THE EFFECT OF SURFACE INDICATIONS ON THE TENSILE PROPERTIES OF CAST STEEL by JEFF HAMBY JOHN A. GRIFFIN, COMMITTEE CHAIR ROBIN D. FOLEY CHARLES A. MONROE A THESIS Submitted to the graduate faculty of The University of Alabama at Birmingham, in partial fulfillment of the requirements for the degree of Master of Science in Materials Engineering BIRMINGHAM, ALABAMA 2013
THE EFFECT OF SURFACE INDICATIONS ON THE TENSILE PROPERTIES OF CAST STEEL JEFF HAMBY MATERIALS ENGINEERING ABSTRACT The objective of this thesis was to study the effect of surface indications on the tensile properties of cast steel. Four cast steel grades were selected for evaluation; these grades include three carbon and low alloy steels (110/80, 165/135, and Eglin) and one high alloy steel (CF8M). Using these steels, tensile specimens were produced, inspected via MT/PT, categorized by surface indications (as-cast or machined), and tested. Bars with natural surface indications were tensile tested and the properties recorded. The presence of a 1 / 16 inch, 1 / 8 inch, or 1 / 4 inch flat-bottomed hole drilled through half the thickness mimicked a similar nonlinear worse-case scenario indication. The 1 / 4 inch indication resulted in an ultimate tensile strength loss ranging from 21.5% to 36.0%, with the more ductile materials being impacted least. The percent elongation loss ranged from 38.5% to 69.9%, with the majority of the alloys showing an approximate 60 percent loss in elongation. The modulus decrease ranged from 2.9% to 17.5%. These results were modeled using ANSYS to observe capability in predicting a decrease in properties. The resulting decrease in properties matched the experimental data to an accuracy of 3±11%. The results provide a previously undocumented relationship between indication size and tensile properties. Keywords: Tensile, Steel, Indications, Surface, Castings, Model ii
TABLE OF CONTENTS Page ABSTRACT... ii LIST OF TABLES... iv LIST OF FIGURES... v LIST OF ABBREVIATIONS... vi BACKGROUND... 1 RESEARCH METHOD... 6 RESULTS... 12 Natural and Machined Surface Indication Lengths at Fracture... 12 0.2% offset YS and UTS... 12 Elongation... 16 Young s Modulus... 18 Percent Indication Area on Fracture Surface... 21 0.2% offset YS and UTS... 21 Elongation... 23 Young s Modulus... 23 Modeled Surface Indications... 26 0.2% offset YS and UTS... 26 Elongation... 32 Young s Modulus... 33 Conclusions... 34 LIST OF REFERENCES... 35 APPENDIX:... 36 A TENSILE DATA... 36 B STEEL CHEMISTRIES AND STRESS-STRAIN CURVES... 41 iii
Table LIST OF TABLES Page 1 Inputs used for ANSYS model... 10 2 Percent decrease of average 0.2% offset YS and UTS vs. indication lengths compared to sound material... 13 3 Percent decrease of average % elongation vs. indication lengths... 16 4 Percent decrease in Young s modulus vs. indication lengths... 18 5 Strength comparison between experimental and model... 31 6 % elongation comparison between experimental and model... 32 7 Young s modulus comparison between experimental and model... 33 iv
Figure LIST OF FIGURES Page 1 Example of tensile bar and plate... 7 2 810 MTS machine... 8 3 IGS models and element meshes... 10 4 0.2% offset YS and UTS vs. indication length at fracture... 15 5 % elongation vs. indication length measured at fracture... 17 6 Young's modulus vs. indication length measured at fracture... 20 7 0.2% offset YS and UTS vs. fracture surface area of indication... 22 8 % elongation vs. fracture surface area of indication... 24 9 Young's modulus vs. fracture surface area of indication... 25 10 Stress-strain curves of experimental data and model data of 165-135 (A)... 27 11 165-135 (A) and 110-80 (B) model outputs... 28 12 CF8M (C) and 110-80 (D) model outputs... 29 13 Eglin (E) model outputs... 30 v
LIST OF ABBREVIATIONS in. kip ksi MT MTS psi PT UTS YS inch or inches kilopounds kilopounds per square inch magnetic particle testing material test system pounds per square inch liquid penetrant testing ultimate tensile strength yield strength vi
1 BACKGROUND Every global industry strives to improve its processes and thus improve its product. This statement is especially true in today s quality-driven market. As a competitor in the global market, the cast steel industry has continuously improved its process and products to manufacture higher quality parts while minimizing costs and production time. Some of this improvement can be accredited to the many standards that have been written to help designers define acceptable product limits to produce required performance. However, some of these standards are workmanship standards and are not directly related to part performance. An example of a workmanship standard for steel castings is the radiographic standard ASTM E-186 [1]. This standard consists of reference radiographs that show examples of discontinuities categorized into severity levels, which allows considerable flexibility for the producer and buyer on how to interpret the radiographic grade of a part. This flexibility is necessary since stricter requirements would demand more information on the service environment of the part. Steel castings are used in an almost infinite variety of service conditions. In essence, the radiographic standards are a yardstick, and it is up to the producer and buyer on how to use the yardstick. Other standards such as ASTM A-903 provide quantitative values but were developed from other manufacturing processes and may be overly conservative [2]. This
2 standard specifies levels of acceptance criteria on the surface of castings using measured lengths and geometries of indications. As the cast steel industry has grown more sophisticated in the use of numerical modeling to predict process quality and part performance, design requirements should be reexamined to see if they actually affect part performance. In an attempt to link designer standards to the performance of materials, this study focused on characterizing the effect of surface/near surface indications on tensile properties using the backdrop of current acceptance standards such as ASTM A- 903 [2]. Most of the steels used in this study, ASTM A-958 Grade 110-80, ASTM A-958 Grade 165-135, ASTM A-351 Grade CF8M, are widely produced and normally used for valves, flanges, fittings, and other pressure containing parts. The only uncommon steel used was Eglin steel, which is a low cost replacement for super alloy steels such as HY- 180 and finds much of its use in military applications. The 110-80, 165-135, and Eglin steel are all low carbon and low alloy steels, the only exception being CF8M. In general, CF8M contains a high percentage of Cr and Ni and is essentially the cast equivalent of 304 type wrought alloys. CF8 may be fully austenitic, but it more commonly contains some residual ferrite (3-30%) in an austenitic matrix. CF8M is a version of CF8 alloy with an addition of 2-3% molybdenum, which increases resistance to corrosion by seawater and improves resistance. These molybdenum-bearing alloys are generally the superior choice for weakly oxidizing environments [3] (p. 20-16). In order to meet demands of buyers, the steel casting industry has used several recordable destructive tests to qualify the material properties. The two most prevalent tests are a cyclical loaded tensile test, also known as fatigue, and a monotonic loaded
