Omar Khalid
Shear strength is a material's internal resistance to applied stress before failure. It is the stress at which a plastic fails. The shear strength of plastics can be determined through a punch test, although this does not account for factors like the rate of shear. Plastic deformation is a result of yield and plastic flow, which is driven by shearing stresses. The deformation can be caused by bending, stamping, drawing, spinning, extruding, and other processes. The deformation is essentially a viscous flow process, similar to liquids.
| Characteristics | Values |
|---|---|
| Shear strength | The ability of a material to withstand shear stress or the stress at which a plastic fails in shear |
| Shear strength value | A material property, independent of thickness |
| Shear strength determination methods | Laboratory and field tests, including the punch test |
| Shear strength in soil engineering design | The most important property, required in the analysis of soil-bearing capacity of embankments, stability of slopes, and lateral pressure exerted by soil on retaining walls |
| Shear strength in geotechnical infrastructures | The most crucial unsaturated parameter in assessing stability |
| Shear stress in plastic parts | Caused by the injection rate as the polymer chains flow past one another during the filling phase |
| Impact of mold design on shear stress | Gate size, location, and quantity must be properly designed to prevent exceeding recommended ranges for shear heating and stress |
| Impact of material selection on shear stress | Each material has a recommended shear rate limit, and violating this can lead to excessive shear heating and potential shear stress |
| Plastic deformation | A non-recoverable process characterized by phenomena such as yielding, strain hardening, and necking |
| Maximum shear stress theory of plastic failure | The maximum shear stress decreases by 4.1% on average, according to the study |
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What You'll Learn
Plastic deformation
In the context of shear stress, plastic deformation and failure are closely related. Shear strength is the ability of a material to withstand shear stress, and it is an important factor in evaluating the structural behaviour of materials. While there is no single method to determine shear strength, various tests, such as the punch test, can provide insights. The shear strength of a material is its internal resistance to applied stress before failure, and it is influenced by factors such as confining pressures, applied stress, and other conditions.
In the case of plastics, excessive shear stress can lead to functionally weak parts and potential failure. This can occur during the injection moulding process due to factors such as flow rates, pressures, and temperatures. Sharp corners in mould design can also contribute to shear stress, as they can generate a shear imbalance and act like a sharp knife cutting through the polymer chains. Additionally, the selection of materials and additives is crucial, as violating the recommended shear rate limits can result in excessive shear heating and potential shear stress.
To prevent plastic parts from failing due to shear stress, design engineers must consider various factors. These include mould design, specifically gate size, location, and quantity, as well as processing parameters such as flow rates, pressures, and cooling. By using equations to calculate the requirements for specific resins and geometries, engineers can avoid shear degradation and maintain the performance of the moulded part. Overall, understanding and managing shear stress are essential to preventing premature failures in plastic parts.
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Sharp corners
Additionally, as plastic parts cool, they shrink. If the plastic has to shrink around a sharp angle, it adds further stress that can lead to cracking. Sharp corners can also cause trapped air during injection moulding, with local high temperatures degrading the parts and causing appearance defects.
To avoid these issues, it is recommended to radius the sharp corners of plastic parts. This helps to ensure a uniform wall thickness, improve the flow of the plastic melt, and reduce stress concentration. It also makes the moulded parts stronger by distributing any stress over a broader area, allowing more plastic material to absorb the load.
Designers and moulders should be careful to avoid corners or edges in their designs and place gating in the proper location to promote a parallel flow of material, reducing the potential for stress and shear imbalance.
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Punch testing
The test specimen is typically square-shaped (50 mm on each side) or disk-shaped (50.8 mm diameter), and a hole of approximately 11 mm diameter is drilled through the centre. This specimen is then placed over a 9.5 mm diameter threaded pin of the punch and fastened with a washer and nut. The specimen and punch are centred and clamped in the support fixture, and the setup is loaded at a crosshead speed of 1.3±0.3 mm/min.
Additionally, the shear strength value obtained from punch testing is independent of thickness and is solely a material property. This means that while it is useful for comparing different plastics, it may not fully represent the behaviour of the material in a specific application where thickness and other factors come into play.
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Shear rate limits
The shear strength of a material refers to its internal resistance to applied stress before failure. Shear strength is a crucial factor in determining the structural integrity of materials, including plastics. While shear strength is an important consideration for various materials, this response will focus specifically on the topic of interest: the shear rate limits relevant to plastic failure.
Plastics undergo shear effects during the filling phase of injection moulding. These shear effects are influenced by the flow rate and pressure exerted on the plastic. If the shear stress or shear rate exceeds certain limits, it can lead to degradation and potential failure of the plastic part. This phenomenon is known as "shear degradation" and can compromise the performance of the final product.
To prevent plastic failure due to shear stress, it is essential to consider the shear rate limits of the specific polymer being used. Each polymer has its own recommended shear rate limit, and violating this limit can result in excessive shear heating and potential shear stress. Properly managing the injection rate is crucial, as it directly impacts the shear effects experienced by the polymer chains during the filling process.
Additionally, mould design plays a significant role in controlling stress within a moulded part. Gate size, location, and quantity must be carefully engineered to prevent exceeding the recommended shear rate and stress limits. Sharp corners in the mould design should be avoided, as they can create stress concentrations and lead to functionally weak parts. By considering the shear rate limits and designing the mould accordingly, the potential for stress can be minimised, resulting in improved part performance and reduced failure chances.
In summary, understanding and adhering to the shear rate limits specific to different polymers are crucial in preventing plastic failure. By carefully managing the injection rate, mould design, and other processing parameters, the potential for shear stress and degradation in plastics can be mitigated, ensuring the final product's integrity and functionality.
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Plastic failure theory
Shear strength is a fundamental concept in material science, referring to a material's internal resistance to applied stress before it fails. In the context of plastics, shear stress can lead to functional failures, such as issues with paint or chrome adhesion to the surface. Additionally, shear stress can cause premature failures, especially if the plastic part needs to perform specific functions.
The design of moulds, including the shape and sharpness of edges, plays a significant role in managing stress within a moulded plastic part. Sharp corners, for instance, can increase stress and lead to functionally weak parts. This is because sharp corners become the weakest link when a load is applied, causing potential failure in this region.
To mitigate these issues, design engineers can control variables such as material selection and processing parameters. Each material has a recommended shear rate limit, and violating this recommendation can result in excessive shear heating and potential shear stress. Additionally, maintaining a consistent flow rate through water channels is critical for even part temperature during ejection.
Several theories have been proposed to understand plastic failure, particularly in fibre-reinforced composite materials. One such theory is the maximum shear stress theory, which suggests that slip trace is found at a 45-degree angle with the tensile direction, which corresponds to the highest shear stress in a uniaxial tensile test. Another theory is the continuum theory for axisymmetric plastic deformation of fibrous composites, which assumes the composite is both plastically extensible and compressible.
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Frequently asked questions
Shear strength is the ability of a material to withstand shear stress or the stress at which a plastic fails in shear. It is the internal resistance of a material to applied stress before failure.
The maximum shear stress theory of plastic failure of fiber-reinforced materials states that the maximum shear stress decreases by 4.1% on average, which provides theoretical guidance for the industrial design of shell-and-tube heat exchangers with helical baffles.
Excessive shear stress can cause plastic failure by exceeding the recommended ranges for shear heating and stress on the material. This can break the molecular chain and lead to part failure.
The factors that influence the shear stress on a plastic part include mold design, gate size, location, and quantity, dimensions, tolerances, and performance criteria, material limitations, processing parameters, and the selection of materials.
