Composites: Part B 53(2013)103-111 Contents lists available at SciVerse Science Direct composites Composites: Part B ELSEVIER journalhomepagewww.elsevier.com/locate/compositesb fracture failure analysis of automotive accelerator pedal arms with Cross Mark polymer matrix composite material Yi Gong, Zhen-Guo Yang Department of Material Science, Fudan University, Shanghai 200433, PR China ARTICLE INFO A BSTRACT The booming automotive industry is undoubtedly not indep of the extensive applications of com- Received 4 January 2013 posite materials, especially the polymer matrix composite aiming to optimize the price versus erformance ratios. Actually, considering safety and reliabil he prior concerns of vehicles, evalu- Available online 25 April 2013 tion of the'performances'must never be stopped, from both laboratories and actual services. In this fracture failure incidents of the automotive accelerator pedal arms with matrix of long glass fiber reinforced polypropylene were systematically investigated. Based on the analysis results of matrix mate- A Polymer-matrix composites(PMCs) rials inspection and fractograph observation, root causes of the failure were identified, and the counter- B Microstructures measures were proposed from the manufacturing point of view. Achievements of this study would provide an actual and vivid example to understand how the processing of polymer matrix composite materials affects the microstructures, the properties and the performances, and would consequently help to prevent similar failures on automotive parts with such materials e 2013 Elsevier Ltd. All rights reserved 1 Introduction the root causes would help to clarify the actual duties. Finally, the countermeasures were proposed on basis of the analysis results. The increasing drive toward smaller and lighter vehicles for the Achievement of this paper would have a reference value for failure sake of higher fuel efficiency has been seeing the expanding appli- prevention of all the lGFPP automotive parts, and would even pro- cation of composite materials, which account for about 10-15% vide a vivid example of the relationship among processing, struc weight of a civilian vehicle nowadays. Among them, the long glass ture, performance, and application for polymer matrix composite fiber reinforced polypropylene(LGFPP)that owns superiorities like materials low-density, low-cost, and recyclable is the most used and faste growing one [1. mainly applied to the front end brackets, dash oards, pedals, and so on. Nevertheless, every coin has two sides. whether these composite automotive parts with optimized price It should be noted firstly that 1100 N was regarded as the qual- versus performance ratio are competent in practice still deserves ified fracture load for the pedal arms. Fig. la displayed the external to be investigated in detai appearances of two fractured samples, one was the failed pedal Besides abundance of experimental studies [2, 3]. failure analy- arm from the automaker, and the other was the unused pe is is relatively seldom reported, but it plays an even more impe tant role since its samples are all acquired from the actual service the manufacturer Learnt from Fig. 1b, the fracture positions were conditions. In this paper, several fracture failure incidents, which both locating at the joints between the pedals and the frames. took place under test loads lower than the design value during Then, referring to our previous experiences of engineeri ing failure tion after whole-vehicle assembly, of the automotive analysis 14-91. investigations were carried out from two aspects ccelerator pedal arms with LGFPP (GF, 50 wt%) polymer matrix n one hand, the matrix material of the failed pedal arm was char pose this fracture occurred in driving. untold sufferings would have lized to verify the chemical constituents; thermogravimetric been engendered From another point of view, as the automaker, analysis(TGA), melt index(MI)measurement, and high-temp the pedal arm manufacturer, and the GF/PP pellets supplier were ture sintering were employed to testify the content, detect the dis three different joint ventures from multiple countries, determining tribution condition, and help to observe the morphologies of the glass fibers; even the gF/PP pellets were examined for their phys- Corresponding author. Tel. +86 21 65642523: fax: +86 21 65103056 ical and mechanical properties. On the other hand, in order for E-mailaddress:zgyang@fudan.edu.cn(Z-G.Yang omparison, the fracture surfaces of both the two samples were 68/s-see front matter 2013 Elsevier Ltd. All rights reserved l/dx doiorg/10. 1016/j- composites. 2013.04.047
Fracture failure analysis of automotive accelerator pedal arms with polymer matrix composite material Yi Gong, Zhen-Guo Yang ⇑ Department of Materials Science, Fudan University, Shanghai 200433, PR China article info Article history: Received 4 January 2013 Accepted 7 April 2013 Available online 25 April 2013 Keywords: A. Polymer–matrix composites (PMCs) A. Glass fibers B. Microstructures D. Fractography abstract The booming automotive industry is undoubtedly not independent of the extensive applications of composite materials, especially the polymer matrix composite materials, aiming to optimize the price versus performance ratios. Actually, considering safety and reliability are the prior concerns of vehicles, evaluation of the ‘performances’ must never be stopped, from both laboratories and actual services. In this paper, fracture failure incidents of the automotive accelerator pedal arms with matrix of long glass fiber reinforced polypropylene were systematically investigated. Based on the analysis results of matrix materials inspection and fractograph observation, root causes of the failure were identified, and the countermeasures were proposed from the manufacturing point of view. Achievements of this study would provide an actual and vivid example to understand how the processing of polymer matrix composite materials affects the microstructures, the properties and the performances, and would consequently help to prevent similar failures on automotive parts with such materials. 