A Low-Temperature Creep Experiment Using Common Solder

Transcription

1 A Low-Temperature Creep Experiment Using Common Solder L. Roy Bunnell, Materials Science Teacer Soutridge Hig Scool Kennewick, WA Copyrigt: Edmonds Community College 2009 Abstract: Tis experiment uses common lead-tin solder as a model material to demonstrate creep in metals at room temperature. By using two specimens loaded wit te same stress but wit different gauge lengts and under te same stress, te concept of te strain is well illustrated. Te data are plotted in a simple manner, but analysis easily sows te effect of increased stress due te reduction in specimen cross-section as strain increases. Objectives: To demonstrate creep as a penomenon in metals, witout using expensive furnaces or equipment, to reinforce te concepts of stress and strain. Student learning objectives: Students see creep actually appening, and by developing a grapical plot, realize te role of stress in determining te creep rate in a metal alloy. MatEd Core Competencies covered: 0.A Demonstrate good communication skills 0.B Prepare tests and analyze data 1.A Carry out measurements of dimensions and pysical properties 7.G Define stress and strengt 7.H Define strain and deformation 16.B Describe te effects of defects on material properties Key words: Creep, solder, stress, strain Type of Module: Laboratory Experiment Time Required: Approximately 1 week, wit experiment conducted at te front of te classroom; observation requires only a few minutes per class period, so te experiment and run concurrently wit oter instruction. Pre-requisite knowledge: Introduction to concepts of stress and strain in mecanical testing would be elpful. Target Grade Levels: 11 t -12 t grade and introductory college courses. Table of Contents

2 Equipment and Supplies Required: Solid-core lead-tin solder (no built-in flux): Te test material used in tis lab was 60/40 tin/lead solder. Be sure tat it is solid core solder wit no built-in flux. You can also try (or compare) oter solder compositions. Solder diameter used in tis experiment was 1/8 inc; larger diameters will need eavier weigts. Also: Brass weigts, meter stick, grap paper. Curriculum overview and notes for te instructor Tis experiment is a modified version, for a ig scool audience, of an experiment originally developed by Prof. Robert Stang and te University of Wasington. His approac, owever, used a tensile testing macine and matematical analysis of te data tat is beyond most introductory tecnology or ig scool classes and students. Te intent of tis work is to adapt tat experiment for use as an introduction witout te matematical finesse. In tis experiment, te mecanical testing macine is replaced by dead weigts, and te gauge lengt of te specimens is measured wen convenient, using a common meter stick. Creep is a slow extension of a material in response to a comparatively low stress. Under a constant load, extension of te material results in a reduction in cross-section area, so stress increases under constant load. Te iger te stress, te iger te creep rate until failure finally occurs. In metals, creep can occur at any temperature iger tan approximately alf te absolute melting point (Celsius melting point + 273). Tus creep is not a problem for common metals used at ordinary temperatures. By coosing te rigt alloy, owever, creep can be demonstrated at room temperature, and te creep of common lead + tin solder is te subject of tis experiment. Lead-tin solder as a melting point of 183 C (456 K), so room temperature at 298K is more tan alf te melting temperature. Tus we would expect creep to occur at room temperature in solder if it is put under sufficient stress. Tis is also wy a soldered electrical connection sould always include a mecanical connection if possible, so tat stresses on te soldered joint are minimized. Creep is also a relevant failure mecanism for ig-temperature engines, suc as jet turbines. Te need for greater efficiency and power output drives temperatures ever iger, and tis makes creep an ever-more-present failure mecanism despite our best efforts to prevent it. As an example of creep-prevention metods, consider te fact tat creep is enanced by fine-grained metals. Turbine blades in te ottest sections of today s engines are now frequently composed of a single grain, a triump of materials processing. Module Procedure: 1. Sample preparation: Te first task was to fabricate specimens tat would not fail due to end effects. One practical metod is to first allow approximately 8 inces of extra wire at eac end of a specimen, and to loop te wire twice around a piece of ¾ ardwood dowel about 1 long, wit a ¼ ole drilled lengtwise troug it (see Figure 1). Once te solder wire is looped around tis end grip, it is wrapped

3 around itself at least 6 times to preclude slippage, and about ½ in. of solder is left projecting perpendicular to te wire to be stressed (to be used as a guide in measuring te sample lengt during te experiment). Tis sould all be done by and, to avoid nicking te solder using tools. Premature failure may occur at any nicks. Te distance between te two ends is measured as a function of time, and is regarded as a gauge lengt. A total of 5 of tese specimens is suggested, four of tem wit a gauge lengt of about 6 and one wit a gauge lengt of about 18. Te longer specimen is used to sow tat te strain calculation eliminates te effect of gauge lengt. 2. Sample testing: Te creep specimens are stressed in te following manner: Specimen #1 as no added weigt on it, and is intended as a control, to sow tat creep requires a minimum stress level to occur. Specimen #2 supports a weigt of 1000 grams, producing a stress level (force/area) of about 200 pounds per square inc. Specimen 3 also supports a weigt of 1 kg, wit specimen 3 about 3 X as long as specimen 2. Specimen 4 supports a weigt of 1.5 kg (stress level about 300 psi), to illustrate te faster failure tat occurs wen te stress is proportionately iger. Finally, specimen 5 supports a weigt of 2000 g (about 400 psi) to be like #4 except more so. Note tat all of tese stress levels are rater trivial for metals, demonstrating tat te stresses do not ave to be very ig for creep to occur once you are above alf te melting temperature. In order to stress te specimens, te appropriate weigts are suspended from teir lower ends, wile te upper ends were suspended from supports at te front of te classroom. 3. Measurement and observation: After te creep experiment is started, measurements sould be taken twice per day, once in te morning and once just before leaving. Eac specimen eventually will fail at some tiny defect suc as a nick were stress was iger, and te effect of tis local overload is seen as necking, wic eventually leads to local overstressing and failure. 4. Data Recording: For convenience, te data may be recorded in an Excel spreadseet, and tis spreadseet projected at te start of eac class for students to review or copy. Strain sould be calculated for eac data point. Te formula for strain is: Strain = (gauge lengt at any time-original gauge lengt)/original gauge lengt Since te units of lengt cancel in te formula above, strain is expressed as a percentage. A sample Excel spreadseet is sown in Table 1, sowing calculated strains as a function of time for te five specimens. Strain versus time for tis example is plotted in Fig. 3. Students sould be required to plot te data in tis manner, and to explain te sapes and ordering of te curves. Note tat, as expected, te specimen wit no added weigt (#1) exibited no strain during tis experiment, and would not be expected to creep because te stress from its own weigt is below te critical creep stress at room temperature for solder. Also, note tat Specimens 2 and 3 plot almost identically even toug differing greatly in lengt, because te strain calculation automatically adjusts for sample lengt. Te creep rate of te specimens is

