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Failure analysis of sport shoe buttons
Engineering Failure Analysis 9 (2002) 185–190
Failure analysis of sport shoe buttons C.K. Chen * Metal Forming Technology Section, Metal Industries Research and Development Centre, 1001 Kao-Nan Highway, Kaohsiung 811, Taiwan Received 20 November 2000; accepted 26 December 2000
Abstract Buttons of sport shoes cracked after shipping to the purchasing agent. The buttons were fabricated by stamping from 0.35 mm brass sheet and were melamine–acrylic enamel coated for decoration. Intergranular facets of the broken buttons reveal brittle fracture. Season cracking susceptibility testing is employed to assess residual stress in the buttons. However, melamine supplementary acrylic enamel may disperse amine compounds that can result in season cracking of brassware. The hard tempered condition of the buttons and the amine environment account for this failure. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Sports equipment failures; Season cracking; Intergranular fracture; Residual stress; Environmentally assisted cracking
1. Introduction A local sport shoe manufacturer fashioned a new design pattern on its sport shoes. Additionally, circular shoe buttons, as shown in Fig. 1, were swapped to oval ones in this modern arrangement. Aluminum alloy was previously used for manufacturing circular buttons: however, copper alloy was substituted in the new design owing to its excellent ductility. Almost 20,000 pairs of shoes were fabricated and shipped to the buyer abroad. After the receipt of goods, some of the buttons on the shoes were found to be cracked as depicted in Fig. 2. This shipment is subject to be rejected. Thus, engineering failure analysis is carried out to examine the possible failure causes. 2. Background The shoe manufacturer ordered these buttons from a particular sheet stamping works. As depicted in Fig. 3, the buttons are made of brass (30Zn–70Cu) by cold stamping with brass sheet of thickness 0.35 mm. However, to provide decoration, the buttons were enamel coated with a specified coloration. These buttons were inserted into the shoe coverings by a specifically designed press machine, and then shoe coverings and bottoms were sewed in and glued together. Nevertheless, the buttons were exclusively observed to be cracked one month after shipment to the purchaser. * Tel.: +886-7-3513121; fax: +886-7-3537530. E-mail address: [email protected] (C.K. Chen). 1350-6307/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(01)00003-6
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C.K. Chen / Engineering Failure Analysis 9 (2002) 185–190
Fig. 1. The practical sport shoe with circular buttons.
Fig. 2. Broken oval buttons on shoes.
Fig. 3. The bare shoe buttons.
2.1. Fabrication of brass buttons Brass buttons were first stamped to form the desired shape and a solvent cleaning process was applied to remove stamping lubrication fat. In addition, acrylic enamel was sprayed evenly onto the buttons, and then baked in the oven within the temperature of 180–200 C for 40 min, dependent on the hardness requirement of the coating layer. The acrylic enamel is composed of 70% acrylic resin, 20% melamine resin
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(H2NCNC(NH2)NC(NH2)N) and 10% epoxy resin in which melamine resin is the hardening agent and the epoxy is for toughening the coating layer. The pigment in the enamel is usually metal oxide such as TiO2 for white paint. 2.2. Damage description As depicted in Fig. 4, cracks in the button initiate at the outer segment and run through to the stem portion. Cracks seem to be straight and through the thickness: nevertheless, crack openings are quite broad. In general, it is observed that one to four cracks are exhibited on one button: however, cracks always initiate from the thinnest section of the outer segment. The crack is then forcibly opened to reveal the fracture surface. In addition, SEM fractography shows predominantly intergranular facets, as illustrated in Fig. 5. Therefore, positively brittle fracture is determined.
3. Investigation Broken buttons are investigated and another three categories of buttons free from cracks are examined as well, namely bare buttons (1), coated buttons (2) and in situ buttons (3) which are inserted in the shoe covering. In this study, all of the test buttons are fabricated from the same heat number of brass sheet.
Fig. 4. Enlarged broken button.
Fig. 5. Intergranular fracture facet of broken button.
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C.K. Chen / Engineering Failure Analysis 9 (2002) 185–190
3.1. Material characterization The enamel coating of broken buttons is dissolved in toluene solvent. Wet chemistry and metallurgical examination are carried out to delineate material characteristics. Chemical contents of shoe button are listed in Table 1; this meets the requirement of ASTM C26000 [1]. Microstructures of shoe buttons are revealed by Cr3O etching solution and a micrograph is shown in Fig. 6, where a-phase and deformed grains are observed. The outer segments and stem portions of bare buttons are examined using a Micro-Vickers hardness test. However, the average hardness of these two regions shows no difference, as tabulated in Table 2. Homogeneous hardness denotes that this stamping process induces uniform strain-hardening in the buttons. Nevertheless, the hardness of brass exceeds HRB91 [2], indicating a hard tempered condition where residual stress may prevail in the button interiors. 3.2. Mercurous nitrate test As described in ASTM B154-95 [3], this method is an accelerated test for detecting the presence of residual stresses in a brass alloy. Residual stress can result in stress corrosion cracking (SCC) failure in parts of Table 1 Chemical analysis of buttons Category
ASTM C26000 Shoe button
Composition (%) Cu
Zn
Al
Pb
Fe
P
68.5–71.5 70.17
Re 28.50
– 0.27
0.07max <0.03
0.05max 0.13
– 0.031
Fig. 6. a-Phase of button microstructure.
