Shaders to show damage?

As it happens, I just had to do the same thing. Here is what I did : In the shader below, I the is the main texture and three masks, for low / mid / high damage, controlled by a float I change in-game (material.SetFloat etc) but enough talk, here is the shader :

 Shader "Custom/Breakables" 
 {
     Properties 
     {
         _Color ("Main Color", Color) = (1,0.5,0.5,1)
         _MainTex ("Base (RGB)", 2D) = "white" {}
         _Crack1 ("Crack 1", 2D) = "white" {}
         _Crack2 ("Crack 2", 2D) = "white" {}
         _Crack3 ("Crack 3", 2D) = "white" {}
         _CrackIndex ("Index", Float ) = 1
     }
     SubShader {
         Tags { "RenderType"="Opaque" }
         LOD 200
         
         CGPROGRAM
         #pragma surface surf Lambert
         
         float4 _Color;
         float _CrackIndex;
         sampler2D _MainTex;
         sampler2D _Crack1;
         sampler2D _Crack2;
         sampler2D _Crack3;
 
         struct Input 
         {
             float2 uv_MainTex;
             float2 uv_Crack1;
             float2 uv_Crack2;
             float2 uv_Crack3;
         };
 
         void surf (Input IN, inout SurfaceOutput o) 
         {
             half4 c = tex2D ( _MainTex, IN.uv_MainTex );
             
             half4 cracks;
             if( _CrackIndex <= 1f ) cracks = (1).rrrr;
             else if( _CrackIndex <= 2f ) cracks = tex2D ( _Crack1, IN.uv_Crack1 );
             else if( _CrackIndex <= 3f ) cracks = tex2D ( _Crack2, IN.uv_Crack2 );
             else cracks = tex2D ( _Crack3, IN.uv_Crack3 );
             
 
             o.Albedo = c.rgb * _Color * cracks.r;
             //o.Albedo = cracks.rgb;
             o.Alpha = c.a;
         }
         ENDCG
     } 
     FallBack "Diffuse"
 }

I'm not really a pro in shaders from unity so it's the simplest possible, but I guess you works things out with that !