4 powerfully affected by stress, wit iger stress producing iger creep rates and failure in sorter times. Note also tat all te curves ave basically te same sape; tis is because, as te specimens stretc, te cross-sectional area must decrease, increasing te stress. Consistent wit te previous statement, te iger stress produces iger creep rates. Te failure time is also very sensitive to te stress level. Figure 2 sows te current ig-strain record-older, a specimen loaded wit 700 grams of weigt (stress = 110 psi) tat as not failed wile undergoing almost 600% strain. Tis strain could undoubtedly be exceeded, wit careful enoug specimen preparation, an even lower stress level, and enoug time. Tis experiment as been performed for several years at Soutridge Hig Scool, and is a good way to sow tis metal failure mecanism at minimum trouble and expense. One possible cange could be to add one more specimen, stressed by 500 grams of weigt. At tis lower stress level, failure times are often more tan a week, depending on ambient temperature. Specimen #5 could also be eliminated, since te idea of iger stress effects is conveyed by specimen #4. For comparison, oter solder compositions could be tried, as could be oter materials suc as nylon fising line. Conclusions from tis experiment: 1. Common lead-tin solder creeps at room temperature, and is tus a convenient model material to demonstrate creep in metals. 2. Strain at failure and time to failure are strongly influenced by te applied stress. Supporting Materials: see attaced Reference: Solid core solder may be available at your local ardware store. Oter options include 1. Cline Glass Co., ttp:// 2. Discount Tools, ttp:// Acknowledgement: Tis experiment is a modified version, for a ig scool audience, of an experiment originally developed by Prof. Robert Stang and te University of Wasington. Evaluation Packet: Student evaluation questions (discussion or quiz): 1. Wy is solder used for tis experiment instead of some oter metal or alloy? 2. Wy do we use strain as te measure of creep? 3. Does te lengt of te specimen determine creep beavior? 4. Wat would appen if we doubled te diameter of te specimen wit oter variable remaining te same? Instructor evaluation questions: 1. At wat grade level was tis module used? 2. Was te level and rigor of te module wat you expected? If not, ow can it be improved?

5 3. Did te lab work as presented? Did tey add to student learning? Please note any problems or suggestions. 4. Was te background material sufficient for your background? Sufficient for your discussion wit te students? Comments? 5. Did te lab generate interest among te students? Explain. 6. Please provide your input on ow tis module can be improved, including comments or suggestions concerning te approac, focus and effectiveness of tis activity in your context. Course evaluation questions (for te students) 1. Was te lab clear and understandable? 2. Was te instructor s explanation compreensive and toroug? 3. Was te instructor interested in your questions? 4. Was te instructor able to answer your questions? 5. Was te importance of materials testing made clear? 6. Wat was te most interesting ting tat you learned?

6 Figure 1. Solder Creep Specimen Ready for Use.

7 Figure 2. Higest-Strain Specimen Tested to Date; Strain is Almost 600%.

8 t=0 t=7 t=23 t=30 t=47 t=54 t=71 t=72 t=78 t=106 t=121 t=143 t=150 t=174 t=182 W=0 W=1000g W=1000g W=1500g W=700g Spec. 1 Spec. 2 Spec. 3 Spec. 4 Spec. 5 cm(0) 6.1 cm 30.9 cm 5.2 cm 4.9 cm cm(0), , , , , cm(0) 7.7, , , 2 cm(0) 8.2, , , , 4 cm(0) 9.1, , , , 18 cm(0) 9.7, , , , 14 cm(0) 10.8, , , , 20 Broke cm(0) 11.5, , , 20 cm(0) 14.7, , 83, 33 cm(0) 16.8, , , 39 cm(0) 20.9, , , 49 cm(0) 22.9, , , 49 cm(0) 29.4, , , 59 Broke, 177 cm(0) 34.8, , 63 Table 1. Typical Data Set. At eac time after test start, eac entry contains te gauge lengt at te time of measurement, ten te calculated strain at tat time.

9 Creep Strain, % Time, X 10 Series2 Series3 Series4 Series5 Series6 Fig. 3. Creep Strain in Lead-Tin Solder as a Function of Stress and Time. Series 2 is Specimen 1, wile Series 3 and 4 are Specimens 2 and 3. Series 5 is Specimen 4, and Series 6 is Specimen 5.