First steps towards a design for impact resistance method
E. Tempelman PhD MSc and R. Wever MSc
Faculty of Industrial Design Engineering, Delft University of Technology
Abstract
For many everyday products, mechanical shock a.k.a. impact is among the main causes for premature failure. One would therefore expect that during the conceptual design of new products, impact resistance is one of the main drivers, to be optimized along with functionality, size and weight, styling and of course cost. However, as this paper shows, impact is at best an issue during design engineering (i.e. after many conceptual choices, such as product lay-out, have already been made) and even then, it is usually treated through trial-and-error. Beyond trivial guidelines, no useful design method seems to exist for the concept phase.
With users increasingly demanding care-free and hence, impact-resistant products, and with technologies such as flexible electronics now emerging from the laboratories, design for impact takes on a new urgency. At Delft University of Technology, a research project has therefore been started that aims at developing a method for designing impact resistant products, encompassing both the conceptual and engineering design phases of new products and addressing their full lifecycle (i.e. production, packaging & transport, use and end-of-life). Active partners include external companies and design agencies as well as BSc and MSc students.
This paper presents the current status of this project, including the first results and conclusions. Feedback from the academic community is greatly appreciated.
1. Introduction
In the design, development and testing of packaging, the actual content is normally treated as a given, as also argued in (Wever et al 2008). In other words: first the product is designed, then its packaging. The research presented here takes a different view and considers product design not as a given, but as a variable instead. In particular, it is concerned with how products can be designed for resistance to
mechanical shocks a.k.a. impacts. This will be considered here primarily for handheld electronic products, where impact resistance must be balanced against many other considerations, especially cost, size and appearance. Examples are cell phones, digital cameras and portable game computers, but also shavers and electric toothbrushes: for such products, impacts due to accidental drops etc. are among the major causes – if not the major cause – of premature product failure (Amy et al 2009).
Packaging design plays an important role in this research. After all, impacts do not only occur during use, but also – some might argue: and especially – during transport. This is one of the reasons why products such as those mentioned above require suitable packaging during transport. From this it follows logically that if products are made more impact-resistant during use, they might also require less packaging during
transport. In fact, the reverse is also true in the sense that packaging engineers may have something to contribute to conceptual product design. This is illustrated by two rare examples of packaging insights leading to product redesigns.(Nielsen 1994) describes a redesign of a fragile computer component. Adding some material improved the impact resistance of the component considerably. The design change cost $1, while the saving in packaging represented $10.80. (Klooster 2002, p. 25) gives an example of a photocopier made by Océ. Packaging designers examined the product to see from which transportation hazards it needed protection. They found that this were mainly vibrations exciting components at their natural frequencies. After redesigning these components, 70% of the copiers could be transported without any packaging whatsoever.
This paper first presents a proposed research project entitled “Creating a method for designing impact resistant products” along with its preliminary studies, now almost completed. Moving on to the results, the outcome of these preliminary studies is presented and discussed. Since the main research project is currently starting, no actual results can be given here. Instead, the expected outcome is briefly touched upon. The paper ends with some tentative conclusions.
2. Methods and materials
2.1. Methods of preliminary studies
At our faculty, design for impact resistance has recently been the subject of four small preliminary studies executed consecutively: desk & field research, two BSc student research projects and one MSc student design engineering project. The methods employed are described below.
Desk & field research
This comprised a literature survey and interviews with Netherlands-based companies for whom product impact resistance is an important aspect during design and
engineering. The companies included three manufacturers (of consumer electronics, copiers and power tools respectively), four design agencies, one electronics R&D institution and one packaging research institute. The main research questions during the interviews were:
What is the current and future relevance of product design for impact resistance?
Which tools and methods are used during product conceptual design to ensure that the product has optimal impact resistance?
Which tools and methods are used during product design engineering to ensure that the product has optimal impact resistance?
BSc student research projects
These projects were executed by two teams of two third-year industrial design engineering students, supervised by the first author. One team focused on user perception of product impact failure. Using a fixed questionnaire, they interviewed 101 people in total. The group consisted of their fellow students at the TU Delft campus as well as a comparable number of randomly-selected visitors of a local
electronics shop. Their research aimed (i) to substantiate the claim found in literature that product impact is a relevant failure mode, (ii) to determine the kinds of impact damage that users acknowledge (from small scratches to ‘total loss’) and (iii) to learn who users blame for impact damage (themselves and/or the manufacturer) as a function of impact height and surface. The products considered were cell phones, digital cameras and laptop computers1.
