|Fall ’95 Volume 1.2||
New int’l design standards
LONDON, 30Th SEPTEMBER
Several new standards were approved; among these, the main DSC standard (ISO 11357-1) and ISO 294-1 through Part 3 for specimen preparation. The issue of mold shrinkage measurement (DIS 294-4) using the new ISO 294-3 plaque has been resolved with the German delegation providing details of the measurement technique. Several new work items were introduced including a new DSC WD 11357-7 for Crystallization Studies.
Dynamic mechanical analysis standards ISO 6721-1 through Part 4 have been published. Parts 5, 6 and 7 have been cleared for the final round of voting.
Hubert Lobo is a member of the US delegation to ISO TC61.
in a collaborative effort with GRC Instruments, Datapoint Testing Services has installed a complete line of instruments for impact and fatigue testing. New capabilities offered include instrumented impact using a Dynatup 8250 impact tower and instrumented Izod and Charpy tests using a P0E2000.
A GRC SP-10 Servo-pneumatic fatigue testing machine permits testing of materials and end products for long term endurance, resistance to cyclic loading, fatigue strength and fatigue life in tensile, compressive, flexural and creep modes.
Call (607) 272-6736 for details.
Datapoint Testing Services now offers analysis-ready data for the following applications;
for those who need impact testing data for more than just comparative purposes, Datapoint Testing Services now offers a wide range of instrumented impact testing capabilities. Instrumented impact is used to determine the load vs. deformation response of materials under multiaxial, high speed deformation; the results may be used for quality control, comparisons between grades or batches, and impact design.
Ensuring product success with ESA
Mr. James Peraro, manager at Montell U.S.A., Inc., is
chairman of the ASTM D20. 10 Subcommittee on Mechanical Properties.
He received a BA in Physics from American International College in Springfield,
MA, and a MS in Physics from Franklin & Marshall College in Lancaster,
PA. He holds three US patents and has authored papers in the field of
plastics and composites.
The primary purpose of this book is to describe the application of modern engineering analysis techniques to the design of components fabricated from thermoplastic materials. The book, the first of its kind to address the unique behavioral characteristics of thermoplastics and their impact on finite element analysis (FEA), points out the need for plastics designers to move on to nonlinear analysis in order to truly simulate the behavior of plastic parts. According to the authors. the easy availability of high speed computing and efficient analysis codes means that it is no longer necessary nor cost-effective to restrict oneself to simple linear analyses
The authors discuss drawbacks to treating plastics like metals for the purpose of FEA. Thermoplastics exhibit complex behavior when subjected to constant, increasing, or cyclical mechanical loads. The typical approximation of a linear relationship between stress and strain is often invalid because of the extremely non-linear behavior of plastics. Failure to account for this phenomenon can lead to over prediction of the stiffness of a plastic part which might then fail in actual use. The current solution to this problem is significant over design, which results in wastage of raw material, and sometimes leads to other unanticipated problems.
The problem of non-Iinearity is further compounded due to the large displacements that tend to occur in plastic components. The elastic modulil of plastics are routinely as much as two orders of magnitude less than those of metals. Plastics can undergo an order of magnitude more strain than metals before incurring damage Consequently, these materials will tend to undergo much larger rotations and displacements so that the deformations carry further into the non-linear regions of the stress-strain curve.
Standard data sheets and most computerized databases commonly provide design engineers with three relevant categories of ‘design properties’: flexural, heat resistance, and impact. While acceptable for comparative purposes, these properties are not useful for predicting the structural performance of plastics components because the data are not independent of the test method, specimen geometry, and conditions of the test. The authors present methodologies for the generation and use of engineering data.
The book seeks to provide a proper understanding of thermoplastic material behavior and its relationship to measured properties, so accurate predictions of component behavior can be made. Because of the widely differing behavioral characteristics of these materials, no general procedure for the design of plastic parts can be proposed; instead, the book suggests practical approaches to handle the design of thermoplastic components. Treatments of stiffness, failure, impact, time dependent behavior, and fatigue are presented. Numerous examples in the book highlight areas of concern for design analysis of thermoplastics and illustrate the expected level of accuracy from such analyses.
Dr. Gerald Trantina is manager and Dr. Ronald Nimmer is
a mechanical engineer at the Mechanics of Materials Program in GE’s
Engineering Physics Research Center. The book was reviewed by Hubert
Lobo, president of Datapoint Testing Services.
A Ithough fatigue data is useful in ranking materials
and perhaps qualitatively influencing the design, the usefulness of
fatigue data for design calculations is rather limited. Theoretically,
fatigue failure is less likely to occur as long as stress is below the
endurance limit. The designer must strive to keep the working stress
well below the endurance limit and allow an adequate safety factor.
Avoiding stress concentrators such as sharp corners, notches, etc. is
essential. Adding ribs or stiffeners by way of gussets also assists
in redistributing the stress. The hysteretic heating due to cyclic loading
at high frequency is minimized by:
The application of fatigue data in design procedure is as follows:
1. Select the design life of the part.
To illustrate this procedure, let us consider an example
of a part which sees cyclic loading of 40 MPa at 1 Hz. Consulting the
S-N curve, let us say we find that the number of cycles to fall is 1.5
Suppose the design life of the part is 2 million cycles. The maximum allowable stress or fatigue endurance limit is then determined from the S-N curve. If the endurance limit is found to be 35 MPa,
working stress = 0.9 x maximum