Table 2 Hardness test of buttons Location
Micro-Vickers hardness HV (100 g/30 s)
Converted to HRB
Outer segment Stem portion
195 194
92.3 92.0
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copper alloys in storage or in service. Bare, coated and in situ buttons are immersed in the mercurous nitrate solution for 30 min. Low power stereo microscopy is used to find out whether induced cracks occurred on the tested buttons. Results show that cracks are observed to initiate from the outer segment of buttons for coated and in situ buttons, but no cracks were found in the bare buttons. Fractography of the cracked buttons exhibited similar brittle intercrystalline fracture facets to those of Fig. 5. Therefore, it is anticipated that the stamping process is not responsible for the failure but the enamel acrylic coating may play an important role. It is noted that season cracking of brassware may occur under high tensile internal/ residual stress. Eventually, the environmental degenerating factors such as oxygen, ammonia, mercury, sulfur dioxide and amine may enhance this failure mechanism [2]. 3.3. Acrylic enamel coating degradation Melamine–acrylic coating has been investigated in reference [4]. In ambient humidity, this coating may be degraded in a photo-oxidation environment which results in a loss of coating material by breaking the melamine–acrylic crosslinks and acrylic copolymer chain backbone without reformation of melamine– melamine crosslinks. In the mean time, formation of amine, hydroxyl and carbonyl compounds are generated. However, amine compounds can jeopardize brassware that may give rise to failure by brass season cracking.
4. Discussion 1. The chemical components and microstructure features of buttons show good characteristics when compared to the specification of brass material. However, a hard tempered state in the buttons indicates that residual stress can exist which is an essential requisite for brass season cracking. 2. Hardness uniformity of shoe buttons reveals proper die design in the stamping process where no extra strain hardening prevails in the buttons. 3. Markedly intergranular brittle facets show the susceptibility of brass to season cracking. Liquid embrittlement cracking is observed in a mercurous nitrate test for the coated buttons, but not for the bare ones. Acrylic enamel coating could be a key reason for this failure. 4. Photo-oxidization may degrade the melamine–acrylic enamel coating and the amine compounds may be involved. Amine-containing surroundings, however, are sure to intensify the susceptibility of brass to season cracking.
5. Conclusions 30Zn–70Cu brass is selected for sport shoe button fabrication. The stamping process may induce a high hardness level in the buttons and produce buttons in a hard tempered condition. Nevertheless, intergranular facets of broken buttons strongly suggests the susceptibility of brass to season cracking. Mercurous nitrate tests reveal that the enamel coating buttons are more susceptible to this stress corrosion cracking failure mechanism. In addition, photo-oxidation degradation of melamine–acrylic coating may result in the formation of amine compounds that may augment the possibility of brass season cracking. Nevertheless, the photo-oxidation condition is always underway in ordinary handling. A hard tempered condition and amine environment are probably the cause of this failure. Brass buttons may be stress relief annealed within the temperature of 250–270 C to diminish the hard tempered condition that is essential to season cracking. Higher Cu bearing brass, pure copper or bronze alternatives are also suggested in the manufacture of shoe buttons to prevent season cracking susceptibility.
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In addition, melamine–acrylic enamel is not a proper coating for brassware since it may aggravate the susceptibility to brass season cracking; silicon modified enamel or epoxy resin enamel may be used instead. The action plan is dependent on the economic considerations.
Acknowledgements Thanks are due to J.C. Chen and C.T. Hsu of the Certification Service Department in the Metal Research and Development Centre for conducting the essential tests that made this analysis possible.
References [1] [2] [3] [4]
Standard specification for brass plate, sheet, strip, and rolled bar. ASTM B36/B36M-91a. Brasses. 11th revised impression, C.D.A. Publication No. 6, 1955. Standard test method for mercurous nitrate test for copper and copper alloy. ASTM B154-95. Nguyen T-LH, Rogers CE. Characterization of the photo-degradation of melamine–acrylic crosslinked network (coatings) using reflection–absorption infrared spectroscopy. Polym Prep 1986;27(2):228–9.
