Lesson 14 Test Method for Product Fragility 第 14 课 产品脆值试验方法

Test Method for Product Fragility A shock machine is used to generate a damage boundary curve A vibration system is used to map out the natural frequencies of a product.

Shock: Damage Boundary Shock damage to products results from excessive internal stress induced by inertia forces - Since F=ma, shock fragility is characterized by the maximum tolerable acceleration level, i. e, how many g’s the item can withstand. - Why damaged? - How to reduce g’s ? The packaging material changes the shock pulse delivered to the product so that the maximum acceleration is greatly reduced (and the pulse duration is many times longer). - The package designer’s goal: To be sure that the g-level transmitted to the item by the cushion is less that the g-level which will cause the item to fail.

Shock: Damage Boundary The damage boundary theory is used to determine which shock inputs will cause damage to a product and which will not. - Two parts of a shock can cause damage: 1. the acceleration level A 2. the velocity change ∆V (the area under the acceleration-time history of the shock, thought as the energy contained in a shock) - The critical velocity change(∆Vc): a minimum velocity change which must be achieved before damage to the product can occur. 1. Below ∆Vc, no damage occurs regardless of the input A 2. Exceeding ∆Vc, does not necessarily imply that damage results. a. If ∆V occurs in a manner which administers acceptable doses of acceleration to the product, the velocity change can be very large without causing damage. b. If ∆Vc and Ac are both exceeded, damage occurs. Figure 14.1: Typical damage boundary curve

Shock: Damage Boundary Implications of Fig.14.1: a. if the input ∆V<the product’s ∆Vc, then the acceleration level of the input can be in the 100 G’s, 1000 G’s, 10,000 G’s, or even without causing damage. In fact, the duration is so short that the product cannot respond the acceleration level of the event, only the energy input. b. if the input ∆V>the product’s ∆Vc, However, the only way to avoid damage is to limit the input A < the product’s Ac. This is usually one of the functions that a cushioned package performs: it translates the high acceleration events experienced on the outside of the container to lower acceleration events experienced inside at the unit.

Shock: Damage Boundary Figure 14.1 Typical damage boundary curve

Shock: Damage Boundary c. For ∆V< ∆Vc, area where damage does not occur even with very high accelerations. Here ∆V (drop height) is so low that the item acts as its own shock isolator. d. <Ac, damage does not occur, even for large ∆V. That’s because the forces generated (F =ma) are within the strength limits of the products. - From Fig. 14.2, a. ∆Vc boundary (vertical boundary line), is independent of the pulse wave shape. b.Ac (to the right of the vertical line) for half sine and sawtooth pulses depends upon ∆V.

Shock: Damage Boundary Figure 14.2 Damage boundary for pulses of same peak acceleration and same velocity change

Shock: Damage Boundary c. The damage boundary generated with use of a trapezoidal pulse encloses the damage boundaries of all the other waveforms. - Fragility testing is the process used to establish damage boundaries of products. a. It is usually conducted on a shock testing machine. The procedure has been standardized (ASTM D3332, Mechanical-Shock Fragility of Products, Using Shock Machines). b. Procedure: the item to be tested is fastened to the top of a shock machine table and the table is subjected to controlled velocity changes and shock pulses. The shock table is raised to a preset drop height. It is then released, free falls and impacts against the base of the machine; it rebounds from the base and is arrested by a braking system so that only one impact occurs.

Shock: Damage Boundary c. For trapezoidal pulses, the programmer is a constant force pneumatic cylinder. The g-level of the trapezoidal pulse is controlled simply by adjusting the compressed gas pressure in the cylinder. The ∆V is controlled by adjusting drop height. A Shock Testing Machine (1)

Conducting a fragility test To determine a damage boundary requires running two sets of tests. - A step velocity test is used to determine the product’s ∆Vc and a step acceleration test is used to determine Ac. 1.Step Velocity Test( Figure 14.3) to determine the vertical line of the damage boundary. 2.Step Acceleration Test(Figure 14.4) to determine the horizontal line of the damage boundary.. a. A new test specimen be attached to the shock table. b.The drop height is set at a level which will produce a velocity change at least 1.57 x ∆Vc. c. The programmer compressed gas pressure is adjusted to produce a low g-level shock.

Conducting a fragility test Figure 14.3 Velocity damage boundary development

Conducting a fragility test A Shock Testing Machine (2)

Conducting a fragility test 3. Plot the damage boundary curve by connecting the vertical velocity boundary line and the horizontal acceleration boundary line. 4. Notes: In a rigorous testing program, damage boundary curves are generated for each orientation of the unit. Compromises are often made to limit the number of units which must be damaged. Figure 14.4 Damage boundary line development

Vibration: Resonance Search & Dwell It is generally accepted that the steady-state vibration environment is of such low acceleration amplitude that failure does not occur due to non-resonant inertial loading. - Damage is most likely to occur when some element or component of a product has a natural frequency which is excited by the environment. - The identification of those frequencies becomes critical in designing a package system. The purpose of the bare product vibration testing is to identify the natural or resonant frequencies of the critical components within the product. - Response of a product or component to input vibration may be represented by a curve similar to that shown in Figure 14.5.

Vibration: Resonance Search & Dwell Figure 14.5 Typical resonant frequency transmissibility curve

Vibration: Resonance Search & Dwell Vibration transmissibility curve shows: a. For very low frequencies, response acceleration is the same as the input; b. For very high frequencies, the response is much less than the input. c. But in between, the response acceleration can be many times the input level. This is the frequency range where damage is most likely to occur. - How to identify the product and component resonant frequencies: a. ASTM Standard Method D3580, Vibration Test of Products. b. The resonance search is run on a vibration test machine (shaker). c. Resonant effects can be seen or heard directly or by use of a stroboscope and/or various sensors - Notes: In general, tests should be performed in each of the three axes. If the product is mounted on a definite skid base, only the vertical axes need to be analyzed.

Vibration: Resonance Search & Dwell A Vibration Testing Machine (1)

Vibration: Resonance Search & Dwell A Vibration Testing Machine (2)