Consider we are testing soil and polymeric material as a stress controlled -shear test.
we will plot stress- strain. In this case after getting peak vale, is it possible to get lower values compared to peak value after the peak value is achieved?
Normally in strain controlled test for example in soil, we will get peak value and lower ultimate value.
Dear Velusamy,
No, it is not possible to get the lower stress values in a stress controlled test. The concept of the test does not allow it. You can only get the peak stress and an approximation of the strain at ultimate failure.
For the full stress-strain curve, a strain-controlled test will be needed.
PS: Maybe in your question you could mention the type of test i.e. compression/shear/triaxial so that some experts on soil testing could give some specific inputs/workarounds.
YES. There are loading rate dependent components - by applying F you can't control the loading rate v... This is a point I'm defending in my Ph.D. thesis:
Equation of motions has 3 components, K, C and M (stiffness, viscosity and mass)
F=K*u + C*v + M*a
If F is the input, then some set of u, v and a will be the output. Therefore, we cannot "isolate" K by applying F, because all three components are generated: (1) static stiffness K*u, (2) dynamic viscosity C*v and (3) inertia M*a.
This is especially problematic in geotechnics, as all three components (K, C and M) act as nonlinear, state / phase dependent functions.
The C component is especially noticeable in partially undrained sand. Pore water takes time to drain, therefore pore-pressure buildup is loading rate (v) dependent. It can generate reduced strength in contracting (liquefying) phase of loading, and/or additional strength during expanding (dilating) phases of loading.
Therefore, it is crucial to keep both v and a under control, at very low values, to measure quasi static stiffness K. It is crucial to recognize the difference between "F as the input" and "F as the output"... It is NOT OK to "input the output"...
If F is the output: u, v and a are the input (v~0, and a~0 is required, to isolate K*u).
If u is the output: F, C*v and M*a are the inputs. F can't be applied without generating C*v and M*a.
This makes a HUGE difference in loading rate sensitive soils (such as partially undrained sand, at the sea floor, reacting to impact loads).
Dear Tomas,
Your answer is quite informative with regard to rate sensitivity during loading, but I am unable to connect it to how the stress can drop during a stress controlled test?
Stress-controlled tests, to my knowledge, mean you apply increasingly high loads on the test specimen and record the amount of deformation that happens at each load. Strain-controlled tests on the other hand mean you cause increasing degrees of deformation by applying higher loads (in case of hardening) or lower loads (in case of softening) respectively. Is there something I'm missing here?
Thank you for asking to clarify, Aashranth B.
Positive pore pressure (contracting soil) - reduced peak strength
Negative pore pressure (dilating soil) - increased peak strength.
Higher velocity generates pore pressures faster (both positive or negative). Thus, amplifying the increase / decrease in peak strength.
I work exclusively with sands. Sands tend to reach peak strength while dilating. Thus, I am biased to assume F tests will increase measured peak strength as well. However, I'm open to the idea that soft clay could reach peak strength while contracting. Therefore, if velocity is not controlled - excess pore water will reduce the strength in under-consolidated clay..?
That's all the difference I see, really. Applying F - loading rate is allowed to increase out of control. Applying U (deformation), we can reduce the loading rate, and the effects of pore pressure accumulated with it.
Working with sand I don't have examples where F loading reduces peak strength. But stiffness curves in sand do get pushed below the drained path while transitioning through liquefied soil states (visible in attached picture). Albeit, in sand, the contractive phase is followed by dilative, where stiffness path shoots up very rapidly. In soils which do not have this dilative phase - F loading could measure lower strength.
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