2013 Elsevier Ltd. All rights reserved. 1. Introduction The increasing drive toward smaller and lighter vehicles for the sake of higher fuel efficiency has been seeing the expanding application of composite materials, which account for about 10–15% weight of a civilian vehicle nowadays. Among them, the long glass fiber reinforced polypropylene (LGFPP) that owns superiorities like low-density, low-cost, and recyclable is the most used and fastest growing one [1], mainly applied to the front end brackets, dashboards, pedals, and so on. Nevertheless, every coin has two sides, whether these composite automotive parts with optimized price versus performance ratio are competent in practice still deserves to be investigated in detail. Besides abundance of experimental studies [2,3], failure analysis is relatively seldom reported, but it plays an even more important role since its samples are all acquired from the actual service conditions. In this paper, several fracture failure incidents, which took place under test loads lower than the design value during the inspection after whole-vehicle assembly, of the automotive accelerator pedal arms with LGFPP (GF, 50 wt%) polymer matrix composite materials were systematically analyzed. Evidently, suppose this fracture occurred in driving, untold sufferings would have been engendered. From another point of view, as the automaker, the pedal arm manufacturer, and the GF/PP pellets supplier were three different joint ventures from multiple countries, determining the root causes would help to clarify the actual duties. Finally, the countermeasures were proposed on basis of the analysis results. Achievement of this paper would have a reference value for failure prevention of all the LGFPP automotive parts, and would even provide a vivid example of the relationship among processing, structure, performance, and application for polymer matrix composite materials. 2. Experimental It should be noted firstly that 1100 N was regarded as the qualified fracture load for the pedal arms. Fig. 1a displayed the external appearances of two fractured samples, one was the failed pedal arm from the automaker, and the other was the unused pedal arm (with qualified fracture load in test) directly obtained from the manufacturer. Learnt from Fig. 1b, the fracture positions were both locating at the joints between the pedals and the frames. Then, referring to our previous experiences of engineering failure analysis [4–9], investigations were carried out from two aspects. On one hand, the matrix material of the failed pedal arm was characterized. Fourier transform infrared spectroscopy (FTIR) was utilized to verify the chemical constituents; thermogravimetric analysis (TGA), melt index (MI) measurement, and high-temperature sintering were employed to testify the content, detect the distribution condition, and help to observe the morphologies of the glass fibers; even the GF/PP pellets were examined for their physical and mechanical properties. On the other hand, in order for comparison, the fracture surfaces of both the two samples were 1359-8368/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.04.047 ⇑ Corresponding author. Tel.: +86 21 65642523; fax: +86 21 65103056. E-mail address: zgyang@fudan.edu.cn (Z.-G. Yang). Composites: Part B 53 (2013) 103–111 Contents lists available at SciVerse ScienceDirect Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Y Gong, Z-G. Yang/Composites: Part B 53(2013)103-11 failed pedal unused frame Fig. 1. External appearances of the two fractured pedal arm samples(a) front side, and (b)back side. analyzed under 3-D stereomicroscope (SM), scanning electron be 50%, while in other regions they may be not. Till now, it was microscope(SEM), and energy disperse spectroscope(EDS). pretty obvious that although the lgFPP matrix material of the ped al arm could be regarded basically qualified uneven distribution of 3. Results and discussion he glass fibers still severely existed within the polypropylene 3.1. FTIR 3. 4. High-temperature sintering In order to identify whether the chemical constituents of the ailed pedal arm were qualified, the organic functional groups of Subsequently, the pedal arm sample was put into furnace for both the pedal and the frame were detected by FtiR at first. As h-temperature sintering As shown in Fig. 4a and b, the rem- 2958 cm-1 was attributed to the valence vibrations of the c-h under SEM, it was obvious that the majority of the glass fibers had bonds, and the peaks at around 1378 cm"Iand 1458 cm-lrepre- lengths ranging from one to several millimeters, while also ac sented the methyl -CH3 and the methylene -CH2- groups respec- panied with a not low content of shorter ones(multi-hundred mi- tively [10]. What's more, the broad and intense band from 847 to crons), seen in Fig. 4c. It should be pointed out that although these crystalline isotactic lengths seemingly did not match the original pellet length 1 cm, it polypropylene[11-13]. Thus, it can be concluded that the polymer was a normal consequence arter processing natrix was dominantly polypropylene, confirming its qualification. 