Figure 7 - T h e o r e t ic a l a n d e x p e r im e n ta l fo r c e vs d is p la c e m e n t d ia g r a m s
Y - 1chg 0,
ARNE HILLERBORG Div. of Building Materials, Lund Institute of Technology,
Box 118, S-221 00 Lund, Sweden
In many types of fracture mechanics testing of concrete, stability problems may occur. Three types of such instabilities are discussed in the paper, viz. tensile and rotational instability in a tension test, and instability in a bending test. The demand on the stiffness of the testing equipment is analysed. Particular attention is paid to the rotational instability in a tension test, as this phenomenon is least understood at the same time as it involves the greatest risk of causing erroneous results. The other types of instabilities may be noticed during the test, whereas the rotational instability in a tension test may pass unnoticed. A stability criterion for this case is given as a simple analytical expression, from which the demand on the stiffness of the testing equipment can be calculated. This demand is very high, and it can be concluded that many reported tests have been unstable, and the corresponding stress-deformation curves inaccurate. An approximate analysis is made of the result of an unstable test.