Failure strength, time-dependent

Failure, however defined, should be a part of a complete constitutive description of a material as discussed in the previous sections. In other, words, the key to dealing effectively with the failure of time dependent or viscoelastic polymers lies in treating failure properties as a termination of a nonlinear viscoelastic process. Perhaps, for this reason, a number of investigators have suggested that modulus and strength laws should be related to each other for polymers (eg., Landel, (1964)). 

Many attempts have been made to obtain mathematical expressions which describe the time dependence of the strength of plastics. Since for many plastics a plot of stress, a, against the logarithm of time to failure, //, is approximately a straight line, one of the most common expressions used is of the form... 

Strength properties are time dependent. The load that timber can sustain without failure decreases with time. If the short-term ultimate load in the 5-minute static bending test is taken as the reference point, then wood will fail, on average, at about 66% of that load after 1 year, at 62% after 10 years and at 56% after about 27 to 200 years depending on the curve used to fit the data there is a dearth of long term experimental evidence (>10 yr). 

Figure 10.17. Time-dependent creep of timber, (a) The creep decelerates where the stress is less than 50% of the short-term failure stress but there is an inflection and accelerating creep where the stress is about 90% of short-term failure stress, (h) Estimated strength ratios for small clearwood specimens expressed as a ratio of the short-term strength, in the 5-minute bending test (Wood, 1951).

It is obvious from the cause of dielectric failure that the measured values of dielectric strength will depend on the magnitude of the applied electric field and on the time of exposure to the field. Since the probability of a flaw and a local leakage current leading ultimately to failure increases with the thickness of the sample, dielectric strength will also be expected to depend on the sample thickness. 

When compared to URPs, the analysis and design of RPs is simpler in some respects and more complicated in others. Simplifications are possible since the stress-strain behavior of RPs is frequently linear to failure and they are less time-dependent. For high performance applications, they have their first damage occurring at stresses just below their high ultimate strength properties. They are also much less temperature-dependent, particularly RTSs (reinforced TSs). 

It is useful to introduce failure statistics into the time dependence of strength and this can be accomplished by stating the initial inert strength in a Weibull form,... 

Stress-probability-time (SPT) diagrams incorporate the time dependence of strength into failure statistics. They give lifetime predictions. An illustration of the use of SPT diagrams is in bioceramrcs. 

Strength and crack size are related by the failure criterion [see, for example Eq. (9) ], and the change of crack length with time is described by Eq. (26). By use of Eqs (26) and (9), the Weibull distribution becomes time-dependent [2, 4, 14] ... 

Long-term durability of adhesively bonded joints may require resistance to a number of individual or combined degradation modes, including environmental attack, fatigue and time-dependent failures. Time-dependent failure mechanisms are often characterized nsing either a strength approach, involving creep and creep-rupture tests, or a fracture approach, in which debond rate is determined. In creep-rupture tests, adhesive joints are subjected to... 

Fatigue is a dynamic and time-dependent phenomenon. When a component is subject to alternating stresses repeatedly, it fails at a much lower stress than the material yield strength due to fatigue. Most mechanical failures are related to dynamic loading therefore, the safety assessment in fatigue life plays a critical role in reverse engineering. 

---