3.5. Examination of GF/PP pellets 3. 2. TGA Finally, the GF/PP pellets that were directly acquired from the supplier were examined, and the results were also compared with fraRespe the ea e d meded 1g, wane the, cha, cte z ed by ada the that from the supplier. As shown in Fig, 5a, normal pellets were displayed in Fig 3a and b, evident reductions in molecular weigl plump polypropylene. However, just through a rough examination of both the two samples started at around 300C, in accordance over several dozens, defective ones like too short, not plump, and vith the initial decomposition temperature of polypropylene in bare gF were detected, seen in Fig 5b. Besides, Table 1 listed so erature[14]. Afterwards, with the increase of temperature, such hysical and mechanical properties of the pellets, which were polypropylene matrix completely decomposed at about 480c apparently a little bit inferior to the standards. Actually, such de- eventually, conforming to the predecessor's research result too fects would not significantly influence the entire qualification of [15].Meanwhile, the 50% weight loss also verified the nominal the pellets, but they would still have the possibility to affect the content of glass fibers. Apparently, all these phenomena have performances of the ultimate pedal arm products strictly confirmed the qualification of the lGFPP again. 3.6. Fractograph observation 33. Melt index measurement 3.6.1. Failed sample milarly, both the pedal and the frame were then measured for Fig 6a displayed the total morphologies of the two counterparts their melt indexes. After preheating at 230C for 6 min, a standard of the failed sample's fracture surfaces under 3-D stereomicro- eight of 2.16 kg was introduced onto the twe les. The re- scope. Totally speaking, it was evident that the distribution of sults showed that the Mi of the pedal was 4.3 g/10 min, and that the glass fibers was a little bit uneven within the polypropylene of the frame was 3.6 g/10 min. It is well known that a lower MI matrix --in some area there were nearly no glass fibers(marked indicates a higher molecular weight. In this case it can be inferred by the red circle), while in other area the glass fibers were that the sample of the frame may have a higher content of polypro- agglomerated (marked by the red arrow ) From another point of pylene, while the sample of the pedal may have a higher content of view, as the loads from the drivers step in service were exerted lass fibers. This conclusion was actually against to the tga results along the yellow arrows, it was not difficult to infer the corner above, but considering the fact that the TGa samples weighed not xceeding just 20 mg, randomness was possibly arisen. That is to I For of color in Figs. 6, 11 and 13, the reader is referred to the web say, the glass fiber contents of the two TGa samples happened to version of thi
analyzed under 3-D stereomicroscope (SM), scanning electron microscope (SEM), and energy disperse spectroscope (EDS). 3. Results and discussion 3.1. FTIR In order to identify whether the chemical constituents of the failed pedal arm were qualified, the organic functional groups of both the pedal and the frame were detected by FTIR at first. As shown in Fig. 2a and b, an intense band extending from 2723 to 2958 cm1 was attributed to the valence vibrations of the CH bonds, and the peaks at around 1378 cm1 and 1458 cm1 represented the methyl CH3 and the methylene CH2 groups respectively [10]. What’s more, the broad and intense band from 847 to 1165 cm1 indicated the polymer matrix was crystalline isotactic polypropylene [11–13]. Thus, it can be concluded that the polymer matrix was dominantly polypropylene, confirming its qualification. 3.2. TGA Respectively sampled 19.6 and 18.8 mg, the pedal and the frame of the failed pedal arm were then characterized by TGA. As displayed in Fig. 3a and b, evident reductions in molecular weight of both the two samples started at around 300 C, in accordance with the initial decomposition temperature of polypropylene in literature [14]. Afterwards, with the increase of temperature, such polypropylene matrix completely decomposed at about 480 C eventually, conforming to the predecessor’s research result too [15]. Meanwhile, the 50% weight loss also verified the nominal content of glass fibers. Apparently, all these phenomena have strictly confirmed the qualification of the LGFPP again. 3.3. Melt index measurement Similarly, both the pedal and the frame were then measured for their melt indexes. After preheating at 230 C for 6 min, a standard weight of 2.16 kg was introduced onto the two samples. The results showed that the MI of the pedal was 4.3 g/10 min, and that of the frame was 3.6 g/10 min. It is well known that a lower MI indicates a higher molecular weight. In this case, it can be inferred that the sample of the frame may have a higher content of polypropylene, while the sample of the pedal may have a higher content of glass fibers. This conclusion was actually against to the TGA results above, but considering the fact that the TGA samples weighed not exceeding just 20 mg, randomness was possibly arisen. That is to say, the glass fiber contents of the two TGA samples happened to be 50%, while in other regions they may be not. Till now, it was pretty obvious that although the LGFPP matrix material of the pedal arm could be regarded basically qualified, uneven distribution of the glass fibers still severely existed within the polypropylene matrix. 