Failure analysis of sport shoe buttons C.K. Chen * Metal Forming Technology Section, Metal Industries Research and Development Centre, 1001 Kao-Nan Highway, Kaohsiung 811, Taiwan Received 20 November 2000; accepted 26 December 2000
Abstract Buttons of sport shoes cracked after shipping to the purchasing agent. The buttons were fabricated by stamping from 0.35 mm brass sheet and were melamine–acrylic enamel coated for decoration. Intergranular facets of the broken buttons reveal brittle fracture. Season cracking susceptibility testing is employed to assess residual stress in the buttons. However, melamine supplementary acrylic enamel may disperse amine compounds that can result in season cracking of brassware. The hard tempered condition of the buttons and the amine environment account for this failure. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Sports equipment failures; Season cracking; Intergranular fracture; Residual stress; Environmentally assisted cracking
1. Introduction A local sport shoe manufacturer fashioned a new design pattern on its sport shoes. Additionally, circular shoe buttons, as shown in Fig. 1, were swapped to oval ones in this modern arrangement. Aluminum alloy was previously used for manufacturing circular buttons: however, copper alloy was substituted in the new design owing to its excellent ductility. Almost 20,000 pairs of shoes were fabricated and shipped to the buyer abroad. After the receipt of goods, some of the buttons on the shoes were found to be cracked as depicted in Fig. 2. This shipment is subject to be rejected. Thus, engineering failure analysis is carried out to examine the possible failure causes. 2. Background The shoe manufacturer ordered these buttons from a particular sheet stamping works. As depicted in Fig. 3, the buttons are made of brass (30Zn–70Cu) by cold stamping with brass sheet of thickness 0.35 mm. However, to provide decoration, the buttons were enamel coated with a specified coloration. These buttons were inserted into the shoe coverings by a specifically designed press machine, and then shoe coverings and bottoms were sewed in and glued together. Nevertheless, the buttons were exclusively observed to be cracked one month after shipment to the purchaser. * Tel.: +886-7-3513121; fax: +886-7-3537530. E-mail address: [email protected] (C.K. Chen). 1350-6307/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(01)00003-6
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C.K. Chen / Engineering Failure Analysis 9 (2002) 185–190
Fig. 1. The practical sport shoe with circular buttons.
Fig. 2. Broken oval buttons on shoes.
Fig. 3. The bare shoe buttons.
2.1. Fabrication of brass buttons Brass buttons were first stamped to form the desired shape and a solvent cleaning process was applied to remove stamping lubrication fat. In addition, acrylic enamel was sprayed evenly onto the buttons, and then baked in the oven within the temperature of 180–200 C for 40 min, dependent on the hardness requirement of the coating layer. The acrylic enamel is composed of 70% acrylic resin, 20% melamine resin
C.K. Chen / Engineering Failure Analysis 9 (2002) 185–190
187
(H2NCNC(NH2)NC(NH2)N) and 10% epoxy resin in which melamine resin is the hardening agent and the epoxy is for toughening the coating layer. The pigment in the enamel is usually metal oxide such as TiO2 for white paint. 2.2. Damage description As depicted in Fig. 4, cracks in the button initiate at the outer segment and run through to the stem portion. Cracks seem to be straight and through the thickness: nevertheless, crack openings are quite broad. In general, it is observed that one to four cracks are exhibited on one button: however, cracks always initiate from the thinnest section of the outer segment. The crack is then forcibly opened to reveal the fracture surface. In addition, SEM fractography shows predominantly intergranular facets, as illustrated in Fig. 5. Therefore, positively brittle fracture is determined.
3. Investigation Broken buttons are investigated and another three categories of buttons free from cracks are examined as well, namely bare buttons (1), coated buttons (2) and in situ buttons (3) which are inserted in the shoe covering. In this study, all of the test buttons are fabricated from the same heat number of brass sheet.
Fig. 4. Enlarged broken button.
Fig. 5. Intergranular fracture facet of broken button.
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C.K. Chen / Engineering Failure Analysis 9 (2002) 185–190
3.1. Material characterization The enamel coating of broken buttons is dissolved in toluene solvent. Wet chemistry and metallurgical examination are carried out to delineate material characteristics. Chemical contents of shoe button are listed in Table 1; this meets the requirement of ASTM C26000 [1]. Microstructures of shoe buttons are revealed by Cr3O etching solution and a micrograph is shown in Fig. 6, where a-phase and deformed grains are observed. The outer segments and stem portions of bare buttons are examined using a Micro-Vickers hardness test. However, the average hardness of these two regions shows no difference, as tabulated in Table 2. Homogeneous hardness denotes that this stamping process induces uniform strain-hardening in the buttons. Nevertheless, the hardness of brass exceeds HRB91 [2], indicating a hard tempered condition where residual stress may prevail in the button interiors. 3.2. Mercurous nitrate test As described in ASTM B154-95 [3], this method is an accelerated test for detecting the presence of residual stresses in a brass alloy. Residual stress can result in stress corrosion cracking (SCC) failure in parts of Table 1 Chemical analysis of buttons Category
ASTM C26000 Shoe button
Composition (%) Cu
Zn
Al
Pb
Fe
P
68.5–71.5 70.17
Re 28.50
– 0.27
0.07max <0.03
0.05max 0.13
– 0.031
Fig. 6. a-Phase of button microstructure.