3 tensile test. The monotonic tensile test is performed by applying an increasing load until failure, whereas the cyclical test applies an oscillating tensile load until failure. These two methods both result in quantifiable material properties. The monotonic tensile test was chosen for this study because of this industrial prevalence. To date, there has not been a study to determine the quantitative effect of surface indications on the monotonic tensile properties of steel castings. There has, however, been studies of the effect of internal indications on mechanical properties. The majority of these studies related fatigue performance to internal radiographic indications. A few studies related internal shrinkage, macro-porosity, and micro-porosity to tensile mechanical properties. In general, reasonable concentrations of internal shrinkage had little effect on 0.2% offset yield strength or YS, ultimate tensile strength or UTS, and elastic modulus, but produced a significantly reduced percent elongation when monotonically tested [5]. It was also observed that monotonically tested specimens with micro-porosity repeat the trend of having little effect on strength but did affect on ductility [6]. However, cyclically loaded fatigue specimens with macro-porosity showed elastic modulus varying as a function of porosity volume [6]. Hardin and Beckermann found that the elastic modulus decreases nonlinearly with porosity when cyclically loaded, and this relationship is dependent on the characteristics of the porosity [7]. These studies reveal that indications can potentially affect all mechanical properties, having the greatest effect on elongation. In order for this study to benefit from these past surface indication studies, a relationship between fatigue and monotonic tensile test must be formed. A comparison of the fatigue and monotonic tensile test is seen in Svoboda s study of fatigue and
4 fracture toughness of five different steels. The study revealed that the YS was lower in fatigue tests than in monotonic tests; however, the UTS was higher in fatigue versus monotonic in four of the five steels [4]. These results reveal that fatigue and monotonic tests are not directly relatable, but they do reveal which material properties will be affected most by surface indications. Thus, only general trends can be carried between surface indication studies using fatigue and studies using monotonic tensile tests. In order to quantitatively define the effect of surface indications on mechanical properties, the term surface indication must first be defined. Surface indication has historically been used to describe any visible inconsistency observed on the casting surface. An example of the current nomenclature, ASTM A903 conveys general acceptance guidelines, but does not reveal a quantitative relationship between the size of the indication and the mechanical properties[2]. With quantitative data, a more defined relationship between surface indications and properties can be developed. This relationship will give designers the ability to properly size a part and produce acceptable performance with a reasonable safety factor. Due to the random nature of surface indications, development of a machinable indication that mimics the effect of naturally occurring indications would be useful for experimental and numerical simulation testing. This technique has been used before by Rudy and Rupert in their study of the mechanical properties of aluminum and its relationship to porosity [8]. This study determined that fine porosity can be as detrimental to a weld as large porosity if the total area of the micro-pores were comparable to the single large pore. Thus, the machined indication replicated a natural indication. These results lead to a second goal of this study, which is to improve testing
5 repeatability in steel castings by using machined notches to mimic naturally occurring indications. The standard means of detecting a surface indication is by visual inspection. In order to improve this inspection, techniques such as magnetic particle testing also known as MT or liquid penetrant testing also known as PT have been developed, which aid the eye in the detection of hard-to-see indications on as cast surfaces. These tools greatly enhance detection, but classification and indication effect on properties are left up to operator interpretation. This study only contains linear and non-linear indication, not cracks from quenching or hot tears. Previous work has shown that linear and non-linear indications typically extend less than 13 mm beneath the surface while cracks developed from quenching or hot tears can run much deeper. By virtue of studying commonly used steels, the noticed effects of the surface indications will be able to directly contribute to real world safety applications. The less common Eglin steel was selected due to its extremely high tensile properties, thus broadening the data range for the study. A long-term use of this study will be the improvement of the quantification of surface indication effects on other mechanical properties, such as bending fatigue.
6 RESEARCH METHOD The four cast steels used included three carbon and low alloy steels and one high alloy steel. These steels provided a range of YS from 40 kilopounds per square inch or ksi up to 160 ksi. The carbon and low alloy steels include a 110/80 (minimum YS 80 ksi, minimum UTS 110 ksi), a 165/135 (minimum YS 135 ksi, minimum UTS 165 ksi), and Eglin steel. A high alloy CF8M cast steel was also included to provide different microstructure and modulus but with tensile properties similar to a 70/40 steel. Plates were cast from these steels yielding approximately 30 potential test bars for each alloy, with exception of the Eglin steel. The only available supply of Eglin steel was in machined billets with no as cast surface and hence no surface indications. In this case, tensile specimens were removed from the billets and artificial indications were machined into the gauge section. The other cast plates had approximately 0.050 inches or in. removed from the cope to remove the as cast surface roughness. Most of the plates were machined to yield 0.500 in. wide standard flat tensile bars [9]; however, the Eglin steel was machined with a thickness of 0.250 in. as opposed to 0.500 in. This reduced thickness was required for the Eglin steel in order to lower maximum load of the test bars to within 50 kilopounds or kips, the maximum load rating of the frame. These test bars were machined from the cope of a cast plate to capture any potential surface indications to the desired shape shown in Figure 1.
