Method for predicting fatigue life of thermoplastic resin

Abstract

To predict a fatigue life of a thermoplastic resin capable of improving prediction accuracy in a range where a stress ratio R is 0 to 1.SOLUTION: A fatigue life prediction method comprising a step of creating a fatigue life prediction diagram of a thermoplastic resin having a stress ratio R of 0 to 1 based on the following formula (1). Where σ a is a stress amplitude, σ m is a mean stress, σ B is a value based on a rupture strength calculated from a result of a tensile test with a stress ratio R of 1, an index x is a value of more than 0 and less than 1.0, and σ w is an intercept obtained by substituting, into the above formula (1), a stress amplitude and a mean stress calculated from a result of a pulsating tensile fatigue test with a stress ratio R of 0 as σ a and σ m, respectively, a value based on the rupture strength as σ B, and a value of more than 0 and less than 1.0 as x. ) SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for predicting the fatigue life of a thermoplastic resin. [Background technology]

[0002] Various equipment and structures are often subjected to repeated loads that change periodically (over time) in addition to a constant static load. Even if the stress fluctuations caused by these repeated loads are smaller than the breaking stress caused by the static load, if this repeated stress continues to act on a component, the component may suffer fatigue failure. For this reason, it is necessary to predict the fatigue life of the component or the material that makes up the component in order to determine the service life of the component.

[0003] Generally, for most steel materials, the 6 ~10 7 Since there exists a lower limit of stress amplitude (fatigue limit) beyond which fracture will not occur even if the number of repetitions is increased, fatigue life predictions for steel materials with different stress ratios are performed using a fatigue limit diagram in which the vertical axis represents stress amplitude and the horizontal axis represents mean stress. Currently, fatigue life predictions for steel materials are performed using, for example, σ based on the following formula (A): T -σ W Fatigue limit diagrams such as the Goodman diagram based on the following formula (B), the modified Goodman diagram based on the following formula (C), the Gerber diagram based on the following formula (D), and the Soderberg diagram based on the following formula (D) have been proposed.

[0004]

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[0005] On the other hand, resins and non-ferrous metals (aluminum, copper alloys, etc.) do not have a fatigue limit like steel materials, so the fatigue limit diagrams for steel materials as described above cannot be applied directly. Currently, there are known examples of carbon fiber reinforced plastics (CFRP) where data on tensile strength, compressive strength, and x (tensile strength / compressive strength) are acquired and fatigue limit diagrams for steel materials are applied to CFRP (see Non-Patent Document 1). Also, a fatigue limit diagram based on the following formula (E), which uses the fatigue strength and creep strength when the stress ratio R is -1, has been proposed for polybutylene terephthalate containing glass fibers (see Non-Patent Document 2).

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[0006] [Non-Patent Document 1] M. Funaki et al., "Fatigue Life of Short Fiber Reinforced Nylon Composites and Its Stress Ratio Dependence," Proceedings of the Ibaraki Conference, Japan Society of Mechanical Engineers, 2016, Vol. 24 [Non-patent document 2] Hideki OKA et al., “Mean-Stress Effect on Fatigue Strength of Short Glass Fiber Reinforced Polybutyleneterephthalate”, Journal of the Society of Material Science, Japan, Vol.55, No.10, pp.951-957, Oct.2006 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in predicting the fatigue life of thermoplastic resins, the methods disclosed in Non-Patent Documents 1 and 2 do not have sufficient prediction accuracy, and the reality is that they cannot be used as fatigue life prediction methods.

[0008] The present invention has been made to solve the above problems, and aims to provide a method for predicting the fatigue life of a thermoplastic resin that can improve prediction accuracy when the stress ratio R is in the range of 0 to 1. [Means for solving the problem]