Table 2 Hardness test of buttons Location
Micro-Vickers hardness HV (100 g/30 s)
Converted to HRB
Outer segment Stem portion
195 194
92.3 92.0
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copper alloys in storage or in service. Bare, coated and in situ buttons are immersed in the mercurous nitrate solution for 30 min. Low power stereo microscopy is used to find out whether induced cracks occurred on the tested buttons. Results show that cracks are observed to initiate from the outer segment of buttons for coated and in situ buttons, but no cracks were found in the bare buttons. Fractography of the cracked buttons exhibited similar brittle intercrystalline fracture facets to those of Fig. 5. Therefore, it is anticipated that the stamping process is not responsible for the failure but the enamel acrylic coating may play an important role. It is noted that season cracking of brassware may occur under high tensile internal/ residual stress. Eventually, the environmental degenerating factors such as oxygen, ammonia, mercury, sulfur dioxide and amine may enhance this failure mechanism [2]. 3.3. Acrylic enamel coating degradation Melamine–acrylic coating has been investigated in reference [4]. In ambient humidity, this coating may be degraded in a photo-oxidation environment which results in a loss of coating material by breaking the melamine–acrylic crosslinks and acrylic copolymer chain backbone without reformation of melamine– melamine crosslinks. In the mean time, formation of amine, hydroxyl and carbonyl compounds are generated. However, amine compounds can jeopardize brassware that may give rise to failure by brass season cracking.
4. Discussion 1. The chemical components and microstructure features of buttons show good characteristics when compared to the specification of brass material. However, a hard tempered state in the buttons indicates that residual stress can exist which is an essential requisite for brass season cracking. 2. Hardness uniformity of shoe buttons reveals proper die design in the stamping process where no extra strain hardening prevails in the buttons. 3. Markedly intergranular brittle facets show the susceptibility of brass to season cracking. Liquid embrittlement cracking is observed in a mercurous nitrate test for the coated buttons, but not for the bare ones. Acrylic enamel coating could be a key reason for this failure. 4. Photo-oxidization may degrade the melamine–acrylic enamel coating and the amine compounds may be involved. Amine-containing surroundings, however, are sure to intensify the susceptibility of brass to season cracking.
5. Conclusions 30Zn–70Cu brass is selected for sport shoe button fabrication. The stamping process may induce a high hardness level in the buttons and produce buttons in a hard tempered condition. Nevertheless, intergranular facets of broken buttons strongly suggests the susceptibility of brass to season cracking. Mercurous nitrate tests reveal that the enamel coating buttons are more susceptible to this stress corrosion cracking failure mechanism. In addition, photo-oxidation degradation of melamine–acrylic coating may result in the formation of amine compounds that may augment the possibility of brass season cracking. Nevertheless, the photo-oxidation condition is always underway in ordinary handling. A hard tempered condition and amine environment are probably the cause of this failure. Brass buttons may be stress relief annealed within the temperature of 250–270 C to diminish the hard tempered condition that is essential to season cracking. Higher Cu bearing brass, pure copper or bronze alternatives are also suggested in the manufacture of shoe buttons to prevent season cracking susceptibility.
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C.K. Chen / Engineering Failure Analysis 9 (2002) 185–190
In addition, melamine–acrylic enamel is not a proper coating for brassware since it may aggravate the susceptibility to brass season cracking; silicon modified enamel or epoxy resin enamel may be used instead. The action plan is dependent on the economic considerations.
Acknowledgements Thanks are due to J.C. Chen and C.T. Hsu of the Certification Service Department in the Metal Research and Development Centre for conducting the essential tests that made this analysis possible.
References [1] [2] [3] [4]
Standard specification for brass plate, sheet, strip, and rolled bar. ASTM B36/B36M-91a. Brasses. 11th revised impression, C.D.A. Publication No. 6, 1955. Standard test method for mercurous nitrate test for copper and copper alloy. ASTM B154-95. Nguyen T-LH, Rogers CE. Characterization of the photo-degradation of melamine–acrylic crosslinked network (coatings) using reflection–absorption infrared spectroscopy. Polym Prep 1986;27(2):228–9.