3.4. High-temperature sintering Subsequently, the pedal arm sample was put into furnace for high-temperature sintering. As shown in Fig. 4a and b, the remnants were white and erect glass fiber clusters. After magnification under SEM, it was obvious that the majority of the glass fibers had lengths ranging from one to several millimeters, while also accompanied with a not low content of shorter ones (multi-hundred microns), seen in Fig. 4c. It should be pointed out that although these lengths seemingly did not match the original pellet length 1 cm, it was a normal consequence after processing. 3.5. Examination of GF/PP pellets Finally, the GF/PP pellets that were directly acquired from the supplier were examined, and the results were also compared with that from the supplier. As shown in Fig. 5a, normal pellets were one-centimeter cylinders in structure of glass fibers clad with plump polypropylene. However, just through a rough examination over several dozens, defective ones like too short, not plump, and bare GF were detected, seen in Fig. 5b. Besides, Table 1 listed some physical and mechanical properties of the pellets, which were apparently a little bit inferior to the standards. Actually, such defects would not significantly influence the entire qualification of the pellets, but they would still have the possibility to affect the performances of the ultimate pedal arm products. 3.6. Fractograph observation 3.6.1. Failed sample Fig. 6a displayed the total morphologies of the two counterparts of the failed sample’s fracture surfaces under 3-D stereomicroscope. Totally speaking, it was evident that the distribution of the glass fibers was a little bit uneven within the polypropylene matrix –– in some area there were nearly no glass fibers (marked by the red circle),1 while in other area the glass fibers were agglomerated (marked by the red arrow). From another point of view, as the loads from the driver’s step in service were exerted along the yellow arrows, it was not difficult to infer the corner Fig. 1. External appearances of the two fractured pedal arm samples (a) front side, and (b) back side. 1 For interpretation of color in Figs. 6, 11 and 13, the reader is referred to the web version of this article. 104 Y. Gong, Z.-G. Yang / Composites: Part B 53 (2013) 103–111
Y. Gong, Z-G. Yang/Composites: Part B 53(2013)103-111 a 88929888803642086 Fudan 4000 3000 2500 1500 1000 wavenumbers (cm-1) 98(b) 9208882 n1311:17:142011 40003500300025002000 1500 Wavenumbers (cm-1) Fig. 2. FTIR results of the failed pedal arm(a)pedal, and(b)fram
Fig. 2. FTIR results of the failed pedal arm (a) pedal, and (b) frame. Y. Gong, Z.-G. Yang / Composites: Part B 53 (2013) 103–111 105
106 Y Gong, Z-G. Yang/Composites: Part B 53(2013)103-11 46007°C4849%L055 48043°C4988%Loss Fig 3. TGA results of the failed pedal arm(a)pedal, and (b)frame. marked by the red rectangular was the stress concentration area, and cracks while shown in Fig. 10d, some of them were twisted. Thus must be the initiation site of fracture. Thus, special attention was in order to identify what such foreign fibers exactly were, subse- then paid to it in Fig. 6b. Two black areas with no glass fibers quently their chemical compositions were analyzed by Eds (marked by the red arrows)were observed around it, which verified According to Fig. 10e-g and table 2, compared with the normal the uneven distribution of the glass fibers again. glass fibers that possessed high contents of inorganic elements, Meanwhile, other distinct flaws besides the uneven distribution the foreign fibers were predominantly composed of carbon and of glass fibers were also detected on areas away from the corner, oxygen elements on the contrary, indicating they were organic fi- including voids(Fig. 7a) and curly foreign fibers(Fig. 7b)within bers instead! As for their concrete constituents, it's not difficult the matrix, which would definitely affect the comprehensive per- to infer they were polypropylene too, but were probably the recy- ormances of the pedal arms too led ones mingled in processing. Actually, this was exactly the effi- On basis of the microscopic observation above, further investi- cient way to decrease cost, and would not have negative effects ations especially into those flaws would then be conducted under chemically. However, just like in Fig. 10a and c, if some extraordi- SEM. As shown in Fig 8, on the convex(right )of the stress concen- narily long fibers existed, they would possibly block the flow of tration corner there were nearly no glass fibers, while on the con- melted GF/PP pellets, and would finally result in voids around, ave (left) although there existed some fibers, their alignments influencing the comprehensive strength of the pedal arms. ere parallel rather than perpendicular to the fracture surface. All these phenomena clearly demonstrated the strength of the 3.6.2. Unused sample stress concentration corner, i.e. the potential fracture initiation For purpose of comparison, the fracture surfaces of the unused site, seemed not to be sufficiently qualified. pedal arm were then observed too. Totally speaking, the glass fi- On other areas, large-scale glass fibers parallel to the