[0009] [1] A method for predicting the fatigue life of a thermoplastic resin, comprising the steps of: conducting a pulsating tensile fatigue test on a test piece of the thermoplastic resin at a predetermined temperature, with a stress ratio R of 0, and repeatedly applying stress at a predetermined frequency, multiple times with different maximum stresses; obtaining multiple relationships between the number of repetitions to failure and the maximum stress when the test piece breaks; plotting the relationships between the multiple numbers of repetitions to failure and the maximum stress to obtain an SN diagram; determining a first-power approximation formula based on the plotted points on the SN diagram; calculating the maximum stress at which the test piece breaks at a predetermined number of repetitions from the first-power approximation formula; and calculating the stress amplitude and mean stress at which the test piece breaks at the predetermined number of repetitions based on the calculated maximum stress. performing a tensile creep test on a test piece of the thermoplastic resin a plurality of times at the predetermined temperature with different stresses at which the stress ratio R is 1, to obtain a plurality of relationships between the time to failure when the test piece breaks and the creep rupture strength; plotting the relationships between the plurality of times to failure and the creep rupture strength to obtain a creep rupture diagram; determining a second-power approximation formula based on the plotted points in the creep rupture diagram; calculating the creep rupture strength at which the test piece breaks in a time corresponding to the predetermined number of repetitions at the frequency from the second-power approximation formula; and creating a fatigue life prediction diagram for the thermoplastic resin with the stress ratio R of 0 to 1 based on the following formula (1):

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[0010] [2] The method according to [1] above, wherein the index x in the formula (1) is 0.5.

[0011] [3] The method according to [1] or [2] above, wherein the thermoplastic resin is a polyphenylene sulfide resin, a polybutylene terephthalate resin, or a polyacetal resin.

[0012] [4] The method according to any one of [1] to [3] above, wherein the temperature is above room temperature or above 23°C.

[0013] [5] The method according to any one of [1] to [3] above, wherein the temperature is equal to or higher than the glass transition temperature of the thermoplastic resin. [Effects of the Invention]

[0014] According to one aspect of the present invention, a method for predicting the fatigue life of a thermoplastic resin that can improve prediction accuracy when the stress ratio R is in the range of 0 to 1 can be provided. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is an SN diagram showing the measured values ​​and power approximation curves when the stress ratio R is 0, 0.2, 0.4, 0.6, and 0.8. [Figure 2] FIG. 2 is a fatigue life prediction diagram of the glass fiber-containing polybutylene terephthalate resin according to Example 1. [Figure 3] FIG. 3 is a fatigue life prediction diagram of the glass fiber-containing polybutylene terephthalate resin according to Example 2. [Figure 4] FIG. 4 is a fatigue life prediction diagram of the glass fiber-containing polybutylene terephthalate resin according to Comparative Example 1. [Figure 5] FIG. 5 is a fatigue life prediction diagram of the glass fiber-containing polybutylene terephthalate resin according to Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, a method for predicting the fatigue life of a thermoplastic resin according to an embodiment of the present invention will be described.

[0017] <Method for predicting fatigue life of thermoplastic resin> First, a test piece of thermoplastic resin to be measured is prepared. Thermoplastic resins include not only a single type of thermoplastic resin but also thermoplastic resin mixtures, which are blends of multiple thermoplastic resins. Furthermore, thermoplastic resins also include those to which desired properties have been imparted by adding additives such as inorganic fillers (e.g., glass fiber, carbon fiber), nucleating agents, pigments (e.g., carbon black, inorganic calcined pigments), antioxidants, stabilizers, plasticizers, lubricants, mold release agents, and flame retardants.

[0018] Examples of thermoplastic resins include polyolefin resins, polyester resins (such as polybutylene terephthalate resins), polyacetal resins, polyphenylene sulfide resins, and polyamide resins.

[0019] The shape of the thermoplastic resin test piece is not particularly limited as long as it can be held by the gripping tools of a hydraulic servo-type fatigue testing machine or a tensile creep testing machine, which will be described later. Examples include a dumbbell shape such as Type 1A as specified in ISO 294-1, or a rectangular shape. Furthermore, when considering the orientation of the inorganic filler, for example, a test piece cut from a flat test piece may also be used. The thickness of the test piece may be, for example, 0.5 mm or more and 8 mm or less.

