The fracture strength (or micro-crack nucleation stress) of a  material was first theoretically estimated by Alan Arnold Griffith in 1921:

  
    
      
        
          σ
          
            
              t
              h
              e
              o
              r
              e
              t
              i
              c
              a
              l
            
          
        
        =
        
          
            
              
                E
                γ
              
              
                r
                
                  o
                
              
            
          
        
      
    
    {\displaystyle \sigma _{\mathrm {theoretical} }={\sqrt {\frac {E\gamma }{r_{o}}}}}
  where: –

  
    
      
        E
      
    
    {\displaystyle E}
   is the Youngs modulus of the material,

  
    
      
        γ
      
    
    {\displaystyle \gamma }
   is the surface energy, and

  
    
      
        
          r
          
            o
          
        
      
    
    {\displaystyle r_{o}}
   is the micro-crack length (or equilibrium distance between atomic centers in a crystalline solid).On the other hand, a crack introduces a stress concentration modeled by

  
    
      
        
          σ
          
            
              e
              l
              l
              i
              p
              t
              i
              c
              a
              l
               
              c
              r
              a
              c
              k
            
          
        
        =
        
          σ
          
            
              a
              p
              p
              l
              i
              e
              d
            
          
        
        
          (
          
            1
            +
            2
            
              
                
                  a
                  ρ
                
              
            
          
          )
        
        =
        2
        
          σ
          
            
              a
              p
              p
              l
              i
              e
              d
            
          
        
        
          
            
              a
              ρ
            
          
        
      
    
    {\displaystyle \sigma _{\mathrm {elliptical\ crack} }=\sigma _{\mathrm {applied} }\left(1+2{\sqrt {\frac {a}{\rho }}}\right)=2\sigma _{\mathrm {applied} }{\sqrt {\frac {a}{\rho }}}}
   (For sharp cracks)where: –

  
    
      
        
          σ
          
            
              a
              p
              p
              l
              i
              e
              d
            
          
        
      
    
    {\displaystyle \sigma _{\mathrm {applied} }}
   is the loading stress,

  
    
      
        a
      
    
    {\displaystyle a}
   is half the length of the crack, and

  
    
      
        ρ
      
    
    {\displaystyle \rho }
   is the radius of curvature at the crack tip.Putting these two equations together gets

  
    
      
        
          σ
          
            
              f
              r
              a
              c
              t
              u
              r
              e
            
          
        
        =
        
          
            
              
                E
                γ
                ρ
              
              
                4
                a
                
                  r
                  
                    o
                  
                
              
            
          
        
        .
Elvitegravir (EVG) is an integrase inhibitor used to treat HIV infection. It was developed by the pharmaceutical company Gilead Sciences, which licensed EVG from Japan Tobacco in March 2008. The drug gained approval by the U.S. Food and Drug Administration on August 27, 2012 for use in adult patients starting HIV treatment for the first time as part of the fixed dose combination known as Stribild. On September 24, 2014 the FDA approved Elvitegravir as a single pill formulation under the trade name Vitekta. On November 5, 2015 the FDA approved the drug for use in patients affected with HIV-1 as a part of a second fixed dose combination pill known as Genvoya.According to the results of the phase II clinical trial, patients taking once-daily elvitegravir boosted by ritonavir had greater reductions in viral load after 24 weeks compared to individuals randomized to receive a ritonavir-boosted protease inhibitor. Medical uses
In the United States, elvitegravir can be obtained either as part of the combination pills Stribild or Genvoya, or as the single pill formulation Vitekta.Vitekta is FDA approved to be used for the treatment of HIV-1 infection in adults who have previous treatment experience with antiretroviral therapy. It must be used in combination with a protease inhibitor that is coadministered with ritonavir as well as additional antiretroviral drug(s). Adverse effects
The most common side effects of taking elvitegravir are diarrhea (in 7% of patients) and nausea (4%). Other side effects that occurred in more than 1% of people are headache, tiredness, rashes, and vomiting.