The other team built a test facility in which small products can be dropped in free-fall onto any surface material as desired by the tester (e.g. carpet, wood or stone), in which the impact between product and surface can be observed separately via a high-speed video camera (type: MiniVis Speedcam, procured from Weinberger) and an accelerometer positioned in or on the product (type: triaxial ceramic shear ICP ® accelerometer, procured from PCB Piezotronics). Such a ‘drop tower’ was chosen because it most closely simulates the impacts that may occur during product use, as opposed to e.g. vibration shock testers or pendulum-swing impact testers. Using the basic equations of packaging design described in (Brandenburg and Lee 2001) and (Klooster 2002), the team made predictions of g-levels during impact inside a solid aluminum ball Ø 50 mm, which were then experimentally verified to determine if the set-up worked correctly for drop heights between 0.1-0.6 m (surfaces: 8 mm office carpet or solid steel plate).
MSc student design engineering project
This project started last January and will run until July this year, as the only preliminary study not yet completed. It is executed by four fourth-year industrial design engineering students as their ‘Project Advanced Products’, supervised by an assistant professor of the faculty and with the first author acting as their client (Tempelman et al 2007).Their focus was and is on refining the drop test facility mentioned above in several ways, in particular by optimizing the use of high speed video and the accelerometer, and by synchronizing the data streams. Also, the product hold-and-release mechanism is being redesigned to allow for easy and reproducible setting of product orientation during impact.
2.2. Methods of proposed research project
The faculties of Industrial Design Engineering (IDE) and Mechanical, Maritime & Materials Engineering (3ME) of the TU Delft have jointly planned a five-year research project that aims to create a method for designing products that are resistant to impact, balancing this property against cost, size and other requirements. The method is primarily intended for the design of handheld and tabletop electric and electronic consumer products and will address conceptual as well as engineering design. It will consider impacts during transport (i.e. on the product plus packaging) as well as during use (i.e. on the product only), categorizing the different kinds of impacts that occur in terms of frequency, drop height and drop surface characteristics. Next to consumer products, the method will address mechatronic components, mainly addressing impact during transport (i.e. packaged).
1
The research will be done by two PhD students and one postdoc working at the TU Delft, supported by the permanent staff. To ensure the practicality of their method, they will cooperate intensively with three industry partners during the start of the project and during validation of the method in the last 18 months. One PhD will start by analyzing existing case study products, the other by analyzing new, self-designed product concepts (i.e. same topic, but different perspectives). The postdoc will support them by making 3ME’s existing hybrid modeling (= modeling using results from simulations as well as from physical characterization testing) method for analyzing vibrations suitable for impact analysis (Rixen 2004, Klerk et al 2008).
In this hybrid modeling approach, the different components (packaging, housing, LCD, battery, PCB’s etc.) will be separately modeled and measured in the time domain in order to create a modular assembly of a full product based on the knowledge of its components. Important issues here, in addition to component characterization for impact, are the assembly through flexible connections and the reduction of sub-models using e.g. decomposition into non-linear unit impulse response functions. This is similar to component mode synthesis in the frequency domain; the approach is also similar to finite element modeling (FEM). Again, suitable case studies will be used to build towards general understanding.
3. Results and Discussion
3.1. Results of preliminary studies
Desk & field study
In the companies interviewed in this study, impact resistance was found to be a driver for the design of many handheld and even some tabletop products. However, a
comprehensive method for dealing with impact during design was found to be lacking. This is underlined by our survey of available literature on design methodology: for instance, the influential (Pahl et al 2007) not so much mentions impact in their otherwise comprehensive list of Design-for-X rules. Instead, a pragmatic but costly and time-consuming cycle of design-test-redesign is followed, as also argued by (Xhou et al 2008), which leaves considerable potential for improvement unexplored. Tools and methods for conceptual design were found to be limited to trivial guidelines, such as “avoid sharp corners”, and key input data for the design process (in terms of acceptable drop height, frequency and contact surface compliance) were conspicuous in their absence. During engineering design, the companies were found to use FEM software to simulate product impact, as well as physical methods, such as free-fall drop testing. Power tools present a noteworthy variation to this theme, as their certification requires them to either survive certain drops without impaired safety or survive such drops not at all – and be tested as such. However, at all companies, correspondence between simulations and test results was found to be poor, and correspondence between testing and actual field data was known to be weaker still. It emerged that because of the range of new components, materials and technologies becoming available (e.g. flexible electronics, multi-layered ceramic embedded systems), past results may no longer apply to new product designs, also because consumer expectancies with regard to impact resistance are on the rise. On the other
hand, improvements in software and hardware were identified also that can contribute to research into product impact resistance, in particular high speed video cameras, strain gauges and accelerometers.
