Introduction

With the increasing demand of high-quality and high-performance components, the process of testing and inspection becomes inevitable in quality control of manufactured parts [1]. Crack or leak occurrence, surface blemishes, and substantial failures are of significant interest in most of the manufacturing scenarios [2]. To this end, leakage testing is considered as one of the important processes in production lines to detect cracks or leaks within an engineering component. Additionally, the process of leak testing triggers most of the bottlenecks during the quality control of any manufactured components [3]. In complex shaped engineering components, like cylinder head, it is rarely possible to measure or detect the irregular and small dimensions of leak. A leak can be referred as flow of gas or liquid from the component due to imperfection such as a hole, crack, or bad seal. Typically, the leakage testing method aims at initially clamping the component on a fixture, pressurizing it with air, and finally submerging the component in water. The presence of leakage and internal cracks if available is visually visible in the form of bubbles [4]. The method of leakage testing is mandatory in a way to localize the leak, hole, or porosity while maintaining the integrity of a manufactured component and thus protects it from failure during its actual operation. In addition, for automobile applications, leakage testing of cylindrical block is essential to prevent poor engine performance as it is the first indication of crack in cylindrical block that can adversely affect the efficiency of vehicles.

In industries, the failure of leak testing process can cause possible failure of industrial component and finally scrapping of the components that may have performed adequately in field. Thus, failure analysis becomes indispensable and can be considered as an effective tool to reveal the causes of failure [5]. Nowadays, the process of leak testing is commonly performed using specific leak testing machine. The component is cautiously clamped on a leak testing machine and tightened using clamping holder on the fixture plate. The leak testing machine is a special purpose machine, especially used for carrying out leakage testing of component in industries. The primary purpose of leak testing machine is leakage detection, measurement of leak rate, and leakage locations. In most of the cases, it is not essential to measure the leak rate, though it may require whether the leak rate is above or below the par level. Since, component with leak rate above a specified level can be rejected directly and can be forbidden from going in the market.

The present study deals with performing effective and correct process of leak detection (bubble leak method) testing a cylinder block/head on leak testing machine. The prime objective of this work is to perform failure analysis of leak testing machine and suggesting an iterative methodology for remedial improvements in different parts for its enhanced performance. Initially, the problem is defined and the Kepner-Tregoe (K-T) method is used to identify possible causes of the problem on leak testing machine. The causes are validated and bifurcated into different parts, and remedial measures are suggested one at a time. Furthermore, innovative and effective design steps are presented for performing leak detection efficiently on leak testing machine. Finally, concluding remarks are presented based on the current failure analysis and about the improvements.

Failure Identification and Description

The present work deals with performing cylinder block leak testing inspection in the industries by clamping and fixing it on the leak testing machine and performing leak bubble test. It is important to perform such test prior to its functioning, as it prevents poor/weak engine performance due to the formation of crack in cylindrical block by prohibiting leakage of fuel during operation. The cylinder block is mounted on a fixture and after closing all ports, air is pressurized into at a required level. If there is leak exit, it will lead to bubble generation. The process flowchart of leak testing process for cylinder block is depicted in Fig. 1. However, during the process, it was found that the effectiveness of the leak testing machine has been reduced due to the enhanced number of rejections in final testing of components (cylinder block). The problem detected in the leak testing process may have been raised due to the imperfection and failure of the leak testing machine. Thus, to identify the area of failure and its root cause a Kepner-Tregoe (KT) method is employed. The KT method is beneficial in identifying and detecting various existing problems and prioritized their solution in an iterative way at manufacturing line and in leak testing machine.

Fig. 1
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Process flow chart for leak testing

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The Kepner-Tregoe (KT) Method

The KT method is a practical and structured methodology used for decision-making, identifying problems, and determining potential risks. It identifies the concerns by listing them one by one followed by bifurcating the level of concern and defined them in terms of the three dimensions listed below [6]. The KT method is one of the multiple thinking domain approaches being used by several manufacturing sectors and thus is an area focused on several researchers [7]. It is one of the clear thinking practices to secure a competitive advantage over others that further helps to flourish as a strategically aligned organization.

Urgency It is the gravity level of the problem and related to the urgency of the time needed to resolve the problem.

Seriousness It is the seriousness level of a problem and related to the impact of the problem to the organization.

Growth It is related to the growth of problems. The faster the problem develops, the higher the growth rate will be.

