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Molded Part Cracking


Fatigue Failure

picture:fatigue failure

If a molded component or material is subjected to a repetitive load in a practical-usage environment, failure (ie, cracking) will take place after a certain number of cycles at a load much lower than its static yield stress. This phenomenon is referred to as material fatigue; and the corresponding failure, as fatigue failure.

Surfaces resulting from fatigue failure can be identified by the clam-shell marking which is distinctive to this phenomenon. Shear stress is the main type of stress that directly affects failure, and as a result of this, sliding occurs between sections of the material; furthermore, disintegration then occurs when countless minute fracture zones are formed.

As a result of this mechanism, a clam shell pattern is generated.

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Components and Locations Where Fatigue Failure Occurs


  • Plastic spring sections from keyboard components or operation-panel switches
  • Detachable snap fits
  • Pressurized water containers for water filtering and compressed air canisters
  • All other types of component which are subjected to repetitive loading

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Measures for Improvement of Fatigue Durability

  • When designing plastic springs, snap fits, and the like, sufficient consideration should be given to spring characteristics (i.e., spring force) and flowability when selecting the material thickness.
  • Fatigue durability is dependent on the corresponding stress value, and for this reason, the occurring stress should be lowered by reducing the load or by implementing design factors which distribute this load.
  • In order to prevent the generation of cracks which constitute the start point for fatigue failure:
    1. - Use a design which avoids stress concentration by, for example, making sure that corner radii are sufficiently large;
    2. - Increase the level of smoothness on molded component surfaces; and
    3. - Round off the edges of rib tips and other locations with high stress levels.
  • Contact with oils, detergents, and any other chemical agents that attack plastic has a significant detrimental effect on fatigue durability, and this should therefore be avoided.
  • Welded sections readily give rise to the start of fatigue failure, and for this reason, measures should be implemented to prevent repetitive loading at such sections.
  • Plastics with superior fatigue durability should be used.

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Fatigue Testing


picture:fatigue testing

Testing standards and conditions Standard: ASTM D671-63T B (cantilever bending, fixed load method)
Conditions: The test piece is to be as shown on the right.
Loading repetition speed: 1,800 times per minute
Load (stress): Normally 14.7 to 29.4 N (13.3 to 26.2 MPa)
Temperature: 23deg.
C Tester:Toyo Seiki model B-50(max. 490-N)


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Device operation principle and testing method

picture:device operation principle and testing method

 When the unbalanced mass is rotated by the motor, centrifugal force is exerted on the axis of rotation. Horizontal oscillation as a result of this centrifugal force is eliminated through the action of a leaf spring, thus ensuring that the test piece is exposed only to an oscillating load in the vertical direction. The stress being exerted on the test piece (or the applied load) can be varied by changing the rotation radius of the unbalanced mass.
 Although this tester generates a constant stress, the corresponding amplitude is not fixed (as this varies in accordance with the elasticity of the test piece). During testing, the number of load repetitions (or oscillations) until fatigue failure occurs in the test piece is measured for each stress (or load), and a graph is created with stress on the y axis and the number of repetitions on the x axis.

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!

 UMG ABS Ltd. accepts no responsibility for the quality or safety of any customer products which use our materials or which have made use of any type of data provided by this company.
 Customers are requested to independently determine the suitability of our materials for their products. We also request that sufficient attention also be paid to laws, regulations, and industrial rights.


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