically desirable. In order to define the region of permissible loads a study of the delayed failure, i.e. of the time-dependent strength, is indispensable. 

Owing to its fundamental importance the time-dependent strength of load-bearing polymers has been extensively investigated with temperature as the principal para- 

A second group of interpretations is based on reaction rate theory and on the gradual exhaustion of the polymer load carrying capability due to the breakage and/or displacement of elements and/or to the formation of defects (cf. Section IV of Chapter 3). At this point the role of molecular backbone chains in delayed fracture of non-oriented polymers is to be elucidated. 

The mode of creep failure in thermoplastic materials is not unique. As an example the creep fracture morphologies of PVC tubes subjected to different stress levels have been reproduced in Chapter 1. At higher stresses (oy = 50 MN/m ) PVC tubes fail in a brittle manner after little creep deformation, and one speaks of delayed brittle frac-ture (Fig. 1.1). At medium stresses (42 MN/m ) and after prolonged times the tubes show large scale plastic deformation, i.e. delayed yielding (Fig. 1.2). At lower stresses Oy 20 MN/m ) failure does either not occur at all within experimental time scales or by a competing mechanism, the formation of a creep crack (Fig. 1.3). 

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Substitution in Eq. (41) yields the total time to failure under constant load ... 

The strength of glass under constant loading also increases with decrease in temperature. Since failure occurs at a lower stress when the glass surface contains surface defects, the strength can be improved by tempering the surface. 

Preliminary research has indicated that a PBT/PBA co-poly(ester ester) is susceptible to environmental stress cracking in water and in phosphoric acid solution, in both cases at 80°C. Time-to-Failure creep experiments were initiated to obtain quantitative data. These tests were performed in water and phosphoric acid solutions (pH = 1.6) at 80°C with notched tensile specimens under constant load (ranging from 0.6-7 MPa). The results have shown that the phosphoric acid solution decreases the lifetime when compared to tests done in water. Both environments decrease the lifetime tremendously when compared to creep tests in air. 

Figure 10.7 (a) Failure lines for grouted and ungrouted granular soils, (b) Drained triaxial test results for silicate grouted coarse and medium sands. (From Ref. 11.15.) (c) Typical stress-strain curve from unconfined compression test on chemically grouted sand, (d) Compression versus time data for creep test on chemically grouted sand, under constant load, (e) Failure time versus percent of unconfined compression failure load. (+) indicates unconfined compression tests, and ( ) indicates triaxial tests with S3 = 25% of Si. 

Fatigue results in a brittle-appearing fracture, with no gross deformation at the fracture. On a macroscopic scale the fracture surface is usually normal to the direction of the principal tensile stress. The crack ejctension depends upon material, amplitudes of alternating stress, frequency of stress, temperature of operation. The process of fatigue failure under a load of a constant amplitude typically includes several stages ... 

All the creep tests were performed under constant load to determine the creep rates, maximum strains and times to failure. Tests were performed at 566°C (1050"F) and at stresses of 55, 69, 83, and 96 MPa (8, 10, 12, and 14 ksi). Four types of specimens were tested. Composites with five and eight infiltrations were examined. Sample specimens infiltrated 5 times were given specimen designations beginning with A while those infiltrated eight times were designated with B . Specimens were tested after a 600°C oxidation exposure in flowing air for 100 hours. The results of this work are summarized in Table 6. 

When failed fuel rods are present in the reactor core, fission product cesium isotopes will also appear in the primary coolant in significant activity concentrations. The high solubility of the cesium compounds deposited in the gap of the fuel rod facilitates the transport to the coolant which, however, is only possible via the liquid phase. This means that under constant-load operating conditions a significant cesium transport will only occur when such fuel rod failures are present in the core that allow a direct contact between fuel and liquid coolant in addition, the shutdown spiking results in a considerable cesium transport to the coolant with almost all types of fuel rod defects. The comparatively low cesium retention on the primary circuit purification resins which are saturated with LiOH occasionally leads to the buildup of activity concentrations of cesium isotopes in the coolant on the same order of magnitude as that of the iodine isotopes 1 and 1, even at comparatively low cesium source strengths or those which are not constant over time. 

Tensile creep also provides a realistic assessment of strength. It is particularly valuable at elevated temperatures where failure of parts under load by creep rupture is a real possibility. Creep rupture is failure by breaking that occurs with time after a material has been placed under constant load, usually at stresses well below the... 

The time constants for this response will vary with the specific characteristics of a type plastic and processing technique. In the rigid section of a plastic the response time is usually on the order of microseconds to milliseconds. With resilient, rubber sections of the structure the response time can be long such as fi-om tenths of a second to seconds. This difference in response time is the cause of failure under rapid loading for certain plastics. 

Figure 21 Comparison between observed and theoretical relationships for the effect of fast neutron fluence on the time to failure of irradiated type 304 stainless steel under constant load in 32 ppm oxygenated water at 288°C.

Pett,T, and Munz,D, Prediction of time to failure of alumina under constant loading as deduced from bending streogth tests,Cfi/Ber.DKG 4/5-84, 190-197. 

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