Based on these dimensions, individual can evaluate that one concern which is relatively more important than another and consequently should be addressed first. In K-T methodology, four rational processes are considered which reflect four fundamental questions that help in making the decision methodologically and effectively. The present work adopts the same by bifurcating four rational processes of the KT method one by one.

Situation Analysis (Outlines Concern)

There is imperfection in leak testing process during performing bubble leak testing of cylinder block.

Problem Analysis (Problem Identification and Root Cause Determination)

For problem identification and analysis, a quality circle team constituted various stakeholder’s departments, i.e. production department, quality assurance, and materials department. Quality circles (QCs) remain feasible and continue to operate effectively [8]. In quality circle team, various stakeholders from different departments are selected and employed seven quality tools to identify possible causes and remedial actions. Various QC tools were employed to identify root causes. An average 4-month rejection of 74 cylinder block components is considered. The multi-variate plot is developed using the data of the number of rejections based on different concerns, obtained from manufacturing division. Figure 2 shows the bar chart for the same representing different reasons with ease. Table 1 reports the detailed analysis of the QC team in detecting the problem by giving a problem ranking on scale from 1 to 10 as seriousness, urgency, and growth.

Fig. 2
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Bar chart providing important concerns for major rejection

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Table 1 K-T analysis
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Based on the knowledge and experience of experts and workers, four important regions are selected for checking and further improvement, the line balancing and clamping pad breakage. From Fig. 2 and Table 1, it is evident that significant rejection is primarily because of the clamping pad breakage and poor line balancing that needs tooling improvement. The former reason can be dealt with K-T analysis as the latter concern can be dealt on production line itself. Based on the problem analysis, it becomes evident that breaking of clamping pad on leak testing machine has higher points as compared to other area. Therefore, the problem of clamping pad should be considered first, and further analysis must be performed.

The QC team brainstormed and come up with root cause of clamping pad breakage in a leak testing machine and their probable solutions. During the engine cylinder block clamping process, two crucial components in fixture play important role in holding and stabilizing the component (see Fig. 3).

Fig. 3
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Component clamping fixture

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  1. 1.

    Clamping holder.

  2. 2.

    Clamping PU pad.

After close look at the clamping pad part, it was clearly seen that cracks were generating as depicted in Fig. 4. While using this fixture and clamping pad during manufacturing and testing of cylindrical block, generated cracks at clamping pad were leading to imperfection in cylindrical block which was the primary reason for leakage testing failure. From further analysis, it was also realized that three different types of clamping pads are being used during the leak testing process for the effective results, leading to cost and time wastage in changing as per the different model. The problems and the adverse effects concerning these issues are explained in Table 2.

Fig. 4
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Generated crack at clamping pad

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Table 2 Problem definition and their impact
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Based on the defined problem above, data were analysed over a period of time and it was observed that there were two reasons for clamping pad change. Either cracks were generated or there was need to change clamp pad because of wear. To compare both reasons, quantitatively data through check sheet were collected and analysed (see Table 3). From Table 3, it is evident that most (75%) of the clamping pads were changed due to the crack generation and less (only 25%) due to wear of the pads.

Table 3 Data analysis for pads replacement
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Decision Analysis (Objectives are Identified Against Potential Problems)

Based on the quantitative analysis in Table 3, three objectives were set and are tabulated in Table 4 based on the problem definition. The first objective was reduction in clamping pad replacement per year. Since, wearing is natural phenomenon that is not easy to forbid; therefore, we only focused on reducing clamping pad breakage to zero. During clamping process, three different types of clamping pad were used based on different models. Objective are set to design new clamping pads which can be used for different models, thus, reducing the cost and changeover time.

Table 4 Objectives formulation based on above analysis
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Failure Analysis To identify root cause for crack generation in clamping pad, QC tool like cause and effect diagram is used. Figure 5 shows the cause and effect diagram prepared by quality circle team that sits together and by rigorous brainstorming under the four factors (man, machine, method, and material), and all probable causes are listed that may lead to crack generation and thus result in rejection of parts. Table 5 provides the inspection report for improper fitment, while, Table 6 shows the comparison between existing and new design.

Fig. 5
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Cause and effect diagram for validation of probable cause

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Table 5 Checking report for improper fitment
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Table 6 Comparison of existing method with new design
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Validation of Probable Cause

In the cause and effect diagram, the QC team lists all possible reasons which may have led to crack generation. It is indispensable to test and validate these probable causes. We selected each of the listed probable cause and verified at the manufacturing line.

1. Man To check improper fitment, sample of five cylindrical blocks are selected from various shifts in which different workers were performing their job. It was found that there were no issues detected with any improper fitment. Therefore, no counter measure is needed for this listed probable cause.

