DATE     94/01/10

    Opérateur CHOC
    --------------

    Description :
    ____________

    The CHOC operator constructs an ATTACHE type object which
 contains the description of an impact type linkage. This
 operator is used for studying the vibrations of structures
 subjected to impacts.

    The model is as follows : if an impact occurs at one point P
 of a structure, a force proportional to the penetration at the
 level of contact is applied on P. Coulomb dry friction may occur.

    There are two ways of using this operator depending on the 
 type of impact considered :

    -------------------
    | 1st possibility |
    -------------------

    Impacts for which the normal to the surface of contact is a 
 fixed direction :
   CHO12=CHOC 'PTCH' | ELSTR1 | 'PTCH' | ELSTR2 | 'NORM' VEC1 'JEU' ...
                      | GEO1   |        | GEO2   |

        .... FLOT1 'RAID' FLOT2  ('FROT'  PROG1) ('AMOR'  PROG2) ;


    Contents :
    _________

   'PTCH'  : key word for describing the points that might be
             subjected to the impact.
             The point I1 of the structure SI1 can hit the point J2
             of the structure SJ2.

    ELSTR1 : structure element (ELEMSTRU type)

    (NOTA : If the impacts between the structures have common
    characteristics, non-elementary ELSTs may be used, in this case,
    ELSTR1 ith point can hit ELSTR2 ith point)

    GEO1  : geometry of a fixed support (POINT or MAILLAGE type)

   'NORM' : key word followed by :
    VEC1  : normal to the impact plane (POINT type)

   'JEU'  : key word followed by :
    FLOT1 : gap in relation to the normal (FLOTTANT type)

   'RAID' : key word followed by :
    FLOT2 : impact stiffness (FLOTTANT type)

   'FROT' : key word followed by :
    PROG1 : friction coefficients (LISTREEL type)

   'AMOR' : key word followed by :
    PROG2 : impact dampers (LISTREEL type)

    -------------------
    | 2nd possibility |
    -------------------

    Impacts for which the normal to the surface of contact pertains
 to a fixed plane. Most of the time it concerns impacts between 
 beams of parallel axes.
    CHO12=CHOC  'PTCH' | ELSTR1 (GEOP1) |  'PROF' SEG2I1 'VECT' VEC1 ...
                       | GEOM1          |

  ... 'TAIL' FLOT1 'PTCH' | ELSTR2 (GEOP2) |  'PROF' SEG2J2 'VECT' VEC2
                           | GEOM2          |

  ... 'TAIL' FLOT2 'CONF' | 'EXTE'   | 'NORM' VEC3 ('JEU' FLOTJEU) ...
                           | 'INTE'   |
                           | ('TRAN') |

   ...  'RAID' FLOT3  ( 'FROT' PROG1 )  ( 'AMOR' PROG2 ) ;

    Contents :
    __________

    CHO12  : result (ATTACHE type)

   'PTCH'  : key word for describing points that might be subjected
             to the impact.
             The point I1 of the structure SI1 can hit the point J2
             of the structure SJ2.

    ELSTR1 : structure element (ELEMSTRU type)

    (NOTA : If the impacts between the structures have common
    characteristics, non-elementary ELSTs may be used, in this case,
    ELSTR1 ith point can hit ELSTR2 ith point)

    GEOM1  : geometry of a fixed support (MAILLAGE type)
    GEOP1  : initial position of the impact point(s) which must
             be specified when the structures are identical (POINT 
             or MAILLAGE type)
            
   'PROF'  : key word followed by :
    SEG2I1 : structure section in the plane xOy (MAILLAGE type)

   'VECT'  : key word followed by :
    VEC1   : impact plane vector used for defining the local axes
             connected with the point of impact I1 (POINT type)

   'TAIL'  : key word followed by :
    FLOT1  : - radius of the greatest circle of centre I1 contained
               in the section, in case of a hollow structure (FLOTTANT
               type)
             - radius of the smallest circle of centre I1 containing
               the section, in case of a solid structure (FLOTTANT
               type) 

   'CONF'  : key word for describing the impact configuration 
   'EXTE'  : key word if we are in the presence of 2 concave structures 
   'INTE'  : key word if we are in the presence of one concave structure
             and one convex
   'TRAN'  : key word if the structures are moving in a translation
             with respect to the normal to the impact plane

   'NORM'  : key word followed by :
    VEC3   : normal to the impact plane (POINT type)

   'JEU'   : key word (compulsory with the word TRAN) followed by :
   FLOTJEU : region of impact length with respect to the normal 
             to the impact plane (FLOTTANT type)

   'RAID'  : key word followed by :
    FLOT3  : impact stiffness (FLOTTANT type)

   'FROT'  : key word followed by :
    PROG1  : friction coefficients (LISTREEL type)

   'AMOR'  : key word followed by :
    PROG2  : impact dampers (LISTREEL type)

    Note :
    _______

    The section of the structure, the corresponding local axes and
 possibly the initial position of the point of impact are associated
 with each point of impact.

    The local axes are defined by the normal to the impact plane and
 by an orthogonal vector V. The origin of the local axes corresponds to
 the point of impact, the axis Oz to the normal, and the axis Ox to the
 vector V.

    The sections are discretized with straight line segments in the
 plane xOy of local axes. MAKE SURE that the mesh of a convex section 
 be oriented anti clockwise, and that the mesh of a concave section 
 be oriented clockwise. 

    If the impacts cannot occur on the part which is not described,
 only a part of the section will be meshed.