The Research On Relaxation Rupture Of Concrete

Stress relaxation is an significant time-dependent property of materials. It wasusually described as that the stress from a high value decreases at a decreasing rate andfinally approaching an asymptotic value greater than zero. So, it would not result tomacroscopic rupture. However, Stress relaxation usually observed in some rupturephenomena.We focus on a system consisting of an elastic part and concrete specimen to study therelaxation creep rupture. The theoretical analysis and numerical calculation show that therelaxation creep rupture appears if the elastic energy stored in elastic part exceeded thefracture energy of damageable part. The lifetime of the system strongly depends on thevalues of applied external strain and initial stiffness ratio k between the elastic part and thedamageable part. We show that a higher stiffness ratio implies a more brittle system. Priorto a complete failure, relaxation creep rupture exhibits a sequence of three stages, similarto creep rupture under constant stress, and the nominal force rate presents a power lawsingularity with a power index-1/2near the global rupture time. The elastic part is testingmachine. Two results of similar creep damage are obtained from test, one is concrete mayoccur damage in stress relaxation, the other is unlimited lifetime. After the analysis of thetest data, we find that in early time the power law is approximately-2/3in logarithm rateof test force change versus logarithm time; in late time the power law is approximately-1/2. The values of initial displacement in stress creep test and stiffness ratio of testmachine and concrete specimen have great influence on happen of stress relaxationdamage. When the initial displacement is between1.05and1.17mm and stiffness ratio isbetween0.61and0.96, concrete specimen may occur stress relaxation damage. During theprocess of stress relaxation, the deformation plays a dominant role on change of energy;the energy release rate of testing machine near the stress relaxation damage is greater thanthat in early stress relaxation. Failure mode of concrete specimen is relative to absorbedenergy per unit volume, the more the absorbed energy is, the more broke fragments is;whereas less. Macro cracks of concrete specimen after stress relaxation damage are mainlytwo forms: one is diagonal crack along diagonal direction; the other is X-type crack along cross diagonal direction; both of them belong to shear crack.