[0020] After preparing a thermoplastic resin test piece, a pulsating tensile fatigue test is performed at a predetermined temperature, where the stress ratio R is 0 and repeated stress is applied to the thermoplastic resin test piece at a predetermined frequency. max The test is carried out multiple times (preferably three or more times) and the number of repetitions until the test piece breaks and the maximum stress σ max In this specification, the "stress ratio R" refers to the maximum stress σ in one cycle of repeated loading in a pulsating tensile fatigue test. max Minimum stress σ for min The ratio (σ min / σ max In the pulsating tension fatigue test, a sinusoidal load is applied to a thermoplastic resin test piece. The pulsating tension fatigue test is performed using a hydraulic servo-type fatigue testing machine (for example, "EHF-EV010k2-0101A" manufactured by Shimadzu Corporation).

[0021] The reason why a pulsating tensile fatigue test was performed on a thermoplastic resin test piece is that if a reciprocating fatigue test is performed on a thermoplastic resin test piece, the test piece buckles when compressed, making it impossible to obtain accurate data, and because thermoplastic resin test pieces do not break when compressed. For this reason, Non-Patent Documents 1 and 2 cannot be applied.

[0022] The temperature at which the pulsating tensile fatigue test is performed is not particularly limited, but may be, for example, room temperature or higher or 23°C or higher. Furthermore, since the strength of a thermoplastic resin differs between temperatures below the glass transition temperature and temperatures above the glass transition temperature, when predicting the fatigue life at temperatures below the glass transition temperature, the pulsating tensile fatigue test is performed at room temperature or higher or at a temperature 23°C or higher but lower than the glass transition temperature of the thermoplastic resin. When predicting the fatigue life at temperatures above the glass transition temperature of the thermoplastic resin, the pulsating tensile fatigue test is performed at a temperature above the glass transition temperature. When performing the pulsating tensile fatigue test at a temperature above the glass transition temperature, it is preferable to perform the test at a temperature below the melting point of the thermoplastic resin in consideration of the actual usage temperature of the thermoplastic resin.

[0023] The frequency for conducting the pulsating tensile fatigue test is preferably 1 Hz or more and 30 Hz or less. If the frequency is 1 Hz or more, it does not take a long time even if the number of repetitions is large, and if the frequency is 30 Hz or less, heat generation in the thermoplastic resin during the pulsating tensile fatigue test can be suppressed.

[0024] Maximum stress σ max can be adjusted by controlling the load in the pulsating tension fatigue test.

[0025] Multiple fracture cycles and maximum stress σ max After obtaining the relationship between the number of cycles and the maximum stress σ max The relationship between these two is plotted to obtain an SN diagram. Then, a first-power approximation formula is found based on the plotted points on this SN diagram. The first-power approximation formula is expressed by the following formula (2). Y=aX b …Formula (2) In the above formula (2), X is the number of repetitions to failure (times), Y is the maximum stress (MPa), and a and b are constants.

[0026] Then, a predetermined number of repetitions is substituted into the first power approximation formula, and the maximum stress σ at which the test piece breaks after the predetermined number of repetitions is calculated. maxThe predetermined number of repetitions is not particularly limited, but for example, 1,000 times (10 3 times), 10,000 times (10 4 times), 100,000 times (10 5 times), 1,000,000 times (10 6 times), 10,000,000 times (10 7 times).

[0027] Then, the maximum stress σ at which the test piece breaks after a predetermined number of cycles is calculated. max The stress amplitude σ at which the test piece breaks at a predetermined number of repetitions based on a and mean stress σ m The maximum stress in one cycle of repeated loading in a pulsating tension fatigue test is calculated as σ max and the minimum stress is σ min When the stress amplitude σ a is expressed by the following formula (3), and the mean stress σ m is expressed by the following formula (4). When the stress ratio R is 0, σ min is 0. σ a =(σ max -σ min ) / 2 …Equation (3) σ m =(σ max +σ min ) / 2 …Equation (4)

[0028] As mentioned above, when the stress ratio R is 0, σ min is 0, so from the above equations (3) and (4), the stress amplitude σ a and mean stress σ m are the same value, and both are σ max / 2.