7 Figure 1: Example of tensile bar and plate Once machined, the carbon and low alloy steel specimens were MT inspected [10] to detect any surface/sub-surface indications present. All specimens were tested with PT [11] to distinguish surface and sub-surface indications, and reveal any indications running perpendicular to the gauge length. Of course, the CF8M specimens were only tested with PT. Indications found within the 2.25 in. reduced gauge section were photographed and measured using Image Pro Plus. According to ASTM A903, an indication is considered relevant if it is equal to or greater than 1 / 16 in. ASTM A903 surface inspection criteria also only considers this 1 / 16 in. relevant if the length of the indication is greater than 3 times its width i.e. linear [2]. For the purposes of this study, all indications detected via MT and PT will be considered relevant. Since the loading direction was known, indication length was measured as the length perpendicular to the loading direction, which will produce inherently conservative results.
8 Many tensile bars had no indications present. Many of these bars were used to provide baseline of properties for this study. However, some of these bars were notched to simulate a naturally occurring nonlinear surface indication. These notches were machined using different drill bit diameters ( 1 / 16 in., 1 / 8 in., and 1 / 4 in.) leaving a flatbottom circular (nonlinear) indication in the bar. Therefore, the created indication falls into the nonlinear class. The depth of drilling was limited to half the thickness of the tensile bar, which results in the surface class of indication as defined by Fatigue design of welded joints and components [12] (p.89). This simulated surface indication was meant to mimic worst case scenario nonlinear indications. These bars were tested according to ASTM E8 & A370 using an 810 material test system or MTS 50 kip frame with hydraulic grips machine seen in Figure 2. Figure 2: 810 MTS machine The tensile test was displacement controlled, while recording the applied force. Stress was determined using the resulting force over the determined cross-sectional area. The strain was recorded using a 2 in. clip-on extensometer utilizing MTS Flex Test software. The YS was obtained by plotting a stress-strain curve and recording the point at which
9 the curve becomes nonlinear. The modulus was obtained by performing a linear regression model on the elastic portion of the stress-strain curve and recording the slope. The 0.2% YS was determined by matching the slope with a parallel line that is transposed on the 0.002 in/in strain reading and recording the intersection of this line and the stressstrain curve. After tensile testing, the bars were studied to see if fracture occurred at an indication. The fracture surfaces were then photographed and the defect s surface area, if present, was measured using Image Pro Plus. The natural and machined indication properties were then compared to the baseline properties to see if a quantitative effect of the measured indications is observable. The tensile properties studied were 0.2% YS, UTS, elongation, and Young s Modulus. Upon completion of testing, the tensile bars without any indications as well as the 0.25 in. machined indications were modeled within ANSYS 14.5 to see if the model predicted a similar property behavior. The model used a 10node187 tetrahedral mesh for an inelastic rate-independent isotropic-hardening bilinear material. The mesh density used was determined to be independent, as a finer mesh yielded an average of less than 1% change in outputs. The mesh density used allowed the model to run quickly without reducing accuracy. These options yielded the mesh seen in Figure 3.
10 Figure 3: IGS models and element meshes The large displacement static solution control was selected over the small displacement static solution, because it accounts for more modeling scenarios. The model ran using 200 substeps to produce sufficient data points for graphing accuracy. The model used load displacement control, similar to tensile testing, and required the inputs found in Table 1. Tensile Bar Group Young s Modulus Table 1: Inputs used for ANSYS model Poisson s Ratio YS (psi) Tangent Modulus Displacement Used (in) 165-135 (A) 29773781 0.27 176257 493115 0.18 165-135 (A 1 / 4 ) 29773781 0.27 176257 493115 0.06 110-80 (B) 28184387 0.27 108656 430977 0.30 110-80 (B 1 / 4 ) 28184387 0.27 108656 430977 0.09 CF8M (C) 25807251 0.27 34449 165815 1.62 CF8M (C 1 / 4 ) 25807251 0.27 34449 165815 0.61 110-80 (D) 31346324 0.27 96472 240232 0.48 110-80 (D 1 / 4 ) 31346324 0.27 96472 240232 0.17 ES-1 (E) 26864791 0.27 162260 1127360 0.15 ES-1 (E 1 / 4 ) 26864791 0.27 162260 1127360 0.09
11 The displacement input was obtained by using the average displacement at failure of the experimental bars being measured. In other words, the average 0.250 in. machined indication 165-135 bars measured displacement was used for the 0.250 in. machined indication 165-135 model. The YS value was obtained by averaging the measured 0.2% offset YS produced by the bars without any detectable indications. Each alloy was categorized into a group A, B, C, D, or E. The models containing the 0.25 in. flat-bottom hole are indicated by the 1 / 4 following the alloy letter.
12 RESULTS Natural and Machined Surface Indications Lengths at Fracture 0.2% offset YS and UTS Naturally occurring indications were present in 3 of the 4 cast steels. Among these 3 cast steels, some test bars had more than one indication present. The Eglin steel test bars did not have any natural surface indications, only machine indications. Figure 4 shows the effects of indication length that instigated fracture on 0.2% YS and UTS. In most cases, fracture occurred at the largest measure indication. It should be noted that all length measurements were taken perpendicular to the load direction. These figures show that each alloy is affected differently by the indication lengths present, confirming an initial assumption that different alloys behave differently. A few data points in the upper right corner of the 165-135 graph seemingly do not follow the same trend as the rest of the data in Figure 4, but these graphs only represent the indication length measured at the location of fracture. Figure 4 does not account for the width or depth of the indication. Common sense would suggest that the longest indication on the bar would be the initiation of fracture. During this study the majority of fractures initiated at the longest measured indication. However, exceptions to this trend occurred in bars with small indications, less than 1 / 16 in.; bars with two or more indications of similar lengths, 0.02 in. difference; or in the more ductile materials CF8M, C and 110-80, D.
