Abstract:
An exemplary containment structure includes an annular wall configured to receive a rotatable portion of a turbomachine. A plurality of apertures extend radially through the annular wall. The apertures have a varying circumferential width.

Description:
BACKGROUND 
     This disclosure relates generally to a containment structure and, more particularly, to a frangible containment structure. 
     Turbomachines extract energy from a flow of fluid as is known. During operation, air is pulled into the turbomachine. The air is then compressed and combusted. The products of combustion expand to rotatably drive a turbine section of the turbomachine. 
     One example turbomachine is an auxiliary power unit (APU). The typical APU is located in the tail section of an aircraft. The APU provides electrical power and compressed air to the aircraft. Another example turbomachine is a gas turbine engine that propels the aircraft. 
     During turbomachine operation, a portion of the turbomachine may become separated from other portions of turbomachine. For example, fragments of failed disks may separate from a turbine section of the turbomachine. The separated fragments possess significant kinetic energy and are quite capable of damaging components lying along the fragment&#39;s trajectory. Turbomachines typically include containment structures that dissipate energy contained in the separated fragments. Absorbing the energy of the separated fragments is difficult, even when using containment structures. 
     SUMMARY 
     An exemplary containment structure includes an annular wall configured to receive a rotatable portion of a turbomachine. A plurality of apertures extend radially through the annular wall. The apertures have a varying circumferential width. 
     An exemplary turbomachine assembly includes a containment structure circumscribing a rotatable assembly of a turbomachine. An array of apertures extend radially through a wall of the containment structure. The array of apertures provide a frangible joint. 
     An example method of absorbing loads within a turbomachine includes applying a load to containment structure, and shearing the containment structure at a location of a frangible joint of the containment structure. 
    
    
     
       DESCRIPTION OF THE FIGURES 
       The various features and advantages of the disclosed examples will become apparent to those skilled in the art from the detailed description. The figures that accompany the detailed description can be briefly described as follows: 
         FIG. 1  shows a section view of an example turbomachine. 
         FIG. 2  shows a section view from a turbine section of the turbomachine of  FIG. 1 . 
         FIG. 3  shows a perspective view of a containment structure of the turbomachine of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a tail section  10  of an example aircraft houses an auxiliary power unit (APU)  14  within an auxiliary power unit compartment  16 . As known, the APU  14  is used to provide power and pressurized air for use in the aircraft. 
     The APU  14  is a type of turbomachine. Another example turbomachine is a gas turbine engine that is used to propel the aircraft. Although shown in the tail section  10  of the aircraft, a person having skill in this art and the benefit of this disclosure will understand that the APU  14  could be located elsewhere within the aircraft. 
     The APU  14  generally includes a compressor section  18 , a combustor section  20 , a turbine section  22 , and an exhaust section  24 . During operation, air is moved through a plenum  26  to the compressor section  18 . The air is compressed in the compressor section  18 . A mixture of the compressed air and fuel is ignited within the combustor section  20 . The products of combustion are expanded within the turbine section  22  to rotatably drive a generator  30 , which provides power to the aircraft. Once expanded, these products are discharged from the APU  14  through the exhaust section  24 . 
     Referring now to  FIGS. 2 and 3  with continuing reference to  FIG. 1 , the turbine section  22  includes turbine blades  34  having tips that seal against a blade outer air seal  38  during operation. A containment structure  42  circumscribes the turbine blades  34  and the blade outer air seal  38 . The containment structure  42  captures fragments expelled from structures of the APU  14 , such as the turbine blades  34  and the blade outer air seal  38 . Fragments may be expelled from structures of the APU  14  during, for example, a turbine failure. 
     The containment structure  42  may be secured directly to a housing within the combustor section  16 , a housing within the exhaust section  24 , or some other area. The forces exerted on the containment structure  42  place a large amount of torque on the containment structure  42 . The torque may damage the containment structure  42 , as well as surrounding components of the APU  14 , especially the components that the containment structure  42  is secured to. 
     The example containment structure  42  includes an annular wall  44  having a plurality of apertures  46 . A first axial end  48  of the containment structure  42  includes a collar  50  that is secured directly to the exhaust section  24  of the APU  14 . An opposing second axial end  52  of the containment structure  42  is secured directly to the combustor section  20  of the APU  14 . 
     The apertures  46  are distributed circumferentially about a rotational axis A of the APU  14 . The apertures  46  of the example containment structure  42  have an arrow- or triangular-shaped profile. Points  56  of the apertures  46  are pointed upstream relative to a direction of flow through the APU  14 . The apertures  46  are 45°-45°-90° triangles in this example with the points  56  having the 90° angle. 
     The points  56  are circumferentially smaller than other areas of the apertures  46 . That is, the circumferential width of the apertures  46  is the smallest at the points  56 . The apertures  46 , in this example, extend from a circumferentially smaller area to a circumferentially wider area. 
     Areas  58  of the containment structure  42  immediately adjacent apertures  46  are weaker than other areas of the containment structure  42  due to the apertures  46 . The weakened area  58  is the most likely area of the containment structure  42  to fail if significant torque is introduced to the containment structure  42 . In this example, the containment structure  42  is weakest in areas  62  where the apertures  46  are circumferentially the closest. As appreciated, the cross-sectional area of the containment structure  42  is the smallest at the areas  62 . 
     The apertures  46  establish a frangible joint within the containment structure  42 . In some examples, the containment structure  42  shears at the frangible joint. The frangible joint facilitates predictable failure in a desired area of the containment structure  42 . Energy imparted to the containment structure  42  by fragments that have separated of the APU  14  can be absorbed in a predictable manner. 
     Features of the disclosed examples, include encouraging the containment structure  42  to fail in a particular area to help contain these forces and avoid damaging other portions of the APU  14  or aircraft. The triangular shape of the example apertures encourage failure in a particular area, minimize the overall weight of the containment structure, and provide enhanced structural properties (lateral and torsional stiffness). 
     The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. Thus, the scope of legal protection given to this disclosure can only be determined by studying the following claims.