User Guide

11
Fig. 4a shows the results of three different products by means of plotting the
measured displacement as a function of the numbers of crack openings. While two
systems would fulfill the requirements concerning the maximum displacement the
other would not fulfill the requirements due to the fact that measured displacement is
larger than the limiting displacement of 3 mm resulting in pullout failure during its life-
time.
The next question would be: “Why do we do such tests?”
As a structure responds to permanent load it experiences displacement and
consequently deformation. This deformation leads to the formation of cracks.
This behavior is schematically given in Fig. 4b. In Fig. 4b the permanent load “g” and
variable load “p” are given as a function of the crack width for a beam. In the life of
the beam probably no cracks will occur if the permanent load is acting on the beam
the first time. However, if the variable load will be considered in combination with the
permanent load (g+p) the deformation will increase and will lead to opening of cracks
in the beam. If the beam will be unloaded to the level of the permanent load again the
deformation will decrease be means of reducing the crack width. However due to the
rough surface of the cracked surface the crack will not be fully, i.e. closed to zero.
Therefore, the lower crack width is around 0.1 mm. During the lifetime of the beam
this crack opening will be repeated.
a) Measured displacement as a function of crack
openings for crack opening test
b) Load acting on a beam as a function of the crack
width
Fig.4 Test results of crack movement test (a) and reasonable explanation for the crack movement test (b)