[0029] Furthermore, a tensile creep test with a stress ratio R of 1 is performed multiple times (preferably three or more times) on a thermoplastic resin test piece at the above-mentioned predetermined temperature under different stresses, and multiple (preferably three or more) relationships between the time to failure when the test piece reaches failure and the creep failure strength are obtained. The tensile creep test is performed by applying a constant load to the thermoplastic resin test piece. For example, the load is initially set to 90% of the tensile strength, and the load is gradually reduced while checking the time to failure. The tensile creep test is performed using a tensile creep tester (e.g., a triple-barrel 10 kN creep tester manufactured by Yonekura Seisakusho Co., Ltd., a 6-barrel 10 kN creep tester manufactured by Aiko Engineering Co., Ltd., or the B-122505900 manufactured by Toyo Seiki Co., Ltd.).

[0030] After obtaining the relationship between multiple rupture times and creep rupture strength, the relationships between multiple rupture times and creep rupture strength are plotted to obtain a creep rupture diagram. Then, a second-power approximation formula is calculated based on the plotted points on this creep rupture diagram. The second-power approximation formula is expressed by the following formula (5). Y=cX d …Equation (5) In the above formula (5), X is the rupture time (hr), Y is the creep rupture strength (MPa), and c and d are constants.

[0031] Then, the creep rupture strength at which the test piece reaches rupture in a time corresponding to a predetermined number of repetitions at the above frequency in a pulsating tensile fatigue test is calculated using a second power approximation formula.

[0032] In this embodiment, the fatigue life of a thermoplastic resin is predicted using the stress amplitude and mean stress calculated based on the results of a pulsating tensile fatigue test and the creep rupture strength calculated based on the results of a tensile creep test. Therefore, the conditions of the tensile creep test must be matched to those of the pulsating tensile fatigue test. Therefore, the tensile creep test is performed at the same temperature as the pulsating tensile fatigue test. However, since the tensile creep test is a test in which a constant load is applied, there is no concept of frequency or number of repetitions. Therefore, based on the following equation (6), the number of repetitions at the frequency in the pulsating tensile fatigue test is replaced with the time in the tensile creep test, and the thermoplastic resin is broken in the tensile creep test at a time equivalent to the number of repetitions at the frequency in the pulsating tensile fatigue test at which it breaks. Tensile creep test time (hr) = number of repetitions of pulsating tensile fatigue test (times) / (frequency (Hz) x 3600 (sec)) ... Equation (6)

[0033] From the above, the stress amplitude and mean stress at a predetermined number of repetitions are calculated based on the results of the pulsating tensile fatigue test, and the creep rupture strength at a time corresponding to the predetermined number of repetitions is also calculated based on the results of the tensile creep test. Then, a fatigue life prediction diagram for the thermoplastic resin with a stress ratio R of 0 to 1 is created based on the following formula (1).

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[0034] In the above formula (1), the index x is a value greater than 0 and less than 1.0. This is because if x is 1.0 or greater, the deviation from the calculated values ​​of stress amplitude and mean stress when the stress ratio R is greater than 0 and less than 1 (particularly, when the stress ratio R is 0.6 or greater and less than 1) becomes large. If x exceeds 1.0, the obtained fatigue life prediction diagram will bend in the opposite direction from the line connecting the calculated values ​​of stress amplitude and mean stress when the stress ratio R is greater than 0 and less than 1 (particularly, when the stress ratio R is 0.6 or greater and less than 1), resulting in a greater deviation from the calculated values ​​when the stress ratio R is greater than 0 and less than 1. The index x varies depending on the type of thermoplastic resin, but from the viewpoint of improving the accuracy of fatigue life prediction, it is preferably 0.3 to 0.7, and more preferably 0.5.

