Nondestructive Computed Tomography Volumetric Void Acceptance Criteria for Heavy Polymer Sections
Computed tomography volumetric void acceptance requires zoning heavy polymer walls by stress field, setting pore limits below 0.1% in tensile skins.
Stress Concentration Factor
Meaning
Mathematical ratios quantify the localized increase in mechanical stress at a geometric discontinuity such as a sharp corner, a notch or a hole in a plastic part. Stress distribution patterns in a loaded component identify the regions most likely to experience premature failure or cracking. The stress concentration factor is the ratio of the peak stress at the discontinuity to the nominal stress in the surrounding material.
It depends on the shape and the size of the feature rather than the material properties themselves. The boundary of the calculation is the point where the material yields or fractures, changing the geometry. This value governs the design rules for radii, fillets and rib attachments in injection moulded parts.
It is used to ensure that a part can withstand its intended service load without failing.
Geometric Impact
Design of a plastic part must avoid sharp internal corners that act as stress raisers. The stress concentration factor increases dramatically as the radius of a corner decreases. A perfectly sharp corner can have a factor of three or more, meaning the local stress is triple the average stress in the part.
This is a common cause of failure in parts made from brittle resins like acrylic or glass-filled grades. By adding a generous radius to every corner, an engineer can distribute the stress over a larger area and lower the peak value. This simple change can make the difference between a part that lasts for years and one that fails on the first impact.
Design Risk
Failures often occur at the point where a rib or a boss meets the main wall of the part. The stress concentration factor helps to identify these high-risk areas during the initial design phase. Engineers use finite element analysis to visualize the stress map and locate the peaks.
If the factor is too high, the part may develop cracks during the cooling phase or when it is first put into use. This is particularly dangerous for parts that are subject to cyclic loading, as the high local stress can lead to fatigue failure. Adding a fillet or changing the thickness of the rib can help to reduce the concentration.
The analysis also reveals how the flow of the plastic around a feature can create residual stresses that add to the external load.
Structural Integrity
Performance of the finished product depends on the careful management of these geometric features. The stress concentration factor is a fundamental tool for predicting the lifespan of a component under load. It allows designers to apply safety factors that account for the reality of the manufacturing process.
For example, a knit line or a weld line near a stress raiser can further weaken the part. Virgin materials typically follow the predicted behavior more closely than regrind, which may have internal voids or contaminants. Understanding the link between geometry and stress is necessary for creating reliable and safe plastic products.
The result of good design is a part that meets all its performance requirements.
Amorphous Thermoplastic Rheology and Extruded Sheet Stretch Dynamics
Amorphous sheet stretch dynamics depend on strain hardening and thermal saturation to prevent localized necking and corner blowout during deep thermoforming.