13 A second notable observation in Figure 4 is how the machined indications trend alongside the naturally occurring surface indications. In all cases, each machined indication represented a worst-case scenario for each indication length group. These results imply that the easily modeled flat-bottomed hole was a valid representation of a natural indication. A percentage decrease of 0.2% offset YS and the UTS as a function of indication length is listed in Table 2. Table 2: Percent decrease of average 0.2% offset YS and UTS vs. indication lengths compared to sound material 0.2 % offset YS Group 0.0001"- 0.0624" 0.0625" - 0.1249" 0.1250" - 0.1874" 0.1875" - 0.2499" > 0.25 Eglin, E - 5% 12% - 31% 165-135, A 1% 1% 12% 9% 17% 110-80, B 4% 1% 2% - 24% 110-80, D 4% 3% 6% 0% 21% CF8M, C 0% 3% 5% 4% 14% UTS Group 0.0001"- 0.0624" 0.0625" - 0.1249" 0.1250" - 0.1874" 0.1875" - 0.2499" > 0.25 Eglin, E - 5% 22% - 36% 165-135, A 2% 3% 17% 15% 21% 110-80, B 4% 4% 11% - 34% 110-80, D 3% 3% 7% 1% 21% CF8M, C 5% 6% 10% 0% 25% Table 2 and Figure 4 shows that for these steel strength levels states, all 0.2% offset YS and UTS are unaffected until indications lengths 0.125 in. or greater are present. It should be noted that due to the variation in the baseline properties seen in Figure 4, any effect less than 10% should be deemed statistically insignificant. Table 2 also suggests that the effect of the indication on 0.2% offset YS and UTS is dependent on the ductility
14 of the material. This correlation is shown by the CF8M, C s and the 110-80, D s resistance to the indications effects until the 0.25 in. size is reached. In summary, any indication less than 1 / 16 in. did not statistically impact the 0.2% offset YS or UTS of any alloy. The machined indication test bars matched similar worstcase scenarios found in the natural indication test bars. The more ductile alloys were less affected by the presence of surface indications, revealing a relationship to strength. Thus, the effect of the indication increased as the strength of the alloy increased.
Figure 4: 0.2% offset YS and UTS vs. indication length at fracture 15
16 Elongation in Table 3. Elongation was significantly more affected by surface indication length as shown Table 3: Percent decrease of average % elongation vs. indication lengths Group 0.0001"- 0.0624" 0.0625" - 0.1249" 0.1250" - 0.1874" 0.1875" - 0.2499" > 0.25" Eglin, E - 17% 33% - 38% 165-135, A 38% 46% 65% 64% 58% 110-80, B 13% 44% 70% - 78% 110-80, D 0% 29% 49% 47% 63% CF8M, C 17% 20% 34% 16% 52% Almost all elongations were affected by the presence of any indication. The only exception to this observed trend was seen in the ductile 110-80. As previously seen, the decrease in elongation is a function of the strength of the material. In other words, the more ductile materials were more resistant to indications. The Eglin steel was an exception to this trend, but this difference is due to the scatter seen in the baseline properties. Similar to the strength, the test bars with machined indications generated data that conservatively matched similarly sized natural indications. Figure 5 shows the effect of indication length on the elongation.
Figure 5: % elongation vs. indication length measured at fracture 17
18 Young s Modulus The Young s modulus was less sensitive to indication size, compared to other material properties. The modulus was obtained by determining the slope of the linear portion of the stress-strain curve using a linear regression model. Table 4 lists the decrease in modulus seen by each alloy. Table 4: Percent decrease in Young's modulus vs. indication lengths Group 0.0001"- 0.0624" 0.0625" - 0.1249" 0.1250" - 0.1874" 0.1875" - 0.2499" > 0.25" Eglin, E - 5% 0% - 11% 165-135, A 8% 1% 1% 1% 7% 110-80, B 0% 0% 0% - 7% 110-80, D 0% 2% 6% 0% 16% CF8M, C 0% 8% 12% 0% 17% Interestingly, Table 4 reveals that all observed moduli were unaffected until the indication lengths reached 0.25 in. or greater. Figure 6 shows the graphs of the data collected. The machined indications again trended alongside the natural indications. Even the observed decreases in the moduli overlapped some of the baseline moduli seen in Figure 6. Previous studies of elastic modulus showed indications had a greater influence than the observed results [7]. This disconnect is most likely due to differences between the compromised length and the total length of the extensometers. The previous study used a 12 millimeter extensometer, and this study used a 2 in. or 50.8 millimeter extensometer. Therefore, 100% of the extensometer length was compromised in the previous experiments; and this study only had approximately 10% of the extensometer length compromised at most. Thus, the observed data does not show as localized a strain
19 increase as seen previously. These greater values of strain would lead to greater reductions in modulus. A second difference in the studies was the method of testing. The previous study used fatigue, whereas this study used monotonic tensile testing. The cyclical loading of fatigue can cause materials to strain-soften, thus lowering the modulus values [6].
Figure 6: Young's modulus vs. indication length measured at fracture 20
21 Percent Indication Area on Fracture Surface 0.2% offset YS and UTS The tensile bar fracture surfaces were examined to determine the total area of indications present, both surface and internal indications previously undetected. The relationship between the percentage of indication area, the 0.2% YS, and the UTS is illustrated in Figure 7. The effect of indication(s) area on the fracture surface had a greater degree of variation than the surface indication length measurements. The location differences of the indications contributed to this observed variation. For example, an indication that is present on the bar surface and penetrates through the entire cross section can be more impactful on mechanical properties than an indication that covers a larger portion of the fracture surface but is not present on the machined surface. Figure 7 shows the relationship between material strength and the fracture surface area of indications. Although the indication fracture surface area had a greater degree of scatter, the strengths followed the same trend as the indication length at fracture. All alloys remain unaffected until a 12.5% area is covered, with the decrease in properties following a function of the materials ductility. The more ductile the material, the less the properties are affected. The machined indications again offer a conservative prediction of loss in properties.
Figure 7: 0.2% offset YS and UTS vs. fracture surface area of indication 22
23 Elongation Following the same trend as the indication length at fracture, the greatest decrease is seen in the elongation of the materials. Practically all alloy elongations were affected by the presence of any form of indications. The more ductile materials, however, showed a greater resistance to the percent fracture area of indications. Although the elongation is affected by the presence of even minor fracture surface areas of indications, the majority of the effect occurs rapidly. In other words, the presence of indications greatly reduces the elongation, but additional indications or increases in the fracture surface area do not enhance this effect. This result is especially evident in the machined indications. The 1 / 4 in. indication is not much worse than the 1 / 16 in. indication. Figure 8 reveals the effect the percent fracture area covered by indications has on the elongation of the alloys studied. Young s Modulus Similar to strength and elongation, the modulus trended in the same manner as the indication length measured at fracture. As expected, a great deal of scatter was again observed in the modulus data. This scatter muddles the effect that fracture surface area of indications has on the elastic modulus. It seems, however; that some degradation does possibly occur at greater observed instances of defect fracture surface area. It seems that the moduli behaved in a similar fashion for all alloys except for 165/135. Again, the Young s modulus was influenced the least by the presence of indications. Figure 9 shows the observed Young s modulus versus the indication surface area at fracture.