[0035] According to this embodiment, a pulsating tensile fatigue test is performed on a thermoplastic resin test piece at a predetermined temperature, in which repeated stress is applied at a stress ratio R of 0 and a predetermined frequency, and the test is performed multiple times with different maximum stresses. A plurality of relationships between the number of repetitions to failure and the maximum stress when the test piece is broken are obtained, and the relationships between the number of repetitions to failure and the maximum stress are plotted to obtain an SN diagram. A first-power approximation formula is found based on the plotted points on the SN diagram, and the maximum stress at which the test piece is broken at a predetermined number of repetitions is calculated from the first-power approximation formula. The stress amplitude and mean stress at which the test piece is broken at the predetermined number of repetitions are calculated based on the calculated maximum stress. Tensile creep tests are performed on specimens multiple times at different stresses at a temperature where the stress ratio R is 1, and multiple relationships between the time to failure and creep rupture strength at which the specimen fails are obtained. These relationships between the time to failure and creep rupture strength are plotted to obtain a creep rupture diagram. A second-power approximation formula is derived based on the plotted points on the creep rupture diagram, and the creep rupture strength at which the specimen fails in the time corresponding to the predetermined number of cycles at the frequency is calculated from the second-power approximation formula. A fatigue life prediction diagram is then created for thermoplastic resins with stress ratios R in the range of 0 to 1 based on the above formula (1). This improves the accuracy of fatigue life predictions for thermoplastic resins with stress ratios R in the range of 0 to 1. Here, since thermoplastic resins do not have a fatigue limit as described above, a fatigue limit diagram is created for each number of fatigue cycles. Furthermore, as mentioned above, when performing alternating-force fatigue testing on thermoplastic resin test specimens, the test specimens buckle when compressed, making it impossible to obtain accurate data. Furthermore, because thermoplastic resin test specimens do not break when compressed, the stress amplitude and mean stress are used, which are the factors that cause the test specimen to break at a certain number of repetitions when the stress ratio R in a pulsating-force fatigue test is 0. Furthermore, because metals are elastic, there is no concept of creep fracture, but thermoplastic resins exhibit viscoelasticity and therefore do experience creep fracture. For this reason, in the above formula (1), instead of tensile strength, a factor called creep fracture strength is used, which is not considered for metals. [Example]

[0036] In order to explain the present invention in detail, the following examples are given, but the present invention is not limited to these. Fig. 1 is an SN diagram showing actual measured values ​​and power approximation curves when the stress ratio R is 0, 0.2, 0.4, 0.6, and 0.8. Fig. 2 is a fatigue life prediction diagram for the glass fiber-containing polybutylene terephthalate resin of Example 1. Fig. 3 is a fatigue life prediction diagram for the glass fiber-containing polybutylene terephthalate resin of Example 2. Fig. 4 is a fatigue life prediction diagram for the glass fiber-containing polybutylene terephthalate resin of Comparative Example 1. Fig. 5 is a fatigue life prediction diagram for the glass fiber-containing polybutylene terephthalate resin of Comparative Example 2.

[0037] Example 1 First, a glass fiber-containing polybutylene terephthalate resin (Duranex (registered trademark) PBT 3300, manufactured by Polyplastics Co., Ltd., glass transition temperature: 40°C, hereinafter referred to as "glass fiber-containing PBT resin") was prepared as a thermoplastic resin. This glass fiber-containing PBT resin was molded into a Type 1A dumbbell shape specified in ISO294-1 with a thickness of 4 mm to obtain a test piece (molded article).

[0038] After preparing a test piece of glass fiber-containing PBT resin, a pulsating tensile fatigue test was performed under the following conditions. max The test was carried out six times, and the number of repetitions until the test piece broke and the maximum stress σ at that time were recorded as shown in Table 1. max The relationship was scored 6 points.

[0039] (pulsating tension fatigue test) Testing machine: Hydraulic servo type fatigue testing machine (Shimadzu Corporation "EHF-EV010k2-0101A") Stress ratio: R=0 Frequency: 10Hz Load: 0kN~20kN Stress type: tension, pulsating, sinusoidal ·Temperature: 80℃ [Table 1]

[0040] Next, the number of repetitions at six points and the maximum stress σ max The relationship between these values ​​was plotted to obtain the SN diagram shown in Figure 1. Then, a power approximation formula was calculated based on the six plotted points on this SN diagram, and the following formula (7) was obtained. Y=69.37X -0.05 …Formula (7) In the above formula (7), X is the predetermined number of repetitions (times), and Y is the maximum stress σ at which the test piece breaks after the predetermined number of repetitions. max (MPa).

[0041] Then, by substituting 10,000,000 repetitions for X in the above formula (7), we obtain the maximum stress σ at which the test piece breaks at 10,000,000 repetitions, which is Y. max When the maximum stress σ max was 31.0 MPa.

