Description of Cracks and the Mechanism of Cracking
If a molded component or material is subjected
to a repetitive load in a practical-usage environment, failure
(i.e., 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.
2.
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
3.
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:
- Use a design which avoids stress concentration by, for example,
making sure that corner radii are sufficiently large;
- Increase the level of smoothness on molded component surfaces;
and
- 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.
4.
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)
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.
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.
Testing standards and conditions 