Figure 8: % elongation vs. fracture surface area of indication 24
Figure 9: Young's modulus vs. fracture surface area of indication 25
26 Modeled Surface Indications 0.2% offset YS and UTS In order to utilize models to analyze the phenomenon caused by natural surface indications, tensile bars were machined halfway through to create flat bottom holes. The flat-bottom hole was chosen because it had been used previously [8], and it is easily modeled. Models were constructed for all alloys studied, both with a 0.25 in. diameter hole in the center of the gauge section and without. These models used experimental data from the tensile bars without any recognizable indications to see if ANSYS could predict the detrimental effect that a 0.25 in. flat bottom hole had on the tensile properties. The model was calibrated by ensuring that the outputs matched the experimental data of the bars without indications. Similar to the actual experiment, the load on the model was controlled by displacement and stopped only when this displacement value was reached. These displacement values were determined by verifying that the model elongations matched the experimental elongations. When input correctly, the model creates data suitable for a stress-strain curve and generates images comparable to the actual tensile test. The inputs used for this model were listed previously in Table 1. An example of two strain-strain curves generated by the model can be seen in Figure 10, where A6 is a sound test bar, A16 is a bar with a 0.25 in. flat-bottom hole, A Full is a model of sound material, and A 1/4 is a model with the flat-bottom hole.
27 Figure 10: Stress-strain curves of experimental data and model data of 165-135 (A) The UTS was determined from the model by averaging the y-stresses at the 4 corner nodes on the surface that was displaced. These averages were then multiplied by 1.5 to account for the change in cross-sectional area of the observed face and the gauge section. The finite element analysis outputs can be seen below in Figures 11-13.
Figure 11: 165-135 (A) and 110-80 (B) model outputs 28
Figure 12: CF8M (C) and 110-80 (D) model outputs 29
30 1 2 Figure 13: Eglin (E) model outputs The resulting data from the models can be seen in Table 5, which shows a comparison of the actual versus predicted 0.2% YS and UTS for the tensile bars.
31 0.2 % offset YS Table 5: Strength comparison between experimental and model Tensile Bar Group Actual (avg.) psi Predicted (avg.) psi % Difference from Actual 165-135 (A) 176,257 182,500 +4% 165-135 (A ¼) 127,318 157,664 +24% % Decrease A to A ¼ 27.8% 13.6% --- 110-80 (B) 108,656 115,000 +6% 110-80 (B ¼) 85,543 104,750 +22% % Decrease B to B ¼ 21.3% 8.9% --- CF8M (C) 34,449 37,000 +7% CF8M (C ¼) 29,664 34,250 +15% % Decrease C to C ¼ 13.9% 7.4% --- 110-80 (D) 96,742 99,500 +3% 110-80 (D ¼) 76,454 87,250 +14% % Decrease D to D ¼ 21.0% 12.3% --- Eglin (E) 162,260 161,000-1% Eglin (E ¼) 112,734 151,750 +35% % Decrease E to E ¼ 30.5% 5.7% --- UTS 165-135 (A) 191,062 193,804 +1% 165-135 (A ¼) 131,739 157,664 +20% % Decrease A to A ¼ 31.0% 18.6% --- 110-80 (B) 135,382 138,913 +3% 110-80 (B ¼) 99,389 111,675 +12% % Decrease B to B ¼ 26.6% 19.6% --- CF8M (C) 77,876 73,032-6% CF8M (C ¼) 58,728 52,862-10% % Decrease C to C ¼ 24.6% 27.6% --- 110-80 (D) 114,786 116,712 +2% 110-80 (D ¼) 90,133 99,140 +10% % Decrease D to D ¼ 21.5% 15.1% --- Eglin (E) 208,382 202,819-3% Eglin (E ¼) 133,293 176,113 +32% % Decrease E to E ¼ 36.0% 13.2% --- The predicted tensile properties of the bars without any defects correlate well with the experimental data. These expected results verify that the correct inputs were chosen in order to replicate the sound tensile bars. The model predicts a decrease in strength
32 caused by the 0.25 in. flat-bottom hole; however, it is less accurate for the 0.2% offset YS. The generated percent decrease in 0.2% offset YS is on average 13 7%, and percent decrease in UTS is on average 8 10%. The less ductile the materials, the less accurate the model becomes. For 3 of the 5 alloys, however, the predicted percentage decrease is less than 7% off from the actual observed decrease. These results reveal that this model is more adequate for ductile materials in terms of predicting losses in strength, and that the model is more effective in predicting UTS than 0.2% offset YS. Elongation The elongation was measured within the model by following the change in displacement of 2 nodes within the gauge section that were approximately 2 in. apart. The resulting data can be seen in Table 6. Table 6: % elongation comparison between experimental and model Actual Elongation % Difference from Actual Tensile Bar Group Predicted Elongation A: 165-135 6.5% 6.7% +3% A ¼: 165-135 2.3% 2.1% -5% % Decrease A to A ¼ 65.4% 68.2% --- B: 110-80 11.5% 11.7% +1% B ¼: 110-80 3.5% 3.7% +5% % Decrease B to B ¼ 69.6% 83.7% --- C: CF8M 46.4% 47.4% +2% C ¼: CF8M 22.0% 22.8% +4% % Decrease C to C ¼ 52.6% 51.8% --- D: 110-80 20.9% 20.0% -4% D ¼: 110-80 7.7% 7.6% -1% % Decrease D to D ¼ 63.2% 61.9% --- E: Eglin 5.2% 5.1% -2% E ¼: Eglin 3.2% 3.2% -1% % Decrease E to E ¼ 38.5% 37.9% ---