[0042] The maximum stress σ at which the test piece breaks after 10,000,000 cycles max After determining this, the maximum stress σ is calculated based on the above equations (3) and (4). max The stress amplitude σ at which the test piece breaks after 10,000,000 cycles a and mean stress σ m The stress amplitude σ a and mean stress σ m was 15.5 MPa.

[0043] Next, a tensile creep test was carried out 13 times under different stresses on a test piece of another glass fiber-containing PBT resin with the same composition as the test piece of the glass fiber-containing PBT resin used in the pulsating tensile fatigue test under the following conditions, and the fracture time when the test piece broke and the creep fracture strength σ B The relationship received a score of 13. (Tensile creep test) Testing machine: Tensile creep testing machine (Yonekura Manufacturing Co., Ltd. "Triple-type 10kN creep testing machine") Stress ratio: R=1 Load capacity: 10kN Stress type: Constant Duration: 278 hours ·Temperature: 80℃

[0044] Next, the fracture time and creep fracture strength σ B The relationship between these values ​​was plotted to obtain a creep strength diagram (not shown). Then, a power approximation formula was calculated based on the 13 plotted points on this creep strength diagram, and the following formula (8) was obtained. Y=76.272X -0.006 ...Formula (8) In the above formula (8), X is the rupture time, and Y is the creep rupture strength at the rupture time σ B is.

[0045] Then, by substituting 278 hours for X in the above formula (8), the creep rupture strength σ B The creep fracture strength σ B The stress was 69.24 MPa. Here, 278 hours corresponds to 10,000,000 repetitions at a frequency of 10 Hz according to the above formula (6).

[0046] The stress amplitude σ in the above formula (1) a and mean stress σ m Substituting 15.5 MPa into each, the creep rupture strength σ B Substitute 69.24 MPa for and 0.5 for x, and then σ w When we calculated σ w was 17.59 MPa.

[0047] The creep rupture strength σ B Substitute 69.24 MPa for , substitute 0.5 for x, and σ w The average stress σ in the following formula (8) is substituted with 17.59 MPa. m Substituting 5MPa, 10MPa, 20MPa, 30MPa, 40MPa, 50MPa, and 60MPa into the stress amplitude σ a The values ​​obtained are shown in Table 2. The results of Table 2 were plotted to obtain the graph in Figure 2.

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[0048] [Table 2]

[0049] Similarly, by substituting the number of repetitions of 1,000, 10,000, 100,000, and 1,000,000 into X in the above formula (7), the maximum stress σ at which the test piece breaks in calculations for each number of repetitions is calculated. max The average stress σ at which the test piece breaks in the calculation was calculated for each repetition. m Stress amplitude σ a Calculate the stress amplitude σ a and mean stress σ m The relationship between these values ​​is also shown in the graph of Fig. 2. The values ​​calculated in this manner are shown as calculated values ​​in Figs.

[0050] Furthermore, for verification, a pulsating tensile fatigue test was conducted at stress ratios R of 0.2, 0.4, 0.6, and 0.8 in the same manner as when the stress ratio R was 0. As shown in Table 3, the number of repetitions at which the test piece broke and the maximum stress σ max Then, similarly to the case where the stress ratio R is 0, the stress amplitude σ leading to fracture at each of the above repetition numbers was obtained. a and mean stress σ m When each of these was calculated, the results shown in Table 4 were obtained. These results are also shown in the graph in Figure 2. The power approximation formulas when the stress ratio R was 0.2, 0.4, 0.6, and 0.8 were as follows: When R=0.2: Y=78.832X -0.048 When R=0.4: Y=85.765X -0.045 When R=0.6: Y=95.494X -0.039 When R=0.8: Y=92.674X -0.022

[0051] [Table 3] [Table 4]

[0052] From the graph in Figure 2, the five fatigue life prediction diagrams with different numbers of cycles show that the stress amplitude σ a and mean stress σ m The line connecting the calculated values ​​of the two almost coincided with the line. This confirmed that the fatigue life of glass fiber-containing PBT resin can be predicted with high accuracy.

