Fall ’95 Volume 1.2

Dynatup 8250 impact tower
New int’l design standards


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.

Now offered: instrumented impact and fatigue testing

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.

Finite element analysis of notched impact. From G. Trantina and R. Nimmer, p. 208.

One-stop shop Supporting more design programs than any other US laboratory

Datapoint Testing Services now offers analysis-ready data for the following applications;

Mold Analysis
Moldf low
Structural Analysis
Polyf low

Simulating impact?

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

The use of composite materials in automotive applications has been increasing rapidly, notably in non- structural applications. The unique ability of these composites to be formed into complex shapes has allowed a decrease in the total number of parts used, and a corresponding reduction in the cost of assembly. Conversely, the use of composites in structural applications has not grown as rapidly, due to a lack of understanding of composite properties. This lack of knowledge can lead to improper selection of materials, insufficient engineering analysis, and inadequate manufacturing process control, resulting in part failure. However, stress failure can often be averted before the product reaches the marketplace by using Experimental Stress Analysis (ESA).

ESA is an technique allowing observation of the stress state in a body under dynamic load. ESA can be performed in several ways. One method begins with the application of a coat of silver paint to the part, which gives a uniform color background. Brittle lacquer paint is then applied to the part. The brittle lacquer coating, which is strain sensitive, cracks under the application of dynamic stress The crack pattern identifies areas of high strain, the direction of the cracks is perpendicular to the direction of the principal strain. Strain gages are then mounted at high strain locations, and are used to measure the strain developed as the part is subjected to dynamic stress.

Photoelastic materials can also be used to model stress distribution in a part. These materials develop colored bands in response to stresses and strains. As stress increases, there is a proportional increase in the number of boundary lines between the colored bands. The sequence of colors and the spacing of the bands also provide information about the stress distribution.

The success of ESA depends on using loading conditions which simulate or duplicate the conditions the part will face in actual use, whether these are as simple as a door slam or as complex as a load simulated by a multiple axis computer-generated profile. The part tested must be representative of production parts, made of the same material and manufactured on production tooling. Conducting

ESA under these conditions provides a definitive analysis of the finished product and eliminates all unknowns and assumptions required by the design process, identifying areas of high strain which may not be predicted by finite element analysis. Although ESA requires additional steps in the development of a product, it can prevent costly failures before the product reaches the marketplace, ultimately reducing development expense.

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.

Structural Analysis of Thermoplastic Components
By G. Trantina and R. Nimmer. McGraw- Hill. Inc. 1993. 384 pp.

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.

Predicted deformed shape of a polycarbonate disk at maximum indentor load.

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.

Using fatigue data for design calculations

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:

• reducing the frequency, if possible
• minimizing wall sections to facilitate better heat transfer.

The application of fatigue data in design procedure is as follows:

1. Select the design life of the part.
2. Consult the S-N curves for the material of interest corresponding to the frequency of cyclic loading the part may be subjected to.
3. Read off the stress level corresponding to the design life of the part. If the part’s design life should exceed 1 million cycles, determine the endurance limit.
4. Calculate the working stress by applying a safety factor of at least 10%.
5. Compare the calculated working stress to the actual stress the part is subjected to. If working stress is greater than the actual stress, the design is satisfactory.
6. If the working stress is less than actual stress, modify the design to lower the stress by means of ribs, gussets, etc.
i.e., increase the section modulus.

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 million cycles.
If the design life is 1 million cycles, the design is adequate.

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
allowable stress = 31.5 MPa

Dr. Ranganath Shastri is a development leader at Dow Chemical. He holds Bachelor’s and Master’s degrees in Chemical Engineering, and received his Ph.D. in Materials Science from the University of Cincinnati.