33 Because the model used runs until the displacement is reached, the elongation directly related to the model inputs. Due to this relationship, the model follows the actual data closely. Young s Modulus The modulus was again obtained by taking the linear portion of the model generated stress-strain curve and determining the slope. This obtained model data was then compared to the experimental and is seen in Table 7. Table 7: Young s modulus comparison between experimental and model Tensile Bar Group Actual E (avg.) Predicted E (avg.) % Difference from Actual A: 165-135 29,773,781 29,792,947 +0% A ¼: 165-135 28,911,217 26,064,508-10% % Decrease A to A ¼ 2.9% 12.5% --- B: 110-80 28,184,387 28,202,236 +0% B ¼: 110-80 25,145,988 24,646,431-2% % Decrease B to B ¼ 10.8% 12.6% --- C: CF8M 25,807,251 20,536,743-20% C ¼: CF8M 21,279,905 22,520,720 +6% % Decrease C to C ¼ 17.5% -9.7% --- D: 110-80 31,346,324 31,353,852 0% D ¼: 110-80 26,391,115 27,336,453 +4% % Decrease D to D ¼ 15.8% 12.8% --- E: Eglin 26,864,791 26,967,907 0% E ¼: Eglin 23,864,716 23,841,044 0% % Decrease E to E ¼ 11.2% 11.6% --- Table 7 reveals that the predicted modulus of each alloy was affected by about the same amount of decrease. For most instances, the generated modulus matched the experimental data. The predicted moduli also reiterated that modulus is affected least by the indication in comparison to the other observed tensile properties. In all cases except
34 for the CF8M, the predicted percent decrease in modulus caused by the 0.25 in. flatbottom hole was off by less than 10%. This CF8M discrepancy is most likely due to the model s first generated data point being after the linear portion of the CF8M stress-strain curve. These results prove that ANSYS models can be used with relative accuracy in predicting the decrease in tensile properties seen by a 0.25 in. flat-bottom hole drilled through half the thickness. Thus the modeled 0.25 in. tensile bars proved useful in predicting the relationship between the defect and its properties. Conclusions In conclusion, alloy strengths were unaffected until the indication length reaches 1 / 8 in. All alloy elongations were greatly affected by the presence of practically any indication, thus revealing that elongation is the governing design factor. Also, the elastic moduli of the observed alloys were unaffected until indication lengths of the 1 / 4 in. or greater. In all observed instances, the more ductile the alloy, the less the impact of an indication. Also, the machined indications generated the most conservative properties in the experiment. ANSYS software was able to predict the percent decrease in properties from sound material to the machined 0.25 in. hole to an average accuracy of 3 11%.
35 References: [1] ASTM E186-10. Standard Reference Radiographs for Heavy-Walled Steel Castings. 2010 [2] ASTM A903/A903M. Standard Specification for Steel Castings, Surface Acceptance Standards, Magnetic Particle and Liquid Penetrant Inspection. 2009 [3] Steel Castings Handbook. Supplement 2: Summary of Standard Specifications for Steel Castings. Steel Founders Society of America. 2009 [4] Svoboda, John M. Fatigue and Fracture Toughness of Five Carbon Low Alloy Steels at Room and Low Climactic Temperatures (Part II) A. Steel Founders Society of America Research Report No. 94A. Carbon and Low Alloy Technical Research Committee Steel Founders Society of America. October 1982. [5] Hamby, Jeff, John Griffin, and Dr. Robin Foley. Verification of the New Radiographic Testing (RT) Standard through Mechanical Testing. Proceedings of Steel Founder s Society of America Technical and Operating Conference UAB. Dec. 2011. [6] Sigl, K.M. et al. Fatigue of 8630 cast steel in the presence of porosity. International Journal of Cast Metals Research 2004 Vol. 17 No.3. University of Iowa 2004. [7] Hardin, R. A., & Beckermann, C. Effect of Porosity on the Stiffness of Cast Steel. Metallurgical and Materials Transactions A. Vol. 38A(12). 2992 3006. The Minerals, Metals, & Materials Society and ASM International. 2007. [8] Rudy, J. F. and Rupert, E. J. Effects of Porosity on Mechanical Properties of Aluminum Welds. Welding Research Supplement. 322-s 335-s. July 1970. [9] ASTM E8/E8M. Standard Test Methods for Tension Testing of Metallic Materials. 2009 [10] ASTM E709. Standard Guide for Magnetic Particle Testing. 2008 [11] ASTM E 165/ E165M. Standard Practice for Liquid Penetrant Examination for General Industry. 2009 [12] Hobbacher, A. Fatigue design of welded joints and components. 1996