[0053] <Example 2> In Example 2, x in (8) was changed from 0.5 to 0.7, and σ w A fatigue life prediction diagram was obtained in the same manner as in Example 1, except that the following formula (9) was used in which the stress of the load was changed from 17.59 MPa to 18.51 MPa.

[0054] Specifically, first, the stress amplitude σ in the above formula (1) a and mean stress σ m Substituting 15.5 MPa into each, the creep rupture strength σ B Substitute 69.24 MPa for and 0.7 for x, and then σ w When we calculated σ w was 18.51 MPa.

[0055] Next, the creep rupture strength σ B Substitute 69.24 MPa for , substitute 0.7 for x, and σ w The average stress σ in the following equation (9) is calculated by substituting 18.51 MPa into m Substituting 5MPa, 10MPa, 20MPa, 30MPa, 40MPa, 50MPa, and 60MPa into the formula, the stress amplitude σ at which the test piece breaks after 10,000,000 cycles is calculated. aThe calculated values ​​are shown in Table 5. The results of Table 5 were plotted to obtain the graph in Figure 3.

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[0056] [Table 5]

[0057] Similarly, when x is 0.7, the mean stress σ at which the test piece breaks after 1,000, 10,000, 100,000, and 1,000,000 cycles is calculated. m Stress amplitude σ a Calculate the stress amplitude σ a and mean stress σ m The relationship is also shown in the graph in Figure 3.

[0058] For verification, the stress amplitude σ for each repetition number when the stress ratio R calculated above is 0.2, 0.4, 0.6, and 0.8 is a and mean stress σ m The graph in Figure 3 also shows the results.

[0059] From the graph in Figure 3, some of the five fatigue life prediction diagrams with different numbers of cycles show that the stress amplitude σ a and mean stress σ m The line connecting the calculated values ​​of the two almost coincided with the line. This confirmed that the fatigue life of glass fiber-containing PBT resin can be predicted with high accuracy.

[0060] <Comparative Example 1> In Comparative Example 1, x in (8) was changed from 0.5 to 1.0, and σ w A fatigue life prediction diagram was obtained in the same manner as in Example 1, except that the following formula (10) was used in which the stress of the load was changed from 17.59 MPa to 19.97 MPa.

[0061] Specifically, first, the stress amplitude σ in the above formula (1) a and mean stress σm Substituting 15.5 MPa into each, the creep rupture strength σ B Substitute 69.24 MPa for and 1.0 for x, and then σ w When we calculated σ w was 19.97 MPa.

[0062] Next, the creep rupture strength σ B Substitute 69.24 MPa for , substitute 1.0 for x, and σ w The average stress σ in the following formula (10) is calculated by substituting 19.97 MPa into m Substituting 5MPa, 10MPa, 20MPa, 30MPa, 40MPa, 50MPa, and 60MPa into the stress amplitude σ a The results were as shown in Table 6. The results of Table 6 were plotted to obtain the graph in Figure 4.

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[0063] [Table 6]

[0064] Similarly, when x is 1.0 and the number of repetitions is 1,000, 10,000, 100,000, and 1,000,000, the average stress σ m Stress amplitude σ a and calculate the stress amplitude σ a and mean stress σ m The relationship is also shown in the graph in Figure 4.

[0065] For verification, the stress amplitude σ for each repetition number when the stress ratio R calculated above is 0.2, 0.4, 0.6, and 0.8 is a and mean stress σ m The graph in Figure 4 also shows the results.

[0066] From the graph in Figure 4, it can be seen that each fatigue life prediction diagram deviates from the line connecting the calculated values ​​for stress ratios R of 0, 0.2, 0.4, 0.6, 0.8, and 1.0 for each number of repetitions. This confirms that when x is 1.0, it is not possible to predict the fatigue life of glass fiber-containing PBT resin.

[0067] <Comparative Example 2> In Comparative Example 2, x in (8) was changed from 0.5 to 2.0, and σ w A fatigue life prediction diagram was obtained in the same manner as in Example 1, except that the following formula (11) was used in which the stress of the load was changed from 17.59 MPa to 25.73 MPa.

