Composites 2004 Convention and Trade Show
American Composites Manufacturers Association October 6-8, 2004
Tampa, FL USA
Compression Testing - Comparison of
Various Test Methods
Arthur R. Wolfe, Goodrich Corporation
Michael Weiner, Goodrich Corporation Abstract
Several test laminates were fabricated for evaluation of mechanical properties under in-plane compression loading. The panels were constructed using a conventional E-glass/vinyl ester knit, a carbon/epoxy prepreg, and an E-glass/epoxy prepreg. The laminates were cured and post-cured according to typical FRP manufacturing practices.
Different types of test specimens were cut from each panel to compare test methods. For example, the
carbon fiber laminate was tested for compressive strength using ASTM and SACMA procedures. All of the methods evaluated are commonly ud to determine the compressive strength of an FRP laminate. Each of the specimens were cut in the same orientation, with the test axis being the 0° reinforcement axis.
It was shown that the results of compressive strength testing of a given laminate will vary according to the method chon. Some procedures are not appropriate for medium-to-high modulus composite laminates at all, due to the type of failure mode. Recent methods designed specifically for high modulus composite materials yield higher, more consistent values for compressive strength. Older, historical methods may have value when testing lower modulus or thicker laminates.
Introduction
Like many composite testing procedures, the determination of compressive properties has long been considered a “black art”. The various methods can produce wildly different results, when the same material is tested. The choice of test method will influence the results – some procedures yield higher strengths, and others produce lower values.
Studies dating back to the 1980’s have looked at the influence of test method on results1. Some of t
he differences between methods include how load is applied (shear vs end loading), whether the gauge length is free or supported, and whether tabs are bonded to the specimen ends.
The “shear” compression failure mode has long been considered ideal for basic laminate properties usage, while a buckling failure is generally considered to be undesirable. But in reality, all compression failures have an element of buckling, although this can be confined to a very localized area. The challenge is to reduce the tendency of the test specimen to buckle on a global scale (column buckling) during the test.
The Test Methods
The different test procedures vary in two main respects – specimen shape and method of loading. In most cas, the test specimen is held or restrained in some sort of “jig” or fixture during loading. It is interesting to note that the methods that work best have been developed relatively recently (specifically for the composites industry), while the older and less desirable methods were adapted for composites from thermoplastic sheet testing procedures.
The common methods in the early days of composites testing were all “end-loaded”, in which a coupon was held erect in the test fixture, with an end protruding slightly from the top. The specimen
was held fairly looly in a supporting fixture to prevent buckling. The fixture was loo enough so that it would not bind up in the fixture during loading, but tight enough to prevent all but the smallest out-of-plane movements. In the tests the end that was loaded would tend to “mushroom”, or “broom”, when relatively high modulus materials were tested. In some cas, the test specimen would be of the dog-bone or dumbbell shape, similar to thermoplastic tensile test specimens. This was done to try to force a failure within the gauge length area. ASTM D695 is an example of this type of test (e Figures 1 & 2).
The Suppliers of Advanced Composite Materials Association (SACMA) developed another end-loaded compression test, SRM 1R-94. This method us the same holding fixture as ASTM D695, however it utilizes a tabbed, straight-sided specimen (e Figure 3). The tabbed ends reduce the probability of mushroom-type failures, while the holding fixture prevents global buckling as in the D695 method. While the short gauge length of the SACMA procedure encourages the preferred failure mode, its very small area precludes the u of a strain gage for modulus determination. A parate, untabbed specimen must be loaded to measure elastic properties. SACMA is no longer in existence, so this method is not reviewed or updated periodically. Shear-loaded test methods u a tabbed specimen, with the tabbed ends held in wedges or grips, similar to tensile
mushroom failures. The gage length of the specimen (distance between tabs) can be opened up to allow the u of a strain gage, if desired. Increasing the gauge length increas the possibility of a buckling failure. Shear-loaded methods include ASTM D3410 and ISO 14126 (e Figures 4 & 5).
A relatively new method us a “combined loading” scheme, with the untabbed specimen held curely between wedge faces, while a cap prevents a mushroom failure. This method (ASTM D6641) has been found very effective for balanced (0/90) laminates. A short specimen gauge length reduces the buckling tendency. The compressive strength of FRP skins on a sandwich panel can be determined using a 4-point bending test. In this instance, the composite skin is restrained only by its adhesion to the core material; no other restraining fixturing is ud. The test is t up so as to result in a compressive failure of the upper skin, between the loading nos. This method is reprented by ASTM D5467 (e Figures 6 &7)
A summary of specimen type and method of loading is provided in Table 1.