First BSc student research project
This project confirmed the expectation that impact is indeed a significant failure mode, adding to this not only that most consumers wish for more impact-resistant products, but also the striking finding that 90% of all interviewees thinks that manufacturers could make tougher products if only they would want to – without compromising product cost, size or appearance. Furthermore, the research revealed that “product failure due to impact” can mean many different things and should be defined in categories – which is notably not the case in e.g. product drop testing standards. Finally, two-thirds of the interviewees expected the products to survive a drop onto a table (blaming the manufacturer if it does not), whereas only one-fourth expected them to survive drops onto the floor (blaming themselves otherwise).
MSc student design engineering project
The second BSc research project essentially laid down the groundwork for the
subsequent MSc design engineering project and the main results (so far) are combined here. It was found that drop testing was certainly possible with the initial set-up. It gave results that correspond well to what was predicted on the basics of the available theory, with peak accelerations spanning a range of 100-600 g depending on drop height and surface. However, drop testing was found to require special attention in order to prevent the accelerometer from unduly affecting the results, particularly for smaller products. Wireless sensors are relatively heavy (with obvious consequences for the test), while wire-based sensors significantly change the product’s effective moments of inertia, affecting rebound and hence, accelerations. Also, vibrations of the drop tower upon impact were found to exert more influence than expected. A
modified design for the drop test facility has been generated that takes these (and other) factors into account, which is currently being realized.
3.2. Results of proposed research project
As the project is currently awaiting financing and go-ahead, only the expected results can be given here. Essentially, these comprise the validated design method plus the knowledge behind it: a quantified understanding of severity and frequency of impacts that products have to withstand, and particularly of how product impact resistance can be modeled even when many details of product and/or its packaging are not yet known. This knowledge will be validated for the design of mechatronic products also.
3.3. Discussion
Returning to the research questions mentioned under sub-section 2.1, it can be stated that for typical consumer products such as cameras, shavers or power tools, design for impact resistance is already relevant to manufacturers as well as to consumers and will become more so in the near future. During the engineering design stage, impact is currently taken along via final validation and – occasionally – simulation plus testing. During the conceptual design stage, however, impact resistance is hardly taken along
at all, and beyond trivial guidelines, no methods or tools seem to exist. From this, there seems to be a definite need for the research outlined in sub-section 2.2 above. Product impact, however, is a complex problem, and the research presented so far has only touched upon it briefly. Even after five years, it cannot be expected that it will have explored the full depth and breadth of the subject. Moreover, the actual impact resistance of products may turn out to be so strongly influenced by choices that can realistically only be made after conceptual design that the method is doomed to failure. Soldered joints present a case in point: (Wonga et al 2008) have found that these can be decisive for a product’s impact resistance, but it can be argued that applying such joints or not may not always be a realistic variable during concept design. However, as a counter-example, for a ‘classic’ product lay-out consisting of strong housing surrounding fragile inner components, there normally is a trade-off between size and impact resistance: the larger the space between housing and components, the more opportunity there is to apply e.g. shock-dampening flexible connections, as argued by (Corben and Burgess 2003). This would be a clear case of a choice made during conceptual design that affects impact resistance.
By extension, it can be stated that product design for impact resistance must address packaging also, since products sustain severe impacts not only during use but also – and sometimes especially – during transport, i.e. while packaged. It remains to be seen how fruitful such an integrated approach will be.
4. Conclusion
The bulk of the research presented here concerns work-in-progress and therefore, only generic conclusions are given. The main one is that for consumer products for which impact resistance is a significant issue, many of the companies designing them currently seem to be struggling to deliver impact performance in a controlled design process, relying more on trial-and-error than on a knowledge-driven approach. The TU Delft hopes to contribute to changing this situation for the better and has recently commenced doing research in the matter, looking in particular how impact can be addressed earlier in the product design process than it is today. The method for
designing impact-resistant products to be developed will not only address use, but also transport, and will therefore have consequences for packaging design.
Acknowledgements
The authors gratefully acknowledge the valuable input and support delivered in this research by our industrial contacts and by our students.
References
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Additional sources, available from the authors:
Cattaneo, S., Homs, S., Knippenberg, N. and Kuiper, E.: Design for impact resistance – Analysis & initial design, MSc student design engineering project report, faculty IDE, 2009
Linden G. van der and Verschoor, J.: Design for impact resistance, BSc student research project report, faculty IDE, 2008
Stricker, E. and Visser, T. 2008: User perception of product failure, BSc student research project report, faculty IDE, 2008
Tempelman, E.: Discussion paper – creating a design for impact resistance method, TU Delft preliminary study, faculty IDE, 2009