fracture bers distributed more evenly than the previous failed sample, seen surface also existed, seen in Fig. 9a; and meanwhile, the flaws of in Fig. 1la. But in some area( Fig. 11b), agglomerated glass fibers voids in size of multi-hundred microns were also observed, seen (marked by the red circle)and foreign fibers (marked by the red ar- Fig 9b. row)could also be detected If only focus on the most significant It must be particularly pointed out that abnormal foreign fibers area, i. e the stress concentration corner, lots of pulled-out glass fi that were in shape of extraordinarily long curls were detected bers perpendicular to the fracture surfaces could be observed obvi well by SEM, seen in Fig. 10a and c As shown in Fig. 10b, compared ously, seen in Fig. 12a. After magnified ( Fig. 12b), the localized with erect and smooth glass fibers, these curly fibers exhibited concave (left) was still covered with glass fibers, and the area with- rough surface morphologies, and were even embedded with micro- out glass fibers on the convex (right) was only 0.5 0.5 mm
marked by the red rectangular was the stress concentration area, and must be the initiation site of fracture. Thus, special attention was then paid to it in Fig. 6b. Two black areas with no glass fibers (marked by the red arrows) were observed around it, which verified the uneven distribution of the glass fibers again. Meanwhile, other distinct flaws besides the uneven distribution of glass fibers were also detected on areas away from the corner, including voids (Fig. 7a) and curly foreign fibers (Fig. 7b) within the matrix, which would definitely affect the comprehensive performances of the pedal arms too. On basis of the microscopic observation above, further investigations especially into those flaws would then be conducted under SEM. As shown in Fig. 8, on the convex (right) of the stress concentration corner there were nearly no glass fibers, while on the concave (left) although there existed some fibers, their alignments were parallel rather than perpendicular to the fracture surface. All these phenomena clearly demonstrated the strength of the stress concentration corner, i.e. the potential fracture initiation site, seemed not to be sufficiently qualified. On other areas, large-scale glass fibers parallel to the fracture surface also existed, seen in Fig. 9a; and meanwhile, the flaws of voids in size of multi-hundred microns were also observed, seen in Fig. 9b. It must be particularly pointed out that abnormal foreign fibers that were in shape of extraordinarily long curls were detected as well by SEM, seen in Fig. 10a and c. As shown in Fig. 10b, compared with erect and smooth glass fibers, these curly fibers exhibited rough surface morphologies, and were even embedded with microcracks; while shown in Fig. 10d, some of them were twisted. Thus in order to identify what such foreign fibers exactly were, subsequently their chemical compositions were analyzed by EDS. According to Fig. 10e–g and Table 2, compared with the normal glass fibers that possessed high contents of inorganic elements, the foreign fibers were predominantly composed of carbon and oxygen elements on the contrary, indicating they were organic fi- bers instead! As for their concrete constituents, it’s not difficult to infer they were polypropylene too, but were probably the recycled ones mingled in processing. Actually, this was exactly the effi- cient way to decrease cost, and would not have negative effects chemically. However, just like in Fig. 10a and c, if some extraordinarily long fibers existed, they would possibly block the flow of melted GF/PP pellets, and would finally result in voids around, influencing the comprehensive strength of the pedal arms. 3.6.2. Unused sample For purpose of comparison, the fracture surfaces of the unused pedal arm were then observed too. Totally speaking, the glass fi- bers distributed more evenly than the previous failed sample, seen in Fig. 11a. But in some area (Fig. 11b), agglomerated glass fibers (marked by the red circle) and foreign fibers (marked by the red arrow) could also be detected. If only focus on the most significant area, i.e. the stress concentration corner, lots of pulled-out glass fi- bers perpendicular to the fracture surfaces could be observed obviously, seen in Fig. 12a. After magnified (Fig. 12b), the localized concave (left) was still covered with glass fibers, and the area without glass fibers on the convex (right) was only 0.5 0.5 mm, Fig. 3. TGA results of the failed pedal arm (a) pedal, and (b) frame. 106 Y. Gong, Z.-G. Yang / Composites: Part B 53 (2013) 103–111
Y. Gong, Z-G. Yang/Composites: Part B 53(2013)103-111 Fig. 4. Macro- and microscopic morphologies of the remnant glass fibers after high-temperature sintering (a) under optical microscope(b) and (c)under SEM. too short bare GF Fig. 5. External appearances of the gF/PP pellets (a)normal ones, and (b) defective ones. le 1 sical and mechanical properties of the GF/PP pellets Density(kg/m) Tensile strength(MPa) Tensile modulus(MPa) Elongation( %) Flexural strength(MPa) Flexural modulus(MPa) 91.3 8379 1.5 From supplier 1340 8400 215 11300 greatly smaller than that on the failed sample(3x 3 mm, Fig 8).As 4. Failure analysis for other areas, pulled-out glass fibers were also densely distrib- uted on the fracture surfaces(Fig. 13a and b). All these phenomena In summary it is not hard to conclude through the analysis re- above evidently indicated a relatively qualified fracture strength of sults above that totally three defects were detected on the the unused pedal arm. But as shown in Fig. 13c and d, long and failed pedal arm. (1)uneven distribution of the glass fibers, i.e. in rly organic fibers (marked by the red arrows) existed within some areas the glass fibers were agglomerated, and in some area the matrix too no glass fibers existed in the polypropylene matrix; (2)inappropI
