Intermittent Audio Failure Analysis of a Remote Speaker-Microphone for a Two-Way Radio
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This paper presents a case study of an intermittent audio failure analysis of a remote speaker-microphone module for a two-way radio. A root cause analysis was undertaken to identify probable causes of the intermittent failure, followed by a series of experiments to determine the strength and the intermittent audio failure load of cable components and the fully assembled cable. The combined experimental and finite element results demonstrated that the main contributor of the intermittent audio failure was the micro surface cracks on the copper conductor strands. In addition, the combination of the component materials and design of the cable have also contributed to the non-uniform state of residual stress induced in the copper conductors which have reduced the ability of the copper conductors to withstand the normal handling load under the influence of micro surface cracks.
KeywordsElectronic cable intermittent contact Experimental analysis Finite element analysis
The reliability of land mobile electronic audio device, such as a two-way radio, is extremely important in public health and security operations, particularly for law enforcers, search and rescue personnel, and armed forces operations . The two-way radio reliability and efficient operation is equally important to consumer and industrial business operations where ground radio communications are usually conducted using portable two-way radios. As the communications operations involve critical situations relating to health and safety of personnel and the success of business operations, it is paramount that the two-way radio can meet a basic function of transmitting and receiving audio signals within a zero tolerance to failure requirement. To meet the strict requirement of the application of the two-way radio, the environmental reliability design conditions are usually based on requirements set by . Although a two-way radio may meet the stipulated design requirements, intermittent or breakdown of audio communications can still occur due to degradation of materials, such as worn connection joints, broken wires, and corroded connections . Loading anomalies subjected to the two-way radio can also contribute to intermittent failure, as discussed in [4, 5].
This paper is centered on the intermittent failure of an RSM connected to a two-way radio. The RSM module is a type of multi-core cable designed for audio applications. The cable is usually reinforced with Kevlar for heavy-duty application and is also a standard in military-grade multi-core cables . Usually, these cables are put through rigorous testing before they are deemed fit for use .
An intermittent audio failure of an RSM module manufactured by a third-party supplier for a commercially available two-way radio has been tested in this failure analysis. The failure report as received by  indicated that a number of remote speaker-microphone units from the same supplied batch had developed intermittent audio during usual application. The report indicated that intermittent audio failures were detected by the users and there were no circumstances of mishandling the RSM cable by the users.
From the RSM module design specification , the reliability requirements stated that the RSM cable should be able to withstand a cyclic bending load of 5 N at 20 cycles/min without impairing the audio function of the RSM module. However, a more important criterion would be the maximum static deformation load permissible for the RSM cable, which was not stated. The requirement for maximum static load was important because the intermittent failure report had indicated that failure occurred due to simple bending of the RSM cable. To understand the static load that can be applied by a user, a reference human grip data applied on typical grounded environments can be referred from . It indicated that the typical pull strength ranges from 249 to 165 N and 165 to 111 N for males and females, respectively. This information has been used as a reference in this analysis to assess the limit of the load that can be carried by the RSM module.
From this prior investigation, the intermittent audio failure analysis has been focused on ascertaining the root cause of the broken copper wires. The severity of the failure toward the safe use of the two-way radio has become a critical issue to the customer and the manufacturer as intermittent audio can endanger the life of the user or the public in cases of emergency when distress calls cannot be transmitted and received appropriately and timely. Although there was clear evidence that the conductor had separated and caused intermittent audio failure, the cause of this breakage was still elusive.
Physical Examination of the RSM Cable
Comparison of RSM cable components after cutting
Dimensions with relaxation (mm)
Dimensions without relaxation (mm)
Percentage difference (%)
The hardened epoxy surrounding the specimen ensures that the components of the RSM cable were not allowed to relax after dissection. The outcome of the analysis indicated that the RSM components were found to be in a compressive fit. There were no design specifications from  to indicate the level of compressive fit of the cable components, but it is speculated that the compressive fit was necessary for the manufacture of the RSM cable. From the four components that made up the RSM cable, the cable jacket showed the largest change in the cross section dimension before and after dissection, indicating the largest source of residual stress. It is speculated that the residual stress can be ascribed to the thermal contraction of the cable jacket.
From the physical examination analysis, it can be postulated that the copper strand may have separated due to residual stress, minor surface crack, or combination of both defects that led to the intermittent audio failure. Detailed experimental analysis and finite element analysis are discussed in the following sections.
Typical loads that the RSM cable can be subjected to during normal use involve tensile, bending, and twisting loads as well as a combination of the loading modes. Based on the failure report , the intermittent audio failure analysis of the RSM cable was limited to direct tensile loading and tensile loading with a bending configuration.
The testing programs include testing the individual components of the RSM cable as well as the entire RSM cable assembly in order to evaluate the effect of the residual stress and micro surface cracks on the copper conductors. The experimental analysis was based on ASTM E8 standards  with specific reference to sections 5 and 7. For the experimental analysis, all specimens were selected from a non-defective RSM cable batch. The strain rate was set to 1 mm/min and the data logging was set to a frequency of 5 Hz.
To exert a maximum bending load to the RSM cable and the components, a critical bending radius was required. In general, the critical bending radius was a function of the diameter of the cable. In the current failure analysis, the bending radius of 10 mm has been identified experimentally to cause a maximum stress on the cable, which was also the smallest attainable bending radius obtained through bending.
