Stress relaxation and creep are the same phenomenon except that one looks at it from a different perspective. Polymers under constant stress creep because the material is constantly attempting to relax the stress. In the same vein, plastics stress-relax under constant strain because the stretched polymer chains are constantly attempting to creep or flow to relieve the stress caused by the stretching. The ideal way to generate stress relaxation data is to take a specimen, put it under a particular strain in a tensile machine and monitor the stress as it decays with time. The problem is that the machine is tied up for long periods of time --- but, it is by far the best way to develop accurate stress relaxation data.
An alternative approach is to use the creep modulus data to estimate the stress relaxation at some point in time. A word of caution, however: compare the apparent creep modulus to the original short term modulus of the material --- donÆt compare the time-dependent creep modulus to the modulus at 100 hours, the typical origin for a logarithmic curve (this is not the modulus at zero time, but the creep modulus at 1 hour --- by this time most polymers will have relaxed about 20% of the initial load). Keep in mind that flexural creep data may be somewhat conservative, and somewhat misleading at strains significantly beyond 5%.
In a paper entitled, ôPull-Out Forces on Joints in Polyethylene Pipe Systemsö, the authors, J.L. Husted and D.M. Thompson, listed the following relationship between apparent modulus and time: Modulus as % of Time Initial (Instantaneous) Modulus
6 min 0.1 hour 100% 1 hour 1.0 hour 80% 10 hours 10 hours 67% 4 days 100 hours 51% 6 weeks 1000 hours 39% 1.1 years 10,000 hours 30% 11 years 100,000 hours 24% 50 years 438,000 hours 21%
These data were developed for polyethylene, however, interestingly enough, these same estimated relaxations have worked well for predicting stress relaxation in other plastic materials including other polyolefins, acetals, and PVC. Even more useful is the fact that these same apparent relaxations work well as a guideline for estimating the relaxation of polymers at various temperatures, when given the short-term modulus of the material at that temperature.
The function of a circuit breaker is to provide overload (thermal) and short-circuit (magnetic) protection to a circuit and its downstream components. A circuit breaker functions like an airbag in a car, protecting circuit components and people by tripping the circuit to interrupt the current flow if it detects a fault condition in the control system. Download Now
As with any new technology, getting into large-format 3D printing begins with investigation. The first question may be a simple one: what does “large-format” mean? For 3D printers, “large” is a relative term. Many extrusion-based (FFF) 3D printers are referred to as desktop machines, because they fit on table space. Some of these have very respectable build volumes – but when it comes to “large-format,” the machines will need their own dedicated floor space. Large-format 3D printers have significant build volumes and are most often found in professional settings, like manufacturing facilities and R&D centers. Download Now
Extend SOLIDWORKS® Simulation with a tightly integrated, cloud-based solution and conduct structural static, frequency, buckling, modal dynamic response, and structural-thermal analysis of parts and assemblies. Run simulations on both your local machine or in the cloud. Download Now
A renaissance of design is underway and CAD systems need to keep up. New technologies are reinvigorating every step of product development, from how engineers create designs, to the review process, and even model validation. Download Now