36 APPENDIX A TENSILE DATA
37 Alloy # % Elong. Modulus, E 0.2% YS (psi) UTS(psi) Strain @ Max Stress Hole in gage Max Indict. Length (in) 165/135 A4 9.0 30,252,866 175019 190737 0.046227697 0.000 0.1 165/135 A5 7.0 28,691,188 177978 191754 0.04447763 0.000 3.5 165/135 A6 7.5 29,364,798 175491 191394 0.045135263 0.000 0.7 165/135 A7 8.0 29,073,879 176588 191361 0.045586135 0.000 0.0 165/135 A8 7.5 31626292.09 176415 191040 0.033652097 0.000 1.3 165/135 A9 7.0 28693570.48 175955 191144 0.039313804 0.000 0.0 165/135 A10 3.5 30,084,072 175446 189976 0.033185799 0.000 0.4 165/135 A11 2.0 33294772.25 175829 183794 0.012656678 0.0625 0.000 6.3 165/135 A12 3.0 32546233.79 174682 182829 0.013380029 0.0625 0.000 6.3 165/135 A13 5.5 28,601,903 176723 190766 0.04301526 0.044 0.2 165/135 A14 3.0 26453681.81 173172 183667 0.015261985 0.0625 0.000 6.3 165/135 A15 3.0 27,094,186 168411 179878 0.024543982 0.320 12.0 165/135 A16 2.0 28641145.74 135646 137494 0.007555771 0.25 0.000 25.0 165/135 A17 2.5 26353446.62 161999 164855 0.009521087 0.125 0.000 12.5 165/135 A18 2.0 28,397,397 152161 152161 0.005935499 0.192 7.2 165/135 A19 2.5 26,015,122 172551 185230 0.021615038 0.020 3.3 165/135 A20 2.5 27,943,811 166603 166603 0.006745987 0.238 6.1 165/135 A21 3.0 27,420,411 133506 133506 0.005484967 0.335 50.8 165/135 A22 4.0 27,637,770 173401 186292 0.023547078 0.063 1.4 165/135 A23 2.0 30013078.44 165122 167566 0.009668754 0.125 0.000 12.5 165/135 A24 5.5 27,623,721 176501 187251 0.017462865 0.092 26.9 165/135 A25 2.0 31,004,845 171892 173814 0.009462523 0.153 32.0 165/135 A26 2.5 31,535,319 160713 166190 0.011038303 0.197 15.9 165/135 A27 3.5 29,245,045 149570 163150 0.020442087 0.165 46.5 165/135 A28 2.5 28674339.46 149606 153096 0.008722876 0.125 0.000 12.5 165/135 A29 2.5 29181287.92 118989 125984 0.008957542 0.25 0.000 25.0 110/80 B1 13.0 27,961,656 107831 134688 0.067877926 0.000 0.8 110/80 B2 12.5 33,086,885 106125 132264 0.072826982 0.030 6.7 110/80 B3 13.5 28,730,595 106269 132766 0.074021488 0.000 0.9 110/80 B4 9.0 28111361.89 109000 136099 0.0707651 0.000 1.6 110/80 B5 12.5 28480141.31 109136 135360 0.065687001 0.000 0.8 110/80 B6 4.5 29428285.73 109017 130995 0.042136353 0.0625 0.000 6.3 110/80 B7 13.0 30,221,569 110253 135836 0.071881428 0.000 5.2 110/80 B8 5.0 33,316,452 110891 130681 0.036337767 0.109 7.2 110/80 B9 6.0 29986745.81 108664 130540 0.041089348 0.0625 0.000 6.3 110/80 B10 7.5 30,320,115 110158 135563 0.068104059 0.072 2.7 110/80 B11 6.0 29714383.44 108782 129228 0.037954699 0.0625 0.000 6.3 110/80 B12 4.5 28257993.3 102319 118292 0.020926585 0.125 0.000 12.5 Fract. Surf. Area (%)
38 110/80 B13 3.5 30810267.77 105175 119799 0.020760607 0.125 0.000 12.5 110/80 B14 3.5 28449006.55 103911 118023 0.019708134 0.125 0.000 12.5 110/80 B15 8.5 31,813,477 109714 135523 0.067677043 0.067 4.4 110/80 B16 5.0 30,456,459 108558 129101 0.033323929 0.085 4.5 110/80 B17 5.5 31,612,967 108301 132700 0.049511354 0.072 5.5 110/80 B18 3.5 25145988.3 85543 99389 0.016362939 0.25 0.000 25.0 110/80 B19 1.5 27,452,271 79368 79368 0.003600626 0.309 27.6 110/80 B20 10.5 28,417,668 108196 135960 0.069382213 0.054 1.3 110/80 B21 1.5 28,942,323 107313 119735 0.01731167 0.161 2.9 110/80 B22 4.0 31,167,327 108365 122190 0.022865729 0.065 11.3 110/80 B23 3.0 28,419,638 107641 118382 0.017986676 0.148 7.7 110/80 B24 10.0 32,000,459 109090 134378 0.067591652 0.000 0.5 110/80 B25 5.0 29,568,335 107331 124334 0.028641639 0.105 11.3 110/80 B26 7.0 28,662,731 103418 127525 0.049730156 0.072 12.3 110/80 B27 7.0 29,545,743 97132 119915 0.049512252 0.054 6.6 110/80 B28 13.0 28,118,132 96352 123447 0.07127481 0.062 21.0 CF8M C1 28.0 23,503,422 31799 65843 0.20145939 0.067 33.4 CF8M C2 52.0 26,321,895 34039 80941 0.42761526 0.068 0.5 CF8M C3 26.0 25,467,449 33352 70549 0.22129148 0.036 11.8 CF8M C4 30.5 25081553.49 33168 73908 0.24403925 0.125 0.000 12.5 CF8M C5 28.5 27514820.62 34561 69098 0.24399422 0.0625 0.000 6.3 CF8M C7 22.5 21066265.22 28991 57423 0.15901543 0.25 0.000 25.0 CF8M C8 51.0 28,259,712 36824 78716 0.48595396 0.000 0.0 CF8M C9 54.0 24,389,059 34163 77775 0.46086529 0.036 0.0 CF8M C10 45.5 26,612,874 34672 78786 0.32751146 0.000 0.0 CF8M C11 39.5 24,078,442 