Experimental Procedure
Typical materials ud for composites parts were chon for test. Several test panels were fabricated using conventional techniques. Separate panels were made using the raw materials:
- carbon/epoxy prepreg (single-skin)
- E-glass/epoxy prepreg (single-skin)
- E-glass/vinyl ester (single-skin & sandwich) The plies of fabric were all laid up in the same orientation with respect to each other; i.e., the 0° reinforcement axis laid down in the same direction for each ply. The prepreg layups were then vacuum bagged, and cured according to manufacturer’s instructions in an autoclave. The E-glass/vinyl ester panel was fabricated using a wet resin infusion technique. A sandwich panel was also infud using the same skin laminate schedule as the E-glass/vinyl ester panel.
After cure, test specimens were cut from each panel. All tests were performed along the 0° reinforcement axis. Great care was taken to maintain this alignment when cutting the specimens. The specimens were machined according to the instructions, schematic diagrams, and tolerances as given in the test procedure. Where tabs were required, they were cut from sheet stock of a commercially available fiberglass material (G-10).
The compression testing was performed on an Instron by the method. The rate of loading was that specified in the procedure. All tests were conducted under ambient lab conditions, tho being 75°F
(+/- 3°F) and 50% relative humidity (+/- 10% RH).
A minimum of 5 specimens was tested for each sample/procedure combination. The results as shown in the tables below reprent the average value for the t of test specimens.
Test Results
In almost all cas the SACMA method yielded the highest compressive strengths. For the E-glass/vinyl ester infusion sample, the 4-point bending test produced at result that was ~20 MPa higher than the SACMA test. The SACMA test has the shortest specimen gage length, and thus the least tendency to buckle.
Generally, the ASTM D695 test produced results 10%-20% lower than the rectangular, tabbed type of test, even when “mushroom” failures were eliminated from the data t.
The test results are summarized in Table 2.
Conclusions & Recommendations
Determination of compressive properties using the 4-point bending test is more time consuming and
expensive than any of the other methods. There would not usually be any advantage to the u of this method for typical composite materials testing.
The legacy test (ASTM D695) will produce lower material strength than the other methods, but may still be of u for thick composites and low modulus materials. No significant difference was en between the SACMA test and the relatively new combined loading test (ASTM D6641).
The choice of test method is dependent on cost, time, ea of sample and specimen preparation, and the potential end u of the data. Composites engineers should beware of attempting to get the highest design values possible, as a few spectacular compression failures of composite parts will slow the introduction of composite materials into new markets.
Figure 1 – ASTM D695 Test Specimens Figure 4 – ASTM D3410 Test Apparatus
Figure 2 – ASTM D695 Support Jig (also ud
for SACMA SRM 1R)
Figure 5 – ASTM D3410 Test Apparatus Figure 3 – SACMA SRM 1R Test Specimens
Carbon/Epoxy Prepreg EGlass/Epoxy
Prepreg
EGlass/Vinyl
Ester一目了然的近义词
Infusion
ASTM
D695
644.5 345.6 331.9
春色的诗句
ASTM
D3410
703.8 337.1 -
ASTM
D5467
- - 443.1
学生时间规划表
ASTM
D6641
- 377.1 -
SACMA
SRM 1R
811.8 378.2 423.4
大气边界层Table 2. Compression Strength Test Results有关宪法的手抄报
(MPa)
Figure 6 – 4-Point Bend Test Apparatus
Note: Values reprent the average of 5-8 test specimens
References
1.“Compression Test Results- A Tough Nut to
Crack”, Advanced Composites, July/August
1989, pp. 57-63.
2.“The Influence of Test Method on The
Compressive Strength of Several Fiber-
Reinforced Plastics”, Journal of Advanced
Materials, Volume 25, No.1, October 1993, pp.复试简历
35-45.
Acknowledgements
Figure 7 – 4-Point Bend Test Apparatus
The authors would like to thank the lab personnel at
Cincinnati Testing Laboratories and OCM Test Labs,
both of whom provided no-cost testing rvices for this
prentation.
Test Method Specimen Type Loading
Condition
ASTM D695 Dog-bone End, side support
ASTM D3410 Rectangular,
tabbed
Shear, no side
support
ASTM D5467 Sandwich panel 4-point bending
ASTM D6641 Rectangular, may
be tabbed
Combined end &
shear
SACMA SRM 1R Rectangular,
tabbed
End, side support
ISO 604 Rectangular
prism
End, no side
xx论坛
support
ISO 14126 Rectangular, may
be tabbed
Shear or end
Authors
Art unwittingly joined the composites industry after
graduating from college in 1983. Unable to escape, he五台山佛母洞
has since worked in a variety of testing laboratories, and
currently coordinates the composites lab at the
Engineered Polymer Products division of Goodrich
Corporation, Jacksonville, FL. Mike is a Senior Engineer
at Goodrich-EPP, and has been involved in the design,
fabrication, and testing of many large-scale composite
structures.
Table 1. Test Methods for Composite Materials