[0068] Specifically, first, the stress amplitude σ in the above formula (1) a and mean stress σ m Substituting 15.5 MPa into each, the creep rupture strength σ B Substitute 69.24 MPa for and 2.0 for x, and then σ w When we calculated σ w was 25.73 MPa.

[0069] Next, the creep rupture strength σ B Substitute 69.24 MPa for , substitute 2.0 for x, and σ w The average stress σ in the following equation (11) is calculated by substituting 19.97 MPa into m Substituting 5MPa, 10MPa, 20MPa, 30MPa, 40MPa, 50MPa, and 60MPa into the stress amplitude σ a The values ​​obtained were those shown in Table 7. The results of Table 7 were plotted to obtain the graph shown in Figure 5.

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[0070] [Table 7]

[0071] Similarly, when x is 2.0 and the number of repetitions is 1,000, 10,000, 100,000, and 1,000,000, the average stress σ m Stress amplitude σ a and calculate the stress amplitude σ a and mean stress σ m The relationship is also shown in the graph in Figure 5.

[0072] For verification, the stress amplitude σ for each repetition number when the stress ratio R calculated above is 0.2, 0.4, 0.6, and 0.8 is a and mean stress σ m The graph in Figure 5 also shows the results.

[0073] From the graph in Figure 5, each fatigue life prediction diagram curved in the opposite direction to the line connecting the calculated values ​​for stress ratios R of 0, 0.2, 0.4, 0.6, 0.8, and 1.0 for each number of cycles. a and mean stress σ m The calculated values ​​of each of the above deviated from the fatigue life prediction diagrams. This confirmed that when x is 2.0, it is not possible to predict the fatigue life of the glass fiber-containing PBT resin.

Claims

1. 1. A method for predicting fatigue life of a thermoplastic resin, comprising: a step of performing a pulsating tensile fatigue test on a test piece of the thermoplastic resin at a predetermined temperature, with a stress ratio R of 0, and repeatedly applying stress at a predetermined frequency, multiple times with different maximum stresses, and obtaining multiple relationships between the number of repetitions to failure and the maximum stress when the test piece breaks; Plotting the relationship between the number of cycles to failure and the maximum stress to obtain an S-N diagram; determining a first power approximation equation based on plotted points in the S-N diagram; calculating a maximum stress at which the test piece breaks after a predetermined number of repetitions from the first power approximation formula; calculating a stress amplitude and an average stress at which the test piece breaks after the predetermined number of repetitions based on the calculated maximum stress; a step of performing a tensile creep test on a test piece of the thermoplastic resin at the predetermined temperature and with different stresses multiple times at the temperature where the stress ratio R is 1, and obtaining multiple relationships between the time to failure when the test piece reaches failure and the creep rupture strength; Plotting a plurality of the relationships between the rupture times and the creep rupture strengths to obtain a creep rupture diagram; determining a second power approximation formula based on plotted points in the creep rupture diagram; calculating, from the second power approximation formula, a creep rupture strength at which the test piece reaches rupture in a time corresponding to the predetermined number of cycles at the frequency; A step of creating a fatigue life prediction diagram of the thermoplastic resin when the stress ratio R is 0 to 1 based on the following formula (1); A method comprising: [Equation 1] (In the above formula (1), σ ａ is the stress amplitude, and σ ｍ is the mean stress, and σ Ｂ is a value based on the creep rupture strength obtained in the tensile creep test, the index x is a value exceeding 0 and less than 1.0, and σ ｗ is added to the above formula (1) as σ ａ and σ ｍ The stress amplitude and the mean stress calculated as above are respectively substituted, and σ Ｂ and a value greater than 0 and less than 1.0 is substituted for x.)

2. The method of claim 1 , wherein the exponent x in formula (1) is 0.

5.

3. The method of claim 1 , wherein the thermoplastic resin is a polyphenylene sulfide resin, a polybutylene terephthalate resin, or a polyacetal resin.

4. The method of claim 1 , wherein the temperature is above room temperature or above 23° C.

5. The method of claim 1 , wherein the temperature is at or above the glass transition temperature of the thermoplastic resin.

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