35112 79285 0.30956766 0.000 1.3 CF8M C12 26.0 22374146.32 33928 68558 0.20041275 0.125 0.000 12.5 CF8M C13 23.5 21776464.53 29697 59943 0.16364397 0.25 0.000 25.0 CF8M C14 28.5 26,764,969 39117 71432 0.22368726 0.047 0.0 CF8M C15 20.0 26,908,895 35937 69248 0.16647017 0.062 47.4 CF8M C16 39.0 27,570,291 33048 78077 0.35273856 0.192 15.6 CF8M C17 45.5 25,625,215 33307 76178 0.39317152 0.091 0.6 CF8M C18 20.0 21514381.28 30280 56602 0.1265161 0.25 0.000 25.0 CF8M C19 54.0 25,233,972 33745 79807 0.38374391 0.000 0.0 CF8M C20 48.5 23585088.31 32851 78104 0.45493639 0.0625 0.000 6.3 CF8M C22 22.5 20762507.38 29688 60943 0.16027179 0.25 0.000 25.0 CF8M C23 45.0 27,384,224 33108 78657 0.38614559 0.000 0.2 CF8M C25 40.0 23,822,520 33403 77752 0.2632235 0.082 0.7 CF8M C26 47.0 24,286,376 31405 75136 0.36255124 0.046 2.2 CF8M C27 37.5 21021843.56 30964 72688 0.29088813 0.125 0.000 12.5 CF8M C28 48.0 27,016,898 33916 75694 0.41851655 0.000 0.0 CF8M C29 28.5 22340494.84 33498 66259 0.2333522 0.125 0.000 12.5
39 CF8M C30 32.5 22019744.01 32793 70017 0.23669343 0.0625 0.000 6.3 CF8M C31 56.5 23,410,103 33074 72530 0.41160625 0.000 0.0 CF8M C32 29.0 18276251.2 35463 72075 0.20579399 0.0625 0.000 6.3 CF8M C34 44.0 22244971.21 35268 77869 0.37655166 0.000 0.0 CF8M C35 35.0 25,580,198 34317 76781 0.35581136 0.000 0.0 CF8M C36 47.0 28251116.45 33921 78449 0.39727163 0.000 0.0 110-80 D1-5 22.5 27,715,332 92905 111615 0.088948622 0.000 2.4 110-80 D1-6 13.0 33606519.33 92482 110226 0.068567723 0.0625 0.000 6.3 110-80 D1-8 10.0 27527446.02 88013 104630 0.04238376 0.125 0.000 12.5 110-80 D1-9 22.0 33,122,194 95722 113496 0.083042622 0.000 0.2 110-80 D1-10 7.5 25491303.38 71779 86658 0.028918706 0.25 0.000 25.0 110-80 D1-12 9.0 27975822.19 91399 106550 0.037625276 0.125 0.000 12.5 110-80 D1-13 12.5 27567138.02 101041 117151 0.06809327 0.0625 0.000 6.3 110-80 D2-4 19.0 32,762,731 97067 115883 0.070279308 0.018 0.8 110-80 D2-5 17.5 32,355,498 96645 115768 0.074118435 0.000 1.4 110-80 D2-6 7.5 24890169.47 75973 89896 0.025466975 0.25 0.000 25.0 110-80 D2-8 8.5 30,094,223.62 94478 109142 0.035086486 0.125 0.000 12.5 110-80 D2-9 21.5 32,192,271 101695 118264 0.07616587 0.000 0.7 110-80 D2-10 12.5 30163883.86 98590 115793 0.065269746 0.0625 0.000 6.3 110-80 D2-11 19.5 31,007,055 98028 116691 0.077785835 0.000 1.3 110-80 D2-12 7.5 26696703.83 75757 89616 0.026572356 0.25 0.000 25.0 110-80 D2-13 8.5 28486281.69 82306 94363 0.024333309 0.25 0.000 25.0 110-80 D3-1 11.0 36,127,462 99579 113540 0.065485843 0.193 6.0 110-80 D3-2 23.0 33,033,785 93355 111510 0.088961989 0.000 0.0 110-80 D3-3 21.5 30,032,985 90193 109662 0.082714073 0.038 0.0 110-80 D3-4 23.0 30,805,009 89514 109258 0.093816414 0.038 0.0 110-80 D3-5 24.0 30,272,877 91861 110186 0.093908153 0.000 0.3 110-80 D3-6 23.0 31,634,592 91739 110758 0.089738443 0.000 0.0 110-80 D3-7 11.0 29284489.46 82624 100583 0.049733803 0.125 0.000 12.5 110-80 D3-8 13.5 31492980.05 87260 105966 0.066313177 0.0625 0.000 6.3 110-80 D3-9 22.5 30,783,910 88861 108094 0.089188881 0.097 0.2 110-80 D3-10 21.5 29,139,459 87668 107510 0.080416195 0.000 0.0 110-80 D3-11 22.5 29,997,490 90492 109881 0.082136124 0.000 0.1 110-80 D3-12 22.5 31,496,735 94044 111729 0.085510574 0.061 0.0 110-80 D3-13 14.5 32,242,414 98979 115430 0.081883222 0.177 6.2 Eglin ES1-2 6.9 26,256,567 163052 215402 0.036350816 0.000 0.0 Eglin ES1-3 4.9 28,012,310 163239 210843.2 0.026835466 0.000 0.0 Eglin ES1-4 3.7 26,325,497 160489 198900 0.020922411 0.000 0.0 Eglin ES1-5 4.2 23,077,987 152269 200521 0.023187885 0.0625 0.000 6.3 Eglin ES1-6 4.4 27,972,467 157057 196704 0.021894567 0.0625 0.000 6.3 Eglin ES1-7 26,098,168 135551 154831 0.015368793 0.125 0.000 12.5 Eglin ES1-8 3.0 31,824,774 148830 170369 0.015213027 0.125 0.000 12.5
40 Eglin ES1-9 3.9 27,081,549 142804 160706 0.016091352 0.125 0.000 12.5 Eglin ES1-10 22,482,321 114494 140782 0.016743625 0.25 0.000 25.0 Eglin ES1-11 3.4 25,189,318 117485 140521 0.015503213 0.25 0.000 25.0 Eglin ES1-12 3.0 23,922,510 106224 118577 0.009456518 0.25 0.000 25.0
41 APPENDIX B STEEL CHEMISTRIES AND STRESS-STRAIN CURVES
Group C Si Mn P S Cr Mo Ni Co Cu Nb Ti V W Zr 165-135, A 0.331 0.5 1.01 0.021 0.0165 0.841 0.460 0.78-0.116 - - - - 0.0144 110-80, B 0.329 0.5 1.00 0.020 0.0171 0.841 0.460 0.78-0.120 - - - - 0.0145 CF8M, C 0.033 1.17 1.14 0.037 0.0069 19.5 0.251 8.59 0.0777 0.348 0.0170 0.0096 0.0631 0.0479-110-80, D 0.301 0.43 1.07 0.018 0.0040 0.565 0.307 0.65-0.107-0.0201 - - - Eglin, E 0.112 0.92 0.65 0.011 0.0024 3.03 0.404 1.05-0.0883 - - 0.0841 0.919-42
43
44
45
46
47
48
49
50
51
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53