You can define a threshold value either for y or for diff(y). Assume you define failure as point when y>0.2:
plot(x,y), hold all
ind=find(y>0.2,1,'first');
plot(x(ind:end),y(ind:end),'rx')
fprintf(1,'Failure at x = %d\n',ind);
If you want to define the threshold on diff(y), do the same but use ind=find(diff(y)>0.2,1,'first'); instead.
