First of all your p1 is not clear in graph despite i used Magnification.

So edit your p1 to be like this. you still can make Style no Thick

p1 = ListLogLogPlot[UvaluesAgainstYSteeltoYRubber, 
  AxesLabel -> {"E_Steel/e_Rubber", "U"}, PlotStyle -> {Thick, Red}, 
  PlotMarkers -> Automatic, 
     Joined -> True, PlotLegends -> {"compound model Ritz"}]

Now as you see that in p8 Show, its there !!!

Now go ahead use little value of Magnify

Magnify[p8, 5]

Actually your Data and Curves are truly fitted on each other. Its means you are so pro Brother in what ever experiment is this !

First of all your p1 is not clear in graph despite i used Magnification.

So edit your p1 to be like this. you still can make Style no Thick

p1 = ListLogLogPlot[UvaluesAgainstYSteeltoYRubber, 
  AxesLabel -> {"E_Steel/e_Rubber", "U"}, PlotStyle -> {Thick, Red}, 
  PlotMarkers -> Automatic, 
     Joined -> True, PlotLegends -> {"compound model Ritz"}]

Now as you see that in p8 Show, its there !!!

Now go ahead use little value of Magnify

Magnify[p8, 5]

Actually your Data and Curves are truly fitted on each other. Its means you are so pro in what ever experiment is this !