Method and apparatus for preventing or arresting crack development and propagation

Abstract

A method and surface treatment apparatus for preventing or arresting the development and propagation of cracks in the material surrounding a fastener. The method includes introducing compressive residual stresses in the material surrounding the fastener. The compressive residual stresses offset the high residual tensile and applied stresses which the material may be subjected to in operation and thereby prevent or arrest the development of cracks. The surface treatment apparatus comprises a compression tool, positioning device and at least one sensor for determining the location of a fastener on the surface being treated. Using the output of the sensor, the surface treatment apparatus is positioned relative to a fastener. The compression tool is impinged against the surface of the material adjacent to the fastener and rotated around the perimeter of the fastener thereby inducing a zone of compressive residual stress.

1 . A method for preventing or arresting crack development and propagation in the material surrounding a fastener, the method comprising the steps of: 
 locating the position of a fastener;    positioning an apparatus for inducing residual compressive stress relative to the fastener; and    inducing compressive residual stress in at least a portion of the material adjacent to the fastener.    
   
   
       2 . The method of  claim 1  wherein the act of locating comprises the use of sensors for detecting the position of the fastener.  
   
   
       3 . The method of  claim 2  wherein the sensors used for locating are selected from the list comprising optical sensors, magnetic sensors, electromagnetic sensors, tactile sensors, inductive sensors, resistive sensors, and capacitive sensors.  
   
   
       4 . The method of  claim 1  wherein the act of inducing compressive residual stress is accomplished by burnishing, deep rolling, controlled impact peening, shot peening, or laser shocking.  
   
   
       5 . The method of  claim 1  wherein the act of inducing compressive residual stress in at least a portion of the material adjacent the fastener includes inducing compressive residual stress in the material circumscribing the fastener.  
   
   
       6 . The method of  claim 1  wherein the act of positioning the apparatus for inducing compressive residual stress is done manually.  
   
   
       7 . The method of  claim 1  wherein the acts of locating, positioning and inducing are performed under computer control.  
   
   
       8 . A metallic article comprising: 
 at least one surface having at least one fastener disposed thereon, the surface having a layer of compressive residual stress circumscribing the fastener, the layer of compressive residual stress having been induced in the surface surrounding the fastener with the fastener in situ, the layer of compressive residual stress extending beneath the surface to at least a depth known to prevent or arrest the development of fatigue cracks and stress corrosion cracking.    
   
   
       9 . The article of  claim 8  wherein the at least one fastener is a plurality of fasteners.  
   
   
       10 . The article of  claim 8  wherein the layer of compressive residual stress extends through the thickness of the article.  
   
   
       11 . The article of  claim 8  wherein the fastener is a screw, rivet, bolt, pin or spot weld.  
   
   
       12 . The article of  claim 8  wherein the fastener is an interference fit fastener.  
   
   
       13 . The article of  claim 8  wherein the metallic article is a skin sheet for the fuselage of an aircraft, the skin sheet attached to a support structure with a fastener.  
   
   
       14 . The article of  claim 8  wherein the metallic article is comprised of titanium alloys or aluminum alloys.  
   
   
       15 . A surface treatment apparatus for the in situ introduction of compressive residual stress in the material surrounding a fastener to prevent or arrest crack development and propagation, the apparatus comprising: 
 a rotational actuator;    a tool holder attached to the rotational actuator, the tool holder for receiving and positioning attachments relative to the axis of rotation of the tool holder;    a tool for inducing residual compressive stresses in the surface of the workpiece, the tool adjustably positioned on the tool holder.    
   
   
       16 . The apparatus of  claim 15  further comprising at least one sensor for locating the position of a fastener on the workpiece.  
   
   
       17 . The apparatus of  claim 15  further comprising a positioning device for positioning the apparatus relative to the workpiece.  
   
   
       18 . The apparatus of  claim 17  wherein the positioning device is a robotic arm or machine tool.  
   
   
       19 . The apparatus of  claim 15  further comprising a control unit operatively connected to the surface treatment apparatus.  
   
   
       20 . A surface treatment apparatus for the in situ introduction of compressive residual stress in the material surrounding a fastener to prevent or arrest crack development and propagation, the apparatus comprising: 
 at least one compression tool for inducing compressive residual stress in the material surrounding a fastener;    a positioning device for positioning the apparatus relative to a fastener; and    at least one sensor for locating a fastener, the at least one sensor operatively connected to the apparatus.    
   
   
       21 . The apparatus of  claim 20  wherein the at least one sensor comprises a plurality of sensors.  
   
   
       22 . The apparatus of  claim 20  wherein the at least one sensor is an optical sensor, magnetic sensor, electromagnetic sensor, tactile sensor, inductive sensor, resistive sensor, and capacitive sensor.  
   
   
       23 . The apparatus of  claim 20  wherein the compression tool is a burnishing tool, deep rolling tool, impact peening tool, laser shock peening tool, or coining tool.  
   
   
       24 . The apparatus of  claim 20  further comprising a control unit operatively connected to the at least one sensor and the positioning device, the control unit configured to control the positioning of the apparatus with respect to a fastener and the operation of the compression tool.