NEW TEST METHOD FOR OBTAINING SOFTENING RESPONSE OF UNNOTCHED CONCRETE SPECIMEN UNDER UNIAXIAL TENSION

A new method was developed to obtain the complete stress-deformation r esponse of an unnotched concrete specimen. This method employs a digit ally controlled closed-loop testing machine and five control channels: stroke LVDT and four LVDTs mounted on the specimen. The test portion of the specimen was fully spanned by these four LVDTs. The outputs of these control channels and the load cell were monitored during the tes t by a computer equipped with LabTech's Notebook software. A 'C' langu age program was written to enable quick switching of the mode of contr ol from one LVDT to another. The problem of uncertainty in the locatio n of the major crack was tackled by the said LVDT arrangement and the computer program. It was demonstrated that it is always possible to ob tain stable post-peak response provided one ensures that at any time d uring the test, the feedback used is the LVDT that exhibits, at that t ime, the largest slope of the response-time curve. An acoustic emissio n (AE) measurement system of six channels was also used in the experim ents. Monitoring of signals from the AE transducers provided valuable information which helped in making the decision to switch control.