During testing, the test board indicated the sequence in which the copper conductors within the cable have undergone separation. As the copper conductors were separated, the circuit becomes open and the LED connected to the wire that has separated will turn off. For all the tests conducted, the intermittent contact was reproducible and was indicated by the blinking of the LEDs. All the tension and bending test results are shown together with the finite element analysis in the subsequent section.
Finite Element Analysis
For the tensile loading, the boundary condition was applied as displacement, U, based on the experimental result onto the free end of the model, while the opposite end was constrained in the z-axis direction leaving the x-axis and the y-axis to move freely. The bending load boundary condition was imposed through a rigid disk modeled with a critical bending radius of 10 mm following the experimental set-up.
The effect of the micro surface crack observed on the copper strand can be modeled with a straight-through edge crack as shown in Fig. 10b. Several micro surface crack depths were examined according to a ratio of a/W = 0.25 and 0.5 to represent shallow and deep micro surface crack problems.
Results and Discussion
The maximum stress experienced by the copper conductor is found to develop nearer to the central Kevlar-Nylon composite filler, while the copper conductor nearer to the cable jacket shows a much lower stress level. For circumferential compressions of 5, 10, and 30%, the peak-to-peak stress difference for the single copper conductor closer to the Kevlar-Nylon core and the cable jacket is 10, 30, and 50%, respectively. This indicates that the central Kevlar-Nylon composite filler is relatively more rigid than the cable jacket and causes the copper conductor nearer to it to experience much higher stress compared to the copper conductor nearer to the cable jacket. This can make the copper conductor nearer to the central Kevlar-Nylon filler more likely to develop the copper conductor critical intermittent failure stress.
The single-core wire damage evolution started with the breakage of the Kevlar strands within the single-core wire, followed by the breakage of the copper conductor which was responsible for the intermittent audio signal. At the same time, the cable jacket, nylon strands, and the insulation material absorbed large amounts of energy through a relatively large elongation as shown in Fig. 11. However, when the load was released back to a load below the intermittent failure load, it was observed that the broken copper conductor made contact again because the cable returned to its original state.
Figure 13a and b shows the finite element analysis for the tension and bending loads, respectively, compared to the experimental data. The intermittent failure load is indicated by the inset circle. The load in Fig. 13a and b was generalized by the appropriate intermittent failure load, P int, but the tension intermittent load was about a third higher than the bending intermittent failure load.
The RSM cable under tension load in Fig. 13a showed that the intermittent failure load occurs at a longer extension in finite element analysis compared to the experimental data, unlike in Fig. 13b where the finite element result matched well with the experimental data. However, the intermittent loads from the finite element analysis were well matched to the experimental results for both the tension and bending loads. It was observed from the tension experimental analysis that the multi-core wires and the cable jacket experience an initial body translation between each other until a critical extension which caused an offset in the load-displacement curve and hence the reduced intermittent failure extension load. Although the extension in tensile analysis was different, the intermittent failure load agreed well with the experimental result. In the bending analysis, the body translation of multi-core wires to the cable jacket was not observed because the load applied was focused on a local area on the RSM wire cable unlike the tension analysis.
The maximum human hand-grip strength for male and female (indicated by the horizontal line) was normalized with the experimental intermittent failure load in each analysis, respectively. Figure 13 shows that the intermittent failure load was higher than the maximum human hand-grip strength, and therefore the effect of local compression on the multi-core cable was unable to simulate a state of residual stress that could cause intermittent failure load due to normal handling load.
Unlike the effect of circumferential compression from the cable jacket, the presence of micro surface crack results in a more severe reduction of intermittent failure load. The estimated intermittent failure load for a crack of geometry a/W = 0.5 for the tension was nearer to the maximum load applicable by an adult but much lower in the case of bending load that can be exerted by an adult using their hands (approximately 300 N) as reported in .
The Kevlar-Nylon filler material must not induce a high stress on the copper conductors. A less rigid configuration should be adopted to offer a balanced stress level for the copper conductors.
The shrink-fit of the cable jacket onto all the single-core wires is a standard requirement in the design of the RSM cable. However, the shrink-fit will cause an inherent residual stress in the RSM cable. Therefore, the combination of micro cracks and residual stress may cause a premature degradation of the strength of the copper conductors.
The intermittent failure load due to micro cracks was found to be more severe in bending load when compared to the tension loading. Therefore, the bending loading configuration must be used as a guide for the maximum limit of load applicable to the RSM cable.
The typical design specification of an RSM cable must not only include the fatigue cycle specification but also include the quasi-static deformation load in tension and bending and combination of loadings to ensure zero tolerance to intermittent failure of the RSM cable.
The authors acknowledge the funding through a grant P17C1-12 from CREST Malaysia and the funding from Motorola Solutions Malaysia in the later stages of the project to Mr. Leong Karh Heng and Mr. Rizman Hariz Abdul Latiff. Thanks are also due to Mr. Kamaruddin Khalid and Mr. Alex Yeo Siang Chew who conducted the initial investigation of the RSM cable failure as part of their undergraduate project and Motorola Solutions Pte. Ltd. Malaysia for the supply of the RSM cable materials used in the testing. Finally, the ABAQUS finite element code was made available under an academic license from Dassault Systemes K.K., Japan.
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