1.  Field of the Invention
The present invention relates generally to detection of external force to an electronic device such as a cellular phone, and particularly to an external force detection method, an external force detector and an electronic device that detect a loading history of external force by using a deformation function for a detection body to have. 
2.  Description of the Related Art
In a portable terminal device as an example of an electronic device, integration of components thereof is progressing to being high along with high functionalization such as mounting a display module and a camera module, etc.  High toughness and strength are requested to protect the device from destruction by external force.  If external force exceeding an allowable limit is received, for example in the field, destruction such as damage to functions thereof can not be avoided.  Even if failure analysis is executed as to the destroyed device, it is difficult to distinguish whether the destruction results from loading external force or an electric system.  For example, there is the case that opening a chassis of the device to identify the cause of destruction thereof has the chassis damaged not to identify the cause thereof. 
Concerning external force detection of an electronic device, there are the structures such as; intensity of shocks such as a fall is detected by a pressure-sensitive sensor, and a detection output thereof is displayed on a display section (Japanese Utility Model Application Laid-Open Publication No.  H6-81155 (Paragraph 0014, FIG.  2, etc.));  being provided with an acceleration sensor, it is tested whether various function units operate normally by sensing shocks exceeding a certain intensity level, in case a breakage therein is detected, the detected result is recorded with time information and the occurrence of the breakage is informed (Japanese Patent Application Laid-Open Publication No.  2004-228976 (Paragraphs 0050 and 0053, FIGS.  1 and 2, etc.));  casing structure provided with an illegal decomposition detecting lug to detect illegal decomposition of a casing (Japanese Patent Application Laid-Open Publication No.  2003-8464 (Paragraphs 0032 to 0034, FIGS.  4 and 5, etc.));  and being provided with an shock detection part detecting a shock value applied to a device, it is confirmed whether the brake of the shock detection part exists to specify the range of the shock value electrically (Japanese Patent Application Laid-Open Publication No.  2003-114180 (Paragraphs 0030 to 0032, FIGS.  2 to 7, etc.)). 
By the way, the structure described in Japanese Utility Model Application Laid-Open Publication No.  H6-81155 needs a pressure-sensitive sensor to detect shocks.  A control means to process an output from the pressure-sensitive sensor electrically and a display section to display shocks electrically are needed.  There are many components to detect shocks, and an electric process is necessary to detect a loading history of shocks, etc.  Thus, destruction of an electric system by shocks disables such shock detection and the process following the detection.  The structure described in Japanese Patent Application Laid-Open Publication No.  2004-228976 records a signal representing shocks detected by an acceleration sensor in a memory, which needs many components.  An electric process is necessary to detect a loading history of shocks, etc.  and destruction of an electric system by shocks disables such shock detection and the process following the detection.  In the structure described in Japanese Patent Application Laid-Open Publication No.  2003-8464, an illegal decomposition detecting lug disposed on a casing can not detect external force such as fall shocks and a press applied to the casing even if illegal decomposition can be detected.  The structure described in Japanese Patent Application Laid-Open Publication No.  2003-114180 includes a plurality of shock detection parts.  Because whether the brake of the shock detection part exists is confirmed to detect the range of the shock value electrically, a plurality of components are needed, and a control means, a record means, a display means, etc.  to process an electric signal are also needed.  The electric process is intervened and is necessary to detect a loading history of shocks, etc.  Or, destruction of an electric system by shocks disables such shock detection and the process following the detection. 
First, even if any one of structures of Japanese Utility Model Application Laid-Open Publication No.  H6-81155, Japanese Patent Application Laid-Open Publication No.  2004-228976, Japanese Patent Application Laid-Open Publication No.  2003-8464 and Japanese Patent Application Laid-Open Publication No.  2003-114180 is used, external force detection needs many components and an electric process.  A loading history representing external force being applied can not be recognized easily from an aspect of a chassis without accompanying an electric process. 
Second, in device development and user support, it is necessary to identify whether the cause of destruction of a destroyed electronic device is external force or an electric system. 
There is no disclosure concerning such problems in any of Japanese Utility Model Application Laid-Open Publication No.  H6-81155, Japanese Patent Application Laid-Open Publication No.  2004-228976, Japanese Patent Application Laid-Open Publication No.  2003-8464 and Japanese Patent Application Laid-Open Publication No.  2003-114180, and no disclosure about the structure to solve them and suggestion thereof are presented.