greatly smaller than that on the failed sample (3 3 mm, Fig. 8). As for other areas, pulled-out glass fibers were also densely distributed on the fracture surfaces (Fig. 13a and b). All these phenomena above evidently indicated a relatively qualified fracture strength of the unused pedal arm. But as shown in Fig. 13c and d, long and curly organic fibers (marked by the red arrows) existed within the matrix too. 4. Failure analysis In summary, it is not hard to conclude through the analysis results above that totally three main defects were detected on the failed pedal arm, (1) uneven distribution of the glass fibers, i.e. in some areas the glass fibers were agglomerated, and in some areas no glass fibers existed in the polypropylene matrix; (2) inappropriFig. 4. Macro- and microscopic morphologies of the remnant glass fibers after high-temperature sintering (a) under optical microscope (b), and (c) under SEM. Fig. 5. External appearances of the GF/PP pellets (a) normal ones, and (b) defective ones. Table 1 Physical and mechanical properties of the GF/PP pellets. Density (kg/m3 ) Tensile strength (MPa) Tensile modulus (MPa) Elongation (%) Flexural strength (MPa) Flexural modulus (MPa) Test results 1265 91.3 8379 1.5 152 9269 From supplier 1340 85 8400 2.3 215 11,300 Y. Gong, Z.-G. Yang / Composites: Part B 53 (2013) 103–111 107
108 Y Gong, Z-G. Yang/Composites: Part B 53(2013)103-11 no GF Fig. 6. Microscopic morphologies of the fracture surfaces(with counterparts)of the failed sample(a)total, and(b stress concentration area. foreign fiber Fig. 7. Microscopic morphologies of other flaws on the fracture surfaces of the failed sample(a)voids, and (b) curly foreign fiber Fig 8. SEM micrographs of the stress concentration corner(with counterpart)of the failed sample. glass fibers, i.e. they were parallel er such conditions, while for the last one, it was possibly the than perpendicul the fracture surface [16, 17: and (3) resulting in the voids. Apparently, all these defects would af auction of some comprehensive strength of the pedal arm under loads. In polypropylene As for the first and the second ones the fact, those defects were also found on the unused pedal arm, but fibers would not sufficiently exhibit their reinforcement function with a relatively lighter extent. That is to say, such defects could
ate alignment of most glass fibers, i.e. they were parallel rather than perpendicular to the fracture surface [16,17]; and (3) introduction of some foreign organic fibers –– probably the recycled polypropylene fibers. As for the first and the second ones, the glass fibers would not sufficiently exhibit their reinforcement function under such conditions, while for the last one, it was possibly the factor resulting in the voids. Apparently, all these defects would affect the comprehensive strength of the pedal arm under loads. In fact, those defects were also found on the unused pedal arm, but with a relatively lighter extent. That is to say, such defects could Fig. 6. Microscopic morphologies of the fracture surfaces (with counterparts) of the failed sample (a) total, and (b) stress concentration area. Fig. 7. Microscopic morphologies of other flaws on the fracture surfaces of the failed sample (a) voids, and (b) curly foreign fiber. Fig. 8. SEM micrographs of the stress concentration corner (with counterpart) of the failed sample. 108 Y. Gong, Z.-G. Yang / Composites: Part B 53 (2013) 103–111
Y. Gong, Z-G. Yang/Composites: Part B 53(2013)103-111 Fig 9. SEM micrographs of other areas away from the stress concentration comer (a)glass fibers parallel to the fracture surface, and (b) voids. L Energ (f) -,m2h03m04056m507104m0m01010mntm4 0. SEM micrographs and EDS results of the bers on the fracture surface of the failed sample (a)one foreign fiber. (b)comparison between glass fiber ar (c)another foreign fiber. (d) twisted morphology, (e)EDS of site A, (n) EDS of site B, and (g)EDS of site C
Fig. 9. SEM micrographs of other areas away from the stress concentration corner (a) glass fibers parallel to the fracture surface, and (b) voids. Fig. 10. SEM micrographs and EDS results of the foreign fibers on the fracture surface of the failed sample (a) one foreign fiber, (b) comparison between glass fiber and foreign fiber, (c) another foreign fiber, (d) twisted morphology, (e) EDS of site A, (f) EDS of site B, and (g) EDS of site C. Y. Gong, Z.-G. Yang / Composites: Part B 53 (2013) 103–111 109