2. Machine Two probable causes were selected for validation.

  1. (a)

    High clamping pressure by machine It was found that pressure at which machine working was within the specified range of 1.5–3 kg/cm2, so no counter measure is needed.

  2. (b)

    Clamping arrangement design In clamping arrangement design at fixture, there was no side support available to clamping pad due to which clamping pad rotates most of the times during clamping process. Thus, a counter measure is needed to rectify this crucial problem.

3. Method In method, it was checked whether the load applied by fixture on the clamping pad is even or not throughout the surface. Due to non-alignment of line of action, force by machine and axis of clamping pad alignment was not proper (see Fig. 6). Hence, counter measure was needed for this cause.

Fig. 6
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Eccentric load at fixture

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4. Material Material of clamping pad was soft with hardness of 60 BHN. To counter this probable cause, material change was needed.

Therefore, after thorough investigation and analysis, it was found that there are mainly three regions where a remedial counter measure is required to prohibit the mode of crack generation and thus failure of the process. Figure 7 clearly illustrates the process flow beginning with defining problem, their causes, essential checks, analysis, and results.

Fig. 7
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Validation of probable causes

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Action Identification and Implementation (Actions Proposed to Minimize Risk/Problem)

Countermeasure 1 In clamping arrangement design, due to unavailability of side support, there was not proper fitment. After brain storming, a new design concept was developed which may provide side support and proper fitment. A 3D concept developed to visualize the idea.

Countermeasure 2 In the second counter measure, developed 3D concept was lacking side support and there was a blind hole in PU pad. In the suggested countermeasure, side support was provided in the concept and through hole was generated at the place of blind hole in PU pad as depicted in Fig. 8.

Fig. 8
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Modified 3D concept

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Countermeasure 3 The soft PU material replaced with hard material having BHN 80 instead of 60.

Effectiveness of Actions Implemented

After implementation of new clamping pad with hard PU material, it reduced clamping pad replacement per year from 24 to 6 (see Fig. 9). At the place of three different types of clamp pad, our new design was serving purpose of all which not only beneficial in terms of monetary benefit but also helps in reducing operator fatigue which was a non-tangible benefit. Data collected over a period of four months from September 2019 to December 2019. There was drastic reduction in rejection after the implementation of identified actions. Rejection rate is reduced by 90% (from 74 to 8) as reported in Table 7. The clamping pad change percentage is reduced significantly by 95%, and pad change due to wear and tear also reduced considerably due to use of hard PU material. Figure 10 shows the comparison of the number of clamping pad replacement before and after implementing suggestive measures. It is worth mentioning that reduction in clamping pad rejection due to crack generation and further failure of leak testing machine has been narrowed down with the help of K-T analysis in this study. Furthermore, cost analysis of implemented solution shows that it is a viable solution.

Fig. 9
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Before and after change once necessary actions implemented

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Table 7 Testing of pad change after corrective action
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Fig. 10
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Reduction in failure of clamping pad after implementing corrective measures

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Standardization

Developing standards for drawings and designs are an essential step for any industrial organization. With proper definition and implementation of standards, defects or failures can be prohibited in production line and at the same time constitute procedures to prevent the occurrence of other errors that could have an impact on production. It is therefore desirable to standardize all processes carried out in the manufacturing sector [3]. After successful trial of clamping pad on fixtures, standardization was done by allotting engineering drawing part code. The standardized clamping and fixture parts are shown in Fig. 11 that can be directly fed to a manufacturing line of any industry and will perform effectively and efficiently in a leak testing process.

Fig. 11
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Final standardized 3D concept

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Conclusions

This article presents a new approach for investigating the problem of clamping pad failure leading to rejection of cylinder block during leak testing process. The failure analysis is carried out to understand the root cause of failure. The Kepner-Tregoe analysis is adopted to solve the problem and suggest preventive measures. It gives a benefit in solving even those problems which are not visible and creating hindrance. The problem detected is mainly the crack generation in clamping pad resulting in failure to hold the part during leak testing process and that lead to rejection of parts in further testing. The step-by-step methodology suggested appropriate preventive actions that successfully reduced the clamping pad replacement by 90% during leak testing process. This method of problem selection at manufacturing line is unique, and problem selection is coming from workers who are working at manufacturing line. The whole approach was unique and easy to adopt which can be implemented in different domains in manufacturing sector to prevent any such problems and issues from continuing to a great extent and thus can improve their services effectively.