Prakt. Met. Sonderband 38 (2006) 129 
on. Such has reduced this problem and is the basis of almost all fine registration algorithms. The 
After the ICP algorithm iteratively performs the following steps: 
face and 
ally. The » For each point of the first model, search for the closest point in the second model. 
ughness Build an error function as the sum of differences between the point pairs. 
1odels of » Compute the Euclidean transformation that minimizes the error function. 
pecimen 
common Since this approach was very successful, many different variants of the original algorithm 
have followed, mainly trying to improve it in terms of speed and robustness [10]. Based on 
[10] and this framework we have developed a variant of this algorithm that is especially tuned to the 
gy [3] in registration of fracture surfaces which may contain several regions of large plastic 
fr actured deformations that do not match properly. To compensate for such regions, the proposed 
vn in the algorithm does not use all points in the model for registration, but rejects all point-pairs 
rmed by whose error lies above a certain threshold value. The error metric we use is the so called 
point-plane metric [10] which is not the Euclidean distance between corresponding points 
n 2), we but the distance between a point and the surface tangent plane of the corresponding point. 
surfaces. This is especially useful for the registration of regions with little surface features, since it 
from two prevents the algorithm from getting stuck in local minima. The transformation is found by 
minimizing the sum of errors from all points that have not been rejected. 
4. MEASUREMENT OF THE LOCAL FRACTURE TOUGHNESS 
SOSCOPIC The local fracture initiation toughness of materials can be described with the critical crack 
ages are tip opening displacement COD. Fig. 1 shows two corresponding fracture surface profiles 
15 ) The of a cast metal matrix composite (MMC) with Al-6061 as matrix material and 10% alumina 
/ Alicona particles. The profiles are arranged so that it shows a cross section perpendicular to the 
irst step crack plane in the moment of fracture initiation. In this moment the first void in front of the 
nd step, fatigue pre-crack coalesces with the blunted crack tip. The left part of the image shows the 
the SEM open fatigue crack. Note that before loading the specimen, the fatigue crack was closed. 
. COD; is measured as the sum of the stretched zone heights, ds + da, at the initial position 
different of the tip of the fatigue crack or by placing a right-angled triangle at the tip [8]. During 
Is mainly further loading, the crack opens more and more until the two fracture surface profiles are 
difficult to totally separated. The right part of the image represents the plastically deformed region of 
3 9 now the fracture surface which should be used only with care during the registration. Other 
1 3 input useful parameters that can be derived from corresponding surface profiles are the crack tip 
opening angle (CTOA), the crack tip opening displacement at the moment of void initiation 
er vision. a r 
> another ~ 
um. The 
aving Six 
approach dik -20 0 20 br #1 8c 
arse. The x [um] 
AcKay [1] Figure 1: Crack profile at the moment of fracture initiation in a cast MMC with alumina 
particles [6]. Extraction of COD;.