Y Gong, Z-G. Yang/Composites: Part B 53(2013)103-1 Table 2 EDS results of the glass fiber and the foreign fibers(wt%). 31.08 15.59 6.92 73.61 1.6 146 Fig. 11. Microscopic morphologies of the fracture surfaces of the unused sample (a evenly distributed glass fibers, and (b)agglomerated glass fibers and foreign fiber. Fig. 12. SEM micrographs of the stress concentration corner(with counterpart) of the unused sample (a)total, and(b)magnification. be regarded universal, even normal for the pedal arm products of seemed to be coincidences, i.e. the defects happened to exist on the this manufacturer. However, why did several fracture failure inci- weakest stress concentration corners. In other words, if those inev- lents including the one in this paper occurred after all? According itable defects did not occur on these areas, the pedal arms could to the fracture surfaces, it is easy to find out the critical point was still exhibit qualified strength, like most of the other products that the joint between the pedal and the e, the destined frac- But anyway the defects did exist, and moreover, even if just one ure position under loads, was coincidentally encountered with pedal arm fractured during actual service, severe traffic accidents Ich defects on its stress concentration corner. Consequently, would be engendered. Consequently, solutions to relieve ever merely under loads far lower than the qualified value 1100 N, eliminate them still need to be proposed. Since the processing of cracks could be initiated from the sites without glass fibers on this the pedal arms was injection molding, three countermeasures corner, and then quickly propagated. ardingly suggested. (1)improving the pellets quality Till oot causes se fracture failures had been identi- the introduction of defective pellets (too short, not fied. Nevertheless, from the probability point of view, such causes plump, bare glass fibers, etc )and organic(probably the recycled
be regarded universal, even normal for the pedal arm products of this manufacturer. However, why did several fracture failure incidents including the one in this paper occurred after all? According to the fracture surfaces, it is easy to find out the critical point was that the joint between the pedal and the frame, the destined fracture position under loads, was coincidentally encountered with such defects on its stress concentration corner. Consequently, merely under loads far lower than the qualified value 1100 N, cracks could be initiated from the sites without glass fibers on this corner, and then quickly propagated. Till now, root causes of those fracture failures had been identi- fied. Nevertheless, from the probability point of view, such causes seemed to be coincidences, i.e. the defects happened to exist on the weakest stress concentration corners. In other words, if those inevitable defects did not occur on these areas, the pedal arms could still exhibit qualified strength, like most of the other products. But anyway, the defects did exist, and moreover, even if just one pedal arm fractured during actual service, severe traffic accidents would be engendered. Consequently, solutions to relieve even eliminate them still need to be proposed. Since the processing of the pedal arms was injection molding, three countermeasures were accordingly suggested, (1) improving the pellets quality, i.e. ensuring the introduction of defective pellets (too short, not plump, bare glass fibers, etc.) and organic (probably the recycled Table 2 EDS results of the glass fiber and the foreign fibers (wt%). Element C O Na Mg Al Si Ca Cl K Site A 31.08 15.59 0.45 1.97 7.65 23.19 20.06 / 2.068 Site B 61.33 16.63 6.75 0.35 0.50 1.11 2.05 6.92 4.36 Site C 73.61 17.71 1.63 0.35 0.64 1.73 1.79 1.46 1.08 Fig. 11. Microscopic morphologies of the fracture surfaces of the unused sample (a) evenly distributed glass fibers, and (b) agglomerated glass fibers and foreign fiber. Fig. 12. SEM micrographs of the stress concentration corner (with counterpart) of the unused sample (a) total, and (b) magnification. 110 Y. Gong, Z.-G. Yang / Composites: Part B 53 (2013) 103–111
Y. Gong, Z-G. Yang/Composites: Part B 53(2013)103-111 Fig. 13. SEM micrographs of other areas away from the stress concentration corner (a, b)pulled-out glass fibers perpendicular to the fracture surfaces, and (c, d) long and polypropylene) fibers as less as possible: (2)adequately adjusting References the process parameters to improve the distribution and alignment of the glass fibers, such as decreasing the injection pressure to pro- [1] Brady P. Brady motive composites: which way are we going? Reinf long the retention time inside the mold for the melt, and/ or mp A, Adam F, Krahl M, et al the mold perature and or the plastici uring concept for crash resistant textile a ture; (3)adequately decrease the localized size of the mold near the joint between the pedal and the frame, for increasing the thick [3 Hufenbach W. Bohm R, Thieme M, winkler A, et al. Polypropylene/glass fibre ness, i.e. enhancing the strength of the joint. The feedback from the 3D-textile reinforced composites for automotive applications. Mater Des pedal arm manufacturer after adopting these suggestions proved 14] Yang ZG, Gong Y, Yuan Jz Failure analysis of leakage on titanium tubes within all of them were effective wer plant. Part 1: Electrochemical corrosion. [5] Gong Y, Yang ZG, Yuan Jz Failure analysis of leakage on titanium tubes within 5 Conclusions corrosion on carbon heat exchanger of ethylene plant. Mater Corros 1:62(10):967-78. basically qualified. However, the defects like uneven distribu- 7 Gong at na diz ed resio ea aiten De s ne tion of the glass ate alis ment of analysis of bursting on the inner pipe of a fibers, and introduction of foreign organic fibers still existe tubula with a not light extent. [9] Gong Y, Cao g XH, Yang ZG. Pitting corrosion on 316L pipes in 2. As for the actual failed pedal arm, those defects coincidentally [101 Rjeb A, Tajounte L Chafik l Idrissi M. et ectroscopy study of occurred together on the stress concentration corner of the joint polypropylene natural aging. J Appl Polym Sci 2000: 77: 1742-8. between the pedal and the frame, so it fractured rather quickly [11 Abu-lsa L Thermal degradation of thin hims of when under the loads far lower than the designed value polypropylene with ketonic additives. J Polym Sci Pol Chem 1970: 8: 961-72 ared determination of the crystallinity of polypropylene. J 3. Based on the analysis results, countermeasures encompassing Polym Sci Pol Chem 1959: 38(134): 545-7. improvement of GF/PP pellets, adjustment of process parame- 13 Luongo JP ters, and tiny modification of the mold structure, etc was pro- andbook of fire protection engineering. Quincy (MA, USA): National Fire posed, and was proved to be effective by the pedal arm manufacturer afterwards the decomposition mechanism of [16] Lu QS, Sun LH, Yang ZG, Li XH, et al. Optimization on the thermal and tensile 1171 Su Acknowledgement contents on tensile properties of PTFE and attapulgite reinforced fabric composites Compos Part A: Appl Sci Manuf 2009: 40: 1785-91 The work was supported by Shanghai Leading Academic disci- pline Project(Project Number: B113)
polypropylene) fibers as less as possible; (2) adequately adjusting the process parameters to improve the distribution and alignment of the glass fibers, such as decreasing the injection pressure to prolong the retention time inside the mold for the melt, and/or increasing the mold temperature and/or the plasticizing temperature; (3) adequately decrease the localized size of the mold near the joint between the pedal and the frame, for increasing the thickness, i.e. enhancing the strength of the joint. The feedback from the pedal arm manufacturer after adopting these suggestions proved all of them were effective. 5. Conclusions 1. Properties of the pedal arms, even of the GF/PP pellets were basically qualified. However, the defects like uneven distribution of the glass fibers, inappropriate alignment of most glass fibers, and introduction of foreign organic fibers still existed with a not light extent. 2. As for the actual failed pedal arm, those defects coincidentally occurred together on the stress concentration corner of the joint between the pedal and the frame, so it fractured rather quickly when under the loads far lower than the designed value. 3. Based on the analysis results, countermeasures encompassing improvement of GF/PP pellets, adjustment of process parameters, and tiny modification of the mold structure, etc. was proposed, and was proved to be effective by the pedal arm manufacturer afterwards. Acknowledgement The work was supported by Shanghai Leading Academic Discipline Project (Project Number: B113). References [1] Brady P, Brady M. Automotive composites: which way are we going? Reinf Plast 2007;51(10):32–5. [2] Hufenbach W, Langkamp A, Adam F, Krahl M, et al. An integral design and manufacturing concept for crash resistant textile and long-fibre reinforced polypropylene structural components. Procedia Eng 2011;10:2086–91. [3] Hufenbach W, Böhm R, Thieme M, Winkler A, et al. Polypropylene/glass fibre 3D-textile reinforced composites for automotive applications. Mater Des 2011;32:1468–76. [4] Yang ZG, Gong Y, Yuan JZ. Failure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part I: Electrochemical corrosion. Mater Corros 2012;63(1):7–17. [5] Gong Y, Yang ZG, Yuan JZ. Failure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part II: Mechanical degradation. Mater Corros 2012;63(1):18–28. [6] Gong Y, Yang C, Yao C, Yang ZG. Acidic/caustic alternating corrosion on carbon steel pipes in heat exchanger of ethylene plant. Mater Corros 2011;62(10):967–78. [7] Gong Y, Yang ZG. Corrosion evaluation of one dry desulfurization equipment – circulating fluidized bed boiler. Mater Des 2011;32(1):671–81. [8] Gong Y, Zhong J, Yang ZG. Failure analysis of bursting on the inner pipe of a jacketed pipe in a tubular heat exchanger. Mater Des 2010;31(9):4258–68. [9] Gong Y, Cao J, Meng XH, Yang ZG. Pitting corrosion on 316L pipes in terephthalic acid (TA) dryer. Mater Corros 2009;60(11):899–908. [10] Rjeb A, Tajounte L, Chafik El Idrissi M, et al. IR spectroscopy study of polypropylene natural aging. J Appl Polym Sci 2000;77:1742–8. [11] Abu-Isa I. Thermal degradation of thin films of isotactic polypropylene and polypropylene with ketonic additives. J Polym Sci Pol Chem 1970;8:961–72. [12] Heinen W. Infrared determination of the crystallinity of polypropylene. J Polym Sci Pol Chem 1959;38(134):545–7. [13] Luongo JP. Infrared study of polypropylene. J Appl Polym Sci 1960;3(9):302–9. [14] Beyler CL, Hirschler MM. Thermal decomposition of polymers. In: SFPE handbook of fire protection engineering. Quincy (MA, USA): National Fire Protection Association, Inc.; 2002. p. 125. [15] Ishiwatari M. Effect of temperature on the decomposition mechanism of polypropylene. J Polym Sci Pol lett 1984;22(2):83–8. [16] Lu QS, Sun LH, Yang ZG, Li XH, et al. Optimization on the thermal and tensile influencing factors of polyurethane-based polyester fabric composites. Compos Part A: Appl Sci Manuf 2010;41:997–1005. [17] Sun LH, Yang ZG, Li XH. Effects of the treatment of attapulgite and filler contents on tensile properties of PTFE and attapulgite reinforced fabric composites. Compos Part A: Appl Sci Manuf 2009;40:1785–91. Fig. 13. SEM micrographs of other areas away from the stress concentration corner (a, b) pulled-out glass fibers perpendicular to the fracture surfaces, and (c, d) long and curly organic fibers. Y. Gong, Z.-G. Yang / Composites: Part B 53 (2013) 103–111 111
